Physical characteristics of a low-dose gas microstrip detector for orthopedic x-ray imaging

A new scanning slit gas detector dedicated to orthopedic x-ray imaging is presented and evaluated in terms of its fundamental imaging characteristics. The system is based on the micromesh gaseous structure detector and achieves primary signal amplification through electronic avalanche in the gas. This feature, together with high quantum detection efficiency and fan-beam geometry, allows for imaging at low radiation levels. The system is composed of 1764 channels spanning a width of 44.8 cm and is capable of imaging an entire patient at speeds of up to 15 cm/s. The resolution was found to be anisotropic and significantly affected by the beam quality in the horizontal direction, but otherwise sufficient for orthopedic studies. As a consequence of line-by-line acquisition, the images contain some ripple components due to mechanical vibrations combined with variations in the x-ray tube output power. The reported detective quantum efficiency (DQE) values are relatively low (0.14 to 0.20 at 0.5 mm(-1)) as a consequence of a suboptimal collimation geometry. The DQE values were found to be unaffected by the exposure down to 7 microGy, suggesting that the system is quantum limited even for low radiation levels. A system composed of two orthogonal detectors is already in use and can produce dual-view full body scans at low doses. This device could contribute to reduce the risk of radiation induced cancer in sensitive clientele undergoing intensive x-ray procedures, like young scoliotic women.     

Comparative study of a low-Z cone-beam computed tomography system

Computed tomography images have been acquired using an experimental (low atomic number (Z) insert) megavoltage cone-beam imaging system. These images have been compared with standard megavoltage and kilovoltage imaging systems. The experimental system requires a simple modification to the 4 MeV electron beam from an Elekta Precise linac. Low-energy photons are produced in the standard medium-Z electron window and a low-Z carbon electron absorber located after the window. The carbon electron absorber produces photons as well as ensuring that all remaining electrons from the source are removed. A detector sensitive to diagnostic x-ray energies is also employed. Quantitative assessment of cone-beam computed tomography (CBCT) contrast shows that the low-Z imaging system is an order of magnitude or more superior to a standard 6 MV imaging system. CBCT data with the same contrast-to-noise ratio as a kilovoltage imaging system (0.15 cGy) can be obtained in doses of 11 and 244 cGy for the experimental and standard 6 MV systems, respectively. Whilst these doses are high for everyday imaging, qualitative images indicate that kilovoltage like images suitable for patient positioning can be acquired in radiation doses of 1-8 cGy with the experimental low-Z system.     

Tomographic mammography using a limited number of low-dose cone-beam projection images

A method is described for using a limited number (typically 10-50) of low-dose radiographs to reconstruct the three-dimensional (3D) distribution of x-ray attenuation in the breast. The method uses x-ray cone-beam imaging, an electronic digital detector, and constrained nonlinear iterative computational techniques. Images are reconstructed with high resolution in two dimensions and lower resolution in the third dimension. The 3D distribution of attenuation that is projected into one image in conventional mammography can be separated into many layers (typically 30-80 1-mm-thick layers, depending on breast thickness), increasing the conspicuity of features that are often obscured by overlapping structure in a single-projection view. Schemes that record breast images at nonuniform angular increments, nonuniform image exposure, and nonuniform detector resolution are investigated in order to reduce the total x-ray exposure necessary to obtain diagnostically useful 3D reconstructions, and to improve the quality of the reconstructed images for a given exposure. The total patient radiation dose can be comparable to that used for a standard two-view mammogram. The method is illustrated with images from mastectomy specimens, a phantom, and human volunteers. The results show how image quality is affected by various data-collection protocols.     

