bluuebunny/crossref_metadata_2025 huggingface dataset:
TU‐C‐350‐03: Medical Physics Licensure Update
Authors: Limmer J
Published: July 2008

To ensure that qualified individuals perform the job of clinical Medical Physics as outlined in the AAPM/ACMP Scope of practice. This is seen as important with or without the passage of the CARE Bill. The existence of the CARE Bill gives us the impetus to do this work. If we do not help the legislators define who is qualified to do our jobs, someone else will.

In this session the activities of the Joint Medical Physics Licensure Subcommittee will be discussed. Topics will range from the rationale for the effort, the new full time position at HQ to help with the effort, the status of licensure efforts, and future goals and plans. In addition, the implications for the clinical practice of medical physics if it became law in more states will be explored.

Educational Objectives:

1. Be exposed to the rationale behind the effort for professional licensure.

2. Understand what has been accomplished related to this effort.

3. Hear what the near and long term goals are for the effort.
TU‐C‐AUD B‐02: Dosimetric Verification of the CCCS Algorithm for Spatially Fractionated Radiation Therapy
Authors: Stathakis S , Gutiérrez A , Mavroidis P , Papanikolaou N
Published: July 2008

Purpose: The comparison between measurements and the collapsed cone convolution superposition (CCCS) using a multileaf collimator (MLC) for grid therapy is demonstrated in this study. Method and Materials: Grids with the projected field openings of 8mm × 8 mm to 20mm × 20 mm were created using multiple MLC‐shaped fields for 6MV and 18MV photon beams. The separation between the grid openings and the open‐to‐blocked area ratio varied from 16mm to 36mm (from center to center) and 0.25 to 0.50 respectively. The deposited doses (profiles) with films at different depths and also the percent depth doses (PDD) were measured in a solid water phantom and compared against calculations using the CCCS algorithm in Pinnacle. Results: The PDDs were in good agreement with the calculated ones. The highest discrepancy was observed at the depth past 10cm and it was in the order of 2% for the smallest grid size. For the larger grid sizes the agreement was with in 1%. On the other hand, there was a higher discrepancy between the measured and calculated profiles. While there was a good agreement at the peaks, there was a difference at the location of the valleys. The difference in the lateral direction was in the order of about 2mm for all grid sizes and at the lowest point of each valley the CCCS algorithm over‐predicted the dose by about 50%. Conclusion: In summary we have demonstrated that the CCCS algorithm can correctly predicted the dose at the openings of the grid fields. The agreement is very good for all grid field sizes and independent of the open‐to‐blocked area ratio. However, the film measurement of the profiles showed that the CCCS algorithm over‐predicts the dose under the blocked area independent of the grid opening and the open‐to‐blocked area ratio.
TU‐C‐AUD B‐04: Dose Perturbations Caused by Implanted Helical Gold Markers Used in Patients Receiving Proton Radiation Therapy for Prostate Cancer
Authors: Giebeler A , Fontenot J , Balter P , Ciangaru G , Zhu X , Newhauser W
Published: July 2008

Purpose: Implanted gold fiducial markers are widely used in radiation therapy to improve targeting accuracy; however recent investigations have revealed that metallic fiducial markers can cause extreme perturbations in dose distributions for proton therapy, suggesting that smaller markers should be considered. This study's objective was to test the dosimetric impact of various small helical, gold markers for tumor localization in patients receiving proton therapy. Method and Materials: Small, medium, and large helical wire markers with lengths of 10 mm and respective diameters of 0.04 mm, 0.25 mm and 0.5 mm were implanted in an anthropomorphic phantom. Radiographic visibility was assessed for a kV x‐ray imaging system, and dosimetric impact was characterized by Monte Carlo simulations and measurements of proton dose. Acceptable dosimetric perturbation was estimated from previous studies to be 10%. Results: Radiographic visibility was confirmed for all markers considered. Monte Carlo simulations indicated that the size of the marker dose perturbation depended on marker size, orientation, and distance from the beam's distal fall off. Simulations also revealed that dose perturbation in the lateral, opposed field treatment‐technique was 31% for large markers and 23% for medium markers in a typical orientation. Perturbation was not observed for the small marker, but it was deemed too fragile for transrectal implantation. Radiochromic film measurements confirmed the accuracy of the Monte Carlo model. Conclusion: Proton dose perturbations from medium and large sized markers exceeded 10%. This suggests that great care should be exercised if these markers are implanted in patients receiving proton therapy for prostate cancer. Conflict of Interest: A similar presentation of this work will be made at the ICRS, International Conference on Radiation Shielding; the ICRS presentation will be more preliminary and delivered to a different audience.
TU‐C‐AUD B‐05: Desired Dose Distributions From Newly‐Designed Low‐Energy Needle‐Based X‐Ray Sources
Authors: Liu Y , Strumban E , Jiang Q , Zhao J , Chen Q , Sozontov E , Gutman G , Guan H , Li S
Published: July 2008

