Omdat men het over 'counts' heeft, ligt het voor de hand te veronderstellen dat men het heeft over becquerels: The becquerel (symbol Bq) is the SI derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to s−1
Verdere context ontbreekt waardoor de vertaling (deels) op gissing moet berusten. Misschien een oplossing:
Arm counts = (Gemeten) radioactiviteit (aan de) arm
'Radioactiviteit' = dan het gemeten aantal atoomkerndesintegraties per tijdseenheid
.... in whom left forearm ischemia was induced by inflating a sphygmomanometer cuff to supra systolic pressure for 4.5 min. Later, dynamic acquisition (2 s frame/min) was initiated after the injection of technetium-99m methoxy-isobutyl isonitril into the dorsal pedal veins. Equivalent regions of interest were drawn on both arms to detect total activity counts during 1 min and the perfusion ratios (left arm/right arm) were calculated. The left arm counts (22,203.3+/-12,372.7) were significantly higher than the right arm counts (9,980.9+/-5,931.9)
http://www.ncbi.nlm.nih.gov/pubmed/16501298
The translation workplace 
