Charles T. Lancée

Arterial wall characteristics determined by intravascular ultrasound imaging: An in vitro study☆

The feasibility of assessing arterial wall configuration with an intravascular 40 MHz ultrasound imaging device was investigated in an in vitro study of 11 autopsy specimens of human arteries. The system consists of a single element transducer, rotated with a motor mounted on an 8F catheter tip. Cross sections obtained with ultrasound were matched with the corresponding histologic sections. The arterial specimens were histologically classified as of the muscular or elastic type. Muscular arteries interrogated with ultrasound presented with a hypoechoic media, coinciding with the smooth muscle cells. In contrast, the media of an elastic artery densely packed with elastin fibers was as echogenic as the intima and the adventitia. On the basis of the cross-sectional image, it was possible to determine the nature of the atherosclerotic plaque. The location and thickness of the lesion measured from the histologic sections correlated well with the data derived from the corresponding ultrasound images. This study indicates that characterization of the type of artery and detection of arterial wall disease are possible with use of an intravascular ultrasound imaging technique.

Higher harmonics of vibrating gas-filled microspheres. Part one: simulations

Abstract The acoustic behaviour of an ideal gas bubble in water is considered and the equation of motion is extended to model an Albunex microsphere. Calculations reveal large differences in non-linear behaviour between ideal gas bubbles and Albunex microspheres, due to the additional restoring force of, and friction inside, the shell that surrounds the Albunex microsphere. Simulations with the Albunex contrast agent further reveal that the optimal driving frequency is 1 MHz, resulting in a second harmonic that is 20 dB below the first harmonic at an acoustic pressure of 50 kPa. The difference increases to 25 dB for a driving frequency of 2 MHz.

Principles and recent developments in ultrasound contrast agents

The behaviour of gas bubbles and gas encapsulated spheres as echographic contrast agents is reviewed. Compared with rigid spheres, gas bubbles are superior scattering agents and they offer a number of useful properties which can be exploited in a variety of ways. The analysis of their velocity of sound, back-scatter intensity, second harmonic emission and resonant frequency opens up new perspectives in the development of contrast agents for echocardiographic research with potential clinical applications.

Early and recent intraluminal ultrasound devices

The history of intraluminal echography dates back to the very beginning of diagnostic ultrasound. Over the years many fascinating ideas and applications of catheter tip or gastroscopic tube tip mounted transducers have been described. This chapter surveys these methods, subdividing them into a) measurements; b) Doppler and c) imaging. The survey ranges from early work of Cieszynski on the feasibility of echocardiography to more recent intra-arterial catheter tip Doppler with guidewire and balloon as described by Serruys.Examples of ultrasound catheter tip echography in combination with other techniques such as angioscopy, laser ablation and spark erosion are also described. Today practical approaches are limited to imaging only. The three major approaches for catheter tip echo imaging are described and compared. This paper concludes with the results of automatic contour analysis of the inner arterial boundaries.

Brandaris 128: A digital 25 million frames per second camera with 128 highly sensitive frames

A high-speed camera that combines a customized rotating mirror camera frame with charge coupled device (CCD) image detectors and is practically fully operated by computer control was constructed. High sensitivity CCDs are used so that image intensifiers, which would degrade image quality, are not necessary. Customized electronics and instruments were used to improve the flexibility and control precisely the image acquisition process. A full sequence of 128 consecutive image frames with 500×292 pixels each can be acquired at a maximum frame rate of 25 million frames/s. Full sequences can be repeated every 20 ms, and six full sequences can be stored on the in-camera memory buffer. A high-speed communication link to a computer allows each full sequence of about 20 Mbytes to be stored on a hard disk in less than 1 s. The sensitivity of the camera has an equivalent International Standards Organization number of 2500. Resolution was measured to be 36 lp/mm on the detector plane of the camera, while under a microscope a bar pattern of 400 nm spacing line pairs could be resolved. Some high-speed events recorded with this camera, dubbed Brandaris 128, are presented.

