Digitally Reconstructed Radiographs Research Articles

Comparison of Similarity Measures for Rigid-body CT/Dual X-ray Image Registrations

A set of experiments were conducted to evaluate six similarity measures for intensity-based rigid-body 3D/2D image registration. Similarity measure is an index that measures the similarity between a digitally reconstructed radiograph (DRR) and an x-ray planar image. The registration is accomplished by maximizing the sum of the similarity measures between biplane x-ray images and the corresponding DRRs in an iterative fashion. We have evaluated the accuracy and attraction ranges of the registrations using six different similarity measures on phantom experiments for head, thorax, and pelvis. The images were acquired using Varian Medial System On-Board Imager. Our results indicated that normalized cross correlation and entropy of difference showed a wide attraction range (62 deg and 83 mm mean attraction range, omega(mean)), but the worst accuracy (4.2 mm maximum error, e(max)). The gradient-based similarity measures, gradient correlation and gradient difference, and the pattern intensity showed sub-millimeter accuracy, but narrow attraction ranges (omega(mean)=29 deg, 31 mm). Mutual information was in-between of these two groups (e(max)=2.5 mm, omega(mean)= 48 deg, 52 mm). On the data of 120 x-ray pairs from eight IRB approved prostate patients, the gradient difference showed the best accuracy. In the clinical applications, registrations starting with the mutual information followed by the gradient difference may provide the best accuracy and the most robustness.

On-Board Imager-based MammoSite treatment verification

Contemporary radiation oncology departments are often lacking a conventional simulator due to common use of virtual simulation and recent implementation of image guided radiation therapy. A protocol based on MammoSite method was developed using CT based planning, a Source Position Simulator (SPS) with a Simulator Wire and a linear accelerator based On-Board Imager (OBI) for daily verification. After MammoSite balloon implantation, the patient undergoes a CT study. The images are evaluated for tissue conformance, balloon symmetry, and balloon surface to skin distance according to the departmental procedure. Prior to the CT study the SPS is attached to the transfer tube that in turn is attached to the balloon catheter. The length from the indexer to the first dwell position is measured using the simulator wire with X-ray markers. After the CT study is performed, the data set is sent to the Varian Eclipse treatment planning system (TPS) and to the Nucletron PLATO brachytherapy planning system. The reference digitally reconstructed radiographs (DRRs) of anterior and lateral setup fields are created using Eclipse TPS and are immediately available on the OBI console via the Varian Vision integrated system. The source dwell position coinciding with the balloon center is identified in the CT dataset, followed by the offset calculation, catheter reconstruction, dose points placement and dwell time calculation. OBI fluoroscopy images are acquired and marked as initial. Prior to each treatment fraction balloon diameter and symmetry are evaluated using OBI fluoroscopy and tools available on the OBI console. Acquired images are compared with reference DRRs and/or initial OBI images. The whole process from initial evaluation to daily verification is filmless and does not undermine the precision of the procedure. This verification time does not exceed 10 min. The balloon diameter correlates well (within 1 mm) between initial CT and OBI verification images. The balloon symmetry is defined with 1 mm accuracy using existing OBI console tools. It is feasible to use OBI based simulation for the MammoSite balloon placement evaluation, balloon integrity daily verification, and treatment dwell position coincidence with balloon center. This verification is a rapid process and is an alternative to the conventional simulator based technique. The simulator wire with X-ray markers for the SPS is the recommended tool for the CT based MammoSite procedure.

A frequency‐based approach to locate common structure for 2D‐3D intensity‐based registration of setup images in prostate radiotherapy

