THE REPORT OF AAPM TASK GROUP 233: Performance Evaluation of Computed Tomography Systems
Table 5c. Image Position Accuracy Purpose
To ensure that the prescribed image location indicated in a CT localizer radiograph correctly corresponds to the image position IEC Method
Testing Devices
Thin absorber, e.g., 1-mm-diameter wire
Setup
Acquire a CT localizer radiograph of the test device.
Scan Protocol
Using the narrowest detector thickness and a table increment ≤1 mm, prescribe an axial scan directly covering the wire (test device) that covers a range of ±3 mm longitudinal to the test object Select image location with the highest CT number
Measurements
Qualitative Criteria Quantitative Criteria References
ACR Method
IAEA Method
A phantom that incorporates A suitable test tool can be easily externally visible radiopaque fiducial fabricated from a PMMA slab at least markers or an image-center indication. 25 cm, and preferably >50 cm long. It can be shaped like a ruler and should have accurately placed markers at a set distance apart (e.g., 50 cm). Another alternative is to use a phantom or material block of a precisely known length. Align phantom in all three directions, Place test tool along the long axis of zero table location, and take a CT the couch and acquire a CT localizer localizer radiograph. Magnify the radiograph of the test device, making image, if possible, and position a single sure that the markers at each end of image at the location of the radiopaque the tool are scanned. fiducial markers. Perform an axial scan using a reconLocate the markers on the CT structed scan width less than 2 mm or localizer radiograph and program two as thin as the scanner can produce in separate scans to create 1 mm (or the axial mode. thinnest available) images directly over each of these markers.
N/A ±2 mm
Determine location where markers are The two acquired CT images of the best visualized markers, based on the CT localizer radiograph, should clearly display each marker. N/A N/A ±2 mm ±2 mm acceptable (±1 mm achievable)
IEC standard 61223-3-5
ACR CT QC manual
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IAEA Series No. 19