Arun Gopal

Verification of multileaf collimator leaf positions using an electronic portal imaging device

An automated method is presented for determining individual leaf positions of the Siemens dual focus multileaf collimator (MLC) using the Siemens BEAMVIEW(PLUS) electronic portal imaging device (EPID). Leaf positions are computed with an error of 0.6 mm at one standard deviation (sigma) using separate computations of pixel dimensions, image distortion, and radiation center. The pixel dimensions are calculated by superimposing the film image of a graticule with the corresponding EPID image. A spatial correction is used to compensate for the optical distortions of the EPID, reducing the mean distortion from 3.5 pixels (uncorrected) per localized x-ray marker to 2 pixels (1 mm) for a rigid rotation and 1 pixel for a third degree polynomial warp. A correction for a nonuniform dosimetric response across the field of view of the EPID images is not necessary due to the sharp intensity gradients across leaf edges. The radiation center, calculated from the average of the geometric centers of a square field at 0 degrees and 180 degrees collimator angles, is independent of graticule placement error. Its measured location on the EPID image was stable to within 1 pixel based on 3 weeks of repeated extensions/retractions of the EPID. The MLC leaf positions determined from the EPID images agreed to within a pixel of the corresponding values measured using film and ionization chamber. Several edge detection algorithms were tested: contour, Sobel, Roberts, Prewitt, Laplace, morphological, and Canny. These agreed with each other to within < or = 1.2 pixels for the in-air EPID images. Using a test pattern, individual MLC leaves were found to be typically within 1 mm of the corresponding record-and-verify values, with a maximum difference of 1.8 mm, and standard deviations of <0.3 mm in the daily reproducibility. This method presents a fast, automatic, and accurate alternative to using film or a light field for the verification and calibration of the MLC.

Study of a prototype high quantum efficiency thick scintillation crystal video-electronic portal imaging device.

Image quality in portal imaging suffers significantly from the loss in contrast and spatial resolution that results from the excessive Compton scatter associated with megavoltage x rays. In addition, portal image quality is further reduced due to the poor quantum efficiency (QE) of current electronic portal imaging devices (EPIDs). Commercial video-camera-based EPIDs or VEPIDs that utilize a thin phosphor screen in conjunction with a metal buildup plate to convert the incident x rays to light suffer from reduced light production due to low QE (<2% for Eastman Kodak Lanex Fast-B). Flat-panel EPIDs that utilize the same luminescent screen along with an a-Si:H photodiode array provide improved image quality compared to VEPIDs, but they are expensive and can be susceptible to radiation damage to the peripheral electronics. In this article, we present a prototype VEPID system for high quality portal imaging at sub-monitor-unit (subMU) exposures based on a thick scintillation crystal (TSC) that acts as a high QE luminescent screen. The prototype TSC system utilizes a 12 mm thick transparent CsI(Tl) (thallium-activated cesium iodide) scintillator for QE=0.24, resulting in significantly higher light production compared to commercial phosphor screens. The 25 X 25 cm2 CsI(Tl) screen is coupled to a high spatial and contrast resolution Video-Optics plumbicon-tube camera system (1240 X 1024 pixels, 250 microm pixel width at isocenter, 12-bit ADC). As a proof-of-principle prototype, the TSC system with user-controlled camera target integration was adapted for use in an existing clinical gantry (Siemens BEAMVIEW(PLUS)) with the capability for online intratreatment fluoroscopy. Measurements of modulation transfer function (MTF) were conducted to characterize the TSC spatial resolution. The measured MTF along with measurements of the TSC noise power spectrum (NPS) were used to determine the system detective quantum efficiency (DQE). A theoretical expression of DQE(0) was developed to be used as a predictive model to propose improvements in the optics associated with the light detection. The prototype TSC provides DQE(0)=0.02 with its current imaging geometry, which is an order of magnitude greater than that for commercial VEPID systems and comparable to flat-panel imaging systems. Following optimization in the imaging geometry and the use of a high-end, cooled charge-coupled-device (CCD) camera system, the performance of the TSC is expected to improve even further. Based on our theoretical model, the expected DQE(0)=0.12 for the TSC system with the proposed improvements, which exceeds the performance of current flat-panel EPIDs. The prototype TSC provides high quality imaging even at subMU exposures (typical imaging dose is 0.2 MU per image), which offers the potential for daily patient localization imaging without increasing the weekly dose to the patient. Currently, the TSC is capable of limited frame-rate fluoroscopy for intratreatment visualization of patient motion at approximately 3 frames/second, since the achievable frame rate is significantly reduced by the limitations of the camera-control processor. With optimized processor control, the TSC is expected to be capable of intratreatment imaging exceeding 10 frames/second to monitor patient motion.