In many radiotherapy clinics, geometric uncertainties in the delivery of 3D conformal radiation therapy and intensity modulated radiation therapy of the prostate are reduced by aligning the patient's bony anatomy in the planning 3D CT to corresponding bony anatomy in 2D portal images acquired before every treatment fraction. In this paper, we seek to determine if there is a frequency band within the portal images and the digitally reconstructed radiographs (DRRs) of the planning CT in which bony anatomy predominates over non-bony anatomy such that portal images and DRRs can be suitably filtered to achieve high registration accuracy in an automated 2D-3D single portal intensity-based registration framework. Two similarity measures, mutual information and the Pearson correlation coefficient were tested on carefully collected gold-standard data consisting of a kilovoltage cone-beam CT (CBCT) and megavoltage portal images in the anterior-posterior (AP) view of an anthropomorphic phantom acquired under clinical conditions at known poses, and on patient data. It was found that filtering the portal images and DRRs during the registration considerably improved registration performance. Without filtering, the registration did not always converge while with filtering it always converged to an accurate solution. For the pose-determination experiments conducted on the anthropomorphic phantom with the correlation coefficient, the mean (and standard deviation) of the absolute errors in recovering each of the six transformation parameters were Theta(x):0.18(0.19) degrees, Theta(y):0.04(0.04) degrees, Theta(z):0.04(0.02) degrees, t(x):0.14(0.15) mm, t(y):0.09(0.05) mm, and t(z):0.49(0.40) mm. The mutual information-based registration with filtered images also resulted in similarly small errors. For the patient data, visual inspection of the superimposed registered images showed that they were correctly aligned in all instances. The results presented in this paper suggest that robust and accurate registration can be achieved with intensity-based methods by focusing on rigid bony structures in the images while diminishing the influence of artifacts with similar frequencies as soft tissue.

SU-FF-T-154: Development of An Immobilization Device for Improving Setup Reproducibility for Patients with Prostate Cancer

Purpose: To design the device improving the setup reproducibility of prostate cancer patients during radiation therapy. Method and Materials: A customized device made of polyurethane was designed to immobilize patient's knee, thigh and foot. It was composed of ‘Foot rest supine’, ‘Knee rest supine’ and ‘Knee rest prone’. 61 patients with prostate cancer were selected and divided into two groups to examine the performance of the devices; patients in one group used immobilization device and patients in the other group did not use that. The setup error was measured with respect to bony landmark in the three axes; medial-lateral (ML), anterior-posterior (AP) and cranial-caudal (CC). 1) The difference between digitally reconstructed radiograph (DRR) and simulation film, 2) the difference between DRR and portal film, and 3) the standard deviation among portal films are measured to evaluate the reproducibility. The setup error over 3 mm and 5 mm were measured. Finally, the results were analyzed statistically by using standard t-test. Results: Without immobilization device, the ML, AP and CC error between DRR and simulation film were 1.5±0.9 mm, 3.6±3.6 mm and 1.6±0.9 mm respectively and between DRR and portal film were 1.6±1.2 mm, 4.0±4.1 mm, and 4.2±5.5 mm respectively. The standard deviations along each axis among the portal films were 1.1 mm, 2.1 mm and 1.0 mm. With immobilization device, the ML, AP and CC error were 1.3±1.9 mm, 1.8±1.5 mm and 1.1±1.1 mm respectively between DRR and simulation film and 1.0±1.8 mm, 1.2±0.9 mm, and 1.2±0.8 mm respectively between DRR and portal film. The standard deviations were 0.9 mm, 1.6 mm and 0.8 mm. The percentage set up error greater than 3 mm and 5 mm were reduced by using immobilization device. The data were statistically significant. Conclusion: The immobilization device improved setup reproducibility and reduced critical set up error.

SU‐FF‐I‐39: Development of Clinical Application Platform Using Digital Tomosynthesis for Target Localization

Purpose: To develop a workstation for clinical application of digital tomosynthesis (DTS) for real‐time target localization. Specifically, to accelerate the reconstruction both digitally‐reconstructed radiograph (DRR) and DTS using ultra‐fast interpolation capability of graphics hardware. Method and Materials: The developed workstation for real‐time target localization using DTS technology includes image acquisition, image reconstruction, and image comparison. A critical step to implement this technology is its reconstruction speed, especially real‐time generation of reference DTSs which require DRRs. The new generation graphics hardware provides a fast implementation of image interpolation and allows the bi‐linear/tri‐linear interpolation and the other image operations to be accomplished within second. We implemented classical ray‐casting algorithm for 2D DRR and filtered back‐projection (FBP) algorithm for DTS reconstruction on graphics hardware based on OpenGL programming interface. Specifically, planning CT images and on‐board projection images were loaded into the memory of hardware and saved as textures which were projected onto frame‐buffer according to the given geometry of imaging system. The final images were generated by blending multiple textures projected onto the frame‐buffer. Difference images and correlation coefficients between hardware and software DRRs and DTSs were calculated to assess their similarity. Results: The mean errors and their standard deviations of the differences DRR and DTS images are 1.0% ± 1.5% and 1.2% ± 2.1%. The correlation coefficients are 99.9% for DRRs and 99.5% for DTS. The reconstructed efficiency was improved by a factor of 67 for DRR and 13 for DTS. Conclusions: The hardware method provides fast reconstructions of DRR and DTS with visibility of anatomical structures comparable to those generated using software methods.This allows real‐time target localization for image registration between on‐board and reference DTSs in the application platform.

