CT and MRI of small renal masses

Abstract

Small renal masses are increasingly detected incidentally at imaging. They vary widely in histology and aggressiveness, and include benign renal tumors and renal cell carcinomas that can be either indolent or aggressive. Imaging plays a key role in the characterization of these small renal masses. While a confident diagnosis can be made in many cases, some renal masses are indeterminate at imaging and can present as diagnostic dilemmas for both the radiologists and the referring clinicians. This article will summarize the current evidence of imaging features that correlate with the biology of small solid renal masses, and discuss key approaches in imaging characterization of these masses using CT and MRI.

Figure 1.

A 68-year-old female with renal pseudotumor secondary to focal parenchymal hypertrophy. (a) Nephrographic phase image shows a bulge in the right lateral renal contour (long arrow) that mimics a tumor. There is adjacent renal cortical scarring (short arrow). (b) Corticomedullary image confirms the bulge in the renal contour represents normal renal parenchyma (long arrow), which appears more prominent due to adjacent scarring (short arrow).

Figure 2.

A 63-year-old female with renal infarct. (a) Initial contrast-enhanced CT performed for abdominal pain shows an ill-defined right renal lesion (long arrow) and small amount of perinephric fat stranding (short arrow). Patient also had history of atrial fibrillation. Given the history, renal infarct was suspected. (b) Contrast-enhanced CT performed 3 months later shows evolution of the right renal lesion with decrease in its size, confirming that the lesion is not a tumor, and is more consistent with renal infarct given clinical history.

Figure 3.

A 65-year-old female with a renal mass. (a) Contrast enhanced CT in the corticomedullary phase shows a small hypodense right renal lesion (arrow) which is obscured by the relatively unenhanced adjacent renal medulla. (b) CT image in the nephrographic phase shows the same lesion (arrow) to a much better advantage surrounded by the homogenously enhancing renal parenchyma. The patient underwent active surveillance for this lesion, which remained stable for one year. The patient was then lost to follow up.

Figure 4.

A 60-year-old female with an angiomyolipoma. (a) Contrast enhanced CT image demonstrates an enhancing mass (long arrow) in the right kidney with questionable area of macroscopic fat (short arrow). (b) Unenhanced CT image demonstrates definite macroscopic fat (short arrow) in the mass (long arrow), allowing a confident diagnosis of an angiomyolipoma.

Figure 5.

A 60-year-old male with a rare papillary RCC containing macroscopic fat and calcification. (a) Unenhanced CT image demonstrates an exophytic right renal mass with a focus of macroscopic fat (short arrow) and amorphous calcifications (long arrow). (b) Image of the more inferior aspect of the renal mass shows dense calcifications (long arrow).

Figure 6.

A 48-year-old female post-ablation of a left renal mass. Contrast-enhanced CT shows the ablated renal tumor (short arrow) surrounded by fat and a thin rim of peritumoral soft tissue attenuation (long arrow).

Figure 7.

A 71-year-old male with renal oncocytoma. (a) CMP CT image shows enhancing left renal mass with two areas of differential enhancement: high enhanced (long arrows) and less enhanced (short arrow). (b) Early excretory phase CT image shows inversion of the enhancement pattern with the highly enhanced area on the CMP becoming less enhancing (long arrows), and the less enhancing area on the CMP highly enhancing (short arrow). (c, d) Show similar findings on contrast-enhanced MRI. CMP, corticomedullar phase.

Figure 8.

A 57-year-old female with renal oncocytoma. Contrast enhanced CT shows an enhancing right renal mass (long arrow) with a central scar (short arrow).

Figure 9.

A 46-year-old female with chromophobe RCC. Contrast-enhanced CT shows a large heterogeneously enhancing left renal mass (long arrow) with central scar (short arrow). RCC, renal cell carcinoma.

Figure 10.

