Revisiting gallium-67 imaging: investigation of the energy photopeaks.
ABSTRACT Ga-67 has multiple energy emissions of different abundances and the quality of images is affected by these inhomogeneous energy distributions. Thus the purpose of this study was to investigate whether two or three energy peaks of Ga-67 would be the most appropriate setting in terms of γ-camera performance and image quality measurements.
The physical performance of the γ-camera using Ga-67 has been tested; in particular, the effect of different gallium energy settings (two vs. three energy photopeaks) on the extrinsic and intrinsic uniformity and resolution of the γ-camera. The multicontrast/resolution phantom (OPRAXMEDICAL model 74-345) has been used to compare the two energy settings. The average contrast of the lesions and the contrast-to-noise ratio were measured and the significance of the result was determined using the paired Student's t-test. To test the effects of size and contrast on the detectability of defects, contrast detail curves have been generated for both the acquisition energy settings. For the optimization of the energy window, multiple static liver phantom images were acquired using the dual-energy photopeak acquisition method. However, each image was taken at different symmetric energy window combinations and the window settings were (5, 10, 15, and 20%) for 92 and 184 keV, respectively.
For the two energy window acquisitions, the intrinsic uniformity calculated by the integral and differential equations for the useful field of view was 2.26 and 1.39%, respectively. However, for the three energy windows, the values were 3.04 and 1.81%, respectively. The integral and differential values for the dual-energy flood images were 10.97 and 3.75%, respectively, whereas the integral and differential values for the triple-energy flood images were 13.3 and 4.13%, respectively. A slight improvement in the system's intrinsic spatial resolution was observed in the dual-window setting with a full width at half maximum of 9.24 mm compared with 10.42 mm for the triple-energy acquisition. The full width at half maximum was not drastically increased in the three window acquisitions, but the full width at tenth maximum was found to be larger. The two energy acquisitions yielded a significant improvement by 20.5 and 22% in the mean contrast of the planar images of the phantom filled with 2 and 5 mCi of Ga-67, respectively (P value <0.01). Single-photon emission computed tomography images acquired for the liver phantom showed an improvement in the mean contrast by 29 and 27.6% using two windows (P<0.001) for 2 and 5 mCi activity concentration, respectively. Unlike planar imaging, single-photon emission computed tomography images yielded a 45% increase in the contrast-to-noise ratio using the two window acquisitions with a mean CNR of 5.24±3.47 and 2.9±2.23 for two and three windows respectively; P value less than 0.05. The contrast detail curves showed better detectibality for small-sized lesions and lower contrast when using dual-energy settings. The best contrast, on average, was achieved using 10% for 92 keV and 5, 10, 15, and 20% for 184 keV, respectively, and it was improved by 16, 24, and 35% compared with other settings.
A significant improvement in image quality can be achieved by simply applying different window widths over the different photopeaks. Our results indicated that two photopeaks with optimized window widths can significantly outperform three windows in terms of spatial and contrast resolution and lesion detectability with a relatively negligible reduction of count sensitivity.