Quality Control Bushong Chapter 10

QC The CT number of water should be 0 Scan a water phantom, select a large enough region of interest (ROI), the average CT number should be within +/- 3 Scan an empty gantry aperture, select a large ROI, the average CT number for air should be within -1000 +/- 5

QC CT noise is evaluated by scanning a water bath. The standard deviation should not exceed +/- 10 Spatial resolution is evaluated by scanning any number of phantoms-hole patters, or line pair patterns Spatial resolution should be at least equal to the manufacturers specifications

QC Most CT imagers can image .5mm (10 lp/cm), some as good as .2mm (20 lp/cm) in a high resolution mode Contrast resolution is evaluated with either a phantom, thick or thin, various hole sizes filled with water, or various hole sizes of various depth for partial volume effects Contrast resolution should be at least 5mm at 5% contrast

QC Distance measurement accuracy is evaluated by imaging the American Association of Physicists in Medicine (AAPM) test object that has pins separated by 10mm The electronic distance calipers must be accurate to within +/- 5%

QC The video monitor should be evaluated for distortion by pressing a clear plastic rules against the screen image of the 10mm pin test object to ensure the same 5% accuracy Scan plane localization, usually a laser light localizer, should be accurate to within 1mm. A number of test devices are available

QC Patient couch indexing can be checked with cardboard wrapped film or marks on the couch and rail. The limit of error is 1% Slice width is usually evaluated with a ramp test object and should be within 50% for nominal widths of 5mm and above and 100% for thinner slices