Renal and adrenal masses containing fat at MRI: Proposed nomenclature by the society of abdominal radiology disease‐focused panel on renal cell carcinoma

Abstract

This article proposes a consensus nomenclature for fat‐containing renal and adrenal masses at MRI to reduce variability, improve understanding, and enhance communication when describing imaging findings. The MRI appearance of "macroscopic fat" occurs due to a sufficient number of aggregated adipocytes and results in one or more of: 1) intratumoral signal intensity (SI) loss using fat‐suppression techniques, or 2) chemical shift artifact of the second kind causing linear or curvilinear India‐ink (etching) artifact within or at the periphery of a mass at macroscopic fat–water interfaces. "Macroscopic fat" is most commonly observed in adrenal myelolipoma and renal angiomyolipoma (AML) and only rarely encountered in other adrenal cortical tumors and renal cell carcinomas (RCC). Nonlinear noncurvilinear signal intensity loss on opposed‐phase (OP) compared with in‐phase (IP) chemical shift MRI (CSI) may be referred to as "microscopic fat" and is due to: a) an insufficient amount of adipocytes, or b) the presence of fat within tumor cells. Determining whether the signal intensity loss observed on CSI is due to insufficient adipocytes or fat within tumor cells cannot be accomplished using CSI alone; however, it can be inferred when other imaging features strongly suggest a particular diagnosis. Fat‐poor AML are homogeneously hypointense on T2‐weighted (T2W) imaging and avidly enhancing; signal intensity loss at OP CSI is uncommon, but when present is usually focal and is caused by an insufficient number of adipocytes within adjacent voxels. Conversely, clear‐cell RCC are heterogeneously hyperintense on T2W imaging and avidly enhancing, with the signal intensity loss observed on OP CSI being typically diffuse and due to fat within tumor cells. Adrenal adenomas, adrenal cortical carcinoma, and adrenal metastases from fat‐containing primary malignancies also show signal intensity loss on OP CSI due to fat within tumor cells and not from intratumoral adipocytes. Level of Evidence: 5 Technical Efficacy Stage: 3 J. Magn. Reson. Imaging 2019;49:917–926.

A Rare Case of Hemorrhagic Giant Adrenal Myelolipoma: Radiographic and Pathologic Correlation

“…Macroscopic fat will lose signal with the application of fat suppression while signal will usually persist with myeloid elements and hemorrhage contained within the tumor [ 14 ]. This distinguishes myelolipomas from adrenal adenomas (most common adrenal lesions) in that adrenal adenomas demonstrate a loss of signal intensity on out-of-phase MRI images compared to in-phase images [ 15 ]. This is due to adrenal adenomas containing microscopic fat which is fat within tumor cells as opposed to intra-humoral adipocytes found in macroscopic fat [ 15 ].…”

“…This distinguishes myelolipomas from adrenal adenomas (most common adrenal lesions) in that adrenal adenomas demonstrate a loss of signal intensity on out-of-phase MRI images compared to in-phase images [ 15 ]. This is due to adrenal adenomas containing microscopic fat which is fat within tumor cells as opposed to intra-humoral adipocytes found in macroscopic fat [ 15 ]. The onset of hemorrhage can be determined on MRI, with early hemorrhage (< seven days onset) showing up iso-tense on T1 and low signal on T2, subacute hemorrhage (seven days-seven weeks onset) showing high signal on T1 and T2, and late hemorrhage (past seven weeks onset) appearing hypo-intense on both [ 1 ].…”

Update on MRI of Cystic Renal Masses Including Bosniak Version 2019

“…23 Fat saturation can be achieved with frequency-selective saturation pulses or with fat-water separation Dixon techniques, the latter offering more homogenous attenuation of the fat signal. 24 Through the application of modest parallel imaging (ie, acceleration factor of 2), breathheld 3D interpolated T 1 W GRE sequences can be acquired in 16-20 seconds. Breath-holds can be performed during end-expiration or end-inspiration; the latter enables a better capacity for the patient to hold their breath while the former enables better fixation of diaphragm positioning.…”

Qualitative Heterogeneous Signal Drop on Chemical Shift (CS) MR Imaging: Correlative Quantitative Analysis between CS Signal Intensity Index and Contrast Washout Parameters Using T1-Weighted Sequences

“…On the other hand, when the tumor cells do not contain a substantial amount of cytoplasmatic lipids, no signal drop on CS out-of-phase images occurs, and usually these lesions are defined as non-adenomas [1][2][3]. However, adrenal adenomas with low fat within their cytoplasm ("lipid-poor") do not show signal drop on CS imaging [4]. Moreover, malignant adrenal tumors, such as primary carcinoma or metastases by renal cell (RCC) and hepatocellular (HCC) carcinomas, may contain small amounts of intracytoplasmic lipids, resulting in heterogeneous signal drop on CS sequence [4].…”