High-contrast Objects Research Articles

Extended focused imaging for digital holograms of macroscopic three-dimensional objects

When a digital hologram is reconstructed, only points located at the reconstruction distance are in focus. We have developed a novel technique for creating an in-focus image of the macroscopic objects encoded in a digital hologram. This extended focused image is created by combining numerical reconstructions with depth information extracted by using our depth-from-focus algorithm. To our knowledge, this is the first technique that creates extended focused images of digital holograms encoding macroscopic objects. We present results for digital holograms containing low- and high-contrast macroscopic objects.

2P2-B15 人工網膜を用いた単眼視差によるの奥行きの推算(ロボットビジョン)

A monocular optical distance sensing vision system was investigated that was based on an algorithm that utilizes motion parallax. The aim of this work is to build a new, robust, absolute distance sensing strategy suitable for unstructured and unprepared environments. In order to apply this strategy only one camera, which was a silicon retina sensor, was used to calculate a contrast enhanced input image together with a temporal change based on difference images on analog hardware. And Kalman filter was also used to decrease uncertainty from measurements and to do an optimal estimate. In this paper an experiment was performed where a static high contrast object was the primary target and the observer was accelerated with the help of gravity. The experimental data was compared with a Monte-Carlo simulation of the algorithm in order to help us build a noise distribution model of the system dynamics.

Benchmarking a novel ultrasound-CT fusion system for respiratory motion management in radiotherapy: Assessment of spatio-temporal characteristics and comparison to 4DCT

Management of respiratory motion during radiation therapy requires treatment planning and simulation using imaging modalities that possess sufficient spatio-temporal accuracy and precision. An investigation into the use of a novel ultrasound (US) imaging system for assessment of respiratory motion is presented, exploiting its good soft tissue contrast and temporal precision. The system dynamically superimposes the appropriate image plane sampled from a reference CT data set with the corresponding US B-mode image. An articulating arm is used for spatial registration. While the focus of the study was to quantify the system's ability to track respiratory motion, certain unique spatial calibration procedures were devised that render the software potentially valuable to the general research community. These include direct access to all transformation matrix elements and image scaling factors, a manual latency correction function, and a three-point spatial registration procedure that allows the system to be used in any room possessing a traditional radiotherapy laser localization system. Counter-intuitively, it was discovered that a manual procedure for calibrating certain transformation matrix elements produced superior accuracy to that of an algorithmic Levenberg-Marquardt optimization method. The absolute spatial accuracy was verified by comparing the physical locations of phantom test objects measured using the spatially registered US system, and using data from a 3DCT scan of the phantom as a reference. The spatial accuracy of the display superposition was also tested in a similar manner. The system's dynamic properties were then assessed using three methods. First, the overall system response time was studied using a programmable motion phantom. This included US video update, articulating arm update, CT data set resampling, and image display. The next investigation verified the system's ability to measure the range of motion of a moving anatomical test phantom that possessed both high and low contrast test objects. Finally, the system's performance was compared to that of a four-dimensional CT (4DCT) data set. The absolute spatial and display superposition accuracy was found to be better than 2 mm and typically 1 mm. Overall dynamic system response was adequate to produce a mean relative positional error of less than 1 mm if an empiric latency correction of 3 video frames was incorporated. The dynamic CT/US display mode was able to assess phantom motion for both high and low contrast test objects to within 1 mm, and compared favorably to the 4DCT data. The 4DCT movie loop accurately assessed the target motion for both of the high and low contrast objects tested, but the minimum intensity and average intensity reconstructions did not. This investigation demonstrated that this US system possesses sufficient spatio-temporal accuracy to properly assess respiratory motion. Future work will seek to demonstrate efficacy in its clinical application to respiratory motion assessment, particularly for sites in the upper abdomen, where low tissue contrast is evident.

