P. Munro

A review of electronic portal imaging devices (EPIDs)

On-line electronic portal imaging devices are beginning to come into clinical service in support of radiotherapy. A variety of technologies are being explored to provide real-time or near real-time images of patient anatomy within x-ray fields during treatment on linear accelerators. The availability of these devices makes it feasible to verify treatment portals with much greater frequency and clarity than with film. This article reviews the physics of high-energy imaging and describes the operation principles of the electronic portal imaging devices that are under development or are beginning to be used clinically.

Clinical use of electronic portal imaging: Report of AAPM Radiation Therapy Committee Task Group 58

AAPM Task Group 58 was created to provide materials to help the medical physicist and colleagues succeed in the clinical implementation of electronic portal imaging devices (EPIDs) in radiation oncology. This complex technology has matured over the past decade and is capable of being integrated into routine practice. However, the difficulties encountered during the specification, installation, and implementation process can be overwhelming. TG58 was charged with providing sufficient information to allow the users to overcome these difficulties and put EPIDs into routine clinical practice. In answering the charge, this report provides; comprehensive information about the physics and technology of currently available EPID systems; a detailed discussion of the steps required for successful clinical implementation, based on accumulated experience; a review of software tools available and clinical use protocols to enhance EPID utilization; and specific quality assurance requirements for initial and continuing clinical use of the systems. Specific recommendations are summarized to assist the reader with successful implementation and continuing use of an EPID.

X‐ray sources of medical linear accelerators: Focal and extra‐focal radiation

A computerized tomography (CT) reconstruction technique has been used to make quantitative measurements of the size and shape of the focal spot in medical linear accelerators. Using this technique, we have measured the focal spots in a total of nine accelerators, including (i) two Varian Clinac 2100cs, (ii) two Atomic Energy of Canada Ltd. (AECL) Therac-25s, (iii) two AECL Therac 6s, (iv) a Siemens KD-2, (v) a Varian Clinac 600c (4 MV), and (vi) an AECL Therac-20. Some of these focal spots were monitored for changes over a 2-yr period. It has been found that (i) the size and shape of the source spot varies greatly between accelerators of different design ranging from 0.5 to 3.4 mm in full width at half maximum (FWHM); and (ii) for accelerators of the same design, the focal spots are very similar. In addition to the measurements of the focal spot, a new technique for measuring the magnitude and distribution of extra-focal radiation originating from the linear accelerator head (flattening filter, primary collimator) has also been developed. The extra-focal radiation produced by a Varian Clinac 2100c accelerator was measured using this technique and it was found that the extra-focal radiation accounts for as much as 8% of the total photon fluence reaching the isocenter. The majority (75%) of this extra-focal radiation originates from within a circle 6 cm in diameter at the target plane. The source MTFs for each of the measured focal spots have been calculated in order to assess their influence on the spatial resolution of verification images. The limiting spatial resolution (i.e., 10% modulation) for all the source MTFs is 1.8 mm-1 or greater when used for transmission radiography at a magnification of 1.2. The extra-focal radiation, which produces a low-frequency drop in the source MTFs of up to 8%, changes with field size. As a result, the source MTFs of linear accelerators depend not only on the design of individual accelerators and image magnification, but also on the field size used when forming an image.

Daily monitoring and correction of radiation field placement using a video-based portal imaging system: A pilot study

We have developed a video-based portal imaging system for radiotherapy localization. The system can acquire high quality portal images automatically using short (1-3 monitor unit) irradiations and immediately display the images. The major advantage of the imaging system is that it can be used routinely to check and correct patient positioning before much of the daily irradiation has been delivered. The portal imaging system has been used in a pilot study to monitor five patients during each of their daily treatments. The study has shown that: (i) image quality is sufficiently high to detect discrepancies in field placement from that prescribed on the simulator film; (ii) discrepancies in field placement occur frequently; and, (iii) routine correction of patient and block positioning can reduce the size of these discrepancies. This is the first time that field placement in radiation therapy has been checked and corrected routinely, before the treatment irradiation. However, limitations in the size of the field of view and in the methods of extracting and presenting the geometric information to the users limits the clinical utility of the imaging system. Solutions to these limitations are currently under development.

X-ray scatter in megavoltage transmission radiography: physical characteristics and influence on image quality.

The physical characteristics of x rays scattered by the patient and reaching the imaging detector, as well as their effect on verification (portal) image quality, were investigated for megavoltage (0.1-20 MeV) x-ray beams. Monte Carlo calculations and experimental measurements were used to characterize how the scatter and primary fluences at the detector plane were influenced by scattering geometry and the energy spectrum of the incident beam. The calculated scatter fluences were differentiated according to photon energy and scattering process. Scatter fractions were measured on a medical linear accelerator (Clinac 2100c, 6 MV) for a typical imaging geometry using an ionization chamber and a silicon diode. After correction for the energy dependence of the chamber and diode, the scatter fractions generated by the Monte Carlo simulations were found to be in excellent agreement with the measured results. In order to estimate the effect of scatter on image quality, the scatter and primary signals (i.e., energy deposited) produced in five different types of portal imaging detectors (lead plate/film, storage phosphor alone, lead plate/storage phosphor, compton recoil-electron detector, and a copper plate/Gd2O2S phosphor) were calculated. The results show that, for a specified geometry, the scatter fraction can vary by an order of magnitude, depending on the sensitivity of the imaging detector to low-energy (< 1 MeV) scattered radiation. For a common portal imaging detector (copper plate/Gd2O2S phosphor), the scattered radiation (i) reduced contrast by much as 50% for a fixed display-contrast system, and (ii) decreased the differential-signal-to-noise ratio (DSNR) by 10%-20% for a quantum-noise-limited portal imaging system. For currently available TV-camera-based portal imaging systems, which have variable display contrast, the reduction in DSNR depends on the light collection efficiency and the noise characteristics of the TV camera. Overall, these results show that scattered radiation can reduce contrast significantly in portal films while deteriorating image quality only moderately in on-line systems.

