We are developing and exploring the imaging performance of, an in vivo, in-line holography, x-ray phase-contrast, micro-CT system with an ultrafast laser-based x-ray (ULX) source. By testing and refining our system, and by performing computer simulations, we plan to improve system performance in terms of contrast resolution and
multi-energy imaging to a level beyond what can be obtained using a conventional microfocal x-ray tube. Initial CT projection sets at single energy (Mo Kα and Kβ lines) were acquired in the Fresnel regime and reconstructed for phantoms and a euthanized mouse. We also performed computer simulations of phase-contrast micro-CT scans for low-contrast, soft-tissue, tumor imaging. We determined that, in order to perform a phase-contrast, complete micro-CT scan using ULX, the following conditions must be met: (i) the x-ray source needs to be stable during the scan; (ii) the laser focal spot size needs to be less than 10 μm for source-to-object distance greater than 30 cm; (iii) the laser light intensity on the target needs to be in the range of 5 × 1017 to 5 × 1019 W/cm2; (iv) the ablation protection system needs to allow uninterrupted scans; (v) the laser light focusing on the target needs to remain accurate during the entire scan; (vi) a fresh surface of the target must be exposed to consecutive laser shots during the entire scan; (vii) the effective detector element size must be less than 12 μm. Based on the results obtained in this research project, we anticipate that the new 10 Hz, 200 TW laser with 50W average power that is being commissioned at ALLS will allow us practical implementation of in vivo x-ray phase-contrast micro-CT.
To assess the feasibility of small soft tissue avascular tumor
micro-CT imaging with x-ray phase-contrast in-line
holography, we have studied micro-CT imaging with in-line geometry of small spheroidal avascular tumor models
with quiescent cell core (< 250 μm) and various distributions of the proliferating cell density (PCD) forming the outer
shell. We have simulated imaging with an ultrafast laser-based x-ray source with a Mo target. We observe
phase-contrast enhancement of the tumor boundaries in the reconstructed transaxial images, resulting in improved
detection of small soft tissue tumors, providing that the PCD density gradient is sufficiently large.
KEYWORDS: 3D modeling, Systems modeling, Single photon emission computed tomography, Reconstruction algorithms, Collimators, Image quality, 3D image reconstruction, 3D image processing, Expectation maximization algorithms, Algorithm development
In order to improve reconstructed image quality for cone-beam collimator SPECT, we have developed and implemented
a fully 3D reconstruction, using an ordered subsets expectation maximization (OSEM) algorithm, along with a
volumetric system model - cone-volume system model (CVSM), a modified attenuation compensation, and a 3D depth- and
angle-dependent resolution and sensitivity correction. SPECT data were acquired in a 128×128 matrix, in 120 views
with a single circular orbit. Two sets of numerical Defrise phantoms were used to simulate CBC SPECT scans, and low
noise and scatter-free projection datasets were obtained using the SimSET Monte Carlo package. The reconstructed
images, obtained using OSEM with a line-length system model (LLSM) and a 3D Gaussian post-filter, and OSEM with
FVSM and a 3D Gaussian post-filter were quantitatively studied. Overall improvement in the image quality has been
observed, including better transaxial resolution, higher
contrast-to-noise ratio between hot and cold disks, and better
accuracy and lower bias in OSEM-CVSM, compared with OSEM-LLSM.
KEYWORDS: Image quality, Single photon emission computed tomography, Expectation maximization algorithms, Reconstruction algorithms, 3D modeling, Signal attenuation, Systems modeling, Chest, 3D image reconstruction, Medical imaging
To improve the speed and quality of ordered-subsets
expectation-maximization (OSEM) SPECT reconstruction, we have
implemented a content-adaptive, singularity-based, mesh-domain, image model (CASMIM) with an accurate algorithm
for estimation of the mesh-domain system matrix. A preliminary image, used to initialize CASMIM reconstruction, was
obtained using pixel-domain OSEM. The mesh-domain representation of the image was produced by a 2D wavelet
transform followed by Delaunay triangulation to obtain joint estimation of nodal locations and their activity values. A
system matrix with attenuation compensation was investigated. Digital chest phantom SPECT was simulated and
reconstructed. The quality of images reconstructed with OSEM-CASMIM is comparable to that from pixel-domain
OSEM, but images are obtained five times faster by the CASMIM method.
