Digital Solid-State SPECT/CT Quantitation of Absolute 177Lu Radiotracer Concentration: In Vivo and In Vitro Validation.

The accuracy of 177Lu radiotracer concentration measurements using quantitative clinical software was determined by comparing in vivo results for a digital solid-state cadmium-zinc-telluride SPECT/CT system with in vitro sampling. Methods: First, image acquisition parameters were assessed for an International Electrotechnical Commission body phantom emulating clinical count rates loaded with a lung insert and 6 hot spheres with a 12:1 target-to-background ratio of 177Lu solution. Then, the data of 28 whole-body SPECT/CT scans of 7 patients who underwent 177Lu prostate-specific membrane antigen radioligand therapy were retrospectively analyzed. Three users analyzed SPECT/CT images for in vivo urinary bladder radiotracer uptake using quantitative software. In vitro radiopharmaceutical concentrations were calculated using urine sampling obtained immediately after each scan, scaled to SUVs. Any in vivo or in vitro identity relations were determined by linear regression (ideally, slope = 1 and intercept = 0), within a 95% confidence interval. Results: Phantom results demonstrated lower quantitative error for acquisitions using the 113-keV 177Lu energy peak rather than including the 208-keV peak, given that only low-energy collimation was available in this camera configuration. In the clinical study, 24 in vivo-in vitro pairs were eligible for further analysis, with 4 having been rejected as outliers (via Cook distance calculations). All linear regressions (R2 ≥ 0.82, P < 0.0001) provided identity in vivo-in vitro relations (95% confidence interval), with SUV averages from all users giving a slope of 0.96 ± 0.13, an intercept of -0.07 ± 0.46 g/mL, and an average residual difference of 19.5%. In acquisitions with the lower-energy 177Lu energy peak, solid-state SPECT/CT imaging provided an accuracy to within approximately 20% of in vivo urinary bladder radiotracer concentrations. Conclusion: This noninvasive in vivo quantitation method can potentially improve diagnosis, patient management, and treatment response assessment and provide data essential to 177Lu dosimetry.

Absolute radiotracer concentration measurement using whole-body solid-state SPECT/CT technology: in vivo/in vitro validation.

The accuracy of recently approved quantitative clinical software was determined by comparing in vivo/in vitro measurements for a solid-state cadmium-zinc-telluride SPECT/CT (single photon emission computed tomography/x-ray computed tomography) camera. Bone SPECT/CT, including the pelvic region in the field of view, was performed on 16 patients using technetium-99m methylene diphosphonic acid as a radiotracer. After imaging, urine samples from each patient provided for the measurement of in vitro radiopharmaceutical concentrations. From the SPECT/CT images, three users measured in vivo radiotracer concentration and standardized uptake value (SUV) for the bladder using quantitative software (Q.Metrix, GE Healthcare). Linear regression was used to validate any in vivo/in vitro identity relations (ideally slope = 1, intercept = 0), within a 95% confidence interval (CI). Thirteen in vivo/in vitro pairs were available for further analysis, after rejecting two as clinically irrelevant (SUVs > 100 g/mL) and one as an outlier (via Cook's distance calculations). All linear regressions (R2 ≥ 0.85, P < 0.0001) provided identity in vivo/in vitro relations (95% CI), with SUV averages from all users giving a slope of 0.99 ± 0.25 and intercept of 0.14 ± 5.15 g/mL. The average in vivo/in vitro residual difference was < 20%. Solid-state SPECT/CT imaging can reliably provide in vivo urinary bladder radiotracer concentrations within approximately 20% accuracy. This practical, non-invasive, in vivo quantitation method can potentially improve diagnosis and assessment of response to treatment. Graphical abstract.

Evaluation of Staff Radiation Exposure during Transthoracic Echocardiography Close to Myocardial Perfusion Imaging.

BACKGROUND: Transthoracic echocardiography (TTE) and myocardial perfusion imaging (MPI) are used in cardiac patients. In this study the radiation exposure of sonographers performing TTE following MPI was evaluated. METHODS: Of 40 study patients, 30 underwent same-day 99mTc sestamibi MPI and TTE, while another 10 underwent only TTE. Patients who underwent both studies were divided into three groups: right-handed TTE performed by an echocardiographer and right- and left-handed TTE performed by a cardiac sonographer. Seven thermoluminescent radiation dosimeter badges monitored the forehead, wrists, anterolateral right and left chest, sternal notch, and umbilical region of each examiner. Group characteristics were compared. Radiation exposures were deemed positive if >0.1 mSv. RESULTS: There were no statistical differences in patient weight and body mass index. The left-handed approach group had higher residual radioactivity (979 ± 73 vs 884 ± 73 MBq [P < .01] and 906 ± 81 MBq [P < .04]), but no statistical difference in duration of TTE, compared with the other two MPI groups. Radiation exposure was positive in the right anterolateral chest and hand (0.45 and 1 mSv, respectively) for the echocardiographer, the right anterolateral chest and wrist and umbilical region (0.59, 1.06, and 0.15 mSv, respectively) for the right-handed sonographer, and the left chest and hand (0.12 and 0.34 mSv, respectively) for the left-handed sonographer. Dosimeters indicated no radiation exposure in the TTE-only group. CONCLUSIONS: Staff members performing TTE after MPI are exposed to radiation that might warrant monitoring. Altering study sequence, adopting a left-handed approach, and using other radiation-reducing techniques can minimize the degree of exposure.