Heinrich Riem

Multiple-gain-ranging readout method to extend the dynamic range of amorphous silicon flat-panel imagers

The dynamic range of many flat panel imaging systems are fundamentally limited by the dynamic range of the charge amplifier and readout signal processing. We developed two new flat panel readout methods that achieve extended dynamic range by changing the read out charge amplifier feedback capacitance dynamically and on a real-time basis. In one method, the feedback capacitor is selected automatically by a level sensing circuit, pixel-by-pixel, based on its exposure level. Alternatively, capacitor selection is driven externally, such that each pixel is read out two (or more) times, each time with increased feedback capacitance. Both methods allow the acquisition of X-ray image data with a dynamic range approaching the fundamental limits of flat panel pixels. Data with an equivalent bit depth of better than 16 bits are made available for further image processing. Successful implementation of these methods requires careful matching of selectable capacitor values and switching thresholds, with the imager noise and sensitivity characteristics, to insure X-ray quantum limited operation over the whole extended dynamic range. Successful implementation also depends on the use of new calibration methods and image reconstruction algorithms, to insure artifact free rebuilding of linear image data by the downstream image processing systems. The multiple gain ranging flat panel readout method extends the utility of flat panel imagers and paves the way to new flat panel applications, such as cone beam CT. We believe that this method will provide a valuable extension to the clinical application of flat panel imagers.

Developments in megavoltage cone beam CT with an amorphous silicon EPID: reduction of exposure and synchronization with respiratory gating.

We have studied the feasibility of a low-dose megavoltage cone beam computed tomography (MV CBCT) system for visualizing the gross tumor volume in respiratory gated radiation treatments of nonsmall-cell lung cancer. The system consists of a commercially available linear accelerator (LINAC), an amorphous silicon electronic portal imaging device, and a respiratory gating system. The gantry movement and beam delivery are controlled using dynamic beam delivery toolbox, a commercial software package for executing scripts to control the LINAC. A specially designed interface box synchronizes the LINAC, image acquisition electronics, and the respiratory gating system. Images are preprocessed to remove artifacts due to detector sag and LINAC output fluctuations. We report on the output, flatness, and symmetry of the images acquired using different imaging parameters. We also examine the quality of three-dimensional (3D) tomographic reconstruction with projection images of anthropomorphic thorax, contrast detail, and motion phantoms. The results show that, with the proper choice of imaging parameters, the flatness and symmetry are reasonably good with as low as three beam pulses per projection image. Resolution of 5% electron density differences is possible in a contrast detail phantom using 100 projections and 30 MU. Synchronization of image acquisition with simulated respiration also eliminated motion artifacts in a moving phantom, demonstrating the systems capability for imaging patients undergoing gated radiation therapy. The acquisition time is limited by the patients respiration (only one image per breathing cycle) and is under 10 min for a scan of 100 projections. In conclusion, we have developed a MV CBCT system using commercially available components to produce 3D reconstructions, with sufficient contrast resolution for localizing a simulated lung tumor, using a dose comparable to portal imaging.

Application of asymmetric cone-beam CT in radiotherapy

In many implementations of cone-beam CT in radiotherapy for target positioning, it is not uncommon that the maximum allowable field of view (FOV) cannot cover the patient because of the limited size of the flat-panel detector. In this situation, the measurements will contain truncated projections, leading to significant artifacts in reconstructed images. Asymmetric cone-beam configurations can be used for increasing the FOV size by displacing the detector panel to one side. From the data acquired with such an asymmetric configuration, the well-known algorithm developed by Feldkamp, Davis, and Kress (FDK) can be modified to reconstruct images. With increasing detector asymmetry, however, the modified FDK algorithm may produce significant aliasing artifacts. In this work, we develop a novel algorithm for image reconstruction in asymmetric cone-beam CT, which can generate images with improved numerical properties and allow for large detector asymmetry. We have employed the asymmetric configuration and the developed algorithm in a cone-beam CT system in radiotherapy to increase the FOV size. Preliminary phantom studies have been conducted to validate the asymmetric configuration and the proposed reconstruction algorithm.

