Imaging PhysicsFree Access

Radiation Dose Reduction at Pediatric CT: Use of Low Tube Voltage and Iterative Reconstruction

Published Online:https://doi.org/10.1148/rg.2018180041

Abstract

Given the growing awareness of and concern for potential carcinogenic effects of exposure of children to ionizing radiation at CT, optimizing acquisition parameters is crucial to achieve diagnostically acceptable image quality at the lowest possible radiation dose. Among currently available dose reduction techniques, recent technical innovations have allowed the implementation of low tube voltage scans and iterative reconstruction (IR) techniques into daily clinical practice for pediatric CT. The benefits of lowering tube voltage include a considerable reduction in radiation dose and improved contrast on images, especially when an iodinated contrast medium is used. The increase in noise, which is attributed to decreased photon penetration, is a major drawback but is not as severe as that at adult CT because of the small body size of children. In addition, use of IR algorithms can suppress increased noise, yielding wider applicability for low tube voltage scans. However, a careful implementation strategy and methodologic approach are necessary to maximize the potential for dose reduction while preserving diagnostic image quality under each clinical condition. The potential pitfalls of and topics related to these techniques include (a) the effect of tube voltage on the surface radiation dose, (b) the effect of window settings, (c) accentuation of metallic artifacts, (d) deterioration of low contrast detectability at low-dose settings, (e) interscanner variation of x-ray spectra, and (f) a comparison with the use of a spectral shaping technique. Appropriate use of low tube voltage and IR techniques is helpful for radiation dose reduction in most applications of pediatric CT.

Online DICOM image stacks are available for this article.

©RSNA, 2018

An earlier incorrect version of this article appeared online. This article was corrected on March 28, 2019.

See discussion on this article by Kanal.

SA-CME LEARNING OBJECTIVES

After completing this journal-based SA-CME activity, participants will be able to:

  • ■ Explain the concern about potential cancer risks associated with pediatric CT.

  • ■ Describe the benefits and pitfalls of low tube voltage scans and IR techniques for pediatric CT.

  • ■ Discuss the approach to optimal pediatric CT protocols for various clinical tasks by using low tube voltage scans and IR techniques.

Introduction

Since the introduction of CT in the 1970s, its use has increased rapidly because of excellent diagnostic accuracy, availability, short acquisition time, and cost-effectiveness (1). Although the increasing use of CT has led to tremendous advances in modern medical practice, this trend has been accompanied by a growing concern for patient safety, because CT delivers higher doses of radiation than do most other diagnostic imaging procedures (1). CT represents only 17% of all radiologic procedures in the United States, but it is the largest medical source of radiation exposure, contributing almost 50% of the collective effective dose (2). In this context, risk model estimations and results of recent large-scale epidemiologic studies have predicted an increased risk of cancer associated with CT radiation exposure in radiosensitive children and adolescents (38).

The true risk of CT remains unclear, and the benefit of an appropriately indicated CT scan may far exceed the associated risk. Nevertheless, increasing awareness of potentially harmful effects led to the justification and optimization of CT scans according to the principle of reducing radiation doses to “as low as reasonably achievable” (ALARA).

Among currently available CT dose optimization techniques, recent hardware and software advances have rendered low tube voltage scans and iterative reconstruction (IR) algorithms to be widely applicable and recommended techniques for low-dose pediatric CT. Investigators have demonstrated utility under various clinical conditions, but a careful implementation strategy and a methodologic approach are necessary to maximize the potential for dose reduction while preserving diagnostic image quality (9,10).

The aim of this article is to describe comprehensively why radiation doses must be reduced, how to use a low–kilovolt peak and IR techniques, and the relevant pitfalls and related topics.

Why We Must Reduce Radiation Dose

Radiation-induced carcinogenesis is generally accepted as a stochastic process, whereby the probability of the occurrence of the effect increases with increasing radiation dose, but the severity of the effect is not influenced by the dose. Children and young adults are more sensitive to the stochastic effects of ionizing radiation because their young bodies undergo rapid cell division. Furthermore, the relatively long remaining life span of children and young adults leaves ample time for expression of potential radiation effects compared with that of adults (1,1113). In addition, children receive higher effective doses because of their smaller body size if dedicated pediatric CT protocols are not applied. Compared with children’s hospitals, community hospitals (where most pediatric patients are likely to undergo CT) are more likely to use adult-calibrated CT protocols (14).

For these reasons, the potential increase in cancer risk is of concern in children who require CT scans, although considerable uncertainty exists in this matter.

Models for Estimation of Radiation Risk and Their Limitations

In 2001, Brenner et al (3) first assessed lifetime cancer risk induced by ionizing radiation exposure from pediatric CT. They projected that 500 of the children who undergo CT each year would ultimately die of cancer attributable to ionizing radiation at CT on the basis of radiologic practices at that time in the United States.

More recently, Berrington de González et al (4) and Miglioretti et al (5) projected that pediatric CT administered in a given year in the United States would result in 4350–4870 future radiation-induced cancers (4,5). These risk estimations are based on data from the Life Span Study of Japanese survivors of the atomic bombs (11). The application of increased risks observed in the Life Span Study to those of CT radiation has been questioned because of differences in the source of radiation, the population exposed, and assumption of a linear nonthreshold association at the low-dose range (<100 mGy).

Some scientists argue that low-dose radiation exposure may not increase the risk for cancer and may even provide health benefits (ie, the hormetic effect) (15). Because of the uncertainty about the appropriateness of hypothetical risk estimation, analysis of observational epidemiologic data on CT-exposed patients is necessary for accurate risk calculation.

Epidemiologic Studies and Potential Biases

Since 2012, large-scale epidemiologic studies (68,1619) have been published in an attempt to reveal direct evidence of carcinogenesis attributable to pediatric CT, and others are underway (20,21). Studies published before 2014 (68) showed potential evidence of a dose-response relationship between pediatric CT and risk for brain tumor and leukemia, which are the most common radiation-induced neoplasms among children (Table 1). However, interpretation of these epidemiologic data is limited by lack of information about the reasons CT scans were performed, probably resulting in “reverse causation” and “confounding by indications” biases.

Table 1: Summary of Early Epidemiologic Studies on Cancer Risk from Pediatric CT

Table 1:

Note.—Numbers in parentheses are reference numbers.

Reverse causation occurs when CT scans are performed because of initial signs or symptoms of cancer. This cancer later may be assumed to be the consequence of CT, rather than the reason for the scan. To minimize this bias, the exclusion interval (ie, lag time) from the time of CT exposure to cancer diagnosis was set at 1–5 years. This period may have been too short to exclude reverse causalities completely (22,23).

Confounding by indications can occur when analyzed patients have conditions that predispose them to cancer (eg, genetic syndromes and immune deficiencies related to carcinogenesis and previous cancer) and undergo CT examinations for diagnosis or monitoring of the conditions themselves. In such conditions, radiation exposure from CT examinations cannot be regarded simply as the cause of cancer. Confounding by indications has become one of the leading concerns and the dominant focus of more recent studies (1619,24). Results of all of these investigations except one (24) have suggested that there was evidence of confounding effects of conditions that predispose a patient to cancer and potential for overestimation of CT risks (Table 2) (1619). Nevertheless, these results still seem to support a dose-response relationship in pediatric CT, although some reported findings did not reach statistical significance, usually because of a small sample size and short observational periods.

Table 2: Epidemiologic Studies Focused on Conditions That Predispose Patients to Cancer

Table 2:

Note.—Numbers in parentheses are reference numbers. NA = not applicable.

