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Physical comparison of gamma camera and two-headed scanner: figures of merit and times required for threshold detection of spherical sources

Medical Physics Department, Royal Postgraduate Medical School, Hammersmith Hospital, London, W.12

Physical comparisons are described between a scintillation camera and a two-headed scanner for their efficiency in detecting radioactive spheres in tanks of water containing radioactivity at different concentrations. For these comparisons, the previously defined figure of merit has to be modified to use "counts recorded per integrating time, i.e. time spent over the abnormal region" instead of "per minute". A new quantity, "merit-time-product", is defined and used for this purpose. It is then possible to compare scanners moving at different speeds with each other or with cameras, on the same basis.

Results are presented showing the total times required for each device to scan an area, so that a given level of significance is achieved in the threshold detection of active spheres in the presence of background radioactivity. The scintillation camera requires shorter times. The differences are small for ¹³¹I, but exceed factors of 20 in overall time for ^99mTc. These factors decrease with depth in the scintillation camera, from 20 to 30 at 7 cm to about 10 at 10 cm depth. Possibly the most striking difference between the camera and the two-head scanner is the high efficiency of the camera for relatively shallow tumours and the rapid decrease of efficiency with depth.

Received for publication January 1, 1968.