KEYWORDS: 3D modeling, Brain, Single photon emission computed tomography, Systems modeling, 3D image processing, Neuroimaging, 3D image reconstruction, Reconstruction algorithms, Monte Carlo methods, Image quality
In order to improve tomographically reconstructed image quality, we have implemented a fully 3D reconstruction, using
an ordered subsets expectation maximization (OSEM) algorithm for fan-beam collimator (FBC) SPECT, along with a
volumetric system model-fan-volume system model (FVSM), a modified attenuation compensation, a 3D depth- and
angle-dependent resolution and sensitivity correction, and a 3D total variation (TV) regularization. SPECT data were
acquired in a 128x64 matrix, in 120 views with a circular orbit. The numerical Zubal brain phantom was used to
simulate a FBC HMPAO Tc-99m brain SPECT scan, and a low noise and scatter-free projection dataset was obtained
using the SimSET Monte Carlo package. A SPECT scan for a mini-Defrise phantom and brain HMPAO SPECT scans
for five patients were acquired with a triple-head gamma camera (Triad 88) equipped with a low-energy high-resolution
(LEHR) FBC. The reconstructed images, obtained using clinical filtered back projection (FBP), OSEM with a line-length
system model (LLSM) and 3D TV regularization, and OSEM with FVSM and 3D TV regularization were
quantitatively studied. Overall improvement in the image quality has been observed, including better axial and transaxial
resolution, better integral uniformity, higher contrast-to-noise ration between the gray matter and the white matter, and
better accuracy and lower bias in OSEM-FVSM, compared with OSEM-LLSM and clinical FBP.
We have investigated experimentally and theoretically the imaging performance of our newly constructed in-line
holography x-ray phase-contrast imaging system with an ultrafast laser-based x-ray source. Projection images of nylon
fibers with diameters in the 10-330 μm range were obtained using an ultrafast (100 Hz, 28 fs, 40 mJ) laser-based x-ray
source with Mo and Ta targets and Be filter, and Gaussian spatial-intensity distribution (FWHMS = 5 μm). A cooled
CCD camera (24 μm pitch) with a Gd2OS2 screen coupled via 1:1 optical taper was used (FWHMD = 50 μm). We have investigated nylon-fiber image quality vs. imaging setup geometry and x-ray spectra. The following parameters were
evaluated: contrast, signal-to-noise ratio (SNR), resolution, and sampling. In addition, we performed theoretical
simulation of image formation for the same objects but within a wide range of geometrical parameters. The rigorous
wave-optical formalism was used for modeling of the free-space propagation of x-rays from the object plane to the
detector, and the "projection approximation" was used. We found reasonable agreement between predictions of our
analytical model and the experiments. We conclude that: a) Optimum magnification maximizing contrast and SNR is
almost independent of the source-to-detector (R) distance and depends strongly on the diameter of the fiber. b) The
corresponding maximum values of the contrast and SNR are almost linear with respect to R; the optimum magnification
decreases with fiber diameter. c) The minimum diameter of fiber defines the minimum source-to-object distance R1 if R is fixed and the object is moved.
We have observed that an expectation maximization (EM) algorithm applied to SPECT reconstruction may produce hotspot
artifacts of varying intensity. Our hypothesis was that scatter caused these artifacts. To test this assumption, we
studied the performance of forward- and back-projection procedures in the EM algorithm for simulated and experimental
SPECT data. First, synthetic scatter-free projections and projections with only one scattered photon in each view were
created for a simulated simple object, and reconstructed with a fully 3D ordered-subsets EM (OSEM) algorithm. Then,
Monte Carlo simulated brain SPECT (with no scatter and with scatter present), a mini-Defrise phantom, and patient
SPECT were reconstructed. We confirmed our hypothesis: hot-spot artifacts appeared only in the reconstruction from
noisy projections but not in the reconstruction from scatter-free projections. We investigated a practical and simple
method, critical path-length control (CPLC), for suppression of the hot-spot artifacts. To this end we performed
reconstructions with or without CPLC and quantitatively evaluated the results including estimation of accuracy, bias,
contrast-to-noise ratio, and uniformity. We found that the OSEM-with-CPLC method significantly reduced hot-spot
artifacts, and yielded a similar or improved image quality. We conclude that the CPLC method provides a useful yet
simple tool to reduce scatter-related hot-spot artifacts.
