KEYWORDS: Lung, Monte Carlo methods, Image quality, Image resolution, Phase modulation, Computed tomography, Tissues, 3D modeling, Natural surfaces, Breast
Using numerical simulations, the influence of various imaging parameters on the resulting image can be determined for various imaging technologies. To achieve this, visualization of fine tissue structures needed to evaluate the image quality with different radiation quality and dose is essential. The present work examines a method that employs simulations of the imaging process using Monte Carlo methods and a combination of a standard and higher resolution voxel models. A hybrid model, based on nonlinear uniform rational B-spline and polygon mesh surfaces, was constructed from an existing voxel model of a female patient of a resolution in the range of millimeters. The resolution of the hybrid model was 500 μm, i.e., substantially finer than that of the original model. Furthermore, a high resolution lung voxel model [(0.11 mm)3 voxel volume, slice thickness: 114 μm] was developed from the specimen of a left lung lobe. This has been inserted into the hybrid model, substituting its left lung lobe and resulting in a dual-lattice geometry model. “Dual lattice” means, in this context, the combination of voxel models with different resolutions. Monte Carlo simulations of radiographic imaging were performed and the fine structure of the lung was easily recognizable.
Imaging x-ray fluorescence generally generates a conflict between the best image quality or highest sensitivity and lowest possible radiation dose. Consequently many experimental studies investigating the feasibility of this molecular imaging method, deal with either monochromatic x-ray sources that are not practical in clinical environment or accept high x-ray doses in order to maintain the advantage of high sensitivity and producing high quality images. In this work we present a x-ray fluorescence imaging setup using a HOPG crystal fan construction consisting of a Bragg reflecting analyzer array together with a scatter reducing radial collimator. This method allows for the use of polychromatic x-ray tubes that are in general easily accessible in contrast to monochromatic x-ray sources such as synchrotron facilities. Moreover this energy-selecting device minimizes the amount of Compton scattered photons while simultaneously increasing the fluorescence signal yield, thus significantly reducing the signal to noise ratio. The aim is to show the feasibility of this approach by measuring the Bragg reflected Kα fluorescence signal of an object containing an iodine solution using a large area detector with moderate energy resolution. Contemplating the anisotropic energy distribution of background scattered x-rays we compare the detection sensitivity, applying two different detector angular configurations. Our results show that even for large area detectors with limited energy resolution, iodine concentrations of 0.12 % can be detected. However, the potentially large scan times and therefore high radiation dose need to be decreased in further investigations.
Due to the increasing number of cone-beam CT (CBCT) devices on the market, reliable estimates of patient doses for these
imaging modality is desired. Cone-beam CT devices differ from conventional CT not only by a larger collimation but also
by different recording modes. In this work, it has been investigated whether reliable patient doses can be obtained for CBCT
devices in partial-fan mode using pre-computed slices. As an exemplary case, chest CBCT scans for the ICRP reference
adult models has been examined. By normalizing organ doses to CTDI100w , the resulting dose conversion coefficients
for CBCT could be well reproduced by precomputed slices, with a relative difference in the effective dose conversion
coefficients of less than 10%.
In radiography there is generally a conflict between the best image quality and the lowest possible patient dose. A proven
method of dosimetry is the simulation of radiation transport in virtual human models (i.e. phantoms). However, while the
resolution of these voxel models is adequate for most dosimetric purposes, they cannot provide the required organ fine
structures necessary for the assessment of the imaging quality.
The aim of this work is to develop hybrid/dual-lattice voxel models (called also phantoms) as well as simulation methods
by which patient dose and image quality for typical radiographic procedures can be determined. The results will provide
a basis to investigate by means of simulations the relationships between patient dose and image quality for various
imaging parameters and develop methods for their optimization.
A hybrid model, based on NURBS (Non Linear Uniform Rational B-Spline) and PM (Polygon Mesh) surfaces, was
constructed from an existing voxel model of a female patient. The organs of the hybrid model can be then scaled and
deformed in a non-uniform way i.e. organ by organ; they can be, thus, adapted to patient characteristics without losing
their anatomical realism. Furthermore, the left lobe of the lung was substituted by a high resolution lung voxel model,
resulting in a dual-lattice geometry model. “Dual lattice” means in this context the combination of voxel models with
different resolution.
Monte Carlo simulations of radiographic imaging were performed with the code EGS4nrc, modified such as to perform
dual lattice transport. Results are presented for a thorax examination.