Purpose: To establish a dosimetry system in developing a low‐energy (17.5 keV) needle‐based miniature x‐ray generator using BANG‐3 Polymer Gel. Method and Materials: A newly‐designed miniature x‐ray generator (Advanced X‐Ray Technology, Inc.) used an off‐shelf X‐ray tube with an Ag‐anode to produce the primary X‐rays of 22 keV Kα that were optically collimated and focused at a cone‐ or wedge‐shaped Mo target in a detachable needle. The Mo‐target absorbed most of the primary X‐rays and emitted the pure Kα X‐rays of 17.5 keV to be used for brachytherapy. The dose distribution at the vicinity of the needle was measured with custom BANG‐3 polymer gel (MGS Research Inc.) cylindrical phantoms according to the calibrated relationship between the dose and R2 (=1/T2) in 3 Tesla MR images with 1.5 mm slice thickness + 0.5 mm gap or 2 mm slice thickness + 1 mm gap. The calibration phantoms had filled with the same gel and scanned under the same conditions but irradiated by the primary beams with doses determined with a parallel‐plate ionization chamber. Results: The gel dosimeter calibration curve was closely approximated by with R2 > 0.9. After 60 min exposures using the cone‐ and wedge‐shaped target needles, the expected full and half ring‐type dose distributions were respectively obtained on the Gel dosimetry phantoms. High dose rate of ∼12 cGy/min at the radius of 0.5 cm was achieved under high input current. Conclusion: Our results demonstrated that the miniature x‐ray generator could be used as a new source in LDR and HDR brachytherapy with electronically controlled dose rate and dose distribution. BANG‐3 Polymer gel dosimetry played an essential role in development of the low energy X‐ray source.
TU‐C‐AUD B‐06: Characterization of Head Scatter Off Axis in Megavoltage Photon Beams
Authors: Novak J , Zhu T
Published: July 2008

Purpose: To investigate the properties of head scatter in an x‐ray beam as a function of off‐axis position. Method and Materials: The head‐scatter off‐axis ratio (HOA) is the ratio of the kerma from head‐scatter photons to the kerma from direct primary photons along the central axis. We determined HOA from measurements made with an ionization chamber within a miniphantom for a 6 MV x‐ray beam. Measurements of HOA were made for square, rectangular, and half‐blocked fields. We developed an empirical model that includes a Gaussian component to account for scatter from the flattening filter and a second component that is a function of the secondary collimator location to account for scatter from the x and y jaws. Results: Outside of beam collimation, the HOA in the y‐jaw direction was larger than in the x‐jaw direction, approximately 0.02 compared to 0.006. The shift of the beam edge, defined for the HOA as (measured FWHM) / (square field size), was approximately 1.0 and 1.15 along the x‐jaw and y‐jaw directions, respectively. The penumbra width of the HOA was approximately equal along the x‐ and y‐jaw directions. Within beam collimation, an additional block was important for the accurate measurement of HOA, however renormalization was also possible using central‐axis data. Outside of beam collimation, an additional block was not necessary. Conclusion: Head scatter off axis is the dominant source of dose outside of beam collimation, and therefore its characterization is important to accurately determine dose for low dose regions, especially when using IMRT fields. The characteristic difference of HOA between Varian and Siemens accelerators with MLCs will be discussed. The model will be used to predict head‐scatter dose for a variety of IMRT test cases.
TU‐C‐AUD B‐10: Evaluation of Accuracy of Field Size Measurements Using Profiler 2 and EPID
Authors: Kurokawa C , Fox C , Simon T , Ozawa S , Li J , Karasawa K , Palta J
Published: July 2008

Purpose: To evaluate the performance of two digital devices, a diode array and an EPID, for routine field size checks (normally performed by using films) by comparing with measurements using an ionization chamber. Method and Materials: Field sizes ranging from 5 × 5 cm2 to 25 × 25 cm2 for 6 and 18 MV photon beams were measured using the Profiler 2 (Sunnuclear) and EPID (iViewGT, Elekta) and compared with those measured with an ionization chamber in a scanning water tank. The Profiler 2 is made up of 83 diode detectors in one direction and 57 diodes in the orthogonal direction with 4 mm spacing, covering lengths of 30 cm and 20 cm in the respective two directions. The physical area of the EPID is 41 × 41 cm2 with 1024 × 1024 pixels, resulting in a resolution of 0.25 mm projected at the isocenter. A QA jig with embedded pins was used to determine the scaling of the EPID. For each measured profile, the width at 50% of the central axis intensity was taken to be the measured field size. Results: The field sizes measured with both detectors agreed to within 1.5% of those measured with the ion chamber for both the 6 MV and 18 MV photon beams. The results for Profiler 2 are slightly better than those of the EPID although the Profiler 2 has a limited spatial resolution of 4 mm. The ratios of field sizes measured with the Profiler 2 and the EPID to those of the ion chamber are almost constant with respect to the field sizes. Conclusion: The field sizes measured with the Profiler 2 and the EPID were comparable to those using the ion chamber. They can be used for routine field size checks in place of films.
TU‐C‐AUD C‐01: Spatio‐Temporal Amplification of Radiation Response Project
Authors: Lindsay P , Moseley D , Hope A , Hill R , Jaffray D
Published: July 2008

