Computed masks in coronary subtraction imaging

In this paper we propose a method to overcome the effects of cardiac and respiratory motion in coronary subtraction imaging. We present the ideas of retrospective gating of masks, where both cardiac and respiratory phases are measured for a set of masks, and are subsequently used in a functional decomposition of motion. Through retrospective gating of masks, we are able to select appropriate images and to perform temporal and spatial processing on them to produce computed masks, subtraction of which should lead to motion-artifact-free images. The computed masks are built from two components: the first includes the time-variant structures related to respiration, such as ribs and soft tissues of the chest wall, and the second incorporates the time-variant structures related to cardiac motion. A preliminary study of the method in the area of digital subtraction angiography produced images which are comparable to but not better than those produced by techniques in current clinical practice: we discuss the reasons for this.     

3例急性冠脉综合征患者CTA、冠脉造影及OCT对比

目的了解急性冠脉综合征患者冠脉CTA、冠脉造影、OCT的表现。方法急性冠脉综合征患者3例,在行冠脉造影前均行64排CT(Somatom Definition)冠脉CTA检查,明确罪犯病变;常规冠状动脉造影后行OCT检查(Lightlab),分别分析冠脉CT、冠脉造影和OCT影像学结果。结果冠脉CTA均提示为软斑块,斑块最低CT值均小于100Hu,1例伴有轻度钙化,斑块负荷较重,均呈正性重构;OCT示2例有极薄的纤维帽(分别为50μm和40μm),纤维帽后有大的脂核。结论冠脉CTA对于血管壁较宏观的观察较有优势,冠脉造影可以动态观察血管的情况,OCT可细致观察血管壁、斑块和血栓。     

An inverse method for imaging the local elasticity of atherosclerotic coronary plaques

The rupture of thin-cap fibroatheroma (TCFA) plaques is a major cause of acute coronary events. A TCFA has a trombogenic soft lipid core, shielded from the blood stream by a thin, possibly inflamed, stiff cap. The majority of atherosclerotic plaques resemble a TCFA in terms of overall structural composition, but have a more complex, heterogeneous morphology. An assessment of the material distribution is vital for quantifying the plaque's mechanical stability and for determining the effect of plaque-stabilizing pharmaceutical agents. We describe a new automated inverse elasticity method, intravascular ultrasound (IVUS) modulography, which is capable of reconstructing a heterogeneous Young's modulus distribution. The elastogram (i.e., spatial strain distribution) of the plaque is the input for the method, and is measured using the clinically available technique, IVUS elastography. Our method incorporates a novel divide-and-conquer strategy, allowing the reconstruction of TCFAs as well as heterogeneous plaques with localized regions of soft, weakened tissue. The method was applied to ex vivo elastograms, which were simulated from the cross sections of postmortem human coronary plaques. To demonstrate the clinical feasibility of the method, measured elastograms from human atherosclerotic coronary arteries were analyzed. One elastogram was measured in vitro; the other, in vivo. The method approximated the true Young's modulus distribution of all simulated plaques, while the in vitro reconstruction was in agreement with histology. In conclusion, the IVUS modulography in combination with the IVUS elastography has strong potential to become an all-encompassing modality for detecting plaques, for assessing the information related to their rupture-proneness, and for imaging their heterogeneous elastic material composition.     

Evaluation of the adaptive speckle suppression filter for coronary optical coherence tomography imaging

During the last few years, optical coherence tomography (OCT) has demonstrated considerable promise as a method of high-resolution intravascular imaging. The goal of this study was to apply and to test the applicability of the rotating kernel transformation (RKT) technique to the speckle reduction and enhancement of OCT images. The technique is locally adaptive. It is based on sequential application of directional masks and selection of the maximum of all outputs. This method enhances the image features by emphasizing thin edges while suppressing a noisy background. Qualitatively, the RKT algorithm provides noticeable improvement over the original image. All processed images are smoother and have better-defined borders of media, intima, and plaque. The quantitative evaluation of RKT performance showed that in terms of average contrast-to-noise ratio, there is a significant improvement in image quality between original and enhanced images. The RKT image enhancement technique shows great promise in improving OCT images for superior boundary identification.     

Improved tomographic reconstruction in seven-pinhole imaging

Cardiac emission tomography using a seven-pinhole collimator has received only little appreciation as a diagnostic imaging technique. The main reasons are the limited angular sampling of the seven-pinhole device and the difficulties encountered in properly positioning the patient relative to the collimator/camera system. In order to overcome these problems, we have developed a modified ART3 algorithm for reconstruction of the radioactivity distribution in the heart. The method is very appropriate for seven-pinhole tomography, as demonstrated by the quality of the reconstructions, by the excellent point source resolution of the system response, and by a comparison to two other suitable reconstruction techniques, viz., SMART and SIRT.     

