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

Tue Jun 24 13:27:58 EDT 2025

Hello,

This is me speculating of course, but there is certainly some expected dependency occurring with angle. I would surmise that at closer distances, you see more angular dependence as per:

[cid:image004.jpg at 01DBE502.2703D720]

where theta is angle of a differential surface element sigma-a with respect to the normal (Lawson p.122):

[cid:image005.png at 01DBE502.2703D720]

So d would have to be sufficiently large in order to ignore the Cos3 dependency.

Although I have 5mCi in my notes from my mentor Jon Anderson, but I typically end up using ~2-3 mCi, which puts me at ~6ft from the heads.

Nima

[cid:image006.png at 01DBE502.2703D720]
Nima Kasraie, PhD, MSc, DABR, DABSNM
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[cid:image007.png at 01DBE502.2703D720]

From: Dylan DeAngelis <ddeangelis at upstatemp.com>
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 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:image008.png at 01DBE502.2703D720]

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

[cid:image006.png at 01DBE502.2703D720]
Nima Kasraie, PhD, MSc, DABR, DABSNM
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[cid:image007.png at 01DBE502.2703D720]

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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
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(Diagnostic Radiological Physics and Medical Nuclear Physics)

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