This study investigates the possibility of using the model observer detectability to represent human detection
performance in differential phase contrast CT (DPC-CT). Five model observers were investigated, including
the prewhitening (PW) observer, the prewhitening observer with eye filter and internal noise (PWEi), the nonprewhitening
(NPW) observer, the non-prewhitening observer with eye filter (NPWE), and the channelized
Hotelling observer (CHO). Human 2AFC experiments involving four physicist observers were also performed to
provide reference to the model observer results. The contrast thresholds required for 92% correct decision rate
in 2AFC tests were evaluated for each observer. While all five model observers show good correlations with
human observers, the CHO generated the best quantitative agreement with human observer results. Compared
with the time-consuming human observer method, the model observer method is of much higher efficiency for
the evaluation and optimization of DPC-CT.
Although the relative ease of implementation and compact nature of grating-based differential phase contrast
CT (DPC-CT) has sparked tremendous enthusiasm for potential medical applications, the pros and cons of this
imaging method remains to be addressed before an actual clinical system can be constructed. To address these
unknowns, either numerical simulations or direct hardware implementations can be used. However, both approaches
have their limitations. It is highly desirable to develop a research method to enable imaging performance
prediction for a future DPC-CT system from the performance of an available absorption CT (ACT) system. In
this paper, a theoretical framework was developed to accurately predict the noise properties and detection performance
of DPC-CT from that of conventional ACT. The framework was derived based on a fundamental noise
relationship between DPC-CT and ACT and was experimentally validated. An example has been given in the
paper on how the framework can be utilized to predict model observer detectability index of a DPC breast CT
constructed based on an existing absorption breast CT. This framework is expected to become a valuable tool in
addressing the following questions: (i) With a fixed radiation dose in a particular clinical application, how well
can a specific detection/discrimination imaging task can be performed provided that an existing ACT scanner is
modified into a DPC-CT by inserting a grating interferometer, which is characterized by a few design parameters
(e.g., pitches and duty cycles of the gratings, relative distance between the gratings, etc.) into the ACT system?
(ii) If a DPC-CT system can outperform an ACT for certain detection/discrimination tasks under the constraint
of identical radiation dose to the image object, how would one optimize design parameters of the gratings in
order to maximize its potential clinical benefits?
This paper concerns an experimental method to quantify the spatial resolution of an experimental DPC-CT
system via modulation transfer function (MTF) measurement. Note that conventional metal wire-based MTF
measurement methods are no long applicable to DPC-CT, because: (i) The refractive signal generated by the
high density metal is out of the dynamic range of DPC-CT, and (ii) A DPC-CT system is not sensitive to
input pulse finer than a detector pixel. These technical challenges were overcome by a new experimental design,
in which a graphite rod was used as the probe to simultaneously measure the MTFs of the DPC-CT and the
associated absorption CT. An appreciable difference in MTFs between DPC-CT and absorption CT was observed
in our experimental benchtop system: At the 10% MTF level, the MTFs of the DPC-CT and the absorption
CT are 4.7 cycles/mm and 5.2 cycles/mm, respectively. Such a difference in MTF leads to the conclusion that
the grating interferometer used by the DPC-CT system has a frequency-dependent response to input DPC-CT
signals.
The effects of beam hardening have previously been extended from absorption imaging to phase contrast imaging,
showing a similar, albeit reduced, effect in the phase images. The effect of beam hardening on the interferometer
performance, however, has not been demonstrated. In this work, the visibility reduction on a differential phase
contrast imaging system due to spectral changes as a result of beam hardening is demonstrated. The implication
of this reduction is an artificial increase in noise for the phase contrast image through highly-attenuating regions
of the object. In addition, false signal will be recorded in the dark-field image, which normally shows only
highly-scattering objects and interfaces. The results show that with added beam filtration, the effect is reduced,
just as with more traditional beam hardening artifacts. However, the effect also means that one must also take
into account the desired imaging task when determining the system’s design energy.
