Investigation of time-resolved proton radiography using x-ray flat-panel imaging system.

Proton beam therapy benefits from the Bragg peak and delivers highly conformal dose distributions. However, the location of the end-of-range is subject to uncertainties related to the accuracy of the relative proton stopping power estimates and thereby the water-equivalent path length (WEPL) along the beam. To remedy the range uncertainty, an in vivo measurement of the WEPL through the patient, i.e. a proton-range radiograph, is highly desirable. Towards that goal, we have explored a novel method of proton radiography based on the time-resolved dose measured by a flat panel imager (FPI). A 226 MeV pencil beam and a custom-designed range modulator wheel (MW) were used to create a time-varying broad beam. The proton imaging technique used exploits this time dependency by looking at the dose rate at the imager as a function of time. This dose rate function (DRF) has a unique time-varying dose pattern at each depth of penetration. A relatively slow rotation of the MW (0.2 revolutions per second) and a fast image acquisition (30 frames per second, ~33 ms sampling) provided a sufficient temporal resolution for each DRF. Along with the high output of the CsI:Tl scintillator, imaging with pixel binning (2 × 2) generated high signal-to-noise data at a very low radiation dose (~0.1 cGy). Proton radiographs of a head phantom and a Gammex CT calibration phantom were taken with various configurations. The results of the phantom measurements show that the FPI can generate low noise and high spatial resolution proton radiographs. The WEPL values of the CT tissue surrogate inserts show that the measured relative stopping powers are accurate to ~2%. The panel did not show any noticeable radiation damage after the accumulative dose of approximately 3831 cGy. In summary, we have successfully demonstrated a highly practical method of generating proton radiography using an x-ray flat panel imager.

Determination of the detective quantum efficiency of a prototype, megavoltage indirect detection, active matrix flat-panel imager.

After years of aggressive development, active matrix flat-panel imagers (AMFPIs) have recently become commercially available for radiotherapy imaging. In this paper we report on a comprehensive evaluation of the signal and noise performance of a large-area prototype AMFPI specifically developed for this application. The imager is based on an array of 512 x 512 pixels incorporating amorphous silicon photodiodes and thin-film transistors offering a 26 x 26 cm2 active area at a pixel pitch of 508 microm. This indirect detection array was coupled to various x-ray converters consisting of a commercial phosphor screen (Lanex Fast B, Lanex Regular, or Lanex Fine) and a 1 mm thick copper plate. Performance of the imager in terms of measured sensitivity, modulation transfer function (MTF), noise power spectra (NPS), and detective quantum efficiency (DQE) is reported at beam energies of 6 and 15 MV and at doses of 1 and 2 monitor units (MU). In addition, calculations of system performance (NPS, DQE) based on cascaded-system formalism were reported and compared to empirical results. In these calculations, the Swank factor and spatial energy distributions of secondary electrons within the converter were modeled by means of EGS4 Monte Carlo simulations. Measured MTFs of the system show a weak dependence on screen type (i.e., thickness), which is partially due to the spreading of secondary radiation. Measured DQE was found to be independent of dose for the Fast B screen, implying that the imager is input-quantum-limited at 1 MU, even at an extended source-to-detector distance of 200 cm. The maximum DQE obtained is around 1%--a limit imposed by the low detection efficiency of the converter. For thinner phosphor screens, the DQE is lower due to their lower detection efficiencies. Finally, for the Fast B screen, good agreement between calculated and measured DQE was observed.

Additive noise properties of active matrix flat-panel imagers.

A detailed theoretical and empirical investigation of additive noise for indirect detection, active matrix flat-panel imagers (AMFPIs) has been performed. Such imagers comprise a pixelated array, incorporating photodiodes and thin-film transistors (TFTs), and an associated electronic acquisition system. A theoretical model of additive noise, defined as the noise of an imaging system in the absence of radiation, has been developed. This model is based upon an equivalent-noise-circuit representation of an AMFPI. The model contains a number of uncorrelated noise components which have been designated as pixel noise, data line thermal noise, externally coupled noise, preamplifier noise and digitization noise. Pixel noise is further divided into the following components: TFT thermal noise, shot and 1/f noise associated with the TFT and photodiode leakage currents, and TFT transient noise. Measurements of various additive noise components were carried out on a prototype imaging system based on a 508 microm pitch, 26 x 26 cm2 array. Other measurements were performed in the absence of the array, involving discrete components connected to the preamplifier input. Overall, model predictions of total additive noise as well as of pixel, preamplifier, and data line thermal noise components were in agreement with results of their measured counterparts. For the imaging system examined, the model predicts that pixel noise is dominated by shot and 1/f noise components of the photodiode and TFT at frame times above approximately 1 s. As frame time decreases, pixel noise is increasingly dominated by TFT thermal noise. Under these conditions, the reasonable degree of agreement observed between measurements and model predictions provides strong evidence that the role of TFT thermal noise has been properly incorporated into the model. Finally, the role of the resistance and capacitance of array data lines in the model was investigated using discrete component circuits at the preamplifier input. Measurements of preamplifier noise and data line thermal noise components as a function of input capacitance and resistance were found to be in reasonable agreement with model predictions.

