What is the partial volume effect in SPECT and PET?
The partial volume effect is the underestimation of true tracer uptake in small structures due to limited spatial resolution. When a structure is smaller than approximately two to three times the system’s spatial resolution, its measured activity is reduced because counts are blurred into surrounding tissue.
The partial volume effect causes underestimation of activity in small structures due to limited spatial resolution and image blurring.
As a result, small lesions may appear less intense than they truly are, and quantitative measurements such as SUV may be underestimated.
The partial volume effect is a direct consequence of finite spatial resolution.
Understanding the physics
Every imaging system has limited spatial resolution. This means that the activity from a point source is not recorded as a perfect point, but rather as a blurred distribution known as the point spread function.
When a lesion is large compared with the system’s resolution, most of its activity remains within its boundaries in the reconstructed image. However, when a lesion is small its activity is spread into adjacent pixels.
This spreading reduces the measured peak activity concentration within the lesion and increases apparent background activity slightly. The smaller the structure relative to the system’s resolution, the greater the underestimation.
In PET, partial volume effects are influenced by positron range, detector resolution, and reconstruction parameters. In SPECT, collimator geometry plays a dominant role.
Partial volume effects do not represent true biological reduction in uptake; they are purely physical and geometric in origin.
Where this matters clinically
Small metastatic lymph nodes, pulmonary nodules, or brain lesions may demonstrate lower apparent uptake than expected. Quantitative metrics such as SUV can be significantly underestimated in small lesions.
Understanding the partial volume effect prevents misinterpretation of low SUV values in small lesions and explains why quantitative accuracy decreases with lesion size.