We have investigated theoretically the mean absorbed dose to the mouse in our newly constructed, in-line
holography, x-ray phase-contrast, in-vivo, micro-CT system with an ultrafast laser-based x-ray (ULX) source. We
assumed that the effective mouse diameter was 30 mm and the x-ray detector required minimum 30 μGy per frame
to produce high quality images. The following laser target-filter combinations were considered: Ag-Ag, Mo-Mo, Sn-
Sn. In addition, we considered narrow-pass multilayer x-ray mirrors. The corresponding ULX spectra were obtained
using a CZT solid-state spectrometer. The approach used for dose computation was similar to human dose
estimation. The mouse was modeled as a tissue-equivalent cylinder located at the isocenter with diameter 30 mm
and density 1g/cm3. A layer of dermis (skin and fur) with 1 mm thickness was also modeled. Imparted energy per
volume was estimated for 1 keV wide x-ray energy intervals in the 6-100 keV range. Monte Carlo simulations were
performed using the SIERRA code previously validated using 30 mm diameter PMMA phantom. The results
obtained indicate that: a) the mean absorbed dose for ULX is less than or equal to that from a W-anode micro-CT
tube operating at 30-40 kVp with 0.5 or 1.0 mm Al; b) for filter thickness above 100 μm, Sn-Sn results in the
highest dose, followed by Ag-Ag and Mo-Mo; c) the multilayer x-ray mirror with FWHM ≤ 10 keV produces
significantly lower dose than metallic foil filters. We conclude that ULX can provide better dose utilization than a
microfocal x-ray tube for in vivo microtomography applications.
KEYWORDS: Single photon emission computed tomography, Signal attenuation, Systems modeling, Monte Carlo methods, Collimators, Lung, Image quality, Tissues, Visualization, Reconstruction algorithms
We have implemented a more accurate physical system representation, a strip-area system model (SASM), for improved fan-beam collimator (FBC) SPECT reconstruction. This approach required implementation of modified ray tracing and attenuation compensation in comparison to a line-length system model (LLSM). We have compared performance of SASM with LLSM using Monte Carlo and analytical simulations of FBC SPECT from a thorax phantom. OSEM reconstruction was performed with OS=3 in a 64×64 matrix with attenuation compensation (assuming uniform attenuation of 0.13 cm-1). Scatter correction and smoothing were not applied. We observe overall improvement in SPECT image bias, visual image quality and an improved hot myocardium contrast for SASM vs. LLSM. In contrast to LLSM, the sensitivity pattern artifacts are not present in the SASM reconstruction. In both reconstruction methods, cross-talk image artifacts (e.g. inverse images of the lungs) can be observed, due to the uniform attenuation map used. SASM applied to fan-beam collimator SPECT results in better image quality and improved hot target contrast, as compared to LLSM, but at the expense of 1.5-fold increase in reconstruction time.
We investigated performance of ultrafast laser-based x-ray source for phase contrast imaging in 2D projection imaging and in enhanced micro-CT imaging. Good quality images were obtained, including images of small animals, in the single energy and multiple energy, in line phase-contrast enhancing geometry using x-ray line energy matching object thickness and density. Phase information has been inferred from images obtained at the same x-ray energy but at different object-to-detector distances and also from images obtained at the same object-to-detector distance but with different K-alpha line energies. Ultrafast laser-based, compact, x-ray source is a promising technique for micro-CT systems. Its utilization might result in faster scans with lower radiation dose, better spatial and contrast resolution and also femtosecond temporal resolution. In addition, it might allow practical implementation of dual-energy and phase-contrast imaging micro-CT that is not possible with conventional micro-CT.
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