What is recovery coefficient in PET?
The recovery coefficient (RC) in PET is a measure of how accurately the measured activity concentration in a structure reflects its true activity concentration. It quantifies the impact of partial volume effects on small objects.
The recovery coefficient is defined as the ratio of measured activity concentration to true activity concentration. For large structures, the recovery coefficient approaches 1. For small structures, it falls below 1 because spatial blurring reduces the measured signal.
The recovery coefficient quantifies how much measured activity underestimates true activity due to partial volume effects, particularly in small lesions.
Recovery coefficients are used to understand and, in some cases, correct for underestimation of uptake in small lesions.
Understanding the physics
Because PET has finite spatial resolution, activity from small objects is blurred into surrounding tissue. As described in the partial volume effect, this reduces the apparent peak activity within the lesion.
If a small sphere containing known activity concentration is imaged, the measured activity in the reconstructed image will be lower than the true activity. The recovery coefficient expresses this relationship:
RC = [measured activity] / [true activity]
For large objects (much larger than system resolution), most of the activity remains within the object’s boundaries in the reconstructed image, and the RC approaches 1.
For small objects, especially those with diameters less than two to three times the system’s full width at half maximum (FWHM), significant blurring occurs and the RC decreases.
Recovery coefficients depend on:
Lesion size
Spatial resolution of the scanner
Reconstruction algorithm
Post-reconstruction smoothing
They are often determined using phantom studies with spheres of known size and activity concentration.
Where this matters clinically
Recovery coefficients explain why small lesions have underestimated SUVs. In research settings, RCs may be applied to improve quantitative accuracy. In clinical practice, understanding recovery behaviour helps avoid underestimating the significance of small but metabolically active lesions.