Validity of the line‐pair bar‐pattern method in the measurement of the modulation transfer function (MTF) in megavoltage imaging

The measurement of the modulation transfer function (MTF) of an imaging device is a common requirement in evaluating radiographic detector performance. Such measurements are considered mandatory in detector development research, and may also be carried out as part of routine quality assurance (QA) checks of image quality. Traditionally, MTF measurement has been performed by imaging either a narrow slit or a sharp edge in order to generate a line spread function, whose Fourier transform provides the MTF on a near-continuous frequency domain. Much less commonly employed is the method of square-wave line-pair modulations, in which the modulation response to bar resolution targets contained in a bar pattern is used to estimate the MTF at discrete spatial frequencies. While the slit and edge methods offer advantages of accuracy and a well-know standardized protocol for measurement based on several decades of development, their major limitation is the difficult and time-consuming experimental setup that is necessary to ensure accurate measurements. On the other hand, the bar pattern offers the advantage of a quick, simple, and easy measurement without the need for a complex experimental setup, with the main disadvantages of the technique being a pseudo-normalization that may lead to an overestimated MTF, and corrections for removing higher-order frequency harmonics that require interpolating between discrete spatial frequencies. Therefore, bar patterns are traditionally used for qualitative imaging applications like detector QA in terms of relative and arbitrarily defined spatial resolution metrics, while slit and edge methods are preferred for quantitative MTF measurements. Compared to diagnostic x rays, MTF measurements using megavoltage x rays are further complicated by low x-ray attenuation and excessive Compton scattering. In this work, a method to measure the MTF of megavoltage x-ray detectors based on imaging square-wave line pairs with improved near-zero-frequency normalization was developed as an adaptation to previously reported methods. Monte Carlo simulations were used to identify an improved normalization condition with which the accuracy of the MTF determined from line-pair modulations could be enhanced considerably compared to previously used techniques. Slit, edge, and bar-pattern measurements were performed to obtain the MTF of commercial megavoltage imaging devices including portal film and electronic portal imaging devices. A comparison of the MTF measurements from the three techniques was used to ascertain the validity of the proposed bar-pattern method for accurate and reliable measurement of MTF for megavoltage imagers. Statistical analyses revealed no significant differences between the bar-pattern method and the standard slit and edge techniques, indicating very good agreement (mean difference within +/- 3%). These results indicated the potential for line-pair bar patterns to be used more effectively than in the past for traditional QA imaging as well as for quantitative MTF measurement in detector development research.

Use of a line-pair resolution phantom for comprehensive quality assurance of electronic portal imaging devices based on fundamental imaging metrics