SU‐FF‐J‐28: A Software Tool for On‐Line Real‐Time Verification of Gated Delivery Using Megavoltage Fluoroscopy

Purpose: To test and to explore the application of a software tool developed for on‐line real‐time treatment verification for 4D/gated radiation delivery using megavoltage fluoroscopy (MVF) acquired with treatment beams. Method and Materials: MVF images were acquired with a flat panel detector installed on a Siemens Linac using 6MV photon beams for phantoms with moving structures and for thoracic cancer patients. A previously developed software, RTReg4D, was used to register acquired MVF with digitally reconstructed radiographs (DRRs) from 4DCT. The MVF frame grabber can be synchronized with the gating signal from Anzai gating device to capture target motion in selected gating window. The isocenter and target contours were transferred from a planning system into RTReg4D and were overlaid on the MVF images, verifying whether the target with residual motion is adequately covered during the gated delivery. Appropriate actions can be initiated should the verification fail. Results: The software tool can effectively and accurately register MVF images with DRRs. The MVF images acquired with a dose as low as 1 cGy were adequate for the registration. In addition, the registration can be performed between the averaged MVF frame of all frames acquired within gating window and the corresponding DRR, further reducing dose required. The accuracy of registration can be better than 3 mm. The entire process is fast. Conclusion: MVF achieves adequate image quality at a low dose. It is practically feasible to use the software tool with MVF for real‐time (prior or during) treatment verification of 4D/gated radiation delivery.Conflict of Interest: This work is sponsored in part by Siemens OCS.

SU‐FF‐J‐32: A Feasibility Study of Image‐Guided Non‐Invasive Single‐Fraction Stereotactic Radiosurgery Using the 2D2D Match of An On‐Board Imaging System

Purpose: To investigate the feasibility of image‐guided non‐invasive single‐fraction stereotactic radiosurgery (SRS) using the 2D2D match of a kilo‐voltage on‐board imager (kVOBI). Method and Materials: An institutional‐approved prospective protocol was developed to evaluate the accuracy of frameless SRS over our current invasive system. Each participating patient was immobilized with a thermoplastic mask covering the face and shoulders. Anterior‐posterior and lateral digitally reconstructed radiographs (DRRs) were generated based on the treatment isocenter identified on the planning CT scan. The patient was then transferred to the treatment room and setup by manually aligning an orthogonal pair of kVOBI projection images with the corresponding DRRs. The patient remained in the treatment position for 20 minutes to simulate a mock SRS treatment. Two cone‐beam CT (CBCT) scans, acquired at the beginning and end of the procedure, were fused with the simulation CT scan to determine the setup accuracy and cumulative intra‐fraction movement. For comparison, the setup accuracy was also measured for the same patient in an invasive SRS frame. Results: Three patients were successfully recruited and studied to date. The setup accuracy (distance between the center of the first CBCT scan and planning isocenter) was 2.25±0.67 mm (mean ± one standard deviation). The cumulative intra‐fractional movement was 1.91±0.30 mm, mainly in the superior‐inferior direction (1.79±0.45 mm). The total setup error at the end of treatment (the second CBCT scan) was 3.89±0.09 mm. The setup accuracy of invasive SRS frame was superior (1.63±1.39 mm) in comparison to the mask system. Conclusion: The setup accuracy of two‐dimensional kVOBI was insufficient for single‐fraction SRS. Volumetric CBCT scanning should be used instead for frameless SRS setup. The observed cumulative intra‐fractional was significant for the mask system and should be monitored and corrected using a realtime position feedback device.