Statistical iterative reconstruction methods have come to the forefront of CT research in recent years, as they
have the ability to incorporate the statistical fluctuations in CT measurements into the image reconstruction
process. While statistical iterative reconstruction methods have been found to be beneficial in CT imaging, they
have not been extensively investigated or applied in other new and promising CT imaging techniques, such as
x-ray differential phase contrast computed tomography (DPC-CT). The purpose of this study is to investigate
and apply statistical image reconstruction to DPC-CT to reduce streaking artifacts caused by strong small-angle
scattering objects.
Interest in grating-based x-ray interferometry has grown rapidly in recent years, due in part to the systems
ability to simultaneously provide images of multiple contrast mechanisms within a single acquisition. In addition
to the well known absorption and phase contrast images, a third image of local small angle scattering known as
the dark-field image is produced. There are limited published results describing applications of this method in
medical imaging. In this work, the dark-field contrast mechanism is examined in the context of medical imaging.
Experimental results demonstrate the relationship of both the size of the scatterers and their electron density
and the resulting dark-field signal strength. The dark-field images are produced using both the well-known phase
stepping technique and the recently reported moir´e technique. The results show that dark-field signal strength
increases with increased difference in electron density from the background and with decreased scatterer size.
They also demonstrate the equivalence of the resulting data using either signal extraction technique.
The demonstration of x-ray differential phase contrast (DPC) imaging with a grating interferometer based setup
using both synchrotron sources1 and conventional low-brilliance
x-ray tubes2 has led to much research interest
within the medical imaging community. DPC imaging provides an opportunity to exploit new contrast mechanisms,
and the corresponding computed tomography (CT) imaging potentially allows higher spatial resolution
imaging at lower radiation dose than conventional absorption imaging.3 Grating interferometer based methods
ease implementation of the method through the use of clinically viable components, e.g., standard x-ray tubes
and detectors.
The noise properties of differential phase contrast CT (DPC-CT) demonstrate some peculiar features. It has
been both theoretically and experimentally demonstrated that the noise variance of DPC-CT scales with spatial
resolution following an inverse first order relationship. This is in stark contrast to absorption CT, where the noise
variance scales with spatial resolution following an inverse third power. In addition to the scaling relationship,
the noise power spectrum (NPS) of DPC-CT is dominated by low spatial frequencies and demonstrates a singular
behavior when approaching zero frequency. This focuses the peak noise power within low spatial frequencies while
high-frequency noise is suppressed. This is again in contrast to the absorption CT case where the NPS smoothly
transitions to zero at zero frequency. The singular behavior of the DPC-CT NPS visually affects image noise
texture and may hinder observer perception. In this paper, a method is proposed to improve the noise properties
in DPC-CT and potentially improve observer performance. Specifically, the low frequency component of the
filtering kernel used in reconstruction has been regularized to modify the noise power at low spatial frequencies.
This results in a high-pass filtering of the image. The high-pass filtered image is combined with the original
image to generate the final image. As a result of these two operations, the noise power is shifted to the high
spatial frequency direction, improving visual perception, while image reconstruction accuracy is maintained.
Experimental phantom results are presented to validate the proposed method.
The development of differential phase contrast imaging using conventional x-ray tubes has spurred great interest
in the medical imaging community. It has been shown to provide higher contrast than absorption imaging in
some cases, and in this work we translate these advantages to tomosynthesis imaging. A general framework for
reconstruction of images from differential phase contrast projection data has been proposed and implemented
using data from a grating-based x-ray phase contrast tomosynthesis system. Reconstructed tomosynthesis images
from differential phase contrast data are shown, using both a direct backprojection (BP) technique and a filtered
backprojection (FBP) reconstruction method. From the results it is seen that phase contrast tomosynthesis can
separate superimposed phase objects while providing complementary information to absorption tomosynthesis.
