New software methods to cope with reduced counting statistics: shorter SPECT acquisitions and many more possibilities

Abstract

Since its inception, radionuclide myocardial perfusion imaging has been criticized for lengthy acquisition times. With single-detector systems a 20-25 minute acquisition was required to obtain adequate counting statistics. With the introduction of dual-headed cameras, acquisition times were cut in half. However, patient comfort and tolerance still remain suboptimal, and the number of patients scanned in a working day is limited to no more than 10-12 per camera. The American Society of Nuclear Cardiology and the Intersocietal Commission for the Accreditation of Nuclear Medicine Laboratories (ICANL) have rightfully insisted that acquisition times not be decreased below those set forth in the ASNC Guidelines in order to maintain adequate image counting statistics, thereby maintaining test sensitivity and minimizing artifacts associated with low count density studies. However, the advantages of shortening image acquisition times even further are considerable. Patient tolerance would be improved, the opportunity for patient motion would be decreased, and laboratory throughput would be enhanced. The Nuclear Cardiology community has responded to this dilemma in two ways. First, new high-speed cameras with novel detector characteristics have been developed. The D-SPECT camera introduced by Spectrum Dynamics performs a preliminary scan of the chest using a scanning multi-detector system, allowing the detectors to be focused on the heart, thereby improving count rate statistics and allowing a markedly reduced image acquisition time. An additional advantage of that system is that cadmium zinc telluride (CZT) semiconductor detectors are substituted for traditional sodium iodide detectors, thereby decreasing the size of the camera head and improving energy resolution. Another novel detector system, the CardiArc , employs focused, multiple slit-hole collimation to improve resolution and markedly reduce image acquisition time. Prototypes of the CardiArc system employed either traditional sodium iodide detectors or CZT semiconductors. General Electric has recently introduced the 530C camera incorporating CZT detector technology with multi-pinhole focused collimators with or without 64-slice CT imaging and attenuation correction. The advantages of theses new cameras include a smaller camera footprint, markedly reduced image acquisition times, and improved energy resolution with CZT, potentially minimizing SPECT degradation by scattered radioactivity. However, the cost of these new cameras (or any new camera for that matter) has become an increasingly important consideration in the present economic environment. An alternate, less costly approach has been the development of new software methods that allow lower count-density cardiac SPECT acquisitions to be processed with resolution recovery and noise-reduction techniques. The advantages of a smaller camera footprint and improved energy resolution described above for new detector systems are not realized with these novel software methods. However, economically they provide a reasonable alternative solution to the nagging problem of long acquisition times and decreased patient throughput. In the current issue of the Journal, Maddahi et al reported the results of reduced-time myocardial perfusion SPECT using the new Digirad nSPEED software in a prospective, multicenter trial of 448 patients in whom full-time gated SPECT acquisitions and simulated half-time acquisitions were compared qualitatively and quantitatively with regards to perfusion defect characteristics and functional parameters. Their impressive results demonstrated that reduced-time acquisition image quality was at least as good as, and often superior to, that obtained with full-time acquisitions. The two techniques provided diagnostically equivalent images. Quantitative perfusion defect severity was not From the St. Luke’s-Roosevelt Hospital and Columbia University College of Physicians and Surgeons, New York, NY. Reprint requests: E. Gordon DePuey, MD, St. Luke’s-Roosevelt Hospital, New York, NY; gdepuey@chpnet.org. J Nucl Cardiol 2009;16:335–8 1071-3581/$34.00 Copyright 2009 by the American Society of Nuclear Cardiology. doi:10.1007/s12350-009-9079-8