Quantitative Measurement of Dual-Radioisotopes of Technetium-99m and Iodine-123 in Blood Samples With a Cadmium-Telluride-Based Counting Device

Our previous study demonstrated that a cadmium telluride (CdTe)-based single-photon emission computed tomography (SPECT) system with good energy resolution could provide dual-radioisotope SPECT images of a patient who had been administered with two kinds of radioisotope simultaneously. To obtain two quantitative physiological parametric images from these radioisotope images by using compartment model analysis, the concentrations of the dual-radioisotope in the blood sample need to be measured. In this paper, a CdTe-based well counter was developed to measure the concentrations of a dual-radioisotope of
^99m
Tc and
¹²³
I, and its performance was evaluated. The counter consists of four detector panels. Each detector panel consists of two detector modules, and each module has 16 × 16 CdTe detector pixels with a pitch of 2.5 mm; the eight detector modules give our system a field of view (x × y × z) of 4 × 4 × 8 cm
³
. The crosstalk correction coefficient α, which is the ratio of the count rate in the
^99m
Tc-energy window to that in the
¹²³
I-energy window, was obtained by measuring the single-radioisotope solution of
¹²³
I. When the dual-radioisotope of
^99m
Tc and
¹²³
I was measured, the true count rate of
^99m
Tc was obtained by subtracting the crosstalk count rate (α × the count rate of
¹²³
I) from the uncorrected count rate of
^99m
Tc. To evaluate the measurement accuracy, dual- and single-radioisotope solutions were measured, and the mean percentage error of the measured count rate of dual-radioisotope solutions to that of the single-radioisotope solution was obtained. The energy resolutions full width at half maximum of the CdTe-based well counter were 6.95% and 6.93% at 140.5 keV and 159 keV, respectively, and were higher than that of a conventional NaI (Tl) scintillation detector (10% at 140.5 keV). The count loss in the CdTe detector was 1.7% at 44 kcps. For the dual-radioisotope measurement, the percentage errors (mean percentage% ± standard deviation%, N = 3) of the count rate of
^99m
Tc and
¹²³
I at a high count rate (
^99m
Tc: 44 kcps,
¹²³
I: 28 kcps) were -1.9% ± 0.0% and -3.5% ± 1.1%, respectively. On the other hand, the percentage errors of the count rate of
^99m
Tc and
¹²³
I at a low count rate (
^99m
Tc: 3 kcps,
¹²³
I: 4 kcps) were -0.4% ± 0.3% and 0.2% ± 0.6%, respectively. The CdTe-based well counter measured each concentration of
^99m
Tc and
¹²³
I in mixed solutions with high accuracy.