SPECT acquisition

A SPECT system consists of one or more scintillation cameras mounted to a gantry that can revolve about a fixed horizontal axis (the axis of rotation) " (Fig. 27.13). SPECT studies are usually acquired over a full 360° arc, although myocardial perfusion studies typically use only data from the 180° arc that minimizes tissue attenuation. SPECT acquisitions are performed with the scintillation camera located at preselected angular locations (step-and-shoot mode), or in a continuous rotation mode. In the step-and-shoot mode, the detector rotates to each angular position and collects data in a preselected frame duration while the detector is motionless. In the continuous-rotation mode, the smdy duration is selected and the rotation speed is adjusted to complete the orbit during this time. Projections are collected as the detector rotates and are binned into 60 to 120 frames over 360°. 

FIGURE 27.13 SPECT acquisition. One or more scintilia-tion cameras collect images at typicaliy 60 to 120 angles around a 360° orbit. The scintillation camera acquires projection images from a large volume simultaneously. 

The heart is continually moving during the SPECT acquisition, and this further compromises spatial resolution. Because the heart beats many times per minute, it is impossible to directly acquire... 

A disadvantage of the coincidence scintillation cameras is that they have low sensitivity due to low detection efficiency of Nal(Tl) crystal for 511-keV photons, which results in a longer acquisition time. To improve the sensitivity, thicker detectors of sizes 1.6-2.5 cm have been used in some cameras, but even then, coincidence photopeak efficiency is only 3-4%. This increase in crystal thickness, however, compromises the spatial resolution of the system in SPECT mode. Fast electronics and pulse shaping are implemented in modern systems to improve the sensitivity. Also, there is a significant camera dead time and pulse pileups due to relatively increased single count rates in the absence of a collimator in PET mode. Low coincidence count rates due to low... 

Note that the sensitivity of a PET scanner increases as the square of the detector efficiency, which depends on the scintillation decay time and stopping power of the detector. This is why LSO, LYSO and GSO detectors are preferred to Nal(Tl) or BGO detectors (see Table 2.1). In 2D acquisitions, system sensitivity is compromised because of the use of septa between detector rings, whereas these septa are retracted or absent in 3D acquisition, and hence the sensitivity is increased by a factor of 4-8. However, in 3D mode, random and scatter coincidences increase significantly, the scatter fraction being 30—40% compared to 15-20% in 2D mode. The overall sensitivities of PET scanners for a small-volume source of activity are about 0.2-0.5% for 2D acquisition and about 2-10% for 3D acquisition, compared to 0.01-0.03% for SPECT studies (Cherry et al, 2003). The greater sensitivity of the PET scanner results from the absence of collimators in data acquisition. 

As mentioned previously, the use of SPECT scanning has, in some smdies, improved resolution when compared with conventional planar scanning but the increased dose and complexity involved makes it debatable whether these techniques can be justified routinely in children. This may change with improved equipment. A further technical problem with SPECT is that the relatively long acquisition time causes problems with radionuclides such as c-sodium pertechnetate that are rapidly cleared from the lungs and subsequently appear in the pulmonary circulation. 

Even if state-of-the-art imaging conditions are met and acquisition as well as reconstruction protocols are optimized, there is a high level of heterogeneity in MAA uptake even in normal liver tissue. MAA SPECT cannot be considered a reliable... 

SPECT studies are acquired over a 10-minute interval. When one acquisition sample is completed, the next begins automatically. In order to minimize time, SPECT systems that perform these studies can alternately reverse the acquisition direction, although at least one SPECT system utilizes slipring technology so that the detectors can rotate continuously in the same direction. 

If low kVp (80 k Vp) and mAs (100 m As) are used for PCT acquisition (Wintermark et al. 2000), the overall effective dose required for PCT (2.0-3.0 mSv) is only shghtly higher than that required for routine head CT (1.5-2.5 mSv). This dose equivalent is less than the dose equivalent obtained with PET or SPECT, and is comparable to that of a single-level xenon CT examination (Eastwood et aL 2003). 

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