Ultrasound is frequently used to evaluate the kidneys and any cysts or masses. It may also be ordered if your doctor is concerned about how well the kidneys are working or if there is blood in the urine. Ultrasound uses high frequency sound waves to form an image. This is a painless test which uses no radiation. The quality of an ultrasound is dependent on the skill and experience of the technologist who performs the test, and the radiology doctor who interprets it. Sometimes the radiologist may want to see the cyst by watching the technologist scan in real time.
A kidney cyst is a benign mass that is filled with fluid. A simple cyst on ultrasound will have simple black appearing fluid inside, with a thin defined wall, and acoustic enhancement along the back wall (a white shadow going beyond the back wall). A cyst can also look complex with areas of thickening or modularity along the wall, or complex looking fluid. In these cases, the radiologist may recommend another test like CT or MRI to further evaluate.
The distinction between a simple cyst and more complex lesion is critical. Simple cysts are left alone and are benign. More complex lesions that contain more then simple fluid may be cancerous at times. Other tests like CT or MRI help assess the risk of cancer in these cysts. There is even a classification system called Bosniak which grades cysts from 1 to 4. Bosniak 1 cysts being benign all the time to 4 which are cancerous.
The ultrasound is sometimes limited in obese patients. The sound waves have a hard time penetrating large amounts of tissue and fat, and may make a simple cyst look more concerning then it really is. A CT or MRI will often help clear up any concern.
Small cysts, say less then 1.5 centimeters can also be a challenge. These small cysts may not have all the classic features of a simple cyst. The uncertainty can often be cleared up with a dedicated CT of the kidneys with thin sections going through the cysts done with and without contrast. In some cases when the cyst is not entirely simple but slightly complex, the radiologist may recommend a 6 month follow up to reevaluate the cyst.