Evaluation of geometric sensitivity for hybrid PET

Hybrid PET systems have spatially varying sensitivity profiles. These profi les are dependent on imaging parameters, namely, number of-heads, head conf iguration, spacing between gantry stops, radius of rotation (RoR), and coin cident head acceptance angle. Methods: Sensitivity profiles were calculated across a 500-mm field of view (FoV) for a representative set of existing a nd theoretic 2-, 3-, and 4-head hybrid PET systems. The head configuration was defined by alpha (n) which describes the angular separation between hea d 1 and head n. Simulated configurations were 2 head ([alpha (2)] = [180 de grees]), 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, mea sured from the perpendicular of the crystal to the surface, were simulated: 90 degrees, 45 degrees, 23 degrees, and 11 degrees. Two RoRs were consider ed: 250 and 300 mm. Each head was rotated through 360 degrees in 128 steps, and no physical collimation was modeled. Results: For a 250-mm RoR and 90 degrees acceptance angle, the sensitivities relative to [alpha (2)]= [180 d egrees] were [alpha (2), alpha (3)] = [120 degrees, 240 degrees], 183%; [al pha (2), alpha (3)] = [90 degrees, 180 degrees], 159%; and [alpha (2), alph a (3), alpha (4)] = [90 degrees, 180 degrees, 270 degrees], 317%. Increasin g RoR to 300 mm decreased [alpha (2) = [180 degrees] sensitivity by approxi mately 12%; all other configurations were decreased by approximately 75% of their 250-mm RoR sensitivities. Decreasing the acceptance angle to 45 degr ees decreased sensitivities to [alpha (2), alpha (3)] = [120 degrees, 240 d egrees], 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 a ffected. The configuration was the most important factor affecting the shap e of the sensitivity profiles. For a 250-mm RoR and 90 degrees acceptance a ngle, [alpha (2) = [180 degrees] concentrated sensitivity in the FoV center , [alpha (2), alpha (3)] = [120 degrees, 240 degrees] had a slightly increa sed peripheral sensitivity, and the profiles for both [alpha (2), alpha (3) ] = [90 degrees, 180 degrees] and [alpha (2), alpha (3), alpha (4)] = [90 d egrees, 180 degrees, 270 degrees] were completely fiat. Conclusion: Sensiti vity profiles are affected strongly by imaging parameters; however, profile s can be shaped to concentrate on an annulus or distribute sensitivity unif ormly over the FoV. Also, the 4-head system showed a markedly higher sensit ivity than:either of the 3-head systems.