A photodiode array x-ray imaging system for digital angiography

A line scanning imaging system that can be used to make low-noise x-ray images to detect low-contrast structure is described. The system makes use of a 1024-element, self-scanning, photodiode array (Reticon RL 1024S) optically coupled to an x-ray image intensifier tube. Low-noise images are obtained by imaging only small areas of interest at a time to reduce the noise resulting from the detection of scattered radiation, and by making use of the very large dynamic range (8000:1) solid-state photodetector. Some performance characteristics of the diode array system are discussed. It was found that while sensitivities of individual elements differed by up to +/- 15% from the average, they could be corrected with a precision of 0.02% to 0.04% of the maximum signal. The limiting spatial resolution of the system in the direction of the diode array was 2.0 cycles/mm, limited by the image intensifier. The system linearity was studied by measuring the attenuation of a monoenergetic x-ray beam by Plexiglas. The measured attenuation agreed with the expected exponential decrease over a range of approximately 1000 to within experimental error. The imaging capabilities of the system were demonstrated by imaging an angiographic phantom consisting of an iodine-filled tube with an asymmetric 20% stenosis. The stenosis was oriented on the tube surface furthermost from the detector resulting in an image with a 2% radiographic contrast change but no decrease of the tube width. The stenosis was clearly imaged using a temporal subtraction technique.     

Algorithms for density and composition-discrimination imaging for fourth-generation CT systems.

This paper shows that if the off-beam idle detectors in the detection ring of a fourth-generation x-ray computed tomography (CT) system are used to measure the scattered radiation, it is numerically feasible to reconstruct electron-density images to supplement the conventional attenuation-coefficient images of transmitted radiation. It is also shown that by combining these two images, composition changes can be detected with the aid of an effective-atomic-number indicator. The required image-reconstruction algorithms are developed and tested against Monte Carlo simulated measurements, for a variety of phantom configurations. In spite of the relatively poor statistical quality of scattering measurements, it is demonstrated that electron-density images of reasonable quality can be obtained. In addition, it is shown that composition discrimination is possible for materials of effective atomic number greater than five, in the photon energy range of a typical medical x-ray CT system operating at 102 kVp. The obtained supplementary electron-density and composition images can be useful in radiotherapy planning and for studying tumour histology, as well as in industrial and security applications where identification of materials based on density and composition is important.     

Vision 20/20: increased image resolution versus reduced radiation exposure

This is a review of methods, currently and potentially, available for significantly reducing x-ray exposure in medical x-ray imaging. It is stimulated by the radiation exposure implications of the growing use of helical scanning, multislice, x-ray computed tomography for screening, such as for coronary artery atherosclerosis and cancer of the colon and lungs. Screening requires high-throughput imaging with high spatial and contrast resolution to meet the need for high sensitivity and specificity of detection and classification of specific imaged features. To achieve this goal beyond what is currently available with x-ray imaging methods requires increased x-ray exposure, which increases the risk of tissue damage and ultimately cancer development. These consequences limit the utility of current x-ray imaging in screening of at-risk subjects who have not yet developed the clinical symptoms of disease. Current methods for reducing x-ray exposure in x-ray imaging, mostly achieved by increasing sensitivity and specificity of the x-ray detection process, may still have potential for an up-to-tenfold decrease. This could be sufficient for doubling the spatial resolution of x-ray CT while maintaining the current x-ray exposure levels. However, a spatial resolution four times what is currently available might be needed to adequately meet the needs for screening. Consequently, for the proposed need to increase spatial resolution, an additional order of magnitude of reduction of x-ray exposure would be needed just to keep the radiation exposure at current levels. This is conceivably achievable if refraction, rather than the currently used attenuation, of x rays is used to generate the images. Existing methods that have potential for imaging the consequences of refracted x ray in a clinical setting are (1) by imaging the edge enhancement that occurs at the interfaces between adjacent tissues of different refractive indices, or (2) by imaging the changes in interference patterns resulting from moving grids which alter the refraction of x rays, that have passed through the body, in a predictable fashion, and (3) theoretically, by an image generated from the change in time-of-flight of x-ray photons passing through the body. Imaging phase shift or change in time-of-flight, rather than attenuation, of x-ray photons through tissues presents formidable technological problems for whole-body 3D imaging. However, if achievable in a routine clinical setting, these approaches have the potential for greatly expanding the use of x-ray imaging for screening. This overview examines the increased contrast resolution and reduced radiation exposure that might be achievable by the above-mentioned methods.     