*Not statistically significant.

Reanalysis of United Kingdom cohort (6).

Estimation of confounding bias in relative risks was based on expert opinion about CT scan patterns among patients with cancer susceptibility syndrome.

Another potential bias in epidemiologic studies is uncertainty in estimations of the absorbed radiation dose, which is not individualized and is based on age-adjusted typical doses for a given device or for CT scans performed during a particular year. Additional absorbed radiation doses from undocumented repeat examinations (eg, those performed because of patient movement) also are not considered for risk estimation (22).

Contrast medium administration could result in higher absorbed doses than expected, although the effect of contrast media on associated risks is currently unknown (25). In addition, none of these studies included doses from other forms of medical imaging (eg, nuclear medicine and fluoroscopy). These uncertainties regarding the absorbed radiation dose in the CT-exposed cohort may have led to overestimation of the cancer risks of CT. The upcoming European Epidemiologic Study to Quantify Risks for Pediatric Computed Tomography and to Optimize Doses (EPI-CT) study, which was designed to include approximately 1 million children exposed to CT, will offer opportunities to better address these limitations of the uncertainties induced by incomplete dosimetry (21).

Benefits of Radiation Dose Reduction at Pediatric CT

Results of several investigations have suggested the potential benefits of reducing radiation dose for prevention of possible future carcinogenesis attributable to CT. Miglioretti et al (5) demonstrated that reducing the highest 25% of doses of pediatric CT to the median level might prevent 43% of future CT-induced cancers. Journy et al (26) estimated that when the dose per CT scan was reduced by 20% and 40%, respectively, compared with past practices in the United Kingdom, the number of future cancers potentially induced at pediatric CT was reduced by 20% and 40%.

Teaching Point The actual benefit of CT dose reduction remains unclear, because estimations are based on risk projection models. However, given that results of recent epidemiologic studies on natural background (27,28) and occupational (29,30) protracted exposure and pediatric CT appear to be consistent with the linear nonthreshold assumption, adherence to the ALARA principle is relevant and beneficial to protect children from possible carcinogenesis caused by CT.

Radiation Dose Reduction at Pediatric CT

Justification and optimization are fundamental principles for protection from medical radiation. Justification implies that CT studies should be performed only when the clinical benefit to the patient clearly exceeds potential adverse effects of radiation exposure. All physicians and radiologists must first ensure the appropriateness of the clinical indication to avoid unnecessary scans, which is the most important step for reducing overall radiation exposure at CT. Even when an imaging examination is justified clinically, alternative modalities that do not involve exposure to ionizing radiation must be used in some clinical settings (eg, US for acute appendicitis). Clinical decision rules (eg, the Pediatric Emergency Care Applied Research Network [PECARN] rule for minor head trauma) provide appropriate CT indications, and use of them helps to avoid unnecessary CT examinations (31). Fortunately, pediatric CT use in the United States has stabilized and declined slightly since 2007 (5), probably because of increased awareness about cancer risks and permeation of the ALARA concept, in part due to the “Image Gently” campaign started in 2007 (32).

After an appropriate clinical indication and justification have been confirmed, imaging settings can be customized to obtain the required clinical information at the lowest dose possible. Application of the scanning parameters used in adults to children must be avoided, because it leads to a substantial increase in the effective dose without providing additional benefits for diagnosis (33,34). If a single-phase scan can provide sufficient diagnostic information, avoidance of multiphase scanning (eg, nonenhanced CT) allows for a drastic reduction in dose (35). The length of the scanning area on the body also should be limited to the area of concern to eliminate unnecessary radiation. Various techniques such as automated exposure control, lowering tube current and tube voltage, high-pitch acquisition, and IR algorithms can help in reducing radiation dose or improving the image quality of low-dose pediatric CT (36).

Low Tube Voltage Scans

Teaching Point The low tube voltage scan has the advantages of reduced radiation dose and improved image contrast. Disadvantages include increased image noise and artifacts. Appropriate settings must be selected to balance these benefits and drawbacks at each examination to achieve diagnostic image quality at the lowest possible radiation dose.

Radiation Dose Reduction.—Radiation output is proportional to the square of tube voltage but shows a linear relationship with tube current. This means that a small reduction in tube voltage leads to substantial dose reduction in comparison with tube current. Reducing tube voltage from 120 kVp to 100 kVp or 80 kVp theoretically yields dose reductions of 33% and 65%, respectively, when all other parameters remain unchanged (37). Even if tube current and exposure time are increased to compensate for reduced photon flux, low tube voltage scans reduce the radiation dose for most pediatric CT examinations, while preserving diagnostic image quality.

Contrast Improvement.—By lowering tube voltage, intertissue contrast tends to be improved to varying degrees. At lower tube voltage settings, CT attenuation of elements with a higher atomic number (eg, calcium and iodine) substantially increases because of greater photoelectric effects. This is not the case for elements with lower atomic numbers (eg, in muscle or fat), because Compton scattering is the dominant interaction in these tissues (9). With the use of a contrast medium, attenuation of iodinated structures steeply increases as average photon energy approaches the iodine k-edge (33.2 keV), which yields better depiction of vascular structures and conspicuity of parenchymal organ lesions showing hyper- and hypovascularity relative to surrounding tissues. In their phantom experiments, Yu et al (9) demonstrated that, on average, iodine attenuation at 100 kVp was 25% higher than that at 120 kVp, and iodine attenuation at 80 kVp was 70% higher than that at 120 kVp. This contrast improvement at low tube voltage settings can compensate for increased noise and provide equivalent or even higher contrast-to-noise ratios than those obtained with standard tube voltage settings.

Increased Image Noise.—The increase in image noise due to less photon penetration may negatively affect image interpretation and compromise diagnostic confidence for images obtained with low tube voltage. However, the degree of this adverse effect is highly dependent on object size (3841). In addition, a tolerable increase in noise (ie, dose reduction potential) also varies depending on the diagnostic task and organ of interest and whether a contrast agent can be used (41). A scanner’s x-ray output is another important factor in determining tube voltage, because higher tube current is usually necessary in low tube voltage scans to compensate for increased noise, especially for larger patients.

Tube Voltage Selection Based on Body Size.—When the scanning object is large, photons need more energy to penetrate, and higher tube voltage may be necessary to obtain a diagnostically acceptable level of noise.

Teaching Point When the object to be imaged is small, photons need less energy to penetrate, and lower tube voltage provides sufficient energy penetration, leading to radiation dose reduction while preserving or even improving image quality (9). Therefore, low tube voltage scans are better suited to and more widely applicable for children than for adults to achieve radiation dose reduction and improvement of image quality.
To take patient body habitus into consideration, a technique chart based on body mass index, lateral diameter, or body circumference is useful in daily clinical practice (38,42).

Tube Voltage Selection Based on Diagnostic Tasks.Clinical tasks and the anatomic region of interest should be taken into consideration to determine optimal tube voltage, because expected contrast improvement and noise tolerability depend on these factors (41).

