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Cases of Radiation Dose at CT Scan Range Center Decreasing with Longer Scan Lengths
X Li*, K Yang , M DeLorenzo , B Liu , Massachusetts General Hospital, Boston, MA
Presentations
MO-RPM-GePD-I-6 (Monday, July 31, 2017) 3:45 PM - 4:15 PM Room: Imaging ePoster Lounge
Purpose: Radiation dose at CT scan range center usually increases with scan length. This presentation was to illustrate cases in which radiation dose at scan range center may decrease with longer scan lengths.
Methods: A constant and two variable mA distributions from a previous paper were adopted in three scan lengths of 10, 28.6, and 50 cm, and all mA distributions had the same average mA over all scan ranges centered at z=0. Using the symmetry based dose calculation algorithms and CT dose equilibration data in the literature, the authors calculated longitudinal dose profile D_L(z) on the central axes and the peripheral axes of 50 water cylinders (diameters 6-55 cm).
Results: With constant tube current or an mA curve peaked at z=0, D_L(z=0) followed the usual trend of asymptotically increasing with scan length. But with an mA curve that had a valley at z=0 and two peaks near scan range edges, D_L(z=0) decreased with longer scan lengths on the phantom peripheral axes. On the phantom central axes, D_L=28.6cm(z=0) was less than D_L=10cm(z=0) for diameter < 23 cm, D_L=50cm(z=0) was less than D_L=10cm(z=0) for diameter < 37 cm, and D_L=50cm(z=0) was less than D_L=28.6cm(z=0) for all diameters.
Conclusion: Radiation dose in CT may be dominantly contributed by scattered radiation from adjacent scans. The dispersion of scatter across the scanned range is limited by solid angle acceptance of scatterings and attenuation. The dose at the scan range center may increase or decrease with scan length, depending on tube current profile, phantom diameter, and phantom axis.
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