We have developed high efficiency, variable-energy x-ray sources based on distributed coupling linacs. These linear accelerators have >2x beam-to-RF efficiency compared to conventional linacs. Separate feeds for cavities allow for their individual optimization for maximum output instead of feeding the RF power through the electron-beam pipe as is done conventionally. In addition, the framework for beam dynamics calculation accounts for beam loading in the cavities and iterates till a steady state solution is achieved. The linac cavities are machined into two symmetric blocks of copper that are diffusion-bonded together. The ports for RF feed, the flanges for the electron gun and the target are then brazed on. Special attention was paid to the mechanical design and assembly process to enable high yield production. Our linacs have an integrated buncher section where the first cavity has a separate feed. By controlling the amplitude and phase of the RF into this port, the beam energy and intensity can be varied. We have completed the design and are in various stages of building and high-power testing linacs for 2.5, 6 and 12 MeV for medical and for security and inspection purposes. We have also designed and are building the RF and electrical power sources including high power modulators and klystrons for supplying the RF and electrical power needed to run these linacs. The modulators are built using Marx-bank capacitor approach. The klystrons design follows a split-structure manufacturing technique, and their design fully accounts for beam loading with iteratively calculated steady state solution.
Complementary metal-oxide-semiconductors (CMOS) flat panel detectors (FPD) have steadily gained acceptance into medical imaging applications1-15. Selecting the proper detector technology for the imaging task requires optimization to balance the cost and the image quality. To facilitate this, fundamental detector performance of CMOS and a-Si panels were evaluated using the following quantitative imaging metrics: X-ray sensitivity, Noise Equivalent Dose (NED,) Noise Power Spectrum (NPS), Modulation Transfer Function (MTF), and Detective Quantum Efficiency (DQE). Imaging task measurements involved high-contrast and low-contrast resolution assessment. Varex FPDs evaluated for this study included: CMOS 3131 (150 μm pixel), a-Si 3030X (194 μm pixel), a-Si XRpad2 3025 (100 μm) and CMOS 2020 (100 μm pixel). Performance comparisons were organized by pixel size: large pixels, 150 μm CMOS and 194 μm a-Si, and small pixels, 100 μm in a-Si and CMOS technology. The results showed high dose DQE of the a-Si 3030X was about 10% higher than the CMOS 3131 between 0 - 1.8 cycles/mm, while beyond 1.8 cycles/mm, the CMOS performed better. The 3030X low dose DQE was higher than the 3131 between 0-1.3 cycles/mm, while the CMOS performance was higher beyond 1.3 cycles/mm. The high dose DQE of 100 μm a-Si was higher than the 100 μm CMOS for all frequencies. However, the low dose DQE of 100 μm CMOS was higher beyond 0.6 cycles/mm, while the 100 μm a-Si pixel had higher DQE only between 0 – 0.6 cycles/mm. Large pixel image quality (IQ) assessment favored a-Si pixel with 7% higher Contrast-to-Noise-Ratio (CNR) results for both high and low contrast-detail at 500 nGy. Small pixel CNR favored CMOS with ~38% better high contrast-detail and 12% greater low contrast-detail at ~500 nGy. Through these measurements that combine imaging metrics and image quality, we demonstrated a practical method for selecting the appropriate detector technology based on the requirements of the imaging applications.
A new high dynamic range CMOS x-ray detector is described. This sensor was designed specifically for x-ray imaging as opposed to the common approach of modifying a 3T optical sensor design. This allowed for a highly linear, wide dynamic range operation that has otherwise been a major drawback of CMOS x-ray detectors. The design is scalable from small tiles to large wafer-scale imagers fabricated on 300mm wafers. The performance of such a detector built using a 9.4cm x 9.4cm tile is reported. The pixel size of this detector is 76 μm and it can be operated in the native resolution or 2x2 binned mode. Measurements were performed with a thallium-doped cesium iodide (CsI(Tl)) scintillator deposited on a reflective aluminum substrate. The imager was operated at 30 frames/second. The linearity, dynamic range, sensitivity, MTF, NPS and DQE at RQA5 were measured using the standard protocols. Linearity was measured to be better than 0.2%. Using 600 μm CsI(Tl) scintillator, the maximum linear dose was 9 μGy with high gain and 56 μGy with low gain settings. This is comparable to conventional amorphous silicon flat panel detectors. The MTF is dominated by the scintillator and is 58% at 1 lp/mm and 28% at 2 lp/mm. The DQE is 70% at 0 lp/mm and 12% at the Nyquist frequency of 6.6 lp/mm. The high resolution combined with the large dynamic range and excellent DQE makes this CMOS detector particularly suitable for dynamic imaging including fluoroscopy, angiography and conebeam CT.
