MR Imaging of Adrenal Lesions

Aoife Kilcoyne, MB BCh BAO, FFR, RCSI; Shaunagh McDermott, MB BCh BAO, FFR, RCSI; Michael A. Blake, MB BCh BAO, MRCPI, FRCR, FFR, RCSI

Disclosures

Appl Radiol. 2017;46(4) 

In This Article

Emerging Techniques

Diffusion Weighting

The data relating to diffusion weighting in the imaging of adrenal tumors has been disappointing thus far in general because of benign adrenal adenomas' propensity to demonstrate restricted diffusion. Miller et al[25] retrospectively evaluated 160 adrenal lesions in 156 patients and found that apparent diffusion coefficient (ADC) values were not useful in accurately distinguishing benign from malignant adrenal lesions. Sandrasegaran et al[26] also found that ADC values are not clinically useful in differentiating adrenal lesions.

MR Spectroscopy

MR spectroscopy is a technique that has been investigated for further characterization of adrenal masses. This method takes advantage of altered metabolite composition of the mass. For example, choline is a metabolite involved in construction of the cell wall, and its concentration is increased in malignancies due to cellular proliferation.[27] Faria et al[28] identifed 60 patients with adrenal tumors larger than 2cm in diameter. They were examined in a 1.5T MR imaging system and point-resolved 1H MR spectroscopy. They found that 1H MR spectroscopy can be used to characterize adrenal masses on the basis of spectral findings for benign adenomas, carcinomas, pheochromocytomas and metastases. Choline-creatine ratios greater than 1.20 yielded 92% sensitivity and 96% specificity. Choline-lipid ratios greater than 0.38 yielded 92% sensitivity and specificity. Both were sufficient for differentiation of adenomas and pheochromocytomas from carcinomas and metastases. In the differentiation of carcinomas and pheochromocytomas from adenomas and metastases, a 4.0 – 4.3 ppm/creatine ratio greater than 1.50 yielded 87% sensitivity and 98% specificity. Simultaneous use of these two analyses for each type of adrenal mass allowed classification of 54 of 60 masses into 4 distinct groups. Limitations of this technique include the fact that lesions smaller than 2cm are generally unsuitable for analysis due to artifacts caused by respiratory movements.

PET/MRI

PET/MRI is the newest clinical hybrid imaging modality (Figure 1). The excellent soft-tissue contrast of MRI and its ability to assess adrenal lesions with chemical shift imaging may provide a superior complement to PET in certain clinical circumstances.[29] A recent study[30] identified a total of 173 patients, with 649 upper-abdominal incidentalomas who underwent 18F-FDG PET/CT and subsequent whole-body or abdominal 18F-FDG PET/MRI for oncological indications. In both cystic and non-cystic masses, fewer findings were rated as indeterminate by 18F-FDG PET/MRI compared with 18F-FDG PET/CT. The organ-specific subgroup analysis of this study showed significantly fewer indeterminate findings in the liver, the kidney and the adrenal glands, pointing to the superior diagnostic performance of 18F-FDG PET/MRI over 18F-FDG PET/CT in these organs. Classification of adrenal incidentalomas is predominantly based on lipid detection techniques.[12,31] Given its high resolution, the three-dimensional VIBE sequence in the Dixon technique is used for MRI-based attenuation correction in integrated PET/MRI scanners.[30] These images can be used to increase the diagnostic accuracy in adrenal incidentalomas. In the case of an adrenal mass, the evaluation of these images for the characteristic signal decrease in the opposed-phase images is useful and can increase the diagnostic certainty without requiring any further resources and without additional radiation exposure.

Figure 1.

PET/MR image demonstrating a left lung carcinoma with a left adrenal metastasis, both showing intense FDG uptake.

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