[Intl_DxMedPhys] [EXTERNAL] Odd CT number calibration issue

[Zhang, Da]

Hi Doug,

Two cents from my experience:

  1.  I observed similar behavior on an old GE Optima – multiple deficiencies on ACR CT number accuracy, and the numbers deviated between annual surveys
  2.  I used the widely known tricks: using a small body FOV, scanning the phantom from head-to-foot and foot-to-head, and adding a scatter block in front of the section-1, but still saw deficiencies from time to time
  3.  Full calibration and multiple daily calibrations will help – according to the FE, the calibration table is updated piece by piece over multiple calibrations
  4.  Sometimes there could be a large air bubble in the GE phantom, affecting the calibration
  5.  I often keep extra scans on the water phantom and air using empty table, when I observed CT number deficiencies on the ACR phantom, to make sure the scanner can be used clinically
Hope these may help!
Da

From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces+da.zhang=childrens.harvard.edu at lists.osu.edu> On Behalf Of Douglas Pfeiffer via Intl_dxmedphys_wd_osu_list
Sent: Monday, January 5, 2026 10:47
To: Januseski, Daniel <DJANUSESKI at VIRTUA.ORG>
Cc: intl_dxmedphys_wd_osu_list at lists.osu.edu
Subject: Re: [Intl_DxMedPhys] [EXTERNAL] Odd CT number calibration issue

I have historically used small body. It’s interesting that the optimal selection can change over time. Best regards, Doug Douglas Pfeiffer, MS, DABR FACR, FAAPM (he/him) Medical Physicist, Radiation Safety Officer Boulder Community Health xraydoug@ me. com
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I have historically used small body. It’s interesting that the optimal selection can change over time.

Best regards,
Doug



Douglas Pfeiffer, MS, DABR FACR, FAAPM (he/him)
Medical Physicist, Radiation Safety Officer
Boulder Community Health
xraydoug at me.com<mailto:xraydoug at me.com>
303.415.7515

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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.


On Jan 5, 2026, at 8:16 AM, Januseski, Daniel <DJANUSESKI at VIRTUA.ORG<mailto:DJANUSESKI at VIRTUA.ORG>> wrote:

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Doug – while I have been adding another phantom to the front of the ACR phantom sometime to get reasonable results from our VCT (and Optimas and original Revolutions), I have also found the scan FOV matters greatly as well.  Worse, sometimes one FOV works and then 6 months later, a different one works better.  Usually MED is a fair bet but not always.

Now I have had more success with an unconventional setting -  using the HEAD FOV on the ACR phantom.  I could not find anything in the ACR guidelines which limited the scan FOVs to body only, but of course using HEAD changes the filters as well (I think?).  If anyone wishes to elaborate or correct please do so.  But it has worked very well (note: some GE HEAD FOVs are 25 cm, newer scanners are 32 cm)


Dan Januseski, MS, DABR
Voorhees, NJ



From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces+djanuseski=virtua.org at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list-bounces+djanuseski=virtua.org at lists.osu.edu>> On Behalf Of Douglas Pfeiffer via Intl_dxmedphys_wd_osu_list
Sent: Friday, January 2, 2026 7:02 PM
To: troyxzhou at icloud.com<mailto:troyxzhou at icloud.com>
Cc: intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>
Subject: [EXTERNAL] Re: [Intl_DxMedPhys] Odd CT number calibration issue

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OK. I’ll try that. It’s just odd that I’ve never needed to do it on this scanner or any other 660 (I didn’t on our old VCT, too). Physics can be very wiley. Best regards, Doug Douglas Pfeiffer, MS, DABR FACR, FAAPM (he/him) Medical Physicist,
OK. I’ll try that. It’s just odd that I’ve never needed to do it on this scanner or any other 660 (I didn’t on our old VCT, too). Physics can be very wiley. Best regards, Doug Douglas Pfeiffer, MS, DABR FACR, FAAPM (he/him) Medical Physicist,
OK. I’ll try that. It’s just odd that I’ve never needed to do it on this scanner or any other 660 (I didn’t on our old VCT, too). Physics can be very wiley.

Best regards,
Doug



Douglas Pfeiffer, MS, DABR FACR, FAAPM (he/him)
Medical Physicist, Radiation Safety Officer
Boulder Community Health
xraydoug at me.com<mailto:xraydoug at me.com>
303.415.7515

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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.



On Jan 2, 2026, at 4:05 PM, troyxzhou at icloud.com<mailto:troyxzhou at icloud.com> wrote:

Optima 660 is like LightSpeed VCT, you would need to stack the GE water phantom right next to ACR phantom module 1 to get the CT number right. It is due to their helical reconstruction.

Cheers

Troy

From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces+troyxzhou=icloud.com at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list-bounces+troyxzhou=icloud.com at lists.osu.edu>> On Behalf Of Douglas Pfeiffer via Intl_dxmedphys_wd_osu_list
Sent: Friday, January 2, 2026 6:00 PM
To: Douglas Pfeiffer <xraydoug at me.com<mailto:xraydoug at me.com>>
Cc: intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>
Subject: Re: [Intl_DxMedPhys] Odd CT number calibration issue

If it makes any difference, I lied. It is a GE Optima 660. (The VCT was their old scanner…) On Jan 2, 2026, at 2: 06 PM, Douglas Pfeiffer via Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list@ lists. osu. edu> wrote: Hi, everyone. Starting
If it makes any difference, I lied. It is a GE Optima 660. (The VCT was their old scanner…)






On Jan 2, 2026, at 2:06 PM, Douglas Pfeiffer via Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto: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. 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<mailto:xraydoug at me.com>
303.415.7515

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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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