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. 1 . Introduction The presence of atherosclerotic lesions (plaque) in the carotid arteries increases the risk of stroke, but surgical removal of the plaque has proven beneficial in reducing the risk of stroke. However, ultrasonic assessment of size, shape and type of the carotid atherosclerotic lesions is often seriously limited by poor visualization of the plaque. There are several reasons for this: i) Some plaque materials, such as certain lipids, are echopoor in nature and will always be difficult to visualize. ii) Other non-isotropic materials, such a fibrous tissues, produce echo signals with a strong angle-dependence resulting in fluctuations in echogenicity when the transducer is moved and/or rotated. iii) Because the pathological thickening of the carotid wall is seldom larger than 4 mm, the region of interest on the ultrasound image is only approximately 20 wavelengths thick (assuming a transducer frequency of 7.5 MHz and a sound speed of 1540 m/s). Due to the speckle noise in this small ultrasound image region, residual lumen, plaque outline and plaque type can often be difficult to determine. A possible solution to some of these problems is to use spatial compound imaging which can reduce the angle-dependence and the speckle noise . Therefore, a digital off-line ultrasound scanner for multi-angle compound imaging (MACI) was build with the purpose of improving visualization of vascular tissue. This paper describes the instrument and the in vitro results obtained when scanning porcine aorta and human atherosclerotic plaque. 2 . Principle of Operation The MACI system utilizes a conventional low-pitch linear array transducer. The active aperture is selected as a subset of the elements and this aperture is then operated in a phased array mode, to create a beam with a given angle, θi. The aperture is then moved along the entire array to record an entire image at that angle. This operation is repeated for all desired beam angles, θi, where i = 1, 2, ..., Nθ. Figure 1 illustrates the scanlines used in this procedure for Nθ = 3. It is seen that the fully compounded region, theoretically, is a triangle. From the geometry, the depth of this region can be found to: Figure 1 Illustration of scan lines for three single-angle images used in creating the multi-angle compound image. zmax(D , θmax, θmin) = D tan(θmax) − tan(θmin) (1) Fully compounded region