Sonographic differentiation of some common renal masses:

Angiomyolioma: Among the commonest solid renal masses is angiomyolipoma. This is a benign mass and is believed to be a variant of hamartoma of the kidneys. Composed of smooth muscle, vascular tissue and above all fat (adipose tissue), an angiomyolipoma is almost invariably echogenic and usually small in size (less than 2 cms.). However, these tumors can be isoechoic to renal tissue in rare cases, where the amount of adipose tissue within the mass is smaller in amount. Visit: http://www.ultrasound-images.com/kidneys.htm#Renal_angiomyolipoma_-%28AML%29_of_right_kidney
These masses are usually benign in the vast majority of cases and rarely pose an symptomatic problems for the patient. However, it is usually in cases where the angiomyolipoma is larger than 4 cms. that, severe hematuria may be an important symptom.
Also visit: http://ultrasound-images.blogspot.com/2011/08/sonography-of-angiomyolipoma.html
  Hypertrophied column of Bertin: of Normal variants of the kidney like hypertrophic column of Bertin can also mimic a renal mass. However, careful sonography of the kidney will display the benign nature of this anatomical variant.  See the ultrasound and color Doppler images below; the column of Bertin  shows a prominent mass of renal cortical tissue extending deeper into the renal medulla separating the pyramids and also indenting the renal sinus. Color Doppler ultrasound imaging shows normal vascularity of this area, unlike the appearances of neovasculature (abnormal vessels) seen in malignant renal masses.  

See: http://www.ultrasound-images.com/kidneys.htm#Hypertrophied_column_of_Bertin_or_Junctional_parenchyma

for more on this topic.


Renal cell carcinoma: is another tumor of the kidney that may pose a diagnostic dilemma to the sonologist. This tumor is usually inhomogenous, relatively hypoechoic and does not produce posterior shadowing, so typically seen in angiomyolipoma of the kidney. Color Doppler ultrasound often helps show abnormal vasculature in the malignant tumor. Besides, follow up ultrasound will show increase in tumor size and possible metastases to adjacent organs. See:
http://www.ultrasound-images.com/kidneys.htm#Case-2_%28Renal_cell_carcinoma%29
The ultrasound image below shows a typical case of renal cell carcinoma of the right kidney. The malignant tumor is seen at the lower pole of the kidney.

Sonography of angiomyolipoma:

This middle aged female patient shows two small, echogenic, rounded lesions in the right kidney. Typical location (the right kidney is the common site for angiomyolipomas) and characteristic ultrasound features are seen in this ultrasound video of the right kidney. The images below show the twin angiomyolipomas, one in the upper pole, and barely visible, and the slightly larger lesion in the lower pole.

     

If close to the renal sinus, renal angiomyolipomas may be confused with the sinus fat echoes. Fortunately, in this patient both lesions are well within the renal cortex.
Also, regarding angiomyolipomas- it may be noted that these kidney tumors may be associated with LAM, or lymphangioleiomyomatosis. This is a progressive lung disease consistent with numerous lung cysts leading to progressive COPD. There are only 2000 known cases in the world today. It is a women's disease presumably associated with estrogen. Women with LAM may present with pneumothorax in pregnancy. Chest x-ray does not show the lung cysts, but CT scan imaging is necessary to confirm the diagnosis. When angiomyolipomas are seen a good clinical history should be taken and the possibility of LAM should be considered. There is no cure however recent research has shown promise in drugs to slow the progression of LAM. 
For more on this topic visit: http://www.ultrasound-images.com/kidneys.htm#Renal_angiomyolipoma_-%28AML%29_of_right_kidney