Grating-based phase contrast mammography can help facilitate breast cancer diagnosis, as several research works have demonstrated. To translate this technique to the clinics, it has to be adapted to cover a large field of view within a limited exposure time and with a clinically acceptable radiation dose. This indicates that a straightforward approach would be to install a grating interferometer (GI) into a commercial mammography device. We developed a wave propagation based optimization method to select the most convenient GI designs in terms of phase and dark-field sensitivities for the Philips Microdose Mammography (PMM) setup. The phase sensitivity was defined as the minimum detectable breast tissue electron density gradient, whereas the dark-field sensitivity was defined as its corresponding signal-to-noise Ratio (SNR). To be able to derive sample-dependent sensitivity metrics, a visibility reduction model for breast tissue was formulated, based on previous research works on the dark-field signal and utilizing available Ultra-Small-Angle X-ray Scattering (USAXS) data and the outcomes of measurements on formalin-fixed breast tissue specimens carried out in tube-based grating interferometers. The results of this optimization indicate the optimal scenarios for each metric are different and fundamentally depend on the noise behavior of the signals and the visibility reduction trend with respect to the system autocorrelation length. In addition, since the inter-grating distance is constrained by the space available between the breast support and the detector, the best way we have to improve sensitivity is to count on a small G2 pitch.
The demands for a large field-of-view (FOV) and the stringent requirements for a stable acquisition geometry
rank among the major obstacles for the translation of grating-based, differential phase-contrast techniques
from the laboratory to clinical applications. While for state-of-the-art Full-Field-Digital Mammography
(FFDM) FOVs of 24 cm x 30 cm are common practice, the specifications for mechanical stability are naturally
derived from the detector pixel size which ranges between 50 and 100 μm. However, in grating-based, phasecontrast
imaging, the relative placement of the gratings in the interferometer must be guaranteed to within
micro-meter precision. In this work we report on first experimental results on a phase-contrast x-ray imaging
system based on the Philips MicroDose L30 mammography unit. With the proposed approach we achieve
a FOV of about 65 mm x 175 mm by the use of the slit-scanning technique. The demand for mechanical
stability on a micrometer scale was relaxed by the specific interferometer design, i.e., a rigid, actuator-free
mount of the phase-grating G1 with respect to the analyzer-grating G2 onto a common steel frame. The
image acquisition and formation processes are described and first phase-contrast images of a test object are
presented. A brief discussion of the shortcomings of the current approach is given, including the level of
remaining image artifacts and the relatively inefficient usage of the total available x-ray source output.
Dark-field imaging has the potential to overcome limitations in computed tomography (CT) investigating relatively
weakly absorbing material. However, an object-position dependence of the visibility loss in dark-field
imaging is observed. This effect might be negligible for small objects, but, for acquisition geometries using fanangle
apertures and field of views as those in human CT scanners, the object-position dependence of visibility
loss has to be taken into consideration if the scattering structure within the object is in the range of the grating
periods, i.e. micrometer. This work examines the effect of object-position dependent visibility loss in dark-field
imaging experimentally, investigates its consequences and presents an algorithm which solves the corresponding
reconstruction problem.
To facilitate systematic calibration and validation of quantitative airway measurements on CT for COPD diagnosis, an
acrylic plastic phantom has been designed with an array of cylindrical tubes varying lumen diameter and wall thickness
in a systematic way, which can be manufactured by inexpensive 3D-printing. Accuracy and reproducibility of the 3Dprinting
have been confirmed by CT measurements. The multipliable, unobtrusive phantom can be scanned simultaneously
with the patient for each exam, allows scan-specific calibration, and can thus improve multicenter study
comparability between differing clinical imaging protocols.
Three different methods to correct the bronchial measurements for partial volume effect and scanner blur were tested.
The correction methods were variants of the physically motivated method suggested by Weinheimer et al. which
integrates the Hounsfield densities in a certain wall area, and derives the corrected values assuming a characteristic
constant Hounsfield density of the bronchial wall. The alternative methods compared here differ in the choice of the
integration boundaries. Analysis of CT scans showed high agreement and good noise robustness of all correction
methods on the one hand, but significant dependency on the choice of the CT reconstruction filter on the other hand,
which emphasizes the benefits of scan-specific calibration.