Translational research in radiation oncology has been in some ways limited by the lack of small animal model systems of common clinical issues. New technology, required to deliver ‘clinically equivalent’ radiation at small animal scale, has been developed to compliment existing small animal imaging systems. In support of such activities, two image‐guided small animal irradiation systems have been developed at Princess Margaret Hospital. Both use a 225 kVp x‐ray tube for treatment and include cone‐beam CT capabilities. Work is underway to refine hardware (collimation and animal immobilization) and software (image registration and dose calculation) systems. Multiple experimental models are being developed to exploit these novel technologies including exploration of tumor control and normal tissue toxicity in lung, brain, and other sites.
TU‐C‐AUD C‐02: CT Scanner‐Based IGRT
Authors: Graves E
Published: July 2008

While small animal models of cancer have evolved to more closely resemble the corresponding human disease, radiation therapy (RT) techniques for small animals are dramatically different than those routinely used in the clinic. Recently there has been considerable interest in developing conformal radiotherapy methods for small animals so as to incorporate more clinically‐relevant treatment modalities into laboratory studies of cancer.

An attractive approach to this engineering problem is to add radiotherapy capabilities to a micro‐computed tomography (microCT) scanner. This strategy exploits the high imaging performance of existing microCT units in order to produce a small animal image‐guided radiation therapy (IGRT) system employing a single X‐ray beam for imaging and treatment. If successful, this small animal IGRT option could be made available as an add‐on option to existing commercial microCT scanners, allowing widespread adoption of small animal conformal radiotherapy throughout the research community.

Initial experiments demonstrated that the dose rate of a GE microCT system is suitable for delivering therapeutic radiation doses in reasonable treatment times. A variable aperture collimator capable of restricting the microCT X‐ray beam to a variable pseudo‐circular profile has been developed and evaluated. This prototype microCT/RT system is now online for experimental use, and is currently being applied towards a variety of biological studies.

In this lecture I will briefly describe the current status and future work for this approach to small animal conformal radiotherapy. I will then discuss initial biological applications of this system, including treatment of orthotopic models of disease and evaluation of positron emission tomography (PET)‐guided radiotherapy strategies.

Educational Objectives

At the end of this lecture, the audience will be able to:

1. Describe the relative advantages and disadvantages of developing small animal radiotherapy within the context of existing microCT scanners.

2. Identify biological applications of small animal IGRT where existing small animal irradiation techniques are inadequate, and

3. Discuss the use of small animal IGRT as both a tool for small animal research and a method for conducting clinical trials in an experimental setting.
TU‐C‐AUD C‐04: Custom Built Small Animal IGRT Platform
Authors: Wong J , Armour E , Kazanzides P , Iordachita I , Ford E , McNutt T , Tryggestad E
Published: July 2008

Over the past 2 decades, there has been a widening technological disparity between laboratory radiation research and clinical radiation therapy. Whereas simple single beam/single fraction techniques are commonly used to irradiate laboratory animals, advanced three‐dimensional (3D) and computer‐controlled delivery technologies are now used clinically to pinpoint fractionated conformal radiation therapy (CRT) and intensity modulated radiation therapy (IMRT). The technological disparity presents a difficult hurdle in the development of novel treatment methods that combine conformal irradiation and other therapeutic agents. There is clearly a pressing need to bridge the technological gap between laboratory radiation research and human treatment methods.

To that end, we have constructed a Small Animal Radiation Research Platform (SARRP) which integrates imaging, radiation delivery and treatment planning capabilities. The SARRP spans 3 ft × 4 ft × 6 ft (W×L×H). A dual‐focal (0.4 mm and 3.0 mm) spot, constant voltage x‐ray source is mounted on isocentric gantry. The source to isocenter distance is 35 cm. Eighty to 100 kVp x‐rays from the smaller focal spot are used for imaging. Both focal spots operate at 225 kVp for irradiation. Robotic translate/rotate stages are used to position the animal. A novel configuration is devised for CBCT imaging by rotating the horizontal animal between the x‐ray source and a flat panel amorphous silicon detector that are fixed at opposite horizontal positions of 90° and 270° respectively. Radiation beams ranging from 0.5 mm in diameter to (60 × 60) mm2 are available. Conformal dose distributions are delivered using a combination of gantry and robotic stage motion. Treatment planning is performed at sub‐mm resolution where Monte Carlo dose calculations are coupled to a research Pinnacle system for visualization. Depending on filtration, the isocenter dose outputs at 1 cm depth in water range from 22 to 375 cGy/min from the smallest to the largest radiation fields. The 20% to 80% dose fall‐off spans 0.16 mm. CBCT with (0.55 × 0.55 × 0.55) mm3 voxel resolution is acquired with less than 1 cGy in 4 min. The ability of our system to focally irradiate a specific anatomic region or target in a mouse subject has generated exciting new collaborations between laboratory and translational research. These include the study of the response of normal tissue and tumor to focal radiation injuries; the development of molecular imaging markers for early assessment of radiation induced toxicity in the lungs; and the study of molecularly targeted therapy in combination with radiation. We are hopeful that our SARRP, and other similar initiatives, will serve to provide the timely and powerful technology to greatly transform future cancer treatment.

