Sensitivity degradation of an anger camera operated in SPECT-like mode under the influence of a strong external magnetic field

Abstract

The purpose of this work was to experimentally determine the degradation in sensitivity of an Anger camera rotated in SPECT-like orbits around the transverse and sagittal planes of the magnetic field produced by a conventional, dual coil, 1T electromagnet. A 74 photomultiplier Siemens Basicam Anger camera with a 29cm radius crystal and an Isotrak 35cm diameter, 46MBq (1.25mCi), Co-57 disk source attached to a low energy general purpose collimator, were used for all measurements. A custom made, air-cooled, dual coil, 1T electromagnet was used to produce the external magnetic field. A map of the magnetic field was obtained by taking intensity measurements around the sagittal and transverse planes of the magnet. Camera sensitivity – defined as the measured count rate for a given activity of a radionuclide in a defined geometry – was first measured around the transverse plane at angles of 0°, 90°, and 270°, with, and without, the magnetic field present. At each angle, three 30min measurements were made and the average count rate was calculated. A similar protocol was used for measurements upon rotation in the sagittal plane: counts per 30min interval were measured for 20 angles, with a 15° increment between measurements. Camera sensitivity as a function of field strength was also determined by collecting counts over 30min intervals at a fixed angle (90°) with magnet currents of 0.00A, 2.65A, and 5.30A. In the transverse plane, at 0° under a field intensity of 21mT, the loss in sensitivity was 18.14%, at 90° (B=37mT) the loss was 30.5%, and at 270° (B=38mT) the loss was 34.9%. Thus for rotation in the transverse plane, the sensitivity is monotonically reduced with an increase in field intensity. On rotation in the sagittal plane, sensitivity degradation ranged between 50.3% at a 22° angle, and 59.1% at 315°. Broad sensitivity peaks were observed at 105° and 195°, with minima at 60°, 135°, and 260°, consistent with our theoretical predictions, as well as previously reported measurements. In this work, the sensitivity variations of a conventional Anger camera rotated in SPECT-like orbits around a conventional dual solenoid electromagnet were measured. Sensitivity variations were studied upon rotations in the transverse and sagittal planes of the magnet. Measured sensitivity losses upon rotation in the transverse plane ranged between 18% and 35%; those in the sagittal plane ranged between 51% and 59%. Upon rotation in the transverse plane, the camera sensitivity is monotonically reduced with an increase in B field intensity. Upon rotation in the sagittal plane, cyclic variations with an angular period of approximately 90° were observed, consistent with theoretical considerations and previously published work.