What is SPECT imaging?
SPECT (Single Photon Emission Computed Tomography) is a tomographic nuclear medicine imaging technique that produces three-dimensional images of radiotracer distribution within the body. Unlike planar imaging, which produces a two-dimensional projection image, SPECT acquires multiple projections around the patient and reconstructs them into cross-sectional slices.
SPECT is a tomographic nuclear medicine technique that reconstructs rotating gamma camera projections into three-dimensional images of tracer distribution.
SPECT uses a rotating gamma camera to detect single gamma photons emitted from within the patient. By collecting data from many angles, it allows the spatial localisation of tracer uptake in three dimensions.
SPECT improves contrast resolution and lesion localisation compared with planar imaging.
Understanding the physics
In planar imaging, the gamma camera records a projection image which is essentially a summation of all detected photons along each line through the body. This can lead to superimposition of structures and reduced depth information.
In SPECT imaging, the gamma camera rotates around the patient, typically acquiring images at multiple angular positions (for example, every 3–6 degrees over 180° or 360°). Each projection represents the distribution of detected photons from a different angle.
These projections are then processed using mathematical reconstruction techniques to generate tomographic slices. The reconstruction algorithms determine how the measured projections relate to the true spatial distribution of radioactivity within the body.
Because SPECT relies on mechanical collimation, it detects single photons (unlike PET, which detects coincident photon pairs). The collimator remains the dominant factor influencing spatial resolution and sensitivity.
SPECT provides improved contrast resolution compared with planar imaging because it reduces the superimposition of overlying structures. However, spatial resolution remains limited by collimator geometry and photon attenuation.
Where this matters clinically
SPECT is widely used in myocardial perfusion imaging, bone scintigraphy, brain perfusion studies, and many other applications. The tomographic approach improves lesion detection and anatomical localisation compared with planar imaging.