Multidimensional tomographic imaging using volume holography

We propose the application of volume holography to four-dimensional (4-D) spatiospectral imaging. The proposed systems use materials and techniques developed for holographic data storage and interconnections to capture three-dimensional (3-D) spatial and one-dimensional (1-D) spectral information about a remote light source or scatterer. We analyze case studies of simple architectures using spherical-reference volume holograms as imaging elements in a fluorescence confocal microscope arrangement and demonstrate the equivalence of the holographic degeneracies with a slicing operation on the reconstructing incoherent source. We develop a general theoretical framework for the diffraction of random fields from volume holograms and show that the formulation can be used as an imaging design tool. Applications and future directions are also discussed     

Wavelet-based multiresolution local tomography

We develop an algorithm to reconstruct the wavelet coefficients of an image from the Radon transform data. The proposed method uses the properties of wavelets to localize the Radon transform and can be used to reconstruct a local region of the cross section of a body, using almost completely local data that significantly reduces the amount of exposure and computations in X-ray tomography. The property that distinguishes our algorithm from the previous algorithms is based on the observation that for some wavelet bases with sufficiently many vanishing moments, the ramp-filtered version of the scaling function as well as the wavelet function has extremely rapid decay. We show that the variance of the elements of the null-space is negligible in the locally reconstructed image. Also, we find an upper bound for the reconstruction error in terms of the amount of data used in the algorithm. To reconstruct a local region 16 pixels in radius in a 256x256 image, we require 22% of full exposure data.     

Computed basis functions for finite element analysis based on tomographic data

In bioelectromagnetics, the structures in which the electromagnetic field is to be computed are sometimes defined by a fine grid of voxels (3-D cells) whose tissue types are obtained by tomography. A novel finite element method is proposed for such cases. A simple, regular mesh of cube elements is constructed, each containing the same, integer number of voxels. There may be several different tissues present within an element, but this is accommodated by computing element basis functions that approximately respect the interface conditions between different tissues. Results are presented for a test model of 128 (3) voxels, consisting of nested dielectric cubes, driven by specified charges. The electrostatic potential computed with the new method agrees well with that of a conventional finite element code: the rms difference along the sample line is 1.5% of the highest voltage. Results are also presented for the potential due to a current dipole placed in a brain model of 181 × 217 × 181 voxels, derived from MRI data. The new method gives potentials that are different to those obtained by treating each voxel as an element by 1% of the peak voltage, yet the global finite element matrix has a dimension which is more than 50 times smaller.     

Medical imaging with a microwave tomographic scanner

A microwave tomographic scanner for biomedical applications is presented. It consists of a 64-element circular array with a useful diameter of 20 cm. Electronically scanning the transmitting and receiving antennas allows multiview measurements with no mechanical movement. Imaging parameters-a spatial resolution of 7 mm and a contrast resolution of 1% for a measurement time of 3 s-are appropriate for medical use. Measurements on tissue-simulating phantoms and volunteers, together with numerical simulations, are presented to assess the system for absolute imaging of tissue distribution and for differential imaging of physiological, pathological, and induced changes in tissues     

Real-time parallel tomographic ultrasound imaging using transputers

An evaluation is described of the application of a transputer array in the real-time parallel implementation of the acoustic backprojection algorithm, which forms the basis for tomographic imaging using backscattered ultrasound. The low-cost, real-time processing implementation described is thus potentially applicable to this form of imaging in a variety of fields: nondestructive testing, noninvasive medical diagnosis, and also in the imaging of industrial processes, such as complex fluid mixtures in pipes. An overview of the parallel processing method is given with sample results which indicate the viability of real-time processing     

A nonlinear estimation method in tomographic imaging

A nonlinear estimation approach to solving the inverse scattering problem, and reconstructing the space-varying complex permittivity of unknown objects is considered. The bilinear operator equations governing the scattering are approximated into finite dimensional spaces on the basis of the finite degrees of freedom of data, and on the simple concept that one cannot expect to reconstruct an arbitrary function from a finite number of independent equations. As a consequence, a discrete model, well suited to numerical inversion, is developed. The particular bilinear nature of the equations, and a suitable choice of contrast and field unknowns allows the functional adopted in the estimation to be minimized in an accurate and numerically efficient manner. Numerical experiments show how the method is capable, when a proper number of searched unknowns is adopted, to manage the possible convergence to local minima (which is a typical question in nonlinear inverse problems), and validate the effectiveness of the proposed approach     

Diffraction tomographic imaging in a monostatic measurementgeometry

Diffraction tomography (DT) is an imaging technique in which spatial variations in refractive index are reconstructed from measured data. The basis for DT is the generalized slice theorem (GST) relating a known function of the measured data to the spatially variable refractive index, subject to a weak scattering approximation. Forms of the GST have been developed for a number of measurement configurations based on bistatic geometries employing arrays of sources and receivers. The problem of imaging with scalar waves for a monostatic measurement geometry is considered. GSTs are derived for two dimensions employing several simplifying assumptions. The quality of the images and limitations of these simplifying assumptions are investigated for several two-dimensional algorithms using simulated data. It is found that one particular monostatic inversion formula yields good image quality and is not substantially limited by the necessary simplifying assumption     

Frequency-domain optical tomographic imaging of arthritic finger joints

We are presenting data from the largest clinical trial on optical tomographic imaging of finger joints to date. Overall we evaluated 99 fingers of patients affected by rheumatoid arthritis (RA) and 120 fingers from healthy volunteers. Using frequency-domain imaging techniques we show that sensitivities and specificities of 0.85 and higher can be achieved in detecting RA. This is accomplished by deriving multiple optical parameters from the optical tomographic images and combining them for the statistical analysis. Parameters derived from the scattering coefficient perform slightly better than absorption derived parameters. Furthermore we found that data obtained at 600 MHz leads to better classification results than data obtained at 0 or 300 MHz.     