Teaching Point In general, low tube voltage is more beneficial for contrast-enhanced CT or diagnostic tasks involving assessment of high-contrast abnormalities than for nonenhanced CT or diagnostic tasks involving low-contrast abnormalities.
For diagnosis of high-contrast abnormalities (eg, with CT angiography and lung or bone CT), noisier images are tolerable, and thus, lower tube voltage provides diagnostic-quality images at low doses of radiation. Low-contrast diagnostic tasks such as nonenhanced brain or abdominal CT require less image noise to maintain lesion conspicuity, and thus, lower tube voltage scans may degrade diagnostic performance at extremely low-dose settings. Even for low-contrast diagnostic tasks, if contrast between the organ of interest or lesions and adjacent tissue can be considerably improved at lower tube voltages (eg, contrast-enhanced hepatic or pancreatic CT), an increase in noise is more permitted, resulting in a reduction in radiation exposure.

Tube Voltage Selection Based on Tube Current Limit.—When tube voltage is decreased, a compensatory increase in tube current usually is required for adequate photon penetration and preservation of acceptable image noise and artifact levels. Therefore, it is essential to take into account the tube current limit of each scanner in selecting tube voltages. Scanners with lower-powered generators may not be able to provide adequate tube current at extremely low tube voltage and may produce images with considerable noise and photon starvation, especially in adolescents with larger bodies (Fig 1). To achieve adequate x-ray penetration, the next-higher tube voltage should be considered in such cases.

Figure 1.

Figure 1. Suspected traumatic liver injury in a 17-year-old boy (weight, 98 kg). Axial CT image was acquired at 120 kVp and 580 mAs (volume CT dose index [CTDIvol], 19.6 mGy). The patient could not elevate both arms because of traumatic shoulder pain, and thus, image quality was somewhat compromised by increased noise and beam hardening artifacts, even at 120 kVp. The maximum tube currents for this scanner were 800 mA at 120 kVp and 633 mA at 100 kVp and 80 kVp. To perform lower tube voltage scans with higher tube current, a scanner equipped with a more powerful x-ray generator is necessary.

Automated Tube Voltage Selection Tool.—Manual selection of the optimal combination of tube voltage and tube current for each examination is time-consuming and sometimes challenging. Automated tube voltage selection systems (CARE kV; Siemens Healthineers, Erlangen, Germany; kV assist; GE Healthcare, Milwaukee, Wis) suggest optimal tube voltage and corresponding tube current to attain predefined image quality (ie, a contrast-to-noise ratio with a limitation of an acceptable image noise level) at the lowest radiation dose, on the basis of (a) patient size (an attenuation profile derived from a scout image), (b) clinical tasks (eg, nonenhanced, parenchymal contrast-enhanced CT, and CT angiography), and (c) tube capability (tube current limit at each kilovolt peak). Because lower voltage increases iodine contrast material enhancement, algorithms select lower voltage more frequently for CT angiography and contrast-enhanced CT than for nonenhanced CT. With the use of this algorithm for pediatric CT, radiation dose could be reduced by up to 50%, without loss of diagnostic image quality, in comparison with the use of a standard 120 kVp protocol (43,44).

IR Techniques

Image reconstruction techniques have a profound effect on image quality and thus radiation dose. Filtered back projection (FBP) is an analytic approach that has been the standard method in CT image reconstruction for a long time. It is simple and fast but unable to handle complicated factors such as scatter, resulting in increased image noise at low-dose CT. IR algorithms allow a significant reduction in the number of required projection views compared with FBP, while still producing acceptable image quality. IR improves the image quality owing to the cyclic iteration process.

First, an initial image approximation is generated from the measured projection data. Second, simulated new projection data are created from the initial image approximation by means of forward projection with incorporation of statistical noise and system geometric models. Third, forward projection data and original projection data are compared. In the case of discrepancy, the initial image approximation is updated on the basis of the characteristics of the underlying algorithm. This correction process is repeated until minimum differences are found between the two data sets, or until a predefined image quality is achieved.

Because of its mathematically demanding properties, until recently IR has not been practical for clinical purposes. However, owing to recent increases in computing power, IR has become an available option in most modern clinical CT scanners.

Teaching Point By using IR techniques, the higher image noise seen at low tube voltage CT can be reduced effectively, allowing dose reduction while preserving or even improving image quality in comparison with standard tube voltage and FBP techniques.
There are several types of IR approaches that incorporate different x-ray photon noise statistics and system optics modeling to generate better image quality (Table 3). Hybrid IR algorithms blend features of analytic and iterative approaches, whereas the model-based IR implements more complex models of the acquisition process, image statistics, and system geometry, yielding further reduction in image noise and artifacts (10).

Table 3: IR Algorithms for Each Vendor

Table 3:

Note.—ADMIRE = advanced modeled IR, AIDR = adaptive iterative dose reduction, ASIR = adaptive statistical IR, FIRST = forward-projected model-based IR, IMR = iterative model reconstruction, IRIS = IR in image space, SAFIRE = sinogram-affirmed IR, 3D = three-dimensional.

How to Use Low Tube Voltage Scans and IR Techniques for Pediatric CT

Similar to other scanning parameters, low tube voltage and IR techniques must be optimally tailored to each examination so that diagnostic accuracy is maintained at the lowest radiation dose. In this article, we show clinical examples of representative diagnostic tasks for pediatric CT. We consistently use the CTDIvol as a dose metric that is readily available, easily comparable, and used for diagnostic reference levels. However, the size-specific dose estimate is a more preferable dose metric for all CT body examinations because it takes into consideration the patient’s body size for an accurate dose estimation. The size-specific dose estimate is not applicable to head CT examinations (45).

Head CT for Brain Parenchymal Assessment

Nonenhanced head CT performed to evaluate trauma or headache is the most common pediatric CT examination (5). Thus, the importance of confirming appropriate CT use cannot be overstated. For most nontraumatic neurologic disorders, CT is not optimal. MRI is much more sensitive and the preferred imaging modality. CT may not be indicated for evaluation of headache in children if the results of neurologic examination are normal and the patient’s clinical history reveals no definitive risk factors. Indeed, CT seldom leads to a diagnosis or contributes to immediate treatment in this setting (46). Overuse of CT for minor head trauma should be minimized by using appropriate clinical decision-making rules (31,47). In conjunction with meticulous evaluation of indications, optimization of the acquisition parameters for each brain CT examination may allow a substantial overall decrease in radiation exposure to children and adolescents.

Generally, this diagnostic task requires relatively high doses of radiation to acquire high-contrast and low-noise images, which are essential for detection of subtle differences in attenuation such as gray matter to white matter contrast (48). Despite the benefits of the potential for dose reduction and contrast improvement with low tube voltage CT, increased noise and beam hardening artifacts due to skull thickness have prevented this technique from being applied to nonenhanced head CT. However, the degree of those problems is not as serious in children because of their smaller cranial mineral density and head circumference (49,50). The combined use of low tube voltage and IR techniques may facilitate radiation saving and better gray matter to white matter contrast without increasing image noise at pediatric head CT (51,52) (Fig 2).

Figure 2a.

Figure 2a. Evaluation of acute headaches in a girl who underwent two CT examinations, with a 3-year interval. (a) Axial CT image acquired at 120 kVp (CTDIvol, 60 mGy) and reconstructed with FBP shows the girl at 12 years old. (b, c) Axial CT images acquired at 100 kVp (CTDIvol, 30 mGy) and reconstructed with FBP (b) and SAFIRE (c) show the girl at 15 years old. Gray matter to white matter contrast is improved at 100 kVp, because of a greater photoelectric effect, whereas the IR technique is used to compensate for increased noise.

Figure 2b.

