[Intl_DxMedPhys] CT Number Deviations on Siemens Force and Alpha – ACR Phantom Failures

[Little, Kevin]

I haven’t had any issues with the water HU yet, but across the board on Somaris 10 scanners (Alpha, go.Top, X.cite, X.ceed, etc), I have had issues with the air measurement inside the ACR phantom. Often the values are too low, and ACR’s limits are very tight on the low side (just a 5 HU margin on the low side compared to 30 HU on the high side for -1005 to -970). If you measure air outside the phantom (like in the image corners), the value is usually very close to -1000. To me, this is more of an indication that ACR’s limits are too tight to account for reconstruction variations rather than that anything is wrong with the scanner. Siemens has obviously made some changes to their reconstruction algorithms, but I don’t see any issue with air having an HU of -1006 inside the phantom when the air outside the phantom is right on target.

Bone is often out of range for head exams. If it helps, only adult abdomen protocols are checked for all five materials for ACR accreditation—for everything else only water matters. This is from the ACR accreditation page<https://urldefense.com/v3/__https://accreditationsupport.acr.org/support/solutions/articles/11000056197-acr-ct-phantom-scanning-instructions__;!!KGKeukY!yF8-5ZRs0JVr9LQ88X2N9BTX3UR7YzC7-XqFxd2ccJbOyCOrtneuEalKTi7rPa_XeuMP2Wt2TbbQUd-EfjMblKhGjaAmSu_qedG1v_jRSPgnC9E$ >:
[cid:image001.png at 01DBFC8B.54D4AF80]
Siemens just links to this ACR accreditation page<https://urldefense.com/v3/__https://accreditationsupport.acr.org/support/solutions/articles/11000056197-acr-ct-phantom-scanning-instructions__;!!KGKeukY!yF8-5ZRs0JVr9LQ88X2N9BTX3UR7YzC7-XqFxd2ccJbOyCOrtneuEalKTi7rPa_XeuMP2Wt2TbbQUd-EfjMblKhGjaAmSu_qedG1v_jRSPgnC9E$ > whenever we have complained about bone being out of range for a head recon. This doesn’t match up exactly with the 2017 accreditation manual, which says the limits apply for any 120 or 130 kVp scan, but it’s become our de facto standard. The 2017 manual allows the physicist to establish new baseline values if it’s verified that the scanner is properly calibrated, so this effectively what we have done.

Kevin


Kevin J Little, PhD, DABR, DABSNM, CIIP
Associate Professor
Imaging Medical Physicist
The Ohio State University Wexner Medical Center
Department of Radiology
395 W 12th Avenue, 4th Fl, Columbus, OH, 43210
ImagingPhysics at osumc.edu<mailto:ImagingPhysics at osumc.edu>
On-call Physicist: Internal link<https://webxchange.osumc.edu/webxchange/ViewProfile.aspx?profileid=08701 > or call 614-293-7243,8701#<tel:+16142937243,8701#.>



From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces at lists.osu.edu> On Behalf Of Imendra Ranatunge via Intl_dxmedphys_wd_osu_list
Sent: Thursday, July 24, 2025 9:37 AM
To: intl_dxmedphys_wd_osu_list at lists.osu.edu
Subject: [Intl_DxMedPhys] CT Number Deviations on Siemens Force and Alpha – ACR Phantom Failures

Hi All, I’m encountering an issue with a couple of my Siemens CT scanners—specifically the Force and Alpha models. Several of my CT number measurements using the ACR phantom are failing for air, water, and bone. Some units fail all three, while

Hi All,

I’m encountering an issue with a couple of my Siemens CT scanners—specifically the Force and Alpha models. Several of my CT number measurements using the ACR phantom are failing for air, water, and bone. Some units fail all three, while others only one or two.

I tried placing a CTDI phantom adjacent to the ACR phantom, but it showed no improvement.

I reported the issue to Siemens. They performed a constancy test and stated that the units were functioning normally. However, my CT number results are still outside acceptable limits. In my experience, service engineers are often reluctant to perform time-consuming calibrations unless specifically requested.

Has anyone experienced this before? If so, do you know which specific calibration or adjustment Siemens needs to perform to correct these CT number inaccuracies?

Imendra
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