Jens E. Wilhjelm

Echo-Lucency of Computerized Ultrasound Images of Carotid Atherosclerotic Plaques Are Associated With Increased Levels of Triglyceride-Rich Lipoproteins as Well as Increased Plaque Lipid Content

BACKGROUND Echo-lucency of carotid atherosclerotic plaques on computerized ultrasound B-mode images has been associated with a high incidence of brain infarcts as evaluated on CT scans. We tested the hypotheses that triglyceride-rich lipoproteins in the fasting and postprandial state predict carotid plaque echo-lucency and that echo-lucency predicts a high plaque lipid content. METHODS AND RESULTS The study included 137 patients with neurological symptoms and > or = 50% stenosis of the relevant carotid artery. High-resolution B-mode ultrasound images of carotid plaques were computer processed to yield a measure of echogenicity (gray-scale level). Lipoproteins were measured before and hourly for 4 hours after a standardized fatty meal. A subgroup of 58 patients underwent endarterectomy. On linear regression analysis, echo-lucency (low gray-scale level) was associated with elevated levels of fasting and postprandial plasma triglycerides (P=.0002 and P=.002), IDL cholesterol (P=.0009 and P=.006), and VLDL/chylomicron remnant cholesterol (P=.0003 and P=.0004) and triglycerides (P=.0003 and P=.003), the area under the plasma triglyceride curve 0 to 4 hours after a fatty meal (P=.001), and body mass index (P=.0001). On ANCOVA, body mass index, fasting IDL cholesterol, and fasting plasma triglycerides were independent predictors of echo-lucency. Echo-lucency was associated with increased relative plaque lipid content (P=.02). CONCLUSIONS Increased plasma levels of triglyceride-rich lipoproteins predict echo-lucency of carotid plaques, which is associated with increased plaque lipid content. Because echo-lucency has been associated with a high incidence of brain infarcts on CT scans, triglyceride-rich lipoproteins may predict a plaque type particularly vulnerable to rupture.

Multi-angle compound imaging

This paper reports on a scanning technique, denoted multi-angle compound imaging (MACI), using spatial compounding. The MACI method also contains elements of frequency compounding, as the transmit frequency is lowered for the highest beam angles in order to reduce grating lobes. Compared to conventional B-mode imaging MACI offers better defined tissue boundaries and lower variance of the speckle pattern, resulting in an image with reduced random variations. Design and implementation of a compound imaging system is described, images of rubber tubes and porcine aorta are shown and effects on visualization are discussed. The speckle reduction is analyzed numerically and the results are found to be in excellent agreement with existing theory. An investigation of detectability of low-contrast lesions shows significant improvements compared to conventional imaging. Finally, possibilities for improving diagnosis of atherosclerotic diseases using MACI are discussed.

Quantitative analysis of ultrasound B-mode images of carotid atherosclerotic plaque: correlation with visual classification and histological examination

This paper presents a quantitative comparison of three types of information available for 52 patients scheduled for carotid endarterectomy: subjective classification of the ultrasound images obtained during scanning before operation, first- and second-order statistical features extracted from regions of the plaque in still ultrasound images from three orthogonal scan planes and finally a histological analysis of the surgically removed plaque. The quantitative comparison was made with the linear model and with separation of the available data into training and test sets. The comparison of subjective classification with features from still ultrasound images revealed an overall agreement of 60% for classification of echogenicity and 70% for classification of structure. Comparison of the histologically determined relative volume of soft materials with features from the still images revealed a correlation coefficient of r=-0.42 (p=0.002), for mean echogenicity of the plaque region. The best performing feature was of second order and denoted contrast (r=-0.5). Though significant, the latter correlation is probably not strong enough to be useful for clinical prediction of relative volume of soft materials for individual patients. Reasons for this is discussed in the paper, together with suggestions for improvements.

Beamforming scheme for 2D displacement estimation in ultrasound imaging

We propose a beamforming scheme for ultrasound imaging leading to the generation of two sets of images, one with oscillations only in the axial direction and one with oscillations only in the lateral direction. Applied to tissue elasticity imaging, this leads to the development of a specific displacement estimation technique that is capable of accurate estimation of two components of the displacement. The mean standard deviation for the axial displacement estimates is 0.0219 times the wavelength of the axial oscillations, and for the lateral estimates, it is equal to 0.0164 times the wavelength of the lateral oscillations. The method is presented and its feasibility is clearly established by a simulation work.

In vitro spatial compound scanning for improved visualization of atherosclerosis.

A new off-line multiangle ultrasound (US) compound scanner has been built with the purpose of investigating possible improvements in visualization of vascular structure. Images of two formalin-fixed human atherosclerotic plaques removed by carotid endarterectomy were recorded from seven insonification angles over a range of 42 degrees and the individual images were combined (averaged) into a single image (spatial compounding). Compared to conventional B-mode imaging, this multiangle compound imaging (MACI) method features images with reduced angle-dependence, reduced random variation (speckle) and improved delineation of the plaque outline. With the MACI approach, it is, thus, easier to assess e.g., a possible residual lumen of an atherosclerotic artery as well as the level of echogenicity for the different plaque constituents.