X-ray phase sensitive imaging methods have seen tremendous growth and increased interest in recent years. Each
method has its advantages and disadvantages, but all have shown the ability to improve the detection of various
objects because of the additional phase measurements. Of the various methods, grating-based differential phase
contrast computed tomography (DPC-CT) imaging has shown greater quantitative and diagnostic capabilities
than traditional absorption CT. Although it has been shown that
DPC-CT provides superior contrast of certain
materials, one question has not been fully addressed to date is whether DPC-CT can provide improved accuracy
in detecting low contrast masses using the same radiation dose as that given in absorption CT. The detectability
is not only related to contrast to noise ratio, but also to the noise texture. The purpose of this study is to
investigate how the peculiar noise texture found in cone-beam DPC-CT affects low contrast objects' detectability
through human observer ROC analysis. Studies for both axial and sagittal planes were carried out, as both could
potentially be used in clinical practice for a 3D image. The results demonstrate that noise texture found in conebeam
DPC-CT strongly affects human visual perception, and that object detectabilities in axial and sagittal
images of DPC-CT are different.
The noise characteristics of x-ray differential phase contrast computed tomography (DPC-CT) were investigated.
Both theoretical derivation and experimental results demonstrated that the dependence of noise variance on spatial
resolution in DPC-CT follows an inverse linear law. This behavior distinguishes DPC-CT from conventional
absorption based x-ray CT, where the noise variance varies inversely with the cube of the spatial resolution.
This anomalous noise behavior in DPC-CT is due to the Hilbert filtering kernel used in the CT reconstruction
algorithm, which equally weights all spatial frequency content. Additionally, we demonstrate that the noise
power of DPC-CT is scaled by the inverse of spatial frequency and is highly concentrated at the low spatial
frequencies, whereas conventional absorption CT increases in power at the high spatial frequencies.
The effects of beam hardening have been an issue from the beginning of x-ray computed tomography. Polyenergetic
beams are attenuated more at lower energies, resulting in the so-called hardening of the beam. Beam
hardening artifacts in diagnostic CT images are a result of data inconsistency in the fundamental imaging equation
in conventional absorption CT. In theory, in phase contrast imaging, the fundamental imaging equation is
related only to a line integral of electron density, which is energy independent. However, due to unaccounted
absorption in the imaging equation for phase contrast, beam hardening artifacts will make their way into phase
contrast images. In this work, we use grating based differential phase contrast imaging, which uses a polyenergetic
source, and extracts phase information from a set of intensity images. The energy dependence in the imaging
equation for differential phase contrast imaging, coupled with the beam hardening present in the measured intensity
data, results in beam hardening artifacts in the reconstructed results. We demonstrate the magnitude of the
beam hardening effects in phase contrast reconstructions and compare it to standard absorption reconstructions.
Helical computed tomography revolutionized the field of x-ray computed tomography two decades ago. The simultaneous translation of an image object with a standard computed tomography acquisition allows for fast volumetric scan for long image objects. X-ray phase sensitive imaging methods have been studied over the past few decades to provide new contrast mechanisms for imaging an object. A Talbot-Lau grating interferometer based differential phase contrast imaging method has recently demonstrated its potential for implementation in clinical and industrial applications. In this work, the principles of helical computed tomography are extended to differential phase contrast imaging to produce volumetric reconstructions based on fan-beam data. The method demonstrates the potential for helical differential phase contrast CT to scan long objects with relatively small detector coverage in the axial direction.
Differential phase contrast imaging has recently been demonstrated using both synchrotron and conventional xray
sources with a grating interferometer. This approach offers the possibility of simultaneous CT reconstructions
of both absorption and index of refraction from a single acquisition. This enables direct comparison of both
types of reconstructed images under identical conditions. One of the most important performance metrics in
CT imaging is that of contrast-to-noise ratio. These results measure the contrast-to-noise ratio for a grating
interferometer-based differential phase contrast imaging system at a range of exposure levels and for several
materials. For three of the four cases measured, the
contrast-to-noise ratio of differential phase contrast CT
images was superior to that of absorption CT images. The most dramatic improvement was noted in the contrast between PMMA and water, where the contrast-to-noise ratio increased from less than 1 in absorption CT images, to approximately 8 in the differential phase contrast CT images. Additionally, a breast tissue specimen
containing a highly malignant carcinoma was scanned and reconstructed using both phase and absorption contrast
reconstructions to illustrate the superior performance of the phase contrast imaging method.
