J Gersh

Evaluation of the spatial dependence of the point spread function in 2D PET image reconstruction using LOR-OSEM.

PURPOSE The use of positron emission tomography (PET) imaging has proved beneficial in the staging and diagnosis of several cancer disease sites. Additional applications of PET imaging in treatment planning and the evaluation of treatment response are limited by the relatively low spatial resolution of PET images. Including point spread function (PSF) information in the system matrix (SM) of iterative reconstruction techniques has been shown to produce improved spatial resolution in PET images. METHODS In this study, the authors sampled the spatially variant PSF at over 6000 locations in the field of view for a General Electric Discovery ST PET/CT (General Electric Healthcare, Waukesha, WI) scanner in 2D acquisition mode. The authors developed PSF blurred SMs based on different combinations of the radial, depth, and azimuthal spatial dependencies to test the overall spatial dependence of the PSF on image quality. The PSF blurred SMs were included in a LOR-OSEM reconstruction algorithm and used for image reconstruction of geometric phantoms. The authors also examined the effect of sampling density on PSF characterization to design a more efficient sampling scheme. RESULTS The authors found that depth dependent change in the amplitude of the detector response was the most important factor affecting image quality. A SM created from a PSF that introduced r (perpendicular to the LOR), d (parallel to the LOR), or r and d dependent blurring across the radial lines of response led to visually identifiable improvements in spatial resolution and contrast in reconstructed images compared to images reconstructed with a purely geometric SM with no PSF blurring. Images reconstructed using a SM with r and d dependent blurring across the radial lines of response showed improved spatial resolution and contrast-noise ratios compared to images reconstructed with a SM that had only r dependent blurring. Additionally, the authors determined that the PSF could be adequately characterized with roughly 85% fewer samples through the use of a better optimized sampling scheme. CONCLUSIONS PET image reconstruction using a SM made from an accurately characterized PSF that accounts for r and d dependencies results in improved spatial resolution and contrast-noise relations, which may aid in lesion boundary detection for treatment planning or quantitative assessment of treatment response.

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.

SU‐E‐T‐468: Gamma Knife Perfexion Dosimetry: A Monte Carlo Model of One Sector

PURPOSE We have implemented a Monte Carlo (MC) based dose computation model of one sector of the Gamma Knife Perfexion (GK PFX) using the Penelope MC dosimetry codes. The single sector simulation was rotated about the z-axis to model all eight GK sectors. 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 along the x- and z-axes, compared to film measurements and dose calculations from the Leksell GammaPlan (LGP) workstation. METHODS We defined the internal GK PFX geometry in Penelope with the aid of vendor-supplied proprietary information. A single source per row was modeled for five rows for each of the 3 collimators (15 beams modeled). MC simulations were carried out on a Linux cluster. Phase space files (PSFs) were collected for the 15 modeled collimators then rotated about the z-axis to model the sector of 24 sources per collimator. 3D dose distributions from the MC model, film, and LGP DICOM-RT dose exports were analyzed using Matlab. For OF calculations, a 16 cm diameter dosimetry sphere was modeled with a virtual detector volume at its center. RESULTS Good agreement is found for row- and total-output factors (greatest deviation of any type < 4%) compared to reference values. Off-axis factors closely follow LGP predicted dose distributions along the x-axis and differ on the inferior side of the z-axis. CONCLUSIONS Detailed geometric representations (radiation source, device components) of the GK PFX are required for high fidelity MC simulations. Calculated GK PFX OF values depend on the simulated detector volume size (4 mm OF most dependent). Our model shows strong agreement for the GK PFX OFs and dose profile curves compared to reference values. Non-disclosure agreement for proprietary information with Elekta AB. No financial contribution.

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‐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‐45: Mathematical Reduction of Artifacts in Limited Projection and Limited Angle Cone Beam CT

Purpose: Modern radiation therapydelivery and imaging systems include flat panel imagers and kV sources. These systems are used to generate projections for cone beam CT(CBCT)reconstructions using filtered backprojection (FBP) algorithms. FBP works best when a large number of projections are taken over 360 degrees; however, the resultant daily dose is of concern. Two methodologies for dose reduction are limited projection number and limited arc acquisition. In this study we examine reconstruction degradation from these two dose reduction methods and present initial solutions to reduce these artifacts. Method: Our initial dataset consisted of a 360 image 360 degree projection dataset acquired on the Nucletron Digital Integrated Brachytherapy Unit (IBU‐D) of a water‐filled Jaszczak Phantom containing spheres of varying contrast. FBP reconstructions were performed on the full dataset, a 16 image limited projection dataset, and a limited angle (60 projections over a 60 degree arc) dataset. Streaking and smearing artifacts from the dose reduction strategies were examined for both limited projection and limited angle. We developed separate techniques for artifact reduction for each scenario: a powered interpolation technique and a view‐by‐view weighting technique, respectively. Images were reconstructed using the new techniques and compared. Results: For the limited projection case, powered interpolation techniques reduce streak artifacts significantly. For the limited angle case, view‐by‐view weighting reduce the smearing artifact at the edges of the projection data arc, which are caused by limited sampling of the Fourier space. Conclusion: With dose reduction comes data reduction, which can lead to unavoidable artifacts in both the limited projection and limited angle CBCT cases due to incomplete data. The new powered interpolation method and weighted projection method can effectively reduce these artifacts and improve image quality. Research supported by Nucletron, BV.

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.