Three-dimensional optical tomographic imaging of breast in a humansubject

We present for the first time a full three-dimensional (3-D) reconstruction of absorption images of breast from continuous-wave (cw) measurements performed on a premenopausal woman. Our 3-D optical images clearly reveal a large primary tumor as well as a small secondary tumor in a separate location of the breast. The multiple tumors identified by our 3-D optical imaging have been confirmed by the subsequent biopsy examination of the breast. Quantitative information of the optical images obtained is provided in terms of the location, size, and absorption coefficient of the tumors.     

Frequency swept tomographic imaging of three-dimensional perfectly conducting objects

The use of frequency swept or frequency diversity techniques to achieve, superresolution in the imaging of three-dimensional perfectly conducting objects is studied and demonstrated by computer simulations. The frequency swept imaging concept is found to be a generalization of the inverse scattering theory. By invoking Fourier domain projection theorems, it is demonstrated analytically that images of separate slices of three-dimensional targets can be obtained, thus establishing the feasibility of a tomographic radar. Computer simulation results that verify these theories for extended and composite point scattering objects are presented.     

Computed tomography for measuring annual rings of a live tree

A portable X-ray computed-tomography (CT) scanner has been developed for the nondestructive measurement of cross sections of live trees, wooden utility poles, wooden building columns, wooden statues, etc., in the field environment. Temporal resolution of CT reconstruction is good enough for observing details of annual rings of trees. The scanner shah be useful in plant physiology, environment assessment, safety assurance of poles, archeology, and dendrochronology.     

Discrete tomography in medical imaging

Discrete tomography (DT) deals with the reconstruction of a function from its projections, when the function has a known discrete range. The knowledge of the discrete range, possibly together with some prior information, can significantly reduce the number of projections required for a high-quality reconstruction. The reconstruction methods used in DT applications are usually based on some formulation as an optimization problem. This paper presents methods and results of DT based on problems of angiography, emission tomography, and electron microscopy (EM).     

Some theoretical aspects of a backscattering based tomographic imaging technique

In Mensa et al. (1983), a particular coherent tomographic technique was proposed to determine the scattering strength density function g inside a body by suitably processing the backscattered field. This method produces the Fourier transform of g on any preassigned number of spectral rings with radii proportional to the frequency values of the exploring radiation. For an experimental implementation of the method, the influence of many of the parameters involved must be studied. The aim of this paper is to evaluate the performance of the reconstruction method with respect to the choice of the exploring frequencies and to the number of explorations made. Finally, the resolution capability of the method was studied for two point scatterers in a homogeneous environment. Particular reference was made to microwave exploration but the criteria derived may also directly be applied to ultrasound based methods.     

Estimation of the local statistical noise in emission computed tomography

A simple modification of the filtered backprojection algorithm is presented for the computation of the local statistical noise in emission computed tomography. The technique is general in that any distribution of radioactivity may be accommodated. When applied to positron emission tomography, it is shown that the effects of photon absorption, random coincidences, radioactive decay, and detector nonuniformity may be included. Calculations have shown the effects of resolution, object size, and photon absorption on the statistical noise of disk-shaped emitters. Comparison of calculation and experiment show close agreement both in magnitude and spatial variation. Measurements of the noise level in tomograms of the brain obtained during continuous inhalation of 150-CO2 demonstrate that estimates of radioactivity concentration with a precision of a few percent are readily attainable.     

Fast 3-d tomographic microwave imaging for breast cancer detection

Microwave breast imaging (using electromagnetic waves of frequencies around 1 GHz) has mostly remained at the research level for the past decade, gaining little clinical acceptance. The major hurdles limiting patient use are both at the hardware level (challenges in collecting accurate and noncorrupted data) and software level (often plagued by unrealistic reconstruction times in the tens of hours). In this paper we report improvements that address both issues. First, the hardware is able to measure signals down to levels compatible with sub-centimeter image resolution while keeping an exam time under 2 min. Second, the software overcomes the enormous time burden and produces similarly accurate images in less than 20 min. The combination of the new hardware and software allows us to produce and report here the first clinical 3-D microwave tomographic images of the breast. Two clinical examples are selected out of 400+ exams conducted at the Dartmouth Hitchcock Medical Center (Lebanon, NH). The first example demonstrates the potential usefulness of our system for breast cancer screening while the second example focuses on therapy monitoring.