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

Dylan DeAngelis ddeangelis at upstatemp.com
Tue Jun 24 10:40:30 EDT 2025


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.
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