362 Improved Respiratory Gated CT Scan Method

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Purpose: In external radiotherapy, portal images are acquired for patient setup verification. Registration of portal images enables setup deviation measurements. In the presence study, a semi-automated method for registration of breast portal images based on distant transform is evaluated. Methods: The method developed is based on computing the similarity between the edges, with the exception of the field edge, detected on the images to be registered. Edges are detected using the Canny edge detector; the parameters of the detector, i.e. high and low sensitivity thresholds and sigma, are interactively set on the reference image (the image not transformed) and then applied on both the reference and mobile (the image registered to the reference) images. On the reference image, the distance of each pixel to the nearest edge pixel is computed by an approximation of the Euclidean distance, thus generating a distance image. A cost function is defined to measure the similarity between the two edges. Two cost functions were evaluated, namely, the mean and the sum of the pixels of the distance image that correspond to the mobile edge pixels. The registration method was applied to 21 pairs of breast portal images acquired on the Varian PortaIVision Mkl V3.8 imaging system. The reference and mobile images were acquired on the I st and 2nd day of treatment respectively. Manual registration was initially carried out to provide the gold standard; both the manual and the evaluated registration algorithms were included in an in-house software developed on MATLAB. Then, perturbation analysis was carried out to evaluate the consistency and reproducibility of the proposed method. The cost functions were evaluated for 400 sets of transformations of the mobile image (transformation range of +_5° and +_lcm for rotation and translation respectively), and the residual error (i.e. difference between induced and computed transformation) was computed. Results: The average (:I:SD) residual error employing the sum cost function was 0.8±0.7 ° , 1.04±0.85mm and 2.45±1.5mm for rotation and translation along the y and x axis respectively. Improved performance was achieved with the mean cost function resulting in average (±SD) residual error of 0.6±0.4 ° , 0.95±0.4mm and 1.8±1.2mm for rotation and translation along the y and x axis respectively. The mean operator smooths out differences between the edges which do not match perfectly, e.g. when the patient's arm position is not the same in the two acquisitions, and this property justifies its improved performance. Conclusion: A semi-automated method to register breast portal images based on distant transform is presented. Perturbation analysis showed that the method is consistent with an average residual error of 1.9mm and 0.6 ° for translation and rotation respectively. The method is useful to allow setup deviation measurements and to enable image analysis. 361 Evaluation of a mobile C - a r m capable o f cone b e a m CT used for real t i m e positioning: accuracy studies using optical tracking

S. Sorensen ~, M. Mitschke 2, N. Agazaryan ~, P. Medin ~, 7-. Solberg ~ ~UCLA, Radiation Oncology, Los Angeles, USA 2Siemens Solutions Inc, Oncology Care Systems, Concord, USA The purpose of this study is to evaluate the possibility of a mobile C-arm for real time patient setup adjustments. The work is supported in part by Research Scholar Grant # 03028-01-CCE from the American Cancer Society and Siemens. A procedure using a commercial optical tracking system allows the image coordinates of a mobile C-arm to be transformed into world/room coordinates. Reflective markers are placed on both the C-arm and a phantom in an initial calibration procedure. The optical tracking system consists of two IR cameras that can locate the markers in world coordinates. After the calibration procedure only the

reflective markers on the C-arm need to be located at the time of image acquisition. From here, the necessary transformation can be made to take points in the reconstructed image space to world space. The technique was used to make real time shifts to a torso phantom which has a 2 mm BB implanted in the spine. The phantom was placed arbitrarily on the treatment couch and imaged with the C-arm. A transformation of coordinates allows the BB to be located in world space, and a calculation can be done to get the shifts (lateral, ant/post, and sup/inf) to position the BB at isocenter. The shift was implemented using the digital readout on the couch. After making the shift the actual position of the BB was located using the EPID capabilities of the linear accelerator, and the difference was recorded. Multiple blind shifts of the torso phantom were made. The total time from image acquisition to implementing the shifts requires approximately 15 minutes. The average difference and standard deviation of the BBs actual position relative to isocenter in the lateral, ant/post, and sup/inf direction was -0.3 mm (1.2 ram), -0.3 mm (0.8 mm), and 0.0 mm (1.1 mm) respectively. The accuracy and time length of the technique shows that a mobile C-arm can be used to make adjustments to patient setup. A methodology to compare cone-beam images with those from the treatment planning CT is presently under investigation.