Educational Objectives:

1. Appreciate the disparity between animal radiation research methods and clinical treatment.

2. Understand the challenges in down‐sizing human treatment methods for small animal.
TU‐C‐351‐02: Alignment Error Due to Speed Artifact in Stereotactic Ultrasound Image Guidance
Authors: Salter B , Wang B , Szegedi M , Cheng C , Fuss M
Published: July 2008

Purpose: US image guidance systems which are calibrated for soft tissue applications will introduce errors in depth‐from‐transducer representation when used in media with a different speed of sound propagation (e.g. fat). In this study we utilized a standard US phantom to demonstrate the existence of the speed artifact when using a commercial US image guidance system to image through simulated layers of body fat, and compared the results with calculated/predicted values. Method and Materials A General Purpose Ultrasound Phantom (speed of sound‐SOS = 1540 m/s) Model ATL 040 (CIRS, Norfolk, VA) was imaged on a LightSpeed RT CT scanner (GE Health Care, Waukesha, WI) at 0.625‐mm slice thickness and 0.5mm pixel. Target‐simulating wires inside the phantom were contoured and later transferred to the BAT Sxi unit (Nomos Corp., Cranberry, PA). Various thicknesses (1–8cm) of fat simulating material (SOS = 1435 m/s), manufactured by CIRS, were placed on top of the phantom to study the depth‐related alignment error. In order to demonstrate that the speed artifact is not caused by adding additional layers on top of the phantom, we repeated these measurements in an identical setup using tissue simulating material (SOS=1540 m/s) for the top layers. Result and Discussion: For the fat simulating material used in this study, we observed the magnitude of the depth‐related alignment errors resulting from this speed artifact to be 0.73 mm per cm of fat imaged‐through. The measured alignment errors caused by the speed artifact agreed with the calculated values within measurement uncertainties for the five different thicknesses of fat simulating material studied here. Conclusion: We demonstrated the depth‐related alignment error due to speed artifact when using US image guidance for radiation treatment alignment. When possible, care should be taken to avoid imaging through a thick layer of fat for larger patients in US alignments.
TU‐C‐351‐03: Can Pre‐ and Post‐Imaging Determine Prostate Intrafraction Motion?
Authors: Adamson J , Wu Q
Published: July 2008

Purpose: Recent studies of prostate intrafraction motion have been inconclusive. One cine‐MRI study demonstrated that pre‐treatment rectal filling status is a significant predictor of intrafraction motion magnitude. However, studies using the Calypso system show that pre‐ and post‐imaging is not an adequate test of prostate intrafraction motion. Our purpose is to determine the effectiveness of pre‐ and post‐imaging in predicting prostate intrafraction motion. Method and Materials: Pre‐ and post‐CBCT images and intrafraction kV fluoroscopy were used to determine the prostate position via fiducial markers of 15 patients over 279 fractions. For each fraction, rectal filling status was documented and bladder volume estimated on both the pre‐ and post‐CBCTs. Also documented was whether a visible change occurred in rectum shape between CBCTs. The ability of rectal and bladder filling to predict the measured intrafraction motion was evaluated using receiver operator characteristic (ROC) analysis. A model assuming a linear motion pattern during the treatment fraction was applied and the RMS error calculated. Results: Initial rectal filling status was a significant predictor of large intrafraction motion while effects from bladder filling during treatment were inconclusive. The sensitivity of detecting intrafraction motion above a given threshold using post‐imaging was improved when change in rectal filling status was taken into account. For detecting motion >5 mm, the sensitivity increased from 0.88 to 0.93 with a decline in specificity from 0.96 to 0.76. The rms‐error decreased from 2.4 mm to 1.4 mm when the linear model was used as opposed to assuming no intrafraction motion (p<0.0001). Conclusion: Pre‐ and post‐imaging is a significant predictor of intrafraction motion especially when rectal filling status is taken into account. A linear model is more accurate than assuming no intrafraction motion. Effects of bladder filling are inconclusive and require further study. Conflict of Interest: Partially supported by NIH Grant CA118037.
TU‐C‐351‐04: Cone Beam CT Acquisition During Volumetric Arc Radiotherapy Delivery: Correction for Induced Artifacts
Authors: Hugo G , Matuszak M , Campbell J , Yan D
Published: July 2008

Purpose: To evaluate the feasibility of kV cone beam CT (CBCT) acquisition simultaneously with volumetric modulated arc therapy (VMAT) delivery, and to test a method to correct for degradation of image quality due to VMAT delivery. Method and Materials: A commercial CBCT system was modified to enable simultaneous CBCT acquisition with VMAT delivery. CBCT scans of an image quality phantom were acquired during VMAT delivery while varying the VMAT parameters. Dose rate, energy, and field size of the VMAT delivery, and phantom size, were varied to evaluate the effect on image quality. The mean and standard deviation of the signal in a known location was quantified both in the raw 2D projection images and also in the reconstructed 3D CBCT images. A nonlinear filter was tested to remove structural artifact and noise. An analytical scatter correction model was developed and used to remove scatter generated by the VMAT beam. Results: Structural artifact was reduced in the CBCT projections with a nonlinear filter. Scatter generated from the VMAT delivery varied with field size and dose rate, and minimally with phantom size. An analytical scatter model was constructed based on the VMAT fluence (field area times dose rate) for each CBCT projection, and applied to reduce scatter per projection. Applying the model improved uniformity from 7.9% to 3.0% and improved the contrast to noise ratio from 0.97 to 1.84. Conclusion: Megavoltage scatter, and its per projection variation, is the largest component contributing to degradation of CBCT image quality during VMAT delivery. The degradation was reduced with a scatter model based on the VMAT delivery. A secondary component was structural artifact related to the repetition rate of the megavoltage beam and the readout mechanism of the kV detector. Conflict of Interest: Research sponsored by Elekta, Inc.
TU‐C‐351‐05: Daily Alignment Results for In‐Room CT‐Guided Stereotactic Body Radiation Therapy for Lung Cancer
Authors: Ikushima H , Balter P , Chang J , Hunjan S , Sadagopan R , Yu Z , Zhang Y , Dong L
Published: July 2008

