William McDicken

Color Doppler Myocardial Imaging: A New Technique for the Assessment of Myocardial function

Color Doppler myocardial imaging is a new technique that has been developed specifically to allow color Doppler imaging of myocardial wall motion rather than blood pool imaging. Such a technique has the potential to interrogate velocities, accelerations, and Doppler signal strength within the myocardial wall. Moreover, the concomitant enhancement of the myocardial Doppler signal after an intravenous injection of a transpulmonary echocardiographic contrast agent could permit the noninvasive assessment of regional myocardial perfusion. Thus this new imaging modality could be a valuable adjunct to the ultrasound assessment of myocardial ischemia.

Colour Doppler velocity imaging of the myocardium

A technique has been developed for producing images of the velocity of tissue motion within the myocardium. It has been demonstrated that Colour Flow Doppler imagers can be operated to depict the velocities within the myocardium rather than moving blood in the cardiac chambers. The technique exhibits the normal advantages of diagnostic ultrasound, i.e., real-time imaging with relatively inexpensive equipment and no hazard to the patient. Further work requires to be done to determine the optimum signal processing algorithms for moving tissue echoes and to ascertain whether the technique is of value in clinical applications.

Myocardial velocity gradients detected by Doppler imaging

Using a scanner whose colour Doppler mode has been adapted to display tissue motion (instead of blood flow), velocity gradients have been detected across the myocardium. A velocity gradient is a gradual spatial change in the value of velocity estimates. Velocity gradients have potential for assessing regional myocardial contractility. 28 M-mode scans were performed on nine normal volunteers at different locations in the left-ventricle posterior wall. In each case simultaneous Doppler M-mode and pulse-echo M-mode images were obtained. Doppler velocity gradient (DVG) was calculated from Doppler M-mode images and rate of change of wall thickness (RCWT) was calculated from pulse-echo M-mode images. In all Doppler M-mode images statistically significant velocity gradients were observed. In all but one scan, cyclically consistent peaks in DVG occur relative to the electrocardiogram waveform. 99% of systolic and 89% of early diastolic peaks in RCWT have a corresponding peak in DVG. Velocity gradients are consistent with wall thickness changes, suggesting that they have potential for assessment of myocardial contractility.

Relationships of human antroduodenal motility and transpyloric fluid movement: non-invasive observations with real-time ultrasound.

To study the relationships between gastric antral and proximal duodenal motor activity, and the movement of liquid across the pylorus, 10 healthy volunteers were given a test meal of dilute orange juice and bran, and events at the gastric outlet monitored by real-time ultrasound. A total of 116 complete gastric peristaltic cycles were observed and in 86% of these, associated proximal duodenal contractions were seen. Transpyloric fluid movement, as reflected by the movement of the bran particles, occurred as brief episodes during the time when the pylorus was open. Distal flow, in episodes lasting 2-4 seconds, was seen to occur in 81% of the 116 complete cycles and 75% of these episodes occurred just after the relaxation of the terminal antrum, pylorus, and proximal duodenum. The remainder occurred shortly before the terminal antral contraction. Retrograde flow, in episodes of up to 5 seconds, occurred in 78% of observed cycles with the majority occurring immediately before contraction of the terminal antrum. Our findings indicate that transpyloric fluid movement occurs in brief episodes lasting a few seconds only and that retrograde flow across the pylorus occurs in normal subjects. This pattern of fluid movement can bear no direct relationship to a steadily advancing antral peristaltic contraction, nor be wholly attributable to constant intragastric pressure.

Assessment of colour doppler tissue imaging using test-phantoms

An investigation has been carried out on the velocity resolution, spatial resolution and accuracy of Doppler images as part of a study into the Doppler display of cardiac tissue motion. Test-phantoms were designed to perform this work and images were captured on a computer. The characteristics of the phantom images and of the image capture process were studied. The smallest spatial detail that was observed in the Doppler image was 3 mm by 3 mm. Doppler receive gain and Doppler ensemble size both affected velocity resolution. Different target materials gave different measures for velocity resolution. This could be related to the different back-scatter intensities of the materials.

Dynamic imaging of the stomach by real-time ultrasound--a method for the study of gastric motility.

The use of real-time ultrasonic imaging of the stomach for the study of gastric contractions in response to a liquid test meal is described. Gastric contractions in the pyloric antrum and distal body of the stomach were observed on closed circuit television, recorded on to cassette tape and also imaged on polaroid and ciné film. Gastric contractions were recorded from the pyloric antrum by longitudinal scanning in the lower epigastrium and reproducible motility tracings were obtained on a fibreoptic chart recorded. Intravenous metoclopramide enhanced the magnitude and frequency of antral movement, which was abolished by intravenous propantheline. Real-time ultrasonic imaging permits the non-invasive study of gastric contractions. It is safe, may be repeated as required, and provides a method for the study of the effect of drugs and disease states on gastric motility.