Automatic Target Localization and Verification for On-Line Image-Guided Stereotactic Body Radiotherapy of the Spine

To facilitate image-guided stereotactic body radiotherapy (IG-SBRT) of spinal and paraspinal tumors, the authors have developed an on-line image registration system for automated target localization and patient position verification with high precision. When rotations are present in a patient's daily setup position, a setup error of a few millimeters can be introduced in localization of the isocenter by using surrounding bony structures. This setup error not only will deteriorate the dose coverage of the tumor, more importantly it will overdose the spinal cord. To resolve this issue, the image registration program developed by the authors detects translational shifts as well as rotational shifts using 3D CT image registration. Unacceptable rotations were corrected by either repositioning the patient or adjusting the treatment couch that was capable of rotational corrections when such a couch was available for clinical use. One pair of orthogonal digitally reconstructed radiographs (DRR) were generated from the daily pretreatment CT scan to compare with the corresponding DRRs generated from the planning CT scan to confirm the target shift correction. After the patient's position was corrected a pair of orthogonal portal images were taken for final verification. The accuracy of the image registration result was found to be within 0.1 mm on a head and neck phantom. Target shifts of a fraction of a millimeter were readily visible in our DRR comparison and portal image verification. The time needed to complete the image registration and DRR comparison was about 3 minutes. An integrated system that combines a high-speed CT scanner and a linear accelerator was used for imaging and treatment delivery. Application of the program in actual IG-SBRT cases demonstrated that it was accurate, fast, and reliable. It serves as a useful tool for image-guided radiotherapy where high precision of target localization is required.

SU‐FF‐T‐51: A Technique for Verification of Isocenter Position in Tangential Field Breast Radiotherapy

Breast irradiation is one of the most challenging problems in radiotherapy due to its complex shape of the target volume and its proximity to the surrounding normal structures. Proper implementation of the planned parameter during treatment is very essential, especially, verification of the planned isocenter position. Small deviations in the positioning of the patient with regard to the beam set up could have a relatively important impact on the treatment volume. Hence, treatment verification and reproducibility of the breast treatment portals is an important step in breast radiotherapy. A simple technique has been proposed in this study to verify the planned isocenter during treatment using electronic portal imaging device. Ten patients were recruited in this study and CT based planning was performed with conventional tangential field technique. For verification purpose, in addition to the standard medial (F1) and lateral (F2) tangential fields, a field (F3) perpendicular to the medial field was used for verification of the treatment portals. Lead markers were placed along the central axis of the two defined fields (F1 & F3) and the separations between the markers were measured on the portal images and verified with the marker separation on the digitally reconstructed radiographs (DRRs). Any deviation will identify the shift in the planned isocenter position during treatment. The average deviation observed between the markers measured from the DRR and portal image was 1.6 mm and 2.1 mm with a standard deviation of 0.4 mm and 0.9 mm for field F1 and F3 respectively. The maximum deviation observed was 3 mm for field F3. This technique will be very useful in patient set‐up for tangential breast radiotherapy.

SU-FF-J-42: Analytical Image Guided Radiotherapy for Head and Neck

Purpose: Present a general cost-effective robust clinical process that can analytically evaluate and correct patient setup error for head and neck radiotherapy by comparing orthogonal megavoltage portal images (PI) with corresponding digitally reconstructed radiographs (DRR). Method and Materials: Adobe Photoshop CS2 is used to interactively segment images. Scripts are available to automate many steps, including image enhancement. MATLAB is a software package with many features for rapid matrix computations and image analysis. The closest point distance (CPD) for each PI point to a DRR point forms a set of homologous points. The translation T that aligns the PI to the DRR is equal to the difference in centers of mass. The original PI points are transformed and the process repeated with an Iterative Closest Point (ICP) algorithm. Analytical results are displayed as a cumulative histogram of the percentage of points exceeding CPD. The total area Σ under the histogram is a general error metric. A color coded image is provided to display the anatomical location of the PI points with respect to the DRR and their CPD. Results: The ICP algorithm always converged. The integral Σ was consistently lower for the analytical IGRT, with a mean reduction of 16%. The process was tested retrospectively by three users. Typical user dependence was below 0.5 mm. The time for segmentation of two images and computations was under ten minutes. The process has been integrated with a Visual Basic Toolbox that guides the user through the required all the steps from importing the daily Portal Image, segmentation, computing the translation, and printing a standard report. Conclusion: Inexpensive commercial software has been shown to be useful for on line analytical IGRT. The process developed is robust, consistently better then IGRT, provides documented error metrics, and can be customized by the user for individual applications.