KEYWORDS: Breast, Mammography, Digital mammography, Monte Carlo methods, Sensors, Image quality, Signal detection, Signal attenuation, Image restoration, Point spread functions
Scattered radiation remains one of the primary challenges for digital mammography, resulting in decreased image contrast and visualization of key features. While anti-scatter grids are commonly used to reduce scattered radiation in digital mammography, they are an incomplete solution that can add radiation dose, cost, and complexity. Instead, a software-based scatter correction method utilizing asymmetric scatter kernels is developed and evaluated in this work, which improves upon conventional symmetric kernels by adapting to local variations in object thickness and attenuation that result from the heterogeneous nature of breast tissue. This fast adaptive scatter kernel superposition (fASKS) method was applied to mammography by generating scatter kernels specific to the object size, x-ray energy, and system geometry of the projection data. The method was first validated with Monte Carlo simulation of a statistically-defined digital breast phantom, which was followed by initial validation on phantom studies conducted on a clinical mammography system. Results from the Monte Carlo simulation demonstrate excellent agreement between the estimated and true scatter signal, resulting in accurate scatter correction and recovery of 87% of the image contrast originally lost to scatter. Additionally, the asymmetric kernel provided more accurate scatter correction than the conventional symmetric kernel, especially at the edge of the breast. Results from the phantom studies on a clinical system further validate the ability of the asymmetric kernel correction method to accurately subtract the scatter signal and improve image quality. In conclusion, software-based scatter correction for mammography is a promising alternative to hardware-based approaches such as anti-scatter grids.
Scintillating Fiber Optic Plates (SFOP) or Fiber Optic Scintillator (FOS) made with scintillating fiber-glass, were
investigated for x-ray imaging. Two different samples (T x W x L = 2cm x 5cm x 5cm) were used; Sample A: 10μm
fibers, Sample B: 50μm fibers both with statistically randomized light absorbing fibers placed in the matrix. A
customized holder was used to place the samples in close contact with photodiodes in an amorphous silicon flat panel
detector (AS1000, Varian), typically used for portal imaging. The detector has a 392μm pixel pitch and in the standard
configuration uses a gadolinium oxy-sulphide (GOS) screen behind a copper plate. X-ray measurements were performed
at 120kV (RQA 9 spectrum), 1MeV (5mm Al filtration) and 6MeV (Flattening Filter Free) for Sample A and the latter 2
spectra for Sample B. A machined edge was used for MTF measurements. The measurements showed the MTF
degraded with increased X-ray energies because of the increase in Compton scattering. However, at the Nyquist
frequency of 1.3lp/mm, the MTF is still high (FOS value vs. Cu+GOS): (a) 37% and 21% at 120kVp for the 10μm FOS
and the Cu+GOS arrays, (b) 31%, 20% and 20% at 1MeV and (c) 17%, 11% and 14% at 6MeV for the 10μm FOS,
50μm FOS and the Cu+GOS arrays. The DQE(0) value comparison were (a) at 120kV ~24% and ~13 % for the 10μm
FOS and the Cu+GOS arrays (b) at 1MV 10%, 10% and 7% and (c) at 6MV 12%, ~19% and 1.6% for the 10μm FOS ,
50μm FOS and Cu+GOS arrays.
X-ray radiography is an important and frequently used NDE method of testing metal structures, such as tube welding
quality, cracks and voids in cast iron or other metals. It gives fast and visible answer for structural defects. The Varian
high energy portal imagers on Clinacs used in cancer treatment were tested for this purpose. We compared the
traditional Gadox (LANEX) screen with and without a 1mm Cu buildup plate as used clinically. We also tested different
hybrid scintillators, which consisted of different phosphor layers deposited onto fiberoptic plates. The last screen tested
was a 2cm thick fiberoptic plate which contained scintillating fibers. The sensitivity (ADU = number of digital counts
per a given X-ray dose), the resolution (MTF – modulation transfer function) and the DQE (detective quantum
efficiency) were compared, with a 1 MV source, for these X-ray conversion screens.
