M Bennett

PET image reconstruction using LOR-OSEM with a 3D spatially variant system matrix

A point source was used to sample the point spread function (PSF) at over 6000 locations within the field of view (FOV) of a General Electric Discovery ST PET scanner (DST) in 2D high sensitivity acquisition mode. These measurements were used to optimize the derivation of a system matrix for the DST. We found for 2D acquisition mode that a system matrix using a PSF with radial, depth, axial, and azimuthal dependence produced reconstructed images with greatly improved spatial resolution and contrast-noise ratios over the entire FOV, as compared to the use of a geometrically derived system matrix. The main improvements in resolution and contrast-noise resulted from the inclusion of depth dependence in the model, which accounts for large variations in sensitivity in the DST that are due to the septa and a span of 11 present in 2D acquisition. Finally, we determined that exploitation of symmetries, particularly along the axial dimension, allow a system matrix of similar quality to what we achieved with over 6000 samples to be created with just over 1000 samples, i.e. with an almost 80% reduction in sample size.

High resolution polymer gel dosimetry for small beam irradiation using a 7T micro-MRI scanner

The use of small field radiation beams has greatly increased with advanced radiation therapy techniques such as IMRT, rotational IMRT, and stereotactic body radiotherapy. In this work small field 3D dose distributions have been measured with high spatial resolution using polymer gels and 7T micro-MR imaging. A MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) polymer gel [1] phantom was used to capture the 3D dose distributions for two small field (5 × 5 mm2 and 10 × 10 mm2) for a 6MV x-ray beam. High resolution 3D T2 maps were obtained with 7T micro-MRI (0.156mm × 0.156mm × 1mm, MSME pulse sequence). For comparison T2 maps, the gel phantom was scanned in a 3T MRI clinical scanner (0.254mm × 0.254mm × 2mm, FSE pulse sequence). Normalized 3D dose maps were calculated in Matlab. Results show that 7T micro-MRI 3D gel dosimetry measurements are much more stable, less noisy, and have higher spatial resolution than those obtained using a 3T clinical scanner for the same amount of scan time. In general, 3D gel dosimetry results also agree with simultaneously-obtained radiochromic film dosimetry. This study indicates that the MAGIC polymer gel with 7T micro-MRI for 3D dose readout could potentially be used for small radiation beams, including measurements for micro-beams (field size ~ 100um).

Improved volumetric imaging in tomosynthesis using combined multiaxial sweeps

This study explores the volumetric reconstruction fidelity attainable using tomosynthesis with a kV imaging system which has a unique ability to rotate isocentrically and with multiple degrees of mechanical freedom. More specifically, we seek to investigate volumetric reconstructions by combining multiple limited‐angle rotational image acquisition sweeps. By comparing these reconstructed images with those of a CBCT reconstruction, we can gauge the volumetric fidelity of the reconstructions. In surgical situations, the described tomosynthesis‐based system could provide high‐quality volumetric imaging without requiring patient motion, even with rotational limitations present. Projections were acquired using the Digital Integrated Brachytherapy Unit, or IBU‐D. A phantom was used which contained several spherical objects of varying contrast. Using image projections acquired during isocentric sweeps around the phantom, reconstructions were performed by filtered backprojection. For each image acquisition sweep configuration, a contrasting sphere is analyzed using two metrics and compared to a gold standard CBCT reconstruction. Since the intersection of a reconstructed sphere and an imaging plane is ideally a circle with an eccentricity of zero, the first metric presented compares the effective eccentricity of intersections of reconstructed volumes and imaging planes. As another metric of volumetric reconstruction fidelity, the volume of one of the contrasting spheres was determined using manual contouring. By comparing these manually delineated volumes with a CBCT reconstruction, we can gauge the volumetric fidelity of reconstructions. The configuration which yielded the highest overall volumetric reconstruction fidelity, as determined by effective eccentricities and volumetric contouring, consisted of two orthogonally‐offset 60° L‐arm sweeps and a single C‐arm sweep which shared a pivot point with one the L‐arm sweeps. When compared to a similar configuration that lacked the C‐arm component, it is shown that the C‐arm improves the delineation of volumes along the transverse axis. The results described herein suggest that volumetric reconstruction using multiple, unconstrained orthogonal sweeps can provide an improvement compared with traditional cone beam CT using standard axial rotations. PACS number: 87.57.nf

