Figure 1: Axial T1-weighted in phase image demonstrates multiple hyperintense masses in the right
kidney (arrows) and one hyperintense mass within the left kidney.

Figure 2: Axial T1-weighted out of phase image demonstrates an “India ink” artifact at the interface
between the multiple renal masses (arrows) and the adjacent renal parenchyma. There are also scattered small areas of signal drop-out within these masses consistent with a macroscopic and microscopic fat component to these masses.

Figure 3: Axial non-contrast VIBE image demonstrates loss of signal within these renal masses (arrows),
again consistent with the presence of fat within these masses.

Figure 4: Axial contrast enhanced VIBE image demonstrates that the renal masses (arrows) have a
heterogeneous enhancement pattern.

Bilateral angiomyolipomas.

Tuberous sclerosis complex (TSC) is an inherited as an autosomal dominant gene. It is characterized by mental retardation, seizures, and cutaneous lesions. Approximately 50% of patients with TSC have renal manifestations: 20-25% have renal cysts; 10-20% have angiomyolipomas (AML), and up to 30% have both. AML lesions in these patients are commonly bilateral and multiple, but they can occur as a single lesion in one kidney. The presence of small cysts and fat-containing AMLs is virtually pathognomonic of TSC, especially if the AMLs are bilateral. .

Angiomyolipoma (AML; renal hamartomas) is a benign tumor composed of variable amounts of thick-walled blood vessels, smooth muscle, and fat. Angiomyolipoma has a prevalence in the general population ranging from 0.3% to 3%, with 80% of cases occurring sporadically and 20% occurring as part of the tuberous sclerosis complex (TSC). The sporadic form of AML usually consists of a solitary tumor, whereas in the TSC they tend to be multiple and bilateral. Patients with the sporadic form of AML most commonly present in the fifth decade of life and are female (female:male ratio of 8:1) Patients with AML as part of tuberous sclerosis complex tend to be younger, with an average age of 30 years. There is also a female predominance in these patients (female to male ratio of 2:1).

Angiomyolipoma most often originates in the renal parenchyma, however it can be attached to the renal capsule as well. In addition, AML can occur in other abdominal organs such as the liver, spleen, fallopian tube, and lymphatic tissue. The blood vessels in these lesions are thick-walled, without elastic tissue and often tortuous, and frequently in angiomatous arrangement. The degree of vascularity and structural rigidity, inelasticity, and tortuosity of these blood vessels predisposes them to hemorrhage.
Various phenotypes of AML have been described including: leiomyoma-like, lipoma-like, epithelioid and atypical. It has been suggested that angiomyolipomas with diffuse areas of epithelioid cell components are potentially malignant and are clinically more aggressive.

Patients with AML are usually asymptomatic and these renal lesions are usually incidentally discovered when a patient has imaging study for another reason. When symptomatic, the main clinical symptoms of AML usually are flank pain, palpable mass, hematuria, and acute retroperitoneal hemorrhage. Hematuria and chronic flank pain are more common signs for smaller lesions, where as acute retroperitoneal hemorrhage, shock, palpable mass and hematuria are more commonly associated with larger lesions. Angiomyolipomas have a greater tendency to increase in size when they are multiple than when they are solitary. Large AMLs can also exert mass effect of adjacent organs and cause symptoms. Although they are benign, AML has great potential complications such as hemorrhage. In fact, AMLs are a leading cause of morbidity in patients with TSC mostly related to complications from bleeding. AMLs can also mimic renal cell cancer. These factors make it necessary to diagnose these lesions early, provide close follow-up and prompt treatment.

The MR imaging characteristics of an individual angiomyolipoma mass depend on the relative amounts of its components (thick-walled blood vessels, smooth muscle, and fat) and on the presence or absence of hemorrhage. The diagnosis of AML rests on demonstrating the presence of macroscopic fat within the lesion. When an AML is composed primarily of fat, it demonstrates a hyperintense signal on T1-weighted images. However, hemorrhagic cysts can also demonstrate similar signal characteristics. Therefore, it is important to compare T1-weighted images with frequency-selected fat-suppression with those without fat-suppression to establish the presence or absence of macroscopic fat. AML can also be diagnosed using chemical-shift imaging techniques which take advantage of the precessional frequency differences of fat and water. This technique will provide images when fat and water signal are in phase (additive) or out of phase (destructive) producing a characteristic “India ink” artifact on the T1-weighted out of phase images. The “India ink” artifact is manifested as a low signal intensity rim at any soft tissue (water) and fat interface. Hemorrhagic cysts and AMLs are both hyperintense on T1-weighted in-phase images, however, they are readily differentiated on T1-weighted out-of-phase images. For AMLs, the “India ink” artifact appears at the interface of the tumor (fat) and the kidney (water). For hemorrhagic cysts, the “India ink” artifact occurs at the interface of the cyst (fluid) and the perirenal fat and not at the interface of the cyst and the kidney.

Although the presence of macroscopic fat in a lesion is very specific of AML, some renal cell carcinomas can contain fat and some AML lesions contain only microscopic fat or minimal fat. In these cases the diagnosis of AML is more complicated and difficult and the goal of MR imaging is to differentiate a benign AML mass from a renal cell carcinoma. Angiomyolipoma with minimal fat does not show hyperintensity on T1-weighted MR images, but on T2-weighted images the lesions frequently show hypointensity. The signal of renal cell carcinoma tends to be slightly high on T1-weighted images. Hypointensity on T2-weighted images in renal cell carcinoma is rare. Renal cell carcinomas also do not lose signal on frequency-selective fat suppressed T1-images. AMLs with minimal fat show homogeneous enhancement on contrast enhanced MR images, but renal cell carcinoma frequently shows heterogeneous enhancement because of necrosis or hemorrhage.

When the diagnosis of AML is established, selected patients can be managed conservatively either by careful imaging follow-up, embolization, or partial nephrectomy. In a small number of cases, when the muscle or vascular components predominate, distinction from renal cell cancer may be difficult on MR imaging. When the diagnosis, based on imaging findings, is certain and tumors are < 4 cm in size and asymptomatic, imaging follow-up is adequate management. AMLs larger than 4 cm in diameter are more likely to become symptomatic due to intratumoral or perinephric hemorrhage. In the case of a ruptured AML, an experienced interventional radiologist may proceed to embolization of the feeding arteries of the tumor. Large AMLs or those that have ruptured require surgical resection.

References:

1. Semelka, Richard. Abdominal-Pelvic MRI. New York: Wiley-Liss Inc, 2002. pp. 766-770.
2. Dunnick NR, Sandler CM, Newhouse JH, and ES Amis. Textbook of Uroradiology, 3rd Edition. Philadelphia: Lippincott Williams & Wilkins, c2001. pp. 150-151.
3. Israel GM and GA Krinsky. MR Imaging of the Kidneys and Adrenal Glands. Radiologic Clinics of North America. 2003; 41: 145-159.
4. Rickhardt PJ, Lonergan GJ, Davis FJ, Kashitani N, and BJ Wagner. Infiltrative Renal Lesions: Radiologic-Pathologic Correlation. RadioGraphics. 2000; 20: 215-243.