Ultrasound waves are mechanical vibrations – which may be generated by a piezoelectric transducer - that have frequencies above the human hearing range, i.e.
above 20 kHz. The piezoelectric transducer is a device that converts electrical energy into mechanical energy and vice versa [85]. These vibrations create high and low pressure areas which travel in a forward direction as sound. The material used in making the transducer depends mainly on two factors: sensitivity and bandwidth.
Sensitivity defines the response of the transducer to a reflected signal (echo).
Transducers with high sensitivity generate higher signal strength (amplitude) from echoes [86]. Bandwidth is the operating frequency range of the transducer. Imaging transducers require wide bandwidth to attain high spatial resolution. Transducers used for fluid velocity measurement require narrow bandwidth to increase selectivity of the desired frequencies [87]. Common materials used are crystal quartz, polymers and ceramics. Crystal quartz is characterised by high sensitivity and narrow bandwidth whereas polymers have low sensitivity and high bandwidth. Ceramic transducers fall between quartz and polymers in terms of sensitivity and bandwidth and are often used in medical applications.
The frequency range often used in medical applications is between 500 kHz and 100 MHz. When ultrasound waves (acoustic signals) travel into the tissues of the body, these waves will be absorbed, refracted, scattered or reflected. This is due to the difference of acoustic impedance between the tissues. Acoustic impedance depends on several factors such as density, speed of sound, absorption coefficient and homogeneity of the tissues. When the acoustic impedance changes at the interface between two
tissues, some of the sound waves are either reflected or refracted. Reflected signals are received by the piezoelectric transducer and interpreted to an image. The strength of the reflected signals depends on the difference between the acoustic impedance of the tissues. Signals of large amplitude are reflected when the difference between the acoustic impedance of the tissues is significant. The refracted signals might also reach another tissue interface, and some of them will be reflected and so on until they are totally attenuated as they reach deeper tissues.
The choice of ultrasound frequency range is dependent on the level of depth (penetration through tissues) and the spatial resolution required. Low-frequency signals have a longer wavelength than high-frequency signals. Low-frequency signals can penetrate tissues further; however, constructed images will have low spatial resolution.
In contrast, high-frequency signals have a short range of penetration; however, they provide higher spatial resolution. A trade-off must be made between depth and resolution in order to obtain satisfactory images of the scanned area of the body.
Usually, for non-invasive imaging and flow rate measurement, a lower frequency range is used, i.e. 1 to 5 MHz; for invasive and intravascular (within the vessel) imaging, a frequency range of 5 to 50 MHz is used and for cuff-type probes (mounted around blood vessels), a frequency range of 45 kHz and 20 MHz is utilised [85].
2.4.2 Principle of Operation
The principle of ultrasound operation is that the time taken for the transmitted ultrasound wave (pulse mode) to be sent and reflected off an object is given by
= 2 / Eq. 2-3
where is the distance between the transducer and the reflector, i.e. body tissue and is the speed of sound in the tissues of the body [85]. The speed of sound in the tissues of
the body (blood, water, muscle, etc) is approximately 1500± 100 m/s. If the transmitted signal is a tone burst (burst mode) with a frequency , the phase of the received signal , measured with respect to the transmitted signal, can also be used to measure the distance between the transducer and the reflector as shown in the expression below [85].
2 =
2 = 2 / Eq. 2-4
where is the wavelength. Eq. 2-3 or Eq. 2-4 is used to determine the distance (1D) between the transducer and the tissue. Measuring several locations simultaneously can create a 2D image of the tissue. This will be explained in detail in Section 2.4.4.
If the object (blood) is moving with respect to the transducer, the velocity of the object can be measured in pulse mode or burst mode. In pulse mode, a single-cycle pulse is transmitted followed by an ‘off’ period, whereas in burst mode, multiple-cycles are sent successively and then followed by an ‘off’ period. The governing equation for velocity measurement in the pulse mode is given below [85].
Δ = 2 Eq. 2-5
Δ is the difference in arrival times between two transducers placed on either side of the blood vessel (refer to Figure 2.8). In other words, it is the time difference between the time taken for a pulse to be sent from transducer 1 to 2 ( ) and the time taken for the pulse to be sent from transducer 2 to 1 ( ) as illustrated in Figure 2.7. Both transducers alternate in sending and receiving ultrasound waves. is the average arrival time, i.e.
( + )/2.
Figure 2.7(a) Pulse mode: difference in arrival time . (b) Burst mode: difference in phase [85]
In burst mode, the phase difference Δ between the transmitted and received signals (transducer 1 to 2 and transducer 2 to 1) is related to the velocity of the flow stream as shown below [85].
Δ
2 = 2 cos Eq. 2-6
where is the distance between both transducers, is the wavelength of the ultrasonic frequency and is the angle between the flow direction and the ultrasound beam which is known as the angle of insonation as shown in Figure 2.8. Eq. 2-5 and Eq. 2-6 are known as the transit-time mode equations for velocity measurement. The transit-time method is illustrated in Figure 2.8. This mode is only used in invasive blood flow measurement as it requires two transducers placed at opposite side of the conduit (vessel) of the moving fluid (blood). Note that for flow rate measurement, the ultrasound beam must entirely cross the blood vessel.
Figure 2.8: Transit-mode ultrasound (two methods): (a) transducers are on opposite sides (b) transducers are on the same side [88]