Effect of radiographic techniques (kVp and mAs) on image quality and patient doses in digital subtraction angiography

We investigated how varying the x-ray tube voltage and image receptor input exposure affected image quality and patient radiation doses in interventional neuroradiologic imaging. Digital subtraction angiography (DSA) images were obtained of a phantom with 1 mm diameter vessels containing iodine at concentrations between 4.5 and 50 mg/cc. The detection threshold concentration of iodine was determined by inspecting DSA images obtained at a range of x-ray tube voltages and input exposure levels. Surface doses were obtained from measured x-ray tube output data, and corresponding values of energy imparted were determined using the exposure-area product incident on the phantom. In one series of experiments, the air kerma at the image intensifier (X) was varied between 0.44 microGy per frame and 8.8 microGy per frame at a constant x-ray tube voltage of 70 kVp. In a second series of experiments, the tube voltage was varied between 50 and 100 kVp, and the mAs adjusted to maintain a constant exposure level at the input of the image intensifier. At a constant x-ray tube voltage, the surface dose and energy imparted were directly proportional to the input exposure per frame used to acquire the DSA images. On our DSA system operated below 2.2 microGy per frame, the threshold iodine concentration was found to be proportional to X(-0.57), which is in reasonable agreement with the theoretical prediction for a quantum noise limited imaging system. Above 2.2 microGy per frame, however, the threshold iodine concentration was proportional to X(-0.26), indicating that increasing the input exposure above this value will only achieve modest improvements in image quality. At a constant image intensifier input exposure level, increasing the x-ray tube voltage from 50 kVp to 100 kVp reduced the surface dose by a factor of 6.1, and the energy imparted by a factor of 3.5. The detection threshold iodine concentration was found to be proportional to kVp(n), where n was 2.1 at 1.1 microGy per frame, and 1.6 at 3.9 microGy per frame. For clinical situations that can be modeled by a uniform phantom, reducing the x-ray tube voltage rather than increasing the exposure level would best achieve improvements on our DSA imaging system performance.     

MicroCT with energy-resolved photon-counting detectors

The goal of this paper was to investigate the benefits that could be realistically achieved on a microCT imaging system with an energy-resolved photon-counting x-ray detector. To this end, we built and evaluated a prototype microCT system based on such a detector. The detector is based on cadmium telluride (CdTe) radiation sensors and application-specific integrated circuit (ASIC) readouts. Each detector pixel can simultaneously count x-ray photons above six energy thresholds, providing the capability for energy-selective x-ray imaging. We tested the spectroscopic performance of the system using polychromatic x-ray radiation and various filtering materials with K-absorption edges. Tomographic images were then acquired of a cylindrical PMMA phantom containing holes filled with various materials. Results were also compared with those acquired using an intensity-integrating x-ray detector and single-energy (i.e. non-energy-selective) CT. This paper describes the functionality and performance of the system, and presents preliminary spectroscopic and tomographic results. The spectroscopic experiments showed that the energy-resolved photon-counting detector was capable of measuring energy spectra from polychromatic sources like a standard x-ray tube, and resolving absorption edges present in the energy range used for imaging. However, the spectral quality was degraded by spectral distortions resulting from degrading factors, including finite energy resolution and charge sharing. We developed a simple charge-sharing model to reproduce these distortions. The tomographic experiments showed that the availability of multiple energy thresholds in the photon-counting detector allowed us to simultaneously measure target-to-background contrasts in different energy ranges. Compared with single-energy CT with an integrating detector, this feature was especially useful to improve differentiation of materials with different attenuation coefficient energy dependences.     

Experimental validation of a simple model capable of predicting the phase contrast imaging capabilities of any x-ray imaging system