Image guided radiation therapy solutions based on megavoltage computed tomography (MVCT) involve the extension of electronic portal imaging devices (EPIDs) from their traditional role of weekly localization imaging and planar dose mapping to volumetric imaging for 3D setup and dose verification. To sustain the potential advantages of MVCT, EPIDs are required to provide improved levels of portal image quality. Therefore, it is vital that the performance of EPIDs in clinical use is maintained at an optimal level through regular and rigorous quality assurance (QA). Traditionally, portal imaging QA has been carried out by imaging calibrated line-pair and contrast resolution phantoms and obtaining arbitrarily defined QA indices that are usually dependent on imaging conditions and merely indicate relative trends in imaging performance. They are not adequately sensitive to all aspects of image quality unlike fundamental imaging metrics such as the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) that are widely used to characterize detector performance in radiographic imaging and would be ideal for QA purposes. However, due to the difficulty of performing conventional MTF measurements, they have not been used for routine clinical QA. The authors present a simple and quick QA methodology based on obtaining the MTF, NPS, and DQE of a megavoltage imager by imaging standard open fields and a bar-pattern QA phantom containing 2 mm thick tungsten line-pair bar resolution targets. Our bar-pattern based MTF measurement features a novel zero-frequency normalization scheme that eliminates normalization errors typically associated with traditional bar-pattern measurements at megavoltage x-ray energies. The bar-pattern QA phantom and open-field images are used in conjunction with an automated image analysis algorithm that quickly computes the MTF, NPS, and DQE of an EPID system. Our approach combines the fundamental advantages of linear systems metrics such as robustness, sensitivity across the full spatial frequency range of interest, and normalization to imaging conditions (magnification, system gain settings, and exposure), with the simplicity, ease, and speed of traditional phantom imaging. The algorithm was analyzed for accuracy and sensitivity by comparing with a commercial portal imaging QA method (PIPSPRO, Standard Imaging, Middleton, WI) on both first-generation lens-coupled and modern a-Si flat-panel based clinical EPID systems. The bar-pattern based QA measurements were found to be far more sensitive to even small levels of degradation in spatial resolution and noise. The bar-pattern based QA methodology offers a comprehensive image quality assessment tool suitable for both commissioning and routine EPID QA.

Analysis of the kinestatic charge detection system as a high detective quantum efficiency electronic portal imaging device

Megavoltage x-ray imaging suffers from reduced image quality due to low differential x-ray attenuation and large Compton scatter compared with kilovoltage imaging. Notwithstanding this, electronic portal imaging devices (EPIDs) are now widely used in portal verification in radiotherapy as they offer significant advantages over film, including immediate digital imaging and superior contrast range. However video-camera-based EPIDs (VEPIDs) are limited by problems of low light collection efficiency and significant light scatter, leading to reduced contrast and spatial resolution. Indirect and direct detection-based flat-panel EPIDs have been developed to overcome these limitations. While flat-panel image quality has been reported to exceed that achieved with portal film, these systems have detective quantum efficiency (DQE) limited by the thin detection medium and are sensitive to radiation damage to peripheral read-out electronics. An alternative technology for high-quality portal imaging is presented here: kinesatic charge detection (KCD). The KCD is a scanning tri-electrode ion-chamber containing high-pressure noble gas (xenon at 100 atm) used in conjunction with a strip-collimated photon beam. The chamber is scanned across the patient, and an external electric field is used to regulate the cation drift velocity. By matching the scanning velocity with that of the cation (i.e., ion) drift velocity, the cations remain static in the object frame of reference, allowing temporal integration of the signal. The KCD offers several advantages as a portal imaging system. It has a thick detector geometry with an active detection depth of 6.1 cm, compared to the sub-millimeter thickness of the phosphor layer in conventional phosphor screens, leading to an order of magnitude advantage in quantum efficiency (>0.3). The unique principle of and the use of the scanning strip-collimated x-ray beam provide further integration of charges in time, reduced scatter, and a significantly reduced imaging dose, enhancing the imaging signal-to-noise ratio (SNR) and leading to high DQE. While thick detectors usually suffer from reduced spatial resolution, the KCD provides good spatial resolution due to high gas pressure that limits the spread of scattered electrons, and a strip-collimated beam that significantly reduces the inclusion of scatter in the imaging signal. A 10 cm wide small-field-of-view (SFOV) prototype of the KCD is presented with a complete analysis of its imaging performance. Measurements of modulation transfer function (MTF), noise power spectrum (NPS), and DQE were in good agreement with Monte Carlo simulations. Imaging signal loss from recombination within the KCD chamber was measured at different gas pressures, ion drift velocities, and strip-collimation widths. Image quality for the prototype KCD was also observed with anthropomorphic phantom imaging in comparison with various commercial and research portal imaging systems, including VEPID, flat-panel imager, and conventional and high contrast film systems. KCD-based imaging provided very good contrast and good spatial resolution at very low imaging dose (0.1 cGy per image). For the prototype KCD, measurements yielded DQE(0)=0.19 and DQE(1 cy/mm)=0.004.

