A simplified approach for real-time detection of arterial wall velocity and distension

Assessing the blood pressure waveform of the carotid artery using an ultrasound image processing method

PURPOSE

The aim of this study was to introduce and implement a noninvasive method to derive the carotid artery pressure waveform directly by processing diagnostic sonograms of the carotid artery.

METHODS

Ultrasound image sequences of 20 healthy male subjects (age, 36±9 years) were recorded during three cardiac cycles. The internal diameter and blood velocity waveforms were extracted from consecutive sonograms over the cardiac cycles by using custom analysis programs written in MATLAB. Finally, the application of a mathematical equation resulted in time changes of the arterial pressure. The resulting pressures were calibrated using the mean and the diastolic pressure of the radial artery.

RESULTS

A good correlation was found between the mean carotid blood pressure obtained from the ultrasound image processing and the mean radial blood pressure obtained using a standard digital sphygmomanometer (R=0.91). The mean absolute difference between the carotid calibrated pulse pressures and those measured clinically was -1.333±6.548 mm Hg.

CONCLUSION

The results of this study suggest that consecutive sonograms of the carotid artery can be used for estimating a blood pressure waveform. We believe that our results promote a noninvasive technique for clinical applications that overcomes the reproducibility problems of common carotid artery tonometry with technical and anatomical causes.

Vibration mode imaging

A new method for imaging the vibration mode of an object is investigated. The radiation force of ultrasound is used to scan the object at a resonant frequency of the object. The vibration of the object is measured by laser and the resulting acoustic emission from the object is measured by a hydrophone. It is shown that the measured signal is proportional to the value of the mode shape at the focal point of the ultrasound beam. Experimental studies are carried out on a mechanical heart valve and arterial phantoms. The mode images on the valve are made by the hydrophone measurement and confirmed by finite-element method simulations. Compared with conventional B-scan imaging on arterial phantoms, the mode imaging can show not only the interface of the artery and the gelatin, but also the vibration modes of the artery. The images taken on the phantom surface suggest that an image of an interior artery can be made by vibration measurements on the surface of the body. However, the image of the artery can be improved if the vibration of the artery is measured directly. Imaging of the structure in the gelatin or tissue can be enhanced by small bubbles and contrast agents.

A simulation study on tissue harmonic imaging with a single-element intravascular ultrasound catheter

Recently, in vivo feasibility of tissue harmonic imaging with a mechanically rotated intravascular ultrasound (IVUS) catheter was experimentally demonstrated. To isolate the second harmonic signal content, a combination of pulse inversion and analog filtering was used. In this paper the development of a simulation tool to investigate nonlinear IVUS beams is reported, and the influence of transducer rotation and axial catheter-to-tissue motion on the efficiency of PI signal processing is evaluated. Nonlinear beams were simulated in homogeneous tissue-mimicking media at a transmit frequency of 20 MHz, which resulted in second harmonic pressure fields at 40 MHz. The competing effects of averaging and decorrelation between neighboring rf lines on the signal-to-noise ratio (SNR) were studied for a single point scatterer. An optimal SNR was achieved when lines were combined over 3 degrees - 3.75 degrees. When the transducer was rotated with respect to point scatterers, simulating the acoustic response of tissue, the fundamental frequency suppression using PI degraded rapidly with increasing interpulse angles. The effect of axial catheter-to-tissue motion on the efficiency of pulse inversion seemed to be of less influence for realistic motion values. The results of this study will aid in the optimization of harmonic IVUS imaging systems.

Plethysmographic arterial waveform strain discrimination by Fisher's method

Plethysmography has been used for over 50 years to measure gross change in tissue blood volume. Over the cardiac cycle, perfused tissue initially expands as the blood flow into the arterioles exceeds the flow through the capillary bed. Later in the cardiac cycle, the accumulated blood drains into the venous vasculature, allowing the tissue to return to its presystolic blood volume. Specific features in the plethysmographic waveform can be used to identify normal and abnormal perfusion. We are developing a Doppler strain-imaging technique to measure the local pulsatile expansion and relaxation of tissue analogous to the gross measurement of tissue volume change with conventional plethysmography. A phantom has been built to generate plethysmographic-style strains with amplitudes of less than 0.1% in a tissue-mimicking material. With Fisher's discriminant analysis, it is shown that normal and abnormal plethysmographic-style strains can be differentiated with high sensitivities using the Fourier components of the strain waveforms normalized to compensate for the variance in the strain amplitude estimate.

Volume-selective 3D turbo spin echo imaging for vascular wall imaging and distensibility measurement

PURPOSE

To use a volume-selective 3D turbo spin echo (TSE) technique to image the carotid artery wall and measure distensibility.

MATERIALS AND METHODS

A high-resolution volume-selective 3D TSE sequence has been developed. Volume selection is accomplished by orthogonal gradients for the 90 degrees and 180 degrees excitations and allows a 3D volume of vessel wall to be imaged in a relatively short time. The technique has been developed to allow imaging at any defined phase of the cardiac cycle so that the vascular function and distensibility can be studied.

RESULTS

Scan efficiency is increased by the reduced phase encode field of view (FOV) (k(y) steps) by the use of selective volume excitation. Significantly more slices (k(z) phase encode steps) for vessel coverage can be acquired with the same scan time as that of the conventional sequence while maintaining signal-to-noise ratio (SNR) levels. The practical value of the technique was demonstrated on 10 normal subjects with high-resolution vessel distensibility measurements of the carotid arteries.

CONCLUSION

A volume-selective TSE method has been used for carotid artery wall imaging and measurement of distensibility in normal subjects. Larger coverage of the vessel, and therefore more information for clinical diagnostics, was achieved with the same overall scan time with an SNR comparable to that of 2D full FOV images.