The flat panel detector consists of 512 × 512 pixels, each of 400 µm-squared, and would provide CBCT images with a limiting voxel resolution of 0.27 mm × 0.27 mm × 0.27 mm at an image magnification of 1.5. For routine application, the CBCT volume was reconstructed as a 256 × 256 × 256 matrix at a voxel resolution of 0.55mm × 0.55mm × 0.55mm. From the initiation of image acquisition to the completion of CBCT reconstruction, the continuous rotate-acquisition procedure took 4 minutes, whereas the “stop and capture” procedure took about 7 minutes. Differences in the reconstructed CBCT quality were insignificant, if at all noticeable. In the continuous rotate-acquisition mode, a complete revolution of the stage took 65 sec, resulting in the animal receiving 0.85 cGy in mid-volume as measured with EBT film in the cylindrical solid water phantom. The surface dose was similar.
shows a sagittal slice of the CBCT of an anesthetized mouse scanned in the prone position. shows a coronal slice of the same mouse. The mouse was immobilized with a custom head holder equipped with ear-pins and bite-block.
Figure 5 (a) A sagittal slice of cone-beam CT of an anesthetized mouse scanned in the prone position. (b) A coronal CBCT slice of the same mouse. The mouse was immobilized with a custom head holder equipped with ear-pins and bite-block. The projection images were (more ...)
lists the dose outputs at 1 cm or 0.5 cm depth in solid water measured with EBT films for different field sizes, and different added filtration. When 4 mm Al was used for added filtration, the output at 1 cm depth was high; about 200 cGy/min for a 1.3 mm diameter beam and 375 cGy/min for a 60 mm × 60 mm beam. The outputs were approximately halved when the aluminum filtration was replaced with 0.5 mm Cu. Addition of 2 mm Al to the 0.5 mm Cu did not alter the output significantly.
Table 1 Radiation output of the SARRP in cGy/min for a range of field dimensions at 35 cm SSD. The Seifert tube was operated at 225 kVp, 13 mA using the larger (3 mm) focal spot, except when noted, with either Al and/or Cu added filtration. A tube current of (more ...)
shows cross-beam profiles measured at different depths in solid water for a 30 mm × 30 mm beam at a SSD of 33.5 cm, in comparison with the Monte Carlo calculations provided by the Pinnacle3 research and EGSnrc codes respectively. The added filtration was 4 mm Al. The Monte Carlo calculations were performed to achieve 2% uncertainty. The agreement is generally good. The largest discrepancies of about 10% occur in the low dose region outside the geometric beam edges where contributions from scattered photons predominate. The measured cross-beam profiles for the smallest beams of 1 mm and 0.5 mm diameters, respectively, at depths of 1, 2 and 4 cm in a solid water phantom are shown in , respectively. The added filtration of 0.5 mm Cu was employed for these measurements. The 1 mm diameter beam was produced by the 3mm focal spot, while the 0.5 mm diameter beam by the 0.4 mm focal spot. A picture of the EBT film at 1 cm exposed by the latter beam is shown in . The span of the 80% to 20% penumbra at 1 cm depth was 0.16 mm for the 0.5 mm beam; and about 0.3 mm for the 1 mm beam. While both x-ray beams were expected to pass through x=0 cm on the graphs, the profiles suggest that the beams were shifted by about 0.5 mm.
Figure 6 (a) Measured cross-beam profiles at depths in plastic water for a 30 mm × 30 mm beam at a SSD of 33.5 cm, in comparison with the Monte Carlo calculations by the Pinnacle3 research (blue) and EGSnrc (pink) codes respectively. The added filtration (more ...)
shows the EGSnrc generated isodose distributions overlaid on a coronal CT slice of one of the mice irradiated by the fifteen 1 mm diameter beams arrangement as shown in . The Monte Carlo calculations were performed to attain 5% statistical uncertainty. The added filtration was 4 mm Al. The presence of dose in air is due to the presence of non-zero density (noisy) voxels encountered by the EGSnrc calculations. The 50% isodose line circumscribes an elliptical region that spans less than 3 mm in its long axis. Dose enhancement is observed in the thicker portion of the skull as photon-electric interactions are more appreciable at 225 kVp than at the megavoltage energies used for human treatment. This observation also motivated the use of 0.5 mm Cu filtration to further harden the beam. shows the corresponding dose volume histogram of the irradiated mouse brain, contoured as the soft tissue density volume encased by the skull. No more than 3% of the brain volume received more than 50% of the prescribed dose. No difference was observed between the T2 weighted MRI scans of the irradiated region acquired prior to irradiation, 1 day post- and 35 days post-irradiation.
Figure 7 (a) The EGSnrc generated isodose distributions overlaid on a coronal CT slice of one of the mouse brains irradiated by the fifteen 1 mm diameter beams arrangement as shown in . The “dose in air” artifacts are due to non-zero density (more ...)
shows a double-exposed EBT film of the mouse brain irradiated in our second feasibility experiments where a single 3 mm × 3mm beam was directed posteriorily to the right hemisphere of mouse. shows a merged image of the sectioned mouse brain that was stained with DAPI for cell nuclei and with antibody against γ-H2AX for correspondence with radiation-induced DNA strand breaks. As expected, the entire section shows DAPI staining. However, there is an apparent sharp demarcation of a region that also shows γ-H2AX staining. This image supports the focal radiation damage resulting from the sharp dose fall-off of the 225 kVp beam. However, the evidence is circumstantial as the experiment did not include rigorous geometric registration that would validate the coincidence of the irradiation and γ-H2AX regions. As such, a beam edge was suggested on the figure. Nevertheless, no other γ-H2AX regions were observed from the sectioned sample.
Figure 8 (a) A double-exposed EBT film of the mouse irradiation where a single 3 mm × 3mm beam was directed posteriorily to the right hemisphere. (b) A merged image of the sectioned mouse brain that was stained with DAPI for cell nuclei and with antibody (more ...)