PubMed · 8046483
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S C Port. 1994. The first-pass ventricular function study: why now?. https://pubmed.ncbi.nlm.nih.gov/8046483/
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Clinical cardiac imaging is hindered by noise due to limited activity and imaging time. Use of 90 degrees dual-detector systems with 90 degrees gantry rotation may provide the best sensitivity/resolution for cardiac imaging, but this option is not readily available to those using a triple-detector system with detectors at 120 degrees intervals. This study utilizes a cardiac/chest phantom to compare several triple-detector orbits, with assessment of sensitivity and resolution. A 180 degrees rotation with reconstruction of two of the three heads was evaluated, resulting in overlapping 180 degrees orbits; use of a starting angle of 165 degrees for the first head placed the overlapping portion of the orbits over the LAO myocardial region, where camera-cardiac distance is most favorable. Use of this overlapping orbit yielded resolution equivalent to a conventional (single-head) 180 degrees rotation. Sensitivity was 87%-90% of that of a 90 degrees dual-detector system, and 16%-20% better than the common practice of using a 120 degrees orbit with reconstruction of 1 1/2 heads to achieve 180 degrees of data. Use of 360 degrees acquisition with reconstruction of all three heads provided the greatest sensitivity, though at the expense of some loss in image quality. Thus, for those centers performing cardiac imaging using a triple-detector system, use of overlapping 180 degrees orbits is the preferred acquisition choice.
Limitations to head movement of dual-head gamma cameras may prevent proper geometry being attained for testing of intrinsic uniformity with a point source. When a point source is placed too near to the uncollimated crystal, the detected flood image will be non-uniform due to the inverse square law effect, oblique incidence, and increased path length traversed through the crystal by oblique rays. A formula has been deduced for constructing a map to correct for these effects. The general formula has been applied to a particular dual-head camera in which a point source has been placed on the face of one head, at 65.5 cm from the opposite uncollimated crystal. It is shown that division of the non-uniform image, acquired with this geometry, by the correction map yields a uniform image which may be used for routine investigation of the uniformity characteristics of the camera.
UNLABELLED: Thallium-201 and 99mTc-sestamibi images of the heart contain a significant amount of scattered events which degrade image quality. Newer generation gamma cameras exhibit enhanced energy resolution and hardware/software to perform scatter correction. The principal aim of this study was to evaluate the effects of these advances in instrumentation on the quantitation of defect size from tomographic images of the heart obtained from a cardiac phantom. METHODS: Tomographic images of a cardiac phantom containing no defect and defects of 5%-70% of total myocardial mass were acquired both with and without scatter correction for 201Tl and 99mTc studies. Data were acquired on a newer generation gamma camera with an energy resolution of 8.7% at 140 keV. From conventional short-axis slices of the heart, circumferential count profiles were generated from five representative slices. Defect size was computed from the fraction of radians that fell below a fixed threshold value in each of the five count profiles. The nadir value (min/max) of the count profiles in each study was used as an index of image contrast. RESULTS: For both 201Tl and 99mTc, threshold values between 55%-60% gave the best correlation (r > 0.99), with the lowest average absolute error in estimating defect size (< 2.1%). Scatter correction reduced the average absolute error to 0.8% for 99mTc and 1.4% for 201Tl, significantly reduced the nadir values for both isotopes (p < 0.0001 for both 201Tl and 99mTc and led to a marked improvement in image quality for both tracers. CONCLUSION: Scatter correction reduces the error associated with measurement of infarct size, increases image contrast and improves image quality for both 201Tl and 99mTc, as assessed in a phantom model.