Non-invasive quantitative reconstruction of tissue elasticity using an iterative forward approach.

A novel iterative approach is presented to estimate Young's modulus in homogeneous soft tissues using vibration sonoelastography. A low-frequency (below 100 Hz) external vibration is applied and three or more consecutive frames of B-scan image data are recorded. The internal vibrational motion of the soft tissue structures is calculated from 2D displacements between pairs of consecutive frames, which are estimated using a mesh-based speckle tracking method. An iterative forward finite element approach has been developed to reconstruct Young's modulus from the measured vibrational motion. This is accomplished by subdividing the 2D image domain into sample blocks in which Young's modulus is assumed to be constant. Because the finite element equations are internally consistent, boundary values other than displacement are not required. The sensitivity of the results to Poisson's ratio and the damping coefficient (viscosity) is investigated. The approach is verified using simulated displacement data and using data from tissue-mimicking phantoms.

Multilevel and motion model-based ultrasonic speckle tracking algorithms.

A multilevel motion model-based approach to ultrasonic speckle tracking has been developed that addresses the inherent trade-offs associated with traditional single-level block matching (SLBM) methods. The multilevel block matching (MLBM) algorithm uses variable matching block and search window sizes in a coarse-to-fine scheme, preserving the relative immunity to noise associated with the use of a large matching block while preserving the motion field detail associated with the use of a small matching block. To decrease further the sensitivity of the multilevel approach to noise, speckle decorrelation and false matches, a smooth motion model-based block matching (SMBM) algorithm has been implemented that takes into account the spatial inertia of soft tissue elements. The new algorithms were compared to SLBM through a series of experiments involving manual translation of soft tissue phantoms, motion field computer simulations of rotation, compression and shear deformation, and an experiment involving contraction of human forearm muscles. Measures of tracking accuracy included mean squared tracking error, peak signal-to-noise ratio (PSNR) and blinded observations of optical flow. Measures of tracking efficiency included the number of sum squared difference calculations and the computation time. In the phantom translation experiments, the SMBM algorithm successfully matched the accuracy of SLBM using both large and small matching blocks while significantly reducing the number of computations and computation time when a large matching block was used. For the computer simulations, SMBM yielded better tracking accuracies and spatial resolution when compared with SLBM using a large matching block. For the muscle experiment, SMBM outperformed SLBM both in terms of PSNR and observations of optical flow. We believe that the smooth motion model-based MLBM approach represents a meaningful development in ultrasonic soft tissue motion measurement.

Feature-adaptive motion tracking of ultrasound image sequences using a deformable mesh.

By exploiting the correlation of ultrasound speckle patterns that result from scattering by underlying tissue elements, two-dimensional tissue motion theoretically can be recovered by tracking the apparent movement of the associated speckle patterns. Speckle tracking, however, is an ill-posed inverse problem because of temporal decorrelation of the speckle patterns and the inherent low signal-to-noise ratio of medical ultrasonic images. This paper investigates the use of an adaptive deformable mesh for nonrigid tissue motion recovery from ultrasound images. The nodes connecting the mesh elements are allocated adaptively to stable speckle patterns that are less susceptible to temporal decorrelation. We use the approach of finite element analysis in manipulating the irregular mesh elements. A novel deformable block matching algorithm, making use of a Lagrange element for higher-order description of local motion, is proposed to estimate a nonrigid motion vector at each node. In order to ensure that the motion estimates are admissible to a physically plausible solution, the nodal displacements are regularized by minimizing the strain energy associated with the mesh deformations. Experiments based on ultrasound images of a tissue-mimicking phantom and a muscle undergoing contraction, and on computer simulations, have shown that the proposed algorithm can successfully track nonrigid displacement fields.

Vibration sonoelastography and the detectability of lesions.

Vibration sonoelastography has been developed for the detection of hard lesions in relatively soft tissue. The basic concept is to propagate low-amplitude and low-frequency shear waves (with displacements below 0.1 mm and frequencies typically below 1000 Hz) through deep organs, and displaying the vibration response in real-time using advanced color Doppler imaging techniques. A hard inhomogeneity, such as a tumor, will produce a localized disturbance in the vibration pattern, forming the basis for detection even when the tumor is isoechoic on B-scan images. This paper focuses on the important quantitative issues concerning the detectability or inherent contrast of lesions. The specific factors of lesion size, relative stiffness and vibration frequency are studied using theoretical models, finite element methods and experimental measurements on tissue-mimicking materials. The results indicate that detectability increases with vibration (shear wave) frequency; however, loss mechanisms ultimately limit the penetration of higher vibration frequencies (in the kHz range).