We found that the additional 1mm Cu plate, which improves the absorption and the contrast at 6 or higher energy MeV
imaging, does not improve the image quality at 1MV. Rather it attenuates the X-rays, resulting in lower sensitivity and a
lower DQE(0) of 2.2% with the additional Cu plate compared to DQE(0) of ~4% without the Cu plate.
The hybrid scintillators with evaporated phosphors on fiberoptic plates tested were too thin resulting in low sensitivity.
The best results were obtained from the thick scintillating fiberoptic screens, which provided the best DQE and high
resolution with the 1MV X-ray beam. Further optimization is planned by changing the thickness of the scintillating
fiber optic plate.
Flat panels imagers based on amorphous silicon technology (a-Si) for digital radiography have been
accepted by the medical community as having several advantages over film-based systems.
Radiotherapy treatment planning systems employ computed tomographic (CT) data sets and
projection images to delineate tumor targets and normal structures that are to be spared from
radiation treatment. The accuracy of CT numbers is crucial for radiotherapy dose calculations.
Conventional CT scanners operating at kilovoltage X-ray energies typically exhibit significant image
reconstruction artifacts in the presence of metal implants in human body. Megavoltage X-ray
energies have problems maintaining contrast sensitivity for the same dose as kV X-ray systems. We
intend to demonstrate significant improvement in metal artifact reductions and electron density
measurements using an amorphous silicon a-Si imager obtained with an X-ray source that can
operate at energies up to 1 MeV. We will investigate the ability to maintain contrast sensitivity at
this higher X-ray energy by using targets with lower atomic numbers and appropriate amounts of Xray
filtration than are typically used as X-ray production targets and filters.
KEYWORDS: Simulation of CCA and DLA aggregates, Image segmentation, Imaging systems, Arteries, Angiography, Computed tomography, Visualization, Brain, Neuroimaging, Silver
Brain perfusion CT using a C-arm angiography system capable of CT-like imaging could optimize patient treatment
during stroke therapy procedures. For this application, an intra-arterial contrast bolus injection at the
aortic arch could be used provided that the location of the injection catheter enables uniform distribution of the
bolus into the two common carotid arteries (CCA). In this work, we present a novel method to support optimal
injection catheter placement by providing additional quantitative information about the distribution of the contrast
bolus into the CCAs. Our fully automatic method uses 2-D digital subtraction angiography (DSA) images
following a test bolus injection. It segments both CCAs and computes the relative contrast distribution. We
have tested the method in DSA data sets from 5 healthy pigs and our method achieved successful segmentation
of both CCAs in all data sets. The results showed that the contrast is uniformly distributed (mean relative
difference less or equal than 10%) if the injection location is properly chosen.
C-arm cone-beam CT (CBCT) is increasingly being used to supplement 2D real-time data with 3D information.
Temporal resolution is currently limited by the mechanical rotation speed of the C-arm which presents challenges
for applications such as imaging of contrast flow in brain perfusion CT (PCT). We present a novel scheme where
multiple scans are obtained at different start times with respect to the contrast injection. The data is interleaved
temporally and interpolated during 3D reconstruction. For evaluation we developed a phantom to generate the range
of temporal frequencies relevant for PCT. The highest requirements are for imaging the arterial input function (AIF)
modeled as a gamma-variate function. Fourier transform analysis of the AIF showed that 90% of the spectral energy
is contained at frequencies lower than 0.08Hz. We built an acrylic cylinder phantom of diameter 1.9 cm, with 25
sections of 1cm length each. Iodine concentration in each compartment was varied to produce a half-cycle sinusoid
variation in HU in version 1, and 2.5 cycles in version 2 of the phantom. The phantom was moved linearly at speeds
from 0.5cm/s to 4cm/s (temporal frequencies of 0.02Hz to 0.09Hz) and imaged using a C-arm system. Phantom CT
numbers in a slice reconstructed at isocenter were measured and sinusoidal fits to the data were obtained. The fitted
sinusoids had frequencies that were within 3±2% of the actual temporal frequencies of the sinusoid. This suggests
that the imaging and reconstruction scheme is adequate for PCT imaging.