A quantitative investigation of additive noise reduction for active matrix flat-panel imagers using compensation lines.

A quantitative investigation of a technique for reducing correlated noise in indirect detection active matrix flat-panel imagers has been reported. Correlated noise in such systems arises from the coupling of electronic noise, originating from fluctuations in external sources such as power supplies and ambient electromagnetic sources, to the imaging array via its address lines. The noise reduction technique involves the use of signals from columns of compensation line pixels located in relatively close proximity to the columns of normal imaging pixels on the array. Compensation line pixels are designed to be as sensitive to externally-coupled noise as columns of normal imaging pixels but are insensitive to incident radiation. For each imaging pixel, correlated noise is removed by subtracting from the imaging pixel signal a signal derived from compensation line pixels located on the same row. The effectiveness of various implementations of this correction has been examined through measurements of signal and noise from individual pixels as well as of noise power spectra. These measurements were performed both in the absence of radiation as well as with x rays. The effectiveness of the correction was also demonstrated qualitatively by means of an image of a hand phantom. It was found that the use of a single compensation line dramatically reduces external noise through removal of the correlated noise component. While this form of the correction increases non-radiation-related uncorrelated noise, the effect can be largely reduced through the introduction of multiple compensation lines. Finally, a position-dependent correction based on compensation lines on both sides of the array was found to be effective when the magnitude of the correlated noise varied linearly across the array.

Strategies to improve the signal and noise performance of active matrix, flat-panel imagers for diagnostic x-ray applications.

A theoretical investigation of factors limiting the detective quantum efficiency (DQE) of active matrix flat-panel imagers (AMFPIs), and of methods to overcome these limitations, is reported. At the higher exposure levels associated with radiography, the present generation of AMFPIs is capable of exhibiting DQE performance equivalent, or superior, to that of existing film-screen and computed radiography systems. However, at exposure levels commonly encountered in fluoroscopy, AMFPIs exhibit significantly reduced DQE and this problem is accentuated at higher spatial frequencies. The problem applies both to AMFPIs that rely on indirect detection as well as direct detection of the incident radiation. This reduced performance derives from the relatively large magnitude of the square of the total additive noise compared to the system gain for existing AMFPIs. In order to circumvent these restrictions, a variety of strategies to decrease additive noise and enhance system gain are proposed. Additive noise could be reduced through improved preamplifier, pixel and array design, including the incorporation of compensation lines to sample external line noise. System gain could be enhanced through the use of continuous photodiodes, pixel amplifiers, or higher gain x-ray converters such as lead iodide. The feasibility of these and other strategies is discussed and potential improvements to DQE performance are quantified through a theoretical investigation of a variety of hypothetical 200 microm pitch designs. At low exposures, such improvements could greatly increase the magnitude of the low spatial frequency component of the DQE, rendering it practically independent of exposure while simultaneously reducing the falloff in DQE at higher spatial frequencies. Furthermore, such noise reduction and gain enhancement could lead to the development of AMFPIs with high DQE performance which are capable of providing both high resolution radiographic images, at approximately 100 microm pixel resolution, as well as variable resolution fluoroscopic images at 30 fps.

Relative dosimetry using active matrix flat-panel imager (AMFPI) technology.

The first examination of the use of active matrix flat-panel arrays for dosimetry in radiotherapy is reported. Such arrays are under widespread development for diagnostic and radiotherapy imaging. In the current study, an array consisting of 512 x 512 pixels with a pixel pitch of 508 microm giving an area of 26 x 26 cm2 has been used. Each pixel consists of a light sensitive amorphous silicon (a-Si:H) photodiode coupled to an a-Si:H thin-film transistor. Data was obtained from the array using a dedicated electronics system allowing real-time data acquisition. In order to examine the potential of such arrays as quality assurance devices for radiotherapy beams, field profile data at photon energies of 6 and 15 MV were obtained as a function of field size and thickness of overlying absorbing material (solid water). Two detection configurations using the array were considered: a configuration (similar to the imaging configuration) in which an overlying phosphor screen is used to convert incident radiation to visible light photons which are detected by the photodiodes; and a configuration without the screen where radiation is directly sensed by the photodiodes. Compared to relative dosimetry data obtained with an ion chamber, data taken using the former configuration exhibited significant differences whereas data obtained using the latter configuration was generally found to be in close agreement. Basic signal properties, which are pertinent to dosimetry, have been investigated through measurements of individual pixel response for fluoroscopic and radiographic array operation. For signal levels acquired within the first 25% of pixel charge capacity, the degree of linear response with dose was found to be better than 99%. The independence of signal on dose rate was demonstrated by means of stability of pixel response over the range of dose rates allowed by the radiation source (80-400 MU/min). Finally, excellent long-term stability in pixel response, extending over a 2 month period, was observed.