Portal Imaging Technology: Past, Present, and Future.

Many different electronic portal imaging devices (EPIDs) have been developed to improve geometric accuracy in radiation therapy. This article describes the two types of EPIDs that have become available commercially-the television camera-based EPID and the matrix ion chamber EPID-as well as describing the amorphous silicon array, a device that may become available in the future for portal imaging. In addition, the various image registration techniques that identify geometric errors from the portal images are described. These include interactive techniques, landmark-based techniques, contrast-based techniques and hybrid techniques. Although great improvements in portal imaging technology have been made, more development needs to be directed towards making portal imaging convenient and reliable. Image quality must be improved further, to improve the robustness of image registration techniques and more thought must be given to integrating and automating the various steps in the image registration process. Otherwise, too much time will have to be devoted to these tasks. Finally, and most importantly, users will have to decide what is the best way of using EPIDs clinically. Much development is required before the full potential of this exciting technology can be realized.

Phase and absorption retrieval using incoherent X-ray sources

X-ray phase contrast imaging has overcome the limitations of X-ray absorption imaging in many fields. Particular effort has been directed towards developing phase retrieval methods: These reveal quantitative information about a sample, which is a requirement for performing X-ray phase tomography, allows material identification and better distinction between tissue types, etc. Phase retrieval seems impossible with conventional X-ray sources due to their low spatial coherence. In the only previous example where conventional sources have been used, collimators were employed to produce spatially coherent secondary sources. We present a truly incoherent phase retrieval method, which removes the spatial coherence constraints and employs a conventional source without aperturing, collimation, or filtering. This is possible because our technique, based on the pixel edge illumination principle, is neither interferometric nor crystal based. Beams created by an X-ray mask to image the sample are smeared due to the incoherence of the source, yet we show that their displacements can still be measured accurately, obtaining strong phase contrast. Quantitative information is extracted from only two images rather than a sequence as required by several coherent methods. Our technique makes quantitative phase imaging and phase tomography possible in applications where exposure time and radiation dose are critical. The technique employs masks which are currently commercially available with linear dimensions in the tens of centimeters thus allowing for a large field of view. The technique works at high photon energy and thus promises to deliver much safer quantitative phase imaging and phase tomography in the future.

A digital fluoroscopic imaging device for radiotherapy localization

We have been developing a digital fluoroscopic imaging system to replace the portal films that are currently used to verify patient positioning during radiotherapy treatments. Our system has a number of modifications compared to previously reported devices. The detector, which consists of a copper plate with Gd2O2S:Tb phosphor bonded directly to the copper, has been designed to maximize light output from the phosphor by increasing the phosphor thickness. The operation of the T.V. camera has been modified so that the light signal is accumulated on the target of the T.V. camera for periods of 0.2-2.0 seconds. Accumulation of the light increases the video signal relative to the fixed noise current generated by the camera, and thus minimizes the camera noise. The resulting image quality is comparable to film, so the imaging system represents a promising alternative to film as a method of verifying patient positioning in radiotherapy.

Low-dose phase contrast tomography with conventional x-ray sources

PURPOSE The edge illumination (EI) x-ray phase contrast imaging (XPCi) method has been recently further developed to perform tomographic and, thus, volumetric imaging. In this paper, the first tomographic EI XPCi images acquired with a conventional x-ray source at dose levels below that used for preclinical small animal imaging are presented. METHODS Two test objects, a biological sample and a custom-built phantom, were imaged with a laboratory-based EI XPCi setup in tomography mode. Tomographic maps that show the phase shift and attenuating properties of the object were reconstructed, and analyzed in terms of signal-to-noise ratio and quantitative accuracy. Dose measurements using thermoluminescence devices were performed. RESULTS The obtained images demonstrate that phase based imaging methods can provide superior results compared to attenuation based modalities for weakly attenuating samples also in 3D. Moreover, and, most importantly, they demonstrate the feasibility of low-dose imaging. In addition, the experimental results can be considered quantitative within the constraints imposed by polychromaticity. CONCLUSIONS The results, together with the methods dose efficiency and compatibility with conventional x-ray sources, indicate that tomographic EI XPCi can become an important tool for the routine imaging of biomedical samples.

Strain estimation in phase-sensitive optical coherence elastography

We present a theoretical framework for strain estimation in optical coherence elastography (OCE), based on a statistical analysis of displacement measurements obtained from a mechanically loaded sample. We define strain sensitivity, signal-to-noise ratio and dynamic range, and derive estimates of strain using three methods: finite difference, ordinary least squares and weighted least squares, the latter implemented for the first time in OCE. We compare theoretical predictions with experimental results and demonstrate a ~12 dB improvement in strain sensitivity using weighted least squares compared to finite difference strain estimation and a ~4 dB improvement over ordinary least squares strain estimation. We present strain images (i.e., elastograms) of tissue-mimicking phantoms and excised porcine airway, demonstrating in each case clear contrast based on the sample’s elasticity.