Mechanisms and prevention of thermal injury from gamma radiosurgery headframes during 3T MR imaging

Magnetic resonance imaging (MRI) is regularly used for stereotactic imaging of Gamma Knife (GK) radiosurgery patients for GK treatment planning. MRI‐induced thermal injuries have occurred and been reported for GK patients with attached metallic headframes. Depending on the specific MR imaging and headframe conditions, a skin injury from MRI‐induced heating can potentially occur where the four headframe screws contact the skin surface of the patients head. Higher MR field strength has a greater heating potential. Two primary heating mechanisms, electromagnetic induction and the antenna effect, are possible. In this study, MRI‐induced heating from a 3T clinical MRI scanner was investigated for stereotactic headframes used in gamma radiosurgery and neurosurgery. Using melons as head phantoms, optical thermometers were used to characterize the temperature profile at various points of the melon headframe composite as a function of two 3T MR pulse sequence protocols. Different combinations of GK radiosurgery headframe post and screw designs were tested to determine best and worst combinations for MRI‐induced heating. Temperature increases were measured for all pulse sequences tested, indicating that the potential exists for MRI‐induced skin heating and burns at the headframe attachment site. This heating originates with electromagnetic induction caused by the RF fields inducing current in a loop formed by the headframe, mounting screws, and the region of the patients head located between any of the two screws. This induced current is then resistively dissipated, with the regions of highest resistance, located at the headframe screw–patient head interface, experiencing the most heating. Significant heating can be prevented by replacing the metallic threads holding the screw with electrically insulated nuts, which is the heating prevention and patient safety recommendation of the GK manufacturer. Our results confirm that the manufacturers recommendation to use insulating nuts reduces the induced currents in the headframe nearly to zero, effectively preventing heating and minimizing the likelihood of thermal injury. PACS numbers: 87.57.‐s, 87.61.‐c, 87.61.Tg, 87.57.c‐

SU‐D‐BRB‐02: Monte Carlo Modeling of the Gamma Knife Perfexion

Purpose: For dosimetric and research irradiation studies, we have implemented a Monte Carlo (MC)dose computation model based on the physical and radiological characteristics of the Gamma Knife Perfexion (GK PFX) using the Penelope MCdosimetry codes. GK dosimetric aspects examined include: 1) output factors (OF) for each of the three GK collimator sizes (4, 8, 16 mm), 2) OFs for each source row and collimator size, and 3) dose distribution profiles. Methods: Vendor proprietary information facilitated our modeling of the GK PFX irradiation geometry, which was mathematically defined within Penelope. MC simulations were carried out on a Linux cluster. 3D dose distributions were analyzed using Matlab. A 16 cm diameter dosimetry sphere was modeled with a virtual detector volume at its center. Detector volume varied from 33 to 590 mm3 to study detector volume effects. A single source per row was modeled for five rows for each collimator (15 beams modeled). Single‐source dose distributions were rotated about the z‐axis of the axially symmetric geometry and summed to simulate all 192 sources. Results: Good agreement is found for row‐ and total‐output factors (greatest deviation <2% for the 4 mm collimator) compared to reference values. Simulated and measured full‐width at half‐ max values of 3D dose distribution profiles show sub‐millimeter differences (0.4 mm, 4 and 8 mm collimators; 0.9 mm, 16 mm collimator). There is excellent agreement for integrated profile shapes. Conclusions: Detailed geometric representations (radiation source, device components) of the GK PFX are required for high fidelity MC simulations. Calculated GK PFX OF values are dependent on the simulated detector volume size (4 mm OF most dependent). Our model shows strong agreement for the GK PFX OFs and dose profile shapes compared to reference values. Acknowledgement: Non‐disclosure agreement for proprietary information with Elekta AB. No financial contribution.