Purpose: To assess daily bone alignment results and changes in soft tissue tumor position during hypofractionated, in‐room computed tomography (CT)‐guided stereotactic body radiation therapy (SBRT) of lung cancer. Method and Materials: Daily alignment results during SBRT were analyzed for 117 tumors in 112 patients. Patients received 40–50 Gy of SBRT in 4–5 fractions to the target using an integrated CT‐LINAC system. The free‐breathing CT scans acquired during treatment set‐up were retrospectively re‐aligned to match with each of the bony references and the gross tumor volume (GTV) defined on the reference CT by rigid registration, and the daily deviations were calculated. Results: The mean (±SD) three‐dimensional (3D) shift from the initial skin marks to the final bone‐aligned positions was 9.4 ± 5.7 mm. The mean daily GTV deviation from the bone position was 0.1 ± 3.8 mm in the anterior‐posterior (AP) direction, −0.01 ± 4.2 mm in the superior‐inferior (SI) direction, and 0.2 ± 2.5 mm in the lateral direction. A statistically significant trend (linear fit with R2>0.8) in the change in GTV position relative to the bone was observed in 15 (13%), 11 (9%), and 21 (18%) cases along the AP, SI, and lateral directions, respectively. There were no significant associations between the trends in GTV movement and clinical factors. A margin of 10 mm around the ITV covered the inter‐fractional organ motion errors in 96.4% of tumors in the AP direction, 100% of tumors in the SI direction, and 100% of tumors in the lateral direction. Conclusion: 3D bone alignment using daily in‐room CT‐guided SBRT has good accuracy. However, a substantial number of tumors showed trends in position changes over 4 or 5 days. An isotropic margin distance of 10 mm around the ITV was necessary for adequate coverage of inter‐fractional organ motion errors of all cases.
TU‐C‐351‐06: Comparison of Multiple 3D‐3D Anatomy‐Based Rigid Image Registration Methods for Prostate Patient Setup Before External‐Beam Radiotherapy
Authors: Wu J , Murphy M
Published: July 2008

Purpose: To assess the variability in translational setup corrections computed via several different 3D to 3D rigid registration methods and conditions using only the image intensity information. Method and Materials: Fifteen pairs of male pelvic CTs were rigidly registered using multiple registration metrics, methods, and image content. Similarity was measured using mutual information and mean‐squared‐difference. The registrations were iterated using two different search algorithms. The effect of image resolution was observed by downsampling in each slice by factors of 2 and 4. The effects of image content were observed by using the entire image and then using only bony landmarks in the similarity measures. The uncertainty associated with the choice of source and target images was revealed by switching the roles of the two registered images. For each image pair the translational shifts for all the registration methods and conditions were compiled and the standard deviation around the mean shifts was computed. Results: Reversibility errors ranged from 0 to 1.8 mm for the various algorithms and conditions. The radial standard deviation of the various methods and conditions was 0.8 mm when the entire image was registered and 0.3 mm when only bony landmarks were used. Downsampling by a factor of 2 improved robustness with only a small loss in precision. Mutual information failed in nearly all cases when the images were thresholded to isolate bony features. Conclusion: Rigid image registrations have an intrinsic uncertainty that depends on the algorithm, the resolution of the images, and the features used to establish rigid congruence. Registration uncertainty should be accounted for in planning margins and patient setup procedures. In the absence of an absolute ground truth test of registration accuracy, the standard deviation in the shifts calculated by several different methods provides a useful estimate of that uncertainty.

Supported in part by NIH P01CA116602.
TU‐C‐332‐02: Concomitant Segmentation and Registration of Liver Anatomy Using SPECT‐CT Imaging
Authors: Fox T , Schreibmann E , Crocker I
Published: July 2008

Purpose: To develop an automatic and accurate technique for concomitant segmentations and registration of liver anatomy using SPECT and CT images for unsealed source radiotherapy. Method and Materials: The link between segmentation and registration is given by the using the level set of a liver segmentation into the registration process. In the combined approach, the liver is automatically segmented from the CT image by evolving an initial seed with a level set until it locks to the liver's border as observed in the CT images. The time‐crossing map of the level set is then used to match gradients in the SPECT image to the level set by using a data structure containing the signed distance values at a small band of neighboring pixels. Results: The technique was applied to three cases of metastatic liver disease treated with unsealed source therapy. Results indicated that the speed map of the level set plays an importance role in obtainng an accurate registration and produce a segmentation that is superior in registration time and accuracy over manual segmentation or the standard registration approach using mutual information. Accuracy measured with the convergence analysis method was of less than 0.5 mm rotation and 1 degree rotation. Conclusion: With the proposed combined segmentation‐registration technique, the uncertainty of soft‐tissue target localization could be greatly reduced ensuring accurate therapy assessment to be precisely delivered as planned. The combined all‐in‐one approach is automated and provides excellent accuracy over manual segmentation and mutual information approaches.
TU‐C‐332‐06: Attenuation Correction for Hybrid PET‐MRI System by Deformable Image Registration
Authors: Schreibmann E , Fox T , Martin D , Votaw J
Published: July 2008

