Faouzi Kallel

Bio: Faouzi Kallel is an academic researcher from University of Texas at Austin. The author has contributed to research in topics: Elastography & Elastic modulus. The author has an hindex of 21, co-authored 35 publications receiving 3236 citations. Previous affiliations of Faouzi Kallel include University of Houston & École Polytechnique de Montréal.

Elastography: ultrasonic estimation and imaging of the elastic properties of tissues.

Abstract: The basic principles of using sonographic techniques for imaging the elastic properties of tissues are described, with particular emphasis on elastography. After some preliminaries that describe some basic tissue stiffness measurements and some contrast transfer limitations of strain images are presented, four types of elastograms are described, which include axial strain, lateral strain, modulus and Poisson's ratio elastograms. The strain filter formalism and its utility in understanding the noise performance of the elastographic process is then given, as well as its use for various image improvements. After discussing some main classes of elastographic artefacts, the paper concludes with recent results of tissue elastography in vitro and in vivo.

A least-squares strain estimator for elastography

Abstract: A least-squares strain estimator (LSQSE) for elastography is proposed. It is shown that with such an estimator, the signal-to-noise ratio in an elastogram (SNRe) is significantly improved. This improvement is illustrated theoretically using a modified strain filter and experimentally using a homogeneous gel phantom. It is demonstrated that the LSQSE results in an increase of the elastographic sensitivity (smallest, strain that could be detected), thereby increasing the strain dynamic range. Using simulated data, it is shown that a tradeoff exists between the improvement in SNRe and the reduction of strain contrast and spatial resolution.

Elastography: Imaging the elastic properties of soft tissues with ultrasound

Abstract: Elastography is a method that can ultimately generate several new kinds of images, called elastograms. As such, all the properties of elastograms are different from the familiar properties of sonograms. While sonograms convey information related to the local acoustic backscatter energy from tissue components, elastograms relate to its local strains, Young's moduli or Poisson's ratios. In general, these elasticity parameters are not directly correlated with sonographic parameters, i.e. elastography conveys new information about internal tissue structure and behavior under load that is not otherwise obtainable. In this paper we summarize our work in the field of elastography over the past decade. We present some relevant background material from the field of biomechanics. We then discuss the basic principles and limitations that are involved in the production of elastograms of biological tissues. Results from biological tissues in vitro and in vivo are shown to demonstrate this point. We conclude with some observations regarding the potential of elastography for medical diagnosis.

Elastographic characterization of HIFU-induced lesions in canine livers.

Abstract: The elastographic visualization and evaluation of high-intensity focused ultrasound (HIFU)-induced lesions were investigated. The lesions were induced in vitro in freshly excised canine livers. The use of different treatment intensity levels and exposure times resulted in lesions of different sizes. Each lesion was clearly depicted by the corresponding elastogram as being an area harder than the background. The strain contrast of the lesion/background was found to be dependent on the level of energy deposition. A lesion/background strain contrast between 22.5 dB and 23.5 dB was found to completely define the entire zone of tissue damage. The area of tissue damage was automatically estimated from the elastograms by evaluating the number of pixels enclosed inside the isointensity contour lines corresponding to a strain contrast of 22.5, 23 and 23.5 dB. The area of the lesion was measured from a tissue photograph obtained at approximately the same plane where elastographic data were collected. The estimated lesion areas ranged between approximately 10 mm 2 and 110 mm 2 . A high correlation between the damaged areas as depicted by the elastograms and the corresponding areas as measured from the gross pathology photographs was found (r 2 5 0.93, p value < 0.0004, n 5 16). This statistically significant high correlation demonstrates that elastography has the potential to become a reliable and accurate modality for HIFU therapy monitoring. © 1999 World Federation for Ultrasound in Medicine & Biology.