An Eigenfunction Method for Reconstruction of Large-Scale and High-Contrast Objects

A multiple-frequency inverse scattering method that uses eigenfunctions of a scattering operator is extended to image large-scale and high-contrast objects. The extension uses an estimate of the scattering object to form the difference between the scattering by the object and the scattering by the estimate of the object. The scattering potential defined by this difference is expanded in a basis of products of acoustic fields. These fields are defined by eigenfunctions of the scattering operator associated with the estimate. In the case of scattering objects for which the estimate is radial, symmetries in the expressions used to reconstruct the scattering potential greatly reduce the amount of computation. The range of parameters over which the reconstruction method works well is illustrated using calculated scattering by different objects. The method is applied to experimental data from a 48-mm diameter scattering object with tissue-like properties. The image reconstructed from measurements has, relative to a conventional B-scan formed using a low f-number at the same center frequency, significantly higher resolution and less speckle, implying that small, high-contrast structures can be demonstrated clearly using the extended method.

Extension of the reconstruction field of view and truncation correction using sinogram decomposition

We propose a novel truncation correction algorithm that completes unmeasured data outside of the scan field of view, which allows extending the reconstruction field of view. When a patient extends outside the detector coverage the projection data are transversely truncated, which causes severe artifacts. The proposed method utilizes the idea of sinogram decomposition, where we consider sinogram curves corresponding to image points outside the field of view. We propose two ways to estimate the truncated data, one based on the minimum value along the sinogram curve, and the other based on the data values near the edge of truncation. Both estimation methods are combined to achieve uniform image quality improvement from the edge of truncation to the outer side of the extended region. In our evaluation with simulated and real projection data we compare the proposed method with existing methods and investigate the dependence on the amount of truncation. The evaluation shows that the proposed method handles cases when truncation is present on both sides of the detector, or when a high-contrast object is located outside the field of view.

Improving Conditioning of Electromagnetic Surface Integral Equations Using Normalized Field Quantities

When the surface integral equation method is applied to study electromagnetic scattering by dielectric or composite metallic and dielectric objects, the unknowns, i.e., the electric and magnetic surface current densities, and the elements of the system matrix, are often of the very different scales. As a consequence, the system matrix may have a high (singular value) condition number. An efficient method is presented to balance the unknowns and the integral equations, and the elements of the system matrix, too. The method is based on the use of normalized field quantities and unknowns, and carefully chosen scaling factors. In the case of dielectric and composite objects the condition numbers of the SIE matrices can be reduced with several orders of magnitudes by the developed method. In the case of high contrast objects, or if the frequency is very low, the developed method leads also to a clear improvement on the convergence of iterative solutions

Image reconstruction using eigenfunctions of a scattering operator

A multiple-frequency iterative inverse scattering method is reviewed. In the method, scattered acoustic fields are described using a scattering operator with eigenfunctions that correspond to far-field patterns of an effective source distribution. Incident-wave patterns determined using the eigenfunctions focus on the distribution. The focusing properties of the eigenfunctions are employed to reconstruct an object by using products of numerically calculated fields defined by weighted products of the eigenfunctions. The full range of frequencies present in an incident pulse waveform is employed. Iteration using a linearized version of the method and calculated scattering from an estimate of the object permits reconstruction of large-scale high-contrast objects. The method is illustrated using calculated and measured data. In the calculations, an exact solution for scattering from nonconcentric lossy cylinders with parallel axes was employed to obtain the scattered field. In the measurements, a novel ring-transducer system was used to obtain the incident and total fields. The results of simulations and experiments show that the method converges and is accurate for high-contrast large-scale tissue-mimicking objects. Reconstructions using measured scattering from random media have considerably less speckle than conventional b-scan images, implying that the inverse scattering method can clearly demonstrate small high-contrast inclusions.