KEYWORDS: Gold, Monte Carlo methods, Sensors, Computed tomography, Signal to noise ratio, Photons, Compton scattering, Tissues, X-ray computed tomography, Signal attenuation
Newly developed spectral CTs with a photon-counting and energy-selective detector provide the possibility to obtain additional information about an object’s absorption properties, the footprint of which can be found in the energy spectrum of the detected photons. These new CT systems are capable of yielding valuable insight into the elemental composition of the tissue and open up the way for new CT contrast agents by detecting element-specific K-edge patterns. Gold could be a promising new CT contrast agent. The major goal of this study is to determine the minimum amount of gold that is needed to use it as a spectral CT contrast agent for medical imaging in humans. To reach this goal, Monte Carlo simulations with EGSnrc were performed.
The energy-selective detector, on which this study is based, has 6 energy bins and the energy thresholds can be selected freely. First different energy thresholds were analyzed to determine the best energy thresholds with respect to detecting gold. The K-edge imaging algorithm was then applied to the simulation results with these energy bins. The reconstructed images were evaluated with respect to signal-to-noise ratio, contrast-to-noise ratio and contrast.
The K-edge imaging algorithm is able to convert the information in the six energy bins into three images, which correspond to the photoelectric effect, Compton scattering and gold content; however, it requires very long computing time. The simulations indicate that at least 0.2w% of gold are required to use it as a CT contrast agent in humans.
Gold nanoparticles (GNPs) were demonstrated as X-ray imaging contrast agents and radiosensitizers in mice. However, the translational medical applications of GNPs in to the clinical practice need further detailed information on the biological effects related to the enhanced doses in malignant and healthy cells. The idea of improving radiotherapy with high atomic number materials, especially gold foils, was initiated in our research unit in the 1980s. Recently, experimental and theoretical efforts were made to investigate the potential improvement of imaging and radiotherapy with GNPs. Initially, the present work attempts to validate the dose enhancement effects of GNPs to cancer cells; secondly, it intends to examine the possible side effects on healthy cells when using GNPs as X-ray contrast agent. In this study, three Monte Carlo simulation programs, namely PENELOPE-2011, GEANT4 and EGSnrc were used to simulate the local energy deposition and the resulting dose enhancement of GNPs. Diameters of the GNPs were assumed to be 2 nm, 15 nm, 50 nm, 100 nm and 200 nm. The X-ray energy spectra for irradiation were 60 kVp, 80 kVp, 100 kVp, 150 kVp with a filtering of 2.7 mm Al for projectional radiography, and 8 mm Al for 100 kVp and 150 kVp for computed tomography. Additional peak energy of 200 kVp was simulated for radiotherapy purpose. The information of energy deposition and dose enhancement can help understanding the physical processes of medical imaging and the implication of nanoparticles in radiotherapy.
The performance of a novel designed x-ray CT scanning geometry is investigated. Composed of a specially designed tungsten collimation mask and a high resolution flat panel detector, this scanning geometry provides high efficient data acquisition allowing dose reduction potentially up to 50%. In recent years a special type of scanning geometry has been proposed. A first prototype of this geometry called CTDOR( CT with Dual Optimal Reading) has already been built. Despite many drawbacks, resulting images have shown promising potential of dual reading. The approach of gaining two subsets of data has anew been picked up and come to terms with a novel designed CT scanner for breast imaging. The main idea consists of collimating the X-ray beam through a specially designed shielding mask thereby reducing radiation dose without compromising image quality. This is achieved by hexagonally sampled Radon transform and image reconstruction with the especially suitable OPED (orthogonal polynomial expansion on disk) algorithm. This work now presents the development and evaluation of the novel designed breast CT system. Therefore simulated phantom data were obtained to test the performance of the scanning device and compared to a standard 3rd generation scanner. Retaining advantages such as scatter-correction potential and 3D-capability, the proposed CT system yields high resolution images for breast diagnostics in low energy ranges. Assuming similar sample size, it is expected that the novel designed breast CT system in conjunction with OPED outperforms the standard 3rd generation CT system combined with FBP (filtered back projection).
A novel designed x-ray CT scanning geometry is proposed. Composed of a specially designed tungsten collimation mask
and a flat panel detector, which is placed inside the mask, this scanning geometry provides high efficient data acquisition
allowing dose reduction potential by a factor of two.
