New software methods to cope with reduced counting

Authors: E Gordon DePuey

Publish Date: 2009/04/28

Volume: 16, Issue: 3, Pages: 335-338

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Abstract

Since its inception radionuclide myocardial perfusion imaging has been criticized for lengthy acquisition times With singledetector systems a 2025 minute acquisition was required to obtain adequate counting statistics With the introduction of dualheaded 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 1012 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 enhancedThe Nuclear Cardiology community has responded to this dilemma in two ways First new highspeed cameras with novel detector characteristics have been developed The DSPECT® camera introduced by Spectrum Dynamics performs a preliminary scan of the chest using a scanning multidetector system allowing the detectors to be focused on the heart thereby improving count rate statistics and allowing a markedly reduced image acquisition time1 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 slithole 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 multipinhole focused collimators with or without 64slice 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 environmentAn alternate less costly approach has been the development of new software methods that allow lower countdensity cardiac SPECT acquisitions to be processed with resolution recovery and noisereduction 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 throughputIn the current issue of the Journal Maddahi et al reported the results of reducedtime myocardial perfusion SPECT using the new Digirad nSPEED® software in a prospective multicenter trial of 448 patients in whom fulltime gated SPECT acquisitions and simulated halftime acquisitions were compared qualitatively and quantitatively with regards to perfusion defect characteristics and functional parameters2 Their impressive results demonstrated that reducedtime acquisition image quality was at least as good as and often superior to that obtained with fulltime acquisitions The two techniques provided diagnostically equivalent images Quantitative perfusion defect severity was not significantly different and there was an extremely good correlation between enddiastolic volumes endsystolic volumes and ejection fractions measured by the two techniques The implication although not specifically stated by the authors is that this new halftime method can be reliably substituted for fulltime SPECT acquisitionPreviously BorgesNeto et al reported similar findings using a standard dualheaded sodium iodide detector system in a single center study of 50 patients employing wide beam reconstruction halftime software developed by UltraSPECT Ltd3 This processing algorithm incorporates resolution recovery and Poisson and Gaussian noise modeling without postprocessing filtering They reported highly significant correlations between perfusion variables using wide beam reconstruction as compared to fulltime filtered back projection processing Subsequently using the same halftime acquisition/processing algorithm DePuey et al demonstrated improved quality of halftime gated perfusion SPECT functional images compared to fulltime filtered back projection gated tomograms in 156 patients studied prospectively with separate fulltime and halftime acquisitions4 There was an excellent correlation between volumetric parameters using the halftime vs fulltime methods although ejection fractions were systematically lower with halftime processing probably due to a more accurate detection of end systolic contours afforded by wide beam reconstruction In a separate prospective trial of 209 patients these investigators recently reported excellent correlations between both perfusion and functional parameters using even shorter “quartertime” acquisitions 4 seconds per stop poststress and 6 seconds per stop postrest with a 90degree angled dualheaded camera processed with the wide beam reconstruction technique compared to fulltime acquisitions processed with ordered subset expectation maximization OSEM5 Although reconstruction artifacts were encountered occasionally in the low count density resting myocardial perfusion SPECT images in general wide beam reconstruction “quartertime” image quality was equivalent to or superior to that in fulltime OSEM studiesUsing a different reducedtime processing algorithm Evolution® software developed by General Electric Healthcare in the same prospective population of 156 patients described above DePuey et al demonstrated image quality similar to that with fulltime filtered back projection SPECT acquisitions and also a good correlation of functional parameters obtained with the two methods4 Reducedtime acquisition protocols developed by other vendors including Phillips and Siemens are under investigation with promising preliminary results67

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