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40 Oral Automatic detection of fiducial markers and bone structures using wavelet transformation
Proffered papers
40
Oral
Automatic detection o f f i d u c i a l markers and bone structures using wavelet transformation D. Buck, M. Alber, F. NOsslin Universit~tsklinikum for Radioonkologie, Medizinische Physik, TObin. gen, Germany Amorphous-silicon flat-panel imagers allow in principle to take high-resolution (0.25mm) portal images with 1-3MU, which could make them valuable for Image-Guided Radiotherapy which requires the fully automatic measurement of setup error and organ motion. The low contrast and noise of the portal images (PI) at low exposure put tools for automatic setup correction to a difficult test. A toolkit for the automatic detection of fiducial markers and pelvic bone anatomy is presented, The automatic extraction of anatomical structures from the PI is complicated by the presence of artefacts, gas pockets and inhomogeneous background due to varying patient diameter. These effects, which are of the same magnitude as bone edge contrasts, as well as the noisy character of the images render ordinary edge detection algorithms useless. We propose a concept based on wavelet transformation to exploit the favourable properties of a multi-scale analysis. The wavelet types Daubechies4 and Haar are employed to generate a multi-scale filter that uses information of different image scales to extract all relevant anatomical structures. Simultaneous edge detection on a hierarchy of wavelet-transformed images of different scales permits to extract even low-contrast bone edge structures with a high tolerance to noise. The filtered images are numerically matched with reference DRRs. In addition, we developed a fully automatic method to detect fiducial spheric tungsten markers. The concept generalises a gradient filter and is based on wavelet-like Mexican-Hat(MH)-filters. These filters are especially adapted to the particular signal of markers in the PI and are obtained by second order derivative of the ideal marker signal. The benefit of this approach is the high selectivity of the MH-filter. Hence it is possible to detect markers with high precision and reliability despite the presence of comparatively larger contrast fluctuations, The presented method of marker detection offers a detection probability higher than 95% reaching a detection resolution down to 1 .Smm, when original patient images are analysed. A series of setup verification PIs of the pelvic region were processed with the wavelet-based filter and matched with the reference DRRs. The average deviation between manually and automatically determined setup errors were 0.6mm in lateral and 1.0mm in ap and cc direction, The method presented allows to do automatic online setup verification and determination of organ motion, 41
Oral
"Gated portal imaging": a new acquisition mode for p o r t a l v i sion as500 D. Vetter/i 1, H. Riem2, P. Manset 3, E.J. Born 1, R. Mini 1, P. ROegsegger3 1/nse/spita/ / University of Berne, Division of Medical Radiation Physics, Berne, Switzerland 2Varian Medical Systems, Baden, Switzerland 3ETH Zurich, Institute for Biomedical Engineering, Zurich, Switzerland Purpose: During the acquisition of a portal image extra dose is delivered to the patient when increasing the treatment field size to make relevant anatomical structures visible. The current readout scheme for the PortalVision aS500 (Varian Medical Systems), where the beam is ON during the entire acquisition of the image, was derived from the experiences with the liquid ionization chamber EPID and the existing Clinac interface. However, this scheme has the drawback that valuable dose is thrown away during detector reset. In order to reduce the dose necessary for the acquisition of a portal image (3 MU) while keeping or even improving the image quality, a new acquisition mode for the aS500, called "Gated Portal Imaging", was developed. Method: By selectively enabling / disabling beam pulses it is possible to stop wasting dose. This new scheme requires different signals from the Clinac and software changes to the IAS2 (Image Acquisition System 2) ACPU program. For "Gated Portal Imaging" the IAS2 needs only control of the Ext.Hold signal that is used to delay gun pulses in order to stop the production of x-rays. After beam ON, the IAS2 asserts the hold line after a definable start delay time and n reset frames are taken; no dose is wasted during this time. The detector read out is stopped and beam is enabled for a definable time (e.g. 300 ms). Beam is disabled again and the detector read out. When multiple frames are required the cycle is repeated. Results: With the new mode two frames can be acquired when setting up the Clinac with 1 MU, i.e. less than 1 MU is actually needed for an image.