The Future of Biomedical UltrasoundElastographic imaging

Abstract: The mechanical attributes of soft tissues depend on their molecular building blocks (fat, collagen etc.), on the microscopic and macroscopic structural organization of these blocks (Fung 1981), and on the boundary conditions involved. These mechanical attributes may include the shear or elastic moduli (Young’s modulus), the Poisson’s ratio, or any of the longitudinal or shear strains that occur in tissues as a response to an applied load. In the normal breast, for example, the glandular structure may be firmer than the surrounding fibrous connective tissue, which in turn is firmer than the subcutaneous adipose tissue. Pathological changes are generally correlated with changes in tissue stiffness as well. Many cancers, such as scirrhous carcinomas of the breast, seem much stiffer and less mobile than benign (fibroadenoma) tumors (Anderson 1953). In many cases, in spite of the difference in stiffness or mobility, the small size of a pathological lesion and/or its location deep in the body impede its detection and/or evaluation by palpation. Moreover, lesions may or may not possess echogenic attributes that make them ultrasonically detectable. Because the echogenicity and the mechanical attributes of tissue are generally uncorrelated, it is expected that imaging some of the latter will provide new information that is related to tissue structure and/or pathology. For example, tumors of the prostate or the breast may be invisible or barely visible in standard ultrasound examinations, yet are much stiffer than the embedding tissue. Diffuse diseases such as cirrhosis of the liver are known to increase the stiffness of the liver tissue significantly (Anderson 1953), yet they may seem normal in conventional ultrasound examination. A clear understanding of tissue stress/strain relationships is necessary for the interpretation of any of these imaged mechanical attributes. Tissue may exhibit viscoelastic and poroelastic behavior such as hysteresis, fluid flow, stress relaxation and creep (Fung 1981). When all these factors are combined, it is evident that describing the mechanical behavior of tissue mathematically requires considerable simplification, if the model is to be useful in a real-time or nearly real-time situation. To a first approximation, most soft tissues have been assumed to be isotropic on the scale of interest (Krouskop et al. 1987; Parker et al. 1990; Sarvazyan et al. 1991), although there is evidence of anisotropic ultrasonic and mechanical attributes in some soft tissues such as muscle (Levinson 1987). Even for relatively small strains (less than 10%), tissue may exhibit nonlinear viscoelastic behavior (Krouskop et al. 1998; Parker et al. 1990). Thus, the mechanical attributes of tissue are often better defined if they are specified over the strain or stress ranges of interest in a specific application (Krouskop et al. 1987; Levinson 1987; Parker et al. 1990). Quantitative measurements of tissue mechanical parameters reported in the past show a wide range of values (Fung 1981; Parker et al. 1990). Most of the research has been done for tissues that undergo tensile loading (muscles, arteries, lung, tendons, bone, skin, ureter). In contrast, very little quantitative information has been collected on the compressive attributes of the tissue in organs. A limited set ofin vitro measurements of the elastic moduli of prostate and liver tissues was described by Parker et al. (1990). In a presentation by Sarvazyan (1993), shear modulus measurements indicated that normal breast tissue is approximately 4 times less stiff than fibroadenoma. Breast cancer showed a wide range of shear moduli that can be up to 7 times higher than those of normal tissue. Walz et al. (1993) presented results of Address correspondence to: Dr. Jonathan Ophir, Ultrasonics Laboratory, Radiology Department, The University of Texas, Medical School, 6431 Fannin Street, Suite 6.168, Houston, TX 77030, USA. E-mail: jonathan.ophir@uth.tmc.edu. Ultrasound in Med. & Biol., Vol. 26, Supplement 1, pp. S23–S29, 2000 Copyright © 2000 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/00/$–see front matter