Segmentation method for three-dimensional visualization of microscopic objects imaged with a confocal laser scanning microscope

This paper is a revision of a paper presented at the SPIE conference on Medical Imaging 2005: Physiology, Function, and Structure from Medical Images, Feb. 2005, San Diego, California. The paper presented there appears (unrefereed) in SPIE Proceedings Vol. 5746. Segmentation, or separating an image into distinct objects, is the key to creating 3-D renderings from serial slice images. This is typically a manual process requiring trained persons to tediously outline and isolate the objects in each image. We describe a template-based semiautomatic segmentation method to aid in the segmentation process and 3-D reconstruction of microscopic objects recorded with a confocal laser scanning microscope (CLSM). The simple and robust algorithm is based on the creation of a user-defined object template, followed by automatic segmentation of the object in each of the remaining image slices. The user guides the process by selecting the initial image slice for the object template, and labeling the object of interest. The algorithm is applied to mathematically defined shapes to verify the performance of the software. The algorithm is then applied to biological samples, including neurons in the common housefly. It is the quest to further understand the visual system of the housefly that provides the opportunity to develop this segmentation algorithm. Further application of this algorithm may extend to other registered and aligned serial section datasets with high contrast objects.

Flying focal spot (FFS) in cone-beam CT

In the beginning of 2004 medical spiral-CT scanners that acquire up to 64 slices simultaneously became available. Most manufacturers use a straightforward acquisition principle, namely an x-ray focus rotating on a circular path and an opposing cylindrical detector whose rotational center coincides with the x-ray focus. The 64-slice scanner available to us, a Somatom Sensation 64 spiral cone-beam CT scanner (Siemens, Medical Solutions, Forchheim, Germany), makes use of a flying focal spot (FFS) that allows for view-by-view deflections of the focal spot in the rotation direction (/spl alpha/FFS) and in the z-direction (zFFS) with the goal of reducing aliasing artifacts. The FFS feature doubles the sampling density in the radial direction (channel direction, /spl alpha/FFS) and in the longitudinal direction (detector row direction or z-direction, zFFS). The cost of increased radial and azimuthal sampling is a two- or four-fold reduction of azimuthal sampling (angular sampling). To compensate for the potential reduction of azimuthal sampling the scanner simply increases the number of detector read-outs (readings) per rotation by a factor two or four. Then, up to four detector readings contribute to what we define as one view or one projection. A significant reduction of in-plane aliasing and of aliasing in the z-direction can be expected. Especially the latter is of importance to spiral CT scans where aliasing is known to produce so-called windmill artifacts. We have derived and analyzed the optimal focal spot deflection values /spl part//spl alpha/ and /spl part/z as they would ideally occur in our scanner. Based upon these we show how image reconstruction can be performed in general. A simulation study showing reconstructions of mathematical phantoms further provides evidence that image quality can be significantly improved with the FFS. Aliasing artifacts, that manifest as streaks emerging from high-contrast objects, and windmill artifacts are reduced by almost an order of magnitude with the FFS compared to a simulation without FFS. Patient images acquired with our 64-slice cone-beam CT scanner support these results.

TU‐C‐ValA‐06: Quantifying the Properties and Accuracy of a Deformable Image Registration Algorithm for 4D Treatment Planning

Purpose: A necessary tool to facilitate automated four‐dimensional and adaptive radiotherapy planning is deformable image registration (DIR). The purpose of the current study was to quantify the accuracy of a DIR algorithm by comparing automatically transferred and manually segmented structures on 4DCT images. Method and Materials: 780 structures were manually segmented on thirteen patient 4DCT image sets each consisting of 10 respiratory phases. A large deformable diffeomorphic DIR algorithm, integrated with a commercial treatment planning system, was used to map each CT set from the inspiration respiratory phase CT image set respiratory phase images. The calculated displacement vector fields were used to deform and transfer structures defined on the inspiration CT to the other respiratory phase CT image sets. The manually and automatically segmented structures were compared using volumetric, displacement, and surface congruence metrics. Results: Deformation with respiration was observed for the lung tumor and normal tissues. This deformation was verified by examining the mapping of high contrast objects, such as the lungs and cord, between image sets. The auto‐ and manual methods showed similar trends, with a smaller difference observed between the GTVs than other structures. The auto‐contoured structures were more consistent both in terms of centroid displacement and volume as a function of respiratory phase than manual contours. 1.6% of the time, deficiencies of manual contouring has been detected using auto contouring. Image artifacts play a crucial role in auto contouring. Conclusion: An automated system is established to auto‐contour structures starting from one 4DCT image phase to other 4DCT image phases. The auto‐contoured structures generally agree with the manually drawn structures. However the auto‐contoured structures are more consistent in trajectory and volume, and also highlighted some large errors in the manually drawn contours. Careful assessment is needed in the presence of 4DCT artifacts.