Wednesday, 18 September 2002 S13
"1 MU"-images acquired with the new mode are compared to images acquired with standard modes (>3 MU). No loss in image quality could be found. As a first application of this new mode we performed the setup verification of an IMRT-patient via two additional orthogonal setup fields. The extra dose delivered to the patient is negligible. Conclusion: A new acquisition mode for PortalVision aS500 allowing to take portal images of excellent quality with only 1 MU was developed. In the meantime the new mode is used in clinical routine for all patients with open field setup verification. Orthogonal setup fields will facilitate correct patient positioning with complex (e.g. non coplanar) treatment field arrangements. 42
Oral
Intra observer accuracy on patient set-up, evaluating DRRS and simulator images as a reference V.N. Hansen 1, M. Bollet2, H. McNait 2, A. Norman 3, U. O'Doherty 2, H. Taylot2-, M. Rose 2, R. Huddart2 1Royal Marsden NHS Trust, physics, Sutton, Surrey, United Kingdom 2Royal Marsden NHS Trust, radiotherapy, Sutton, Surrey, United Kingdom 3Royal Marsden NHS Trust, Statistics, Sutton, Surrey, United Kingdom Background: With the increased use of 3D planning systems where good quality Digitally Reconstructed Radiographs (DRRs) can be produced, a study was designed to evaluate the precision of using DRRs of either 3 mm or 6 mm slice separation versus using simulator images for the set-up of prostate patients. In addition the study will reveal if a transfer error, (i.e., difference in the set-up using DRRs, derived from CT) existed between CT and simulator. Methods: 20 patients were CT scanned with 3mm slice spacing/3ram slice width. DRRs were generated for both 3mm and 6ram separation; these DRRs and a simulator image of an anterior and a lateral field were used as reference images. 5 observers matched each of the reference images to treatment images using the Theraview~ "Target check" facility. Automatic field edge detection and field edge placement was performed. The observers had to draw an anatomical template on each of the reference images to use for manual overlay on the treatment image to assess the setup accuracy. Results and discussion: The choice of anatomy outlined by each of the observers is very similar for the anterior fields, however, for the lateral fields the observers differ in choice of anatomy. Probably the differences in template play a major role in the difference of the standard deviations found for the set-up errors. The intra observer standard deviation was ~1 mm for the anterior fields in both cranio-caudal and lateral directions and for all three reference images. The intra observer standard deviation was -2ram for the lateral fields in both cranio-caudal and and ant-post directions for all three reference images. The set-up errors found were not statistically significant different using any of the 3 types of reference images. However, all observers found it more dif-
ficult to make the anatomical template for the DRRs of 6mm separation due to the poorer image quality. The transfer error detected was very small, 1.4ram, and only in ihe craniocaudal direction. This may be due to use of CT information at time of simulation. Conclusion: The small differences in intra-observer accuracy suggest that the precision of evaluating the set-up error using DRR's is equivalent to using simulator films.
40
Oral
Automatic detection o f f i d u c i a l markers and bone structures using wavelet transformation D. Buck, M. Alber, F. NOsslin Universit~tsklinikum for Radioonkologie, Medizinische Physik, TObin. gen, Germany Amorphous-silicon flat-panel imagers allow in principle to take high-resolution (0.25mm) portal images with 1-3MU, which could make them valuable for Image-Guided Radiotherapy which requires the fully automatic measurement of setup error and organ motion. The low contrast and noise of the portal images (PI) at low exposure put tools for automatic setup correction to a difficult test. A toolkit for the automatic detection of fiducial markers and pelvic bone anatomy is presented, The automatic extraction of anatomical structures from the PI is complicated by the presence of artefacts, gas pockets and inhomogeneous background due to varying patient diameter. These effects, which are of the same magnitude as bone edge contrasts, as well as the noisy character of the images render ordinary edge detection algorithms useless. We propose a concept based on wavelet transformation to exploit the favourable properties of a multi-scale analysis. The wavelet types Daubechies4 and Haar are employed to generate a multi-scale filter that uses information of different image scales to extract all relevant anatomical structures. Simultaneous edge detection on a hierarchy of wavelet-transformed images of different scales permits to extract even low-contrast bone edge structures with a high tolerance to noise. The filtered images are numerically matched with reference DRRs. In addition, we developed a fully automatic method to detect fiducial spheric tungsten markers. The concept generalises a gradient filter and is based on wavelet-like Mexican-Hat(MH)-filters. These filters are especially adapted to the particular signal of markers in the PI and are obtained by second order derivative of the ideal marker signal. The benefit of this approach is the high selectivity of the MH-filter. Hence it is possible to detect markers with high precision and reliability despite the presence of comparatively larger contrast fluctuations, The presented method of marker detection offers a detection probability higher than 95% reaching a detection resolution down to 1 .Smm, when original patient images are analysed. A series of setup verification PIs of the pelvic region were processed with the wavelet-based filter and matched with the reference DRRs. The average deviation between manually and automatically determined setup errors were 0.6mm in lateral and 1.0mm in ap and cc direction, The method presented allows to do automatic online setup verification and determination of organ motion, 41
