[Intl_DxMedPhys] [EXTERNAL] Re: ACR CT test question

Wed Sep 16 12:35:17 EDT 2026

I had struggled a while with this very question as trying to find the average patient from some recent scans did not seem very representative at all.  The comments I agree with that each health system has a different “average” patient.  So I moved to using DMS data by looking at the WED (water-equivalent diameter) vs. modulated output.  The data was restricted to the WED for which our medium abdomen protocols were set (we have mainly GE scanners and use sm/med/lg auto modulated protocols to compensate for extreme body types), which include diameters from 25 – 32 cm.

Six months of data is analyzed, about 10,000 exams, and the mean and median patient weight for this sample is always around 70-74 kg.  So I felt pretty good that this was our “average” patient.  The corresponding mAs values are then used for the CTDI phantom test and image quality test.  It has also helped to properly set the manual backup techniques, which I usually pick the 75%ile values because there is enough dose headroom to stay under the DRL and guarantee decent image quality for patients at the extreme of the size limit.  The small and large protocols are then adjusted by noise index from these data.  So far, unless I am missing something fundamental, out DIR results have been very decent and I believe this is a representative sample of our average patients.

One caveat I do see, though, is that our average patient is bigger than the ACR phantom by maybe 6-8 cm, and so the image quality result is overstated.  Would it be more appropriate to scale the test mAs down from our 27cm “average” patient dose to the 20cm ACR phantom and see if the CNR is still acceptable?

Dan

Daniel J. Januseski, M.S., DABR
Director, Department of Diagnostic Imaging Physics
Radiation Safety Officer
(856) 247-4436 | f (856) 247-2819
djanuseski at virtua.org<mailto:djanuseski at virtua.org>

From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces+djanuseski=virtua.org at lists.osu.edu> On Behalf Of Donna Davis-Urgo via Intl_dxmedphys_wd_osu_list
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Subject: [EXTERNAL] Re: [Intl_DxMedPhys] ACR CT test question

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Just as the average BMI could be larger at a given facility, so can the amount of pediatrics patients in the targeted weight/age range. Many times, facilities might only do a handful of these pediatric exam types/ages a year (and need to substitute
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Just as the average BMI could be larger at a given facility, so can the amount of pediatrics patients in the targeted weight/age range. Many times, facilities might only do a handful of these pediatric exam types/ages a year (and need to substitute
Just as the average BMI could be larger at a given facility, so can the amount of pediatrics patients in the targeted weight/age range.  Many times, facilities might only do a handful of these pediatric exam types/ages a year (and need to substitute another type of clinical exam or an older patient when the accreditation comes up).

Donna Davis-Urgo, One Physics

From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces+murgo571=verizon.net at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list-bounces+murgo571=verizon.net at lists.osu.edu>> On Behalf Of Bob Kobistek via Intl_dxmedphys_wd_osu_list
Sent: Wednesday, September 16, 2026 10:23 AM
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Cc: DxMedPhys List <intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>>
Subject: Re: [Intl_DxMedPhys] ACR CT test question

The downside I see with basing the phantom scans on the “average” patient drawn from the institutions patient population is that if the average for the local population does not align with whatever the ACR considers average, then pass / fail
The downside I see with basing the phantom scans on the “average” patient drawn from the institutions patient population is that if the average for the local population does not align with whatever the ACR considers average, then pass / fail criteria would not be appropriate for that institution. I used to consult in rural West Virginia where the average patient would be heavy when compared to most other definitions of average. So if a higher CTDIvol is needed to produce acceptable image quality in this high-BMI population, the facility may fail if the CTDIvol is above the failure limit. The flip side to that is that the standard ACR image quality phantom was designed with a specific body model in mind. A high BMI population would lead to the use of increased mAs or mAs and kV combination when scanning the phantom, which would result in image quality scores that do not represent the image quality of the clinical scans.