Driver contrast sensitivity and reaction times as measured through a salt-covered windshield

Background The objective of this study was to use contrast sensitivity measurements to determine the effect of windshield salt film on driver vision. This effect simulates road salt spray that dries in place on a vehicle windshield during winter driving conditions. Methods Ten emmetropic subjects were tested to assess contrast sensitivity functions through windshields that were clear compared with the same stimuli as viewed through windshields coated with salt film. Achromatic stimuli were generated by a VisionWorks system and were presented at 4 spatial frequencies (0.5, 3, 10, and 20 cycles per degree). Results A significant reduction in contrast sensitivity through the salt film was found at all spatial frequencies. In addition, reaction time to detect the stimuli was also found to be increased significantly at 2 of the 4 spatial frequencies through the saline-covered windshield. Conclusion The results of this study indicate that windshield salt film reduces the ability to detect high and low contrast objects in a simulated driving environment as well as increases the subjects’ reaction times.

Pose estimation of known objects during transmission tomographic image reconstruction

We address the problem of image formation in transmission tomography when metal objects of known composition and shape, but unknown pose, are present in the scan subject. Using an alternating minimization (AM) algorithm, derived from a model in which the detected data are viewed as Poisson-distributed photon counts, we seek to eliminate the streaking artifacts commonly seen in filtered back projection images containing high-contrast objects. We show that this algorithm, which minimizes the I-divergence (or equivalently, maximizes the log-likelihood) between the measured data and model-based estimates of the means of the data, converges much faster when knowledge of the high-density materials (such as brachytherapy applicators or prosthetic implants) is exploited. The algorithm incorporates a steepest descent-based method to find the position and orientation (collectively called the pose) of the known objects. This pose is then used to constrain the image pixels to their known attenuation values, or, for example, to form a mask on the "missing" projection data in the shadow of the objects. Results from two-dimensional simulations are shown in this paper. The extension of the model and methods used to three dimensions is outlined.

A simple and stable autofocusing protocol for long multidimensional live cell microscopy

Focus maintenance is a challenging problem in multidimensional wide-field microscopy. Most automated microscopes use software algorithms, which are applied to z-sections of the object, to select for the plane with the best signal to noise ratio. When applied automatically in multidimensional imaging applications, auto focus routines significantly increase light exposure and can become cytotoxic if applied too frequently. In addition, automated focusing procedures can readily focus on unwanted high contrast objects. By labelling a defined position with a fluorescent marker, we were able to separate the focusing procedure from the actual image acquisition positions and therefore overcome some of the major drawbacks of routine auto focus procedures. To implement this method in a multidimensional acquisition experiment, we created a visual basic-based program, which is run prior to each image acquisition. This technique allows tight control of focus whilst keeping light toxicity in live cell imaging experiments to a minimum.