SU‐FF‐I‐69: A 2D‐3D Registration Quality Evaluator for Patient Positioning in Radiotherapy

Purpose: To construct a registration quality evaluator (RQE) for 2D‐3D registration to automatically determine the “goodness” of a registration for user‐defined error tolerance based on average pixel shift in the region of interest, a more consistent quantification of registration error, rather than conventional units in mm and degree. Method and Materials: RQE was automatically constructed from two orthogonal kilovoltage portal images and their corresponding CT dataset of an anthropomorphic cranial phantom. Digitally reconstructed radiographs (DRRs) were generated from a registered CT dataset by adding known arbitrary displacements. Then normalized mutual information (NMI) values between each pair of DRRs and their corresponding portal images were computed, and associated with the known registration error. Based on a user‐defined error tolerance, each sample, which includes a NMI value and its known registration error, was classified as a successful or unsuccessful registration. Finally, supervised learning was performed to calculate a decision threshold on NMI. To determine the goodness of a registration, the registration error can be estimated by calculating the NMI related to the registration output and comparing the value with the RQE threshold. To estimate RQE performance, radio‐opaque markers were attached to the phantom and marker‐based registrations were carried out independently to establish a gold standard. In the absence of markers, this standard can be established by using multiple runs of a registration algorithm alongside visual verification. Results: RQE training yielded a sensitivity and a specificity of 0.9804 (0.8955–0.9995) and 0.9388 (0.8313–0.9872) respectively at 95% confidence interval. Using test dataset, the sensitivity and the specificity of RQE were 0.939 and 0.937, respectively. Conclusion: Our phantom study showed RQE had very good performance in identifying registration errors in 2D‐3D registrations. As part of an automated patient positioning system, RQE can be combined with a 2D‐3D registration algorithm to avoid local optima and improve robustness.

SU-FF-J-40: Image-Guided Body Stereotactic Radiotherapy Using Cone-Beam Computer Tomography for Treatment of Spinal Tumors

Purpose: To report the technique and result of image‐guided stereotactic body radiotherapy (IG‐SBRT) for treatment of spinal tumors using a conebeam computer tomography(CT) for daily pretreatment imaging.Method and Materials: A spine phantom was first used to study the feasibility and accuracy of image‐guided stereotactic body radiotherapy using cone‐beam CT before one patient was treated with a prescription of 2700 cGy to the lesion at the level of L5 in three fractions. Planning CTs were taken using regular CT machines, and pretreatment daily CT scans were taken using the cone‐beam CT equipped on a Varian Trilogy Linac treatment machine. Pretreatment daily CTimages were first registered with the planning CTimages to find out the difference of the daily isocenter from its planning position, then shifts were made by moving the treatment couch to align the patient's daily isocenter to that of the treatment machine. A set of three reference points was used in each case to add accuracy to the localization and alignment of the daily isocenter. As final verification, the KV on‐board imaging (OBI) device was used to take orthogonal portal images to be registered with planning digitally reconstructed radiographs(DRR) before treatments were delivered. Results:Image registration found shifts of the spine phantom to be 16.4 mm laterally, 25.1 mm vertically, and 16.8 mm longitudinally. After shifting the couch accordingly, portal images showed perfect match in vertical and longitudinal directions, only 0.5 mm shift was detected in the lateral direction. For all the three treatments of the patient, daily shifts in any direction were found to be between 2–28 mm. Portal images demonstrated positioning accuracy within 1 mm. Conclusion: Phantom study and real patient treatment for IG‐SBRT of spinal tumor were successfully performed using cone‐beam CT. Satisfactory accuracy of patient positioning was achieved.

SU-FF-J-79: Quantification of Pitch Angle Deviations in Patients Receiving IMRT for Prostate Cancer, Without the Use of Fiducial Seed Markers

Purpose: To demonstrate the strength of kV cone beam CT(CBCT) over MV portal imaging in the detection and quantification of pitch errors in prostate patients without use of internal fiducial markers. Additionally the cause and subsequent correction of large pitch variations was investigated as the daily use of CBCT had highlighted a small number of patients with deviations is set‐up of more than three degrees. Method and Materials: By using CBCT six variables can be corrected — three translations in the orthogonal axes as well as pitch, yaw, and roll. Five patients and RANDO phantom were studied, with significant differences being seen in pitch angle for two patients in the same direction as couch sag. Slight alterations were made in the position of the patients' knees and ankles in an attempt to reduce the pitch deviations. For all patients right lateral MV portal images and corresponding digitally reconstructedradiographs(DRR) were shown to the attending physician and a senior therapist with the question to determine if pitch greater than 3 degrees could be determined. Results: The deviation needed to create a pitch angle of 5 degrees in the RANDO phantom is significant so couch sag does not explain the deviations seen in a small population of the prostate patients imaged with CBCT. MV images were judged as showing insufficient information to determine pitch and Conclusion: Our clinical practice is that when any angular deviation over 3 degrees is observed in a patient set‐up the attending physician is called to decide how to proceed — treat or set‐up again. The phantom work demonstrated that a pitch variation in a patient set‐up cannot be attributed to couch sag and patient studies show that a few centimeters difference in knee separation lead to up to 3 degrees increase in pitch angle.