Figure 2b. Evaluation of acute headaches in a girl who underwent two CT examinations, with a 3-year interval. (a) Axial CT image acquired at 120 kVp (CTDIvol, 60 mGy) and reconstructed with FBP shows the girl at 12 years old. (b, c) Axial CT images acquired at 100 kVp (CTDIvol, 30 mGy) and reconstructed with FBP (b) and SAFIRE (c) show the girl at 15 years old. Gray matter to white matter contrast is improved at 100 kVp, because of a greater photoelectric effect, whereas the IR technique is used to compensate for increased noise.

Figure 2c.

Figure 2c. Evaluation of acute headaches in a girl who underwent two CT examinations, with a 3-year interval. (a) Axial CT image acquired at 120 kVp (CTDIvol, 60 mGy) and reconstructed with FBP shows the girl at 12 years old. (b, c) Axial CT images acquired at 100 kVp (CTDIvol, 30 mGy) and reconstructed with FBP (b) and SAFIRE (c) show the girl at 15 years old. Gray matter to white matter contrast is improved at 100 kVp, because of a greater photoelectric effect, whereas the IR technique is used to compensate for increased noise.

Head CT for Hydrocephalus

Every year, approximately 4500 children in the United States undergo initial placement of a ventriculoperitoneal shunt for treatment of hydrocephalus (53). Because complications such as infection, obstruction, and overdrainage are common in children with a shunt (54) and patients often undergo multiple follow-up head CT examinations, the need for optimization of acquisition parameters for this diagnostic task must be emphasized. Contrary to detection of brain parenchymal abnormalities, evaluation of ventricular size and shape is a higher-contrast diagnostic task. Therefore, larger dose reduction could be achievable by using lower tube voltage and IR techniques without any deterioration in diagnostic performance (Fig 3).

Figure 3a.

Figure 3a. Two follow-up CT examinations (2-year interval) in a boy who underwent placement of a cerebrospinal fluid shunt for hydrocephalus. (a) Axial image acquired at 120 kVp (CTDIvol, 53.1 mGy) and reconstructed with FBP shows the boy at 3 years old. (b) Axial image acquired at 100 kVp (CTDIvol, 12.7 mGy) and reconstructed with SAFIRE shows the boy at 5 years old. Although image noise is higher on b (even with the use of SAFIRE), the high inherent contrast between the brain parenchyma and ventricles allows drastic dose reduction without loss of diagnostic image quality. Beam-hardening artifacts near the skull are not perceivable, most likely because of the thin cranial bone and low cranial mineral density.

Figure 3b.

Figure 3b. Two follow-up CT examinations (2-year interval) in a boy who underwent placement of a cerebrospinal fluid shunt for hydrocephalus. (a) Axial image acquired at 120 kVp (CTDIvol, 53.1 mGy) and reconstructed with FBP shows the boy at 3 years old. (b) Axial image acquired at 100 kVp (CTDIvol, 12.7 mGy) and reconstructed with SAFIRE shows the boy at 5 years old. Although image noise is higher on b (even with the use of SAFIRE), the high inherent contrast between the brain parenchyma and ventricles allows drastic dose reduction without loss of diagnostic image quality. Beam-hardening artifacts near the skull are not perceivable, most likely because of the thin cranial bone and low cranial mineral density.

Although head CT is the current practice standard for imaging evaluation of shunt malformation, MRI protocols with reduced acquisition time provide a radiation-free and accurate diagnostic alternative. However, longer imaging time, higher cost, and less availability of equipment and/or trained staff may limit the use of quick MRI protocols, especially in emergency settings. Nevertheless, increased familiarity with and availability of quick MRI protocols may lead to a substantial decrease in overall radiation exposure among children with shunted hydrocephalus (55,56).

Head CT for Cranial Synostosis

Cranial synostosis is the premature fusion of cranial sutures, which causes cranial deformity and sometimes, function impairment due to increased intracranial pressure and restricted brain growth. Volume-rendered CT is a valuable diagnostic tool for accurate evaluation, therapy planning, and postsurgical follow-up. For evaluation of synostosis, the inherent high contrast between the skull and cranial sutures or intracranial structures allows for more image noise and thus, the use of low-dose CT is indicated (57). This contrast is further enhanced with lower tube voltage settings because of increased bone attenuation, and combination with IR allows for the preservation of diagnostic image quality (Figs 4, 5). Ernst et al (58) demonstrated that combined use of 80-kVp CT (fixed tube current, 10 mA; rotation time, 0.8 seconds; helical pitch, 0.53; beam collimation, 32 × 0.625 mm) and model-based IR (Veo; GE Healthcare) enables a 97% radiation dose reduction without loss of diagnostic quality for cranial synostosis compared with the dose required for 120-kVp scans (fixed tube current, 276 mA; rotation time, 0.5 seconds; helical pitch, 0.69; beam collimation, 16 × 0.625 mm) with FBP reconstruction (CTDIvol, 0.94 mGy vs 32.18 mGy).

Figure 4a.

Figure 4a. Metopic synostosis in a 3-day-old neonate. Volume-rendered image (a) and sagittal CT image (b) acquired at 100 kVp (tube current, 350 quality reference milliampere second with automated exposure control; rotation time, 1.0 seconds; helical pitch, 0.9; beam collimation, 64 × 0.6 mm; CTDIvol, 11 mGy) and reconstructed with SAFIRE reveal both the presence of metopic synostosis and the absence of substantial intracranial abnormalities such as increased intracranial pressure and restricted brain growth at a low radiation dose.

Figure 4b.

Figure 4b. Metopic synostosis in a 3-day-old neonate. Volume-rendered image (a) and sagittal CT image (b) acquired at 100 kVp (tube current, 350 quality reference milliampere second with automated exposure control; rotation time, 1.0 seconds; helical pitch, 0.9; beam collimation, 64 × 0.6 mm; CTDIvol, 11 mGy) and reconstructed with SAFIRE reveal both the presence of metopic synostosis and the absence of substantial intracranial abnormalities such as increased intracranial pressure and restricted brain growth at a low radiation dose.

Figure 5a.

Figure 5a. Postoperative CT examination in a 20-month-old boy who underwent surgery for unilateral coronal synostosis. Volume-rendered (a) and axial (b, c) CT images were acquired at 80 kVp (tube current, 36 mA; rotation time, 0.5 seconds; helical pitch, 0.89; beam collimation, 64 mm × 0.6 mm; CTDIvol, 1.2 mGy) and reconstructed with IMR (a, c) and FBP (b). A diagnostically adequate volume-rendered image was obtained for global evaluation of the skull. Diagnostic evaluation of intracranial abnormalities is limited at this low dose, although the image quality of c was dramatically improved by using IMR.

Figure 5b.

Figure 5b. Postoperative CT examination in a 20-month-old boy who underwent surgery for unilateral coronal synostosis. Volume-rendered (a) and axial (b, c) CT images were acquired at 80 kVp (tube current, 36 mA; rotation time, 0.5 seconds; helical pitch, 0.89; beam collimation, 64 mm × 0.6 mm; CTDIvol, 1.2 mGy) and reconstructed with IMR (a, c) and FBP (b). A diagnostically adequate volume-rendered image was obtained for global evaluation of the skull. Diagnostic evaluation of intracranial abnormalities is limited at this low dose, although the image quality of c was dramatically improved by using IMR.

Figure 5c.

