Brain SPECT with short focal-length cone-beam collimation.

Single-photon emission-computed tomography (SPECT) imaging of deep brain structures is compromised by loss of photons due to attenuation. We have previously shown that a centrally peaked collimator sensitivity function can compensate for this phenomenon, increasing sensitivity over most of the brain. For dual-head instruments, parallel-hole collimators cannot provide variable sensitivity without simultaneously degrading spatial resolution near the center of the brain; this suggests the use of converging collimators. We have designed collimator pairs for dual-head SPECT systems to increase sensitivity, particularly in the center of the brain, and compared the new collimation approach to existing approaches on the basis of performance in estimating activity concentration of small structures at various locations in the brain. The collimator pairs we evaluated included a cone-beam collimator, for increased sensitivity, and a fan-beam collimator, for data sufficiency. We calculated projections of an ellipsoidal uniform background, with 0.9-cm-radius spherical lesions at several locations in the background. From these, we determined ideal signal-to-noise ratios (SNRCRB) for estimation of activity concentration within the spheres, based on the Cramer-Rao lower bound on variance. We also reconstructed, by an ordered-subset expectation-maximization (OS-EM) procedure, images of this phantom, as well as of the Zubal brain phantom, to allow visual assessment and to ensure that they were free of artifacts. The best of the collimator pairs evaluated comprised a cone-beam collimator with 20 cm focal length, for which the focal point is inside the brain, and a fan-beam collimator with 40 cm focal length. This pair yielded increased SNRCRB, compared to the parallel-parallel pair, throughout the imaging volume. The factor by which SNRCRB increased ranged from 1.1 at the most axially extreme location to 3.5 at the center. The gains in SNRCRB were relatively robust to mismatches between the center of the brain and the center of the imaging volume. Artifact-free reconstructions of simulated data acquired using this pair were obtained. Combining fan-beam and short-focusing cone-beam collimation should greatly improve dual-head brain SPECT imaging, especially for centrally located structures.