Maria Siebes

Segmentation of intravascular ultrasound images: a knowledge-based approach

Intravascular ultrasound imaging of coronary arteries provides important information about coronary lumen, wall, and plaque characteristics. Quantitative studies of coronary atherosclerosis using intravascular ultrasound and manual identification of wall and plaque borders are limited by the need for observers with substantial experience and the tedious nature of manual border detection. We have developed a method for segmentation of intravascular ultrasound images that identifies the internal and external elastic laminae and the plaque-lumen interface. The border detection algorithm was evaluated in a set of 38 intravascular ultrasound images acquired from fresh cadaveric hearts using a 30 MHz imaging catheter. To assess the performance of our border detection method we compared five quantitative measures of arterial anatomy derived from computer-detected borders with measures derived from borders manually defined by expert observers. Computer-detected and observer-defined lumen areas correlated very well (r=0.96, y=1.02x+0.52), as did plaque areas (r=0.95, y=1.07x-0.48), and percent area stenosis (r=0.93, y=0.99x-1.34.) Computer-derived segmental plaque thickness measurements were highly accurate. Our knowledge-based intravascular ultrasound segmentation method shows substantial promise for the quantitative analysis of in vivo intravascular ultrasound image data.

Investigation of plaque biomechanics from intravascular ultrasound images using finite element modeling

Biomechanical properties of atherosclerotic plaque are not well known due to the difficulties involved in experimental testing. In this study an iterative finite element program was developed that estimates the elastic material properties of diseased arteries and plaque based on deformations from intravascular ultrasound images. The program successfully determined the elastic modulus of latex tubes with artificial plaque and of the ex vivo human coronary arteries. The authors conclude that this finite element method provides a way to estimate the material properties of the plaque an wall in stenosed arteries from clinically available data, i.e., ultrasound images and blood pressure measurements.

Automated detection of wall and plaque borders in intravascular ultrasound images

Intravascular ultrasound is a minimally invasive tomographic technique which produces 2-D cross-sectional images depicting vessel wall architecture and plaque morphology. Currently, no reliable automated approaches exist that offer segmentation of blood and vascular wall. We have developed a method for automated segmentation of intravascular ultrasound images to differentiate among plaque, wall, and blood. To achieve reliable border detection in noisy intravascular ultrasound images, a priori knowledge is incorporated in the edge detection process using heuristic graph searching. The method was validated using images from two phantoms that were imaged under several pressure conditions. In the first image set, our automated border detection method correctly identified the wall and plaque borders in 69/91 images. In the second image set, our method successfully identified external and internal wall and plaque borders in all 36 images. Lumen cross-sectional areas correlated very well with distending pressure in both sets of images. By comparison with the micrometer determined average wall thickness, mean absolute error of wall thickness was 0.02 +/- 0.01 mm.

Mechanical effect of contrast injection on coronary artery diameter during angiography

The authors performed an angiographic study in six healthy anaesthetized pigs to investigate possible mechanical effects of contrast injection on coronary artery pressure and diameter during the period of image acquisition. Angiograms were acquired at 30 f/sec for repeated injections of 8 ml of contrast material by hand and with a power injector at rates of 2, 4, and 6 ml/sec. A 1.5 F micro-tip pressure transducer was used to measure intracoronary pressure at 1, 2, and 4 cm downstream from the injection site for each injection rate. The results show a progressive increase in intracoronary pressure with increasing rate of injection (p<0.005), ranging from 3 to 117 mmHg. Corresponding diameters were distended by 0.02 to 0.6 mm (p<0.05) during the injection, as evaluated by quantitative coronary angiography (QCA). These mechanical effects were related to the mean coronary pressure prior to injection (p<0.02) in a manner consistent with the nonlinear compliance of the arterial wall. The authors conclude that coronary dimensions obtained from QCA critically depend on the mode of contrast injection, which may represent a previously unrecognized source of error in the analysis and interpretation of clinical angiographic studies.<<ETX>>

Biomechanical characterization of blood vessel and plaque from intravascular ultrasound images

The elastic properties of arteries and plaque are difficult to assess due to the technical problems associated with obtaining the required dimensional and stress parameters in vivo. This paper describes a method to determine the biomechanical properties of blood vessels and plaque based on dimensional information obtained from the analysis of intravascular ultrasound images. As a pilot study, two models of a stenotic artery were made that included an eccentric, flexible plaque of known size. These models were submerged in water and imaged at up to 15 axial locations over a static pressure range from 0 to 200 mmHg. Plaque and wall borders were automatically detected using a heuristic graph searching technique. Cross-sectional area changes, dimensional parameters and elastic properties were determined for each location and intraluminal pressure. The incremental elastic modulus of the tube wall was within 8% of the value determined from micrometer measurements. Plaque stiffness was close to that of a fibrous arterial plaque. Cross-sectional changes indicated a radial expansion of the compliant plaque with pressure.