Intravascular elasticity imaging using ultrasound: Feasibility studies in phantoms

A technique is described for measuring the local hardness of the vessel wall and atheroma using intravascular ultrasound. Strain images were constructed using the relative local displacements, which are estimated from the time shifts between gated echo signals acquired at two levels of intravascular pressure. Time shifts were estimated using one-dimensional correlation with bandlimited interpolation around the peak. Tissue-mimicking phantoms with typical morphology and hardness topology of some atherosclerotic vessels were constructed. Hard and soft regions could be distinguished on the strain image, independently of their contrast in echogenicity. Thus, the potential of ultrasonic hardness imaging to provide information that may be unavailable from the echogram alone was demonstrated. The strain images of the homogeneous and layered phantoms showed some artifacts that need to be corrected for, to obtain images of the modulus of elasticity. For in vitro and in vivo experiments, the spatial resolution of the technique needs to be improved. Furthermore, two-dimensional correlation techniques may be necessary in case of nonradial expansion and an off-centre catheter position.

Higher harmonics of vibrating gas-filled microspheres. Part two: measurements

Abstract Acoustic measurements on diluted Albunex at two driving frequencies (1 and 2 MHz) are reported which show that the level of the second-harmonic response is about 20 dB below the first harmonic at an acoustic pressure amplitude of 50 kPa. It is demonstrated that, under controlled conditions, it is possible to discriminate between Albunex and other scattering/reflecting objects using the non-linear behaviour of the microspheres.

An ultrasonic intracardiac scanner

Abstract Ultrasound cathetertip echo techniques may be applied to obtain internal dimensions of the heart. So far it has been difficult to study the movement of structures in a cross-sectional plane. This paper describes a 32-element cathetertip system (outer diameter 3 mm) specially developed for instantaneous study of moving cardiac structures. The system is based on the sonar-type phase-corrected circular array techniques. It is shown that such a catheter can be constructed. A brief discussion on the possible applications, the limitations and the preliminary in vitro results is given.

Intravascular echographic assessment of vessel wall characteristics: a correlation with histology

SummaryIn vivo application of intravascular high frequency ultrasonic imaging for peripheral and coronary artery disease is a promising technique for vascular surgeons, radiologists and cardiologists. This report demonstrates in vitro results obtained with a high frequency imaging catheter (40 MHz) in 70 human specimens including arteries with and without atherosclerosis, veins, coronary artery bypass grafts and vascular prosthetic material. Correlation between the ultrasonic images and the histologic characteristics of the corresponding vessel wall tissue and lumen geometry was established. In addition, the effect of intervention techniques i.e. balloon angioplasty, spark erosion and laser were studied with ultrasound and histology. It is anticipated that development of such a catheter imaging technique has potential for diagnostic imaging and for combination with therapeutic systems.

Assessment of medial thinning in atherosclerosis by intravascular ultrasound

This study investigated the in vitro (40 MHz) and in vivo (30 MHz) feasibility of intravascular ultrasound to document the influence of atherosclerotic lesions on the typical 3-layered appearance of muscular arteries. The in vitro images of 39 arteries were compared with the corresponding histologic sections. Media and lesion thickness were measured at the areas of minimal and maximal lesion thickness. The median media thickness was 0.8 mm in the absence of a lesion, decreasing to 0.3 mm in the area of maximal atherosclerosis. The ultrasonic data correlated closely with histologic measurements (0.6 and 0.3 mm, respectively). The in vivo study was performed in 29 patients undergoing coronary or peripheral vascular procedures. A total of 150 still-frames were selected for quantitative analysis. The median media thickness was 0.6 mm in the absence of a lesion, decreasing to 0.1 mm in the area with maximal atherosclerosis. This study revealed that intravascular ultrasound imaging accurately determines that media thickness of muscular arteries is inversely related to lesion thickness. In vitro data, verified with histology, can be translated to humans in vivo.