DOPPLER4 (Раздаточные материалы)

4. DOPPLER IMAGING

4. DOPPLER IMAGING

The three previous chapters have attempted to explain the basis of operation of a Doppler flowmeter and how the signals received from blood moving within a blood vessel are analysed and displayed. If such a Doppler device is linked to a position sensing arm then it is possible to produce two-dimensional images of the moving column of blood within the blood vesseI. This type of imaging system is beginning to play an important role in the clinical management of patients.

The simplest Doppler imaging system consists of a continuous wave Doppler flow detector, a storage monitor, and a probe position resolver. The position resolver is mechanically linked to the Doppler probe and delivers signals to the storage monitor. When a Doppler shift signal is detected a bright spot, corresponding to the probe position at which the signal was detected, is stored on the monitor. As the probe is moved over the skin . overlying the blood vessel an image of the projection of the blood vessel on the skin surface is generated on the screen. The limitation of this simple system is that this is the only projection that can be obtained. In order to study the three-dimensional structure of a blood vessel it is necessary to pulse the transmitted ultrasonic signal, and time-gate the received signal.

Pulsed-Doppler imaging systems, therefore, require a position resolver which combines the probe position signals, and the signals corresponding to the position of the detected flow point in front of the transducer. It is then possible to produce images in three orthogonal planes of the internal lumen of the blood vesseI. A more recent development in the held of Doppler imaging is the multichannel system, capable of detecting and measuring blood velocity at a number of positions across the lumen of the blood vessel. simultaneously. A block diagram of this system is shown in Fig. 4.1.

The MAVIS Doppler scanner is a range-gated, direction resolving system consisting of 30 range gates, operating at 5 MHz. Apart from having the capability to sample the variation in blood-velocity over the cardiac cycle in all the gates simultaneously, it is capable of producing a Doppler m…p of the distribution of moving blood inside the vessel. The Doppler probe is mechanically linked to the position resolver which has three degrees of movement (a, b, c), as shown in Fig.4.1. Potentiometers give electrical outputs proportional to the probe movement. The position of the 30 gates is indicated on a storage monitor which receives X and Y deflection signals from a position computer. This position computer receives information on the ultrasound beam position from the probe position resolver and information on the position of the flow detection points along the beam from the flow detector. The flow detector delivers a signal to the Z input of the monitor and the spot is intensified and stored. As the probe is scanned over the area of interest a flow map of the distribution of moving blood is produced. The flow detector also determines whether the direction of blood flow is towards or away from the probe so that flow in one direction only may be imaged. Usually arterial and venous flow are in the opposite direction at a particular site. This allows the separation of arterial Flow from venous flow on the display, although both can be imaged and displayed simultaneously, using a split screen technique, if required.

It is also possible to measure the depth of a blood vessel below the skin surface, by writing a spot corresponding to the end of the probe and producing either a cross-sectional or lateral scan.

To produce the three orthogonal projections of the vessel lumen, namely, the antero-posterior, lateral and cross-sectional views, it is necessary to align the probe position resolver in different ways; see Fig.4.1. For an A-P view coordinates b and c are aligned to correspond with Y and X and the 30 flow detection points are arranged to appear coincidentally on the screen. For the lateral view coordinates a and b are aligned with Y and X and the beam is scanned along the b direction. The cross-sectional image is achieved by aligning coordinate b parallel to the vessel axis, and a and c selected to correspond to coordinates Y and X respectively.

The position of the 30 gates can be altered with respect to their distance from the transducer face and also their separation from each other. The best resolution available is approximately 1 mm. The scanning technique involves the coordination of hand movements, to control the scan, vision, to see the display, and the audio analysis of Doppler-shift signals from any of the 30 gates.

The latest development in Doppler imaging systems is MAVIS C which combines the imaging capability with the ability to compute the velocity profile across the lumen of the blood vesseI, and calculate the absolute volume flow. In order to measure the blood velocity absolutely, the angle of inclination of the ultrasound beam to the direction of blood flow must be known. MAVIS C achieves this in the following way.

Two scans are made across the vessel at approximately 1 cm separation; see
Fig. 4.2. The coordinates of the centres of these images are computed from the coordinates of the points at which flow has been detected and the orientation of the line joining these two centres is calculated. The line is an estimate of the vessel axis. The orientation of the ultrasound beam in the mid-position (2) is gained from the position resolver, and combined with the previous information concerning the vessel axis. The absolute velocity measurement can be measured in this position, in each gate, and a velocity profile produced. However, because the sample volume of each gate is a finite size, and, because of the variation of velocity streamlines within the sample volume, the measured velocity is the weighted mean of the velocities of streamlines passing through the sample volume. The weighting function is the variation in sensitivity within the sample volume, and its effect is to smooth out the velocity profile. By measuring the sample volume sensitivity function, and deconvolving this with the actual measured profile, the apparent profile, that would have resulted had a smaller sample volume been used, is obtained. This results in an improvement by a factor of two in resolution. The velocity profile can be displayed in 25 ms intervals over the whole of the cardiac cycle.

Volume flow is measured by calculating the corrected velocity profile and by assuming that the particular vessel cross-section is circularly symmetric. Symmetry of the vessel cross-section can be checked using the imaging capability of the equipment. An example of the full facility of the MAVIS C Doppler imaging system is shown in Fig. 4.3. This instrument is the only commercially available system which can image blood vessels and calculate velocity profiles and volume flow, non-invasively.

The accuracy of this type of imaging system when compared with contrast arteriograms will he discussed in Chapter 5, and examples of Doppler images will be shown in Chapters 5, 6 and 8.