Differential phase-contrast imaging in the x-ray domain provides three physically complementary signals:1, 2 the
attenuation, the differential phase-contrast, related to the refractive index, and the dark-field signal, strongly
influenced by the total amount of radiation scattered into very small angles. In medical applications, it is of
the utmost importance to present to the radiologist all clinically relevant information in as compact a way as
possible. Hence, the need arises for a method to combine two or more of the above mentioned signals into
one image containing all information relevant for diagnosis. We present an image composition algorithm that
fuses the attenuation image and the differential phase contrast image into a composite, final image based on the
assumption that the real and imaginary part of the complex refractive index of the sample can be related by a
constant scaling factor. The merging is performed in such a way that the composite image is characterized by
minimal noise-power at each frequency component.
Cardiac CT image reconstruction suffers from artifacts due to heart motion during acquisition. In order to mitigate these effects, it is common practice to choose a protocol with minimal gating window and fast gantry rotation. In addition, it is possible to estimate heart motion retrospectively and to incorporate the information
in a motion-compensated reconstruction (MCR). If shape tracking algorithms are used for generation of the heart motion-vector field (MVF), the number and positions of the motion vectors will not coincide with the number and positions of the voxels in the reconstruction grid. In this case, data interpolation is necessary for
MCR algorithms which require one motion vector at each voxel location. This work examines different data interpolation approaches for the MVF interpolation problem and the effects on the MCR results.
In this contribution, the results of a phantom study for in-stent restenosis imaging with ECG gated continuous
circular acquisition and reconstruction are summarized. Different rotation speeds and angular ranges are used
to enable high resolution 3D and 4D reconstruction of objects covered by the cone at a high temporal resolution.
Though the detector coverage of today's CT scanners is not large enough to irradiate the complete human heart,
the coverage is sufficient to image smaller objects like conventional stents. We applied the proposed method to
the visualization of an in-stent re-stenoses phantom covered by a clinical stent, attached to a dynamic heart
phantom. The method delivers images of stents in vitro at an excellent visibility and is able to rule out in-stent
occlusions.
Energy-resolved fan-beam coherent scatter computed tomography (CSCT) is a novel X-ray based tomographic imaging
method revealing structural information on the molecular level, namely the momentum-transfer dependence of the
coherent scatter cross-section. Since the molecular structure is the source of contrast a very good material discrimination
and possibly also medical diagnosis of structural changes of tissue can be achieved with this technique. For the design of
a medical or baggage inspection CSCT-scanner acquisition speed is of particular importance. Several performance
improvements for CSCT were investigated. The multi-slit fan beam collimator and multi-line scatter detector allow
increasing the detected photon flux without compromising angular resolution. Analysis of the noise in reconstructed data
leads to the possibility to adjust scan time to the size of the objects to be analyzed. Improved energy resolution of the
detector improves momentum-transfer resolution such that angular resolution becomes the limiting factor. Overall, the
implemented improvements now enable the real-world application of CSCT.
In this contribution, we introduce a retrospectively gated cardiac cone-beam reconstruction scheme for continuous circular acquisition with parallel ECG recording. The technique is capable of handling variable angular ranges and enables high resolution 3D and 4D reconstruction of objects covered by the cone at a high temporal resolution. Though the detector coverage of nowadays CT scanners is not large enough to cover the complete human heart, it is sufficient to image smaller objects like conventional stents. We applied the proposed reconstruction method to the visualization of an in-stent re-stenoses phantom covered by a clinical stent, attached to a dynamic heart phantom. The method delivers images of stents in vitro at an excellent visibility and is able to rule out in-stent occlusions.
KEYWORDS: Sensors, Polymethylmethacrylate, Photons, Monte Carlo methods, Fluctuations and noise, X-rays, Computed tomography, Scattering, Signal attenuation, Aluminum
Coherent Scatter Computer Tomography (CSCT) is a novel x-ray imaging method revealing structural information on the molecular level. More precisely, the momentum transfer dependence of the coherent scatter cross section of the object material in each voxel of an object slice under investigation is determined. Compared to other x-ray diffraction techniques very large objects can be investigated which allows to apply the technique in medical imaging, material analysis or baggage inspection. The ratio of multiple scattered radiation over single scatter increases with object size. For large objects multiple scatter can become the dominant contribution. Since this part of the measured radiation cannot be reconstructed correctly, artifacts in the resulting images occur. The amount of multiple scattered radiation in CSCT and its dependence on the object size and material have been investigated by means of Monte Carlo simulations. A method to correct for multiple scattered radiation in energy-resolved CSCT is introduced. The benefit of this correction method to the quality of reconstructed data is demonstrated.
