What is attenuation correction in SPECT?
Attenuation correction in SPECT is the process of compensating for the reduction in detected photon counts caused by absorption and scatter within the patient’s body. Without correction, deeper structures appear artificially less active because fewer photons reach the detector.
Attenuation correction compensates for photon absorption within the body, improving quantitative accuracy and reducing false defects in SPECT imaging.
Attenuation correction adjusts reconstructed images to account for tissue-dependent photon absorption, improving quantitative accuracy and contrast. It is commonly performed using CT-based attenuation maps in hybrid SPECT/CT systems.
Understanding the physics
Gamma photons emitted within the body must travel through varying amounts of tissue before reaching the detector. As photons pass through tissue, some are absorbed or scattered. This process reduces the number of photons that reach the detector, particularly from deeper regions.
The probability of attenuation depends on:
Photon energy
Tissue composition
Path length through tissue
In SPECT, attenuation results in underestimation of activity in deeper structures.
Attenuation correction requires knowledge of how much tissue each photon has passed through. In hybrid SPECT/CT systems, a low-dose CT scan is acquired to generate an attenuation map. This map provides a spatial distribution of attenuation coefficients, which is incorporated into the iterative reconstruction process.
By modelling attenuation during reconstruction, the system compensates for expected photon losses. This improves uniformity and quantitative accuracy.
However, attenuation correction can introduce artefacts if there is misregistration between the SPECT and CT datasets, such as from patient motion or breathing differences.
Where this matters clinically
Attenuation correction improves diagnostic confidence, particularly in myocardial perfusion imaging and abdominal SPECT studies. It reduces false-positive defects caused by tissue attenuation but requires careful alignment to avoid artefacts.