[Intl_DxMedPhys] Odd CT number calibration issue

[Douglas Pfeiffer]

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
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 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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