PET/CT systems
Discovery MI
The Discovery MI is a combination of LBS, an SiPM-PET scanner and a 64-slice CT scanner. The LBS includes four blocks of detectors aligned in the axial direction, each comprising 19,584 crystals (3.95 × 5.3 × 25-mm) in a 4 × 9 matrix. The scanner has 36 detector units per ring and 9,792 SiPM channels. The DMI enables axial and transaxial FOV of 20 and 70 cm, respectively, with 71 image planes spaced at 2.79-mm intervals. The spatial resolution according to NEMA NU 2-2012 is 3.91 mm at full width at half maximum (FWHM) (3).
Discovery PET/CT 710
The Discovery PET/CT 710 is a combination of LBS with PMT-PET and 64-slice CT scanners. The PET scanner has 13,824 LBS crystals in a 4.2 × 6.3 × 25-mm3 block. The D710 enables a 150.42-mm axial FOV and a 700-mm transaxial FOV with 47 image planes spaced at 3.27-mm intervals. The timing resolution is 500 ps. The spatial resolution according to a NEMA NU 2-2007 was 4.52 mm at FWHM (8).
PET reconstruction condition
Data acquired using SiPM-PET and PMT-PET were reconstructed under the following conditions: three dimensional-ordered subset-expectation maximization (3D-OS-EM) with TOF; 4 iterations; 16 subsets; Gaussian filter, 2.5 mm (FWHM); 128 × 128 matrix size; FOV, 25.6 cm; 2.0 mm/pixel. The PET images of the Hoffman 3D brain phantom acquired by both scanners were reconstructed as described without TOF to evaluate TOF gain in background counts.
Phantom study
Data acquisition
Images from Hoffman 3D brain phantom (Data Spectrum Corporation, Hillsborough, NC, USA) and pool phantom (Itoi Plastics Co. Ltd., Kobe, Japan) containing 20 MBq of [18F]FDG were acquired for 30 min in list mode using the SiPM-PET and PMT-PET systems. Phantom conditions and the scan duration were determined according to the Japanese Society of Nuclear Medicine (JSNM) phantom test procedure (9). We extracted a time frame of 0 – 7 min from 30 min of data derived from the Hoffman and pool phantoms that was equivalent to the count statistics for [18F]FDG clinical brain images at the Tokyo Metropolitan Institute of Gerontology (TMIG) as described below.
Data processing
The physical indices for phantom tests proposed by the JSNM were used to evaluate the image quality: the ratio of grey-to-white matter contrast (contrast) calculated from images of Hoffman phantom, image noise (coefficient of variation, CV [%]) and uniformity (standard deviation, SD) calculated from images of pool phantom (9). The SD was also calculated from the pool phantom image with a scan duration of 30 min. The contrast, CV and SD were respectively calculated as described using images acquired from Hoffman and pool phantoms (9). Eight 10-mm circular volumes of interest (VOI) were placed on images of the acrylic plate at the bottom of the Hoffman phantom that were reconstructed using 3D-OS-EM with and without the TOF in the background (BG) (Fig. 1). The TOF gain (%) in the background counts was calculated as:
[Please see the supplementary files section to view the equation.] ,
where BGnon-TOF and BGTOF are mean background counts [kBq/mL] with and without TOF, respectively.
Clinical protocol
Data acquisition
The present study proceeded in accordance with the Declaration of Helsinki, and was approved by the Ethics Committee at the TMIG (Approval No. 28077). All applicants provided written informed consent to participate in the present study after physicians provided a detailed explanation of the study. The individuals rested comfortably in a quiet, dimly-lit room for several minutes, then were placed in the supine position for intravenous [18F]FDG injection and uptake. Low-dose CT images for attenuation and scatter correction were acquired before starting PET image acquisition. The first set of PMT-PET images were acquired for 10 min starting from 40 min after [18F]FDG administration, then the second set of SiPM-PET images was acquired, also for 10 min. We assessed 22 individuals using two PET/CT scanners between April 2017 and July 2018. Twenty two individuals were confirmed without degenerative neurological disorder on [18F]FDG and brain magnetic resonance (MR) images acquired using a Discovery MR750w 3.0T scanner (GE Healthcare) in the three dimensional mode (spoiled gradient recalled acquisition in the steady state: repetition time, 7.648 ms; echo time, 3.092 ms; matrix size, 196 × 256 × 256; voxel size, 1.2 × 1.0547 × 1.0547 mm3). Table 1 shows the characteristics of these individuals who were cognitively normal. The 4 of 22 individuals were normal volunteer and 18 of 22 individuals had visual problem (visual snow, 12; blepharospasm, 2; visual disturbance, 1; photophobia, 1; Charles Bonnet syndrome, 1; traffic injury, 1).
Table 1. Characteristics of individuals (n = 22)
Age (y; minimum - maximum)
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41.1 ± 18.9, (21-75)
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Male (n)
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11
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Height (cm)
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166.1 ± 7.5
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Weight (kg)
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58.5 ± 9.1
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Glucose (mg/dL)
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101.0 ± 13.7
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Injected dose (MBq)
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155.8 ± 14.7
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Uptake duration (min, PMT-PET/SiPM-PET)
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40.1 ± 0.6/55.3 ± 1.2
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Data are shown as means ± standard deviation. PET, positron emission tomography; PMT, photomultiplier tube; SiPM, Silicon photomultiplier.
Data processing
All [18F]FDG images were separately normalized to a standard [18F]FDG PET and MR template using a MIMneuro (MIM Software Inc. Cleveland, OH, USA). Anatomical VOI of MIMneuro were automatically placed on the caudate nucleus, cerebellum, frontal, occipital, parietal and temporal lobes, putamen, thalamus and whole brain. Mean standardized uptake values (SUVmean) were measured using these VOIs (10). Statistical image analysis was performed using Cortex ID Suite (GE Healthcare)(11-13). Cortex ID Suite includes original normal database, the VOI values of individuals and those of the normal database for were statistically compared. Anatomical VOI of Cortex ID Suite comprised the lateral and medial frontal, inferior and superior parietal, and lateral and medial temporal lobes, the anterior and posterior cingulate cortex, occipital lobe, sensorimotor, precuneus, primary visual cortex, and cerebellum. The SUV ratio (SUVR) was calculated using the value of the pons as a reference region. Z-scores for anatomical VOI-based analyses were calculated from anatomically normalized SUVR images using the formula,
[Please see the supplementary files section to view the equation.] ,
where SUVRindividual and SUVRnormal are the mean SUVR of the individuals and the normal database in the VOI, respectively and SDnormal is the SD of the SUVR of the normal database in the VOI.
Data analyses
Data were statistically analyzed using GraphPad Prism 8 Version 8.4.0 (GraphPad Software Inc., San Diego, CA, USA). The SUVmean of all regions for SiPM- and PMT- PET acquisitions were statistically compared using two-tailed paired Student t tests. Spearman rank correlation coefficients were calculated to evaluate relationships among different SUVmean in the whole brain and intervals between acquisitions. Z-scores were statistically analyzed for both acquisitions using Wilcoxon matched-pairs signed rank tests. Values with P < 0.05 were considered significant.