Edward J. Seppi

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.

Integrating respiratory gating into a megavoltage cone-beam CT system.

We have previously described a low-dose megavoltage cone beam computed tomography (MV CBCT) system capable of producing projection image using one beam pulse. In this study, we report on its integration with respiratory gating for gated radiotherapy. The respiratory gating system tracks a reflective marker on the patients abdomen midway between the xiphoid and umbilicus, and disables radiation delivery when the marker position is outside predefined thresholds. We investigate two strategies for acquiring gated scans. In the continuous rotation-gated acquisition, the linear accelerator (LINAC) is set to the fixed x-ray mode and the gantry makes a 5 min, 360 degree continuous rotation, during which the gating system turns the radiation beam on and off, resulting in projection images with an uneven distribution of projection angles (e.g., in 70 arcs each covering 2 degrees). In the gated rotation-continuous acquisition, the LINAC is set to the dynamic arc mode, which suspends the gantry rotation when the gating system inhibits the beam, leading to a slightly longer (6-7 min) scan time, but yielding projection images with more evenly distributed projection angles (e.g., approximately 0.8 degrees between two consecutive projection angles). We have tested both data acquisition schemes on stationary (a contrast detail and a thoracic) phantoms and protocol lung patients. For stationary phantoms, a separate motion phantom not visible in the images is used to trigger the RPM system. Frame rate is adjusted so that approximately 450 images (13 MU) are acquired for each scan and three-dimensional tomographic images reconstructed using a Feldkamp filtered backprojection algorithm. The gated rotation-continuous acquisition yield reconstructions free of breathing artifacts. The tumor in parenchymal lung and normal tissues are easily discernible and the boundary between the diaphragm and the lung sharply defined. Contrast-to-noise ratio (CNR) is not degraded relative to nongated scans of stationary phantoms. The continuous rotation-gated acquisition scan also yields tomographic images with discernible anatomic features; however, streak artifacts are observed and CNR is reduced by approximately a factor of 4. In conclusion, we have successfully developed a gated MV CBCT system to verify the patient positioning for gated radiotherapy.

40 x 30 cm flat-panel imager for angiography, R&F, and cone-beam CT applications

Preliminary results are presented from the PaxScan 4030A; a 40x30cm, 2048 x 1536 landscape, flat panel imager, with 194um pixel pitch. This imager builds on our experience with the PaxScan 2520, a 127um real-time flat panel detector capable of both high-resolution radiography and low dose fluoroscopy. While the PS2520 has been applied in C-arms, neuroangiography, cardiac imaging and small area radiographic units, the larger active area of the PaxScan 4030A addresses the broader applications of angiography, general RF however, a number of innovations have been incorporated into the 4030A to increase its versatility. The most obvious change is that the data interface between the receptor and command processor has been reduced to one very flexible and thin fiber-optic cable. A second new feature for the 4030A is the use of split datalines. Split datalines facilitate scanning the two halves of the array in parallel, cutting the readout time in half and increasing the time window for pulsed x-ray delivery to 15ms at 30fps. In addition, split datalines result in lower noise, which, coupled with the larger signal of the 194um pixels, enables high quality imaging at lower fluoroscopy doses rates.

Low-dose megavoltage cone-beam computed tomography for lung tumors using a high-efficiency image receptor.

We report on the capabilities of a low-dose megavoltage cone-beam computed tomography (MV CBCT) system. The high-efficiency image receptor consists of a photodiode array coupled to a scintillator composed of individual CsI crystals. The CBCT system uses the 6 MV beam from a linear accelerator. A synchronization circuit allows us to limit the exposure to one beam pulse [0.028 monitor units (MU)] per projection image. 150-500 images (4.2-13.9MU total) are collected during a one-minute scan and reconstructed using a filtered backprojection algorithm. Anthropomorphic and contrast phantoms are imaged and the contrast-to-noise ratio of the reconstruction is studied as a function of the number of projections and the error in the projection angles. The detector dose response is linear (R2 value 0.9989). A 2% electron density difference is discernible using 460 projection images and a total exposure of 13MU (corresponding to a maximum absorbed dose of about 12cGy in a patient). We present first patient images acquired with this system. Tumors in lung are clearly visible and skeletal anatomy is observed in sufficient detail to allow reproducible registration with the planning kV CT images. The MV CBCT system is shown to be capable of obtaining good quality three-dimensional reconstructions at relatively low dose and to be clinically usable for improving the accuracy of radiotherapy patient positioning.