Phase contrast (PC) imaging is one of the most exciting emerging x-ray imaging techniques, with the potential of removing some of the main limitations of conventional radiology. After extensive experimentation carried out particularly at synchrotron radiation (SR) facilities, the scientific community agrees that it is now time to translate these ideas towards the first clinical implementations. In this framework, a complete model, based on Fresnel/Kirchoff diffraction integrals, was devised. This model accounts for source dimensions, beam spectrum and divergence and detector point spread function (PSF), and can thus be applied to any x-ray imaging system. In particular, by accepting in input the above parameters along with the ones describing the sample, the model can be used to optimize the geometry of the set-up, i.e. to assess the source-to-sample and sample-to-detector distances which maximize feature detection. The model was evaluated by acquiring a range of images of different samples with a laboratory source, and a good agreement was found between simulated and experimental data in all cases. In order to maximize the generality of the results, all acquisitions were carried out using a polychromatic source and an energy-resolving detector; in this way, a range of monochromatic images could be obtained as well as polychromatic images, which can be created by integrating different parts of the acquired spectra. One of the most notable results obtained is that in many practical cases polychromatic PC imaging can provide the same image quality as its monochromatic counterpart. This is an important step in the wider application of PC using conventional sources.     

Evaluation of edge effect due to phase contrast imaging for mammography

It is well-known that the edge effect produced by phase contrast imaging results in the edge enhancement of x-ray images and thereby sharpens those images. It has recently been reported that phase contrast imaging using practical x-ray tubes with small focal spots has improved image sharpness as observed in the phase contrast imaging with x-ray from synchrotron radiation or micro-focus x-ray tubes. In this study, we conducted the phase contrast imaging of a plastic fiber and plant seeds using a customized mammography equipment with a 0.1 mm focal spot, and the improvement of image sharpness was evaluated in terms of spatial frequency response of the images. We observed that the image contrast of the plastic fiber was increased by edge enhancement, and, as predicted elsewhere, spectral analysis revealed that as the spatial frequencies of the x-ray images increased, so did the sharpness gained through phase contrast imaging. Thus, phase contrast imaging using a practical molybdenum anode tube with a 0.1 mm-focal spot would benefit mammography, in which the morphological detectability of small species such as microcalcifications is of great concern. And detectability of tumor-surrounded glandular tissues in dense breast would be also improved by the phase contrast imaging.     

X-ray micro-tomography system for small-animal imaging with zoom-in imaging capability

Since a micro-tomography system capable of microm-resolution imaging cannot be used for whole-body imaging of a small laboratory animal without sacrificing its spatial resolution, it is desirable for a micro-tomography system to have local imaging capability. In this paper, we introduce an x-ray micro-tomography system capable of high-resolution imaging of a local region inside a small animal. By combining two kinds of projection data, one from a full field-of-view (FOV) scan of the whole body and the other from a limited FOV scan of the region of interest (ROI), we have obtained zoomed-in images of the ROI without any contrast anomalies commonly appearing in conventional local tomography. For experimental verification of the zoom-in imaging capability, we have integrated a micro-tomography system using a microfocus x-ray source, a 1248 x 1248 flat-panel x-ray detector, and a precision scan mechanism. The mismatches between the two projection data caused by misalignments of the scan mechanism have been estimated with a calibration phantom, and the mismatch effects have been compensated in the image reconstruction procedure. Zoom-in imaging results of bony tissues with a spatial resolution of 10 lp mm(-1) suggest that zoom-in micro-tomography can be greatly used for high-resolution imaging of a local region in small-animal studies.     

Effect of scattered radiation on image noise in cone beam CT

Cone beam CT has a capability for the 3-dimensional imaging of large volumes with isotropic resolution, and has a potentiality for 4-dimensional imaging (dynamic volume imaging), because cone beam CT acquires data of a large volume with one rotation of an x-ray tube-detector pair. However, one of the potential drawbacks of cone beam CT is a larger amount of scattered x-rays, which may enhance the noise in reconstructed images, and thus affect the low-contrast detectablity. Our aim in this work was to estimate the scatter fractions and effects of scatter on image noise, and to seek methods of improving image quality in cone beam CT. First we derived a relationship between the noise in a reconstructed image and in an x-ray intensity measurement. Then we estimated the scatter to primary ratios in x-ray measurements using a Monte-Carlo simulation. From these we estimated the image noise under relevant clinical conditions. The results showed that the scattered radiation made a substantial contribution to the image noise. However, focused collimators could improve it by decreasing the scattered radiation drastically while keeping the primary radiation at nearly the same level. A conventional grid also improved the image noise, though the improvement was less than that of focused collimators.     