A study of IMRT planning parameters on planning efficiency, delivery efficiency, and plan quality

PURPOSE To improve planning and delivery efficiency of head and neck IMRT without compromising planning quality through the evaluation of inverse planning parameters. METHODS Eleven head and neck patients with pre-existing IMRT treatment plans were selected for this retrospective study. The Pinnacle treatment planning system (TPS) was used to compute new treatment plans for each patient by varying the individual or the combined parameters of dose/fluence grid resolution, minimum MU per segment, and minimum segment area. Forty-five plans per patient were generated with the following variations: 4 dose/fluence grid resolution plans, 12 minimum segment area plans, 9 minimum MU plans, and 20 combined minimum segment area/minimum MU plans. Each plan was evaluated and compared to others based on dose volume histograms (DVHs) (i.e., plan quality), planning time, and delivery time. To evaluate delivery efficiency, a model was developed that estimated the delivery time of a treatment plan, and validated through measurements on an Elekta Synergy linear accelerator. RESULTS The uncertainty (i.e., variation) of the dose-volume index due to dose calculation grid variation was as high as 8.2% (5.5 Gy in absolute dose) for planning target volumes (PTVs) and 13.3% (2.1 Gy in absolute dose) for planning at risk volumes (PRVs). Comparison results of dose distributions indicated that smaller volumes were more susceptible to uncertainties. The grid resolution of a 4 mm dose grid with a 2 mm fluence grid was recommended, since it can reduce the final dose calculation time by 63% compared to the accepted standard (2 mm dose grid with a 2 mm fluence grid resolution) while maintaining a similar level of dose-volume index variation. Threshold values that maintained adequate plan quality (DVH results of the PTVs and PRVs remained satisfied for their dose objectives) were 5 cm2 for minimum segment area and 5 MU for minimum MU. As the minimum MU parameter was increased, the number of segments and delivery time were decreased. Increasing the minimum segment area parameter decreased the plan MU, but had less of an effect on the number of segments and delivery time. Our delivery time model predicted delivery time to within 1.8%. CONCLUSIONS Increasing the dose grid while maintaining a small fluence grid allows for improved planning efficiency without compromising plan quality. Delivery efficiency can be improved by increasing the minimum MU, but not the minimum segment area. However, increasing the respective minimum MU and/or the minimum segment area to any value greater than 5 MU and 5 cm2 is not recommended because it degrades plan quality.PURPOSE To improve planning and delivery efficiency of head and neck IMRT without compromising planning quality through the evaluation of inverse planning parameters. METHODS Eleven head and neck patients with pre-existing IMRT treatment plans were selected for this retrospective study. The Pinnacle treatment planning system (TPS) was used to compute new treatment plans for each patient by varying the individual or the combined parameters of dose∕fluence grid resolution, minimum MU per segment, and minimum segment area. Forty-five plans per patient were generated with the following variations: 4 dose∕fluence grid resolution plans, 12 minimum segment area plans, 9 minimum MU plans, and 20 combined minimum segment area∕minimum MU plans. Each plan was evaluated and compared to others based on dose volume histograms (DVHs) (i.e., plan quality), planning time, and delivery time. To evaluate delivery efficiency, a model was developed that estimated the delivery time of a treatment plan, and validated through measurements on an Elekta Synergy linear accelerator. RESULTS The uncertainty (i.e., variation) of the dose-volume index due to dose calculation grid variation was as high as 8.2% (5.5 Gy in absolute dose) for planning target volumes (PTVs) and 13.3% (2.1 Gy in absolute dose) for planning at risk volumes (PRVs). Comparison results of dose distributions indicated that smaller volumes were more susceptible to uncertainties. The grid resolution of a 4 mm dose grid with a 2 mm fluence grid was recommended, since it can reduce the final dose calculation time by 63% compared to the accepted standard (2 mm dose grid with a 2 mm fluence grid resolution) while maintaining a similar level of dose-volume index variation. Threshold values that maintained adequate plan quality (DVH results of the PTVs and PRVs remained satisfied for their dose objectives) were 5 cm(2) for minimum segment area and 5 MU for minimum MU. As the minimum MU parameter was increased, the number of segments and delivery time were decreased. Increasing the minimum segment area parameter decreased the plan MU, but had less of an effect on the number of segments and delivery time. Our delivery time model predicted delivery time to within 1.8%. CONCLUSIONS Increasing the dose grid while maintaining a small fluence grid allows for improved planning efficiency without compromising plan quality. Delivery efficiency can be improved by increasing the minimum MU, but not the minimum segment area. However, increasing the respective minimum MU and∕or the minimum segment area to any value greater than 5 MU and 5 cm(2) is not recommended because it degrades plan quality.