Developments in megavoltage cone beam CT with an amorphous silicon EPID

We have studied the feasibility of a low-dose megavoltage cone beam computed tomography (MV CBCT) system for visualizing the gross tumor volume in respiratory gated radiation treatments of nonsmall-cell lung cancer. The system consists of a commercially available linear accelerator (LINAC), an amorphous silicon electronic portal imaging device, and a respiratory gating system. The gantry movement and beam delivery are controlled using dynamic beam delivery toolbox, a commercial software package for executing scripts to control the LINAC. A specially designed interface box synchronizes the LINAC, image acquisition electronics, and the respiratory gating system. Images are preprocessed to remove artifacts due to detector sag and LINAC output fluctuations. We report on the output, flatness, and symmetry of the images acquired using different imaging parameters. We also examine the quality of three-dimensional (3D) tomographic reconstruction with projection images of anthropomorphic thorax, contrast detail, and motion phantoms. The results show that, with the proper choice of imaging parameters, the flatness and symmetry are reasonably good with as low as three beam pulses per projection image. Resolution of 5% electron density differences is possible in a contrast detail phantom using 100 projections and 30 MU. Synchronization of image acquisition with simulated respiration also eliminated motion artifacts in a moving phantom, demonstrating the systems capability for imaging patients undergoing gated radiation therapy. The acquisition time is limited by the patients respiration (only one image per breathing cycle) and is under 10 min for a scan of 100 projections. In conclusion, we have developed a MV CBCT system using commercially available components to produce 3D reconstructions, with sufficient contrast resolution for localizing a simulated lung tumor, using a dose comparable to portal imaging.

SU‐CC‐J‐6C‐07: Helical Cone‐Beam CT for Radiation Therapy

Purpose: To develop the helical cone‐beam scanning capability on a simulator CTsystem and apply a recently developed backprojection‐filtration (BPF) algorithm to reconstruct exact 3D images from data obtained in this system.Method and Materials: Helical cone‐beam CT is an emerging technology with many advantages over conventional CT scans, such as fast volume coverage speed, efficient use of x‐ray power, and sufficient data for exact 3D image reconstruction. We developed the helical cone‐beam capability on a simulator CT device (Acuity, Varian Medical Systems), which includes a kV x‐ray source, a patient couch, and an amorphous silicon flat‐panel detector (Varian PaxScan 4030CB). We fabricated a small motorized leadscrew mechanism on the couch of the Acuity, which allows for longitudinal translation of the patient couch at a constant speed as the gantry rotates. We applied the BPF algorithm to reconstruct exact 3D images from data obtained in this system.Results: A CatPhan phantom was used for data acquisition. 683 projection data were collected during a one‐turn gantry rotation while the couch was translated longitudinally with a helical pitch of 187.2 mm. The x‐ray source was operated at 125 kVp and 560 mAs. Before projection data were used for reconstruction, a number of corrections were performed, such as bad pixel, dark field, flood field, detector sag. We also applied simple corrections for beam hardening and scattering. 3D images were reconstructed from the corrected data by use of the BPF algorithm. Conclusion: We have developed for the first time the helical scanning capability on a simulator cone‐beam CTsystem and performed a preliminary phantom study. The BPF algorithm was used to reconstruct exact 3D images from the helical cone‐beam data. Such approach can readily be extended to cone‐beam CT components on a LINAC to provide more accurate image representations of the patient.

Developments in megavoltage cone beam CT with an amorphous silicon EPID: Reduction of exposure and synchronization with respiratory gating: Gated megavoltage cone beam CT with aS500 EPID

We have studied the feasibility of a low-dose megavoltage cone beam computed tomography (MV CBCT) system for visualizing the gross tumor volume in respiratory gated radiation treatments of nonsmall-cell lung cancer. The system consists of a commercially available linear accelerator (LINAC), an amorphous silicon electronic portal imaging device, and a respiratory gating system. The gantry movement and beam delivery are controlled using dynamic beam delivery toolbox, a commercial software package for executing scripts to control the LINAC. A specially designed interface box synchronizes the LINAC, image acquisition electronics, and the respiratory gating system. Images are preprocessed to remove artifacts due to detector sag and LINAC output fluctuations. We report on the output, flatness, and symmetry of the images acquired using different imaging parameters. We also examine the quality of three-dimensional (3D) tomographic reconstruction with projection images of anthropomorphic thorax, contrast detail, and motion phantoms. The results show that, with the proper choice of imaging parameters, the flatness and symmetry are reasonably good with as low as three beam pulses per projection image. Resolution of 5% electron density differences is possible in a contrast detail phantom using 100 projections and 30 MU. Synchronization of image acquisition with simulated respiration also eliminated motion artifacts in a moving phantom, demonstrating the systems capability for imaging patients undergoing gated radiation therapy. The acquisition time is limited by the patients respiration (only one image per breathing cycle) and is under 10 min for a scan of 100 projections. In conclusion, we have developed a MV CBCT system using commercially available components to produce 3D reconstructions, with sufficient contrast resolution for localizing a simulated lung tumor, using a dose comparable to portal imaging.