Monte Carlo simulation of the image formation process in portal imaging

We have written Monte Carlo programs to simulate the formation of radiological images. Our code is used to propagate a simulated x-ray fluence through each component of an existing video-based portal imaging system. This simulated fluence consists of a 512 x 512 pixel image containing both contrast-detail patterns as well as checker patterns to assess spatial resolution of the simulated portal imager. All of the components of the portal imaging system were modeled as a cascade of eight linear stages. Using this code, one can assess the visual impact of changing components in the imaging chain by changing the appropriate probability density function. Virtual experiments were performed to assess the visual impact of replacing the lens and TV camera by an amorphous silicon array, and the effect of scattered radiation on portal images.     

Dual-energy mammography: initial experimental results

Dual-energy x-ray techniques may be able to enhance the detectability of calcifications in mammographic examinations by removing the background "clutter" caused by contrast between adipose and glandular tissue. This hypothesis is examined experimentally by implementation of dual-energy imaging on a prototype digital scanned projection radiography system developed in our laboratory. A model of the propagation of signal and noise in dual-energy processing for a given radiation dose is validated by measurements from phantom images. The experimental imaging system has low spatial resolution and cannot be operated at dose-optimum energies; however, since both the single- and dual-energy images are subject to the same technical limitations, a comparison of such images allows an assessment of the benefits of dual energy. Experimental images of breast tissue specimens, showing improved detectability of calcifications when obscuring background clutter is removed, are presented. The dose required for a given signal-to-noise ratio can be reduced by smoothing the higher energy image prior to dual-energy processing. For practical implementation, it is reasonable to smooth the higher energy image such that its variance is reduced fourfold.     

6 CONCLUSIONS

The two purposes of patient dosimetry for medical x-ray imagingare to set and check standards of good practice and to assistin assessing detriment or harm. The present situation in patientdosimetry for medical x-ray imaging clearly indicates the needsfor dose quantities recommended for different applications andfor using the same, self-consistent system for names, symbols,and units. The radiation quality of an x-ray beam can be characterizedby the x-ray spectrum. Although x-ray spectra can be measured,it is recommended that the radiation quality of x-ray beamsused for medical imaging be characterized by a combination ofvarious parameters. These include the first HVL, HVL₁, the second,HVL, HVL₂, the ratio of HVL₁ to HVL₂, referred     

Performance characterization of megavoltage computed tomography imaging on a helical tomotherapy unit

Helical tomotherapy is an innovative means of delivering IGRT and IMRT using a device that combines features of a linear accelerator and a helical computed tomography (CT) scanner. The HI-ART II can generate CT images from the same megavoltage x-ray beam it uses for treatment. These megavoltage CT (MVCT) images offer verification of the patient position prior to and potentially during radiation therapy. Since the unit uses the actual treatment beam as the x-ray source for image acquisition, no surrogate telemetry systems are required to register image space to treatment space. The disadvantage to using the treatment beam for imaging, however, is that the physics of radiation interactions in the megavoltage energy range may force compromises between the dose delivered and the image quality in comparison to diagnostic CT scanners. The performance of the system is therefore characterized in terms of objective measures of noise, uniformity, contrast, and spatial resolution as a function of the dose delivered by the MVCT beam. The uniformity and spatial resolutions of MVCT images generated by the HI-ART II are comparable to that of diagnostic CT images. Furthermore, the MVCT scan contrast is linear with respect to the electron density of material imaged. MVCT images do not have the same performance characteristics as state-of-the art diagnostic CT scanners when one objectively examines noise and low-contrast resolution. These inferior results may be explained, at least partially, by the low doses delivered by our unit; the dose is 1.1 cGy in a 20 cm diameter cylindrical phantom. In spite of the poorer low-contrast resolution, these relatively low-dose MVCT scans provide sufficient contrast to delineate many soft-tissue structures. Hence, these images are useful not only for verifying the patient's position at the time of therapy, but they are also sufficient for delineating many anatomic structures. In conjunction with the ability to recalculate radiotherapy doses on these images, this enables dose guidance as well as image guidance of radiotherapy treatments.     

