[Intl_DxMedPhys] Odd CT number calibration issue

[Junguo Bian]

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

-- 
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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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