Attenuation correction

Attenuation is the loss of PET or SPECT signal that occurs when photons emitted inside the body are absorbed or scattered before they escape and reach the detector; because photons emitted deep in the body must traverse more tissue than those emitted near the surface, uncorrected images make deep structures appear falsely cold and superficial tissue falsely hot. Attenuation correction is the processing step that compensates for this, turning PET or SPECT from a qualitative picture into a quantitatively meaningful measurement of activity concentration, the basis for standardised uptake value (SUV) and for dosimetry calculations.

In PET, an annihilation event produces two 511 keV photons travelling in opposite directions; because attenuation depends on the total tissue thickness traversed by both photons along their combined path (the line of response), the attenuation correction factor for that path can be computed independent of exactly where along the path the decay occurred, making PET attenuation correction mathematically straightforward once the tissue attenuation map is known. In modern PET/CT and SPECT/CT, this attenuation map is derived from the CT scan: the CT's x-ray attenuation values (measured at a low, polyenergetic effective energy around 70 keV) must be mathematically rescaled to the much higher, monoenergetic energy of the emission photons (511 keV for PET, the specific gamma energy for a given SPECT radionuclide, such as 208 keV for 177Lu) before being applied as a correction, since attenuation is strongly energy-dependent.

Attenuation correction is one of the largest and most consequential physical corrections applied to PET and SPECT data, essential for accurate SUV measurement, lesion detectability, and any dosimetry that depends on absolute activity quantification; historically it was performed with a rotating external radioactive transmission source before CT-based methods became standard.