David T. Linker

Two-Dimensional Intravascular Ultrasound: Technical Development and Initial Clinical Experience

This article reviews the development and current status of catheter-based, intravascular ultrasound imaging. The first section provides an introduction to some of the general technical issues encountered in the design of the catheter imaging systems and compares the potential merits of the multiple-element versus the mechanical approach. The second section of the article reviews the data from early studies correlating the intravascular ultrasound images with histologic sections (in vitro) and angiography (in vivo). The article concludes with a discussion of potential clinical applications and future technical developments.

A new three-dimensional echocardiographic method of right ventricular volume measurement: In vitro validation

Right ventricular volume is difficult to measure accurately from one or two views because the complexity of right ventricular shape invalidates simplifying geometric assumptions. This article describes a new three-dimensional echocardiographic reconstruction method of right ventricular volume calculation, and reports the results of testing this method in vitro using normal animal hearts and pathologic specimens from infants and children who died with aortic stenosis or hypoplastic left heart. The correlation with reference volumes was excellent for both groups (r = 0.98, n = 25 for the animal data; r = 0.97, n = 15 for the human data). Given the calculated echocardiographic volume (Vc), the reference volume (Vr) was best estimated by the equation Vr = 1.16 Vc for the animal data and Vr = 0.92 Vc for the human data. Three-dimensional echocardiographic measurement of right ventricular volume is an accurate method that deserves further study and application in a clinical setting.

Clinical applications of intravascular ultrasound imaging in atherectomy

This paper discusses the potential application of intravascular ultrasound imaging in the context of catheter-based atherectomy. The advantages and limitations of ultrasound in this application are discussed, and representative cases are presented.

The Velocity Distribution in the Aortic Anulus in Normal Subjects: A Quantitative Analysis of Two-dimensional Doppler Flow Maps

The velocity distribution in the aortic anulus is commonly assumed to be uniform. A skewed velocity profile may have consequences for the accuracy of volume flow estimates by the Doppler echocardiographic technique. To assess this issue, the velocity distribution in the aortic anulus in 12 normal subjects was studied by computer analysis of digital velocity data from two-dimensional Doppler ultrasound flow maps. The velocity profiles in the aortic anulus were found to be flat but slightly skewed, with the highest velocities toward the septum. There was little interindividual variation. Our findings imply that the centerline velocity is the best estimate for the spatial mean velocity at the aortic anulus in normal subjects. The importance of this finding in patients is unknown. In normal subjects, the results suggest that stroke volume might be overestimated by approximately 15% by Doppler echocardiography if the cross-sectional velocity profile is not accounted for.

Mesenteric blood flow velocity and its relation to circulatory adaptation during the first week of life in healthy term infants.

ABSTRACT: We investigated early postnatal changes of the mesenteric circulation and its relationship to the systemic circulation in two groups of newborn infants. Group I (n = 10) was studied before the first feeding at 1 h and preprandially at 6 and 24 h. Group II (n = 10) was studied before the first feeding at 2 h of age and preprandially and postprandially at d 3, 4, and 5. Blood flow velocity was measured with ultrasound Doppler in the superior mesenteric artery (SMA), middle cerebral artery, subclavian artery, and aortic orifice for cardiac out-put (CO) calculations. Blood pressure and heart rate were monitored. SMA mean velocity (Vmean) decreased from 1 [0.33 ± 0.07 m/s (mean ± SD)] to 6 h (0.23 ± 0.08 m/s,p < 0.005) in group I, probably due to ductal steal, returning to the 1-h value at 24 h. In contrast, middle cerebral artery Vmean remained unchanged in the first 24 h. From d 3, SMA Vmean increased 92% postprandially, with no relation to increasing amounts of food. The postprandial increase in SMA Vmean was not associated with changes in CO and blood pressure; however, a fall in relative mesenteric vascu-lar resistance suggested regional redistribution of CO. Middle cerebral artery Vmean increased from h 2 to d 3 with a further increase on d 4 (p < 0.01). This increase was associated with an increase in blood pressure. The relative fraction of CO to middle cerebral artery increased during the first days of life, suggesting a redistribution of blood flow to the metabolically active organs in the neonatal period.

Analysis of backscattered ultrasound from normal and diseased arterial wall

Intra-arterial ultrasonic imaging has several features which affect the feasibility of clinical tissue characterization when compared with trans-thoracic ultrasound. The short distance from transducer to tissue, fluid path, high frequencies, and special characteristics of the tissues of interest all contribute to making practical tissue characterization by measurement of the backscattered signal more probable in intra-arterial imaging. The properties of backscattered ultrasound, and methods of characterizing such signals, are discussed with special reference to intra-arterial applications.

Cross sectional early mitral flow velocity profiles from colour Doppler.

