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The performance and dosimetric characteristics of the first U.S- installed hybrid headscanner developed commercially by CGR have been evaluated. Using a translate- rotatemotion with a xenon gas detector array, the scanner produces excellent low contrast reso-lution at low dose levels (single slice surface dose of 1 rad). Console selection ofscanning parameters, viz.: scan time, kV, mA, beam filtration, slice width, and pixel sizepermits optimization of image quality for various exams. For optimizing either spatialor low contrast resolution, a push- button choice of reconstruction algorithms is available.Using the AAPM CT phantom, spatial resolution and line spread functions for both algo-rithms were determined from the graphical display of CT numbers. Low contrast resolutionwas measured at various contrast levels as a function of kV, dose, beam width, pixel size,and algorithm. Values for noise (o) were also recorded for these scanning parameters.Patient dose distributions for various operator -selectable beam widths (3,6,9 mm) wereobtained using film and TLD's. CT number linearity and contrast scale were determinedusing different plastic rods.IntroductionThe proliferation of CT scanners of differing designs requires that performance evalua-tions be made in a standardized manner so comparisons will be meaningful. With this inmind, the present evaluation follows the guidelines published in AAPM Report #1(1) anduses a phantom similar to that recommended in the report.(2) Modern scanners present theoperator and radiologist with a number of choices to be made for the physical parametersof the scan. In the present study, the effects of these various options were measuredwith regard to image quality and patient dose.System DescriptionThe CGR ND 8000 is a translate- rotate head scanner utilizing an array of 31 xenon detec-tors and a 12° fan beam collimated to 31 separate beams. Three scanning modes are opera-tor selectable: 1800-20 seconds, 1800-40 seconds and 3600-80 seconds; reconstructiontimes are 30, 40 and 60 seconds respectively. The x -ray tube is a fixed anode 1 x 5 mmnominal focal spot with a 2.4 kilowat rating. The operator is capable of selecting kV,mA, and beam widths (3, 6, and 9 mm) in addition to three beam filters. Two algorithmsare available: "Density" or "Spatial ", along with two matrix sizes 256 x 256 (1 mm pixelsize) and 512 x 515 (0.5 mm pixel size).Spatial ResolutionSpatial resolution has been shown to depend on various operating parameters,(3'4) themost critical being:1. Display Matrix (pixel size)2. Sampling Frequency3. Algorithm4. Detector ApertureThe ND 8000 has the capability of utilizing two algorithms and two pixel sizes while boththe sampling frequency (0.5 mm) and the detector apertures (4 mm) are kept constant.High contrast resolution was evaluated using the methods available on the AAPM CTphantom, the line spread function (LSF) and the disappearance spacing of air filled holesin Plexiglas. The disappearance spacing method is somewhat subjective, depending on win-dow and level settings and observer interpretation. Another problem is that results arenot independent of phantom position when resolution approaches pixel size.(5) The bestscans were taken as representative of maximum resolution. The LSF was measured by scan-ning the 14 mil wire and plotting CT number versus distance. The CT reconstructs usingfinite square shaped pixels, resulting in a histogram rather than a smooth gaussian dis-tribution because the LSF is rapidly changing in a short distance. By scanning the wireat several locations, it was possible to obtain an average LSF. The resolution can betaken to be the full -width at half- maximum (FWHM) of the LSF (1,6,7). The resolution wardetermined for all the operator selectable parameters, however, only pixel size and |
