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Perinephric fat, also known as perirenal fat or the adipose capsule of the kidney, is a layer of fatty material that surrounds the kidneys. It plays an important role because it helps to cushion and protect the kidneys. Evaluation of this layer of tissue can be done either by ultrasonography or computed tomography (CT) scanning. Pathologic processes such as infection and cancers can affect this type of fat.
The human kidneys are surrounded by perinephric fat, which is composed of adipose tissue — a collection of fat cells bound together by connective tissue and supplied by blood vessels. This layer of tissue is found outside of the outer surface of the kidney, but under a layer of connective tissue called the renal fascia. Typically, it is less than 1 inch (2.54 cm) in thickness.
Perhaps the most important function of this fat is to protect the kidneys. Since the kidneys play a critical role in helping the body maintain a consistent internal environment, from filtering out toxins to balancing the concentration of electrolytes within the blood, these organs need to be safe from damage. The perinephric fat, as well as the paranephric fat that is found outside of the renal fascia, helps to cushion and protect the kidneys.
In order to examine this layer of fat, a number of imaging studies can be done. One of the cheapest and most sensitive imaging techniques that can be used to examine this layer of adipose tissue is an ultrasound. This technology uses sound waves to elucidate the structures underlying the external surface of the human body, and is therefore cost-effective and relatively harmless. Computed tomography (CT) scanning, which is typically more expensive and more harmful because it exposes patients to ionizing radiation, can also be performed to investigate this tissue.
A number of conditions can cause abnormalities of this fat layer. One of the most common conditions is pyelonephritis, an infection of the kidneys. The fat can become inflamed as a result of this kidney infection. Alternatively, a severe infection could spread throughout the kidney and cause an abscess to develop within this fat. Patients with an infection this severe can be very sick, and typically require hospitalization and intravenous antibiotics; in some cases the abscesses have to be drained surgically in order for patients to fully recover from their infections.
Another important pathologic process involving the perinephric fat is cancer. A malignancy originating in the kidneys, such as a renal cell carcinoma, can grow and invade into this layer of fat. Knowing whether a cancer has spread into this tissue can have important implications in the treatment and prognosis of a patient with this type of cancer.