RT Journal Article SR Electronic T1 Evaluation of Geometric Sensitivity for Hybrid PET JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 1116 OP 1120 VO 42 IS 7 A1 Robert Z. Stodilka A1 Stephen J. Glick YR 2001 UL http://jnm.snmjournals.org/content/42/7/1116.abstract AB 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. Methods: 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 αn, which describes the angular separation between head 1 and head n. Simulated configurations were 2 head ([α2] = [180°]), 3 head ([α2, α3] = [120°, 240°] and [90°, 180°]), and 4 head ([α2, α3, α4] = [90°, 180°, 270°]). Four transverse acceptance angles, measured from the perpendicular of the crystal to the surface, were simulated: 90°, 45°, 23°, and 11°. Two RoRs were considered: 250 and 300 mm. Each head was rotated through 360° in 128 steps, and no physical collimation was modeled. Results: For a 250-mm RoR and 90° acceptance angle, the sensitivities relative to [α2] = [180°] were [α2, α3] = [120°, 240°], 183%; [α2, α3] = [90°, 180°], 159%; and [α2, α3, α4] = [90°, 180°, 270°], 317%. Increasing RoR to 300 mm decreased [α2] = [180°] sensitivity by approximately 12%; all other configurations were decreased by approximately 75% of their 250-mm RoR sensitivities. Decreasing the acceptance angle to 45° decreased sensitivities to [α2, α3] = [120°, 240°], 100%; [α2, α3] = [90°, 180°], 105%; and [α2, α3, α4] = [90°, 180°, 270°], 210%. The 2-head [α2] = [180°] 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° acceptance angle, [α2] = [180°] concentrated sensitivity in the FoV center, [α2, α3] = [120°, 240°] had a slightly increased peripheral sensitivity, and the profiles for both [α2, α3] = [90°, 180°] and [α2, α3, α4] = [90°, 180°, 270°] were completely flat. Conclusion: 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.