IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 48, NO. 5, OCTOBER 1999 909 Timing Characteristics of a Cd Zn Te Detector-Based X-ray Imaging System George C. Giakos, Senior Member, IEEE, S. Vedantham, S. Chowdhury, J. Odogba, A. Dasgupta, R. Guntupalli, S. Suryanarayanan, V. Vega-Lozada, M. Sridhar, M. Khyati, and N. Shah Abstract—The timing characteristics of a planar Cd Zn Te sample at each frequency of a scanning square-wave test pattern, has been measured. This study is aimed at evaluating the speed characteristics of a Cd Zn Te detector for X-ray imaging and computed tomographic (CT) applications. The experimental results of this study indicate that the temporal response of a Cd Zn Te detector based X-ray system, improves significantly by optimizing the X-ray tube and detector parameters. I. INTRODUCTION S EVERAL detectors have been proposed for digital radi- ography [1]–[38], [44]–[50], [59]–[70] although there is no single technology of choice that addresses all the issues for optimal imaging. The technology of choice depends on several image quality criteria such as high quantum and energy absorption efficiency, high detector quantum efficiency (DQE), high spatial resolution, negligible scatter acceptance, detector geometry, fast readout, high dynamic range, image correction, display capabilities, and acceptable cost. Flat-panel image sensor arrays are being developed for medical imaging applications [1], [2], [5], [6]. These systems consist of large area pixel arrays which use matrix addressing to read out charges resulting from X-ray absorption in the detector medium. There are two methods for making flat panel image sensors. In one method, the indirect method [1], [2], a phosphor converter absorbs the incident X-rays and emits visible light which is converted by an a–Si : H p-I-n photodiode to an electronic image. This process is inefficient and can lead to increased image noise, particularly when signals are low. The other approach, the direct method [1], [5], [8], [9], [16], [44]–[50], [69], [70], uses a photoconductive layer to absorb X-rays and collect the ionization charge which is subsequently read-out by an active matrix array. The direct method has a higher intrinsic resolution compared to the indirect method because it avoids the X-ray to light conversion stage. The primary advantages of the photoconductors for good quality imaging are: efficient radiation absorption, large band gap energy limiting the thermal generation of charge carriers in the bulk, good linearity, good charge transport properties, high stability, high sensitivity and wide dynamic range [44]–[60]. Lead iodide (PbI ), cadmium zinc telluride (Cd Zn Te), and amorphous selenium, (a–Se) are good candidates [74]. Significant progress has been achieved in the growth of high Manuscript received July 27, 1999. The authors are with the Department of Biomedical Engineering, University of Akron, Akron OH 44325 USA (e-mail: gcgiakos@uakron.edu). Publisher Item Identifier S 0018-9456(99)08376-X. quality Cd Zn Te semiconductor crystals using the high pressure Bridgman (HPB). Specifically, by alloying CdTe with Zn the bulk resistivity of this new semiconductor becomes approximately 10 -cm. Several detector parameters such as space charge, electronic and ionic diffusion, mobility dispersion, trapping and recom- bination, space charge, detector tolerances, and electric field nonuniformities, can deteriorate the timing characteristics of a system. A careful, general analysis of all these parameters and their impact on the overall system response is extremely difficult and extends beyond the scope of this study. In this study, the timing modulation transfer function (MTF) of X-ray detector system has been measured. Specifically, a scanning square-wave test pattern [72] was utilized to modulate the X-ray beam passing through a slit-aperture. As a result, spatial information was converted into timing information via the test pattern scanning speed. Therefore, the timing performance of the detector at each frequency of a scanning square-wave test pattern has been measured for different detector parameters. II. DETECTOR CHARACTERISTICS Solid state detector arrays are potentially attractive for digital radiography because of their direct conversion of X- rays to electrical signals [1], [5], [6], [45]–[70]. Cd Zn Te [45]–[70], [74] has a high stopping power due to its high mass density (5.8 g/cm ) and effective atomic number, , of 49.6 (Cd : 48, Zn : 30, Te: 52). As a result, the proposed Cd Zn Te based imaging technology has a high energy absorption efficiency which allows the fabrication of thin detector substrates. Therefore, detector performance parameters such as spatial, temporal, and contrast resolution will be improved significantly, giving rise to an enhanced specificity, sensitivity, and sample rates. High-quality resistive Cd Zn Te semiconductor detectors utilizing high resistivity-high grade material are fabricated using the high pressure Bridgman (HPB) technique [61]–[63]. Specifically, by alloying CdTe with Zn the bulk resistivity of this new semiconductor is approximately 10 -cm. This high resistivity is due to the wide band gap of this ternary semiconductor (wider than 1.5 eV for CdTe and increases with Zn percentage) which results in low leakage currents, and consequently, low noise characteristics. Resistive detectors exhibit linear dc-voltage current characteristics. The contacts are quasiohmic in the sense that their dynamic resistance is lower than the bulk resistance of the CdZnTe substrate. 0018–9456/99$10.00  1999 IEEE