The use of cone-beam computed tomography (CBCT) has been proposed for guiding the delivery of radiation therapy, and investigators have examined the use of both kilovoltage (kV) and megavoltage (MV) x-ray beams in the development of such CBCT systems. In this paper, the inherent contrast and signal-to-noise ratio (SNR) performance for a variety of existing and hypothetical detectors for CBCT are investigated analytically as a function of imaging dose and object size. Theoretical predictions are compared to the results of experimental investigations employing largearea flat-panel imagers (FPIs) at kV and MV energies. Measurements were performed on two different FPI-based CBCT systems: a bench-top prototype incorporating an FPI and kV x-ray source (100 kVp x rays), and a system incorporating an FPI mounted on the gantry of a medical linear accelerator (6 MV x rays). The SNR in volume reconstructions was measured as a function of dose and found to agree reasonably with theoretical predictions. These results confirm the theoretically predicted advantages of employing kV energy x rays in imaging soft-tissue structures found in the human body. While MV CBCT may provide a valuable means of correcting 3D setup errors and may offer an advantage in terms of simplicity of mechanical integration with a linear accelerator (e.g., implementation in place of a portal imager), kV CBCT offers significant performance advantages in terms of image contrast and SNR per unit dose for visualization of soft-tissue structures. The relatively poor SNR performance at MV energies is primarily a result of the low x-ray quantum efficiencies (approximately a few percent or less) that are currently achieved with FPIs at high energies. Furthermore, kV CBCT with an FPI offers the potential of combined volumetric and radiographic/fluoroscopic imaging using the same device.
The role of scatter in a cone-beam computed tomography system using the therapeutic beam of a medical linear accelerator and a commercial electronic portal imaging device (EPID) is investigated. A scatter correction method is presented which is based on a superposition of Monte Carlo generated scatter kernels. The kernels are adapted to both the spectral response of the EPID and the dimensions of the phantom being scanned. The method is part of a calibration procedure which converts the measured transmission data acquired for each projection angle into water-equivalent thicknesses. Tomographic reconstruction of the projections then yields an estimate of the electron density distribution of the phantom. It is found that scatter produces cupping artefacts in the reconstructed tomograms. Furthermore, reconstructed electron densities deviate greatly (by about 30%) from their expected values. The scatter correction method removes the cupping artefacts and decreases the deviations from 30% down to about 8%.
In their tomotherapy concept Mackie and co-workers proposed not only a new technique for IMRT but also an appropriate and satisfactory method of treatment verification. This method allows both monitoring of the portal dose distribution and imaging of the patient anatomy during treatment by means of online CT. This would enable the detection of inaccuracies in dose delivery and patient set-up errors. In this paper results are presented showing that a single electronic portal imaging device (EPID) could deliver all data necessary to establish such a complete verification system for tomotherapy and even other IMRT techniques. Consequently it has to be shown that it is able to record both the low-intensity photon fluences encountered in tomographic imaging and the intense photon transmission of each treatment field. The detector under investigation is a video-based EPID, the BIS 710 (manufactured by Wellhöfer Dosimetrie, Schwarzenbruck, Germany). To examine the suitability of the BIS for CT at 6 MV beam quality, different phantoms were scanned and reconstructed. The agreement between a diamond detector and BIS responses is quantitative. Tomographic reconstruction of a complete set of these transmission profiles resulted in images which resolve 3 cm large objects having a (theoretical) contrast to water of less than 9%. Three millimetre objects with a 100% contrast are clearly visible. The BIS signal was shown to measure photon fluence distributions. The reconstructed images possess a spatial and contrast resolution sufficient for accurate imaging of the patient anatomy, needed for treatment verification in many clinical cases.