Instantaneous cross sectional flow velocity profiles from early mitral flow in 10 healthy men were constructed by time interpolation of the velocity data from each point in sequentially delayed two dimensional digital Doppler ultrasound maps. This interpolation allows correction of the artificially produced skewness of velocities across the flow sector caused by the time taken to scan the flow sector for velocity recording of pulsatile blood flow. These results suggested that early mitral flow studied in an apical four chamber view is variably skewed both at the leaflet tips and at the annulus. The maximum flow velocity overestimated the cross sectional mean velocity at the same time by a factor of 1.2-2.2. Also the maximum time velocity integral overestimated the cross sectional mean time velocity integral to the same extent. This cross sectional skew must be taken into account when calculation of blood flow is based on recordings with pulsed wave Doppler ultrasound from a single sample volume.

Tissue characterization with intra-arterial ultrasound: special promise and problems.

Although we are able to identify many tissue types based on the screen image in intravascular ultrasound, there is additional information in the ultrasound signal which could be of assistance in characterization and identification of tissue. Intravascular ultrasound has several special characteristics which affect tissue characterization. These include the high transducer frequency, small transducers, short and relatively uniform path to the tissue, and limited tissue types to identify. These characteristics influence the results obtained by absolute backscatter, local statistics, frequency dependent backscatter, and angle dependency of backscatter. These effects are both positive and negative, and in many cases can be observed in clinical imaging. Another area of tissue characterization which can be performed with ultrasound is measurement of arterial wall elasticity. This can be of importance in the evaluation of mechanisms of dilatation, and the potential for complications.

Intravascular ultrasound imaging for guidance of atherectomy and other plaque removal techniques

Intravascular ultrasound imaging provides a direct view of atherosclerotic disease, generatingin vivo information about the depth and mechanical characteristics of plaque at any point in the vessel wall. For this reason, ultrasound has significant potential to serve as a guidance modality for catheter-based techniques designed to remove or ablate plaque. Although the current generation mechanical atherectomy, laser ablation and ultrasound pulverization techniques all have some specificity for attacking plaque as opposed to normal vessel wall, it appears that in practice all of these devices will continue to carry a risk of traumatizing or even perforating arteries. In addition, it seems highly likely that aggressive ‘debulking’ of plaque will require some type of guidance beyond angiography — a role which ultrasound is theoretically well suited to play.The purpose of this review is to consider the theoretical and practical applications of ultrasound imaging as a guide to catheter-based plaque removal and ablation techniques. Specific uses will be discussed with respect to both directional and coaxial therapeutic devices.

Significance of Automated Stenosis Detection During Quantitative Angiography Insights Gained From Intracoronary Ultrasound Imaging

BACKGROUND Automated stenosis analysis is a common feature of commercially available quantitative coronary angiography (QCA) systems, allowing automatic detection of the boundaries of the stenosis, interpolation of the expected dimensions of the coronary vessel at the point of obstruction, and angiographically derived estimation of atheromatous plaque size. However, the ultimate meaning of this type of analysis in terms of the degree of underlying atherosclerotic disease remains unclear. We investigated the relationship between stenosis analysis performed with QCA and the underlying degree of atherosclerotic disease judged by intracoronary ultrasound (ICUS) imaging. METHODS AND RESULTS In 40 coronary stenoses, automated identification of the sites of maximal luminal obstruction and the start of the stenosis was performed with QCA by use of curvature analysis of the obtained diameter function. Plaque size at these locations also was estimated with ICUS, with an additional ICUS measurement immediately proximal to the start of the stenosis. Crescentlike distribution of plaque, indicating an atheroma-free arc of the arterial wall, was recorded. At the site of the obstruction, total vessel area measured with ICUS was 16.65 +/- 4.04 mm2, whereas an equivalent measurement obtained from QCA-interpolated reference dimensions was 7.48 +/- 3.30 mm2 (P = .0001). Plaque area derived from QCA data was significantly less than that calculated from ICUS (6.32 +/- 3.21 and 13.29 +/- 4.22 mm2, respectively; mean difference, 6.92 +/- 4.43 mm2; P = .0001). At the start of the stenosis identified by automated analysis, ICUS plaque area was 9.38 +/- 3.17 mm2, and total vessel area was 18.77 +/- 5.19 mm2 (50 +/- 11% total vessel area stenosis). The arterial wall presented a disease-free segment in 28 proximal locations (70%) but in only 5 sites (12%) corresponding to the start of the stenosis and none at the obstruction (P = .0001). At the site of obstruction, all vessels showed a complete absence of a disease-free segment, and the atheroma presented a cufflike or all-around distribution with a variable degree of eccentricity. CONCLUSIONS At the site of maximal obstruction, QCA underestimated plaque size as measured with ICUS. Atherosclerotic disease was consistently present at the start of the stenosis and was used as a reference site by automated stenosis analysis. At the start of the stenosis, ICUS demonstrated a mean 50 +/- 11% total vessel area stenosis, with a characteristic loss of disease-free arcs of arterial wall present in proximal locations. Thus, the site identified by automated stenosis analysis as the start of the stenosis does not represent a disease-free site but rather the place where compensatory vessel enlargement fails to preserve luminal dimensions, a phenomenon that seems related to the observed loss of a remnant arc of normal arterial wall.