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

Wed Jun 25 10:18:10 EDT 2025

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
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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*
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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
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>
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> *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
>
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