Technical needs vary widely across CT scanners. This article addresses some of the more common special considerations that may arise related to CT accreditation phantom testing.
‘Exposure Time’ DICOM Tags Using Values Other Than Time for One Rotation
Some manufacturers use a value other than the time for one complete rotation in the ‘exposure time’ DICOM tag (0018, 1150). Unfortunately, this is set at the factory and the user does not have control over this. You can check and see if the scanner reports the ‘revolution time’ DICOM tag (0018,9305), as this is reported on an increasing number of scanners. If so, this can be pointed out in your submission. ACR is making its CT reviewers aware of this potential discrepancy as well.
Parameters for Scans Done in Overscan Mode
If the scan is done in a 420 overscan mode, then the rotation time scan parameters should be recorded in the phantom data form and dose calculator spreadsheets as the actual time used for each scan along with the resultant mA. For example, if 200 mAs with a 1.0 second rotation time is used but the actual time for each scan is 1.167 making the mA approximately 171, record the rotation time as 1.167 seconds and the mA as 171.
Exposure Time Tag (0018, 1150) Discrepancies
Some manufacturers use a value other than the time for one complete rotation in this particular exposure time tag. Unfortunately, this is set at the factory and the user does not have control over this. You can check to see if the scanner reports the “revolution time” DICOM tag (0018, 9305) as this is reported on an increasing number of scanners. If so, this can be pointed out in your submission. ACR is making its CT reviewers aware of this potential discrepancy.
Discrepancies Between Water Calibration and CT Number
In some cases, a scanner may pass the manufacturer’s QA specification for water calibration (e.g., water is 0 +/- 5 HU), but the CT number of some of the cylinders in module 1 of the ACR phantom are not within the appropriate range. In such cases, the ACR phantom can be scanned in the opposite direction (from section 4 toward section 1 instead of from section 1 to section 4). This will result in the phantom images appearing in the reverse order, but this will be acceptable.
Alternatively, a water phantom or CTDI phantom can be positioned on the patient table in a manner that effectively extends the ACR phantom somewhat and fills the air gap that causes the problem (see figure 1). The water or CTDI phantom may have to be raised a bit to match the ACR phantom height; anything that is not a major attenuator will serve as a shim (such as a few towels or folded up bed linens). In this case, the scan acquisition directions could be followed per the original instructions (from section 1 toward section 4).
Additionally, phantom reviewers may use an image that is closest to Module 2, further reducing the impact of the helical interpolation. If this is indicated, please include a note to this effect with your submission so that the reviewer is alerted.
It is worth noting that some bleed-through of the CT number module into the low contrast module may occur on models such as the Aquilion One in Volume Mode. This effect can be eliminated by applying VCOR. VCOR lets the reconstruction engine know the object on the table is not a patient, but rather an artificial construct about which no clinical assumptions can be made. VCOR can be activated by your service engineer or via a dropdown menu, depending on software version.
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Article created; FAQs incorporated; No criteria changes
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