The impact of acquisition time on quantitative bone single photon emission computed tomography using the SwiftScan technique

Objectives: Recently, the SwiftScan technique was released by GE Healthcare and has been used in clinical settings. In single photon emission computed tomography (SPECT), the SwiftScan mode enhances sensitivity, allowing data acquisitions during heads rotation. In previous studies, performance of the SwiftScan has been evaluated; however, the effect of the acquisition time for quantitative bone SPECT using SwiftScan has not been fully understood yet. Thus, this study aimed to clarify the effect of the acquisition time on the quantitative value of bone SPECT using the SwiftScan technique.Methods: Phantom SPECT image acquisition was performed on a Discovery NM/CT 860 (GE Healthcare) with a low-energy high-resolution and sensitivity collimator. Main and sub energy window acquisitions were 140 keV ± 10% and 120 keV ± 5%, respectively. The phantom was a SIM2 Bone Phantom (Kyoto Kagaku). The diameters of the spheres installed in the phantom were 13, 17, 22, 28, and 35 mm. The background was assumed normal area while the sphere was assumed vertebra tumor area. The radioactivity concentration of the background and hot sphere were 50 kBq/mL and 300 MBq/mL, respectively. Each SPECT acquisition time was 6, 10, 30, and 60 s per projection. All acquired images were reconstructed using a three-dimensional iterative ordered subsets expectation maximization algorithm with attenuation correction, scatter correction, and resolution recovery (10 iterations, 4 subsets, no post-filter, matrix size of 128 × 128, and pixel size of 4.42 × 4.42 mm). The relative error between the theoretical and measured standardized uptake value (SUV) was calculated, and the sums of the relative errors of all the spheres at each acquisition time were compared.

Results: In the 35-mm reference part, relative errors between theoretical and measured SUVs were 50.6%, 28.9%, 11.6%, and 6.6% in 6, 10, 30, and 60 s per projection of the acquisition time, respectively. Similarly, in the 28-mm sphere, the relative errors were 35.2%, 21.2%, 3.3%, and 9.6%; 22-mm sphere, 46.8%, 24.0%, 9.5%, and 0.1%; 17-mm sphere, 30.2%, 14.3%, 19.2%, and 23.6%; and 13-mm sphere, 6.5%, 10.5%, 37.4%, and 35.7%. The sum of the relative errors in each sphere size were 169.4%, 98.9%, 81.1%, and 75.7% in 6, 10, 30, and 60 s per projection of the acquisition time, respectively. As the acquisition time was shorter, the relative error of SUV was larger.

Conclusions: This study investigated the impact of acquisition time on quantitative bone SPECT using the SwiftScan technique. Our results suggest that the SwiftScan cannot obtain an accurate SUV with a short acquisition time. Therefore, it is recommended to use longer acquisition times of at least 30 s per projection for quantitative bone SPECT to stabilize the SUV. The use of an optimal acquisition time would improve bone metastases diagnostic performance.