Figure 5c. Postoperative CT examination in a 20-month-old boy who underwent surgery for unilateral coronal synostosis. Volume-rendered (a) and axial (b, c) CT images were acquired at 80 kVp (tube current, 36 mA; rotation time, 0.5 seconds; helical pitch, 0.89; beam collimation, 64 mm × 0.6 mm; CTDIvol, 1.2 mGy) and reconstructed with IMR (a, c) and FBP (b). A diagnostically adequate volume-rendered image was obtained for global evaluation of the skull. Diagnostic evaluation of intracranial abnormalities is limited at this low dose, although the image quality of c was dramatically improved by using IMR.

Temporal Bone CT

Temporal bone CT is a useful diagnostic tool with which to evaluate abnormalities of middle ear and inner ear structures, as observed in patients with congenital hearing deficits, infection, and trauma. In this clinical task, the depiction of fine anatomic structures is crucial for accurate diagnosis. Lowering tube voltage provides higher attenuation of skeletal structures, yielding better contrast between osseous structures and surrounding tissue. This may allow substantial radiation dose reduction while preserving the image quality for accurate assessment of middle and inner ear anatomy (59,60). IR techniques may improve image quality in low-dose temporal bone CT (61), but deterioration of spatial resolution caused by a strong noise reduction mode should be considered in this anatomic area (Fig 6) (62).

Figure 6a.

Figure 6a. Two follow-up CT examinations (1-year interval) in a boy who underwent tympanoplasty for cholesteatoma. (a) Temporal bone CT image acquired at 120 kVp (CTDIvol, 25.3 mGy) and reconstructed with FBP shows the boy at 10 years old. Black arrow = osseous spinal lamina, white arrow = long crus of the incus. (b, c) Temporal bone CT images acquired at 80 kVp (CTDIvol, 12.8 mGy) and reconstructed with three-dimensional AIDR with high-spatial-resolution mode (b) and FIRST with strong noise reduction (c) show the boy at 11 years old. Higher osseous attenuation of inner- and middle-ear structures was attained at 80 kVp with high-spatial-resolution IR (b), yielding sufficient depiction of fine structures such as osseous spinal lamina (black arrow) and the long crus of the incus (white arrow) with a 50% reduced dose. However, when the 80-kVp image was reconstructed with strong noise reduction, IR fine anatomic structures became unclear because of the deterioration of spatial resolution, in spite of the noise-free appearance.

Figure 6b.

Figure 6b. Two follow-up CT examinations (1-year interval) in a boy who underwent tympanoplasty for cholesteatoma. (a) Temporal bone CT image acquired at 120 kVp (CTDIvol, 25.3 mGy) and reconstructed with FBP shows the boy at 10 years old. Black arrow = osseous spinal lamina, white arrow = long crus of the incus. (b, c) Temporal bone CT images acquired at 80 kVp (CTDIvol, 12.8 mGy) and reconstructed with three-dimensional AIDR with high-spatial-resolution mode (b) and FIRST with strong noise reduction (c) show the boy at 11 years old. Higher osseous attenuation of inner- and middle-ear structures was attained at 80 kVp with high-spatial-resolution IR (b), yielding sufficient depiction of fine structures such as osseous spinal lamina (black arrow) and the long crus of the incus (white arrow) with a 50% reduced dose. However, when the 80-kVp image was reconstructed with strong noise reduction, IR fine anatomic structures became unclear because of the deterioration of spatial resolution, in spite of the noise-free appearance.

Figure 6c.

Figure 6c. Two follow-up CT examinations (1-year interval) in a boy who underwent tympanoplasty for cholesteatoma. (a) Temporal bone CT image acquired at 120 kVp (CTDIvol, 25.3 mGy) and reconstructed with FBP shows the boy at 10 years old. Black arrow = osseous spinal lamina, white arrow = long crus of the incus. (b, c) Temporal bone CT images acquired at 80 kVp (CTDIvol, 12.8 mGy) and reconstructed with three-dimensional AIDR with high-spatial-resolution mode (b) and FIRST with strong noise reduction (c) show the boy at 11 years old. Higher osseous attenuation of inner- and middle-ear structures was attained at 80 kVp with high-spatial-resolution IR (b), yielding sufficient depiction of fine structures such as osseous spinal lamina (black arrow) and the long crus of the incus (white arrow) with a 50% reduced dose. However, when the 80-kVp image was reconstructed with strong noise reduction, IR fine anatomic structures became unclear because of the deterioration of spatial resolution, in spite of the noise-free appearance.

Lung CT

Optimization of a lung CT protocol is important because of direct exposure to some of the most radiosensitive structures, such as the thyroid and mammary glands. Low-dose CT is well suited to imaging the chest, because the lung parenchyma absorbs fewer x-rays and inherently shows high contrast, which allows images with high image noise to be diagnostic. Although reduction in tube current is a common method of reducing radiation dose, the combination of low tube voltage and IR techniques can provide diagnostic image quality at a low dose (Figs 7, 8). In addition, combined use of high-pitch acquisition may yield further reduction in radiation dose and motion artifacts (63).

Figure 7a.

Figure 7a. Assessment for chronic lung disease in two preterm infants (both 3-month-old girls; weight, 3 kg [a] and 2.9 kg [b]). (a) Coronal CT image acquired at 120 kVp (CTDIvol, 2.52 mGy) was reconstructed with FBP. (b) Coronal CT image acquired at 80 kVp (CTDIvol, 0.32 mGy) was reconstructed with SAFIRE. Although perceptible image noise is somewhat higher on b, diagnostically adequate image quality was attained with an 87% reduced radiation dose, compared with that in a.

Figure 7b.

Figure 7b. Assessment for chronic lung disease in two preterm infants (both 3-month-old girls; weight, 3 kg [a] and 2.9 kg [b]). (a) Coronal CT image acquired at 120 kVp (CTDIvol, 2.52 mGy) was reconstructed with FBP. (b) Coronal CT image acquired at 80 kVp (CTDIvol, 0.32 mGy) was reconstructed with SAFIRE. Although perceptible image noise is somewhat higher on b, diagnostically adequate image quality was attained with an 87% reduced radiation dose, compared with that in a.

Figure 8a.

Figure 8a. Collagen vascular disease–related intestinal pneumonia (juvenile idiopathic arthritis) in a girl who underwent two CT examinations, with a 3-year interval. (a) Coronal image acquired at 120 kVp (CTDIvol, 12.1 mGy) and reconstructed with FBP shows the girl at 7 years old. (b, c) Coronal CT images acquired at 80 kVp (CTDIvol, 0.8 mGy) and reconstructed with FBP (b) and FIRST (c) show the girl at 10 years old. Compared with 120 kVp, 80 kVp with FIRST yielded compatible diagnostic lung image quality with a 93% reduced dose. IR provides even better image quality at the shoulder and subdiaphragmatic levels. (Full DICOM image stacks for ac are available online.)

Figure 8b.

Figure 8b. Collagen vascular disease–related intestinal pneumonia (juvenile idiopathic arthritis) in a girl who underwent two CT examinations, with a 3-year interval. (a) Coronal image acquired at 120 kVp (CTDIvol, 12.1 mGy) and reconstructed with FBP shows the girl at 7 years old. (b, c) Coronal CT images acquired at 80 kVp (CTDIvol, 0.8 mGy) and reconstructed with FBP (b) and FIRST (c) show the girl at 10 years old. Compared with 120 kVp, 80 kVp with FIRST yielded compatible diagnostic lung image quality with a 93% reduced dose. IR provides even better image quality at the shoulder and subdiaphragmatic levels. (Full DICOM image stacks for ac are available online.)

Figure 8c.