SU‐E‐T‐116: Measuring Dose Distribution Accuracy in Stereotactic Radiosurgery and Gamma Knife Treatment Using MR Or CT Imaging

Purpose: This study aimed to establish a standard dosimetry protocol for HDR Ir‐192 sources using an ion chamber calibrated with a Co‐60 beam. We developed a dedicated device for ion chambermeasurements with a sandwich method and examined its measurement accuracy. Methods: A microSelectron‐v2 HDR Ir‐192 source was modeled with the EGSnrc/egs_chamber code. The accuracy of modeling was confirmed by comparing calculated results for gL (r) and F(r, angle) with those of TG‐43. First, an optimal source‐to‐chamber (SCD) separation for Ir‐192 dosimetry was determined from measurements with a PTW 31010 chamber at distances of 1.5–5 cm from the source center in water. The measuredionization chamber reading was corrected with the Monte Carlo‐calculated energy response for Co‐60 and Ir‐192, and was converted to the absorbed dose to water. The measured doses were compared with TPS values based on TG‐43. We developed a dedicated device for ion chambermeasurements with a sandwich method at the optimal SCD separation. The average dose measured with two EXRADIN A1SL chambers was compared with the TPS value. Results: Calculated gL (r) and F(r, angle) values agreed well with those of TG‐43. The absorbed dose to water measured with the PTW31010 chamber was 3% lower than that of TPS at a distance of 5 cm and was 3%‐7% lower at distances less than 5 cm. This was addressed to the uncertainty of the chamber positioning. We made a sandwich measurement device with the separation of 5 cm, considering the uncertainty of positioning and measurement time. The dose to water with the sandwich method was in agreement with that of TG‐43 within −1.2%. Conclusions: The optimal distance for ion chambermeasurements was at 5 cm from the Ir‐192 source. The dose to water measurement with the sandwich method is useful for daily dose management for Ir‐192 sources.

SU‐E‐J‐80: Small Beam Dosimetry Using MAGIC Gel with a 7T Micro‐MRI Scanner

Purpose: Dose distribution characteristics for small radiation fields can be very difficult to determine. In this work a MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) 3D polymergel is combined with 7T micro‐MR imaging for high resolution measurements of the small field (<1cm) 3D dose distributions. Methods: MAGIC(Gelatin 9%; Methacrylic acid 4%; CuSO4 0.1 mM; Ascorbic ascid 2mM; Glucose 22%) 3D gel phantoms were irradiated with very small 6MV x‐ray beams (5x5 mm2 and 10 × 10 mm2 square fields; 2mm diameter round field). Gel dose measurements were performed in Bruker 7T mirco‐MRI and GE Signa 3T scanners and with simultaneously‐obtained radiochromic films. T2 maps were acquired using a 10‐echo‐Multi‐Spin Multi‐Echo (MSME) pulse sequence on both MR scanners. Normalized 3D dose maps were calculated in Matlab. Results: Dose distributions determined from 30 minute scans for the 5×5 mm2 and 10×10 mm2 square fields on the 7T MR unit were superior to 3T MR unit in spatial resolution (7T: 0.156mm × 0.156mm × 1mm voxel ; 3T: 0.254mm × 0.254mm × 2mm voxel). For the very small field size (2mm diameter), the MAGIC gel with 7T MRI provided even better quality dose distribution images (1.5hour scan; spatial resolution 79um ×79um × 1mm and ; 12 hour scan: spatial resolution 38um × 38um × 1mm), 3T MRI was not able to read accurate dose profile due to a low SNR in the same 1.5hour scan time. Conclusions: This study indicates that the MAGIC polymergel with 7T micro‐MRI for 3D dose readout can potentially be used for dosimetric characterization of very small radiation beams, including measurements for micro‐beams (field size ∼ 100um). Techniques and limitations for MAGIC geldosimetry via high field MR imaging for dosimetric assessment are detailed in this work.

SU‐GG‐T‐512: Causes and Prevention of MR‐Induced Skin Heating for Patients with Attached Headframes for Gamma Radiosurgery