Differential phase contrast computed tomography (DPC-CT) is a novel X-ray imaging method that uses the
wave properties of imaging photons as the contrast mechanism. It has been demonstrated that differential phase
contrast images can be obtained using either synchrotron radiation or a conventional X-ray tube and a Talbot-
Lau-type interferometer. These data acquisition systems offer only a limited field of view and thus, are prone
to data truncation. In this work, we demonstrated that a small region of interest (ROI) of a large object can be
accurately and stably reconstructed using fully truncated projection datasets provided that a priori information
on electron density is known inside the ROI. The method reconstructs an image iteratively to satisfy a group
of physical conditions using a projection onto convex set (POCS) algorithm. This POCS algorithm is validated
using numerical simulations.
Compared to single energy CT, which provides information only about the x-ray linear attenuation coefficients,
dual energy CT is able to obtain the electron density and effective atomic number for different materials in
a quantitative way. In this study, as an alternative to dual energy CT, a novel quantitative imaging method
based on phase contrast CT is described. Rather than requiring two scans with different x-ray photon energies,
diffraction grating-based phase contrast CT is capable of reconstructing images of both the linear attenuation
and refractive index decrement from a single scan. From the two images, quantitative information of both the
electron density and effective atomic number can be extracted. Experimental results demonstrate that: (1)
electron density can be accurately determined from refractive index decrement through a linear relationship;
and (2) effective atomic number can be explicitly derived from the ratio of linear attenuation to refractive index
decrement, using a simple function, i.e., a power function plus a constant. The presented method will shed
insight into the field of material separation and find its use in medical and non-medical applications.
Dark-field x-ray projection imaging and dark-field neutron computed tomography have both recently been demonstrated. Such techniques provide insight into the small-angle scattering properties of the image objects. In this work, the dark-field x-ray imaging method is extended to x-ray computed tomography in order to provide unique
and complementary information to the previously reported phase contrast and absorption contrast CT images.
Dark-field reconstructions are presented and compared to these two other contrast mechanisms, all three of which
can be obtained from a single acquisition using an x-ray grating interferometer setup. Objects which provide
little absorption contrast, but have a significant small-angle scattering component, are better visualized in a dark-field CT reconstruction.
Recently, differential phase contrast computed tomography (DPC-CT) imaging methods have been successfully
implemented using either synchrotron or x-ray tube generated x-rays. As far as image reconstruction is concerned, FBPtype
reconstruction algorithms have been proposed. However, due to the intrinsic low photon efficiency of the system
and the sampling requirement of the FBP reconstruction algorithms, the x-ray exposure time is unacceptably long, on
the order of hours. In order to significantly shorten the data acquisition time, we proposed to acquire projection data at
significantly fewer view angles. This poses a challenge in image reconstruction. In this work, we aimed at using the
newly developed compressed sensing (CS) image reconstruction method to accurately reconstruct phase contrast CT
images from vastly under-sampled data. Experimental data were utilized to validate our new reconstruction method.
Our results demonstrate that the CS method enables acceptable image reconstruction in DPC-CT using significantly
fewer view angles when compared with the FBP image reconstruction method.
Purpose: To achieve three dimensional isotropic dynamic cardiac CT imaging with high temporal resolution for
evaluation of cardiac function with a slowly rotating C-arm system.