Figure 8c. Collagen vascular disease–related intestinal pneumonia (juvenile idiopathic arthritis) in a girl who underwent two CT examinations, with a 3-year interval. (a) Coronal image acquired at 120 kVp (CTDIvol, 12.1 mGy) and reconstructed with FBP shows the girl at 7 years old. (b, c) Coronal CT images acquired at 80 kVp (CTDIvol, 0.8 mGy) and reconstructed with FBP (b) and FIRST (c) show the girl at 10 years old. Compared with 120 kVp, 80 kVp with FIRST yielded compatible diagnostic lung image quality with a 93% reduced dose. IR provides even better image quality at the shoulder and subdiaphragmatic levels. (Full DICOM image stacks for ac are available online.)

Spine CT

Spine CT is a crucial diagnostic tool in the management of spinal abnormalities such as spina bifida and adolescent idiopathic scoliosis. Similar to that for the lungs, the radiation dose for the assessment of bone structure may be substantially reduced, because increased noise is tolerable owing to high intrinsic contrast between the bone structure and surrounding soft tissue. Results of previous studies suggested that 80-kVp CT provides diagnostic image quality for evaluation of adolescent idiopathic scoliosis at submillisievert levels without IR (64), and application of IR can improve image quality (65) (Figs 9, 10).

Figure 9a.

Figure 9a. Spina bifida and myelomeningocele in a 1-day-old newborn. Presurgical CT images were acquired at 80 kVp (CTDIvol, 0.5 mGy) and reconstructed with SAFIRE. (a) Volume-rendered image clearly shows the abnormality of the spine. (b) Coronal CT image clearly shows the myelomeningocele.

Figure 9b.

Figure 9b. Spina bifida and myelomeningocele in a 1-day-old newborn. Presurgical CT images were acquired at 80 kVp (CTDIvol, 0.5 mGy) and reconstructed with SAFIRE. (a) Volume-rendered image clearly shows the abnormality of the spine. (b) Coronal CT image clearly shows the myelomeningocele.

Figure 10a.

Figure 10a. Evaluation for suspicion of a thoracic deformity in a 16-year-old girl. Coronal CT images acquired at 80 kVp (CTDIvol, 0.8 mGy) were reconstructed with FBP (a) and FIRST (b). Application of FIRST can reduce image noise and artifacts dramatically, especially at the shoulder and subdiaphragmatic levels, leading to substantial improvement in overall image quality with a low radiation dose.

Figure 10b.

Figure 10b. Evaluation for suspicion of a thoracic deformity in a 16-year-old girl. Coronal CT images acquired at 80 kVp (CTDIvol, 0.8 mGy) were reconstructed with FBP (a) and FIRST (b). Application of FIRST can reduce image noise and artifacts dramatically, especially at the shoulder and subdiaphragmatic levels, leading to substantial improvement in overall image quality with a low radiation dose.

CT Angiography

CT angiography is the most beneficial diagnostic task for low tube voltage imaging, because vascular contrast is substantially improved and increased noise is tolerable. Especially for chest CT angiography, a large dose reduction is possible because there is little increase in noise owing to the presence of air in the lung parenchyma. Use of IR can allow further improvement of image quality (Fig 11). For currently available CT scanners, 70 kVp is the lowest tube voltage setting that yields the greatest improvement in iodine attenuation. Acquisition at 70 kVp is particularly relevant for CT angiography in smaller patients, such as infants with congenital heart disease (66), because the noise increase is minimal and the increase in contrast is substantial (Fig 12).

Figure 11a.

Figure 11a. Pulmonary arteriovenous fistula (hereditary hemorrhagic telangiectasia) in a 14-year-old girl (weight, 54 kg). (a) Coronal maximum intensity projection CT angiographic image was acquired before embolization at 120 kVp (CTDIvol, 22.5 mGy) and reconstructed with FBP. (b) After embolization, follow-up CT images were acquired at 80 kVp (CTDIvol, 2.2 mGy) and reconstructed with SAFIRE. No substantial deterioration in vascular depiction or increase in noise was observed on b, despite a 90% reduction in radiation dose.

Figure 11b.

Figure 11b. Pulmonary arteriovenous fistula (hereditary hemorrhagic telangiectasia) in a 14-year-old girl (weight, 54 kg). (a) Coronal maximum intensity projection CT angiographic image was acquired before embolization at 120 kVp (CTDIvol, 22.5 mGy) and reconstructed with FBP. (b) After embolization, follow-up CT images were acquired at 80 kVp (CTDIvol, 2.2 mGy) and reconstructed with SAFIRE. No substantial deterioration in vascular depiction or increase in noise was observed on b, despite a 90% reduction in radiation dose.

Figure 12a.

Figure 12a. Evaluation for heterotaxy (polysprenia) in a newborn girl. Chest and abdominal CT angiographic images were acquired at 70 kVp (CTDIvol, 0.38 mGy) and postprocessed with SAFIRE. Coronal maximum intensity projection image (a) and volume-rendered image (b) provide sufficient information of abnormal vascular and organ anatomy at a low radiation dose.

Figure 12b.

Figure 12b. Evaluation for heterotaxy (polysprenia) in a newborn girl. Chest and abdominal CT angiographic images were acquired at 70 kVp (CTDIvol, 0.38 mGy) and postprocessed with SAFIRE. Coronal maximum intensity projection image (a) and volume-rendered image (b) provide sufficient information of abnormal vascular and organ anatomy at a low radiation dose.

Contrast-enhanced Abdominal and Pelvic CT

After CT of the head, abdominal and pelvic CT is the second most frequently performed pediatric CT examination (5). Optimization of acquisition parameters for each abdominal or pelvic examination is crucial, because a relatively high effective dose is delivered for many radiosensitive organs such as the colon wall and breast (5). When radiation dose is reduced, diagnostic confidence is compromised by increased noise because of the inherently lower contrast of abdominal organs compared with that of the chest and bone. However, increased noise seen at low tube voltage CT is not as severe in children as in adults because children are smaller, and improved iodine contrast enhancement can compensate for smaller body size. Furthermore, by combining IR techniques, low tube voltage CT examinations such as those at 80 kVp may be performed routinely in most children and thin adolescents at the lowest radiation doses (Figs 13, 14). For larger adolescents, an intermediate tube voltage of 100 kVp may be more widely applicable and effective for this diagnostic task.

Figure 13a.

Figure 13a. Assessment of acute abdominal pain in two girls. (a) Contrast-enhanced abdominal-pelvic CT image acquired at 120 kVp (CTDIvol, 6.5 mGy) in a 13-year-old girl (weight, 40 kg) was reconstructed with FBP. (b) Contrast-enhanced CT image in a 12-year-old girl (weight, 45 kg) acquired at 80 kVp (CTDIvol, 3.5 mGy) was reconstructed with SAFIRE. Higher organ and vessel contrast is seen on b than on a, without a substantial increase in noise or loss of diagnostic quality, despite a 46% reduction in radiation dose.

Figure 13b.

Figure 13b. Assessment of acute abdominal pain in two girls. (a) Contrast-enhanced abdominal-pelvic CT image acquired at 120 kVp (CTDIvol, 6.5 mGy) in a 13-year-old girl (weight, 40 kg) was reconstructed with FBP. (b) Contrast-enhanced CT image in a 12-year-old girl (weight, 45 kg) acquired at 80 kVp (CTDIvol, 3.5 mGy) was reconstructed with SAFIRE. Higher organ and vessel contrast is seen on b than on a, without a substantial increase in noise or loss of diagnostic quality, despite a 46% reduction in radiation dose.