Use of local noise power spectrum and wavelet analysis in quantitative image quality assurance for EPIDs

PURPOSE To investigate the use of local noise power spectrum (NPS) to characterize image noise and wavelet analysis to isolate defective pixels and inter-subpanel flat-fielding artifacts for quantitative quality assurance (QA) of electronic portal imaging devices (EPIDs). METHODS A total of 93 image sets including custom-made bar-pattern images and open exposure images were collected from four iViewGT a-Si EPID systems over three years. Global quantitative metrics such as modulation transform function (MTF), NPS, and detective quantum efficiency (DQE) were computed for each image set. Local NPS was also calculated for individual subpanels by sampling region of interests within each subpanel of the EPID. The 1D NPS, obtained by radially averaging the 2D NPS, was fitted to a power-law function. The r-square value of the linear regression analysis was used as a singular metric to characterize the noise properties of individual subpanels of the EPID. The sensitivity of the local NPS was first compared with the global quantitative metrics using historical image sets. It was then compared with two commonly used commercial QA systems with images collected after applying two different EPID calibration methods (single-level gain and multilevel gain). To detect isolated defective pixels and inter-subpanel flat-fielding artifacts, Haar wavelet transform was applied on the images. RESULTS Global quantitative metrics including MTF, NPS, and DQE showed little change over the period of data collection. On the contrary, a strong correlation between the local NPS (r-square values) and the variation of the EPID noise condition was observed. The local NPS analysis indicated image quality improvement with the r-square values increased from 0.80 ± 0.03 (before calibration) to 0.85 ± 0.03 (after single-level gain calibration) and to 0.96 ± 0.03 (after multilevel gain calibration), while the commercial QA systems failed to distinguish the image quality improvement between the two calibration methods. With wavelet analysis, defective pixels and inter-subpanel flat-fielding artifacts were clearly identified as spikes after thresholding the inversely transformed images. CONCLUSIONS The proposed local NPS (r-square values) showed superior sensitivity to the noise level variations of individual subpanels compared with global quantitative metrics such as MTF, NPS, and DQE. Wavelet analysis was effective in detecting isolated defective pixels and inter-subpanel flat-fielding artifacts. The proposed methods are promising for the early detection of imaging artifacts of EPIDs.

SU-F-J-32: Do We Need KV Imaging During CBCT Based Patient Set-Up for Lung Radiation Therapy?

PURPOSE To evaluate the role of 2D kilovoltage (kV) imaging to complement cone beam CT (CBCT) imaging in a shift threshold based image guided radiation therapy (IGRT) strategy for conventional lung radiotherapy. METHODS A retrospective study was conducted by analyzing IGRT couch shift trends for 15 patients that received lung radiation therapy to evaluate the benefit of performing orthogonal kV imaging prior to CBCT imaging. Herein, a shift threshold based IGRT protocol was applied, which would mandate additional CBCT verification if the applied patient shifts exceeded 3 mm to avoid intraobserver variability in CBCT registration and to confirm table shifts. For each patient, two IGRT strategies: kV + CBCT and CBCT alone, were compared and the recorded patient shifts were categorized into whether additional CBCT acquisition would have been mandated or not. The effectiveness of either strategy was gauged by the likelihood of needing additional CBCT imaging for accurate patient set-up. RESULTS The use of CBCT alone was 6 times more likely to require an additional CBCT than KV+CBCT, for a 3 mm shift threshold (88% vs 14%). The likelihood of additional CBCT verification generally increased with lower shift thresholds, and was significantly lower when kV+CBCT was used (7% with 5 mm shift threshold, 36% with 2 mm threshold), than with CBCT alone (61% with 5 mm shift threshold, 97% with 2 mm threshold). With CBCT alone, treatment time increased by 2.2 min and dose increased by 1.9 cGy per fraction on average due to additional CBCT with a 3mm shift threshold. CONCLUSION The benefit of kV imaging to screen for gross misalignments led to more accurate CBCT based patient localization compared with using CBCT alone. The subsequently reduced need for additional CBCT verification will minimize treatment time and result in less overall patient imaging dose.

