[Intl_DxMedPhys] Odd CT number calibration issue [EXTERNAL]

[Junguo Bian]

He did not say and he was not exactly sure of the source of the
problems, either.  We actually had a difficult time convincing the first
FSE that their scanner had a problem because the water HU measured with GE
water phantom is normal.  But the first FSE still performed a detailed
calibration and after that he performed a few GE tests and also several QA
scans.  It passed all GE tests and the water HU for regular scans passed.
When I looked at his QA images,  I saw a GSI image series at 70 kev and the
water HU  in that image series was quite high.  That was why I was able
to escalate the problem further.   The FSE from GE headquarter had my
screen captures of the GSI image results when he began to look into the
problem.

Junguo

On Tue, Jan 13, 2026 at 12:05 PM Zhang, Da <Da.Zhang at childrens.harvard.edu>
wrote:

> Thank you Junguo for sharing! Did the FSE from GE headquarter offer
> information about what was wrong with the high voltage tank? Was the kV
> accurate, or the waveforms as expected?
>
>
>
> Da
>
>
>
> *From:* Intl_dxmedphys_wd_osu_list <
> intl_dxmedphys_wd_osu_list-bounces at lists.osu.edu> *On Behalf Of *Junguo
> Bian via Intl_dxmedphys_wd_osu_list
> *Sent:* Tuesday, January 13, 2026 12:36
> *To:* intl_dxmedphys_wd_osu_list at lists.osu.edu
> *Subject:* Re: [Intl_DxMedPhys] Odd CT number calibration issue [EXTERNAL]
>
>
>
> ** External Email - Caution **
>
>
>
> Hi Doug, We have several GE VCT and HD750 scanners, and water HU issues
> with the ACR phantom are fairly common on these systems. In many cases, the
> issue can be mitigated by placing a scatter block in front of Section 1 of
> the phantom and using
>
> Hi Doug,
>
> We have several GE VCT and HD750 scanners, and water HU issues with the
> ACR phantom are fairly common on these systems. In many cases, the issue
> can be mitigated by placing a scatter block in front of Section 1 of the
> phantom and using the small body scan FOV—particularly when the problem
> occurs only in helical protocols and axial scans are within limit.
>
> However, in some cases, the issue may indicate an underlying hardware
> problem. We recently installed a new GE HD750 scanner that exhibited
> persistent water HU issues with the ACR phantom, despite passing QC with
> the GE water phantom. Upon reviewing the uniform water-equivalent section
> of the ACR phantom, the water HU values were consistently at the ACR
> tolerance limits. Even after applying the scatter block and small body FOV,
> results only marginally passed.
>
> We compared these results with another HD750 using the same ACR phantom
> and observed significantly better performance on our existing system,
> suggesting a scanner-specific issue. A GE field engineer performed a
> detailed recalibration with no improvement. The case was then escalated to
> GE Healthcare, and the service engineer from the headquarter ultimately
> replaced the high-voltage tank. After replacement, the water HU issue was
> fully resolved.
>
> Just sharing for your awareness.
>
> Best
>
> Junguo
>
> --
>
> ---------------------------------------------------------------------------------------------------
>
> Junguo Bian, PhD, DABR
>
> Associate Professor, Medical Physicist
>
> Department of Radiology, Stritch School of Medicine
>
> Loyola University Medical Center
>
> 2160 S. First Ave.
>
> Maywood, IL. 60153
>
>
>
>
>
> On Fri, Jan 2, 2026 at 3:07 PM Douglas Pfeiffer via
> Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu>
> wrote:
>
> Hi, everyone. Starting off the new year with an oddity! GE VCT has been
> installed for years. CT number testing has never had an issue. I performed
> my annual testing and found that the water CT number is high for all four
> of the standard protocols.
>
> Hi, everyone.
>
>
>
> Starting off the new year with an oddity! GE VCT has been installed for
> years. CT number testing has never had an issue. I performed my annual
> testing and found that the water CT number is high for all four of the
> standard protocols. They are fine in axial mode. After several
> calibrations, including the deep one, the service dude sent me an image of
> the water phantom in spec using the adult abd clinical protocol. I return
> to document the correction and get series needed for accreditation. Again I
> find that the water CT number is unacceptable for all four protocols.
> HOWEVER, when I scan the GE water phantom with exactly the same protocols,
> the water is fine.
>
>
>
> I have used this ACR phantom for as long as they’ve been made and never
> had an issue like this on any scanner. I should also state that nothing was
> done to the scanner since last testing. No software change, no tube change.
>
>
>
> Does anyone have an idea of what’s going on? I’m tapped out of ideas. I
> might put my Catphan on it to see what true water in that looks like.
>
>
>
>
>
> Best regards,
>
> Doug
>
>
>
>
>
>
>
> Douglas Pfeiffer, MS, DABR FACR, FAAPM (he/him)
>
> Medical Physicist, Radiation Safety Officer
>
> Boulder Community Health
>
> xraydoug at me.com
>
> 303.415.7515
>
>
>
> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
>
>
>
> The opinions expressed in this message are the product of the gray and
> white matter loitering in my cranium. I speak for myself and no one else,
> unless I say otherwise.
>
>
>
>
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