The behaviour of individual contrast agent microbubbles

In recent years, our knowledge of the behaviour of ultrasonic microbubble contrast agents has improved substantially through in vitro experiments. However, there has been a tendency to use high concentrations of contrast agents in suspension, so that ultrasonic backscatter data are generated by a cloud of microbubbles. Such experiments involve a variety of assumptions with validity that is open to question. In addition, high concentrations of microbubbles cannot be used to understand the behaviour of individual microbubble scatterers. This paper proposes a technique that minimises the number of assumptions that need to be made to interpret in vitro experimental data. The basis of the technique is a dilute suspension of microbubbles that makes single scattering events distinguishable. A commercial scanner was used to collect radio frequency (RF) data from suspensions of two different contrast agents, Quantison and Definity. Backscatter data were collected over a range of acoustic pressures. It was found that Definity provided a constant number of scattering events per unit volume of suspension for almost all applied acoustic pressures. Quantison demonstrated an increasing number of scattering events per unit volume with increasing acoustic pressure. Below 0.6 MPa, Quantison scatterers were not individually detectable and provided levels of backscatter similar to those of a blood-mimicking fluid, which suggests that Quantison microbubbles had almost linear scattering behaviour. At acoustic pressures greater than 0.6 MPa, both agents appeared to provide echoes from free bubbles. The change in the number of scatterers per unit volume with acoustic pressure cannot be demonstrated using high concentrations of contrast agent.

In vitro acoustic characterisation of four intravenous ultrasonic contrast agents at 30 MHz

The acoustic properties of four ultrasonic contrast agents (Optison, Definity, SonoVue and Sonazoid) were studied at 30 MHz using a Boston Scientific ClearView Ultra intravascular ultrasound (US) scanner modified to allow access to the unprocessed US data. A range of contrast agent concentrations were studied using either saline or glucose as the diluent of choice. Mean backscatter power was measured over regions-of-interest (ROI) at distances of 1, 1.5, 2, 3, 4 and 5 mm from the centre of the intravascular probe and normalised to the US data collected from a standard glass reflector. For all of the agents, the mean backscatter power at 30 MHz varied in a linear manner with concentration between 0.01 million microbubbles/mL and 1 million microbubbles/mL. Furthermore, for two of the agents, mean backscatter enhancement was detectable at concentrations as low as 2 microbubbles/sample volume.

A computer controlled flow phantom for generation of physiological Doppler waveforms

A flow phantom for the generation of physiological Doppler waveforms is described. The suspension of scattering particles is driven by a gear pump powered by a stepping motor. The speed of the stepping motor is controlled by a BBC microcomputer. The waveform shape is selected from a library of waveforms from disc. Use of the microcomputer allows the waveform shape and mean flow to be easily changed. Sephadex particles suspended in a solution of glycerol were used as artificial blood. Thin walled heat shrink tubing which had been moulded around metal rods was used. Distortions in the waveforms caused by reflections from the end of the tubing were largely removed by reducing the pipe diameter to half of its value for 30 cm from the end of the pipe. There was good agreement between the control waveforms and the Doppler waveforms over a wide range of waveform pulsatility.

Quantification of microbubble destruction of three fluorocarbon-filled ultrasonic contrast agents

The assessment of myocardial blood velocity using ultrasonic contrast agents is based on the premise that the vast majority of contrast microbubbles within a myocardial region can be destroyed by an acoustic pulse of sufficient magnitude. Determination of the period of time after destruction that a region of myocardium needs to reperfuse may be used to assess myocardial blood velocity. In this study, we investigated the acoustic pressure sensitivity of three solutions of intravenous fluorocarbon-filled contrast agents and the magnitude of acoustic pulse required to destroy the contrast agent microbubbles. A novel tissue-mimicking phantom was designed and manufactured to investigate the relationships between mean integrated backscatter, incident acoustic pressure and number of frames of insonation for three fluorocarbon-filled contrast agents (Definity(R), Optison(R), and Sonazoid(R), formerly NC100100). Using a routine clinical ultrasound (US) scanner (Acuson XP-10), modified to allow access to the unprocessed US data, the contrast agents were scanned at the four acoustic output powers. All three agents initially demonstrated a linear relationship between mean integrated backscatter and number of frames of insonation. For all three agents, mean integrated backscatter decreased more rapidly at higher acoustic pressures, suggesting a more rapid destruction of the microbubbles. In spite of the fact that there was no movement of microbubbles into or out of the beam, only the results from Definity(R) suggested that a complete destruction of the contrast agent microbubbles had occurred within the total duration of insonation in this study.