In recent years a first prototype of the CTDOR geometry (CT with Dual Optimal Reading) has been evaluated. It
consisted of a discontinuous ring of detectors fixated on X-Ray absorbing material. The source and an outer detector
were mounted on a gantry rotating around the inner static detector and the patient. Despite many drawbacks, resulting
images have shown promising potential of dual reading. Based on those results, the present work presents further
development and improvement of the recommended scanner geometry. The main idea consists of collimating the X-ray
beam through a specially designed shielding mask thereby reducing radiation dose and structuring data without
compromising image quality. An especially developed high precision laser-beam cutting process assures an accurate
mask crafting with tungsten shielding and window sizes of 300μm.
Additionally, simulation data were obtained with Monte Carlo calculations to test the dose reduction potential of the
scanning device. Retaining advantages of the CTDOR geometry such as 3D-capability, built-in capacity of scatter
correction and radiation structuring, a high-precision manufactured collimation mask of novel designed CT-scanner
enables high resolution images for breast-imaging in low energy ranges.
KEYWORDS: Sensors, Fluctuations and noise, Photons, Image quality, X-ray computed tomography, Computed tomography, Data acquisition, Reconstruction algorithms, Medical imaging, Lead
In order to decrease the patient's radiation exposure from Computed Tomography, the new CT geometry CTDOR has
been invented. It consists of two data sets: A conventional arc or flat panel detector and a mask ring with shieldings on
the outside and detectors on the inside separated by windows. Combined with the reconstruction algorithm OPED, it has
the theoretical potential to decrease the dose about 50% while providing the same image quality as conventional systems.
First steps to evaluate this theory were done with a mask ring demonstrator combined with a conventional C-arm device.
Although the quality of the demonstrator is limited, this set-up was supposed to demonstrate how the combination of the
two data sets works in principle. Preliminary results from earlier studies, however, provided images of rather poor quality.
This work presents better images obtained with an optimized data treatment. We showed that most artifacts are eliminated
and that we get sharper images with higher contrast compared to the images reconstructed from the single data sets and
compared to the earlier study. Regarding the limitations of the set-up, the resulting images were remarkably good. CTDOR
is therefore a promising method, which is worth to perform further studies.
Reliable estimates for patient doses for CT examinations are desirable for the patients themselves as well as for new
epidemiological studies. It has been shown that dose conversion coefficients normalized to CTDIvol provide rather scanner
independent quantities. In this work, it is demonstrated that this normalization provides also tube voltage independent
values by simulating axial CT scans of a seven-year old infant and an adult. The differences in the effective dose conversion
coefficients per CTDIvol between 80 and 120 kV is for most body heights below 5%. Only at the height of the testes and
the thyroid the difference can be as large as 15%. This results in differences of the effective dose conversion coefficient per
CTDIvol between 80 and 120 kV of less than 6-7% for typical CT examinations.
KEYWORDS: Medical imaging, Ionizing radiation, Computed tomography, Tissues, Data modeling, Medical diagnostics, Monte Carlo methods, Sensors, X-ray computed tomography, Particles
Radiation exposure due to medical imaging is a topic of emerging importance. In Europe this topic has been dealt with
for a long time and in other countries it is getting more and more important and it gets an aspect of public interest in the
latest years. This is mainly true due to the fact that the average dose per person in developed countries is increasing
rapidly since threedimensional imaging is getting more and more available and useful for diagnosis. This paper
introduces the most common dose quantities used in medical radiation exposure characterization, discusses usual ways
for determination of such quantities as well as some considerations how these values are linked to radiation risk
estimation. For this last aspect the paper will refer to the linear non threshold theory for an imaging application.
KEYWORDS: Mammography, Monte Carlo methods, X-rays, X-ray sources, Optical simulations, Image acquisition, Medical imaging, X-ray imaging, Physics, Current controlled current source
Since overcoming some of the inherent limitations of x-ray tubes becomes increasingly harder, it is important to consider new ways of x-ray generation and to study their applications in the field of medical imaging. In the present work we investigate a novel table-top-sized x-ray source, developed in a joint project within the Cluster of Excellence "Munich Center for Advanced Photonics". It uses laser-accelerated electrons emitting x-ray radiation in a short period undulator. This source has the potential to deliver tunable x-rays with a very narrow spectral bandwidth. The main purpose of this contribution is to investigate the performance of this source in the field of mammography and to compare it to that of conventional x-ray tubes. We simulated the whole imaging process from the electron beam dynamics through the generation of the synchrotron radiation in the undulator up to the x-ray-matter interaction and detection in the mammographic setting. A Monte Carlo simulation of the absorption and scattering processes based on the Geant4 software toolkit has been developed that uses a high-resolution voxel phantom of the female breast for the accurate simulation of mammography. We present simulated mammograms generated by using quasi-monochromatic undulator radiation and by using the polychromatic spectrum of a conventional x-ray tube.
