D Wiant

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.

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‐

Evaluation of offline adaptive planning techniques in image-guided brachytherapy of cervical cancer

Abstract Modern three‐dimensional image‐guided intracavitary high dose rate (HDR) brachytherapy is often used in combination with external beam radiotherapy (EBRT) to manage cervical cancer. Intrafraction motion of critical organs relative to the HDR applicator in the time between the planning CT and treatment delivery can cause marked deviations between the planned and delivered doses. This study examines offline adaptive planning techniques that may reduce intrafraction uncertainties by shortening the time between the planning CT and treatment delivery. Eight patients who received EBRT followed by HDR boosts were retrospectively reviewed. A CT scan was obtained for each insertion. Four strategies were simulated: (A) plans based on the current treatment day CT; (B) plans based on the first fraction CT; (C) plans based on the CT from the immediately preceding fraction; (D) plans based on the closest anatomically matched previous CT, using all prior plans as a library. Strategies B, C, and D allow plans to be created prior to the treatment day insertion, and then rapidly compared with the new CT. Equivalent doses in 2 Gy for combined EBRT and HDR were compared with online adaptive plans (strategy A) at D 90 and D 98 for the high‐risk CTV (HR‐CTV), and D 2 cc for the bladder, rectum, sigmoid, and bowel. Compared to strategy A, D 90 deviations for the HR‐CTV were −0.5 ± 2.8 Gy, −0.9 ± 1.0 Gy, and −0.7 ± 1.0 Gy for Strategies B, C, and D, respectively. D 2 cc changes for rectum were 2.7 ± 5.6 Gy, 0.6 ± 1.7 Gy, and 1.1 ± 2.4 Gy for Strategies B, C, and D. With the exception of one patient using strategy B, no notable variations for bladder, sigmoid, and bowel were found. Offline adaptive planning techniques can shorten time between CT and treatment delivery from hours to minutes, with minimal loss of dosimetric accuracy, greatly reducing the chance of intrafraction motion.

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‐T‐538: A Quantitative Evaluation of TomoTherapy Lung SBRT Set‐Up Accuracy

Purpose: The TomoTherapy Hi Art system uses daily megavoltage cone beam computed tomography (MVCT) to help ensure accurate patient positioning for lung stereotactic body radiotherapy(SBRT). In this study we evaluate the accuracy and precision associated with registering daily TomoTherapy MVCTs to kilovoltage CT (kVCT) images acquired at simulation for lungSBRT. Methods: Volumetric images were retrospectively analyzed in MIMVista for 10 patients who received lungSBRT at our institution. The MVCTs were used to define gross tumor volume (GTV). Internal target volumes (ITV) were defined on kVCT from 10 phase‐binned 4DCT. The MVCTs were fused to the kVCTs by a mutual‐information technique and cross‐checked for accuracy. Registration was evaluated by 1) displacement of the GTV centroids from the ITV centroids and 2) measuring the extent of the GTV outside of the ITV. This analysis was also carried out on a novel 4D lung phantom with 3 non‐ coplanar lesions moving at 18 breaths per minute with +/−5mm displacement. One reference kVCT and 10 MVCTs were acquired, and fusion was achieved using 3 fixed radio‐opaque markers. Results: The average extent of the patient GTVs outside of the ITVs was 3.5 +/− 1.1 mm, with a maximum extent of 8.3 mm. The average centroid variation was 3.3 +/− 1.3 mm, with a maximum variation of 8.0 mm. The average extent of the phantom GTVs outside of the ITVs was 1.9 +/− 0.5 mm, with a maximum extent of 4.4 mm. The average centroid variation was 2.4 +/− 0.6 mm, with a maximum variation of 3.1 mm. Conclusions: The phantom setup showed an uncertainty of 2–3 mm for the TomoTherapy MVCT registration process. The patient data showed an uncertainty of 3–4 mm. This suggests that a margin of at least 3–4 mm around the ITV is required for TomoTherapy lungSBRT cases.

SU-E-T-889: Target Definition in TomoTherapy Lung SBRT Treatment Plans

Purpose: An unresolved question in lung stereotactic body radiotherapy(SBRT)treatment planning is whether a free breathing computed tomography(CT)image set, an image set with the electron density of the internal target volume (ITV) set to water, or an averaged 4DCT scan provides the most realistic representation of the physical situation. Methods: TomoTherapy Hi Art treatment plans created on free breathing scans for three patients receiving lungSBRT at our institution were compared to plans created on 1) a scan with the ITV electron density set to water, 2) a scan with the planning target volume (PTV) electron density set to water, 3) a phase averaged 4DCT, and 4) a time averaged 4DCT. The comparison plans were made by creating TomoTherapy delivery quality assurance (DQA) phantoms from the additional image sets, then recalculating the free breathing plan on these phantoms. All doses were exported to MIMVista for analysis. Results: The maximum isodose line covering the entire PTV and ITV showed little change among the different plans for each patient. The maximum point dose to the ITV and the PTV was similar for the free breathing and average scans. The maximum point doses showed a slight increase for both the ITV and PTV override plans for all patients. The PTV override plan pushed the maximum point dose out of the ITV into the PTV. Conclusions: Plans calculated on the scans with density overridden target volumes show a marked change in dose distribution and target coverage from plans calculated on free breathing and average scans. The different treatment planning techniques are clearly not equivalent. The use of a poor model could lead to over or under‐dosing of the target. Further study is needed to determine which plan most accurately models the true physical situation.

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.