Yuichiro Ueno

Pulsed Bias Voltage Shutdown to Suppress the Polarization Effect for a CdTe Radiation Detector

Pulsed bias shutdown operation, in which the bias voltage supply is periodically shut down for a certain time, is suitable for suppressing the polarization effect in a CdTe radiation detector. The duration of the bias voltage shutdown should be short to decrease the dead time and ensure the continuity of the measurement; however, the polarization effect may not be suppressed if the duration is too short. Therefore, we investigated bias voltage shutdown durations and intervals that stabilize the detector performance. A stacked type detector consisting of four 1-mm-thick CdTe diodes was used. At 35degC, the energy resolution for the 511 keV photopeak of the 22 Na source saturates at around 30 min after the measurement start and then does not change up to 300 min when the bias voltage is shut down for 10 ms every 5 min. Similarly at 60degC, the energy resolution for the 511 keV photopeak is unchanged for 300 min when the bias voltage is shut down for 10 ms every 10 s.

Monte Carlo-based scatter correction considering the tailing effect of a CdTe detector for dual-isotope brain SPECT imaging

A Monte Carlo (MC)-based scatter correction method considering the tailing effect of a CdTe detector was developed for dual-isotope brain single-photon emission computed tomography (SPECT) imaging using technetium-99m (99mTc) and iodine-123 (123I), and its accuracy was validated by measuring phantoms. The tailing effect was modeled by convolutions of energy spectra obtained by geometry and tracking (GEANT) simulation with energy smoothing kernels. In our experimental phantom studies, quantitative accuracy and image contrast in the reconstructed image of dual-isotope-filled phantoms with our MC-based scatter correction method (Dual_SC) were compared with those of single-isotope-filled phantoms (Single_SC). The quantitative accuracy was evaluated by the percent error between the estimated activity concentration and true activity concentration. In our six-compartment phantom study with six different activity concentrations, the mean absolute percent errors of 99mTc for Single_SC and Dual_SC were 1.7% and 2.8%, respectively, while those of 123I for Single_SC and Dual_SC were 4.8% and 5.6%, respectively. In our striatal phantom study, the percent errors in the background regions for Single_SC and Dual_SC were less than 2% for both 99mTc and 123I. The image contrast was evaluated by the percent contrast of a cold or hot spot region to a background region. In our cold rod phantom study, the mean percent contrasts in the cold rod regions of 99mTc for Single_SC and Dual_SC were 66.2% and 65.0%, respectively, while those of 123I for Single_SC and Dual_SC were 59.3% and 61.6%, respectively. In our striatal phantom study, the percent contrasts in the striatal regions of 99mTc for Single_SC and Dual_SC were 56.1% and 56.1%, respectively, while those of 123I for Single_SC and Dual_SC were 39.8% and 40.0%, respectively. In conclusion, dual-isotope imaging with the CdTe-based brain SPECT system and our MC-based scatter correction method can provide comparable quantitative accuracy and image contrast to those of single-isotope imaging.

Semiconductor Detector-Based Scanners for Nuclear Medicine

Semiconductor detectors have the potential to improve the quantitative accuracy of nuclear medicine imaging with their better energy and intrinsic spatial resolutions than those of conventional scintillator-based detectors. The fine energy resolution leads to a better image contrast due to better scatter rejection. The fine intrinsic spatial resolution due to a pixelated structure leads to a better image contrast and lower partial volume effect. Their pixelated structures also improve the count-rate capability. The authors developed CdTe semiconductor detector-based positron emission tomography (CdTe-PET) and single-photon emission computed tomography (CdTe-SPECT) in order to test the potential of using semiconductor detectors in nuclear medicine. The physical performances of both systems were measured in several phantom experiments. The capability of using CdTe-PET to measure the metabolic distribution of tumors was evaluated through scans of cancer patients and rat tumor models. The feasibility of using CdTe-SPECT for simultaneous dual-radionuclide imaging was evaluated through scans of phantoms and healthy volunteers. The results suggest that the prototype CdTe-PET can identify intratumoral metabolic heterogeneous distribution and that CdTe-SPECT can accurately acquire dual-radionuclide images simultaneously. Although there are still problems to be solved, semiconductor detectors will play significant roles in the future of nuclear medicine.

Collimator for Variable Sensitivity and Spatial Resolution Without the Need for Exchange

A new design of collimator is proposed that has variable sensitivity and spatial resolution, eliminating the need for exchanging collimators in a gamma camera. Using Monte Carlo simulations, the present article evaluates the shielding of undesirable gamma rays in a parallel-hole collimator. It consists of a number of layers of rectangular holes. These layers consist of alternately stacked fixed and movable collimators. In high-resolution mode, the movable collimators are shifted by half the aperture pitch along the diagonal direction. The first collimator (type A) has 50 layers with fixed thicknesses of 1.2 mm. The second collimator (type B) has 25 layers with a thickness of 1.0 mm on the object side and 25 layers with a thickness of 1.4 mm on the opposite side. The third collimator (type C) has 20 layers with non-uniform thicknesses. The ratios of the maximum artificial peak to the main-peak are calculated for point-source responses. The ratios for types A, B, and C collimators are 0.78, 0.08, and 0.03, respectively. The same performance for shielding undesirable gamma rays is achieved in the type C collimator as for a conventional collimator.