Magnetic Resonance–Based Treatment Planning for Prostate Intensity-Modulated Radiotherapy: Creation of Digitally Reconstructed Radiographs

To develop a technique to create magnetic resonance (MR)-based digitally reconstructed radiographs (DRR) for initial patient setup for routine clinical applications of MR-based treatment planning for prostate intensity-modulated radiotherapy. Twenty prostate cancer patients' computed tomography (CT) and MR images were used for the study. Computed tomography and MR images were fused. The pelvic bony structures, including femoral heads, pubic rami, ischium, and ischial tuberosity, that are relevant for routine clinical patient setup were manually contoured on axial MR images. The contoured bony structures were then assigned a bulk density of 2.0 g/cm(3). The MR-based DRRs were generated. The accuracy of the MR-based DDRs was quantitatively evaluated by comparing MR-based DRRs with CT-based DRRs for these patients. For each patient, eight measuring points on both coronal and sagittal DRRs were used for quantitative evaluation. The maximum difference in the mean values of these measurement points was 1.3 +/- 1.6 mm, and the maximum difference in absolute positions was within 3 mm for the 20 patients investigated. Magnetic resonance-based DRRs are comparable to CT-based DRRs for prostate intensity-modulated radiotherapy and can be used for patient treatment setup when MR-based treatment planning is applied clinically.

Novel image registration quality evaluator (RQE) with an implementation for automated patient positioning in cranial radiation therapy

In external beam radiation therapy, digitally reconstructed radiographs (DRRs) and portal images are used to verify patient setup based either on a visual comparison or, less frequently, with automated registration algorithms. A registration algorithm can be trapped in local optima due to irregularity of patient anatomy, image noise and artifacts, and/or out-of-plane shifts, resulting in an incorrect solution. Thus, human observation, which is subjective, is still required to check the registration result. We propose to use a novel image registration quality evaluator (RQE) to automatically identify misregistrations as part of an algorithm-based decision-making process for verification of patient positioning. A RQE, based on an adaptive pattern classifier, is generated from a pair of reference and target images to determine the acceptability of a registration solution given an optimization process. Here we applied our RQE to patient positioning for cranial radiation therapy. We constructed two RQEs-one for the evaluation of intramodal registrations (i.e., portal-portal); the other for intermodal registrations (i.e., portal-DRR). Mutual information, because of its high discriminatory ability compared with other measures (i.e., correlation coefficient and partitioned intensity uniformity), was chosen as the test function for both RQEs. We adopted 1 mm translation and 1 degree rotation as the maximal acceptable registration errors, reflecting desirable clinical setup tolerances for cranial radiation therapy. Receiver operating characteristic analysis was used to evaluate the performance of the RQE, including computations of sensitivity and specificity. The RQEs showed very good performance for both intramodal and intermodal registrations using simulated and phantom data. The sensitivity and the specificity were 0.973 and 0.936, respectively, for the intramodal RQE using phantom data. Whereas the sensitivity and the specificity were 0.961 and 0.758, respectively, for the intermodal RQE using phantom data. Phantom experiments also indicated our RQEs detected out-of-plane deviations exceeding 2.5 mm and 2.50. A preliminary retrospective clinical study of the RQE on cranial portal imaging also yielded good sensitivity > or = 0.857) and specificity (> or = 0.987). Clinical implementation of a RQE could potentially reduce the involvement of the human observer for routine patient positioning verification, while increasing setup accuracy and reducing setup verification time.