Focused review: physiatric management of HIV-related disability.

Since 1981, the epidemic of acquired immunodeficiency syndrome (AIDS), due to infection with human immunodeficiency virus (HIV), has grown and challenged health care providers in the United States. Although the total number of AIDS cases diagnosed each year appears to be reaching a plateau, cases attributable to heterosexual transmission are rising. With improved treatment, survival of persons with HIV infection is expected to increase; this change suggests that the prevalence of HIV-related physical disability will also increase. This article outlines the pathophysiology and systemic manifestations of HIV infection. The more common neuromuscular and neurological complications at each stage of the disease are presented, and appropriate rehabilitation interventions are discussed. The indications for aerobic exercise in persons with HIV infection and the approach to disability management in pediatric HIV infection are presented. Psychosocial considerations related to access to rehabilitations services, discrimination on the basis of HIV infection or membership in HIV risk behavior groups, and vocational rehabilitation are discussed. The review concludes that rehabilitation interventions in persons with HIV infection are based on functional deficits, rather than disease processes, suggesting that a general physiatric fund of knowledge should be adequate to manage most HIV disability.

Imaging of the elastic properties of tissue--a review.

Recently, a number of methods have been developed that make it possible to image the elastic properties of soft tissues. Because certain types of tissues such as malignant lesions, for example, have elastic properties that are markedly different from surrounding tissues, elasticity imaging could provide a significant adjunct to current diagnostic ultrasonic methods. Further, elasticity imaging techniques could be used to augment the study of tissues that change their elastic properties, such as skeletal and cardiac muscle. In this paper, we survey some of the previous work done in the related field of biomechanics, and we review measurement techniques from the 1950s to the 1980s. Different approaches to elastic imaging and signal processing are then discussed and a lexicography for elastic imaging is introduced. It is hoped that this nomenclature will provide a meaningful categorization of various approaches and will make evident the inherent parameters displayed and conditions applied in deriving the resulting images. Key assumptions and signal processing approaches are also reviewed. Finally, directions for future work are suggested.

Sonoelastic determination of human skeletal muscle elasticity.

It is not currently practical to directly measure viscoelastic parameters in human muscles in situ. Methods used in vitro cannot readily be applied, and motion analysis provides only a gross estimate. We report on the application of a hybrid approach, sonoelastography, which uses ultrasound to measure the propagation of shear waves induced by externally applied vibrations. Because shear waves predominate in incompressible viscoelastic media at low frequencies, sonoelastic data should be comparable to those obtained using conventional means. We recorded vibration propagation speeds as a function of applied load in the quadriceps muscles of ten volunteers as they underwent a series of static contractions. Data collection during dynamic contractions, not possible with the current equipment, will be the subject of future experimentation. Although statistically significant correlations were not uniformly obtained above 60 Hz nor for propagation perpendicular to the muscle fibers, this is felt to have resulted from deviations from the applied plane wave model. Calculated values of Young's modulus for 30 Hz propagation parallel to the muscle fibers were 7 +/- 3, 29 +/- 12 and 57 +/- 37 x 10(3) Nm-2 for applied loads of 0, 7.5 and 15 kg, respectively. The corresponding values at 60 Hz were 25 +/- 6, 75 +/- 61 and 127 +/- 65. These values were statistically significant and linearly correlated with the applied load, as expected. Our data represent the first in situ human measurements of their kind. It is anticipated that sonoelastography will provide a useful adjunct to the study of human biomechanics.

Structural characterization and specificity of expression of E2F-5: a new member of the E2F family of transcription factors.

Members of the E2F gene family are transcription factors that have been implicated in the control of genes essential for cell cycle progression. Regulation of E2F function is finely tuned by the retinoblastoma tumor suppressor gene product and a small family of related "pocket proteins," with the participation of a number of cyclins and cyclin-dependent kinases. Perturbations of this regulatory network can lead to oncogenic transformation and, in certain systems, to the loss of the ability to maintain terminal differentiation. We describe here the cloning, structural characterization, and tissue expression pattern of a new member of the E2F family, E2F-5. We show that this protein is highly conserved between human and rat but exhibits considerable divergence from E2F-1, E2F-2, or E2F3. Together with the recently reported E2F-4, E2F-5 defines a new branch of the E2F family. The distribution of E2F-5 mRNA among adult rat tissues and the temporal pattern of its expression during the cell cycle of vascular smooth muscle cells are distinctly different from that of E2F-1. The structural divergence between the two branches of the E2F family may thus reflect participation in different regulatory networks.