Figure 14a.

Figure 14a. Two follow-up CT examinations (5-year interval) in a boy who underwent liver transplantation for biliary atresia. (a) Coronal CT image acquired at 120 kVp and reconstructed with FBP shows the boy at 3 years old (weight, 13 kg). (b) Coronal image acquired at 80 kVp and reconstructed with FIRST shows the boy at 8 years old (weight, 30 kg). On b (CTDIvol, 2.3 mGy), the radiation dose was reduced by 73% compared with that on a (CTDIvol, 8.5 mGy), with preservation or even improvement of image quality.

Figure 14b.

Figure 14b. Two follow-up CT examinations (5-year interval) in a boy who underwent liver transplantation for biliary atresia. (a) Coronal CT image acquired at 120 kVp and reconstructed with FBP shows the boy at 3 years old (weight, 13 kg). (b) Coronal image acquired at 80 kVp and reconstructed with FIRST shows the boy at 8 years old (weight, 30 kg). On b (CTDIvol, 2.3 mGy), the radiation dose was reduced by 73% compared with that on a (CTDIvol, 8.5 mGy), with preservation or even improvement of image quality.

Relevant Pitfalls and Areas of Concern

In these sections, we focus on potential pitfalls and related topics with the use of low tube voltage and IR techniques, including (a) the effect of tube voltage on surface radiation dose, (b) the effects of window settings, (c) accentuated metallic artifacts, (d) deterioration of low contrast detectability, (e) interscanner variation of x-ray spectra, and (f) comparison with spectral shaping technique.

Effect of Tube Voltage on Surface Radiation Dose

A potential concern regarding low tube voltage CT is an increased surface dose (ie, higher radiation absorption by radiosensitive tissues such as the mammary glands and red bone marrow) because of reduced penetration of x-rays. Phantom studies have shown that the increase in surface or peripheral doses that occurs at low tube voltages is minimal and not significant, especially for small objects, when the CTDIvol or noise level is consistent throughout kilovolt peak values (40,67). This concern may be further mitigated through the use of an IR technique, because it allows for a reduction in the CTDIvol without increasing image noise. Nevertheless, most dose-efficient tube voltages should be further investigated on the basis of accurate in vivo dosimetry and achievable image quality. This is especially true for nonenhanced CT, where the benefit of contrast improvement is smaller than that for contrast-enhanced CT.

Effect of Window Display Settings

Lowering tube voltage leads to a different image impression if the window setting used is equivalent to that used for standard tube voltage levels. Images obtained with low tube voltage show a wider range of CT numbers, mainly because of increased attenuation of structures with a high atomic number, such as bone or iodinated vessels and organs. Therefore, the window display should be adjusted optimally so that images that are similar in appearance can be obtained, and accurate image interpretation can be performed (68). Use of wider and higher window settings may minimize unfamiliar visual impressions caused by excessive iodine or bone contrast and a noisy appearance observed on lower tube voltage images. Subtle intertissue contrast, which is unrecognizable with standard window settings, can be depicted clearly (Figs 15, 16). This window adjustment may be within the realm of common sense for radiologists. Nevertheless, it is desirable to provide images at optimally preset window display settings, because clinicians may not be accustomed to adjusting the window display and mistakenly may consider the low–kilovolt peak image to be of insufficient quality for diagnosis.

Figure 15a.

Figure 15a. Hepatic tumors (hemangiomas) in a 3-month-old girl. Axial CT images were acquired at 80 kVp (CTDIvol, 1.2 mGy). Compared with a standard soft-tissue window display (window level, 30 HU; width, 300 HU) (a), use of a broader window width and higher window level (level, 100 HU; width, 600 HU) (b) can modulate the excessive iodine attenuation and noisy appearance.

Figure 15b.

Figure 15b. Hepatic tumors (hemangiomas) in a 3-month-old girl. Axial CT images were acquired at 80 kVp (CTDIvol, 1.2 mGy). Compared with a standard soft-tissue window display (window level, 30 HU; width, 300 HU) (a), use of a broader window width and higher window level (level, 100 HU; width, 600 HU) (b) can modulate the excessive iodine attenuation and noisy appearance.

Figure 16a.

Figure 16a. Interrupted aortic arch in a 15-day-old boy. Sagittal maximum intensity projection images obtained at 70 kVp (CTDIvol, 0.37 mGy) show that aortic attenuation reached 790 HU. Optimization of the window display level and width changed values from 300 HU and 800 HU (a) to 460 HU and 1400 HU (b). The difference in iodine contrast enhancement between the ascending aorta and pulmonary artery is clearer on b, and the interrupted part of the aortic arch is seen (arrow in b).

Figure 16b.

Figure 16b. Interrupted aortic arch in a 15-day-old boy. Sagittal maximum intensity projection images obtained at 70 kVp (CTDIvol, 0.37 mGy) show that aortic attenuation reached 790 HU. Optimization of the window display level and width changed values from 300 HU and 800 HU (a) to 460 HU and 1400 HU (b). The difference in iodine contrast enhancement between the ascending aorta and pulmonary artery is clearer on b, and the interrupted part of the aortic arch is seen (arrow in b).

Accentuated Metallic Artifacts

For patients with metallic hardware, metallic artifacts caused by beam hardening and photon starvation are accentuated at lower tube voltage because of reduced photon penetration (Fig 17). To minimize this adverse effect, imaging with a higher tube voltage and tube current is preferable; however, this approach substantially increases the radiation dose. In most cases, this artifact may be reduced through use of metal artifact reduction algorithms, which are provided by each equipment vendor (69). However, metal artifact reduction algorithms may introduce new artifacts in some conditions (70). In such cases, model-based IR may have the advantage of reducing this artifact compared with FBP or a metal artifact reduction algorithm (Fig 18) (71).

Figure 17a.

Figure 17a. Two CT examinations (1-year interval) after placement of a metallic device for idiopathic scoliosis in a boy. Sagittal CT images acquired at 120 kVp (CTDIvol, 8.1 mGy) when the boy was 12 years old (a) and at 100 kVp when he was 13 years old (CTDIvol, 5.7 mGy) (b) show that artifacts from the metallic device are significantly accentuated on the 100-kVp image.

Figure 17b.

Figure 17b. Two CT examinations (1-year interval) after placement of a metallic device for idiopathic scoliosis in a boy. Sagittal CT images acquired at 120 kVp (CTDIvol, 8.1 mGy) when the boy was 12 years old (a) and at 100 kVp when he was 13 years old (CTDIvol, 5.7 mGy) (b) show that artifacts from the metallic device are significantly accentuated on the 100-kVp image.

Figure 18a.

Figure 18a. CT examination after wire fixation for a maxillary alveolar fracture in a 9-year-old girl. (a) Axial image from the initial CT examination at 120 kVp (CTDIvol, 17.5 mGy) was reconstructed with FBP. (b–d) Follow-up study images at 80 kVp at a substantially reduced dose (CTDIvol, 2.5 mGy) were reconstructed with FBP (b), metallic artifact reduction (c), and FIRST (d). Metallic artifacts are accentuated on b when compared with a. Use of the metal artifact reduction algorithm introduces new artifacts that compromise diagnostic quality. Among the images provided by all reconstruction algorithms, FIRST provides the image with the fewest artifacts (d).

Figure 18b.

