Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

detector array modules. With a traditional single detector array CT system, thedetectors are quite wide (e.g., 15 mm) and the adjustable collimator determinesslice thickness, typically between 1 and 13 mm. With these systems, the spacingbetween the collimator blades is adjusted by small motors under computer control.With multiple detector arrays, slice width is determined by the detectors, not by thecollimator (although a collimator does limit the beam to the total slice thickness).To allow the slice width to be adjustable, the detector width must be adjustable. Itis not feasible, however, to physically change the width of the detector arrays per se.Therefore, with multislice systems, the slice width is determined by grouping oneor more detector units together. For one manufacturer, the individual detector elementsare 1.25 mm wide, and there are 16 contiguous detectors across the module.The detector dimensions are referenced to the scanner's isocenter, the point at thecenter of gantry rotation. The electronics are available for four detector array channels,and one, two, three or four detectors on the detector module can be combinedto achieve slices of 4 X 1.25 mm, 4 X 2.50 mm, 4 X 3.75 mm, or 4 X 5.00 mm. Tocombine the signal from several detectors, the detectors are essentially wiredtogether using computer-controlled switches. Other manufacturers use the samegeneral approach but with different detector spacings. For example, one manufactureruses I-mm detectors everywhere except in the center, where four 0.5-mmwidedetectors are used. Other manufacturers use a gradually increasing spacing,with detector widths of 1.0, 1.5, 2.5, and 5.0 mm going away from the center.Increasing the number of active detector arrays beyond four (used in the examplediscussed) is a certainty.Multiple detector array CT scanners make use of solid-state detectors. For athird-generation multiple detector array with 16 detectors in the slice thicknessdimension and 750 detectors along each array, 12,000 individual detector elementsare used. The fan angle commonly used in third-generation CT scanners is about60 degrees, so fourth-generation scanners (which have detectors placed around 360degrees) require roughly six times as many detectors as third-generation systems.Consequently, all currently planned multiple detector array scanners make use ofthird-generation geometry.13.4 DETAILS OF ACQUISITIONSlice Thickness: Single DetectorArray ScannersThe slice thickness in single detector array CT systems is determined by the physicalcollimation of the incident x-ray beam with two lead jaws. As the gapbetween the two lead jaws widens, the slice thickness increases. The width of thedetectors in the single detector array places an upper limit on slice thickness.Opening the collimation beyond this point would do nothing to increase slicethickness, but would increase both the dose to the patient and the amount of scat-, tered radiation.There are important tradeoffs with respect to slice thickness. For scans performedat the same kV and mAs, the number of detected x-ray photons increaseslinearly with slice thickness. For example, going from a I-mm to a 3-mm slicethickness triples the number of detected x-ray photons, and the signal-to-noiseratio (SNR) increases by 73%, since V3 = 1.73. Increasing the slice thickness from