Iodine contrast cone beam CT imaging of breast cancer

An iodine contrast agent, in conjunction with an X-ray cone beam CT imaging system, was used to clearly image three, biopsy verified, cancer lesions in two patients. The lesions were approximately in the 10 mm to 6 mm diameter range. Additional regions were also enhanced with approximate dimensions down to 1 mm or less in diameter. A flat panel detector, with 194 μm pixels in 2 x 2 binning mode, was used to obtain 500 projection images at 30 fps with an 80 kVp X-ray system operating at 112 mAs, for an 8-9 mGy dose - equivalent to two view mammography for these women. The patients were positioned prone, while the gantry rotated in the horizontal plane around the uncompressed, pendant breasts. This gantry rotated 360 degrees during the patients 16.6 sec breath hold. A volume of 100 cc of 320 mg/ml iodine-contrast was power injected at 4 cc/sec, via catheter into the arm vein of the patient. The resulting 512 x 512 x 300 cone beam CT data set of Feldkamp reconstructed ~(0.3 mm)3 voxels were analyzed. An interval of voxel contrast values, characteristic of the regions with iodine contrast enhancement, were used with surface rendering to clearly identify up to a total of 13 highlighted volumes. This included the three largest lesions, that were previously biopsied and confirmed to be malignant. The other ten highlighted regions, of smaller diameters, are likely areas of increased contrast trapping unrelated to cancer angiogenesis. However the technique itself is capable of resolving lesions that small.

TU‐FF‐A3‐04: An In Vivo Comparative Study of the MV and KV Cone Beam Computed Tomography Image Quality of a Lung Patient

Purpose: To compare image quality, reconstruction artifacts and tumorvisibility for kV and MV cone‐beam computed tomography(CBCT) scans reconstructed with the same algorithm. Method and Materials: A protocol lung‐cancer patient was set up in the identical treatment position for kV and MVCBCT using a Varian On‐Board ImagerCBCT and an inhouse MVCBCT imaging system. For both scans the gantry made a 1‐minute, 360° continuous rotation. For the MVCBCT, ∼460 projection images were acquired at 6MV for ∼13 MU; for kVCBCT ∼600 projections were acquired using 125 kVp, 80 mA and 25‐ms exposure time per projection, resulting in ∼2cGy at isocenter. Reconstruction was performed using the Feldkamp back projection algorithm. Both scans were registered to the treatment plan CT. The visibility of three selected regions (bronchus, vertebrae, heart) is compared using the corresponding signal‐to‐noise ratio (SNR). The contrast ratio (CR) and contrast‐to‐noise ratio (CNR) at the tumor are also compared for ease of tumor identification. Results: The SNR of bronchus, vertebrae and heart are 25, 34 and 33 respectively for MVCBCT while the corresponding values in kV scan are 17, 33 and 42. For tumor identifiability, CNR and CR are 11 and 2 respectively for MV scan, and 10 and 2 for kV scan. The CNR of the vertebrae in MV and kV cases are 2 and 6. Time to register the kV image is approximately 50% less than MV image. Similar breathing artifacts are present in both scans. Conclusions: Both kV and MV scans deliver usable images. The tumor can be discriminated from the lung background. Higher bone contrast in kV scan helps to reduce time required to register the scan with the planning CT.Conflict of Interest: Research sponsored by NCI Grant P01‐CA59017 and Varian Medical Systems; Research agreement with Varian Medical Systems.