Visual and quantitative evaluation of selected image combination schemes in ultrasound spatial compound scanning

Multi-angle spatial compound images are normally generated by averaging the recorded single-angle images (SAIs). To exploit possible advantages associated with alternative combination schemes, this paper investigates both the effect of number of angles (N/sub /spl theta//) as well as operator (mean, median, mean-excluding-maximum (mem), root-mean-square (rms), geometric mean and maximum) on image quality (tissue delineation and artifacts), speckle signal-to-noise ratio (SNR/sub s/) and contrast. The evaluation is based on in vitro SAI (/spl plusmn/21/spl deg/ in steps of /spl Delta//spl theta/=7/spl deg/) of formalin fixed porcine tissue containing adipose, connective and muscular tissue. Image quality increased with number of angles up to /spl plusmn/14/spl deg/ after which the improvements became debatable. The mem and median operators, which try to render the images more quantitatively correct by suppressing strong echoes from specular reflectors, provide some improvement in this regard. When combining the SAI with the mean operator, the SNR/sub s/ increases-in general-with N/sub /spl theta//. For N/sub /spl theta//=2, the SNR/sub s/ increases with /spl Delta//spl theta/ as expected. When N/sub /spl theta//=7, the highest SNR/sub s/ is obtained for the mem, rms, and geometric mean operators, while the lowest SNR/sub s/ is obtained for the maximum operator. When comparing SNR/sub s/ for adipose and fibrous tissue, the level is close to 1.91 for adipose tissue but only 1.7 for fibrous tissue which contain relatively few organized scattering structures.

Investigation on the load-displacement curves of a human healthy heel pad: In vivo compression data compared to numerical results

The aims of the present work were to build a 3D subject-specific heel pad model based on the anatomy revealed by MR imaging of a subjects heel pad, and to compare the load-displacement responses obtained from this model with those obtained from a compression device used on the subjects heel pad. A 30 year-old European healthy female (mass=54kg, height=165cm) was enrolled in this study. Her left foot underwent both MRI and compression tests. A numerical model of the heel region was developed based on a 3D CAD solid model obtained by MR images. The calcaneal fat pad tissue was described with a visco-hyperelastic model, while a fiber-reinforced hyperelastic model was formulated for the skin. Numerical analyses were performed to interpret the mechanical response of heel tissues. Different loading conditions were assumed according to experimental tests. The heel tissues showed a non-linear visco-elastic behavior and the load-displacement curves followed a characteristic hysteresis form. The energy dissipation ratios measured by experimental tests (0.25±0.02 at low strain rate and 0.26±0.03 at high strain rate) were comparable with those evaluated by finite element analyses (0.23±0.01 at low strain rate and 0.25±0.01 at high strain rate). The validity and efficacy of the investigation performed was confirmed by the interpretation of the mechanical response of the heel tissues under different strain rates. The mean absolute percentage error between experimental data and model results was 0.39% at low strain rate and 0.28% at high strain rate.

Spiral computed tomographic imaging related to computerized ultrasonographic images of carotid plaque morphology and histology.

Echolucency of carotid atherosclerotic plaques, as evaluated by computerized B‐mode ultrasonographic images, has been associated with an increased incidence of brain infarcts on cerebral computed tomographic scans. We tested the hypotheses that characterization of carotid plaques on spiral computed tomographic images correlates with that on computerized B‐mode ultrasonographic images and that spiral computed tomographic imaging predicts the histomorphometric plaque content.

Target velocity estimation with FM and PW echo ranging Doppler systems. II. Systems analysis

For part I see ibid., vol.40, no.4, pp.366-372 (1993). In Part I, the encoding of the velocity and range information into the received and demodulated signals based on transmission of coherent repetitive linear sweep signals, was discussed. In the present work, two different implementations of FM Doppler systems that can be used to obtain velocity profiles are presented. The first implementation is similar to the implementation of a conventional pulsed wave (PW) Doppler system, based on measurement of phase shift (correlation based system): the second implementation is a frequency-domain analog to the PW Doppler system, based on time shift measurements (cross correlation-based system).<<ETX>>

The influence of roughness, angle, range, and transducer type on the echo signal from planar interfaces

The received electrical echo signal from a pulse-echo system insonifying a planar interface was measured for varying degrees of rms roughness [0 to 0.29 mm (0 to 1.7 /spl lambda/)], angles of incidence, /spl theta/, (-7/spl deg/ to 7/spl deg/), and ranges to a planar or focused transducer. The effect of varying a is quantified in terms of the energy of the received signal, E(/spl theta/), and the normalized spectrum of the received signal. E(/spl theta/) is approximately Gaussian when using a planar transducer or a focused transducer with the reflecting interface located at or beyond the focal point. For focused transducers with the interface located closer than the geometrical point of focus, two maxima can sometimes be observed when varying the incident angle. As is generally known, the width of E(/spl theta/) is strongly dependent on transducer type, e.g., for a smooth interface, the -3 dB width for a 25.4 mm diameter 5-MHz planar and focused transducer was approximately 0.5/spl deg/ and 4/spl deg/ (at the focal point), respectively. E(0/spl deg/) as a function of surface roughness, R/sub q/, was nearly linear on a decibel scale, with a slope of -109 dB/(R/sub q///spl lambda/) and -61 dB/(Rq//spl lambda/) for planar and focused transducers, respectively. The characteristic nulls present in the normalized spectra of the echo signal at non-normal incidence tend to vanish with increasing R/sub q/ when using planar transducers. For focused transducers, the normalized spectra change from relatively flat to monotonically decreasing as R/sub q/ increases, and they exhibit reduced amplitude with increased incident angle.