How does SPECT differ from planar imaging?
SPECT differs from planar imaging in that it produces three-dimensional tomographic images, whereas planar imaging produces a two-dimensional projection image.
In planar imaging, all detected photons are summed along each line through the body, resulting in superimposition of structures at different depths. In SPECT, the gamma camera rotates around the patient, acquiring multiple projections from different angles. These projections are mathematically reconstructed into cross-sectional slices.
SPECT differs from planar imaging by acquiring rotating projections and reconstructing them into three-dimensional slices, improving depth localisation and contrast.
As a result, SPECT improves depth localisation and contrast resolution compared with planar imaging, although it requires longer acquisition times and more complex reconstruction.
Understanding the physics
In planar imaging, each pixel represents the total number of detected photons along a straight path through the body. This means that activity from deeper structures is superimposed on activity from superficial structures. While planar imaging is simple and fast, it does not provide depth information.
SPECT overcomes this limitation by acquiring multiple projection images at different angles around the patient. Each projection contains directional information from a different viewpoint.
Using reconstruction algorithms, these projections are combined to estimate the original three-dimensional distribution of radioactivity. This process is analogous to CT reconstruction, but instead of measuring transmitted X-rays, SPECT reconstructs emitted gamma photons.
Because SPECT separates overlapping structures into slices, it reduces background interference and improves lesion contrast. However, spatial resolution is still limited by collimator geometry, and photon attenuation within the body can degrade accuracy if not corrected.
SPECT requires longer acquisition times than planar imaging because data must be collected at multiple angles. This increases susceptibility to motion artefacts.
Where this matters clinically
SPECT provides improved lesion localisation in myocardial perfusion imaging, brain perfusion studies, and skeletal imaging. It allows differentiation of overlapping structures that may appear indistinct on planar images, improving diagnostic confidence.