Oral
"Gated portal imaging": a new acquisition mode for p o r t a l v i sion as500 D. Vetter/i 1, H. Riem2, P. Manset 3, E.J. Born 1, R. Mini 1, P. ROegsegger3 1/nse/spita/ / University of Berne, Division of Medical Radiation Physics, Berne, Switzerland 2Varian Medical Systems, Baden, Switzerland 3ETH Zurich, Institute for Biomedical Engineering, Zurich, Switzerland Purpose: During the acquisition of a portal image extra dose is delivered to the patient when increasing the treatment field size to make relevant anatomical structures visible. The current readout scheme for the PortalVision aS500 (Varian Medical Systems), where the beam is ON during the entire acquisition of the image, was derived from the experiences with the liquid ionization chamber EPID and the existing Clinac interface. However, this scheme has the drawback that valuable dose is thrown away during detector reset. In order to reduce the dose necessary for the acquisition of a portal image (3 MU) while keeping or even improving the image quality, a new acquisition mode for the aS500, called "Gated Portal Imaging", was developed. Method: By selectively enabling / disabling beam pulses it is possible to stop wasting dose. This new scheme requires different signals from the Clinac and software changes to the IAS2 (Image Acquisition System 2) ACPU program. For "Gated Portal Imaging" the IAS2 needs only control of the Ext.Hold signal that is used to delay gun pulses in order to stop the production of x-rays. After beam ON, the IAS2 asserts the hold line after a definable start delay time and n reset frames are taken; no dose is wasted during this time. The detector read out is stopped and beam is enabled for a definable time (e.g. 300 ms). Beam is disabled again and the detector read out. When multiple frames are required the cycle is repeated. Results: With the new mode two frames can be acquired when setting up the Clinac with 1 MU, i.e. less than 1 MU is actually needed for an image.
Wednesday, 18 September 2002 S13
"1 MU"-images acquired with the new mode are compared to images acquired with standard modes (>3 MU). No loss in image quality could be found. As a first application of this new mode we performed the setup verification of an IMRT-patient via two additional orthogonal setup fields. The extra dose delivered to the patient is negligible. Conclusion: A new acquisition mode for PortalVision aS500 allowing to take portal images of excellent quality with only 1 MU was developed. In the meantime the new mode is used in clinical routine for all patients with open field setup verification. Orthogonal setup fields will facilitate correct patient positioning with complex (e.g. non coplanar) treatment field arrangements. 42
Oral
Intra observer accuracy on patient set-up, evaluating DRRS and simulator images as a reference V.N. Hansen 1, M. Bollet2, H. McNait 2, A. Norman 3, U. O'Doherty 2, H. Taylot2-, M. Rose 2, R. Huddart2 1Royal Marsden NHS Trust, physics, Sutton, Surrey, United Kingdom 2Royal Marsden NHS Trust, radiotherapy, Sutton, Surrey, United Kingdom 3Royal Marsden NHS Trust, Statistics, Sutton, Surrey, United Kingdom Background: With the increased use of 3D planning systems where good quality Digitally Reconstructed Radiographs (DRRs) can be produced, a study was designed to evaluate the precision of using DRRs of either 3 mm or 6 mm slice separation versus using simulator images for the set-up of prostate patients. In addition the study will reveal if a transfer error, (i.e., difference in the set-up using DRRs, derived from CT) existed between CT and simulator. Methods: 20 patients were CT scanned with 3mm slice spacing/3ram slice width. DRRs were generated for both 3mm and 6ram separation; these DRRs and a simulator image of an anterior and a lateral field were used as reference images. 5 observers matched each of the reference images to treatment images using the Theraview~ "Target check" facility. Automatic field edge detection and field edge placement was performed. The observers had to draw an anatomical template on each of the reference images to use for manual overlay on the treatment image to assess the setup accuracy. Results and discussion: The choice of anatomy outlined by each of the observers is very similar for the anterior fields, however, for the lateral fields the observers differ in choice of anatomy. Probably the differences in template play a major role in the difference of the standard deviations found for the set-up errors. The intra observer standard deviation was ~1 mm for the anterior fields in both cranio-caudal and lateral directions and for all three reference images. The intra observer standard deviation was -2ram for the lateral fields in both cranio-caudal and and ant-post directions for all three reference images. The set-up errors found were not statistically significant different using any of the 3 types of reference images. However, all observers found it more dif-
ficult to make the anatomical template for the DRRs of 6mm separation due to the poorer image quality. The transfer error detected was very small, 1.4ram, and only in ihe craniocaudal direction. This may be due to use of CT information at time of simulation. Conclusion: The small differences in intra-observer accuracy suggest that the precision of evaluating the set-up error using DRR's is equivalent to using simulator films.