Purpose: To develop an accurate patient‐specific PET attenuation coefficients map to be used in hybrid MRI‐PET systems for brain tumor imaging. The attenuation maps are obtained by warping a general (atlas) CT dataset to the patient‐specific MRI dataset using a deformable registration model. Method and Materials: Patient MR images and the atlas CT images are registered using a B‐Spline deformable model and the Mattes formulation of the mutual information metric as registration criterion. The registration establishes a voxel‐to‐voxel correspondence that maps each voxel in the CT atlas voxels to the MRI dataset, creating an artificial, individualized CT scan of the patient's anatomy as observed in the MRI dataset. To evaluate the accuracy of the deformable‐based attenuation correction, ten clinical brain tumor cases are studied with MR‐CT image sets. For each case an artificial CT is computed by warping the atlas to the MRI datasets. This artificial CT is compared to the true patient's CT in terms of geometrical accuracy of the deformation module as well as a voxel‐to‐voxel comparison of HU units. Results: In all cases, mapping form the atlas CT to the individual MR was achieved with great geometrical accuracy as visually judged using the qualitative visual inspection tools. The mean distance between the artificial and true CT's external contour and bony anatomy was 1 mm and 1.5 mm, respectively. In terms of HU unit comparison, the mean voxel‐to‐voxel difference was less than 5%. Conclusion: Attenuation correction for hybrid MRI‐PET scanners can be easily achieved by individualizing an atlas CT to the MRI dataset using the BSpline deformable model, with no user interaction required. The method provides clinical accuracy while eliminating the need for an additional CT scan for PET attenuation correction.
TU‐C‐332‐08: Comparison of Pulse‐Echo Methods for Measuring Ultrasound Attenuation in the Liver
Authors: Kim H , Musack R , Varghese T , Kliewer M , Jiang J , Hall T , Zagzebski J
Published: July 2008

Purpose: Ultrasound attenuation in liver differs from normal values when diffuse disease is present. Additionally, clinical images suggest that many masses in the liver have attenuation that varies from that of background tissue. In this presentation we compare four attenuation measurement algorithms to determine strengths and weaknesses of each for determining global and local values of attenuation in liver. Method and Materials: Full frames of RF echo data were acquired from test phantoms using a Siemens Antares scanner equipped with the Axis Direct research interface. Signals were acquired using linear array transducers operating at a 6 MHz frequency. Data were analyzed offline using 4 different algorithms: “Video Signal Analysis” (VSA), a centroid frequency shift method (FS), Diffraction Corrected Spectral Cross‐correlation (DCSC), and a conventional Reference Phantom Method (RPM). Both global estimates of ultrasound attenuation and low resolution attenuation coefficient images of uniform and “inclusion” phantoms were obtained. In a similar manner, attenuation is being measured in the livers of patients (under an IRB approved protocol) who are undergoing needle biopsies under ultrasound guidance. Results: The RPM provides accurate attenuation estimates, with acceptable variance, for uniform regions in phantoms. All methods are subject to attenuation image artifacts when the backscatter is not uniform, as shown with the inclusion phantoms. However, the DCSC appears to be the least susceptible to backscatter variations. Initial attenuation results in liver were obtained from a patient with a hemangioma, a tumor that exhibited lower attenuation and higher backscatter than background liver. Because of elevated backscatter, the DCSC method performed best. Additional liver samples are being processed. Conclusion: Modern machines provide RF data that can be used to measure acoustical properties of tissues. When measuring attenuation, the RPM performed best in uniform regions of phantoms, but the DCSC technique appeared to be least susceptible to backscatter variations.
TU‐C‐342‐01: Image Guidance During Minimally Invasive Cardiac Interventions : Beyond Fluoroscopy
Authors: Fahrig R
Published: July 2008

Minimally invasive cardiac procedures using endovascular access have enabled treatment of patients that previously would have required open‐heart surgery. Such procedures are typically carried out in the interventional suite, with x‐ray fluoroscopy used to guide the placement of a catheter, followed by a phase of the procedure in which x‐ray fluoroscopy provides little to no information on the treatment. C‐arm CT has been under development since the early 90's, but has now entered a new phase as the ability to visualize soft tissue and low‐contrast lesions improves. By generating 3D images utilizing the x‐ray system in the interventional suite, the error prone and time‐consuming spatio‐temporal registration with prior CT or MRI exams can be avoided. However, C‐arm CT imaging in the presence of cardiac motion remains a challenge due to the slow rotation speed of the gantry, and the slow readout rates of the flat‐panel detectors. Three main data acquisition approaches to circumvent these limitations will be discussed: multi‐sweep retrospective ECG gating for myocardial visualization, single‐ or double‐sweep with volume image fusion for left atrium and pulmonary vein visualization and single‐sweep with retrospective gating for coronary artery visualization. Approaches to improve image quality, both hardware and software, such as optimized acquisition timing, motion compensation approaches and algorithms for non‐idealities such as x‐ray scatter, detector lag, limited field‐of‐view or angular coverage, data insufficiency and noise will be presented. With these developments the ultimate goal is to accomplish faster and more accurate catheter based interventions with an equivalent success rate as surgical procedures (80–90%). The C‐arm CT technology described here could eventually impact many minimally invasive procedures such as RF ablation for atrial fibrillation and coronary stenting, and also future procedures as they become available, such as endocardial injection of stem cells for treatment of myocardial infarction.

