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

[Nima Kasraie]

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 01DBE4F0.BF270110]

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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Nima Kasraie, PhD, MSc, DABR, DABSNM
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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)

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