Fundamental image quality limits for microcomputed tomography in small animals

Small-animal imaging has become increasingly more important as transgenic and knockout mice are produced to model human diseases. One imaging technique that has emerged is microcomputed tomography (micro-CT). For live-animal imaging, the precision in the images will be determined by the x-ray dose given to the animal. As a result, we propose a simple method to predict the noise performance of an x-ray micro-CT system as a function of dose and image resolution. An ideal, quantum-noise limited micro-CT scanner, assumed to have perfect resolution and ideal efficiency, was modeled. Using a simplified model, the coefficient of variation (COV) of the linear attenuation coefficient was calculated for a range of entrance doses and isotropic voxel sizes. COV calculations were performed for the ideal case and with simulated imperfections in efficiency and resolution. Our model was validated in phantom studies and mouse images were acquired with a specimen scanner to illustrate the results. A simplified model of noise propagation in the case of isotropic resolution indicates that the COV in the linear attenuation coefficient is proportional to (dose)(-1/2) and to the (isotropic voxel size)(-2) in the reconstructed volume. Therefore an improvement in the precision can be achieved only by increasing the isotropic voxel size (thereby decreasing the resolution of the image) or by increasing the x-ray dose. For the ideal scanner, a COV of 1% in the linear attenuation coefficient for an image of a mouse exposed to 0.25 Gy is obtained with a minimum isotropic voxel size of 135 microm. However, the same COV is achieved at a dose of 5.0 Gy with a 65 microm isotropic voxel size. Conversely, for a 68 mm diameter rat, a COV of 1% obtained from an image at 5.0 Gy would require an isotropic voxel size of 100 microm. These results indicate that short-term, potentially lethal, effects of ionizing radiation will limit high-resolution live animal imaging. As improvements in detector technology allow the resolution to improve, by decreasing the detector element size to tens of microns or less, high quality images will be limited by the x-ray dose administered. For the highest quality images, these doses will approach the lethal dose or LD50 for the animals. Approaching the lethal dose will affect the way experiments are planned, and may reduce opportunities for experiments involving imaging the same animal over time. Dose considerations will become much more important for live small-animal imaging as the limits of resolution are tested.     

Evaluation of sparse-view reconstruction from flat-panel-detector cone-beam CT

Flat-panel-detector x-ray cone-beam computed tomography (CBCT) is used in a rapidly increasing host of imaging applications, including image-guided surgery and radiotherapy. The purpose of the work is to investigate and evaluate image reconstruction from data collected at projection views significantly fewer than what is used in current CBCT imaging. Specifically, we carried out imaging experiments using a bench-top CBCT system that was designed to mimic imaging conditions in image-guided surgery and radiotherapy; we applied an image reconstruction algorithm based on constrained total-variation (TV)-minimization to data acquired with sparsely sampled view-angles and conducted extensive evaluation of algorithm performance. Results of the evaluation studies demonstrate that, depending upon scanning conditions and imaging tasks, algorithms based on constrained TV-minimization can reconstruct images of potential utility from a small fraction of the data used in typical, current CBCT applications. A practical implication of the study is that the optimization of algorithm design and implementation can be exploited for considerably reducing imaging effort and radiation dose in CBCT.     

X-ray scatter removal by deconvolution

The distribution of scattered x rays detected in a two-dimensional projection radiograph at diagnostic x-ray energies is measured as a function of field size and object thickness at a fixed x-ray potential and air gap. An image intensifier-TV based imaging system is used for image acquisition, manipulation, and analysis. A scatter point spread function (PSF) with an assumed linear, spatially invariant response is modeled as a modified Gaussian distribution, and is characterized by two parameters describing the width of the distribution and the fraction of scattered events detected. The PSF parameters are determined from analysis of images obtained with radio-opaque lead disks centrally placed on the source side of a homogeneous phantom. Analytical methods are used to convert the PSF into the frequency domain. Numerical inversion provides an inverse filter that operates on frequency transformed, scatter degraded images. Resultant inverse transformed images demonstrate the nonarbitrary removal of scatter, increased radiographic contrast, and improved quantitative accuracy. The use of the deconvolution method appears to be clinically applicable to a variety of digital projection images.