Farhad A. Ghelmansarai

Soft tissue visualization using a highly efficient megavoltage cone beam CT imaging system

Recent developments in two-dimensional x-ray detector technology have made volumetric Cone Beam CT (CBCT) a feasible approach for integration with conventional medical linear accelerators. The requirements of a robust image guidance system for radiation therapy include the challenging combination of soft tissue sensitivity with clinically reasonable doses. The low contrast objects may not be perceptible with MV energies due to the relatively poor signal to noise ratio (SNR) performance. We have developed an imaging system that is optimized for MV and can acquire Megavoltage CBCT images containing soft tissue contrast using a 6MV x-ray beam. This system is capable of resolving relative electron density as low as 1% with clinically acceptable radiation doses. There are many factors such as image noise, x-ray scatter, improper calibration and acquisitions that have a profound effect on the imaging performance of CBCT and in this study attempts were made to optimize these factors in order to maximize the SNR. A QC-3V phantom was used to determine the contrast to noise ratio (CNR) and f50 of a single 2-D projection. The computed f50 was 0.43 lp/mm and the CNR for a radiation dose of 0.02cGy was 43. Clinical Megavoltage CBCT images acquired with this system demonstrate that anatomical structures such as the prostate in a relatively large size patient are visible using radiation doses in range of 6 to 8cGy.

SU‐FF‐J‐07: A Low‐Z Target with No Flattener and Reduced Energy for Improved Contrast in Megavoltage Cone‐Beam CT

Purpose: To improve contrast in MV cone‐beam CT using a low‐Z target (LZT). Method and Materials: A high‐Z tungsten target (HZT) at 6 MV and low‐Z carbon targets of different thicknesses were used with no flattener. The maximum energy was used with LZT while eliminating primary electron leakage into the monitor chamber. Output and dose distributions were measured. Images were acquired at 7 frames/sec with a 1024×1024 400‐micrometer pixel flat panel detector with 1mm Cu build‐up and a Lanex fast scintillator. Phantoms were QC‐3V, a contrast/spatial resolution phantom, a sheep head and a cadaver. Results: Beam energy was 3.5±0.5 MV and 4.5±0.5 MV for the 1.016 cm and 1.35 cm carbon targets, respectively. The higher energy was more stable with higher output: 0.299 cGy/sec compared to 0.084 cGy/sec. Surface dose was 80%. Field diameter at isocenter was limited to 36 cm by the electron monitor, when present. Computed contrast‐to‐noise ratio (CNR) for the contrast‐spatial resolution phantom for LZT was 3.5 times that of HZT at 10 cGy. Computed CNR for HZT for the 2D QC‐3V phantom images is 24 and for LZT is 100 with dose per projection of 0.035cGy. The f50 of LZT is 0.41 lp/mm. Adaptive noise filtering with a kernel of 8×8 increased the CNR by a factor of 2.4 without degrading resolution. Preliminary cone beam images with the low Z target show remarkable bone contrast and indicate improved soft tissue contrast.Conclusion: A stable 4.5 MV beam was produced on a standard treatment linac equipped with a carbon target, showing improved CNR over images taken with the treatment target and sufficient output for cone‐beam CT. A direct comparison to images taken with the carbon and tungsten targets on the same cadaver, expected to show an improvement in soft tissue contrast, will be available at the meeting.

SU-FF-J-81: Clinical Integration of a MV Conebeam CT System for Image-Guided Treatment

Purpose: To perform the integration of a newly developed image‐guidance system and to describe the main advantages and performance of the first Megavoltage Conebeam CT (MV CBCT) system. Method and Materials: The MV CBCT system, consisting of a new a‐Si flat panel adapted for MV imaging and an integrated workflow application allowing the automatic acquisition of projection images, conebeam CTimage reconstruction,CT to CBCTimage registration and couch position adjustment was recently introduced in clinic. Template protocols can be used for the acquisition of CBCTimages at different dose ranging from 1 to 60 M.U. Geometrical calibration, gain image adjustment and defect pixels correction procedures are performed off‐line. Results: For a typical case, 200 projection portal images and a total exposure of 5 to 8 M.U. are acquired with the 6 MV beam in 45 seconds and the 256×256×256 MV CBCTimage is reconstructed less than two minutes after the start of the acquisition. Examples of the image‐guided treatment process including the acquisition of projections images, the reconstruction of the MV CBCTimage and its registration with the planning CT, followed by the couch position correction and dose delivery will be presented. Conclusion: MV CBCT provides a 3D patient anatomy volume in the actual treatment position, relative to the treatment isocenter, moments before the dose delivery, that can be tightly aligned to the planning CT, allowing verification and correction of the patient position. Research supported by Siemens Oncology Care Systems.