Rehabilitation dimensions of AIDS: a review.

With the spread of human immunodeficiency virus (HIV) infection and of the acquired immune deficiency syndrome (AIDS), many rehabilitation professionals are faced with new challenges. This report covers two basic problems that are becoming common in rehabilitation practice: the management of the patient who suffers from disability resulting from HIV infection or AIDS, and the management of the traditional rehabilitation patient who may coincidentally be infected with HIV. Common manifestations of HIV infection and associated secondary infections and neoplasms are discussed, as well as are the complications of current medical treatments. This report also explores specific neurologic and musculoskeletal disorders and the fatigue associated with AIDS. In addition, potential approaches to rehabilitation management are evaluated, and consideration is given to the management of the asymptomatic HIV carrier. The differences between HIV infection in adults and children are explored, and the social implications of HIV rehabilitation are discussed. After consideration of the basic pathophysiology of infection and the modes of transmission, the significance and ethics of serologic testing are examined, and the concept of universal precautions is described. The paper concludes with a discussion of the use of safe sex practices by the disabled population.

Experience with rehabilitation in the acquired immunodeficiency syndrome.

Patients with the acquired immunodeficiency syndrome (AIDS) represent a novel referral population for rehabilitation services. Limited information about the rehabilitation needs of individuals with human immunodeficiency virus infection is available. We reviewed 51 consecutive patients with AIDS referred to a rehabilitation consult service. Common problems encountered included generalized deconditioning (27%) and neurologic dysfunction (45%). Neurologic presentations were diverse and included hemiparesis, diffuse cognitive dysfunction and dementia, myelopathy, myopathy and peripheral neuropathy. Other patients were referred for wound care as well as the management of the local effects of Kaposi's sarcoma, various musculoskeletal syndromes and new onset blindness. Problems identified included impaired mobility (76%), difficulty with self-care (57%), impaired cognition (29%) and uncontrolled pain (37%). Among the rehabilitation interventions utilized were therapeutic exercise (73%), gait aids (45%), bathroom and safety equipment (45%), orthotics (29%), vocational counseling (4%), pain management (29%) and whirlpool treatments (10%). Five patients were too ill or refused treatment. We conclude that AIDS patients referred for rehabilitation have a wide variety of physical deficits, demonstrate a considerable degree of functional impairment and may require multiple rehabilitation interventions.

Phase detection in acoustic microscopy.

The scanning laser acoustic microscope provides a relatively simple means by which speed of sound data can be collected from biological specimens. By employing a phase detection circuit, phase-contour lines can be superimposed on acoustic micrographs and digitized for direct speed of sound calculations. Because of the existence of mild field non-uniformities, the phase reference must be mathematically modeled. This is accomplished by employing a polynomial function and simple linear regression. Scattering can be studied by using a frequency-selective "dark field" approach. Speed of sound maps are readily produced from the phase data by applying the simple geometric relationships of wave propagation; however the theoretical limitations of the technique must be kept in mind. Using this approach, useful data have been obtained.

Ultrasound propagation in anisotropic soft tissues: the application of linear elastic theory.

Linear elastic theory has served well in modeling the mechanical properties of numerous materials. In modeling ultrasonic wave propagation in biological soft tissues, an isotropic model has usually been employed. Many tissues, however, possess a lower order of symmetry, and the speed of sound in muscle is known to vary with the direction of propagation. In this study, by applying linear regression to acoustic microscopic data from seven frog sartorius specimens, four observable elastic constants associated with a transversely isotropic model were obtained. The average values of these constants were c11 = 2.64, c13 = 3.39 and c33 = 4.40 Nm-2 for resting muscles and c11 = 2.65, c13 = 3.43 and c33 = 4.57 Nm-2 for muscles undergoing tetanic contraction, where '1' and '3' represent the transverse and longitudinal axes, respectively. In all cases, c44 was 0, indicating a minimal contribution from longitudinal shear. For all seven specimens, the model of transverse isotropy provided a better fit of the data than that of isotropy.