What’s the difference? Deciphering the alphabet soup of imaging tests

What's the difference? Imaging tests
CT scans, MRIs, PET scans and others may create a confusing alphabet soup of imaging procedures for patients.

In the winter of 1895, while experimenting with electric currents and cathode rays, German scientist Wilhelm Röntgen stumbled across a discovery that would change medicine forever. It was called the X-ray and its ability to take images of inside the human body has developed into a critical tool to help diagnose broken bones, lung disease and many other conditions. When it comes to cancer, X-rays are among many imaging procedures used to help diagnose and stage the disease and determine the effectiveness of treatments. CT-scans, MRIs, PET scans and others may create a confusing alphabet soup of procedures for patients. But whichever test or tests are used, they often are the foundation of a cancer diagnosis and are used to help determine treatment options.

“When you first get a diagnosis, you get a baseline scan,” says Dr. Alexander Trebelev, Chief of Diagnostic and Interventional Radiology at our Philadelphia hospital. “If you’re diagnosed with a malignancy, you get a scan to make sure it hasn’t spread anywhere. Treatments will differ based on that. If the disease has spread, you might need chemotherapy or radiation … or it may be able to be removed by surgery.”

The type of imaging tests, or scans, doctors order vary based on a number of considerations including the cancer type and location as well as the patient’s general health and whether a particular scan’s benefits outweigh its potential risks.

CT Scan

A computed tomography scan, or CT scan, is a painless imaging test often used to see soft tissue, the brain, stomach, intestines, liver and biliary system, pancreas, spleen and kidneys. It can be given with or without contrast, a dye designed to enhance the image. “Different tumors behave differently based on the enhancement,” Dr. Trebelev says. “We can tell what tumor it likely is. For example, liver cancer has certain enhancement characteristics that are almost diagnostic in certain patients. It’s bright early and darker later. It enhances early on and then washes out. You can identify it’s there, its size and you can usually identify what it is, in certain tumors. In other tumors you can’t say exactly what it is, but you can know the location and determine if it can be biopsied, cut out, resected or so forth.”


MRI, which stands for magnetic resonance imaging, doesn’t use radiation and shows soft tissue definition, and is often used by doctors to diagnose brain tumors, according to Dr. Trebelev. “Since there’s no radiation, there are so many advantages,” he says. “With MRI we can also look at other organs, like the liver.”

He says patients should not experience any pain during an MRI, though the tube you lie in during the procedure, which typically lasts about 45 minutes, is narrow and may make some patients claustrophobic. There are open MRI machines, but Dr. Trebelev says the imaging quality is not as good as a closed machine. “People can be sedated, in fact anesthesia is often used,” he says.


Ultrasound is another radiation-free imaging test. This technology captures images of soft tissue structures inside the body with high-frequency sound waves. A hand-held device called a transducer sends and receives ultrasound signals. Ultrasound is useful for differentiating solid tumors from cysts filled with fluid. The drawbacks of ultrasound, according to the American Cancer Society, are that its images are less detailed than some other types of scans; the technology cannot tell if a tumor is cancer; and it can’t be used in certain parts of the body because the sound waves can’t penetrate bone or air, the latter precluding it from being used on the lungs.

PET scan

A PET— positron emission tomography—scan is a noninvasive nuclear imaging test that uses radioactive material injected in the body — intravenously, inhaled or swallowed — which then gets absorbed by cells. It shows how “hot” an area is since the radioactive chemical, called a tracer, collects more in cancerous cells than in non-cancerous ones, making those cells appear brighter. PET scans are commonly ordered during treatment to see if the treatment is effective, as well as after treatment has concluded to assess the outcome. Patients who have completed treatment and show no evidence of disease may have periodic PET scans as follow ups to make sure the cancer has not returned or spread elsewhere in the body.  “A PET scan provides unique information that often cannot be obtained using other imaging procedures and it offers the potential to identify disease in its earliest stages,” according to Dr. Trebelev.

Dr. Trebelev cautions that there are limitations for every modality. “PET scans are very good for finding active disease because you know it’s hot there, but MRI and CT scans give a better delineation of lesions. A PET scan tells you there’s disease, but it’s not specific for what that disease is.” For that reason, doctors may rely on more than one type of imaging test to get a comprehensive look at a patient’s cancer.

Medical tests are rapidly innovating, something that Dr. Trebelev says is leading to earlier and earlier cancer diagnoses. “The quality of imaging is such that we can find much smaller lesions and tumors at an earlier stage, before they are so large and have spread to other tissues,” he said. ”We’re probably biopsying things that are so small that we may never have even been able to see in the past.”

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