Educational Objectives:

1. Learn how 3D cardiac imaging is done in the interventional suite: acquisition protocols, injection protocols, challenges and solutions.

2. Learn how image quality of 3D cardiac images can be improved using hardware and algorithm approaches.
TU‐C‐342‐02: Dual Energy CT: Promises and Challenges for Cardiovascular Imaging
Authors: McCollough C
Published: July 2008

Dual‐energy CT can be expected to play a new and evolving role in cardiovascular imaging. Clinical uses already reported include 1) direct CT angiography, whereby the dual energy algorithm identifies and removes bone in a 3‐D CT angiographic data set, allowing direct visualization of iodinated vessels without the need for user intervention to remove overlying bony anatomy, 2) removal of smaller hard plaques within vessels, allowing more rapid and clearer visualization of patent lumens in MIP projections, and 3) visualization of the perfused blood volume, also referred to as blood pool imaging, to demonstrate focal perfusion deficits. Dual energy bone and plaque removal techniques can be applied in retrospectively‐gated cardiac imaging, suppressing the appearance of calcified plaque and providing improved visualization of stenotic lumens. Additionally, dual‐energy CT can be used to identify and remove calcified plaque prior to image reconstruction in order to reduce the effects of calcium blooming, which creates artifactual elevation of CT numbers in voxels adjacent to calcified objects, often obscuring the dimensions of the true lumen for large or dense coronary artery calcifications. Finally, using projection‐space dual‐energy methods, mono‐energetic CT images can be calculated, which greatly reduces beam hardening effects and increases the accuracy of CT number measurements. The use of dual‐energy CT techniques is being explored for the evaluation of myocardial perfusion deficits.

Attendees of this presentation can expect to learn about:

1) the technical approaches to dual‐energy cardiac CT currently implemented or under investigation on commercial CT systems,

2) current clinical uses of dual‐energy CT in cardiovascular imaging and

3) areas of future investigation.
TU‐D‐352‐01: Beyond the Limits of Active Matrix Flat‐Panel Imagers: A Comparative Performance Assessment of X‐Ray Converter Enhancement Versus Innovative Active Pixel Sensor Architectures
Authors: Antonuk LE , El‐Mohri Y , Zhao Q , Koniczek M , Wang Y , Behravan M
Published: July 2008

Purpose: Despite the many advantages of Active Matrix Flat‐Panel Imagers (AMFPIs), these devices suffer from modest system gain relative to additive noise, as well as restrictions on maximum frame rates and charge trapping. A quantitative comparison of the performance of two of the strategies being developed to overcome these limitations is presented. One strategy employs significant enhancement of direct detection converter properties while the other employs complex, polycrystalline silicon (poly‐Si) pixel circuits to form active pixel sensor (APS) architectures. Method and Materials: Theoretical upper limits for DQE and for other metrics that determine maximum frame rate and charge trapping effects were computed under fluoroscopic and radiographic conditions using a combination of cascaded systems analysis and circuit simulation. The cascaded systems calculations employed empirical measurements, published data and analytical calculations, while the simulations used representative a‐Si:H and poly‐Si transistor models. Results: The potential of high sensitivity photoconductors, such as , to offer up to a factor of 10 increase in system gain results in significant improvement to DQE performance under conditions of low exposures and/or for very small pixel sizes. Poly‐Si APS designs are capable of equivalent, or even greater DQE improvement through a combination of gain provided by in‐pixel amplifiers, along with correlated double sampling of the pixel signal. Furthermore, APS circuit designs allow substantially higher frame rates as well as reduction in charge trapping effects such as ghosting. Conclusion: While both approaches offer substantial improvements in DQE, and thus imaging performance, under conditions of low exposure and/or for small pixel pitches, converter enhancement offers the potential advantage of compatibility with existing AMFPI array designs. Conversely, poly‐Si APS architectures offer flexibility of design, function and operation, providing for the possibility of variable gain as well as addressing frame rate limitations and charge trapping issues.
TU‐D‐350‐02: Civil Law, Ethics and the Medical Physicist
Authors: Serago C
Published: July 2008

Ethics Case Reports.