Figure 18b. CT examination after wire fixation for a maxillary alveolar fracture in a 9-year-old girl. (a) Axial image from the initial CT examination at 120 kVp (CTDIvol, 17.5 mGy) was reconstructed with FBP. (b–d) Follow-up study images at 80 kVp at a substantially reduced dose (CTDIvol, 2.5 mGy) were reconstructed with FBP (b), metallic artifact reduction (c), and FIRST (d). Metallic artifacts are accentuated on b when compared with a. Use of the metal artifact reduction algorithm introduces new artifacts that compromise diagnostic quality. Among the images provided by all reconstruction algorithms, FIRST provides the image with the fewest artifacts (d).

Figure 18c.

Figure 18c. CT examination after wire fixation for a maxillary alveolar fracture in a 9-year-old girl. (a) Axial image from the initial CT examination at 120 kVp (CTDIvol, 17.5 mGy) was reconstructed with FBP. (b–d) Follow-up study images at 80 kVp at a substantially reduced dose (CTDIvol, 2.5 mGy) were reconstructed with FBP (b), metallic artifact reduction (c), and FIRST (d). Metallic artifacts are accentuated on b when compared with a. Use of the metal artifact reduction algorithm introduces new artifacts that compromise diagnostic quality. Among the images provided by all reconstruction algorithms, FIRST provides the image with the fewest artifacts (d).

Figure 18d.

Figure 18d. CT examination after wire fixation for a maxillary alveolar fracture in a 9-year-old girl. (a) Axial image from the initial CT examination at 120 kVp (CTDIvol, 17.5 mGy) was reconstructed with FBP. (b–d) Follow-up study images at 80 kVp at a substantially reduced dose (CTDIvol, 2.5 mGy) were reconstructed with FBP (b), metallic artifact reduction (c), and FIRST (d). Metallic artifacts are accentuated on b when compared with a. Use of the metal artifact reduction algorithm introduces new artifacts that compromise diagnostic quality. Among the images provided by all reconstruction algorithms, FIRST provides the image with the fewest artifacts (d).

Deterioration of Low Contrast Detectability at Low-Dose Settings

The radiation dose of CT for high-contrast diagnostic tasks such as CT angiography, CT urography, or CT enterography may be reduced dramatically through use of IR, without loss of spatial resolution or diagnostic performance (72). However, when CT is performed with considerably reduced doses, diagnostic performance deteriorates for clinical tasks involving low contrast resolution (eg, brain CT, hepatic CT, and pancreatic CT), even if image noise and contrast-to-noise ratio are maintained or improved with IR algorithms (73). The maximum dose reduction potential and imaging texture (IR-related artifacts, such as a pixelated or plastic appearance) differ according to the IR algorithm used (74,75), and there has been controversy over how much the radiation dose can be reduced with each IR algorithm without loss of low contrast detectability (76,77). However, the decision whether to increase the radiation dose substantially to depict subtle lesions in most children, who may have more conspicuous lesions or no lesions at all, should be carefully considered, according to each clinical condition.

Interscanner Variation of X-Ray Spectra

CT systems have different x-ray spectra and photon energy, even at the same tube voltage settings. Accordingly, x-ray absorption, CT attenuation, and contrast enhancement at each anatomic area vary among CT scanners (78,79). These differences may affect image interpretation and decision making in some clinical conditions. For instance, higher iodine attenuation on images obtained with a lower-photon scanner may lead to higher frequency and magnitude of pseudoenhancement in renal cystic lesions (80). Variations in photon energy also may affect the radiation dose and the distribution of radiation delivered to patients. Therefore, when low tube voltage CT is used, scanner properties should be taken into account to obtain consistent image quality and facilitate image interpretation.

Comparison with Spectral Shaping Technique

The lower photon energy range in the polyenergetic x-ray spectrum does not penetrate the patient and may not be used for diagnostic purposes in a nonenhanced CT examination involving a high-contrast abnormality. The latest dual-source CT scanners are equipped with a tin filter to remove most lower-energy photons. Application of a tin filter for 100-kVp lung CT shifts mean photon energy from 66.4 keV to 78.7 keV, leading to a higher percentage of x-ray photons exiting the patient and thus, higher dose efficiency. Weis et al (81) demonstrated that pediatric lung CT performed at 100 kVp with a tin filter reduces radiation dose (up to 75%) in comparison with 70-kVp imaging, without loss of diagnostic quality (Fig 19). Thus, if available, spectral shaping is the preferred technique for dose reduction, compared with low tube voltage for nonenhanced lung CT. In addition, this approach may reduce the dose for other nonenhanced CT examinations involving high-contrast diagnostic tasks, such as sinus CT (82).

Figure 19a.

Figure 19a. Acute respiratory distress syndrome in a 20-month-old patient (a) and neuroendocrine cell hyperplasia in a 19-month-old patient (b). Axial nonenhanced CT images obtained at 70 kVp (CTDIvol, 0.51 mGy) (a) and at 100 kVp (CTDIvol, 0.14 mGy) with spectral shaping (b) show that the image quality is comparable between the images despite substantial dose reduction with the use of spectral shaping. (Reprinted, with permission, from reference 81.)

Figure 19b.

Figure 19b. Acute respiratory distress syndrome in a 20-month-old patient (a) and neuroendocrine cell hyperplasia in a 19-month-old patient (b). Axial nonenhanced CT images obtained at 70 kVp (CTDIvol, 0.51 mGy) (a) and at 100 kVp (CTDIvol, 0.14 mGy) with spectral shaping (b) show that the image quality is comparable between the images despite substantial dose reduction with the use of spectral shaping. (Reprinted, with permission, from reference 81.)

Conclusion

Cumulative evidence has elucidated the potential risk for carcinogenesis and dose-response relationships in pediatric CT. Accordingly, optimization of acquisition parameters is crucial, and those involved in radiation imaging must incorporate strategies for dose reduction into clinical practice.

The low tube voltage technique yields substantial reduction in radiation dose while improving the image contrast, especially when an iodinated contrast medium can be used. Increased image noise due to less photon penetration is a major drawback of this technique; however, the degree of this adverse effect is highly dependent on patient body size and thus is not much of a problem in pediatric CT.

In conjunction with body habitus, the diagnostic task and the anatomic region of interest should be taken into consideration for selection of optimal tube voltage settings, because the degree of contrast improvement and noise tolerability depends on these factors. Combined use of IR algorithms can effectively suppress image noise, which maximizes the benefit of a low tube voltage technique and yields wider applicability to obtain diagnostic image quality at the lowest radiation dose. As outlined in this article, appropriate use of low tube voltage imaging and IR techniques enables dramatic dose reductions for pediatric CT while maintaining or improving diagnostic image quality for most clinical conditions.

Acknowledgments

The authors acknowledge the great assistance of radiological technologists at Kumamoto City Hospital (Shunsuke Hirokawa, Chiemi Kashiwagi, Takashi Sakamoto, Kiyohiro Hayashida, Shinya Ueda, Hisao Murakami) and Kumamoto University Hospital (Daisuke Sakabe, Takafumi Emoto, Yuki Kawamata, Kiyotaka Kakei, Masahiro Hatemura) in image collection.

Presented as an educational exhibit at the 2017 RSNA Annual Meeting.

For this journal-based SA-CME activity, the authors, editor, and reviewers have disclosed no relevant relationships.

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Article History

Received: Feb 26 2018
Revision requested: Apr 2 2018
Revision received: Apr 28 2018
Accepted: May 9 2018
Published online: Sept 12 2018
Published in print: Sept 2018