Mediastinal and Pleural MR Imaging in Daily Practice

Published Online:https://doi.org/10.1148/rg.2018180011

Editor:

In the January-February 2018 issue of RadioGraphics, Raptis et al (1) report on the use of magnetic resonance (MR) imaging in daily practice for assessment of the mediastinum and pleura. Although the importance of diffusion-weighted (DW) imaging in thoracic MR imaging protocols is widely discussed in their review, basic information on how to obtain reliable measurements of the apparent diffusion coefficient (ADC) are missing. First, about b values, it should be stressed that the b value of 0 sec/mm2 must be excluded from ADC maps and ADC calculations to avoid significant ADC overestimation due to tissue perfusion. Indeed, Padhani et al (2) reported a perfusion-induced overestimation of up to 21% by using b values of 0 and 500 sec/mm2 in tissues with a volume fraction of flow of 0.055 (ie, 5.5% of signal intensity derived from flowing blood). Likewise, by comparing perfusion-induced ADC measurements (obtained with b values of 0 and 800 sec/mm2) and perfusion-free ADC measurements (obtained with b values of 150 and 800 sec/mm2) in the anterior mediastinum, we found an overestimation of ADC values that was significantly higher for malignancies than for benign conditions (22.4% vs 5.3%, respectively) (3).

Second, it should be pointed out that adequate selection of other technical parameters such as echo time is pivotal for obtaining ADC values that reflect true diffusion (4,5). In DW MR imaging of the thorax, the echo time should be kept as short as possible, typically 50–65 msec, to avoid ADC underestimation. When b values of 100 and 1000 sec/mm2 are used for compartments with a T2 relaxation time of 400 msec, ADC measurements could be up to 30% lower when an echo time of 92 msec is used versus an echo time of 52 msec (6).

Last, Raptis et al (1) state that MR imaging can be used “to determine whether a suspected lesion [detected at CT] has properties that make it certainly or near certainly benign.” We do not fully agree with this sentence, because there is an increasing awareness of the potential role of MR imaging as a first-line technique in selected conditions, such as for assessing thymic abnormalities in patients with newly diagnosed autoimmune disorders (eg, myasthenia gravis) or monitoring residual fibrotic tissue after treatment in the surveillance of patients with primary mediastinal lymphoma.

References

  • 1. Raptis CA, McWilliams SR, Ratkowski KL, Broncano J, Green DB, Bhalla S. Mediastinal and pleural MR imaging: practical approach for daily practice. RadioGraphics 2018;38(1):37–55. LinkGoogle Scholar
  • 2. Padhani AR, Liu G, Koh DM, et al. Diffusion-weighted magnetic resonance imaging as a cancer biomarker: consensus and recommendations. Neoplasia 2009;11(2):102–125. Crossref, MedlineGoogle Scholar
  • 3. Priola AM, Priola SM, Gned D, et al. Diffusion-weighted quantitative MRI to diagnose benign conditions from malignancies of the anterior mediastinum: improvement of diagnostic accuracy by comparing perfusion-free to perfusion-sensitive measurements of the apparent diffusion coefficient. J Magn Reson Imaging 2016;44(3):758–769. Crossref, MedlineGoogle Scholar
  • 4. Ackman JB. A practical guide to nonvascular thoracic magnetic resonance imaging. J Thorac Imaging 2014;29(1):17–29. Crossref, MedlineGoogle Scholar
  • 5. Priola AM, Gned D, Veltri A, Priola SM. Chemical shift and diffusion-weighted magnetic resonance imaging of the anterior mediastinum in oncology: current clinical applications in qualitative and quantitative assessment. Crit Rev Oncol Hematol 2016;98:335–357. Crossref, MedlineGoogle Scholar
  • 6. Schmidt H, Gatidis S, Schwenzer NF, Martirosian P. Impact of measurement parameters on apparent diffusion coefficient quantification in diffusion-weighted-magnetic resonance imaging. Invest Radiol 2015;50(1):46–56. Crossref, MedlineGoogle Scholar

References

1. Raptis CA, McWilliams SR, Ratkowski KL, Broncano J, Green DB, Bhalla S. Mediastinal and pleural MR imaging: practical approach for daily practice. RadioGraphics 2018;38(1):3755. LinkGoogle Scholar
2. Padhani AR, Liu G, Koh DM, et al. Diffusion-weighted magnetic resonance imaging as a cancer biomarker: consensus and recommendations. Neoplasia 2009;11(2):102125. Crossref, MedlineGoogle Scholar
3. Priola AM, Priola SM, Gned D, et al. Diffusion-weighted quantitative MRI to diagnose benign conditions from malignancies of the anterior mediastinum: improvement of diagnostic accuracy by comparing perfusion-free to perfusion-sensitive measurements of the apparent diffusion coefficient. J Magn Reson Imaging 2016;44(3):758769. Crossref, MedlineGoogle Scholar
4. Ackman JB. A practical guide to nonvascular thoracic magnetic resonance imaging. J Thorac Imaging 2014;29(1):1729. Crossref, MedlineGoogle Scholar
5. Priola AM, Gned D, Veltri A, Priola SM. Chemical shift and diffusion-weighted magnetic resonance imaging of the anterior mediastinum in oncology: current clinical applications in qualitative and quantitative assessment. Crit Rev Oncol Hematol 2016;98:335357. Crossref, MedlineGoogle Scholar
6. Schmidt H, Gatidis S, Schwenzer NF, Martirosian P. Impact of measurement parameters on apparent diffusion coefficient quantification in diffusion-weighted-magnetic resonance imaging. Invest Radiol 2015;50(1):4656. Crossref, MedlineGoogle Scholar


Dr Raptis and colleagues respond:

We appreciate the interest of Dr Priola and colleagues in our article, “Mediastinal and Pleural MR Imaging: Practical Approach for Daily Practice.” Their comments on the technical aspects of DW imaging in the chest are welcomed and do extend beyond what is reviewed in our article. In our practice, we do not routinely perform quantitative measurements of ADC for most thoracic lesions. The main purposes of DW imaging for our basic thoracic MR imaging protocols are to provide a heavily T2-weighted sequence at low b values as well as a means of qualitative assessment of diffusion restriction using higher b values and the ADC map. As understanding of the implications of DW imaging of the chest evolves, quantitative analysis of DW imaging may take on a greater role, but it is not essential or warranted at present for interpretation of most thoracic MR imaging examinations.

Regarding the second comment by Dr Priola and colleagues, pertaining to our use of thoracic MR imaging to determine whether a suspected lesion has properties that make it certainly or near certainly benign, this is our most common indication for thoracic MR imaging. As stated in our article, we did not intend to provide a comprehensive list of all potential indications for MR imaging of the chest, and we did not imply that the evaluation of whether a lesion is certainly or near certainly benign is the only indication for thoracic MR imaging. The indications listed by Dr Priola et al in their letter are newer niche first-line indications that are promising. We sincerely hope that these and other indications for thoracic MR imaging continue to grow and contribute to the expansion of thoracic MR imaging in clinical practice.

Article History

Published online: May 14 2018
Published in print: May 2018