Elastic Moduli of Breast and Prostate Tissues under Compression

Abstract: To evaluate the dynamic range of tissue imaged by elastography, the mechanical behavior of breast and prostate tissue samples subject to compression loading has been investigated. A model for the loading was validated and used to guide the experimental design for data collection. The model allowed the use of small samples that could be considered homogeneous; this assumption was confirmed by histological analysis. The samples were tested at three strain rates to evaluate the viscoelastic nature of the material and determine the validity of modeling the tissue as an elastic material for the strain rates of interest. For loading frequencies above 1 Hz, the storage modulus accounted for over 93 percent of the complex modulus. The data show that breast fat tissue has a constant modulus over the strain range tested while the other tissues have a modulus that is dependent on the strain level. The fibrous tissue samples from the breast were found to be 1 to 2 orders of magnitude stiffer than fat tissue. Normal glandular breast tissue was found to have an elastic modulus similar to that of fat at low strain levels, but the modulus of the glandular tissue increased by an order of magnitude above fat at high strain levels. Carcinomas from the breast were stiffer than the other tissues at the higher strain level; intraductal in situ carcinomas were like fat at the low strain level and much stiffer than glandular tissue at the high strain level. Infiltrating ductal carcinomas were much stiffer than any of the other breast tissues. Normal prostate tissue has a modulus that is lower than the modulus of the prostate cancers tested. Tissue from prostate with benign prostatic hyperplasia (BPH) had modulus values significantly lower than normal tissue. There was a constant but not significant difference in the modulus of tissues taken from the anterior and posterior portions of the gland.

High-intensity focused ultrasound in the treatment of solid tumours.

Abstract: Traditionally, surgery has been the only cure for many solid tumours. Technological advances have catalysed a shift from open surgery towards less invasive techniques. Laparoscopic surgery and minimally invasive techniques continue to evolve, but for decades high-intensity focused ultrasound has promised to deliver the ultimate objective - truly non-invasive tumour ablation. Only now, however, with recent improvements in imaging, has this objective finally emerged as a real clinical possibility.

EFSUMB Guidelines and Recommendations on the Clinical Use of Ultrasound Elastography. Part 1: Basic Principles and Technology

Abstract: The technical part of these Guidelines and Recommendations, produced under the auspices of EFSUMB, provides an introduction to the physical principles and technology on which all forms of current commercially available ultrasound elastography are based. A difference in shear modulus is the common underlying physical mechanism that provides tissue contrast in all elastograms. The relationship between the alternative technologies is considered in terms of the method used to take advantage of this. The practical advantages and disadvantages associated with each of the techniques are described, and guidance is provided on optimisation of scanning technique, image display, image interpretation and some of the known image artefacts.

Elastography: ultrasonic estimation and imaging of the elastic properties of tissues.

Abstract: The basic principles of using sonographic techniques for imaging the elastic properties of tissues are described, with particular emphasis on elastography. After some preliminaries that describe some basic tissue stiffness measurements and some contrast transfer limitations of strain images are presented, four types of elastograms are described, which include axial strain, lateral strain, modulus and Poisson's ratio elastograms. The strain filter formalism and its utility in understanding the noise performance of the elastographic process is then given, as well as its use for various image improvements. After discussing some main classes of elastographic artefacts, the paper concludes with recent results of tissue elastography in vitro and in vivo.

On the feasibility of remote palpation using acoustic radiation force.

Abstract: A method of acoustic remote palpation, capable of imaging local variations in the mechanical properties of tissue, is under investigation. In this method, focused ultrasound is used to apply localized (on the order of 2 mm3) radiation force within tissue. and the resulting tissue displacements are mapped using ultrasonic correlation based methods. The tissue displacements are inversely proportional to the stiffness of the tissue, and thus a stiffer region of tissue exhibits smaller displacements than a more compliant region. In this paper, the feasibility of remote palpation is demonstrated experimentally using breast tissue phantoms with spherical lesion inclusions, and in vitro liver samples. A single diagnostic transducer and modified ultrasonic imaging system are used to perform remote palpation. The displacement images are directly correlated to local variations in tissue stiffness with higher contrast than the corresponding B-mode images. Relationships between acoustic beam parameters, lesion characteristics and radiation force induced tissue displacement patterns are investigated and discussed. The results show promise for the clinical implementation of remote palpation.