Robert J. Kobistek, MS, FACR, DABR, MRSE(MRSC™)
Medical Physicist
RJK Medical Physics, Inc.
440-463-7879

From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces+bob=rjkmedphys.com at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list-bounces+bob=rjkmedphys.com at lists.osu.edu>> On Behalf Of Tyler Fisher via Intl_dxmedphys_wd_osu_list
Sent: Tuesday, September 15, 2026 1:36 PM
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Subject: Re: [Intl_DxMedPhys] ACR CT test question

Hi Everyone, This has been a very good discussion. I'll add my two cents but also state up-front that I am the current chair of the ACR CT Physics Accreditation Sub-Committee, so there may be a pro-ACR bias coming through. In general, the
Hi Everyone,
This has been a very good discussion.  I'll add my two cents but also state up-front that I am the current chair of the ACR CT Physics Accreditation Sub-Committee, so there may be a pro-ACR bias coming through.

In general, the ACR technical standards, QC Manuals, and Accreditation program requirements are overlapping but independent.  The QC manuals are assembled and maintained by ACR staff and committees of volunteer members who represent the accreditation programs and other stakeholders.  Slight variations in language or specifics like this are to be expected.

The intent of the "average-sized" language in the accreditation program, as I interpret it, is site and scanner specific.  When I test a CT system, I will look through the patients scanned recently for what I think is their standard adult abdomen protocol.  I check the scout first to make sure they're not tiny or bariatric, then I'll scroll through the primary axial images until I get to mid-liver and record the mA.  I'll take an average of 6 or 8 patients.  I don't look at patient sex or age.  If a site has additional data (ACR DIR or dose-tracking software) that will give me techniques for an average-sized patient, I'll use that, but more often than not, the sites that I visit do not.  This method would be more accurate than my average of 6 or 8 scans, but I think we have to remember that the majority of scanners in the country aren't being monitored like this.  The ACR would love for everyone to participate in the ACR DIR, but this is not a requirement for accreditation.  Also, we're measuring and reporting a single CTDI value for a protocol whose standard deviation of CTDI values is huge.

For new scanners, Scott's suggested methods all seem reasonable.  I tend to take my knowledge of similar (or exactly the same make/model) scanners, the site's standard protocols for other scanners if available, default protocols, and my own personal expertise to come up with an average technique.  I will note in my report that the protocols tested may not match the clinical protocols established during applications training and the next time I lay hands on that scanner, I'm going to closely review what they're actually doing.  Also, I'll make sure at acceptance testing that the reported CTDI values match what I measure, so that if the protocols are different than what I tested, at least they'll know that the doses are accurate.

The discussion about finding a "21 or 23 cm effective diameter patient" to set as the "average-patient" is wrong.  The "DFOV close to 21 cm" is strictly based on the size of the image quality phantom - it has nothing to do with an average patient.  If the image quality phantom was 30 cm diameter, we'd tell you to scan with a DFOV close to 31 cm.

Last thing I'll say is this:  The ACR believes that qualified diagnostic medical physicists have the expertise and integrity to perform quality medical physics testing on scanners.  Can someone iteratively increase dose to get CNR to pass?  Of course.  Can we cheat the average technique down to make sure that we're within the dose thresholds?  Sure.  The accreditation programs work on the assumption that we're all doing our best.  The programs try to balance being overly prescriptive on how to do every single test vs. allowing physicists free-reign to do whatever we want.  Over 25 years of the CT accreditation program, I think we've gotten pretty close to the right balance, but as the chair, I'm open to suggestions of how we can improve.

Thanks.
Tyler Fisher
Therapy Physics, Inc.

On Mon, Sep 14, 2026 at 1:53 PM Matt Wait 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:
To clarify: I had been going on the assumption that 70 kg correlated with a patient with an abdomen of about 21 cm in diameter. Maybe I had gotten it mixed up with the phantom diameter somewhere down the line. The point is, I don't really have
To clarify: I had been going on the assumption that 70 kg correlated with a patient with an abdomen of about 21 cm in diameter. Maybe I had gotten it mixed up with the phantom diameter somewhere down the line.