Low-dose 2.5 MV cone-beam computed tomography with thick CsI flat-panel imager

Most of the treatment units, both new and old models, are equipped with a megavoltage portal imager but its use for volumetric imaging is limited. This is mainly due to the poor image quality produced by the high‐energy treatment beam (>6 MV). A linac at our center is equipped with a prototype 2.5 MV imaging beam. This study evaluates the feasibility of low‐dose megavoltage cone‐beam imaging with the 2.5 MV beam and a thick cesium iodide detector, which is a high‐efficiency imager. Basic imaging properties such as spatial resolution and modulation transfer function were assessed for the 2.5 MV prototype imaging system. For image quality and imaging dose, a series of megavoltage cone‐beam scans were acquired for the head, thorax, and pelvis of an anthropomorphic phantom and were compared to kilovoltage cone‐beam and 6X megavoltage cone‐beam images. To demonstrate the advantage of MV imaging, a phantom with metallic inserts was scanned and the image quality was compared to CT and kilovoltage cone‐beam scans. With a lower energy beam and higher detector efficiency, the 2.5 MV imaging system generally yields better image quality than does the 6 MV imaging system with the conventional MV imager. In particular, with the anthropomorphic phantom studies, the contrast to noise of bone to tissue is generally improved in the 2.5 MV images compared to 6 MV. With an image quality sufficient for bony alignment, the imaging dose for 2.5 MV cone‐beam images is 2.4−3.4 MU compared to 26 MU in 6 MV cone‐beam scans for the head, thorax, and pelvis regions of the phantom. Unlike kilovoltage cone‐beam, the 2.5 MV imaging system does not suffer from high‐Z image artifacts. This can be very useful for treatment planning in cases where high‐Z prostheses are present. PACS number(s): 87.57.Q‐Most of the treatment units, both new and old models, are equipped with a megavoltage portal imager but its use for volumetric imaging is limited. This is mainly due to the poor image quality produced by the high-energy treatment beam (>6 MV). A linac at our center is equipped with a prototype 2.5 MV imaging beam. This study evaluates the feasibility of low-dose megavoltage cone-beam imaging with the 2.5 MV beam and a thick cesium iodide detector, which is a high-efficiency imager. Basic imaging properties such as spatial resolution and modulation transfer function were assessed for the 2.5 MV prototype imaging system. For image quality and imaging dose, a series of megavoltage cone-beam scans were acquired for the head, thorax, and pelvis of an anthropomorphic phantom and were compared to kilovoltage cone-beam and 6X megavoltage cone-beam images. To demonstrate the advantage of MV imaging, a phantom with metallic inserts was scanned and the image quality was compared to CT and kilovoltage cone-beam scans. With a lower energy beam and higher detector efficiency, the 2.5 MV imaging system generally yields better image quality than does the 6 MV imaging system with the conventional MV imager. In particular, with the anthropomorphic phantom studies, the contrast to noise of bone to tissue is generally improved in the 2.5 MV images compared to 6 MV. With an image quality sufficient for bony alignment, the imaging dose for 2.5 MV cone-beam images is 2.4-3.4 MU compared to 26 MU in 6 MV cone-beam scans for the head, thorax, and pelvis regions of the phantom. Unlike kilovoltage cone-beam, the 2.5 MV imaging system does not suffer from high-Z image artifacts. This can be very useful for treatment planning in cases where high-Z prostheses are present. PACS number(s): 87.57.Q.

New method for breast tumor tracking

Mammographic imaging is a very well-known and useful method to discover cancer lesions in the breast at an early stage. Treatment options are better than at later stages, when the tumor is larger and frequently has already spread to other body organs (metastasis). However, detection is difficult in dense breasts. Often mammographers prescribe biopsy, because they cannot visually predict the probability that the lesion is benign or malignant. A new method is proposed in this article, which may determine the malignancy of the tumor without invasive biopsy.

