Magnetic Resonance Imaging (MRI) of the kidneys is a non-invasive medical test that uses a powerful magnetic field and radio waves to create detailed pictures. This technology allows doctors to see the kidneys, their blood vessels, and the surrounding tissues with clarity. The primary purpose is to assess the structure and function of the kidneys without using the ionizing radiation found in other imaging methods like CT scans.
Reasons for a Kidney MRI
A physician may order a kidney MRI for several reasons, often after another test like an ultrasound or CT scan has revealed an abnormality. One of the most common uses is to characterize a mass found on a kidney. An MRI can help determine if a mass is a simple cyst or a more complex solid mass that could be a tumor.
Beyond identifying masses, a kidney MRI provides information about kidney function and blood flow. It can be used to diagnose conditions that obstruct the urinary tract, which causes swelling of the kidneys (hydronephrosis). For patients with high blood pressure, an MRI can check for narrowing of the arteries that supply blood to the kidneys, known as renal artery stenosis.
The test is also employed to evaluate the health of a transplanted kidney, monitoring for any signs of rejection or complications with blood supply. In some cases, it can help diagnose congenital abnormalities, which are structural problems with the kidneys that have been present since birth.
The Kidney MRI Procedure
You will likely be asked to fast, avoiding food and drink for several hours before the scan. It is important to inform the technologist about any metal in or on your body, including jewelry, piercings, and internal devices like pacemakers or aneurysm clips, as these can be affected by the strong magnetic field. Before the procedure, you will change into a hospital gown.
During the scan, you will lie on a motorized table that slides into the center of the large, tube-shaped MRI machine. To obtain clear images, you must lie as still as possible. The machine produces loud, repetitive noises, such as banging and buzzing, so you will be offered earplugs or headphones. In some cases, an intravenous (IV) line may be placed in your arm to administer a contrast agent to enhance image clarity.
The technologist will be in an adjacent room but can see and communicate with you through an intercom system. The entire procedure can take between 30 to 60 minutes, depending on the specifics of the scan. After the scan is complete, the table will slide out of the machine, and if an IV was used, it will be removed.
Most kidney MRI procedures are performed on an outpatient basis, meaning you can go home the same day. There are generally no restrictions on your activities afterward, and you can typically eat, drink, and drive as normal. The images will be analyzed by a radiologist, and the results will be sent to your referring physician.
Interpreting Kidney MRI Findings
After a kidney MRI, a radiologist analyzes the images to identify any abnormalities in the kidneys’ structure or function. The findings can range from normal to indicating a specific condition. A common finding is a simple cyst, which appears as a well-defined, fluid-filled sac; these are almost always non-cancerous and may not require further action.
In contrast, the images might reveal a complex cyst or a solid mass. A complex cyst has thicker walls or internal components, while a solid mass is composed of tissue. These types of findings often require further evaluation to determine if they are malignant. The MRI provides details about the size, shape, and composition of these masses to help in planning subsequent steps.
Other potential findings include signs of infection, such as pyelonephritis, which may cause areas of inflammation in the kidney tissue. The MRI can also show evidence of blockages in the urinary tract, which cause the kidneys to swell, a condition known as hydronephrosis. Additionally, the scan can detect issues with blood flow, such as narrowing of the renal arteries, which can affect kidney function.
Safety and Alternative Imaging
A significant advantage of an MRI is that it does not use ionizing radiation, unlike CT scans. The primary safety consideration is the powerful magnetic field, which can interact with any metal inside the body. For this reason, it is not recommended for individuals with certain implants like pacemakers or metal fragments in their eyes.
For some kidney MRIs, a gadolinium-based contrast agent is injected to improve image clarity. While generally safe, these agents pose a small risk for individuals with severe pre-existing kidney disease. In rare cases, it can lead to a condition called nephrogenic systemic fibrosis, so a blood test to check kidney function is often required beforehand.
Doctors have other imaging tools for evaluating the kidneys, including ultrasound and CT scans. An ultrasound is often used for initial screening because it is widely available and does not involve radiation. A CT scan is faster than an MRI and provides excellent detail of the urinary tract, but an MRI is often preferred when superior soft-tissue detail is needed to characterize a mass.