The point is, I don't really have any idea what the abdomen of a 70 kg patient "looks like". It would be much easier to have a rough estimate of an abdomen diameter range to use.

Matt Wait, MS, DABR, DABSNM, MRSE
Senior Diagnostic Physicist

Associate Radiation Safety Officer

Assistant Residency Director

Kaiser Permanente

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Medical Imaging Technology and Informatics
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________________________________
From: Douglas Pfeiffer <xraydoug at me.com<mailto:xraydoug at me.com>>
Sent: Monday, September 14, 2026 1:47 PM
To: Matt Wait <Matt.Wait at kp.org<mailto:Matt.Wait at kp.org>>
Cc: DxMedPhys List <intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>>; Bob Kobistek <bob at rjkmedphys.com<mailto:bob at rjkmedphys.com>>
Subject: Re: [Intl_DxMedPhys] ACR CT test question

The 21 cm DFOV is to make it easier to analyze - nothing to do with patient size. It doesn’t impact dose in any way. Using a small Scan FOV is to reduce non-uniformities due to the bowtie not being matched to the object size on some scanners.

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<tel:(303)%20415-7515>

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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 Sep 14, 2026, at 2:42 PM, Matt Wait <Matt.Wait at kp.org<mailto:Matt.Wait at kp.org>> wrote:

That's good to hear, that's the practice I've been using. But I have never found a good reference for correlating the abdomen diameter (21 cm?) with 70 kg.

Matt Wait, MS, DABR, DABSNM, MRSE
Senior Diagnostic Physicist
Associate Radiation Safety Officer
Assistant Residency Director
Kaiser Permanente
Southern Permanente Medical Group
Medical Imaging Technology and Informatics
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________________________________
From: Douglas Pfeiffer <xraydoug at me.com<mailto:xraydoug at me.com>>
Sent: Monday, September 14, 2026 1:35 PM
To: Matt Wait <Matt.Wait at kp.org<mailto:Matt.Wait at kp.org>>
Cc: DxMedPhys List <intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>>; Bob Kobistek <bob at rjkmedphys.com<mailto:bob at rjkmedphys.com>>
Subject: Re: [Intl_DxMedPhys] ACR CT test question

Going through the list of scanned patients on the scanner and looking for abdomens that look about the right size, and looking at the mA at mid-liver (belly button-ish).

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<tel:(303)%20415-7515>

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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 Sep 14, 2026, at 2:16 PM, Matt Wait <Matt.Wait at kp.org<mailto:Matt.Wait at kp.org>> wrote:

Doug, how did you envision determining the automatic tube current selection for Standard Man?

Matt Wait, MS, DABR, DABSNM, MRSE
Senior Diagnostic Physicist
Associate Radiation Safety Officer
Assistant Residency Director
Kaiser Permanente
Southern Permanente Medical Group
Medical Imaging Technology and Informatics
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________________________________

From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces+matt.wait=kp.org at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list-bounces+matt.wait=kp.org at lists.osu.edu>> on behalf of 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>>
Sent: Monday, September 14, 2026 1:12 PM
To: Bob Kobistek <bob at rjkmedphys.com<mailto:bob at rjkmedphys.com>>
Cc: DxMedPhys List <intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>>
Subject: Re: [Intl_DxMedPhys] ACR CT test question

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For what it’s worth now, the intent when I was working on the manual was Standard Man (i.e., 70 kg). It was an unfortunate oversight on my part that I did not catch that and state it explicitly way back when. I’m not sure what is assumed now by the ACR, but that was the original thinking.

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<tel:(303)%20415-7515>

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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 Sep 9, 2026, at 1:45 PM, Bob Kobistek 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.

I also beat my head against a wall searching for answers to these questions when the ACR CT accreditation program was started. I stopped overthinking it, and now I pretty much wing it.

