[Intl_DxMedPhys] Gamma Camera Max Count Rate Perpendicular versus Off-Angle

Wed Jun 25 11:56:18 EDT 2025

P. S. The follow-up question would be which UFOV dimension should be used
in calculating the 5 UFOV distance ...

On Wed, Jun 25, 2025 at 9:18 AM Bill Erwin <ervster57 at gmail.com> wrote:

> I think you are on the correct track, Michael.  While performing that test
> intrinsically by moving a source inward from a starting distance is quite
> convenient (i.e., fast), the source radiation pattern transitions from
> closer to parallel (essentially uniform flux across the detector UFOV) to
> spherical (non-uniform flux weighted to that part of the detector closest
> to the source at a given distance).  The degree of non-uniformity in the
> flux, and how much of the UFOV begins to experience paralysis due to the
> Anger logic (some modern cameras have "fancy-schmancy" pulse pileup
> correction, multiple pulse integrators or other digital corrections), may
> depend on not only distance but the angle at which the source approaches
> the detector.
>
> This may be why NEMA specifies the decay and copper plates methods with
> the source at a fixed and at the "magic" 5 UFOV or greater distance
> (although they allow the manufacturer to report results at a closer
> distance, but noting clearly that is the case).
>
> "A camera under test shall have the camera crystal masked to the UFOV.
> The source shall be placed within a source holder, as in Figure 2-5, and
> shall be arranged as in Figure 2-1, except that the distance from the
> source to the detector surface may be less than five times the UFOV. The
> manufacturer shall report the distance that will produce the reported peak
> count rate value (note that the detector may not be fully irradiated at
> this distance). The source holder with the source shall be placed in
> front of the detector, so that the collimated cone of radiation is
> centered within the UFOV."
>
> (It might be interesting to find out what happens with a multi-element,
> a.k.a. pixelated, detector.)
>
> A good question for students or physics residents is why 5 UFOV is a
> "magic" (i.e., acceptable) distance, since ideal parallel flux is only
> obtained at infinite distance.
>
>
> On Tue, Jun 24, 2025 at 11:38 AM Martin, Michael via
> Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu>
> wrote:
>
>> A guess(?): When the source is perpendicular/centered to the face of the
>> camera, some number of PMTs directly in front of the source will be
>> paralyzed, while those on the periphery will still be counting. When the
>> source is at an angle, a fewer
>>
>> A guess(?):
>>
>>
>>
>> When the source is perpendicular/centered to the face of the camera, some
>> number of PMTs directly in front of the source will be paralyzed, while
>> those on the periphery will still be counting.
>>
>>
>>
>> When the source is at an angle, a fewer number of PMTs near the edge with
>> the source are paralyzed, while a larger number are still counting, due to
>> the 1/r^2.
>>
>>
>>
>> I’ve normally done ~ 1 mCi and roughly get the same distance (a few
>> feet).
>>
>>
>>
>> Best,
>>
>>
>>
>> Michael
>>
>>
>>
>>
>>
>> *T. Michael Martin* PhD, DABSNM, DABHP, LMP  |  Sr. Health Physicist and
>> Assistant RSO
>> Environmental Health & Safety | Texas A&M University
>> 4472 TAMU | College Station, TX 77843-4472
>>
>> ph: 979.458.0365  |   *michael.martin at tamu.edu* <michael.martin at tamu.edu>
>> - - - - - - - - - - - - - - - - - - - - - - - -
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>>
>>
>> *From:* Intl_dxmedphys_wd_osu_list
>> <intl_dxmedphys_wd_osu_list-bounces+michael.martin=tamu.edu at lists.osu.edu>
>> *On Behalf Of *Dylan DeAngelis via Intl_dxmedphys_wd_osu_list
>> *Sent:* Tuesday, June 24, 2025 11:16 AM
>> *To:* Nima Kasraie <Nima.Kasraie at UTSouthwestern.edu>; '
>> intl_dxmedphys_wd_osu_list at lists.osu.edu' <
>> intl_dxmedphys_wd_osu_list at lists.osu.edu>
>> *Subject:* Re: [Intl_DxMedPhys] Gamma Camera Max Count Rate
>> Perpendicular versus Off-Angle
>>
>>
>>
>> Hi Nima, What do you consider ‘too close’? The quickest reference I could
>> find is TG 177, which says to use 1 mCi, but doesn’t say about how close
>> that should get you to the detector surface. We aim for between 0. 5 and 1
>> mCi so that we can
>>
>> Hi Nima, What do you consider ‘too close’? The quickest reference I could
>> find is TG 177, which says to use 1 mCi, but doesn’t say about how close
>> that should get you to the detector surface. We aim for between 0. 5 and 1
>> mCi so that we can use
>>
>> Hi Nima,
>>
>>
>>
>> What do you consider ‘too close’? The quickest reference I could find is
>> TG 177, which says to use 1 mCi, but doesn’t say about how close that
>> should get you to the detector surface. We aim for between 0.5 and 1 mCi so
>> that we can use the same point source for sensitivity testing. I’d say we