For the first time, a reconstruction technique based on filtered back-projection (FBP) using curved 3D back-projection lines is applied to energy-resolved coherent-scatter projection data. Coherent-scatter computed tomography (CSCT) yields information about the molecular structure of an object. It has been shown that the relatively poor spectral resolution due to the application of a polychromatic X-ray source can be overcome, when energy-resolved detection is used. So far, the energy-resolved projection data, acquired with a CSCT scanner, are reconstructed with the help of
algebraic reconstruction techniques (ART). Due to the computational complexity of iterative reconstruction, these methods lead to relatively long reconstruction times. In this contribution, a reconstruction algorithm based on 3D FBP is introduced and applied to
projection data acquired with a demonstrator setup similar to a multi-line CT scanner geometry using an energy-resolving CdTe-detector. Within a fraction of the computation time of algebraic reconstruction methods, an image of comparable quality is generated when using FBP reconstruction. In addition, the FBP approach has the advantage, that sub-field-of-view reconstruction becomes feasible.
This allows a selective reconstruction of the scatter function for a region of interest. The method is based on a high-pass filtering of the scatter data in fan-beam direction applied to all energy channels. The 3D back-projection is performed along curved lines through a volume defined by the in-plane spatial coordinates and the wave-vector transfer.
Energy-resolved fan beam coherent scatter computed tomography (CSCT) is a novel X-ray based imaging method revealing structural information on the molecular level of tissue or other material under investigation with high resolution of the momentum-transfer dependent coherent scatter cross-section. Since the molecular structure is the source of contrast a very good material discrimination and possibly also medical diagnosis of structural changes of tissue can be achieved with this technique. Poor spectral resolution as found in previous work due to the application of a polychromatic X-ray source can be overcome when energy-resolved detection is used. In this paper experimental results on phantoms using an energy-resolving CdTe-detector are shown. With the present setup the spatial resolution was found to be 4.5 mm (FWHM) and a spectral resolution of 6% was achieved. Applications of this technique can be found in medical imaging, material analysis and baggage inspection.
In this contribution, we investigate the spatial and temporal resolution of retrospectively gated cardiac cone-beam CT. Data of a static and a dynamic resolution phantom are acquired for various heart rates, table speeds and scanner rotation times. The projection data are reconstructed in different motion states with the help of a
retrospectively gated helical cardiac cone-beam reconstruction method. This multi-cycle method automatically adapts the number of heart cycles used for the reconstruction, based on the scan parameters and the ECG data. The spatial resolution is derived from a resolution phantom by multi-planar reformation (MPR) along the scan
direction.
Fan-beam coherent scatter computed tomography (CSCT) is a novel X-ray based imaging method revealing structural information of tissue under investigation. The source of contrast is the angular-dependent coherent scatter cross-section, which is determined by the molecular structure. In this work a phantom consisting of water, tricalcium phosphate, collagen and fat was used to investigate the contrast resolution of these four tissue constituents. Scatter projections were measured in fan-beam 3rd generation CT-geometry using an experimental demonstrator set-up equipped with a 4.5 kW DC power X-ray tube and photon-counting detectors. Reconstruction was performed using two algorithms, one based on algebraic reconstruction technique (ART) and the other based on filtered back-projection (FBP). The reconstruction results of the two techniques are compared. Furthermore, scatter functions of the four components were extracted from the 3D data sets and compared to previous measurements. The applicability of this technique for medical diagnosis is discussed.
For the first time, a reconstruction technique based on filtered back-projection (FBP) using curved 3D back-projection lines is applied to 2D coherent-scatter computed tomography (CSCT) projection data.
It has been demonstrated, that CSCT yields information about the molecular structure of an object. So far, the acquired projection data are reconstructed with the help of algebraic reconstruction techniques. Due to the computational complexity of iterative reconstruction, these methods lead to relatively long reconstruction times. In this contribution, a reconstruction algorithm based on 3D FBP is introduced and tested with simulated projection data as well as with projection data acquired with a demonstrator setup similar to a multi-line CT scanner geometry. Within a fraction of computation time at least a comparable image quality is achieved when using FBP reconstruction. In addition, it has the advantage, that - in contrast to iterative reconstruction schemes - sub-field-of-view reconstruction becomes feasible. This allows a selective reconstruction of the scatter function for a region of interest. The method is based on a row by row high-pass filtering of the scatter data, with or without fan to parallel beam rebinning. The 3D back-projection is performed along curved lines through a volume defined by the in-plane spatial coordinates and the wave-vector transfer.
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