The problem of reconstructing incident radiotherapy beam profiles from electronic portal images recorded behind a phantom is addressed. To this end an iterative algorithm is presented, which is able to extract the input beam profile from a portal image by compensating for the attenuation of the beam and subtracting the amount of scatter emitted by the phantom. The algorithm requires only a thickness map of the phantom. Scatter is estimated using a superposition method based on precalculated Monte Carlo scatter kernels. The method is tested for a homogeneous water-equivalent slab phantom for simple rectangular and complex multileaf collimated fields. It is shown that the method produces a stable result within four iterations yielding an accuracy for the incident beam distribution of better than 3%.
There is significant current interest into the use of electronic portal imaging systems for radiotherapy dosimetry. Most work presented to date assumes idealized models for the spectral sensitivity of imaging systems, such as Compton, photopeak or photon counting. In this paper it is shown that a typical portal imaging detector, comprised of thin metal converter plate (1 mm Cu) with a 134 mg/cm2 Gd202S:Tb phosphorescent screen, exhibits highly non-linear energy response with a sharp rise in sensitivity at low energies.The presence of scattered radiation significantly reduces the mean energy of a radiotherapy beam, leading to overestimation of the fluence measured by the imaging device. Two simple methods are studied for reducing this low-energy contamination effect: (i) increasing the air gap between the patient and detector, and (ii) increasing the metal converter plate thickness in order to absorb the lower energies. Monte Carlo (MC) simulations are presented showing both the spectral response of detectors with varying converter plate thickness and the photon spectra expected at varying distances from a scattering object. The effects of low-energy multiplyscattered photons are also investigated.Experimental results are presented showing the effects of air gap, converter plate thickness and multiple-scattering.Results are compared to the MC predictions for an ideal Compton detector model, showing fluence over-estimates of more than 30% for the worst case. Excellent agreement is seen between the experimental and MC results for the variable air gap, variable converter plate thickness and multiplescattering studies.
From single-crystal 27Al NMR experiments, the full tensors for both the electrical field gradient (EFG) and the chemical shift (CS) for the aluminum atoms in γ-LiAlO2 have been determined. A simultaneous fit of the quadrupolar splittings observed for the four 27Al in the unit cell gave the EFG tensor in the crystal frame, from which a quadrupolar coupling constant of χ = C Q = 3.330 ± 0.005 MHz and an asymmetry parameter of ηQ = 0.656 ± 0.002 were derived. The experimentally determined quadrupolar splittings were sufficiently sensitive to quantify small deviations of both rotation axis direction and starting direction by the data fitting routine. For determination of the CS tensor, the evolution of the outer satellite centers over the crystal rotation was tracked, and the contribution of the quadrupolar shift was subtracted according to the previously determined EFG tensor. The resulting CS tensor of 27Al yields an isotropic chemical shift of δiso = 81.8 ± 0.25 ppm and an asymmetry parameter of ηCS = 0.532 ± 0.004, in good agreement with the fit of a MAS NMR spectrum acquired at B 0 = 21.1 T. From both experiments and DFT calculations using the Castep code, we find the eigenvectors of the EFG and CS tensors to be practically colinear.
This thesis investigates the feasibility of the application of flat‐panel imagers (FPIs) based on amorphous silicon to radiotherapy verification. Intensity modulated radiotherapy imposes new demands on verification procedures. A contrast resolution of 1% may be required from portal imaging systems to resolve the target structure and the organs at risk (OAR). Furthermore a spatial resolution of 1 mm is desired. The imaging characteristics, spatial resolution, efficiency, and noise characteristics of the FPI for megavoltage imaging were investigated regarding these demands. Possible improvements from scatter rejection by a modified detector design were investigated by simulating the spectral response of the converter plates of these detectors to photons of different energies and the response to scattered photons. Since low contrast objects like the target structure and the OAR can be detected in radiographic transmission images only for some rare cases, tomographical imaging was tested for position verification. This was also done using an FPI and the megavoltage (MV) source of the treatment accelerator. A different approach to position verification is the integration of kilovoltage (kV) computed tomography (CT) into a clinical linear accelerator. Although this approach has higher demands on the hardware side, it will deliver higher contrast images at a lower dose. A comparison of MVCT and kVCT and an outlook to new possibilities using integrated CT therefore completes this thesis.
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