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The upper limit of the amount of x-rays that are scattered from a SPECT/CT room and are acquired by an adjacent
gamma camera is estimated using physical principles and approximations. Methods: We first estimated the amount of xrays
scattered from the patient to the ceiling of the SPECT/CT room, then the amount scattered from the ceiling through
the gap between the ceiling and the top of lead walls to reach outside of the room, and finally the amount acquired by an
adjacent gamma camera into the Tl-201 data. Results: The counts of scattered x-ray photons acquired in the Tl-201
energy window can reach 0.12% of the CT primary counts when the standard 2.13 m high lead walls are used for the
SPECT/CT room. Due to the high CT counts, contamination to the Tl-201 data cannot be ignored. It is not effective to
reduce the contamination by increase the lead height or change the floor plan because the scattered x-rays reduce
moderately with increasing lead height or different floor plans. When the lead height increases from 2.13 m to 2.74 m,
for example, the amount of scattered x-rays only decreases by 20%. With the same 2.13 m lead height, there is little
difference in the amount of scattered x-rays for three different floor plans. Conclusions: The standard lead walls for a
SPECT/CT room cannot prevent scattered x-rays from severe contamination to the Tl-201 data acquired by an adjacent
gamma camera. Since dramatic increase of lead height is costly and often prohibitive due to the heavy load, we
recommend that Tl-201 studies be stopped when an adjacent CT scanner is in operation.
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