Design and development of an electromechanical ultrasound probe holder for urodynamics testing

A prevalent difficulty in urodynamics studies employing ultrasonography is associated with the manual application of the imaging transducer to the perineum. We have developed an electromechanically operated device for remote positioning of an ultra-sound probe during voiding studies of the lower urinary tract. The mechanical arm holds the probe inside a funnel that is mounted underneath a modified portable commode on which the patient is seated. External manually operated mechanical slides are used to translate the probe along the three primary axes for initial lateral and vertical positioning. Backwards/forwards and left/right pivoting of the transducer is then accomplished via linear stepper motors that are operated with a hand-held controller. A preliminary evaluation has shown that the device is easy to use, safe, and allows excellent visualization of the bladder outlet and proximal urethra in both male and female patients. The capability to remotely adjust the imaging angle allows the patient to void in a more private setting behind a drawn curtain, thereby minimizing the psychological distress associated with this test and facilitating the acquisition of more physiological test results.

Mathematical model of pressure-related changes in stenosis cross-sectional area during the cardiac cycle

The purpose of this study was to investigate the effect of pressure losses during the cardiac cycle on the cross-sectional shape of a partially compliant coronary artery stenosis. A mathematical model of pulsatile flow through a partially collapsible stenosis was used to calculate changes in minimum stenosis diameter in response to changes in intraluminal pressure. Nine stenoses were simulated, ranging from 50 to 90% area reduction with different fractions of flexible wall circumference (1/4, 1/3, and 1/2). For typical flow rates during maximum vasodilation the minimum diameter of simulated severe stenoses decreased by over 50% from systole to diastole, thereby substantially altering the cross-sectional area of the stenosis. These results may explain some of the discrepancies that have been reported from pathologic and angiographie studies of coronary artery stenosis morphology.

Model of coronary hemodynamics during radiographic contrast injection

Serial quantitative angiographic studies are frequently employed to assess the effect of dietary or mechanical interventions on the severity of coronary artery stenosis and angiographic conditions that may alter coronary diameter via changes in pressure need to be avoided. The purpose of this study was to investigate the extent of hemodynamic changes introduced during the injection of radiographic contrast into the coronary ostium. A linear resistance model was developed to simulate this event for different rates of injection (2-6 ml/sec) and resistance ratios (Rcor/Rcath=40 and 80). The results show an increase in coronary pressure between 7 and 44 mmHg at the time of injection, which depends primarily on the injection rate and the resistance posed by the guiding catheter in the coronary ostium. These data are consistent with earlier experimental results and point to the importance of standardizing contrast injections for QCA studies.

Comparison of conventional and CO2 quantitative angiography: fiz or fizzle?

Typically, quantitation of 2D angiographic data has been limited to assessment of luminal edges from a single or, at most, two views. Use of densitometric image information, which can give data more closely related to the 3D geometry of the vessel, has been largely ignored because of imaging problems associated with conventional contrast agents. Recently, in phantom studies, we have demonstrated that the use of CO2 as an angiographic contrast agent may eliminate many of the obstacles previously limiting the use of densitometric approaches to quantitating 2D angiograms. This may allow us to provide an evaluation more closely related to the 3D geometry of the vasculature of interest. In this paper, we discuss our approach to CO2 angiography and provide illustrative angiographic studies demonstrating the potential breakthrough CO2 angiography may provide in extracting 3D information from 2D data sets.

Effect of contrast injection on coronary artery pressure and dimension during angiography

Digital coronary angiography and intracoronary pressure recordings were performed in 17 patients to quantitate the mechanical effects of contrast injection on coronary artery pressure and diameter in arterial segments free of stenotic lesions at the moment of image acquisition. A progressive increase in intracoronary pressures by 9.2/spl plusmn/6.6 mmHg for a power injection rate of 2.5 ml/s, 14.4/spl plusmn/10.9 mmHg for 5 ml/s and 23.9/spl plusmn/24.2 mmHg for 10 ml/s was documented Corresponding diameters were distended by net -0.13 to +0.65 mm (p<0.005) during the injection, as evaluated by digital quantitative coronary angiography (DQCA). These mechanical phenomena result in changes of coronary artery, diameter from -4.9% to +19.5% at 10 ml/s (p<0.005) and unmask the individual compliance of the arterial wall. It is concluded that coronary dimensions obtained from DQCA critically depend on the intracoronary pressure with contrast medium injection. It may represent an additional source of error in the analysis and interpretation of clinical angiographic studies.