362 I m p r o v e d R e s p i r a t o r y Gated CT Scan M e t h o d

S. Mori 1, M. Endo I, H. Asakura 2 1National Institute of Radiological Sciences, Medical Physics, Chia, Japan 2Accelerator Engineering Corporation, Chiba, Japan For the 4D radiationtherapy and IGRT, respiratory gated CT scan method was performed cine scan mode using respiratory signals to obtain all respiratory phases in the wide scan range. This method performed cine scan and the couch moved to the adjacent position. Thus, this process was repeated until the entire scan range. Therefore CT images have different x-ray tube angle to obtain the projection data for each couch position. These different tube angles may degrade image quality. While the 256-detector row CT could obtain volumetric images with approximately 100 mm in CC direction with 0.5 mm slice thickness in one rotation, therefore their x-ray tube positions are same per rotation and there are neither sorting image errors nor respiratory sensing system errors. Therefore, we investigated the fundamental nature of the motion artifact in physical and clinical studies using the 256-detector row CT scanner. We evaluated artifacts and phantom length using the moving phantom AP and CC motions with a velocity of 0 (static), 10, 15, and 20 mm/s at constant. The phantom contained an acrylic ball in the water-filled acrylic cylinder. Scan conditions were 120 kV, 200 mA, 256 x 0.5 mm beam collimation, 25 s scan time, and cine mode. Reconstruction parameters were a voxel size of 0.47 x 0.47 x 0.47 mm 3. For the AP motion, the phantom length in the moving direction is depended on the x-ray tube angle. While those for the CC motion show the phantom diameter plus the moving distance per gantry rotation. With regard to the clinical study, we used domestic pigs, the respiratory motions were tracked by a video camera. The coronal and sagittal images for mid-exhale are shown in Figs. (a) and (b), respectively. These images are same tube angle. Motion artifact of the pulmonary vessel and interlobular septa become visible as double structures. We arranged CT images randomly to simulate the respiratory gated CT method using MDCT (8 × 0.5 mm slice collimation) (Figs. (c) and (d)). These images are obtained at different tube angle. The surface of the abdomen shows smooth in coronal and sagittal sections, however, the motion artifact at the diaphragm shows indentation. In conclusion, the length of the moving object in transverse

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motion is depended on the tube angle, therefore, the respiratory gated CT technique should be explicitly considered of both respiratory phase and tube angle to obtain exactly anatomical structures. These results can be applied to other MDCT.

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363 Investigations into the determination of target volumes using 18FDG-PET-CT images for radiotherapy treatment planning A. Hounsell I K. Carson 2, A. Zatari ~, V. Cosgrove ~, R. Eakin 3, J. Clarke4, D. Stewart 3, L. Fleming 3, P. Jarritt 2 ~Belvoir Park Hospital, Medical Physics, Belfast, UK 2Royal Victoria Hospital, Medical Physics, Belfast UK 3Belvoir Park Hospital, Oncology, Belfast, UK 4Royal Victoria Hospital, Radiology, Belfast, UK Co-registered 18FDG-PET-CT images have been reported as impacting significantly on the staging and selection of patients for radiotherapy. The use of these images for the delineation of target volumes in radiotherapy planning studies has shown that there can be significant differences between target volumes delineated using CT images alone and target volumes delineated using PET and/or PET-CT images. These differences can be due to PET identifying nodal involvement but may also be due to effects such as: respiration; different characteristics of the images; or the PET and CT images showing different morphological and physiological areas of the tumour. An ethically approved study investigating the use of PET-CT images for treatment planning for non-small cell lung cancer patients is underway. All patients recruited have already undergone a staging PET-CT scan and have been referred for radical radiotherapy. As part of this study, the patients have a planning PET-CT scan followed by a respiratory gated PET scan, on a GE Discovery LS PET-CT scanner. A respiratory belt is used to provide the trigger for the gated scans. Experiments have been performed using a phantom containing fillable spheres of different volumes. Various contrasts between these objects and background have been simulated, in order to determine the appropriate threshold value for automatic segmentation of the images. Radiotherapy planning scans were carried out successfully on a combined PET-CT scanner. Practical problems in acquiring gated PET scans have been addressed and gated scans have been performed. These scans have provided information about movement of tumours during respiration. Phantom experiments showed that the threshold required for segmentation of the lesion depends on both lesion size and contrast between lesion and background. Target volumes outlined by clinicians and automatically segmented using thresholding were compared. As the patients had already had a staging PET-CT scan, these volume comparisons were not affected by unsuspected nodal involvement being shown on the planning PET. In conclusion, the combination of planning PET-CT scans on patients who had already had staging PET scans, and respiratory gated studies have allowed investigations into the differences between volumes defined on PET and CT images to be carried out.

