Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

nal. For other applications, photographic film is used to record the light the scintillatorsemit. Many years ago, in physics research and medical fluoroscopy, the lightfrom scintillators was viewed directly with dark-adapted eyes.Semiconductor detectors are especially pure crystals of silicon, germanium, orother semiconductor materials to which trace amounts of impurity atoms have beenadded so that they act as diodes. A diode is an electronic device with two terminalsthat permits a large electrical current to flow when a voltage is applied in one direction,but very little current when the voltage is applied in the opposite direction.When used to detect radiation, a voltage is applied in the direction in which littlecurrent flows. When an interaction occurs in the crystal, electrons are raised to anexcited state, allowing a momentary electrical current to flow through the device.Detectors may also be classified by the type of information produced. Detectors,such as Geiger-Mueller (GM) detectors, that indicate the number of interactionsoccurring in the detector are called counters. Detectors that yield informationabout the energy distribution of the incident radiation, such as NaI scintillationdetectors, are called spectrometers. Detectors that indicate the net amount of energydeposited in the detector by multiple interactions are called dosimeters.Many radiation detectors produce an electrical signal after each interaction of a particleor photon. The signal generated by the detector passes through a series of electroniccircuits, each of which performs a function such as signal amplification, signalprocessing, or data storage. A detector and its associated electronic circuitryform a detector system. There are two fundamental ways that the circuitry mayprocess the signal-pulse mode and current mode. In pulse mode, the signal fromeach interaction is processed individually. In current mode, the electrical signals fromindividual interactions are averaged together, forming a net current signal.There are advantages and disadvantages to each method for handling the signal.GM detectors are operated in pulse mode, whereas most ionization chambers,including the dose calibrators used in nuclear medicine and those used in somecomputed tomography (CT) scanners, are operated in current mode. Scintillationdetectors are operated in pulse mode in nuclear medicine applications but in currentmode in digital radiography, fluoroscopy, and x-ray CT.The main problem with using a radiation detector or detector system in pulse modeis that two interactions must be separated by a finite amount of time if they are toproduce distinct signals. This interval is called the dead time of the system. If a secondinteraction occurs during this time interval, its signal will be lost; furthermore,if it is close enough in time to the first interaction, it may even distort the signalfrom the first interaction. The fraction of counts lost from dead-time effects tendsto be small at low interaction rates and increases with interaction rate.The dead time of a detector system is largely determined by the component inthe series with the longest dead time. For example, the detector usually has thelongest dead time in GM counter systems, whereas in multichannel analyzer systems(see later discussion) the analog-to-digital converter often has the longest deadtime.