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INTRODUCTION

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Nuclear cardiology instrumentation, including SPECT, PET, and their transmission imaging options, has evolved at an unprecedented pace in recent years. Contemporary SPECT systems are notably different in physical size and detector configuration offering new patient positioning options and flexible facility placement. Solid-state detector technologies and new aperture designs with advantages described decades ago are available in these new systems. Hybrid SPECT and PET systems now have transmission imaging options for attenuation correction (AC) using x-ray CT, radionuclide, or novel fluorescence x-ray sources. The greater majority of PET systems manufactured today have integrated x-ray CT imaging capabilities although "dedicated" cardiac PET systems remain available. This chapter presents an overview of current SPECT, PET, and related CT instrumentation options for nuclear cardiology, including new detector design, and quality control (QC) considerations; traditional QC assessment is addressed in detail in the next chapter.

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SPECT INSTRUMENTATION

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Anger Camera

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Modern cardiac SPECT imaging relies extensively on the Anger camera concept using NaI(Tl) crystal detectors and circuitry as originally developed in the 1950s.1,2 This approach uses lead collimators placed over the large crystal surface that "focus" incident photons by excluding those not traveling nearly perpendicular to the detector surface. The range of photon acceptance is determined primarily by the collimator hole design parameters. Immediately behind the lead collimator is the NaI(Tl) crystal where photons are absorbed producing a scintillation event.2 A burst of secondary photons is then released with energy in the optical frequency range and an array of photomultiplier (PM) tubes, positioned behind the crystal, detects and amplifies the scintillation light. An electronic signal containing partial information on the location and energy of the scintillation is produced from each PM. A mathematical weighting of the pulses from all PM tubes detecting the scintillation produces X, Y (spatial location), and Z (energy) values (Fig. 3-1). The energy value is tested by a discrimination circuit that will exclude the signal if it is not within the permitted energy range. The net efficiency of this process is measured by the energy resolution defined by the full-width at half-maximum (FWHM) of the measured energy distribution relative to the Tc-99m photopeak (140 keV)2 (Fig. 3-1). As energy resolution degrades, image contrast degrades proportionally as improving energy resolution provides improving contrast resolution.3 System sensitivity is another key imaging performance measure expressed in units of counts/min/μCi (cpm/μCi) of Tc-99m in a standard measurement protocol.2,4 It is highly dependent on the collimator properties.4 Higher system sensitivity implies higher image signal-to-noise and more efficient acquisition but is also associated with decreased spatial resolution (FWHM of a point activity source), which also depends on the collimation parameters. System sensitivity has unique implications for multidetector systems. If system sensitivity differs significantly between detectors, image artifacts in the reconstructed images may result. Planar and SPECT performance measurements are well standardized for conventional systems ...

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