364 Linac cone-beam-CT option: A useful tool? A. Stuessi~ C. yon Briel, P. Cossmann Hirslanden Klinik Aarau, Institute of Radiation Oncology, Aarau, Switzerland Purpose: For image-guided radiotherapy (IGRT) the different vendors of linear accelerators offer new kV imaging tools. These systems include - besides a radiographic and fluoroscopic mode - CT functionality. The aim of this study was to evaluate the future potential of a cone beam CT option for patient repositioning as well as therapy planning and to determine image quality in comparison with data from diagnostic CT scanners. Materials and Methods: The Varian On-Board ImagerTM (OBI) cone-beam-CT option consists of a kV-source and a kV-Imager mounted on robotic arms perpendicular to the MV therapy beam. In a single 360 ° rotation a volumetric CT data set of 17cm length can be acquired by collecting fluoroscopic images with up to 900 projections. Two different modalities with either full or half fan setup offer a 26.6 or 48 cm fieldof-view, both with a physical aperture of 88 cm. In order to widen the dynamic range and to prevent saturation of the imager so called bowtie-filters are used to modify the beam profile. Results: Comparisons of data between a diagnostic CT scanner and the cone-beam-CT (CBCT) with regard to topography and Hounsfield unit representation for a humanoid phantom indicate good accordance, the latter within 5%. Low contrast resolution in CBCT slices turns out to be even better than for diagnostic CT scans, i.e. for 0.5% contrast difference an object with 4mm diameter can be detected. The spatial resolution of CBCT data is inferior to a diagnostic CT scan; maximum line pair number per cm is 2 for half fan and 4 for full fan setup. Central axis doses applied for acquisition of one volumetric data set are between 3 and 8 cGy. Preliminary treatment planning studies using CBCT data with a standard Hounsfield unit calibration already give good results, i.e. monitor units compared to planning based on diagnostic CT data are within 2% and the relative dose distributions are nearly identical. Conclusion: This work illustrates that the OBI cone-beamCT option allows a real 3D setup verification followed by a 3D matching which takes also soft tissue information into account. Cone beam CT also is suitable to serve as a control CT for monitoring during the therapy period. As the acquisition time for the scan is around 1 minute, blurring due to organ movements is a limitation. Furthermore the planning studies indicate that CBCT data could probably in the future be used for planning and even allow - in a later stage - online re-planning. 365

On-line patient positioning using On-Board Imager, OBI B. Sorcini, A.C. Severin, A. Isoz, M. Furberg, P. Wers~ll, I. N~slund, R. Odh, W. Wyrsch Karolinska University Hospital, Department of Radiation Oncology and Medical Physics, Stockholm, Sweden Introduction: The use of image guided radiotherapy will provide greater confidence of both conformal targeting and avoidance, whether this is carried out prior to treatment or on the treatment unit may depend on time constraints within departments. The On-Board Imager (Varian medical systems, Palo Alto, CA) is in clinical use since June 2004 at the Karolinska University Hospital. This system used for on-line set-up correction of the patients with and without implanted gold marker. In terms of image quality, radiographic mode with a set of orthogonal kV-kV images seems to be an ideal procedure to set-up the patient accurately. Methods and Materials: In the treatment room the patient is positioned according to the laser marks on the skin. Before