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Half-Fanbeam Collimators Combined with Scanning Point Sources for Simultaneous Emission-Transmission Imaging

Department of Nuclear Medicine, Image Sciences Institute, University Hospital Utrecht, Utrecht, The Netherlands; Department of Medical Physics, Westmead Hospital, Sydney, France

Correspondence: For correspondence or reprints contact: Freek J. Beekman PhD, University Hospital, Utrecht, E 02.222, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands.

ABSTRACT

One type of SPECT system often used for simultaneous emission-transmission tomography is equipped with parallel-hole collimators, moving line sources (MLS) and electronic windows that move in synchrony with the sources. Although downscatter from the emission distribution is reduced by the use of the electronic window, this still can represent a sizable fraction of the transmitted counts. These systems have relatively poor spatial resolution and use costly transmission sources. Methods: Using a two-head SPECT system, with heads at right angles, two ¹⁵³Gd line sources (5800 MBq each) were replaced by two ¹⁵³Gd point sources of only 750 MBq each and positioned to move along the focal lines of two half-fanbeam collimators. A suitable acquisition protocol for a moving point source (MPS) system was selected by considering the results of a simulation study. With this protocol, physical phantom experiments were conducted. Results: Simulations showed that by using two half-fanbeam collimators, a gantry rotation of 90°, such as used for 180° acquisition with parallel-beam collimators for cardiac imaging, was insufficient. A gantry rotation of 180° resulted in attenuation maps where only an area to the posterior of a 400-mm wide thorax phantom was affected by truncation. The MPS system had a 14.7 times higher sensitivity for transmission counts than the MLS system. Despite the smaller sources in the MPS system, the number of acquired transmission counts was a factor 1.91 times higher compared with the MLS system, resulting in reduced noise. The relative downscatter contribution from ^99mTc(140 keV) in the ¹⁵³Gd moving electronic window (100 keV) was reduced by a factor of 1.81. Transmission images of a rod phantom with segments containing acrylic rods of different diameters showed an improvement of resolution in favor of the MPS system from about 11 mm to about 6 mm (five instead of two segments of rods were clearly visible). In addition, the noise level in the MPS thorax transmission images was significantly lower. Conclusion: The MPS system has important advantages when compared with the MLS system. The use of low-activity point sources is economically beneficial when compared with line sources and reduces radiation exposure to staff and patients.

Key Words: quantitative SPECT • attenuation correction • transmission scanning • fanbeam collimator

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				H. Zaidi and B. Hasegawa Determination of the Attenuation Map in Emission Tomography J. Nucl. Med., February 1, 2003; 44(2): 291 - 315. [Abstract] [Full Text] [PDF]