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Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

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TABLE 22-2. PROPERTIES OF SEVERAL INORGANIC SCINTILLATORS OFINTEREST IN PETDecay Peak Attenuation Conversionconstant wavelength Atomic Density coefficient efficiencyScintillator (nsec) (nm) numbers (g/cm 3 ) 511 keV (cm-') relative to NalNal(TI) 230 415 11,53 3.67 0.343 100%BGO 300 460 83,32,8 7.17 0.964 12-14%GSO(Ce) 56 430 64,14,8 6.71 0.704 41%LSO(Ce) 40 420 71,14,8 7.4 0.870 75%Decay constants, peak wavelengths of emitted light, densities, and conversion efficiencies of BGO, GSO,and LSO from Ficke DC, Hood JT, Ter-Pogossian MM. A spheroid positron emission tomograph <strong>for</strong> brainimaging: a feasibility study. J Nucl Med 1996;37:1222.BGO, bismuth germanate; GSO, gadolinium oxyorthosilicate; LSO, lutetium oxyorthosilicate; PET,positronemission tomography.of the Compton continuum, increasing the sensitivity, but also increasing the numberof scattered photons detected.<strong>The</strong> time signals of interactions not rejected by the energy discrimination circuitsare used <strong>for</strong> coincidence detection. When a coincidence is detected, the circuitryor computer of the scanner determines a line in space connecting the twointeractions (Fig. 22-14). This is called a line of response. <strong>The</strong> number of coincidencesdetected along each line of response is stored in the memory of the computer.Figure 22-17 compares the acquisition of projection data by SPECT andPET systems. Once data acquisition is complete, the computer uses the projectiondata to produce transverse images of the radionuclide distribution in the patient, asin x-ray CT or SPECT.FIGURE 22-17. Acquisition of projection data by a SPECT system(left) and a PET system (right), showing how each system collectsdata <strong>for</strong> two projections. However, the PET system collects data <strong>for</strong> allprojections simultaneously, whereas the scintillation camera head ofthe SPECT system must move to collect data from each projectionangle.

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