Initial performance evaluation of an indirect-detection, active matrix flat-panel imager (AMFPI) prototype for megavoltage imaging.

PURPOSE: The development of the first prototype active matrix flat-panel imager (AMFPI) capable of radiographic and fluoroscopic megavoltage operation is reported. The signal and noise performance of individual pixels is empirically quantified. Results of an observer-dependent study of imaging performance, using a contrast-detail phantom, are detailed and radiographic patient images are shown. Finally, a theoretical investigation of the zero-frequency detective quantum efficiency (DQE) performance of such imagers, using a cascaded systems formalism, is presented. METHODS AND MATERIALS: The imager is based on a 508-microm pitch, 26 x 26 cm2 array which detects radiation indirectly via an overlying copper plate + phosphor screen converter. RESULTS: Due to its excellent optical coupling, the imager exhibits sensitivity superior to that of video-based systems. With an approximately 133 mg/cm2 Gd2O2S:Tb screen the system is x-ray quantum-noise-limited down to approximately 0.3 cGy, conservatively, and extensions of this behavior to even lower doses by means of reduced additive electronic noise is predicted. The observer-dependent study indicates performance superior to that of conventional radiotherapy film while the patient images demonstrate good image quality at 1 to 4 MU. The theoretical studies suggest that, with a 133 mg/cm2 Gd2O2S:Tb screen, the system would provide DQE performance equivalent to that of video-based systems and that almost a factor of two improvement in DQE is achievable through the incorporation of a 400 mg/cm2 screen. CONCLUSION: The reported prototype imager is the first megavoltage AMFPI having performance characteristics consistent with practical clinical operation. The superior contrast-detail sensitivity of the imager allows the capture of high-quality 6- and 15-MV images at minimal dose. Moreover, significant performance improvements, including extension of the operational range up to full portal doses, appear feasible. Such capabilities could be of considerable practical benefit in patient localization and verification.

The effect of medial meniscectomy and coronal plane angulation on in vitro load transmission in the canine stifle joint.

The purpose of this study was to determine the in vitro load transmission characteristics of the canine stifle joint, paying particular attention to the positioning effect of the meniscus in the coronal plane. The intact joint was first loaded, and then tested under two different loading conditions after a complete medial meniscectomy. The first set of test conditions attempted to simulate those used by previous investigators, by ignoring the spacer effect of the meniscus and not repositioning the joint after its removal. The second set of tests was carried out after the joint was repositioned in the coronal plane to allow initial contact to occur in both tibiofemoral compartments. It is presumed that this occurs subsequent to a meniscectomy in vivo, following the application of any weight-bearing load. As with previous investigators, it was found that after meniscectomy the joints produced slightly larger displacements and lower stiffnesses than when intact (no significant differences from intact). However, repositioning the meniscectomized joint produced markedly smaller displacements (35-49%, p less than 0.01) and greater stiffnesses (47-123%, p less than 0.05) over the range of forces analyzed, compared with the intact joint. The ratio of dissipated to input energy was 42% for the intact joint, and rose following meniscectomy to 54% (p less than 0.05) with repositioning and 55% (p less than 0.05) without repositioning. Measured contact area decreased by 17% (p less than 0.05) following meniscectomy alone, and by 12% (p less than 0.05) following meniscectomy with repositioning. Since repositioning of the joint subsequent to meniscectomy (accounting for the loss of the meniscal spacer) resulted in an increase in structural stiffness, it was concluded that the medial meniscus decreases the structural stiffness of the intact stifle joint. In addition, the meniscus has a role in elastic energy storage and increasing contact area. This study is intended to serve as a baseline comparison for future in vivo studies on meniscectomy, meniscal repair, and meniscal replacement, in addition to more fully elucidating the mechanism of load transmission. A model is presented to explain both the decrease in stiffness after meniscectomy without repositioning and the increase in stiffness after meniscectomy with repositioning, employing linear springs of unequal length and different stiffnesses. After removal of the softer meniscal element and allowing joint approximation to occur, loading of the stiffer articular element results in an initially stiffer preparation.