>> usually hit the max count rate around 2-3 feet from the detector.
>>
>>
>>
>> *Dylan DeAngelis, MS, DABR*
>>
>> *NYS Licensed Medical Physicist*
>>
>> *(Diagnostic Radiological Physics and Medical Nuclear Physics)*
>>
>>
>>
>> *Upstate Medical Physics – *Diagnostic Radiology, Medical Nuclear &
>> Medical Health Physics, P.C.
>>
>> W: 585-924-0350
>>
>> C: 585-441-5096
>>
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>> *From:* Intl_dxmedphys_wd_osu_list <
>> intl_dxmedphys_wd_osu_list-bounces at lists.osu.edu> *On Behalf Of *Nima
>> Kasraie via Intl_dxmedphys_wd_osu_list
>> *Sent:* Tuesday, June 24, 2025 11:14 AM
>> *To:* 'intl_dxmedphys_wd_osu_list at lists.osu.edu' <
>> intl_dxmedphys_wd_osu_list at lists.osu.edu>
>> *Subject:* Re: [Intl_DxMedPhys] Gamma Camera Max Count Rate
>> Perpendicular versus Off-Angle
>>
>>
>>
>> I’ve also noticed angular dependencies. But I usually ignore, since I
>> treat this as more of a consistency check. For my Symbia Intevo Bolds, I’ve
>> set the pass/fail criteria at +/- 5% of Siemens reference value: One also
>> notices variations depending
>>
>> I’ve also noticed angular dependencies. But I usually ignore, since I
>> treat this as more of a consistency check. For my Symbia Intevo Bolds, I’ve
>> set the pass/fail criteria at +/- 5% of Siemens reference value:
>>
>> [image: cid:image001.png at 01DBE501.A58F6DD0]
>>
>>
>>
>> One also notices variations depending on what window you use as well.
>>
>>
>>
>>
>>
>> Plus, are you maybe too close to the head? I use 5mCi.
>>
>>
>>
>>
>>
>> Nima
>>
>>
>>
>> [image: cid:image005.png at 01DBE4FB.E6472CB0]
>>
>> *Nima Kasraie, PhD, MSc, DABR, DABSNM*
>>
>> Lead Physicist, Children’s Medical Center Dallas
>> <https://urldefense.com/v3/__https:/www.childrens.com/__;!!KGKeukY!xmCo1aMNjkpqsHQud5hp1rl55EO3z0pSJokJKaTiOsB48aGqljvm-B-wiY7c6jw4L0ysTtlAYgMK017QAK_SV6UlD5joLP5_JsgMP246FAZd3KudalHipg$>
>>
>> Associate Professor, Department of Radiology, UT Southwestern
>> <https://urldefense.com/v3/__https:/profiles.utsouthwestern.edu/profile/187443/nima-kasraie.html__;!!KGKeukY!xmCo1aMNjkpqsHQud5hp1rl55EO3z0pSJokJKaTiOsB48aGqljvm-B-wiY7c6jw4L0ysTtlAYgMK017QAK_SV6UlD5joLP5_JsgMP246FAZd3Kuw8vs8nA$>
>>
>> Phone: 214-648-7978
>>
>> [image: cid:image003.png at 01DBE501.A58F6DD0]
>>
>>
>>
>> *From:* Intl_dxmedphys_wd_osu_list <
>> intl_dxmedphys_wd_osu_list-bounces+nima.kasraie=utsouthwestern.edu at lists.osu.edu>
>> *On Behalf Of *Dylan DeAngelis via Intl_dxmedphys_wd_osu_list
>> *Sent:* Tuesday, June 24, 2025 9:41 AM
>> *To:* 'intl_dxmedphys_wd_osu_list at lists.osu.edu' <
>> intl_dxmedphys_wd_osu_list at lists.osu.edu>
>> *Subject:* [Intl_DxMedPhys] Gamma Camera Max Count Rate Perpendicular
>> versus Off-Angle
>>
>>
>>
>> Good morning, We assess max count rate on a gamma camera by moving a
>> point source (usually 0. 5-1 mCi) closer to the detector until we
>> observe the peak rate, before it starts to fall off again due to the
>> paralyzable dead time. I’ve
>>
>>
>>
>> Good morning, We assess max count rate on a gamma camera by moving a
>> point source (usually 0. 5-1 mCi) closer to the detector until we
>> observe the peak rate, before it starts to fall off again due to the
>> paralyzable dead time. I’ve always
>>
>> Good morning,
>>
>>
>>
>> We assess max count rate on a gamma camera by moving a point source
>> (usually 0.5-1 mCi) closer to the detector until we observe the peak rate,
>> before it starts to fall off again due to the paralyzable dead time. I’ve
>> always done this with the activity perpendicular to the surface of the
>> detector. But we recently noticed that if you approach the detector at an
>> angle, you can reach a higher count rate. I don’t know if this is true for
>> every unit – I noticed it first on a Siemens Symbia and then made the same
>> observation on an old Philips Vertex. The change is pretty substantial. On
>> the Vertex, measuring perpendicular I observed around 205 kcps, but from an
>> angle I managed to get 236 kcps.
>>
>>
>>
>> In both cases, the dead time of the counting system should be fixed, so
>> there should only be one max count rate, right? How could I be getting two
>> different values based on the angle of approach? Has anyone else seen this?
>>
>>
>>
>> *Dylan DeAngelis, MS, DABR*
>>
>> *NYS Licensed Medical Physicist*
>>
>> *(Diagnostic Radiological Physics and Medical Nuclear Physics)*
>>
>>
>>
>> *Upstate Medical Physics – *Diagnostic Radiology, Medical Nuclear &
>> Medical Health Physics, P.C.
>>
>> W: 585-924-0350
>>
>> C: 585-441-5096
>>
>> https://urldefense.com/v3/__http://www.upstatemp.com__;!!KGKeukY!xvHBMBrfTr6GgnBXMf48gJTbnsoxmMYLfRIGVhKbODYGFv55d8x6PGxwkuT1HyDkmlqd0Yw3Yc50MePToqCUgRDhyBZGTNK2OJZvBqyk$ 
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>>
>>
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