KEYWORDS: X-rays, Monte Carlo methods, Tungsten, Optical spheres, Electron beams, Chemical species, Reflection, Photon transport, Absorption, Grazing incidence
The angular probability distribution of x-rays from a single interaction of a high energy electron with a target atom is a
function of the incident-electron energy, direction and target material. This distribution can be quite directional, which
suggests that x-ray tube efficiency might be increased if this effect was used. This can be important for novel tubes that
use scanning electron beams or carbon nanotubes that have low output flux. We performed Monte Carlo(MC)
simulations for studying how this angular distribution is affected by the interactions in thick targets. The theoretical
distribution for single-atom interaction was verified using a 4nm tungsten (W) sphere. Contributions of the various
processes undergone by the electrons and x-ray photons were analyzed individually. The angular distributions of x-rays
generated by electrons incident normally and at a grazing angle to a 4mm thick target were calculated. The results for a
12μm transmission target were also simulated. For single interactions, the theoretical peak for 120keV electron at 28°
was measured to be 29° for the MC simulations. The transmission target was found to have 26% higher x-ray output in
the forward direction compared to a conventional tube for E≥30keV. When x-ray flux per unit heat was considered for
E≥30, grazing incidence of electrons and the associated reflection beam was found to be 41% more efficient than a
conventional tube.
KEYWORDS: X-rays, Electron beams, Magnetism, Finite element methods, X-ray imaging, Magnetic resonance imaging, Computer simulations, Sensors, 3D modeling, Surgery
In order to achieve a truly hybrid, high quality X-ray/MR system one must have a rotating anode x-ray source as close as
possible to the bore of the high-field MR magnet. Full integration between a closed bore MR system and an x-ray
fluoroscopy system presents two main challenges that must be addressed: x-ray tube motor operation and efficiency in
an external field, and focal spot deflection. Regarding the first challenge our results have shown that an AC induction
motor operating in external fields will experience a drop off in efficiency. Specifically, fields on the order of 100 Gauss
perpendicular to the rotor decrease the rotation speed from 2450 RPM to below 1800 RPM. We are currently
developing an alternate brushless DC motor design that would exploit the presence of the external MR fringe field and
our initial finite element results indicate that the necessary amount of torque is produced. Regarding the second
challenge our results show that an external field of 195 Gauss perpendicular to the anode-cathode axis (BR direction)
produces a focal spot deflection of 5 mm. For the fields at which we want to operate the x-ray tube (~to 1000 Gauss
along BR) this deflection will be significantly larger than 5 mm and must be corrected for. We propose a design that
includes active deflection coils which serve to counteract the presence of the external field and reduce the focal spot
deflection to less than 1 mm in our simulations.
A high-resolution (198 μm) C-arm CT imaging system (Axiom Artis dTA, Siemens Medical Solutions, Forchheim, Germany) was optimized for imaging superficial femoral artery (SFA) stents in humans. The SFA is susceptible to the development of atherosclerotic lesions. These are typically treated with angioplasty and stent deployment. However, these stents can have a fracture rate as high as 35%. Fracture is usually accompanied by restenosis and reocclusion. The exact cause of breakage is unknown and is hypothesized to result from deforming forces due to hip and knee flexion. Imaging was performed with the leg placed in both straight and bent positions. Projection images obtained during 20 s scans with ~200° of rotation of the C-arm were back-projected to obtain 3D volumes. Using a semi-automatic software algorithm developed in-house, the stent centerlines were found and ellipses were fitted to the slice normals. Image quality was adequate for calculations in 11/13 subjects. Bending the leg was found to shorten the stents in 10/11 cases with the maximum change being 9% (12 mm in a 133 mm stent), and extend the stent in one case by 1.6%. The maximum eccentricity change was 36% with a bend angle of 72° in a case where the stent extended behind the knee.
