Computed tomography (CT) and Positron Emission Tomography (PET) scans are tools used in medicine to visualize the inner workings of the human body. A CT scan is primarily a structural imaging technique, using X-rays to create detailed pictures of physical anatomy. Conversely, a PET scan is a functional imaging technique, designed to map out biochemical and metabolic activity within cells. The difference lies in their focus: CT reveals what the tissues look like, while PET shows what the tissues are doing.
What a CT Scan Reveals
A CT scan uses a rotating X-ray beam and specialized detectors to generate cross-sectional “slices” of the body. These images are compiled by a computer to create detailed, three-dimensional views of internal structures. CT images excel at demonstrating anatomical detail, providing clear differentiation between bone, soft tissue, and blood vessels.
The strength of CT lies in its ability to quickly measure the size, shape, and precise location of masses or abnormalities. It is highly effective for visualizing bone fractures, assessing organ damage following trauma, and pinpointing the dimensions of a tumor or lesion.
The Unique Metabolic Insight of PET
The information a PET scan provides goes beyond physical structure, offering insight into cellular function and metabolism. This is achieved by introducing a radioactive compound called a radiotracer into the patient’s bloodstream, most commonly fluorodeoxyglucose (FDG). FDG is a glucose analog, meaning the body treats it similarly to regular sugar.
Cells with high metabolic demands, such as rapidly dividing cancer cells or highly active brain regions, absorb this FDG tracer at a much faster rate than less active or healthy cells. Once inside the cell, the FDG becomes temporarily trapped, allowing the PET scanner to detect the resulting radioactive emissions. The resulting image is a functional map that highlights areas of intense biochemical activity, showing where the body is consuming the most glucose.
This metabolic mapping is what a CT scan cannot provide. PET scans can detect abnormal cellular function long before structural changes become large enough to be visible on a CT image. For example, a small cluster of highly active cancer cells may not form a detectable mass on a structural scan, but their aggressive glucose consumption will cause a bright signal on a PET scan.
Applications Where PET Excels
The unique functional data provided by a PET scan makes it invaluable in medical contexts where structural information alone is insufficient. A primary application is the early detection and characterization of cancer.
Cancer Detection and Staging
While a CT scan can identify a lung nodule, a PET scan helps determine if that nodule is likely malignant by assessing its metabolic rate. Malignant tumors frequently exhibit hypermetabolic activity, causing them to “light up” brightly on the PET image.
PET scans are also routinely used in cancer staging and monitoring treatment response. By scanning the entire body for areas of high FDG uptake, the physician can determine if the cancer has metastasized to distant sites, which is a process known as staging. After treatment, a repeat PET scan assesses effectiveness; a successful treatment shows a significant decrease in the tumor’s metabolic activity, often before the tumor has substantially shrunk in size on a CT scan.
Neurological Conditions
PET imaging is highly useful in neurological conditions that involve functional rather than structural changes. Disorders like Alzheimer’s disease and certain forms of dementia are characterized by altered glucose metabolism in specific areas of the brain. An FDG-PET scan can reveal distinct patterns of reduced glucose uptake, or hypometabolism, aiding in the differential diagnosis of various dementias. Specialized PET tracers are also available to directly visualize the protein deposits, such as amyloid or tau, that are hallmarks of Alzheimer’s disease.
Epilepsy Evaluation
PET scans can help locate the seizure focus in the brain. Between seizures, the hyperactive area that causes the seizures often shows a lower-than-normal metabolic rate, or hypometabolism. This functional abnormality is undetectable on a structural CT scan. This mapping capability allows physicians to precisely target the source of the disorder.