CT scan (computer tomography) and PET scan (positron emission tomography) are different but related imaging techniques. A PET scan uses nuclear medicine imaging to produce a three-dimensional picture of functional processes in the body. PET scans provide metabolic information and are increasingly read alongside CT or MRI (magnetic resonance imaging) scans, which provide anatomic information.
A CT scan provides good detail about bony structures and some detail of soft tissues. It answers the question ‘What does it look like?’. For example, an abnormal growth like a tumor can be easily detected using a CT scan. A PET scan on the other hand is more useful in providing a good detail of functioning of body parts. It answers the question ‘How is it working?’. For example, staging and monitoring treatment of cancers.
CT Scanning is useful for screening diseases such as colon cancer, detecting injuries and abnormalities in the head, chest, heart, abdomen and extremities. This technique is often combined with other techniques such as MRI, and ultrasonography.
PET scanning is used effectively in oncology (study and treatment of cancer), neurology, cardiology, Cognitive neuroscience, Psychiatry, Pharmacology and Muscular-Skeletal Imaging.
Principle and Procedure
A CT scan relies on a series of X-rays and is usually completed within 5-10 minutes. On the other hand, a PET scan takes anywhere between 2 to 4 hours.
The basic principle behind a CT scan relies on the reconstruction of a three dimensional image of an organ, by a computer. During a CT scan, the patient is moved along the scanning system, during which multiple X-ray images of the organ being examined are acquired from different angles. These images are obtained by passing multiple beams of X-rays through the region of interest. Sophisticated computer algorithms combine all these images to create a view of the organ, which is available soon after the scan is complete.
This video explains how a CT scan is performed:
During a PET scan, a radioactive substance like fluorine-18 (F-18), fluorodeoxyglucose (FDG) or oxygen-15 is injected into the patient’s body. This is called a tracer or radiotracer. Each tracer has the ability to get absorbed by the particular organ or tissue being studied. It takes 30 minutes to an hour for the tracers to get absorbed. Only then can the patient be moved into the PET scanner and imaging can start. The tracer is injected into the body on a biologically active molecule and emits pairs of gamma rays. The PET scan system detects these gamma rays and thereby determines the movement of the tracer in the body over time. This movement is then reconstructed via a CT scan.
This video provides a good overview of how PET scans work:
CT Scan costs range from $1,200 to $3,200 and the cost of PET scan depends on the area examined and usually ranges form $3,000 to $6,000. The cost might vary in different countries.
Advantages of PET vs. CT scan
CT scans have advantages in viewing anatomical structures. Due to the high contrast resolution of CT scanning, differences between the tissues are more apparent compared to other techniques. Further, imaging in CT scan removes the superimposition of structures other than the area of interest. The data from a single procedure can be viewed in different planes and thus increases diagnostic ability. This technique is also popular because it can be used to diagnose a number of conditions. This might eliminate the need for other diagnostic techniques such as colonoscopy.
PET scans have an advantage over regular CT scans in determining the functioning of biological processes. In a PET scan the imaging technique gets down to the cellular level of the body, hence it can detect the early onset of disease like cancer, before they start showing up in CT scan. It is also very useful in finding out how effective the treatment is.
CT scans are associated with the risk of causing cancers, such as lung cancer, colon cancer and leukemia. This is mainly due to the use of X-rays. There are other safety concerns associated with the use of contrast agents which are administered intravenously. However, these disadvantages can be reduced with the use of lower doses.
There is exposure to ionization radiation in a PET scan. Pregnant or breastfeeding women should not get this scan done. Some mild allergic reaction may occur in some people because of the radioactive substance used. Sometimes abnormal blood sugar and insulin levels may result in an erroneous PET report.