Method and Materials: A recently introduced extension to compressed sensing, viz. Prior Image Constrained
Compressed Sensing (PICCS), in which a prior image is used as a constraint in the reconstruction has enabled this
application. An in-vivo animal experiment (e.g. a beagle model) was conducted using an interventional C-arm system.
The imaging protocol was as follows: contrast was injected, the contrast equilibrated, breathing was suspended for ~14
seconds during which time 420 equally spaced projections were acquired. This data set was used to reconstruct a fully
sampled blurred image volume using the conventional FDK algorithm (e.g. the prior image). Then the data set was
retrospectively gated into 19 phases according to the recorded ECG signal (heart rate ~ 95bpm) and images were
reconstructed with the PICCS algorithm.
Results: Cardiac MR was used as the gold standard due to its high temporal resolution. The same short-axis slice was
selected from the PICCS-CT data set and the MR data set. Manual contouring on the peak systolic and peak diastolic
frames was performed to assess the ejection fraction contribution from this single plane. The calculated ejection
fractions with PICCS-CT agreed well with the MR results.
Conclusion: We have demonstrated the ability to use a slowly rotating interventional C-arm system in order to make
measurements of cardiac function. The new technique provides high isotropic spatial resolution (~0.5 mm) along with
high temporal resolution (~ 33 ms). The evaluation of cardiac function demonstrated a great agreement with single slice
cardiac MR.
The current x-ray source trajectory for C-arm based cone-beam CT is a single arc. Reconstruction
from data acquired with this trajectory yields cone-beam artifacts for regions other than the central slice. In
this work we present the preliminary evaluation of reconstruction from a source trajectory of two concentric arcs
using a flat-panel detector equipped C-arm gantry (GE Healthcare Innova 4100 system, Waukesha, Wisconsin).
The reconstruction method employed is a summation of FDK-type reconstructions from the two individual arcs.
For the angle between arcs studied here, 30°, this method offers a significant reduction in the visibility of cone-beam
artifacts, with the additional advantages of simplicity and ease of implementation due to the fact that it is
a direct extension of the reconstruction method currently implemented on commercial systems. Reconstructed
images from data acquired from the two arc trajectory are compared to those reconstructed from a single arc
trajectory and evaluated in terms of spatial resolution, low contrast resolution, noise, and artifact level.
In this paper, we present shift-invariant filtered backprojection (FBP) cone-beam image reconstruction algorithms
for a cone-beam CT system based on a clinical C-arm gantry. The source trajectory consists of two
concentric arcs which is complete in the sense that the Tuy data sufficiency condition is satisfied. This scanning
geometry is referred to here as a CC geometry (each arc is shaped like the letter "C"). The challenge for image
reconstruction for the CC geometry is that the image volume is not well populated by the familiar doubly
measured (DM) lines. Thus, the well-known DM-line based image reconstruction schemes are not appropriate
for the CC geometry. Our starting point is a general reconstruction formula developed by Pack and Noo which
is not dependent on the existence of DM-lines. For a specific scanning geometry, the filtering lines must be
carefully selected to satisfy the Pack-Noo condition for mathematically exact reconstruction. The new points
in this paper are summarized here. (1) A mathematically exact cone-beam reconstruction algorithm was formulated
for the CC geometry by utilizing the Pack-Noo image reconstruction scheme. One drawback of the
developed exact algorithm is that it does not solve the long-object problem. (2) We developed an approximate
image reconstruction algorithm by deforming the filtering lines so that the long object problem is solved while
the reconstruction accuracy is maintained. (3) In addition to numerical phantom experiments to validate the
developed image reconstruction algorithms, we also validate our algorithms using physical phantom experiments
on a clinical C-arm system.