Central Visual Persistences: I. Visual and Kinesthetic Interactions

Phenomena associated with 'central visual persistences' (CPs) are new to both medical and psychological literature. Five subjects have reported similar CPs: positive afterimages following brief fixation of high-contrast objects or drawings and eye closure. CPs duplicate shapes and colors of single objects, lasting for about 15 s. Unlike retinal afterimages, CPs do not move with the eyes but are stable in extrapersonal space during head or body rotations. CPs may reflect sustained neural activity in neurons of association cortex, which mediate object perception. A remarkable finding is that CPs can be moved in any direction by the (unseen) hand holding the original seen object. Moreover, a CP once formed will 'jump' into an extended hand and 'stick' in that hand as it moves about. The apparent size of a CP of a single object is determined by the size of the gap between finger and thumb, even when no object is touched. These CPs can be either magnified or minified via the grip of the extended hand. The felt orientation of the hand-held object will also determine the orientation of the CP seen in that hand. Thus, kinesthetic signals from hand and arm movements can determine perceived location, size, and orientation of CPs. A neural model based on physiological studies of premotor, temporal, parietal, and prefrontal cortices is proposed to account for these novel phenomena.

Microwave-tomographic imaging of the high dielectric-contrast objects using different image-reconstruction approaches

Microwave tomography is an imaging modality based on differentiation of dielectric properties of an object. The dielectric properties of biological tissues and its functional changes have high medical significance. Biomedical applications of microwave tomography are a very complicated and challenging problem, from both technical and image reconstruction point-of-views. The high contrast in tissue dielectric properties presenting significant advantage for diagnostic purposes possesses a very challenging problem from an image-reconstruction prospective. Different imaging approaches have been developed to attack the problem, such as two-dimensional (2-D) and three-dimensional (3-D), minimization, and iteration schemes. The goal of this research is to study imaging performance of the Newton and the multiplicative regularized contrast source inversion (MR-CSI) methods in 2-D geometry and gradient and MR-CSI methods in 3-D geometry using high-contrast, medium-size phantoms, and biological objects. Experiments were conducted on phantoms and excised segment of a pig hind-leg using a 3-D microwave-tomographic system operating at frequencies of 0.9 and 2.05 GHz. Both objects being of medium size (10-15 cm) possess high dielectric contrasts. Reconstructed images were obtained using all imaging approaches. Different approaches are evaluated and discussed based on its performance and quality of reconstructed images.

Po-Poster - 02: Generation of MV cone beam CT - a feasibility study

The objective of this work was to study the feasibility of acquiring mega-voltage cone-beam CT (MV CBCT) images of sufficient quality for setup verification from 2 D projection images. The generation of mega-voltage cone-beam CT with a standard radiation therapy Primus linear accelerator and an amorphous silicon electronic portal imaging device has been attempted in this work. 2 D projection images of the head section of a Rando phantom were acquired at an interval of 2° from gantry angle of 258° to 102°. Thus 103 2D projection images of the Rando phantom were acquired with 6 MV beam with 1 MU/projection exposure at 300MU/minute dose rate. The estimated dose to the center of the phantom placed at the isocentre of the linac was about 75cGy. The cone angle was only 14.2° and hence for simplicity the images were reconstructed assuming a parallel beam geometry using the IRADON function implemented in MATLAB. High contrast objects are clearly seen on the reconstructed images thus showing the potential of MV CBCT for patient alignment during external beam radiotherapy. From the available image data set, reconstructions were also performed with fewer projections i.e with 52 and 26. Image quality was acceptable with 103 and 52 projections. Reconstruction with Feldkamp cone beam algorithm or iterative algorithm such as Algebraic Reconstruction Techniques (ART) could further improve the image quality.

SU-FF-I-53: The Impact of Portal Imager Shifts and the Assumption of Rigid Isocentricity On Megavoltage Cone Beam CT Images

