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Making Sense of CT, MRI and PET Scans for Melanoma Skin Cancer

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Updated June 19, 2014

CT, MRI and PET scanning for melanoma patients

Imaging technologies help to identify metastases in patients with melanoma

photo © A.D.A.M.
If your physician suspects that you have metastatic melanoma (stage III or IV), there are several tools available to confirm the diagnosis, determine the exact stage and check if it has spread to local or distant lymph nodes, lungs, brain, bones or other areas of the body. These include a blood test for LDH (lactate dehydrogenase) level, a sentinel lymph node biopsy, or imaging studies using technologies such as computed tomography (CT), magnetic resonance imaging (MRI) and positron emission tomography (PET). This introduction will help you make sense of the jargon and understand what to expect during these sometimes intimidating scanning procedures.

Computed Tomography (CT)

CT is an imaging method that uses multiple x-rays to create cross-sectional pictures of the body. Small detectors inside the scanner measure the amount of x-rays that make it through the part of the body being studied. A computer takes this information and uses it to create several individual images, called "slices." Three-dimensional models of organs can be created by stacking the individual slices together. If you have stage III or IV, "in transit," or local recurrent melanoma, you will likely have a CT scan of the chest, because the lungs often are the first site of metastatic disease. The physician may also order a CT scan of your brain, abdomen or pelvis, depending on the stage and symptoms.

What to expect. During your appointment, you will be asked to lie on a narrow table that slides in to the center of the CT scanner. You may receive an intravenous (IV) injection of a dye, or radiocontrast agent, which helps better outline structures in your body. Depending on the study being done, you will lie on your stomach, back or side. Once inside the scanner, the machine's x-ray beam rotates around you. (Modern "spiral" scanners can perform the exam in one continuous motion.) You must be still during the exam, because movement causes blurred images. You may be told to hold your breath for short periods of time. Generally, complete scans take only a few minutes. The newest multidetector scanners can image your entire body, head to toe, in less than 30 seconds.

Magnetic Resonance Imaging (MRI)

MRI is a noninvasive way to take pictures of the body. Unlike x-rays and CT scans, which use radiation, MRI uses powerful magnets and radio waves to construct the image. A MRI is useful for melanoma staging and may be required to detect additional distant metastases, particularly in the brain or spinal cord.

What to expect. To perform the scan, you may be asked to wear a hospital gown or clothing without metal fasteners (such as sweatpants and a T-shirt). You will lie on a narrow table, which slides in to the middle of the MRI machine. If you fear confined spaces (claustrophobia), tell your doctor before the exam. You may be prescribed a mild sedative or your doctor may recommend an "open" MRI, in which the machine is not as close to the body. Small devices, called "coils," may be placed around the head, arm, leg or other area to be studied. These devices help send and receive the radio waves, and improve the quality of the images. Some exams require a special dye (contrast). The dye is usually given before the test through an intravenous line in your hand or forearm. The dye helps the radiologist see certain areas more clearly. During the MRI, the person who operates the machine will watch you from another room. Several sets of images are usually needed, each taking 2 to 15 minutes. Depending on the areas being studied and type of equipment, the exam may take one hour or longer.

Positron Emission Tomography (PET)

PET is an imaging test that uses a radioactive substance to look for disease in the body. Unlike MRI and CT scans, which reveal the structure of organs, a PET scan shows how the organs and tissues are functioning. The technique has proven to be useful in screening for metastases of unknown location, determining the stage of the melanoma, and finding recurring tumors. It is considered more sensitive than a CT scan for seeing the small tumors typical of metastatic melanoma, but still can't match the accuracy of a sentinel lymph node biopsy for detecting metastases in lymph nodes.

What to expect. PET scans use a small amount of a radioactive "tracer" injected into a vein, usually on the inside of the elbow. The substance travels through the blood and collects in organs or tissues with high activity. You will be scanned approximately 60 minutes after receiving the radioactive substance. You lie on a table that slides in to a tunnel-shaped hole in the center of the PET scanner. The PET machine detects energy given off by the radioactive substance and converts it into three-dimensional pictures. The images are sent to a computer, where they are displayed on a monitor for the physician to read. You must lie still during the PET scan, so that the machine can get clear images of your organs. The test takes about 30 minutes.

Sources:

Dancey AL, Mahon BS, Rayatt SS. A review of diagnostic imaging in melanoma. J Plast Reconstr Aesthet Surg. 2008 61(11):1275-83. 18 November 2008.

"How is Melanoma Diagnosed?" American Cancer Society. 18 November 2008.

"Melanoma" National Comprehensive Cancer Network. V.2.2009. 18 November 2008.

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