Evaluation of geometric sensitivity for hybrid PET

Hybrid PET systems have spatially varying sensitivity profiles. These profiles are dependent on imaging parameters, namely, number of heads, head configuration, spacing between gantry stops, radius of rotation (RoR), and coincident head acceptance angle.

Sensitivity profiles were calculated across a 500-mm field of view (FoV) for a representative set of existing and theoretic 2-, 3-, and 4-head hybrid PET systems. The head configuration was defined by alpha(n), which describes the angular separation between head 1 and head n. Simulated configurations were 2 head ([alpha(2)] = [180 degrees ]), 3 head ([alpha(2), alpha(3)] = [120 degrees, 240 degrees ] and [90 degrees, 180 degrees ]), and 4 head ([alpha(2), alpha(3), alpha(4)] = [90 degrees, 180 degrees, 270 degrees ]). Four transverse acceptance angles, measured from the perpendicular of the crystal to the surface, were simulated: 90 degrees, 45 degrees, 23 degrees, and 11 degrees. Two RoRs were considered: 250 and 300 mm. Each head was rotated through 360 degrees in 128 steps, and no physical collimation was modeled.

For a 250-mm RoR and 90 degrees acceptance angle, the sensitivities relative to [alpha(2)] = [180 degrees ] were [alpha(2), alpha(3)] = [120 degrees, 240 degrees ], 183%; [alpha(2), alpha(3)] = [90 degrees, 180 degrees ], 159%; and [alpha(2), alpha(3), alpha(4)] = [90 degrees, 180 degrees, 270 degrees ], 317%. Increasing RoR to 300 mm decreased [alpha(2)] = [180 degrees ] sensitivity by approximately 12%; all other configurations were decreased by approximately 75% of their 250-mm RoR sensitivities. Decreasing the acceptance angle to 45 degrees decreased sensitivities to [alpha(2), alpha(3)] = [120 degrees, 240 degrees ], 100%; [alpha(2), alpha(3)] = [90 degrees, 180 degrees ], 105%; and [alpha(2), alpha(3), alpha(4)] = [90 degrees, 180 degrees, 270 degrees ], 210%. The 2-head [alpha(2)] = [180 degrees ] system sensitivity was not affected. The configuration was the most important factor affecting the shape of the sensitivity profiles. For a 250-mm RoR and 90 degrees acceptance angle, [alpha(2)] = [180 degrees ] concentrated sensitivity in the FoV center, [alpha(2), alpha(3)] = [120 degrees, 240 degrees ] had a slightly increased peripheral sensitivity, and the profiles for both [alpha(2), alpha(3)] = [90 degrees, 180 degrees ] and [alpha(2), alpha(3), alpha(4)] = [90 degrees, 180 degrees, 270 degrees ] were completely flat.

Sensitivity profiles are affected strongly by imaging parameters; however, profiles can be shaped to concentrate on an annulus or distribute sensitivity uniformly over the FoV. Also, the 4-head system showed a markedly higher sensitivity than either of the 3-head systems.