PET Scan vs. CT Scan

Computed tomography (CT) and positron-emission tomography (PET) scans are two medical imaging techniques that differ in several key ways. The main difference is that a CT scan creates a detailed image of organs, bones, and tissues, while a PET scan shows how the tissues in your body work on a cellular level.

Both can be used to diagnose or evaluate many different medical conditions. The two techniques can also be combined into a PET-CT scan to provide more detailed information about cancer.

This article will review the basics of PET and CT scanning, including their differences and their medical uses.

What Is a CT Scan?

A CT scan composites multiple X-rays to create a detailed image of the internal structures of the body. The cross-sectional images generated during the scan can be viewed as multiple "slices" of an organ, bone, or tissue, or reformatted to create a three-dimensional image.

As with X-rays, CT involves exposure to low-dose ionizing radiation. The devices, called CT scanners, can vary in their designs but are comprised of a table and a rotating X-ray tube.

A CT scan has many different medical uses, including the diagnosis and monitoring of diseases as well as preventive screening of diseases. This includes:

• Detecting internal injuries or bleeding
• Diagnosing musculoskeletal (muscle and bone) problems, such as bone fractures or tumors
• Diagnosing vascular (blood vessel) problems, like blood clots, ruptures, or blockages
• Pinpointing the location of an abscess, tumor, or hemorrhage
• Detecting and monitoring diseases like stroke, heart disease, lung disease, cancer, and liver disease
• Guiding medical procedures such as surgery, biopsy, and radiation therapy
• Monitoring the effectiveness of medical treatments, such as cancer therapy or heart valve surgery

Abnormalities are detected based on areas of hypodensity (areas that are abnormally light) or hyperdensity (areas that are abnormally dark). For instance, hypodensity may indicate edema (tissue fluid overload), while hyperdensity can occur with hemorrhages or bone fractures.

Some CT scans involve a contrast dye delivered orally (by mouth) intravenously (into a vein). These help sharpen the contrast between organs, tissues, bones, or blood vessels so that abnormalities are better spotted.

What Is a PET Scan?

A PET scan is a medical imaging technique that uses radioactive substances, called radiotracers, to visualize and measure changes in metabolism (the chemical processes that take place in cells to keep the body functioning normally). This includes evaluating where blood flow is being directed, where energy is being consumed, and how waste products are being eliminated.

A PET scanner is comprised of a table that moves in and out of a doughnut-shaped or tube-shaped port that houses the detection apparatus.

PET scans have several different medical uses, including:

• Diagnosis and monitoring of cancer, including the spread (metastasis) of cancer to other parts of the body, including bone
• Diagnosis and monitoring of brain disorders, such as tumors, epilepsy, and Alzheimer's disease
• Diagnosis and/or monitoring of cardiovascular problems, such as coronary artery disease (CAD), heart attacks, and heart failure

Different radiotracers, delivered intravenously, can provide different pieces of information based on the part of the body they target. Any abnormal changes in metabolism may suggest a medical concern. These are indicated by different colors or degrees of brightness on a PET image.

By way of example, cancer cells have a higher metabolic rate as they grow faster than other cells and cause the formation of new blood vessels to feed them. These metabolic changes will appear as dark-colored spots on the PET scan.

PET scans can often identify cancer sooner than a CT scan because of the rapid uptake of a sugar-based radiotracer by the fast-growing tumor.

Comparing CT Scans to PET Scans

CT scans and PET scans are important tools for medical imaging with specific benefits, uses, risks, and limitations.

From the broadest perspective, CT scans help detect structural abnormalities in the body, while PET scans detect functional abnormalities in the body.

Despite their differences, CT scans and PET scans have certain similarities, namely that:

• They can both be performed on an outpatient basis.
• They both are non-invasive and relatively painless.
• They can both detect cancer.
  
• They both can eliminate the need for exploratory surgery.
  
• They both can be used in pregnancy (with limitations) if medically indicated.

PET-CT Scan

CT and PET technology can be combined into a single scanner, called a PET-CT scanner. Combined PET-CT scanners perform almost all PET scans today. Together, they may be able to provide a more accurate diagnosis than the two scans performed separately.

This is particularly true with respect to cancer in which a PET-CT scan can:

• Diagnose and stage cancer, particularly smaller tumors that a CT might miss
• Identify metastasis, including early metastasis
• Determine which type of treatment is needed, including surgery or radiation
• Monitor your response to treatment
• Confirm whether your cancer is in remission or if there is a recurrence

By combining the two, the diagnosis time may be shortened (an important consideration for aggressive, later-stage cancer).

The only major limitation is the cost of a PET-CT scan, which can cost upwards of $1,000 or more. A CT scan can cost one-half to two-thirds less.

Summary

CT scans and PET scans are both essential tools in the diagnosis and monitoring of many health conditions, including cancer. They use different types of radiation, with PET using radioactive tracers injected into a vein and CT scans using external X-rays.

PET scans detect functional changes in metabolism that indicate disease. CT scans detect structural changes in bones, organs, and tissues indicative of disease or injury. A PET-CT scan is a combination of these two techniques that offer more information and can help deliver earlier diagnoses.