In this study we develop a novel ECG-gated method of HYPR (HighlY constrained backPRojection) CT reconstruction for low-dose myocardial perfusion imaging and present its first application in a porcine model. HYPR is a method of reconstructing time-resolved images from view-undersampled projection data. Scanning and reconstruction techniques were explored using x-ray projections from a 50 sec contrast-enhanced axial scan of a 47 kg swine on a 64-slice MDCT system. Scans were generated with view undersampling factors from 2 to 10. A HYPR reconstruction algorithm was developed in which a fully-sampled composite image is generated from views collected from multiple cardiac cycles within a diastolic window. A time frame image for a heartbeat was produced by modifying the composite with projections from the cycle of interest. Heart rate variations were handled by automatically selecting cardiac window size and number of cycles per composite within defined limits. Cardiac window size averaged 35% of the R-R interval for 2x undersampling and increased to 64% R-R using 10x undersampling. The selected window size and cycles per composite was sensitive to synchrony between heart rate, gantry rate, and the view undersampling pattern. Temporal dynamics and perfusion metrics measured in conventional short-scan (FBP) images were well-reproduced in the undersampled HYPR time series. Mean transit times determined from HYPR myocardial time-density curves agreed to within 8% with the FBP results. The results indicate potential for an order of magnitude reduction in dose requirement per image in cardiac perfusion CT via undersampled scanning and ECG-gated HYPR reconstruction.
There exists a strong desire for a platform in which researchers may investigate planar tomosynthesis (i.e. all
source positions reside in a single plane that is parallel to the reconstructed image planes) trajectories directly
on an interventional C-arm system. In this work we describe an experimental system designed to accomplish this
aim, as well as the potential of this system for testing multiple aspects of the tomosynthetic image acquisition
process. The system enables one to evaluate the effect of the physical imaging parameters on the image quality,
as well as the effect of the reconstruction algorithm utilized. The experimental data collection for this work is
from the Innova 4100 (Flat-panel based interventional C-arm system manufactured by GE Healthcare). The
system is calibrated using a phantom with known geometrical placement of multiple small metallic spheres.
Initial performance was assessed with three physical phantoms and performance was assessed by varying: the
reconstruction algorithm (backprojection, filtered backprojection), the half tomographic angle (15°, 25°, 35°),
and the angular sampling (20,40,80 views / acquisition). Initial results demonstrate the ability to well differentiate
simulated vessels separated by 1 cm, even with the modest half tomographic angle of 15° and modest sampling
of 20 views/acquisition.
The first results from an interventional C-arm based computed tomography system where a complete source trajectory was used are presented. A scan with two arcs which are joined approximately at the center of their paths (CC trajectory) is utilized here. This trajectory satisfies Tuy's sufficiency condition for a large volume, but is not well
populated with PI-lines. Therefore, a non-PI-line based reconstruction
method is required. The desire for high dose efficiency led to the selection of an equal weighting based method. An FBP type reconstruction algorithm which was derived for two orthogonal concentric circles was utilized for reconstruction.
The concept of a virtual image object was used to relate the projections from the two acquired non-orthogonal arcs to projections of a virtual object from two orthogonal arcs. Geometrical calibration is vital when performing tomography from an interventional system, and was incorporated here with the use of a homogeneous virtual projection matrix. The results demonstrate a significant reduction in cone-beam artifacts when the complete source trajectory is utilized.
X-ray cone-beam computed tomography (CBCT) is of importance in image-guided intervention (IGI) and image-guided radiation therapy (IGRT). In this paper, we present a cone-beam CT data acquisition system using a GE INNOVA 4100 (GE Healthcare Technologies, Waukesha, Wisconsin) clinical system. This new cone-beam data acquisition mode was developed for research purposes without interfering with any clinical function of the system. It provides us a basic imaging pipeline for more advanced cone-beam data acquisition methods. It also provides us a platform to study and overcome the limiting factors such as cone-beam artifacts and limiting low contrast resolution in current C-arm based cone-beam CT systems. A geometrical calibration method was developed to experimentally determine parameters of the scanning geometry to correct the image reconstruction for geometric non-idealities. Extensive phantom studies and some small animal studies have been conducted to evaluate the performance of our cone-beam CT data acquisition system.
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