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

Tue Jun 24 12:36:36 EDT 2025

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
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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
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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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(Diagnostic Radiological Physics and Medical Nuclear Physics)

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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:

[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

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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)

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