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Synthetic Aperture Imaging : Applications In High-Frequency Ultrasound

Purpose: To explore the possibility of separately quantifying the leakage and scatter components of the peripheral radiation dose resulting from Leksell GammaKnifeR Perfexion™ radiotherapy. Methods and Materials: In this study, peripheral dose was defined as the dose arising at points outside the paths of the primary beams. Consider a spherical skull phantom centered at Leksell coordinates (100,100,100). Peripheral dose may arise from: (1) Inphantom scattered radiation originating from primary beams; (2) Radiation first scattered within the collimator structure (3) Photons undergoing multiple scattering and exiting through adjacent collimator openings and (4) Leakage radiation that passes directly through the collimator body. Contributions (1) and (2) were separately quantified via Monte‐Carlo simulations. Types (3) and (4) were measured using EBTII Gafchromic film inserts and placing the sources in“beam‐off” position (i.e. between collimator openings). Additionally, the total peripheral dose arising from a 16‐mm shot was measured using film. The combination of simulated type_1 and type_2 doses and measured type_3 and type_4 were compared to the measured total dose at 40 mm superior and inferior to the isocentre. Results: At the isocentre of the 16‐mm shot, the dose‐rate determined by Monte Carlo simulations agreed with measured dose‐rates to 0.8%, thereby establishing consistency between measurement and simulation. The simulated sum of Type_1 and Type_2 contributions at 40 mm inferior and superior to the isocentre was 0.40% and 0.35% of the isocentre dose‐rate, respectively. Of those totals, 87% was Type_1 a 13% came from Type 2. The measured total peripheral dose‐rate was 0.40% and 0.38% at 40 mm inferiorly and superiorly, respectively. The measured Type_3 and Type_4 dose‐rates were less than 0.01% of the isocentre dose‐rate in either direction. Conclusion: The results of this study indicate that peripheral dose arising from a single 16‐mm shot arises primarily from Type_1, with <0.01% contribution from leakage radiation.

Purpose: To quantify inter‐fraction variation in upper‐spine (C1–C3) positioning when using a novel vacuum bite‐block relocatable head‐frame (RHF), that was originally designed for use in intra‐cranial stereotactic radiotherapy (SRT). Methods and Materials: The RHF (Extend™) was designed for multi‐session treatments on Perfexion™. As part of an ethics‐approved study, the RHF was used to set up and immobilize patients undergoing linac‐based intra‐cranial SRT. For each fraction, cone‐beam CT (CBCT) was used to guide the skull to the treatment room isocentre based on anatomical 3D image‐matching. In this study, the same CBCT data were analyzed to quantify the relative displacement of the upper‐spine to an ideally well‐aligned skull as follows. For each patient, each daily CBCT image set was first co‐registered to the planning CT images based on bony anatomy in the skull. From this reference position, the mean difference and standard deviation between planning and daily treatment was determined for each of the geometrical centroid of the C2 process, as well as the spinal canal at level of C3. Results: Data from three patients were analyzed (total of 80 fractions). The mean relative displacement of the C2 process after skull alignment was 0.9±0.78 (range: −1.84–2.7) and 1.2±0.57 (range: −1.15–3.2) mm in the lateral and anterior‐posterior directions, respectively. The mean displacement at the level of C3 was 0.35±2.04 (range: −4.1–6.5) and 1.59±1.2 (range: −4.2–4.6) mm in the lateral and anterior‐posterior directions, respectively. Conclusion:_Relative to an ideal skull‐based alignment as determined with 3D‐CBCT, systematic differences in upper‐spine position were observed, as indicated by the non‐zero mean deviations. Furthermore, a larger variability in daily positioning of C3 compared to C1/C2 was observed, as indicated by the larger standard deviations. These results indicate that applying translational corrections intended for skull localization may be insufficient for localization of the upper‐spine when using the RHF.

Purpose: To evaluate the immobilization accuracy of a novel vacuum bite block re‐locatable head frame (RHF) system in patients receiving intracranial stereotactic radiation therapy (SRT) by means of an infrared optical tracking system (OTS). Methods and Materials: The RHF was designed for multi‐session treatments on the Leksell GammaKnife Perfexion™. In the present REB approved study, the RHF was used for patients undergoing linac‐based SRT. A passive optical marker was placed on the bite block as a surrogate for patient motion. The OTS monitored patients continuously throughout each fraction. For each fraction, cone‐beam computed tomography (CBCT) was used to verify the patient position at three time points: after the initial setup, pre‐delivery following the automatic couch adjustments and post‐delivery. To investigate the reliability of the optical measurements, the OTS measurements were compared against the implemented couch shifts for patient setup. Intra‐fractional anatomical motion, as measured from the second and third CBCT scans, was compared with the difference in OTS marker positions. Real‐time OTS data throughout each fraction was assessed to determine if patients moved by more than 1 mm during treatments. Results: Currently, data from three patients have been analyzed. The couch motion measured from the OTS strongly correlated with the actual reported couch shift (Pearson correlation coefficient of 0.99). Mean differences between OTS and true couch motion were 0.01, 0.0, and 0.04 mm in the lateral, anterior‐posterior, and superior‐inferior directions, respectively. Mean differences between OTS and CBCT reported motion were 0.37, 0.07, and 0.01 mm respectively with a mean vector magnitude of 0.41 mm. The real‐time positional data indicated that there were few instances (3) of patient motion greater than 1mm. Conclusion: The OTS is a reliable tool for evaluation of intra‐fraction patient motion. The results of this study indicate that the RHF provides sufficient immobilization accuracy for SRT.