What is scatter correction and why is it needed in PET and SPECT?
Scatter correction is the process of identifying and compensating for photons that have undergone Compton scatter before reaching the detector. Without correction, scattered photons are incorrectly assigned to positions in the image, reducing contrast and quantitative accuracy.
Scatter correction compensates for mispositioned Compton-scattered photons, improving contrast and quantitative accuracy in PET and SPECT.
In both PET and SPECT, scatter adds unwanted background signal and degrades lesion detectability. Modern reconstruction algorithms incorporate scatter modelling to improve image quality and restore more accurate activity distribution.
Understanding the physics
When gamma photons travel through tissue, some undergo Compton scatter. During this interaction, the photon changes direction and loses energy.
In SPECT, scattered photons may still fall within the energy window and be detected. Because they have changed direction, their recorded position does not correspond to their true origin. This introduces image blurring and reduces contrast.
In PET, scatter can affect one or both photons in a coincidence pair. If a scattered photon is still detected within the timing and energy window, it may be incorrectly assigned to a line of response. This results in mispositioned events and artificially elevated background activity.
Scatter increases with:
Patient size
Lower photon energy
Increased path length through tissue
Without correction, scatter reduces lesion-to-background contrast and can lead to overestimation of activity in low-uptake regions.
Scatter correction methods estimate the contribution of scattered photons and subtract or model them during reconstruction. In SPECT, this may involve energy window-based techniques. In PET, scatter is typically estimated within iterative reconstruction algorithms using physical modelling.
Where this matters clinically
Scatter correction improves contrast resolution and quantitative accuracy, particularly in large patients or deep structures. In PET, accurate SUV measurement depends on proper scatter correction.
Failure to correct for scatter can reduce lesion detectability and compromise quantitative assessment.