Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

PhotomultipliertubesLucite light pipeNal(TI) crystalCollimatortor is essential; a scintillation camera without a collimator does not generate meaningfulimages. Figure 21-2 shows a parallel-hole collimator.The lead walls, called septa, between the holes in the collimator absorb mostphotons approaching the collimator from directions that are not aligned with theholes. Most photons approaching the collimator from a nearly perpendicular directionpass through the holes; many of these are absorbed in the sodium iodide crystal,causing the emission of visible and ultraviolet light. The light photons are convertedinto electrical signals and amplified by the PMTs. These signals are furtheramplified by the preamplifiers (preamps). The amplitude of the pulse produced byeach PMT is proportional to the amount of light it received from an x- or gammarayinteraction in the crystal.Because the collimator septa intercept most photons approaching the camerafrom nonperpendicular directions, the pattern of photon interactions in the crystalforms a two-dimensional projection of the three-dimensional activity distributionin the patient. The PMTs closest to each photon interaction in the crystal receivemore light than those that are more distant, causing them to produce larger voltagepulses. The relative amplitudes of the pulses from the PMTs following each interactioncontain sufficient information to determine the location of the interaction inthe plane of the crystal.Until the late 1970s, scintillation cameras formed images as shown in Fig. 21-3, using only analog circuitry. The pulses from the preamps were sent to two circuits.The position circuit received the pulses from the individual preamps aftereach x- or gamma-ray interaction in the crystal and, by determining the centroid ofthese pulses, produced an X-position pulse and a Y-position pulse that togetherspecified the position of the interaction in the plane of the crystal. The summingcircuit added the pulses from the individual preamps to produce an energy (Z) pulseproportional in amplitude to the total energy deposited in the crystal. The Z pulsefrom the summing circuit was sent to a single-channel analyzer (SCA). The SCA,described in Chapter 20, produced a logic pulse (a voltage pulse of fixed amplitudeand duration) only if the Z pulse received was within a preset range of energies. Aninteraction in the camera's crystal was recorded in the image only if a logic pulse was