Søren Kragh Jespersen

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.

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.

Some imaging strategies in multi-angle spatial compounding

Multi-angle compound images were generated with four schemes: mean, median, root-mean-square and geometric mean. The in vitro images, based on formalin fixed porcine tissue, were analyzed by visual inspection and by calculation of speckle contrast and contrast between different tissues. The mean and rms images featured the largest enhancement of distributed specular reflections. The median image appears somewhat noisy. However, the quantitative analysis did not indicate large differences between the images.

Influence of tissue preservation methods on arterial geometry and echogenicity.

Thoracic porcine aortas from 5 pigs were investigated with 7.5-MHz ultrasound in vitro at low and high transmural pressure before and after the following tissue preservation methods were applied: 1. Storage in frozen condition (-12 degrees C) for 24 h followed by thawing; 2. fixation in formalin at zero transmural pressure for 24 h; and 3. fixation with formalin for 24 h while applying 74 mmHg of transmural pressure from within the lumen and a tensile force to longitudinally stretch the artery. Fixation in formalin at zero transmural pressure resulted in swelling of the arterial wall (25 +/- 40%, p < 0.02, at low transmural measurement pressure) and in decreased echogenicity (-23 +/- 38%, p < 0.01) of the arterial vessel wall. No changes in this respect were found after storage in a frozen condition nor after fixation in formalin at high transmural pressure which, therefore, are more appropriate procedures for fixation of arteries prior to in vitro ultrasound examination if geometry is important.

Ultrasound spatial compound scanner for improved visualization in vascular imaging

A new off-line multi-angle ultrasound compound scanner has been built with the aim of improving visualization of vascular tissue. Images are recorded from 3 to 11 insonification angles over a range of 40 to 50 degrees and the individual images are combined (averaged) into a single image (spatial compounding). Compared to conventional B-mode imaging, this multi-angle compound imaging (MACI) method features less angle-dependent images, since more scan lines are nearly perpendicular to the tissue interfaces. Further, the spatial compounding strongly diminishes the speckle pattern. These improvements are illustrated with in vitro images of porcine aorta and human atherosclerotic plaque.

Correlation between ultrasound B-mode images of carotid plaque and histological examination

This paper reports on a study on 69 patients where image features extracted from B-mode ultrasound images of atherosclerotic plaque in the carotid arteries were compared to histological results obtained from the same plaque after carotid endarterectomy. The study also investigated the correlation between image features and visual classification of the plaque appearance as revealed by ultrasound scanning. The study found a few texture features to be significantly correlated to the histological findings within the training set (p=0.002-0.04). However, the correlation found may not be reliable for clinical determination of plaque composition. The correlation between image texture features and the visual classification of the B-mode images was, not surprisingly, found to be much better. Thus, computer analysis is likely to be useful in future investigations as many problems with intra- and inter-observer variations that inherently exist in the subjective visual classification can be eliminated.

Influence of insonification angle on echogenicity of B-mode images of atherosclerotic plaque in vitro

A newly developed (off-line) spatial compound scanner was used to scan formalin-fixed atherosclerotic carotid plaques. Forty-eight B-mode images were recorded using 7 insonification angles. All calculations were done on the envelope-detected image data. The mean amplitude level (MAL) in (relative) volts was calculated for the plaque region in each image. The standard deviation over the 48 MAL values were for each of the 7 angles between 0.12 V and 0.18 V. For each scan plane, the standard deviation was also calculated over the 7 images. The mean and standard deviation of these 48 numbers were 0.07 V and 0.04 V, respectively. Thus based on these data, the variation from scan plane to scan plane is normally larger than the variation over angle for a single scan plane.