Takuzo Takayama

The CdTe detector module and its imaging performance

In recent years investigations into the application of semiconductor detector technology in gamma cameras have become active world-wide. The reason for this burst of activity is the expectation that the semiconductor-based gamma camera would outperform the conventional Anger-type gamma camera with a large scintillator and photomultipliers. Nevertheless, to date, it cannot be said that this expectation has been met.Methods: While most of the studies have used CZT (Cadmium Zinc Telluride) as the semiconductor material, we designed and fabricated an experimental detector module of CdTe (Cadmium Telluride). The module consists of 512 elements and its pixel pitch is 1.6 mm. We have evaluated its energy resolution, planar image performance, single photon emission computed tomography (SPECT) image performance and time resolution for coincidence detection.Results: The average energy resolution was 5.5% FWHM at 140 keV. The intrinsic spatial resolution was 1.6 mm. The quality of the phantom images, both planar and SPECT, was visually superior to that of the Anger-type gamma camera. The quantitative assessment of SPECT images showed accuracy far better than that of the Anger-type camera. The coincidence time resolution was 8.6 ns. All measurements were done at room temperature, and the polarization effect that had been the biggest concern for CdTe was not significant.Conclusion: The results indicated that the semiconductor-based gamma camera is superior in performance to the Anger-type and has the possibility of being used as a positron emission computed tomography (PET) scanner.

Application of transmission scan-based attenuation compensation to scatter-corrected thallium-201 myocardial single-photon emission tomographic images

Abstract. A practical method for scatter and attenuation compensation was employed in thallium-201 myocardial single-photon emission tomography (SPET or ECT) with the triple-energy-window (TEW) technique and an iterative attenuation correction method by using a measured attenuation map. The map was reconstructed from technetium-99m transmission CT (TCT) data. A dual-headed SPET gamma camera system equipped with parallel-hole collimators was used for ECT/TCT data acquisition and a new type of external source named ”sheet line source” was designed for TCT data acquisition. This sheet line source was composed of a narrow long fluoroplastic tube embedded in a rectangular acrylic board. After injection of 99mTc solution into the tube by an automatic injector, the board was attached in front of the collimator surface of one of the two detectors. After acquiring emission and transmission data separately or simultaneously, we eliminated scattered photons in the transmission and emission data with the TEW method, and reconstructed both images. Then, the effect of attenuation in the scatter-corrected ECT images was compensated with Chang’s iterative method by using measured attenuation maps. Our method was validated by several phantom studies and clinical cardiac studies. The method offered improved homogeneity in distribution of myocardial activity and accurate measurements of myocardial tracer uptake. We conclude that the above correction method is feasible because a new type of 99mTc external source may not produce truncation in TCT images and is cost-effective and easy to prepare in clinical situations.

Improvement of detection efficiency in coincidence data acquisition with CdTe detector

Describes a new method to improve the sensitivity of a CdTe detector for coincidence imaging. The problem in the use of the semiconductor detector is low sensitivity for high-energy photons such as 511 keV photons. To increase the number of events we record the time, location and energy of an event whose energy ranges from 166 keV to 350 keV. The event list is then checked to see if the succeeding two events occur at almost the same time and if the sum of the energy for these two events is nearly equal to 511 keV and if the locations of these two events are in neighboring detector elements. If the above conditions are satisfied we consider these two events as a single event. Because most of the photons that are scattered once in the semiconductor detector loss their energy in the detector elements next to the detector element in which an incident photon is scattered, we can treat these scattered photons just like primary photons. The results of Monte Carlo simulations showed that the proposed method increased the sensitivity by a factor of 1.48 compared with the detection of 511 keV photons with a single energy window.

The compensation method for the effects of system resolution to the left ventricular volume measurements using QGS program

We evaluated the accuracy of left ventricular volume calculations performed using the left ventricular function automatic analysis software (QGS program), which automatically extracts the left ventricular profile from ECG-gated myocardial SPECT data and then measures the left ventricular volume. We used a myocardial phantom for this evaluation. The results of our experiments showed that the left ventricular volume was underestimated by up to 13% as the SPECT spatial resolution deteriorated. To overcome this problem, we have developed a method for correcting the left ventricular volume measured by the QGS program according to the SPECT spatial resolution of the detectors. This new correction method permits the left ventricular volume of the phantom to be calculated with an error of less than 1.4%.

Determination of energy window width and position for scintigraphic imaging using different energy resolution detection with the triple energy window (TEW) scatter compensation method

Since primary photons can provide information concerning the position of radioisotope (RI) accumulation and the energy of the photons, it would seem reasonable to vary the position and width of the energy window depending on the type of RI and the energy resolution of the detector to collect as many of the primary photons as possible. The authors propose a method for determining energy window width and position for scintigraphic imaging to collect as many of the primary photons as possible, and studied the influence on the Triple Energy Window (TEW) scatter compensation method of setting such energy window levels for Tc-99m (single photopeak) and Tl-201 (multiple photopeaks) using detector with different energy resolution through simulation. The Monte Carlo simulations were verified by comparing the regional energy spectrum at the phantom obtained from the simulation against experimental measurements. The energy window with the authors proposed method for Tc-99m is 20% and 47.3% for Tl-201 using gamma camera, and 9.8% for Tc-99m using a semiconductor detector with a theorized energy resolution of 7.0 keV.