I. Introduction II. Energy Spectrum Performance III. Imaging Performance IV. Nuclear Medicine Applications V. Conclusion The application of room-temperature semiconductor detectors to nuclear medicine imaging is presented here. Cadmium telluride (CdTe) and cadmium-zinc-telluride (CdZnTe, or CZT) are compound semiconductors that have been researched extensively in the fields of medical imaging, astronomy, and general gamma-ray spectroscopy. Much is understood about the growth, surface and contact treat- ment, and charge transport phenomena in this family of crystals. Improvements in the control of crystal-growing processes have produced steadily increasing spectroscopic crystal yields over the years. Advances in microelectronics, packaging, data-acquisition electronics, and computer power have combined to allow CdTe and CZT materials to be assembled into multiple-pixel modules. These modules can themselves be configured into convenient, flexible geome- tries for medical imaging and other applications. The appeal of this family of detector materials for nuclear medicine is based on several factors. Most important is the elimination of scintillation photons and the need for photo- multiplier tubes (PMTs). The PMTs in the conventional gamma camera design occupy considerable volume compared with semiconductors, and this volume must be shielded by lead. The complete semiconductor detector assembly there- fore has considerable advantages in weight and maneuverabil- ity. Also, the compound semiconductor has extremely good energy resolution with modest or even no cooling. This allows for scatter rejection and multiple isotope imaging in nuclear medicine applications. Another attractive feature of the room- temperature semiconductors is the possibility of dialing the intrinsic spatial resolution due to the small cloud of electron– hole pairs created by the interaction of the gamma-ray or X-ray photon. This small size provides the detector designer the option of determining the location of the photon interaction to within a 0.5 mm × 0.5 mm × 0.5 mm cubic volume. This option comes with a price: the harnessing of thousands, even tens of thousands, of individual channels of electronics. Solutions have been developed and are making their way to imaging applications. In turn, the combinations of one or more of these features: size, energy resolution, intrinsic spatial resolution, and direct pixel readout, will give the designer the necessary tools to expand the current realm of nuclear medicine. This chapter reviews the history of these detectors and the research behind the important aspects of their performance. Charge transport and signal generation schemes have been studied by many groups, and the current work describes those efforts that most directly impact medical imaging performance. Finally, the recent efforts of medical imaging equipment manufacturers to create prototypes from these materials are presented, along with comments 269 Copyright 2004 Elsevier Science (USA). All rights reserved. EMISSION TOMOGRAPHY C H A P T E R CdTe and CdZnTe Semiconductor Detectors for Nuclear Medicine Imaging DOUGLAS J. WAGENAAR Siemens Medical Solutions, Nuclear Medicine Group, Hoffman Estates, Illinois 15

Emission Tomography Volume 950 || CdTe and CdZnTe Semiconductor Detectors for Nuclear Medicine Imaging

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Page 1: Emission Tomography Volume 950 || CdTe and CdZnTe Semiconductor Detectors for Nuclear Medicine Imaging

I. IntroductionII. Energy Spectrum Performance

III. Imaging PerformanceIV. Nuclear Medicine ApplicationsV. Conclusion

The application of room-temperature semiconductordetectors to nuclear medicine imaging is presented here.Cadmium telluride (CdTe) and cadmium-zinc-telluride(CdZnTe, or CZT) are compound semiconductors that havebeen researched extensively in the fields of medical imag-ing, astronomy, and general gamma-ray spectroscopy. Muchis understood about the growth, surface and contact treat-ment, and charge transport phenomena in this family ofcrystals. Improvements in the control of crystal-growingprocesses have produced steadily increasing spectroscopiccrystal yields over the years. Advances in microelectronics,packaging, data-acquisition electronics, and computer powerhave combined to allow CdTe and CZT materials to beassembled into multiple-pixel modules. These modules canthemselves be configured into convenient, flexible geome-tries for medical imaging and other applications.

The appeal of this family of detector materials for nuclearmedicine is based on several factors. Most important is theelimination of scintillation photons and the need for photo-multiplier tubes (PMTs). The PMTs in the conventionalgamma camera design occupy considerable volume compared

with semiconductors, and this volume must be shielded bylead. The complete semiconductor detector assembly there-fore has considerable advantages in weight and maneuverabil-ity. Also, the compound semiconductor has extremely goodenergy resolution with modest or even no cooling. This allowsfor scatter rejection and multiple isotope imaging in nuclearmedicine applications. Another attractive feature of the room-temperature semiconductors is the possibility of dialing theintrinsic spatial resolution due to the small cloud of electron–hole pairs created by the interaction of the gamma-ray orX-ray photon. This small size provides the detector designerthe option of determining the location of the photon interac-tion to within a 0.5 mm × 0.5 mm × 0.5 mm cubic volume.This option comes with a price: the harnessing of thousands,even tens of thousands, of individual channels of electronics.Solutions have been developed and are making their wayto imaging applications. In turn, the combinations of oneor more of these features: size, energy resolution, intrinsicspatial resolution, and direct pixel readout, will give thedesigner the necessary tools to expand the current realm ofnuclear medicine.

This chapter reviews the history of these detectors andthe research behind the important aspects of their per-formance. Charge transport and signal generation schemeshave been studied by many groups, and the current workdescribes those efforts that most directly impact medicalimaging performance. Finally, the recent efforts of medicalimaging equipment manufacturers to create prototypesfrom these materials are presented, along with comments

C H A P T E R

CdTe and CdZnTe Semiconductor Detectors for NuclearMedicine Imaging

DOUGLAS J. WAGENAARSiemens Medical Solutions, Nuclear Medicine Group, Hoffman Estates, Illinois

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