This study was performed with the approval of the institutional ethics committee for clinical research of Osaka University. Written informed consent was obtained from each subject, after they were provided a detailed explanation about the procedures of the study.
A total of 5 healthy volunteers (three males and two females) participated in the present study after they received a detailed explanation about the radiotracer drug, the purpose and contents of the study. The mean age of the 5 healthy volunteers was 34 years (range, 21 to 56), and the mean height and weight were 167 cm (range, 160 to 174 cm) and 61 kg (range, 48 to 66 kg), and the mean body mass index were 21.7 ± 1.8 (range, 18.8 to 24.5), respectively. None of the healthy volunteers had any prior history of any major medical illness.
Preparation of radiotracers
FBPA was prepared by a previously described method, with several modifications [7, 9, 19]. The F-1 synthesizer (Sumitomo Heavy Industries, Tokyo, Japan) was used. 18F-acetylhypofluorite in Ne was bubbled at a flow rate of 600 mL/min at room temperature into 5 mL of trifluoroacetic acid containing 30 mg of 4-borono-L-phenylalanine (Matrix SCIENTIFIC, COLUBIA, SC, USA). Trifluoroacetic acid was removed by passing N2 under reduced pressure at a flow rate of 200 mL/min. As in previous studies, the residue was also dissolved in 3 mL of water containing 0.1 % acetic acid, and the solution was applied to a high-performance liquid chromatography column, YMC-Pack ODS-A S-5 (20 × 150 mm; YMC, Kyoto, Japan), with water for injection containing 0.1 % acetic acid as the mobile phase, flow rate of 10 mL/min, ultraviolet detection at 280 nm, and a radioactivity detector. The FBPA fraction (retention time = 19 to 21 min) was collected by adding 25 % ascorbic acid injection and 10 % sodium chloride injection. The specific activity of FBPA was 49.7 ± 17.3 GBq/mmol as determined by HPLC. The radiochemical purity of FBPA was >98 %.
PET image acquisition
All the healthy volunteers were instructed to fast for at least 4 h before the radiotracer injection [20, 21]. Before injecting each radiotracer, the volunteers were asked to void their bladders. Whole-body PET scans were performed using Eminence SOPHIA SET-3000 BCT/X (Shimadzu Co, Kyoto, Japan) in the three-dimensional acquisition mode. Transmission data using a rotating 137Cs point source for attenuation correction were acquired. The amount of injected activity was measured in a dose calibrator (196 ± 16 MBq). Whole-body emission scans were initiated simultaneously with injection of the radiotracer into the antecubital vein at the rate of 5.2 mL/min. Seven repeated whole-body PET scans from the parietal crown to the groin were performed in each of the five healthy volunteers. The data consisted of seven scans with an acquisition time of 455 sec, and an interval between scans of 45 sec. Radioactivity decay during the PET scan was corrected for the FBPA injection time. All the images were reconstructed using Dynamic Row-Action Maximum Likelihood Algorithm (DRAMA) with an image matrix of 128 × 128, resulting in a voxel size of 4.0 × 4.0 × 3.25 mm3 . The axial field of view was 26 cm. After the PET scanning, whole-body CT (80 kV, 135 mAs) was performed for image fusion.
Time-activity curves (TACs) of blood sampling radioactivity
Before and during the PET study, nine arterialized venous blood samplings were obtained: at background, 30 sec, and 1, 3, 5, 10, 20, 30 and 50 min after the tracer injection from the antecubital vein contralateral to the intravenous FBPA injection side, heated in a heating blanket. The radioactivity (cps/g) of the whole blood was measured in a cross-calibrated well-type scintillation counter (Shimadzu Co, Kyoto, Japan), and fixed in attenuation correction to the FBPA injection time.
TACs of PET image-derived blood radioactivity
The radioactivity in various blood pools was obtained from reconstructed PET images by averaging the activities in each blood pool, since the radioactivity distribution within a blood pool can be considered to be uniform . TACs of the image-derived blood radioactivity were obtained using volumes of interest (VOIs) over the ascending aorta, aortic arch, pulmonary artery, left and right ventricles, inferior vena cava and abdominal aorta. Spherical VOIs with a diameter of 10 mm were set on these blood pools of the PET images referring to the individual CT images. VOIs were drawn on 3 cross-sections of the tomographic images. Data were obtained for each of the organs using PMOD software, ver.3.6. TACs were created using the average values of five VOIs consisting of regions of interest drawn over each blood-pool area.
FBPA metabolite analysis
The proportion of 18F radioactivity in the plasma present as FBPA was measured in 5 healthy subjects. Blood samples were collected at 20 min and 50 min after the FBPA injection from the antecubital vein contralateral to the intravenous FBPA injection side. Radiochemical purity was analyzed by HPLC .
We verified whether the image-derived blood radioactivity was correlated with the blood sampling radioactivity in the healthy subjects. TACs of the blood sampling radioactivity were compared with those of the image-derived blood radioactivity. The TAC of the image-derived radioactivity within each blood pool was calculated as the average of five radioactivities (cps/g), respectively. The blood sampling radioactivity at 3, 20, 30 and 50 min after the tracer injection and the image-derived blood radioactivity at each VOI (ascending aorta, aortic arch, pulmonary artery, left and right ventricles, inferior vena cava, and abdominal aorta) in mid-scan time at 3.8, 20.5, 28.8 and 53 min after the tracer injection were compared.
The magnitude of underestimation of blood radioactivity counts was examined. The timing of the minimal reduction of image-derived blood FBPA radioactivity was examined from TAC, and reduction of blood radioactivity of FBPA in each blood pool was calculated and compared, respectively.
The relationships between the blood sampling radioactivity levels and the radioactivity level within each blood pools on the PET images were analyzed by with linear regression and Spearman’s correlation tests. The correlation coefficient was also estimated as a statistic to express the comparability of these values.
The proportion of blood sampling radioactivity and image-derived radioactivity and that of reduction rate was described as the mean ± SD. Mean values were compared using a one-way analysis of variance. In all the statistical analyses, significance was defined as a P value of less than 0.05. All the statistical analyses were performed with StatMate IV (ATMS Co., Ltd., Tokyo, Japan).