KEYWORDS: Sensors, Data acquisition, Fluctuations and noise, Computed tomography, Reconstruction algorithms, Modulation transfer functions, Photons, Monte Carlo methods, Image quality, Collimation
The sampling geometry of CT-scanners plays an important role in the reconstruction of images. We have
previously reported a test-device that directly collects the Radon data within a special scanning geometry, whose
acquired data can be efficiently treated with series expansion algorithms such as, for example, OPED (Orthogonal
Polynomial Expansion on Disc). This geometry has the potential of reducing the radiation exposure of the patient
by about a factor of two. However, a fourth of the data must be obtained by interpolation within the measured
projections. In this contribution, we show by a Monte Carlo simulation that this interpolation has no significant
influence on the quality of the reconstructions.
For optimisation in diagnostic medical imaging it is important to consider the relation between diagnostic image quality
and patient dose. In the past, schematic representations of the human body were commonly used for dosimetric
simulations together with Monte Carlo codes. During the last two decades, voxel models were introduced as an
improvement to these body models. Studies performed by various research groups have shown that the more realistic
organ topology of voxel models constructed from medical image data of real persons has an impact on calculated doses
for external as well as internal exposures. As a consequence of these findings, the ICRP decided to use voxel models for
the forthcoming update of organ dose conversion coefficients. These voxel models should be representative of an
average population, i.e. they should resemble the ICRP reference anatomical data with respect to their external
dimensions and their organ masses. To meet the ICRP requirements, our group at the Helmholtz Zentrum München
(formerly known as GSF-National Research Center for Environment and Health) constructed voxel models of a male
and female adult, based on the voxel models of two individuals whose body height and weight resembled those of the
male and female ICRP reference adult. The organ masses of both models were adjusted to the ICRP reference anatomical
data, without spoiling their realistic anatomy. The paper describes the method used for this process and the resulting
voxel models.
The new scanner geometry CT D'OR ("CT with Dual Optimal Reading"), developed at the Helmholtz Zentrum München
(former GSF-National Research Center for Environment and Health), consists of a discontinuous ring of detectors facing
toward the ring center, which are fixated on an x-ray absorbing material. The x-ray source and an additional outer detector
are mounted on a gantry which rotates around the inner static detector and thus the patient. When the source is moving,
the detectors are alternately exposed and shielded from the source. Data recorded during periods of direct exposure can
be combined and are used for the reconstruction of the image. When the detectors are shielded, their signal is solely
caused by scatter. Therefore, direct scatter correction is possible. This can be used to considerably improve the image
quality, when scatter radiation yields a strong deterioration of the reconstructed image. The advantage of CT D'OR is thus,
that information about scatter radiation is obtained without additional effort or exposure. This property of CT D'OR is
investigated and its feasibility is demonstrated by the use of Monte Carlo simulations.
KEYWORDS: Sensors, Scattering, Signal detection, Photons, Monte Carlo methods, X-ray computed tomography, Signal attenuation, Data modeling, Image restoration, Image processing
An important step in assessing the quality of an image reconstruction algorithm is the simulation of the medical imaging process. For that purpose, the patient's anatomical structure is substituted in general by more or less simple geometrical objects, as, e.g., the Shepp-Logan phantom. Furthermore, the attenuation of the human body and thus the resulting detector image (e.g., the sinogram in CT) is often computed by integrating the attenuation coefficient along various rays without considering the contribution of scattered photons in the detector signal. We therefore decided to improve the simulation by using an existing Monte Carlo code (EGSnrc) to model the transport of numerous photons from the x-ray tube through the body to the detector. The deflection of photons and creation of secondary particles in scattering events occurs naturally in this program, but can also be avoided artificially. Besides the improved simulation of the irradiation process, this allows us to quantify the amount of scattered radiation in the detector image. The patient is represented by a so-called voxel phantom, which is based on tomographic image data of a real person, adopted to represent the ICRP Reference Man. Our improved modeling process is being applied to determine the amount of scatter radiation in helical multi-slice CT of the thorax compared to a planned circular CT with large flat panel detectors. The new reconstruction algorithm OPED (orthogonal polynomial expansion on disc), developed at GSF and the University of Oregon, might reduce the scatter radiation considerably.
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