SU‐E‐T‐26: Determination of Field Parameters for Unflattened Photon Beams Using a Universal Scaling Factor

Purpose: To investigate unflattened and flattened photon beam profiles and to propose a new method in determining field parameters for unflattened beams. Methods: For flattened beams, field parameters of field size, etc., are defined according to the profile scaled the dose at central axis (D_CAX) to 100%. For unflattened beams, Siemens presents a table of scaling factors (to D_CAX) which are field size and energy dependent. The M. D. Anderson group presented a method to match the dose level at the inflection point for an unflattened beam to that for the corresponding flattened beam. By investigating cross‐plane and in‐plane profiles for the flat beams of Siemens Artiste 6 and 10 MV, Varian 21EX 6 and 10 MV, Trilogy 6 and 18 MV at different field sizes and depths, it was observed that the ratio of the dose at the inflection point (D_IP) to D_CAX remains unchanged within an uncertainty. Therefore, it is proposed to scale any unflattened beam profile by defining its D_IP to this value. After the scaling, the common definitions of field size, penumbra, and asymmetry remain the same. Results: The ratio (D_IP/D_CAX) approaches a Gaussian distribution, yielding a mean of 54% with standard deviation of 2% for all data. The differences between the field sizes determined by this new method and those by Siemens scaling factors are mostly within 1 mm for Artiste 7 UF and 11 UF unflattened beams in a wide range of field sizes and depths. Varying this scaling factor in the 95% confidence interval leads to a variation within ±1 mm for field size determination.Conclusion: This study demonstrates reasonability in use of single “54%” scaling factor to the inflection point for all unflattened beam profiles. The proposed method would provide simplification in determining field parameters for unflattened beams. This work is supported in part by a Siemens research fund.

SU‐E‐T‐01: Comparison of Head Scatters in Flattening‐Filter‐Free and Flattened Photon Beams

Purpose: To decouple and quantify scatter contributions in air and in phantom for flattening‐filter‐free (FFF) photon beams in comparison with those for flattened beams from a medical linac in radiotherapy. Methods: The 7 UF FFF beam on a Siemens Artiste linac shows equivalent depth dose distribution as the 6 MV flat beam. An ionization chamber was used to measure doses along central axis in air (with a 3 cm diameter buildup cap) and in phantom (SSD = 100 cm and depth = 1.5 cm) for both beams in a range of field size, yielding head scatter factors (Sc) and output factors (Scp). A third‐order polynomial function was used to fit the data within the fields of 3×3 to 12×12 cm2 for each Sc or Scp distribution. An extrapolation of the fitted function to 0×0 field yielded the dose component due to primary radiation. Scatter components for different field sizes can be derived by subtracting this primary component from measured total doses. Results: A primary FFF beam produces only 5% scatters in 10× 10 cm2 field through the linac head while 43% scatters are produced by a flat primary beam, resulting in a reduction of scatter contributions by a factor of 6 in the measured doses in air. Comparable scatter contributions (∼10% in 10× 10 field) produced in the phantom at the same depth were observed for both primary beams as expected. Overall, the significantly reduced head scatter for the FFF beam leads to a total scatter reduction of 60% in phantom at 1.5 cm depth (14% and 34% scatter contributions in 10× 10 field for the FFF and flat beams, respectively). Conclusion: Our method works well in decoupling scatter components from measured total doses. The significant reduction of head scatters for FFF beams would provide dosimetric benefits to patients with reduced shallow‐dose. This work is supported in part by a Siemens research fund.