Registration of DRRs and portal images for verification of stereotactic body radiotherapy: a feasibility study in lung cancer treatment

Image guidance has become a pre-requisite for hypofractionated radiotherapy where the applied dose per fraction is increased. Particularly in stereotactic body radiotherapy (SBRT) for lung tumours, one has to account for set-up errors and intrafraction tumour motion. In our feasibility study, we compared digitally reconstructed radiographs (DRRs) of lung lesions with MV portal images (PIs) to obtain the displacement of the tumour before irradiation. The verification of the tumour position was performed by rigid intensity based registration and three different merit functions such as the sum of squared pixel intensity differences, normalized cross correlation and normalized mutual information. The registration process then provided a translation vector that defines the displacement of the target in order to align the tumour with the isocentre. To evaluate the registration algorithms, 163 test images were created and subsequently, a lung phantom containing an 8 cm3 tumour was built. In a further step, the registration process was applied on patient data, containing 38 tumours in 113 fractions. To potentially improve registration outcome, two filter types (histogram equalization and display equalization) were applied and their impact on the registration process was evaluated. Generated test images showed an increase in successful registrations when applying a histogram equalization filter whereas the lung phantom study proved the accuracy of the selected algorithms, i.e. deviations of the calculated translation vector for all test algorithms were below 1 mm. For clinical patient data, successful registrations occurred in about 59% of anterior-posterior (AP) and 46% of lateral projections, respectively. When patients with a clinical target volume smaller than 10 cm3 were excluded, successful registrations go up to 90% in AP and 50% in lateral projection. In addition, a reliable identification of the tumour position was found to be difficult for clinical target volumes at the periphery of the lung, close to backbone or diaphragm. Moreover, tumour movement during shallow breathing strongly influences image acquisition for patient positioning. Recapitulating, 2D/3D image registration for lung tumours is an attractive alternative compared to conventional CT verification of the tumour position. Nevertheless, size and location of the tumour are limiting parameters for an accurate registration process.

ESTERR‐PRO: A Setup Verification Software System Using Electronic Portal Imaging

The purpose of the paper is to present and evaluate the performance of a new software-based registration system for patient setup verification, during radiotherapy, using electronic portal images. The estimation of setup errors, using the proposed system, can be accomplished by means of two alternate registration methods. (a) The portal image of the current fraction of the treatment is registered directly with the reference image (digitally reconstructed radiograph (DRR) or simulator image) using a modified manual technique. (b) The portal image of the current fraction of the treatment is registered with the portal image of the first fraction of the treatment (reference portal image) by applying a nearly automated technique based on self-organizing maps, whereas the reference portal has already been registered with a DRR or a simulator image. The proposed system was tested on phantom data and on data from six patients. The root mean square error (RMSE) of the setup estimates was 0.8 ± 0.3 (mean value ± standard deviation) for the phantom data and 0.3 ± 0.3 for the patient data, respectively, by applying the two methodologies. Furthermore, statistical analysis by means of the Wilcoxon nonparametric signed test showed that the results that were obtained by the two methods did not differ significantly (P value > 0.05).

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Enhancing Depth Perception in Translucent Volumes

We present empirical studies that consider the effects of stereopsis and simulated aerial perspective on depth perception in translucent volumes. We consider a purely absorptive lighting model, in which light is not scattered or reflected, but is simply absorbed as it passes through the volume. A purely absorptive lighting model is used, for example, when rendering digitally reconstructed radiographs (DRRs), which are synthetic X-ray images reconstructed from CT volumes. Surgeons make use of DRRs in planning and performing operations, so an improvement of depth perception in DRRs may help diagnosis and surgical planning.

Po-Thur Eve General-12: ProLoc: An image-guided localization software for target positioning

An image‐guided localization system has been developed for daily localization of the prostate using electronic portal images, digitally reconstructedradiographs(DRR) and implanted radio‐opaque fiducial markers or anatomic landmarks. This paper describes the computer program (ProLoc) and its potentials with particular application to the daily localization of the prostate during radiation therapytreatment. Three radio‐opaque markers are implanted in the prostate before radiotherapytreatment. Reference positions of the markers are established from orthogonal DRRimages and the center of mass determined. Before treatment each day, orthogonal images are acquired using the electronic portal image devices (EPID). Projected marker positions and the center of mass are determined and align to the reference position of the center of mass. Couch position adjustment is then performed to reposition the patient to the reference position. This software has the potential to determine prostate movement over the course of routine radiotherapy using implanted markers or anatomic landmarks. It also has the potential to determine marker migration in the prostate and gives an indication of weather the prostate is increasing or decreasing in size. Localization of the prostate within the patient and proper alignment with the radiation beam are critical components of radiotherapy. Daily target of the prostate has been shown to be technically feasible. This system provides the ability to significantly reduce treatment margins for cancers confined to the prostate. ProLoc may be used to measure the marker shifts and positions on the daily portal films with an accuracy of less than 1.5mm.