Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

of the diode and the negative polarity on the n-type side, the holes in the p-typematerial and the mobile conduction-band electrons of the n-type material aredrawn to the junction. There, the mobile electrons fall into the valence band to fillholes. Applying an external voltage in this manner is referred to asforward bias. Forwardbias permits a current to flow with little resistance.On the other hand, if an external voltage is applied with the opposite polarity-thatis, the with the negative polarity on the p-type side of the diode and thepositive polarity on the n-type side-the holes in the p-type material and themobile conduction-band electrons of the n-type material are drawn away from thejunction. Applying the external voltage in this polarity is referred to as reverse bias.Reverse bias draws the charge carriers away from the n-p junction, forming a regiondepleted of current carriers. Very little electrical current flows when a diode isreverse-biased.A reverse-biased semiconductor diode can be used to detect visible and ultravioletlight or ionizing radiation. The photons of light or ionization and excitationproduced by ionizing radiation can excite lower-energy electrons in the depletionregion of the diode to higher-energy bands, producing hole-electron pairs. The electricalfield in the depletion region sweeps the holes toward the p-rype side and theconduction band electrons toward the n-type side, causing a momentary pulse ofcurrent to flow after the interaction.Photodiodes are semiconductor diodes that convert light into an electrical current.As mentioned previously, they are used in conjunction with scintillators asdetectors in CT scanners. Scintillation-based thin-film transistor radiographicimage receptors, discussed in Chapter 11, incorporate a photodiode in each pixel.Semiconductor detectors are semiconductor diodes designed for the detection ofionizing radiation. The amount of charge generated by an interaction is proportionalto the energy deposited in the detector by the interaction; therefore, semiconductordetectors are spectrometers. Because thermal energy can also raise electronsto the conduction band, many types of semiconductor detectors used for x-rayand gamma-ray spectroscopy must be cooled with liquid nitrogen.Semiconductor detectors are seldom used for medical imaging devices becauseof high expense, low quantum detection efficiency in comparison to scintillatorssuch as NaI(Tl) (Z of iodine = 53, Z of germanium = 32, Z of silicon = 14), becausethey can be manufactured only in limited sizes, and because many such devicesrequire cooling.Efforts are being made to develop semiconductor detectors of higher atomicnumber than germanium that can be operated at room temperature. The leadingcandidate to date is cadmium zinc telluride (CZT). A small-field-of-view nuclearmedicine camera using CZT detectors has been developed.The energy resolution of germanium semiconductor detectors is greatly superiorto that ofNaI(Tl) scintillation detectors. Liquid nitrogen-eooled germanium detectorsare widely used for the identification of individual gamma ray-emitting radionuclidesin mixed radionuclide samples because of their superb energy resolution.Many radiation detectors, such as scintillation detectors, semiconductor detectors,and proportional counters, produce electrical pulses whose amplitudes are propor-