Purpose: We have investigated the potential for magnetic resonance imaging(MRI) induced skin heating for gamma radiosurgery patients with attached rigid headframes. MRI‐induced heating through three mechanisms may be possible where the four headframe screws contact the skin surface of the patients head. Method and Materials: Using melons as head phantoms, optical thermometers were inserted sub‐surface at selected points to measure the temperature profile of the melon‐headframe composite as a function of the applied 3T MR pulse sequence. Multiple headframe post and screw combinations, representing possible clinical scenarios, were evaluated for MRI‐induced heating.Results: The potential exists for a range of MRI‐induced skin heating from 2–10 C or more at the attachment sites of the radiosurgical headframe. This localized heating originates with the RF fields inducing current in a loop formed by the headframe, mounting screws and the region of the patients head located between any of the two screws, with the loop in a position perpendicular to the RF field. This current is then resistively dissipated, with the regions of highest resistance, the screw‐patient interface, experiencing the most heating. Thus skin heating, including burns, is a potential hazard for gamma radiosurgery patients during MRI scans. However, this hazard is easily prevented by replacing the metallic threads holding the screws with electrically insulated nuts that prevent the formation of current loops. This method has been confirmed and is a recommendation of the gamma units manufacturer. Conclusion: MRI‐induced heating of the skin has been investigated for patients with rigidly attached headframes. Using a melon‐phantom system the cause of heating and potential burns has been determined for selected 3T MR imaging sequences and headframe‐screw combinations. The recommended method for prevention of MRI‐induced skin heating with an attached gamma radiosurgery headframe has been verified. Disclosure: Supported in part by NIH T32‐CA113267.

SU‐GG‐T‐532: A Method for Dose Calculation and Collision Detection in Gamma Plan Pre‐Planning Mode

Purpose: The latest version of the Gamma Plan treatment planningsystem allows for treatment “pre‐planning” using an image set with no headframe. However, in pre‐planning mode the user is not able to define the Gamma Knife coordinate system, so collision checks and dose time calculations cannot be performed. This restriction is particularly limiting when pre‐planning head and neck cases or cases for lesions located in the posterior fossa. We have developed a simple method to establish the Gamma Knife coordinate system and to therefore run collision checks and dose time calculations. Method and Materials: An image set of the Gamma Knife headframe and fiducial box is imported into Gamma Plan and opened in treatment mode. An MR or CT patient image set without a headframe is imported into the same treatment planning window. The two image sets are then co‐registered and fused. The resulting composite images may then be used to plan a treatment with the full Gamma Plan functionality, including collision monitoring and dose time calculations. Results: The headframe image set was successfully co‐registered and fused to patient image sets and used for treatment planning. The fused image set was then able to be checked for collisions and dose delivery times were able to be calculated. Conclusion: We have developed a simple method that allows for the Gamma Knife coordinate system to be established in Gamma Plan pre‐planning mode. This technique lets the user check for collisions and calculate dose times prior to headframe placement. It also may serve as an aid for determination of headframe placement on treatment day. The main limitation of this method is that it does not allow for gamma angles not equal to 90. Supported in part by NCI T‐32 CA113267.

SU-GG-I-27: Adaptive Projection Weighting in CBCT Using Laser Profilometry

PURPOSE: Laser profilometry is a three‐dimensional surface reconstruction technique which bases volume delineation on data acquired using a laser range‐finding system.Fusion of such a system with cone‐beam CT(CBCT)imagingsystems shows promise in improving imaging fidelity without imagingdose escalation. In filtered back‐projection (FBP), the algorithm most commonly used in computed tomography,images are formed by smearing projection pixel intensities throughout the reconstruction volume. Imagecontrast and edge definition can suffer as a result of pixel incrementation outside of where the imaged objects actually lie. By providing information on imaged object location (such as table and body location), laser profilometry can be used to design per‐projection adaptive weighting filters which result in the accentuation of object image data (the patient) and deemphasize non‐object data (the imaging table). MATERIALS AND METHODS: A laser profilometry system was developed and attached to a Nucletron Digital Integrated Brachytherapy Unit (IBU‐D), orthogonally offset from axis where the kV source and digital flat‐panel detector lie. This system is able to detect surfaces with +/−1.5 mm uncertainty. Imaging proceeds on a water‐filled Jaszczak Phantom containing spheres of varying contrasting materials using reduced‐image‐set CBCT (18‐images). Surface data and kV imaging are acquired in immediate succession. RESULTS: A comparison of laser‐weighted FBP‐based reconstructions with nonweighted FBP‐reconstructions shows that the laser‐weighted techniques yield higher uniformity of voxel‐intensity values. CONCLUSION: In CBCT, voxel nonuniformity in an image reconstruction (especially near the surface) is usually reduced with the use of a bowtie filter; an accessory which requires more dose to the patient. This increased dose can possibly be reduced with the used of weighted projection data acquired using laser profilometry. Research supported in part by NIH T32‐CA113267 and Nucletron, B.V.