Purpose: Linear accelerators and the current flat panel positioners do not represent a rigid isocentric imaging system. To correct for this effect in the reconstruction process of Megavoltage Cone Beam CT, the group uses projection matrices that are obtained from geometric calibration. This project studies the effect of deviations in detector position from the calibrated geometry which may occur over time. Method and Materials: To simulate the effect of portal imager shifts, we translated projection images before performing the reconstruction. Synthetic projection matrices that assume perfect isocentricity were also produced to study the utility of our geometric calibration. To accentuate the observed effects, we first used noise-free simulated projections of a CT phantom as well as reconstructions of small, high-contrast ball bearings. We also acquired anatomical images of a Rando head with an acceptable clinical dose (8 MU) to verify our capacity to identify the previously observed artifacts. Results: A pure flat panel shift of 2 mm along the longitudinal direction causes the same shift in the reconstruction volume. A 2 mm shift in the lateral direction however, greatly degrades the image quality with streaking and half-moon shadow artifacts. The orientation of those artifacts depends on the start and end angle of the acquisition. Since most of the flat panel flex was previously measured to be in the longitudinal direction, the use of synthetic projection matrices caused a blurring of the image in the longitudinal direction. Conclusion: The calibrated projection matrices play an important role at conserving the image quality around high contrast objects. A 2 mm lateral shift away from the calibration will likely be detectable in high contrast regions of anatomical images. Longitudinal shifts will not degrade the image but will cause a positioning error. Conflict of Interest: This research is supported by Siemens OCS.

SU-FF-J-09: Comparison of Auto-Contouring with Manual Contouring: A First Step Towards Automated 4D Treatment Planning

Purpose: Four-dimensional (4D) radiotherapy is the explicit inclusion of the temporal changes in anatomy during the imaging, planning and delivery of radiotherapy. One key component of 4D radiotherapy planning, is the ability to auto contour the individual respiratory phase CT datasets (up to ten in total) comprising a 4D computed tomography (CT) scan. A tool that can be used to automatically generate such contours (based on contours manually drawn on a single CT phase) is deformable image registration. The purpose of the current study was to compare automatically generated contours with manually drawn contours. Method and Materials: Two out of ten patient 4D CT data sets have completed this study. The 4D CT scans consisted of a series of ten 3D CT image sets acquired at different respiratory phases. Large deformable diffeomorphic image registration was performed to map each CT set from the peak-inhale respiration phase to the CT image sets corresponding with subsequent respiration phases. The calculated displacement vector fields were used to deform contours defined on the peak-inhale CT automatically to the other respiratory phase CT image sets. Auto-contouring was performed on each of the ten 3D image sets via automated scripts. Treatment planning system, Pinnacle version 7.7 was interfaced with automated scripts to view the resulting images and to obtain the volumetric and displacement information. Results: Deformation with respiration was observed for the lung tumor and normal tissues. This deformation was verified by examining the mapping of high contrast objects, such as the lungs and cord, between image sets. Conclusion: An automated system is established to auto - contour the ROI's starting form the ROI's in the inhale phase to the other phases of the respiratory motion using Pinnacle Treatment planning system. Auto-contoured organs and the GTV in the Thorax agree with the manually drawn ROI's.

Four‐dimensional radiotherapy planning for DMLC‐based respiratory motion tracking

Four-dimensional (4D) radiotherapy is the explicit inclusion of the temporal changes in anatomy during the imaging, planning, and delivery of radiotherapy. Temporal anatomic changes can occur for many reasons, though the focus of the current investigation is respiration motion for lung tumors. The aim of this study was to develop 4D radiotherapy treatment-planning methodology for DMLC-based respiratory motion tracking. A 4D computed tomography (CT) scan consisting of a series of eight 3D CT image sets acquired at different respiratory phases was used for treatment planning. Deformable image registration was performed to map each CT set from the peak-inhale respiration phase to the CT image sets corresponding to subsequent respiration phases. Deformable registration allows the contours defined on the peak-inhale CT to be automatically transferred to the other respiratory phase CT image sets. Treatment planning was simultaneously performed on each of the eight 3D image sets via automated scripts in which the MLC-defined beam aperture conforms to the PTV (which in this case equaled the GTV due to CT scan length limitations) plus a penumbral margin at each respiratory phase. The dose distribution from each respiratory phase CT image set was mapped back to the peak-inhale CT image set for analysis. The treatment intent of 4D planning is that the radiation beam defined by the DMLC tracks the respiration-induced target motion based on a feedback loop including the respiration signal to a real-time MLC controller. Deformation with respiration was observed for the lung tumor and normal tissues. This deformation was verified by examining the mapping of high contrast objects, such as the lungs and cord, between image sets. For the test case, dosimetric reductions for the cord, heart, and lungs were found for 4D planning compared with 3D planning. 4D radiotherapy planning for DMLC-based respiratory motion tracking is feasible and may offer tumor dose escalation and/or a reduction in treatment-related complications. However, 4D planning requires new planning tools, such as deformable registration and automated treatment planning on multiple CT image sets.

