SPECT
Meaning of SPECT: Often described as a type of CT scan, which it is not: the radiation comes from inside the patient, not from a tube.
Definition of SPECT
SPECT, or single-photon emission computed tomography, is a nuclear medicine scan that builds cross-sectional images from gamma rays given off by a radioactive tracer inside the body. A camera rotates around the person, collecting views from many angles, and a computer reconstructs them into slices. The picture shows where the tracer collected, which reflects blood flow and cell activity in the tissue.
Is it a type of CT scan?
No, although the name invites the confusion. A computed tomography scan fires x-rays from a tube outside the body and measures what passes through, so it maps density and shows structure in fine detail. SPECT has no x-ray tube. The radiation comes from a radioactive drug already inside the person, and the camera counts the gamma rays leaving the body. All the two share is the word tomography, meaning a computer turns many angled views into slices. Modern machines often pair them in one gantry so a faint functional image can be laid over a sharp anatomical one, but they remain separate techniques doing separate jobs.
What can it show?
Function rather than form. The tracer is chosen so that it gathers in tissue doing a particular job: a heart tracer collects where blood reaches the heart muscle, a bone tracer collects where bone is being remodeled, a brain tracer follows blood flow or a specific chemical target. Regions taking up too little or too much stand out as cold or hot spots. Because the tracer reports on activity, a change can appear before anything looks different in shape or size. Most tracers are injected into a vein; a few are breathed in instead.
How does it differ from a PET scan?
| Feature | SPECT | PET |
|---|---|---|
| What is detected | Single gamma rays | Paired photons flying apart |
| Image sharpness | Lower | Higher |
| Tracer supply | Longer lived, shipped or drawn from a generator | Short lived, usually needs a nearby cyclotron |
| Availability | Widespread | More limited |
Because each gamma ray arrives alone, its point of origin can only be estimated from the direction it entered the camera. Positron emission tomography registers two photons leaving in opposite directions and can place the source along a line between them, which is why it resolves finer detail. SPECT holds its ground on cost, on tracers that survive being shipped, and on decades of routine use in heart, bone and thyroid imaging.
Also known as
Used in a sentence
SPECT imaging of the heart was performed after the tracer injection.
Good to know
SPECT records radiation emitted by a tracer inside the body rather than x-rays passed through it, so it maps tissue function; the anatomical detail in a combined study comes from the CT component, not from the SPECT images.