For average size patient, I look through patient scans to find one with an A-P dimension through the abdomen of around 23 cm. If I can find more than one (hard to find nowadays), I’ll toss out the outliers and take an average of the mAs values.

For new scanners I use nearly all the options you presented. If it’s a scanner with a similar model in use at the facility, I refer to patients scanned on the existing scanners. If it’s a scanner that I see at other facilities, I’ll make adjustments for differences in protocols and use that. If it’s a scanner I’ve never seen before, I’ll pull up the manufacturer’s suggested protocols and take a guess at the mAs when TCM is used. If I use the second or third option, I’ll take a look at patient scans once the scanner has been operating for a few weeks. If I can make adjustments to my measurements based on the clinical data, I will. If I have to, I’ll repeat the measurements using the clinical protocols. If the site is going to be ACR accredited I will always make a second trip so I can use the actual protocols and clinical mAs values for accreditation, not matter which of the three options above that I use.

There is very little guidance on how to chose mAs (and kVp when applicable) for automated technique selection. So we use out best judgment.

Robert J. Kobistek, MS, FACR, DABR, MRSE(MRSC™)
Medical Physicist
RJK Medical Physics, Inc.
440-463-7879<tel:(440)%20463-7879>

From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list-bounces at lists.osu.edu>> On Behalf Of David S. Winter via Intl_dxmedphys_wd_osu_list
Sent: Monday, September 7, 2026 4:02 PM
To: DxMedPhys List <intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>>
Subject: [Intl_DxMedPhys] ACR CT test question

All, I apologize as this has no doubt been discussed in the past, but I have a two part inquiry related to ACR CT testing. I am happy to clarify directly with the ACR, but I figured I would touch base with this forum first. (Especially since
All,

I apologize as this has no doubt been discussed in the past, but I have a two part inquiry related to ACR CT testing. I am happy to clarify directly with the ACR, but I figured I would touch base with this forum first. (Especially since only part 1 would be good to ask the ACR directly.)

Part 1:  The ACR’s scanning instructions webpage and the CT QC manual both give a weight range for the pediatric abdominal exam of 40-50lbs, but only the QC manual has a weight for the adult abdomen (i.e. 70 kg, ~154 lbs). The webpage merely says “average sized patient”. The exclusion of the 70 kg reference could be an accidental omission, but that requires the same omission to have been repeated in multiple different locations on their webpage.

“Average sized” could be interpreted to mean average across the entire US population, the region a CT scanner is in, or the specific patient population a specific CT scanner sees (which could be massively different if the scanner is the go-to scanner for bariatric patients.) The CDC says the current adult average weights are 199 lbs for men and 171.8 lbs for women. (Both significantly more than 154). In addition, the ACR’s mandatory dose calculation sheet calculates the adult abdomen SSDE for a 35 cm water equivalent diameter. While you can’t exactly convert abdominal water equivalent diameter to a patient weight, that value would likely correspond to a patient significantly heavier than 154 lbs.

Has anyone specifically seen anything that dictates which is definitively correct for determining what techniques to set, and by extension, determine whether or not the scanner is really passing the CNR test?

Part 2: In practice, I am curious how people are determining the mA that CT scanners are selecting for the “average”, particularly for brand new scanners without much/any historic patient data. I can imagine four different general approaches. 1) Analysis of historical data aggregated by a dose metric monitoring system like Radimetrics. 2) Manually looking for a few historical patients that look to be roughly “average size” and then averaging the mA from a representative slice from a few of those studies. 3) For new systems, using values from a similar system with similar settings. 4) Conducting scans of a standard (e.g. their CTDI phantom), seeing what the CT scanner picks for mA, and then applying a scaling factor to that mA to adjust for the size difference.

Sincerely,
Scott

D. SCOTT WINTER

CHIEF DIAGNOSTIC MEDICAL PHYSICIST
O:
228-865-3475<tel:228-865-3475>
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