A CT (computed axial tomography) scan uses X-rays, an MRI (Magnetic resonance imaging) scan uses magnetic and radio waves, a PET (Positron emission tomography) scan uses a radioactive substance injected in the body and gamma rays, and SPECT (Single photon emission computed tomography) scan uses a radioactive substance injected into the body and a gamma camera. Tomography is a technique for displaying a representation of a cross section through a human body or other solid object using a penetrating wave.
CT Scanner
A CT scan is better suited to cancer, pneumonia, abnormal chest x-rays, and bleeding in the brain, especially after an injury. A CT scan shows organ tear and organ injury more quickly, so is more suitable for trauma cases. Broken bones and vertebrae are more clearly visible on a CT scan. CT scans provide a better image of the lungs and organs in the chest cavity between the lungs.
MRI Scanner
An MRI is better for examining the spinal cord. An MRI show a more visible brain tumor.
A CT scan does not show tendons and ligaments, but an MRI does.
A CT or MRI scan can assess the size and shape of body organs and tissue, but they cannot assess how these work.
PET Scanner
The PET system detects pairs of rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body. Three-dimensional images of tracer concentration within the body are then constructed by computer analysis. A PET scan can show how an organ works, and is often used with a CT or MRI scan. PET scans are used to diagnose a condition or to track how it is developing. PET scans are used to investigate epilepsy, Alzheimer's disease, cancer, and heart disease.
The SPECT system works like a PET, but uses gamma rays to show a tracer dose of radioactive material injected into the body. The material moves to areas of bone and elsewhere highlighting healing or cancer progression as it is usually lit up on SPECT scans.