Introduction of a novel ultrahigh sensitivity collimator for brain SPECT imaging.

PURPOSE: Noise levels of brain SPECT images are highest in central regions, due to preferential attenuation of photons emitted from deep structures. To address this problem, the authors have designed a novel collimator for brain SPECT imaging that yields greatly increased sensitivity near the center of the brain without loss of resolution. This hybrid collimator consisted of ultrashort cone-beam holes in the central regions and slant-holes in the periphery (USCB). We evaluated this collimator for quantitative brain imaging tasks. METHODS: Owing to the uniqueness of the USCB collimation, the hole pattern required substantial variations in collimator parameters. To utilize the lead-casting technique, the authors designed two supporting plates to position about 37 000 hexagonal, slightly tapered pins. The holes in the supporting plates were modeled to yield the desired focal length, hole length, and septal thickness. To determine the properties of the manufactured collimator and to compute the system matrix, the authors prepared an array of point sources that covered the entire detector area. Each point source contained 32 µCi of Tc-99m at the first scan time. The array was imaged for 5 min at each of the 64 shifted locations to yield a 2-mm sampling distance, and hole parameters were calculated. The sensitivity was also measured using a point source placed along the central ray at several distances from the collimator face. High-count projection data from a five-compartment brain phantom were acquired with the three collimators on a dual-head SPECT/CT system. The authors calculated Cramer-Rao bounds on the precision of estimates of striatal and background activity concentration. In order to assess the new collimation system to detect changes in striatal activity, the authors evaluated the precision of measuring a 5% decrease in right putamen activity. The authors also reconstructed images of projection data obtained by summing data from the individual phantom compartments. RESULTS: The sensitivity of the novel cone-beam collimator varied with distance from the detector face; it was higher than that of the fan-beam collimator by factors ranging from 2.7 to 162. Examination of the projections of the point sources revealed that only a few holes were distorted or partially blocked, indicating that the intensive manual fabrication process was very successful. Better reconstructed phantom images were obtained from the USCB+FAN collimator pair than from either LEHR or FAN collimation. For the left caudate, located near the center of the brain, the detected counts were 9.8 (8.3) times higher for UCSB compared with LEHR (FAN), averaged over 60 views. The task-specific SNR for detecting a 5% decrease in putamen uptake was 7.4 for USCB and 3.2 for LEHR. CONCLUSIONS: The authors have designed and manufactured a novel collimator for brain SPECT imaging. The sensitivity is much higher than that of a fan-beam collimator. Because of differences between the manufactured collimator and its design, reconstruction of the data requires a measured system matrix. The authors have demonstrated the potential of USCB collimation for improved precision in estimating striatal uptake. The novel collimator may be useful for early detection of Parkinson's disease, and for monitoring therapy response and disease progression.

Development of a cost-effective modular SPECT/CT scanner.

PURPOSE: To develop a combined SPECT/CT based on a conventional gamma camera with the following design goals: (i) the subject is scanned on the same bed without moving between SPECT and CT scans, (ii) maintenance of full diagnostic utility of both scanners, (iii) a minimal equipment footprint which is adaptable to different site configurations, (iv) a modular design so that either scanner can be upgraded without impacting the other scanner and (v) cost-effectiveness. METHODS: A Philips SKYLight gamma camera was combined with a single-slice helical CT. The CT bed has been modified by mounting on wheels and is attached to a track in the floor when required. The CT bed is used for both the CT and the SPECT scans. The CT bed is parked outside the SKYLight's active area when not required. The CT data are used for anatomical localisation, attenuation correction and definition of anatomical boundaries (e.g. lobar boundaries in the lungs). RESULTS: The combined SPECT/CT system is installed in a 6.2 mx4.6 m room. The CT can scan up to 1.3 m of the subject, which is usually sufficient for registration with the SPECT scan. More than 200 subjects have had SPECT/CT scans in the first year of operation. Initially, the main outcome has been improved confidence of localisation of abnormalities. Attenuation correction for all relevant radionuclides has been implemented. CONCLUSION: We have developed a combined SPECT/CT using a novel approach based on a low-cost CT scanner. The design is compact, is readily upgradeable and does not compromise either machine's diagnostic or functional capabilities.