WE‐D‐I‐6B‐01: Soft Tissue Visualization Using a Highly Efficient Megavoltage Cone Beam CT Imaging System

Purpose: Recent developments in two‐dimensional x‐ray detector technology have made volumetric Cone Beam CT(CBCT) a feasible approach for integration with conventional medical linear accelerators. The requirements of a robust image guidance system for radiation therapy include the challenging combination of soft tissue sensitivity with clinically reasonable doses. Previously, low contrast objects have generally not been perceptible with MV energies due to relatively poor signal to noise ratio (SNR) performance. We have developed an improved imagingsystem that is optimized for MV CBCT and acquire CBCTimages containing soft tissuecontrast using a 6MV x‐ray beam. Method and Materials: Many factors, such as imagenoise, x‐ray scatter, improper calibration and acquisitions have a profound effect on the imaging performance of CBCT. In this study attempts were made to optimize these factors in order to maximize the SNR. A QC‐3V and contrast/resolution phantoms were used to determine the contrast to noise ratio (CNR) and f50 of a single 2‐D projection and the contrast and spatial resolution of the reconstructed images. Results: The computed f50 was 0.43 lp/mm and the CNR for a radiationdose of 0.02cGy was 43. Relative electron density as low as 1% can be resolved with clinically reasonable radiationdoses. Clinical Megavoltage CBCTimages acquired with this system demonstrate that anatomical structures such as the prostate and optic nerves are visible using radiationdoses in range of 4 to 8cGy. Conclusion: We have developed an imagingsystem that is optimized for MV and acquire Megavoltage CBCTimages containing soft tissuecontrast using a 6MV x‐ray beam and irradiation doses in range of 4 to 8cGy. This system can also be used for routine portal imaging applications without risk of saturation for high dose/high energy treatment/verification imaging, or dosimetry applications. Conflict of Interest : Sponsored by Siemens.

Electronic readout of a-Si EPIDs for optimum signal-to-noise ratio

Three acquisition schemes for a-Si flat panels are described for radiation therapy imaging. The goal of all three acquisition modes is to acquire images with the highest achievable SNR (signal to noise ratio). The acquisition modes are Single mode for low dose acquisition (used for patient positioning), external continuous mode used for patient treatment (verification), and Cone Beam mode for mega-voltage computed tomography (MVCT). During single mode acquisition, a few frames are readout prior to the start of irradiation. During this cycle, the accumulated dark current and residual data are cleared. During the radiation delivery no readout occurs, and the signal is integrated over the entire exposure period. After the irradiation readout occurs. The advantages of this readout scheme are to reduce the effects of readout noise and eliminate the linear accelerator (linac) pulsing effects on the final image. There is no readout during the exposure; therefore, no beam pulsing artifacts occur. Since the signal is integrated during the exposure time and the readout is performed after the exposure, this improves the SNR compared to acquiring a few frames during the radiation and averaging these frames to create the final image. The single mode acquisition is used clinically routinely and allows the acquisition of clinical images with a small amount of exposure (<=2 MU). During external trigger continuous mode, the linear accelerator pulsing artifacts are removed by synchronizing the frame readout with linear accelerator pulses. The pulsing artifacts reduce the signal to noise ratio. This degradation is in the range of 70% for a single frame acquisition with 6MV, 300MU/min X-ray beam. Frame averaging reduces the degradation. The Cone beam acquisition mode is used to perform volume MVCT in the cone beam geometry to visualize 3D (three dimensional) anatomy during patient positioning. In this mode the image acquisition is synchronized with the linear accelerator, which enables the imager to remove linear accelerator pulsing artifacts from the image and also provides the charge integration during low dose imaging. This synchronization improves the SNR.

High-resolution video-based EPID for cone beam CT

A high-resolution video (HRV) based EPID that is capable of matching the high spatial resolution and SNR (signal to noise ratio) of a-Si flat panel devices was developed at Siemens OCS (Oncology Care Systems) for cone beam CT. This system is using a high resolution CCD camera (1300 x 1030). The optical components and scintillator screen were modified to generate high-resolution images. A Pips-Pro QC3 phantom was used to compare the spatial resolution and the contrast to noise ratio (CNR) of a-Si flat panel (1024x1024) and HRV system. The QC phantom was placed at the linear accelerator iso-center, and the detectors were placed 40cm below iso-center. The measured f50 for the Siemens a-Si flat panel and HRV are 0.49 lp/mm, and 0.43 lp/mm; respectively. The image of the flat panel had already been corrected for Offset, Gain and Defective pixels; however, no correction was performed on the HRV system. Due to the fast readout of HRV, small pixel size, and adjustable camera lens aperture, a thicker scintillator screen would have resulted in an increase in SNR without sensor saturation during radiation treatment imaging. This is one of the main advantages of this system compared to the flat panels, and it makes the system ideal for cone beam reconstructions as well as regular therapy imaging. A new electronic readout is implemented in HRV control circuit that will synchronize the image acquisition with the linear accelerator, and thereby increases the SNR.