SU‐C‐214‐05: 2.5 MV Cone‐Beam Computed Tomography with Thick CsI EPID

Purpose: To evaluate the feasibility of CsI detector for low dose 2.5 MV cone‐beam computed tomography (MVCB). Methods: The new Varian TrueBeamTM at our center is equipped with a prototype non‐clinical 2.5 MV imaging system with a 9 mm thick cesium iodide (CsI) scintillator EPID. Both static portal radiographs and MVCB images can be obtained. The minimum dose per beam pulse or frame is ∼0.0025 cGy. Approximately 450 projections were collected at up to 9 frames/second in a full 360 degrees rotation that may take 60–90 seconds. MVCB scans of a RANDO phantom were acquired using 2.5 MV and 6 MV x‐rays, the latter taken with conventional EPID (aS1000, Varian Medical Systems). KVCB scans were also taken for comparative purposes. An acrylic phantom containing high‐Z inserts was imaged with all beams to assess image artifacts. Results: The new 2.5X prototype system required only ∼2.4–3.4 cGy to achieve clinically acceptable image quality in the head, thorax, and pelvis, while standard 6X MVCB required ∼26 cGy. Image quality approaching kVCB can be obtained with 2.5X MVCB using doses of ∼ 9cGy, although images acquired using 2.4 cGy were sufficient for bony alignment purposes. As expected, MVCB images in the head and thorax are superior to pelvis. In addition, the 2.5 MV imaging beam greatly reduces high‐Z image artifacts that can significantly perturb kV image quality. Conclusions: 2.5X MVCB with high quantum efficiency CsI detector can replace kVCB in most clinical situations at greatly reduced cost and fewer high‐Z artifacts. Unlike kV, treatment planning systems can easily incorporate MV dose into plan optimization. Further, patients having metallic implants (e.g. dental fillings, hip prosthesis, etc.) can benefit from MVCB due to its insensitivity to high‐Z materials and its ability to yield true electron densities for treatment planning. Research partially supported by Varian Medical Systems.

WE-E-201B-07: Megavoltage Cone-Beam Computed Tomography Using 2.5MV X-Rays

Purpose: To evaluate the feasibility of 2.5MV cone‐beam computed tomography (MVCB) for patient setup and verification. Method and Materials: A new linac (Varian Trilogy Mx) produces an unfiltered 2.5MV imaging beam (2xMVCB) in addition to the higher energy treatment beams. In conjunction with a standard electronic portal imaging device(EPID), 2xMVCB scans were acquired of CTcalibration and anthropomorphic phantoms. Approximately 500 projections were collected in each 360‐degree scan using a total of 7.5 – 50 monitor units (MU). Depending on total MU, scanning time was as low as 1.5 minutes. Intrinsic calibration corrections were applied to all raw projections prior to reconstruction. Reconstructed images were further processed to correct for imaging artifacts and compared to kilovoltage cone‐beam (KVCB) and 6MV MVCB scans (6xMVCB). Results: Preliminary results demonstrate that it is possible to obtain 3D images with adequate image quality using relatively low doses (7.5 MU, approximately 4 cGy to isocenter) which are comparable to either a pair of double‐exposed MV portal images or a KVCB. Although image quality is inferior to KVCB, 2xMVCB still permits assessment of bony alignment for patient setup and has better tissuecontrast than 6xMVCB. Soft tissue registration may also be possible for 2xMVCB in the thorax and other anatomical regions. These results provide evidence that 2xMVCB may be an efficient image guidance procedure for clinical routine. Conclusion: MVCB using 2.5MV x‐rays can provide volumetric images with adequate quality for patient setup verification. An improved EPID optimized for 2.5MV beam having ∼14 times higher quantum efficiency than conventional EPIDs is being tested. This will enable even lower imaging doses, and also improve imagecontrast in the abdominal and pelvic regions. Reconstruction algorithms optimized for 2.5MV are also under development. Acknowledgements: Research sponsored by Varian Medical Systems.