Questionable ethical behavior may or may not also entail questionable lawful behavior. Several case reports are presented to differentiate legal and ethical issues. There is often an overlap of ethical and legal boundaries; so many situations may have both legal and ethical implications. The ramifications of legal vs. ethical violations are obviously different. Hypothetical case reports will include: hostile work environment, sexual harassment, graduate student progress impediments, substance abuse, and questionable practice.
TU‐D‐AUD B‐08: Clinical Validation of a Novel Adaptive Approach for Patients Concurrently Treated with the Prostate and Pelvic Lymph Nodes
Authors: Xia P , Hwang A , Ludlum E , Aubin M , Pouliot J , Roach M
Published: July 2008

Purposes/Objectives: One of the challenges for concurrent irradiation of the prostate and pelvic lymph nodes with IMRT is the independent movement of the prostate and the pelvic nodes. To resolve this challenge, we proposed a novel adaptive approach using multiple adaptive IMRT plans (MAP). This study is to validate dosimetric accuracy of this adaptive approach. Method and Materials: MAP Plans for a patient included five IMRT plans with one planning prostate position and the four shifted prostate positions. Prior to each treatment, daily mega‐voltage cone beam CT (MV‐CBCT) was acquired to determine the prostate position relative to the pelvic bones, and one of the five MAP plans was selected for the treatment. Seventeen daily MV‐CBCTs were available for analysis. Seventeen verification plans were retrospectively created by applying delivery beams to daily MV‐CBCTs. For each verification plan, the daily prostate contour was created by shifting the planning contour with the measured prostate movement and the daily pelvic lymph nodal volume was transferred from the planning CT to the MV‐CBCT using rigid body image registration. Typical dosimetric endpoints were used to assess the accuracy of this approach. Results: The prostate moved 0.4–0.7 cm superior in 38% days, > 0.8 cm superior in 19%, 0.4–0.7 cm posterior in 12%, and less than 0.3 cm in all directions in 31%. Of these days, 12 (65%) of the daily dose to the 95% of the prostate was greater than 98% of the prescribed dose, 4 (24%) in the range of 93% to 97%, and 2 (11%) less than 90% of the prescription dose. Among these days, the pelvic lymph node volume received 100% of the planned dose. Conclusion: The MAP adaptive approach is a clinically feasible strategy. Verification plan calculated with daily MV‐CBCT as a dosimetric monitoring tool provides patient specific dose guidance.
TU‐D‐AUD B‐09: Dose Uncertainties of Head and Neck IMRT by Dental Implant Artifacts in CT Images
Authors: Lee S , Kim C , Luo C
Published: July 2008

Purpose: The IMRT in the head and neck region is practiced frequently in order to reduce toxicity to the OARs such as parotid glands while escalating dose to the tumor volume. In spite of common use of this modality, streak artifact in CT caused by dental implants is problematic because this leads to errors in dose calculation. This study quantifies uncertainty ranges of dose and provides a way for better dose calculation. Method and Materials: In this study, five different H&N IMRT plans were chosen, and then for each plan two different CT sets were manipulated using the original CT images. Those three sets are: i) images without any manipulation (original CT image), ii) images with the same Hounsfield Unit equivalent to water for whole treatment body, and iii) images with HU = 0 only for the part with artifacts. After the adjustment, each image set was used for the dose calculations. The Varian's Eclipse® treatment planning system was used in this study with pencil beam algorithm. With and without heterogeneity correction options were also observed in this study. The results were analyzed using the DVH data. Results: The difference of overall mean doses for each set was minimal. The hot and cold spots were varied not only with the disease site but also with the selection of image sets. The variations were close to 2% in PTVs, and OARs usually a little less. Conclusion: Using the different image manipulation sets, mean dose variation relative to the calculation based on the original CT set was insignificant but cold and hot spots were varied due to the artifact's location. The dose calculation using this image manipulation technique can reduce the dose to the OARs and leads to improvement of accuracy of dose calculation.
TU‐D‐AUD C‐01: Performance of An In‐House Laser CT Scanner and Dose Verification for IMRT Using Gel
Authors: Ravindran P , Subramanian B , Babu E Suman
Published: July 2008

Purpose: To evaluate the performance of an in‐house laser CT scanner and study its usefulness for dose verification in IMRT. Method and Materials A laser CT scanner has been developed based on the principle of a first generation X‐ray CT. This scanner consists of an aquarium, a turntable with angular graduation, photodiode as detector and laser as light source. The laser is swappable between a red laser and a green laser. The laser attenuation data acquired using a data acquisition board and a Personal computer is reconstructed using IRADON function implemented in MATLAB. The spatial accuracy of measurement, resolution and reproducibility of the scanner were investigated in this study. The spatial accuracy of the scanner was verified by scanning needles placed in phantom with known geometry. A capillary tube with 1mm bore was scanned to check the resolution of the scanner. Calibration was performed by exposing the gel for a few known doses. Three IMRT plans were verified using normoxic polymer gels and the dose distribution were compared with that of the treatment planning system. Results: The spatial measurement accuracy of the scanner was one millimeter. The scan of the 1 mm capillary showed that the resolution of the scanner was 1 mm. The repeated scanning of a gel produced identical results suggesting that the dose mapping with the scanner is reproducible. The dose distribution obtained with gel measurements agree with the treatment plans within the 5%‐3mm. Conclusion: The performance of laser CT scanner developed in‐house has been found to be adequate for reading dose distribution embedded in gel. The comparison of the IMRT plans showed that it could be used for patient specific quality assurance of IMRT plans.