Cancer Diagnostic Tools: Understanding the Difference Between a CT, PET and More

Imaging studies allow your oncologist a view inside the body without the need for more invasive measures. Imaging is a vital part of cancer diagnosis and treatment because it allows a physician to determine the presence of cancerous cells and to what degree (if any) the cancer has spread. However, the imaging type recommended can vary based on the kind and location of the cancer you have. Here are some brief explanations of why your doctor may choose a PET scan over an ultrasound or an MRI over a mammogram (or vice-versa).
What It Is
: A mammogram is a type of X-ray that detects tumors or calcifications in the breast.
Best for Detecting: Mammograms are used for detecting breast cancer, determining the size of a tumor or determining if a tumor has shrunk following breast cancer treatments. Mammograms can be either screening or diagnostic. Screening mammograms are performed in women as a precautionary measure to identify potentially cancerous cells. Diagnostic mammograms are performed after a doctor has diagnosed a woman with breast cancer.
Limitations: Mammograms can have false-positive results and false-negative results. For example, the National Cancer Institute estimates diagnostic mammograms miss about 20 percent of breast cancers present at screening time. However, the NCI recommends women age 40 and older have a mammogram every 1 to 2 years. If you are at higher risk for breast cancer due to family history, this recommendation tends to be earlier and/or more frequently.
Computed Tomography (CT) Scans
What It Is
: Also called a "cat" scan, a CT scan uses X-ray imaging to create pictures of the body in "slices," which allows a physician to look at an imaging scan in three dimensions as well as two dimensions.
Best for Detecting: The three-dimensional capabilities of a CT scan mean a physician can better identify the exact location of a cancerous tumor. In addition to cancer diagnosis, the locating abilities help physicians identify the exact place to make an incision for cancer biopsy, where to target radiation treatments or where to place internal radiation treatments. The scans are also used to detect if a tumor is responding to treatment.
Limitations: CT imaging does involve the use of ionizing radiation, which can increase a person's risk for cancer. A CT does expose a person to more radiation than a typical X-ray. The likelihood of a person developing a fatal cancer from CT exposure is an estimated 1 in 2,000, according to the National Cancer Institute.
Positron Emission Tomography (PET) Scan
What It Is
: A PET scan is a real-time imaging scan that helps a physician determine how well the body is functioning. PET scans can measure blood flow, oxygen usage and how well the body uses glucose.
Best for Detecting: PET scans can detect metabolic changes at the cellular level in an imaging scan. These changes are typically the first signs of cancer. This means PET scans can detect cancerous changes before the cancerous cells start to group together, which is what a CT or MRI could detect.
Limitations: PET scans can offer false positive results, just as X-rays can. Also, PET scans are not typically used to determine treatment planning, but are instead diagnostic and prognostic, meaning they tell how well a treatment is working. This is why PET and CT imaging are often combined in a single scan.
Magnetic Resonance Imaging (MRI) Scan
What It Is: An MRI scan uses radio waves and magnetic fields to create images. This means no radiation is involved during the scan. However, the test is very loud due to the magnetic wave generation. Like a CT scan, an MRI is used to create slices of the body to re-create the body in three-dimensional images.
Best for Detecting: MRIs are used to diagnose tumors, especially those in the brain and spinal cord. The images tend to be more detailed than a CT scan.

Limitations: MRIs tend to be more costly that CT scans, and those who are claustrophobic can become anxious due to the loud noise and enclosed space. Additionally, patients who have any kind of metal in their bodies, such as pacemakers or aneurysm clips, cannot have the procedure because the magnetic force could dislodge any metal in the body.