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Differential Effect of 180° and 360° Acquisition Orbits on the Accuracy of SPECT Imaging: Quantitative Evaluation in Phantoms

Section of Cardiovascular Medicine, Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut

View larger version (35K): [in a new window]   FIGURE 1. Reconstructed short-axis slices of healthy volunteer, including images acquired using 180° circular acquisition orbit (A) and images acquired with 360° acquisition orbit (B). Images reconstructed from 180° acquisition data show inhomogeneity (arrows) mimicking small anterior and inferior defects (variability, 13%). Images reconstructed from 360° acquisition data show more homogeneous and normal radiotracer distribution (variability, 8%).

View larger version (45K): [in a new window]   FIGURE 2. (A) Schematic of cardiac phantom with hollow inner and outer cylinders. Fillable defect inserts are 5, 10, and 20 mL, which respectively represent 4%, 8%, and 16% of simulated left ventricular myocardium (space between 2 cylinders). (B) Cardiac phantom secured in wooden frame. Numbers are distances from center of acquisition orbit, in centimeters.

View larger version (25K): [in a new window]   FIGURE 3. Schematics of 4 phantom positions relative to center of orbit, including 180° acquisition setup with phantom in straight orientation (A), 360° acquisition setup with phantom in straight orientation (B), 180° acquisition setup with phantom in angled orientation (C), and 360° acquisition setup with phantom in angled orientation (D).

View larger version (38K): [in a new window]   FIGURE 4. SPECT short-axis slices and corresponding circumferential count profiles for normal phantom in center position and straight orientation using 180° and 360° acquisition orbits. Visually, all images appear homogeneous. Quantitative analysis by circumferential count distribution of apical, midventricular, and basal slices show mild inhomogeneity, possibly caused by slightly imprecise positioning of phantom. Patterns of circumferential count profiles are similar for 180° and 360° acquisition orbits.

View larger version (40K): [in a new window]   FIGURE 5. SPECT short-axis slices and corresponding circumferential count profiles for normal phantom in 15-cm off-center position and straight orientation using 180° and 360° acquisition orbits. Visually, images acquired with 180° orbit are inhomogeneous (top). Mild defects are present (arrows). These segments of phantom wall were closest to rotating detectors. Images acquired with 360° orbit are visually homogeneous (bottom). Quantitative analysis by circumferential count profiles of apical, midventricular, and basal slices shows marked inhomogeneity of images obtained with 180° orbit. Arrows indicate troughs in profiles that correspond to visual defects in images. Circumferential count profiles of images acquired with 360° acquisition display only mild inhomogeneity similar to that in Figure 4.

View larger version (25K): [in a new window]   FIGURE 6. Mean percentage of variability in circumferential profiles of reconstructed slices of normal phantoms in straight and angled orientations acquired with 180° and 360° orbits. Using 180° acquisition orbit, variability increased significantly (P < 0.0001, ANOVA) with increasingly off-center position in both straight and angled orientations. In contrast, variability did not significantly differ using 360° acquisition orbit (P = not statistically significant [ns], ANOVA). Variability in straight versus angled phantom orientations was significantly different only in center and 5-cm off-center positions (P < 0.01, unpaired t test).

View larger version (39K): [in a new window]   FIGURE 7. Correlations between true defect size and quantified SPECT defect size for SPECT acquisition using 180° and 360° orbits, in center position (A) and in 5- (B), 10- (C), and 15-cm (D) off-center positions. Each phantom position was analyzed with phantom in straight and angled orientations. Data label () denotes multiple data points with same (overlapping) SPECT defect value for 180° and 360° acquisitions. Dotted-and-dashed line represents line of identity. With 180° acquisition orbit, quantified SPECT defect size was increasingly overestimated when phantom was placed more eccentrically. With 360° acquisition orbit, quantified SPECT defect size remained in same range of underestimation regardless of whether phantom was positioned in center or off center.