Two SPECT-CT scanners used; Discovery NM/CT670 Q.suite Pro (GE Healthcare, Tokyo, Japan) with low-energy high resolution (LEHR) or LEHRS collimators, and Symbia Intevo 16 (Siemens, Tokyo, Japan) with a LEHR collimator. The collimator design is shown in Table 1. The flat plastic dish phantom (diameter 15cm), line source and cylindrical phantom in accordance with Japanese industrial standards (JIS) Z4922 phantom (KYOTO KAGAKU, Co., Ltd, Kyoto, Japan) were used to measure the basic performance of SwiftScan planar and SPECT. Each acquisition parameter of system spatial resolution, system planar sensitivity, SPECT reconstructed spatial resolution, system volume sensitivity and image uniformity with SPECT image were determined with reference to National Electrical Manufacturers Association (NEMA) NU-1 2018 or Japanese Engineering Standards of Radiological Apparatus (JESRA) X-0051*C-2017 guidelines [10, 11]. All images were analyzed using Prominence Processor ver. 3.1 (Prominence conference, Japan) or Daemon research image processor (FUJIFILM Toyama Chemical Co., Ltd., Tokyo, Japan).
System spatial resolution
A line source (inner diameter: 1.5 mm, and length: 200 mm) filled with 784 MBq/mL of 99mTc solution was created and was set at 10 cm from the detector to measure the system spatial resolution. The photo-peak window of GE and Siemens was set to 140.5 keV ± 7.5% and 140 keV ± 7.5%. The anterior view was acquired using 512×512 matrices for both GE LEHR and LEHRS, and Siemens LEHR, with a pixel size of 1.1 and 1.2 mm for GE and Siemens, respectively, and average counts of line source image were 10 k counts/pixel. The count profile drawn on the center of the line source image was used to analyze the full width at half maximum (FWHM). Five planar images with a blend ratio of 0%, 20%, 40%, 60% and 80% were created with GE LEHRS collimator using Clarity 2D processing.
System planar sensitivity
System planar sensitivity was measured by scanning the flat plastic dish phantom, 15 cm in diameter, filled with 187 MBq of 99mTc activity positioned at the center of each detector.
A planar image of each detector was acquired using 256×256 matrices for both GE and Siemens collimators, with a pixel size of 2.2 and 2.4 mm for GE and Siemens, respectively, and total acquisition counts was 4000 k counts. System planar sensitivity was calculated by dividing the total counts in the detector by the measured activity (1).
where STOT and Ct are decay-corrected count rate and summed counts on the region of interest (ROI) over the entire image, respectively, Ts, Tcal, Tacq and Thalf are start time of the acquisition, time of the activity calibration, duration of the acquisition and half-life of radionuclide, respectively, and Acal is the amount of radioactivity measured in the phantom at time Tcal after correcting for residual activity in the syringe.
SPECT reconstructed spatial resolution without scatter
The same phantom used in the measurement of system spatial resolution was located the center of rotation to measure the SPECT reconstructed spatial resolution without scatter. The photo-peak windows with GE and Siemens cameras were set to 140.5 keV ± 10% and 140 keV ± 7.5%. SPECT acquisition was performed using 256 ×256 matrices for both GE and Siemens cameras, with a pixel size of 2.2 and 2.4 mm for GE and Siemens, respectively. The step-and-shoot or SwiftScan modes of a 360° circular orbit (radius of rotation, 15 cm) and 60 projections of step angle 4°, and average pixel counts of line source image of approximately 110 k counts/pixel. Projection data were reconstructed by filtered back projection (FBP) and ordered subset expectation maximization (OSEM) incorporating Flash 3D [12, 13] or Evolution for Bone [14, 15] algorithms, and pre- or post-filters were not used. The number of iterations varied from 1 to 20 (subset number fixed 12) for Flash 3D and from 1 to 25 (subset number fixed 10) for Evolution for Bone, respectively. Neither scatter correction (SC) nor attenuation correction (AC) were performed. The count profiles of x- and y-axes were drawn on a transverse image of five slices across the center of line source were also used to analyze the average FWHM on x- and y- axes.
System volume sensitivity
A cylindrical phantom (inner diameter, 20 cm; and length, 20 cm) filled with 99mTc solution of 40 kBq/mL was used to evaluate system volume sensitivity. The photo-peak window of GE and Siemens cameras was set to 140.5 keV ± 10% and 140 keV ± 10%, respectively. Other acquisition parameters were the same as the reconstruction conditions of the SPECT for spatial resolution without scatter. System volume sensitivity was calculated (2):
where A is the average counts/sec for the SPECT acquisition divided by the total counts imaged by the total elapsed time, and B is the source activity concentration (MBq/cm3) at halfway of total time through the 360° SPECT acquisition. A proper source decay correction factor for the radionuclide was applied.
Image uniformity with SPECT image
Image uniformity with SPECT image was analyzed by the same cylindrical phantom for the system volume sensitivity. Projection data were reconstructed by FBP and OSEM incorporating Flash 3D or Evolution for Bone as well as the parameter of the SPECT reconstructed spatial resolution without scatter. Differential uniformity was calculated using the maximum and minimum values in 75% circular areas drawn on five contiguous transverse images (3).