A novel hardware design for image and dose guided radiotherapy

Conventional radiation therapy assumes that possible movements of organs during therapy can either be neglected or taken into account in the planning phase. In this presentation, we describe a novel imaging hardware set-up to overcome this limitation. The imaging hardware was designed and integrated into a standard medical accelerator to provide a tomographic and a beams eye view imaging which provides a radiographic localisation of bone and soft-tissue targets. Furthermore this novel imaging hardware provides also a system for verifying of the delivered dose by means of entrance dosimetry. For imaging purposes a kV X-ray tube (POWERPHOS, SIEMENS) was integrated with a SIEMENS Primus linear accelerator. The kV X-ray tube was mounted opposite to the source of the therapy beam. As a detector a flat-panel device RID1640 (Perkin Elmer Optoelectronics, Germany) was mounted on the side of the therapy beam in front of the patient. The amorphous silicon flat-panel imager provides 1024 × 1024 pixels with 400 mm pitch and an active area of 40.96cm × 40.96cm. While the flat-panel is irradiated with the kV x-ray from its front side, the detector is irradiated from the back side with the MV therapy beam. This imaging set-up provides on one hand a tomographic cone beam imaging system which allows monitoring the patient’s anatomy in treatment position just before the start of the treatment fraction. On the other hand it allows acquiring an image of a volume of interest of the patient e.g. the treatment volume in fluoroscopic mode during therapy beam application utilising the pulse recess of the Linac beam. Because both the therapy beam and the diagnostic x-ray beam are mounted on the same beam axis the fluoroscopic images can be used to match these fluoroscopic views with images of the shape of the treatment. These images are acquired nearly at the same time just with the same flat-panel detector by an interlaced read out of the detector between MV and kV X-ray beam. For CT image reconstruction we use a standard 3D algorithm (Feldkamp, Kress). The flat-panel imager is used as a 2D beam monitor to record the intensity modulated fluence distribution of incoming photons of the therapy beam. Different sample IMRT plans based on a solid water phantom and an anthropomorphic head and body phantom were generated using the Virtuos/KonRad inverse planning system (DKFZ / SIEMENS, Heidelberg, Germany). The IMRT beams from these pre-calculated treatment plans were delivered and recorded with the flat-panel imager by acquiring sequences of images. The treatment set-up of the phantom was verified by the imaging system. Compared to the dose of a conventional CT first results of the tomographic cone beam images show with a similar dose a contrast resolution of about 1% for objects larger than 3mm and a spatial resolution of about 1mm for high contrast objects. A very good agreement could be seen between planned and measured entrance fluence distributions for each IMRT beam. The error was calculated to be less than ±2%. An error of less then ±2mm was seen for isodose lines down to the 50% line. For the lower isodose lines or regions with steep dose gradients the error somewhat increases. Different dose measurements in a water tank indicate that the therapy beam quality is not significant affected by the flat-panel. Only the absolute dose is reduced by 10% because of the additional attenuation. We have integrated an imaging system with a medical linear accelerator for kV tomographic and beam’s eye view imaging which provides a radiographic target localisation and for delivered therapy beam verification. Preliminary results show that image acquisition in tomographic and radiographic mode gives high quality images, sufficient for patient set-up and for target localisation before and during therapy beam application. A method of in vivo dose reconstruction based on measurements of entrance beams was developed and tested by means of comparisons with sample IMRT plans. Generally good agreement was seen between planning, reconstruction and direct measurement of the three-dimensional dose distribution