A 63-year-old male with an incidentally detected renal mass. (a) Unenhanced CT shows an hyperattenuating right renal mass (arrow). (b) Contrast-enhanced CT shows the mass to be homogenously enhancing. The imaging features suggest but are not diagnostic for a lipid-poor angiomyolipoma. The patient subsequently underwent a percutaneous renal biopsy which confirmed the diagnosis of a lipid-poor angiomyolipoma.

Figure 11.

A 65-year-old male with papillary RCC. (a) Unenhanced CT image shows an exophytic right renal mass that is intermediate in attenuation measuring 40 HU. (b) Corticomedullary phase and (c) nephrographic phase images show the mass to have progressive low-level enhancement with attenuation increase to 58 HU in corticomedullary phase, and to 77 HU in nephrographic phase. RCC, renal cell carcinoma.

Figure 12.

A 55-year-old female with renal angiomyolipoma. (a) Post-gadolinium enhanced image shows a well-defined enhancing right renal mass (long arrow) with a small area of hypointensity (short arrow). In-phase (b) and opposed-phase (c) images confirm the presence of macroscopic fat (short arrow) in the mass with T₁ hyperintensity on the in-phase image, and rim of signal loss on the opposed-phase image.

Figure 13.

A 70-year-old male with papillary RCC. (a) T₁ weighted fat saturated pre-contrast image shows a right renal mass (arrow) with intermediate T₁ intensity. (b) T₁ weighted fat saturated post-contrast image shows the mass with indeterminate enhancement. (c) Subtraction image shows clear enhancement in the mass consistent with a tumor which was later resected and proven to a papillary RCC.

Figure 14.

A 71-year-old male with clear cell RCC. (a) T₂ weighted, fat suppressed image shows the mass (arrow) has heterogeneous high T₂ signal. (b) T₁ weighted dual echo in-phase and (c) opposed-phase images show a left renal mass (arrows) that demonstrates signal drop in the opposed-phase image (b), consistent with the presence of microscopic fat. (d) Gadolinium-enhanced image shows avid enhancement of the mass. The combination of findings is suggestive of a clear cell renal cell carcinoma, and was proven at pathology.

Figure 15.

A 73-year-old female with a clear cell RCC. (a) T₂ weighted, fat suppressed image shows the mass (arrow) has heterogeneous high T₂ signal. (b) T₁ weighted dual echo in-phase and (c opposed-phase (c) images show the mass (arrows) demonstrating signal drop in the opposed-phase image, consistent with the presence of microscopic fat. (d) Gadolinium-enhanced image shows heterogeneous enhancement of the mass. The combination of findings is suggestive of a clear cell RCC, and was proven at pathology.

Figure 16.

A 38-year-old male with papillary RCC. (a) T₂ weighted image shows a hypointense mass (arrow). (b) T₁ weighted post-contrast image shows low-level enhancement in the mass (arrow). The combination of findings is suggestive of a papillary RCC, which was proven at pathology. RCC, renal cell carcinoma.

Figure 17.

A 55-year-old male with papillary RCC. (a) In-phase and (b) opposed-phase images show a right renal mass (arrow) with loss of signal on the in-phase (longer TE sequence) when compared to opposed-phase, consistent with presence of hemosiderin in the mass. (c) T₂ weighted MR image shows the mass to be T₂ hypointense (arrow). (d) T₁ weighted post-contrast image obtained in the nephrographic phase shows the mass with low-level enhancement. The combination of the findings is highly suggestive of a papillary RCC, and was proven at pathology. RCC, renal cell carcinoma.

Figure 18.

A 32-year-old male with lipid poor angiomyolipoma. (a) T₂ weighted, fat-suppressed image shows the mass (arrows) to be hypointense. T₁ weighted post-gadolinium image shows the mass (arrows) to be avidly enhancing in the corticomedullary phase (b), and washing out in the early excretory phase (c).

Figure 19.

Algorithm that can be used to diagnose or suggest histology of the more common subtypes of RCCs and benign renal tumors using multiparametric MRI features. *, the imaging features are suggestive but not diagnostic. AML, angiomyolipoma; RCC, renal cell carcinoma.