Under certain assumptions the detectability of the ideal observer can be defined as the integral of the system Noise Equivalent Quanta multiplied by the squared object spatial frequency distribution. Using the detector Noise-Equivalent-Quanta (NEQD) for the calculation of detectability inadequately describes the performance of an x-ray imaging system because it does not take into account the effects of patient scatter and geometric unsharpness. As a result, the ideal detectability index is overestimated, and hence the efficiency of the human observer in detecting objects is underestimated. We define a Generalized-NEQ (GNEQ) for an x-ray system referenced at the object plane that incorporates the scatter fraction, the spatial distributions of scatter and focal spot, the detector MTFD, and the detector Normalized-Noise-Power-Spectrum (NNPSD). This GNEQ was used in the definition of the ideal detectability for the evaluation of the human observer efficiency during a two Alternative Forced Choice (2-AFC) experiment, and was compared with the case where only the NEQD was used in the detectability calculations. The 2-AFC experiment involved the detection of images of polyethylene tubes (diameters between 100-300 um) filled with iodine contrast (concentrations between 0-120 mg/cm3) placed onto a uniform head equivalent phantom placed near the surface of a microangiographic detector (43 um pixel size). The resulting efficiency of the human observer without regarding the effects of scatter and geometric unsharpness was 30%. When these effects were considered the efficiency was increased to 70%. The ideal observer with the GNEQ can be a simple optimization method of a complete imaging system.
C-arm CT first emerged as a useful high-contrast imaging modality in the late 1990s, using an XRII as the large area x-ray detector. To date, the C-arm approach to intra-procedural 3D imaging has primarily been used for high-contrast imaging tasks. The emerging goal for these systems is to extend the imaging range into the area of soft-tissue, and it is thought that digital flat-panel detectors may help. Flat panels replace the analog image intensifier, the camera optics, the pickup tube and the analog-to-digital converter with an all-digital detector. Flat panel detectors have a linear response, do not require distortion correction, do not suffer from veiling glare or blooming, and have higher dynamic range that current XRIIs. On the other hand, XRIIs have greater flexibility in FOV, and could support higher frame rates at high resolution, thereby reducing the effects of view aliasing. We have experience with a typical XRII-based C-arm imaging system and a new high-end C-arm equipped with a large flat-panel detector. Initial investigations show that when projection pixel size, acquisition geometry and focal spot size are matched, the flat-panel-based system produces reconstructions with improved MTF, primarily due to the additional interpolation step required for XRII warp correction. Investigations of artifact levels and comparison with in vivo CT images are presented.
New neuro-interventional devices such as stents require high spatial-resolution image guidance to enable accurate localization both along the vessel axis as well as in a preferred rotational orientation around the axis. A new high-resolution angiographic detector has been designed with capability for micro-angiography at rates exceeding the 5 fps of our current detector and, additionally, with noise low enough and gain high enough for fluoroscopy. Although the performance requirements are demanding and the detector must fit within practical clinical space constraints, image guidance is only needed within a approximately 5 cm region of interest at the site of the intervention. To achieve the design goals, the new detector is being assembled from available components which include a CsI(Tl) phosphor module coupled to a fiber-optic taper assembly with a two stage light image intensifier and a mirror between the output of the fiber taper and the input to a conventional high performance optical CCD camera. Resulting acquisition modes include 50-micron effective pixels at up to 30 fps with the capability to adjust sensitivity for both fluoroscopy and angiography. Estimates of signal at the various stages of detection are made with quantum accounting diagrams (QAD).
Minimally invasive image-guided interventions require very high image resolution and quality, specifically over regions-of-interest (ROI) crucial to the procedure. An ROI high quality image allows limited patient radiation deposition while permitting rapid frame transfer rates. Considering current developments in direct conversion Flat Panel Detectors (FPD), advantages of such an imager for ROI angiography were investigated. The performance of an amorphous-selenium based FPD was simulated to evaluate improvements in MTF and DQE under various angiographic imaging conditions. The detector envisioned incorporates the smallest pixel size of 70 mm, reported to date, and a photoconductor thickness of 1000 mm to permit angiography. The MTF of the FPD is calculated to be 60% at the Nyquist frequency of 7.1 lp/mm compared to 6% for a previously reported CsI(Tl)-based ROI CCD camera. The DQE(0) of the FPD at 0.7 mR and 70 kVp is 74% while for the CCD camera is 70%. At 7.1 lp/mm, the FPD's DQE is 26% while for the CCD camera it is 12%. Images of an undeployed stent with 70 mm pixel mammography FPD prototype, compare favorably with images acquired with the CCD camera. Thus a practical direct flat-panel ROI detector with both improved performance and physical size is proposed.
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