Laser Doppler flowmetry has found a multitude of applications for the noninvasive monitoring of blood flow. The main application of LDF is the continuous monitoring of skin blood flow changes and the assessment of the status of the microcirculation by analysing the response to a vascular occlusion [31, 34]. Among the numerous ap-plications, the technique has been used for diagnosis of peripheral arterial or venous disorders [31, 34], flaps and grafts monitoring [28, 31, 35, 36], assessment of endothelial function [34], skin diseases and tumours [28], skin ulcers and burns [28]. LDF has also been investigated for the measure of changes in blood flow directly from organs such as the brain [37,38] and the liver [39].
Although it has been widely used for monitoring blood flow in different settings, Laser Doppler flowmetry suffers from some limitations. The estimated Flux is presented in arbitrary units, it is not predictable nor reproducible [28, 29], and it has an inter- and intra-subject variability [29]. Therefore, the users should observe the results as changes from a baseline (trends) rather than absolute values of Flux [29, 36]. LDF does not provide an indication of the direction of motion [30] and there is no actual knowledge of the penetration depth [29,40]. The technique is also considerably sensitive to minimum pressures applied on the sensor [40] or to movement artefacts [28,29,40]. Finally, laser Doppler measurements do not have an absolute zero, because of the residual signal (biological zero) that is measured even after complete occlusion of arterial and venous circulation [28,29]. It was also previously reported that LDF measurements alone may not be sufficient for distinguishing between venous and arterial occlusions [36,41].
3.4 Ultrasound Doppler Flowmetry
The Doppler effect introduced in Section 3.3 is not used with laser light only, but it can be applied to ultrasound waves as well. Ultrasound Doppler flowmetry is a noninvasive technique, which applies the Doppler principle to ultrasound waves for the measurement of blood flow. Sound waves are mechanical waves that propagate trough a medium such as air, liquid or tissue, and are usually known for constructing sounds like voices or music [42]. Sound waves propagate through a medium in a certain frequency and waves propagating at a frequency between 20 Hz to 20 kHz can be detectable by the human ear [42]. Ultrasound waves used for medical application have propagating
3.4. Ultrasound Doppler Flowmetry
frequencies comprised between 2 MHz and 40 MHz [42], thus far beyond the audible range of the human ear. Ultrasound waves are used in medical applications for their ability to propagate through tissues noninvasively and for the capability of providing information from their interaction with structures within the tissues.
When ultrasound waves are emitted in tissue, they undergo reflection with stationary objects, whereas a Doppler shift in frequency is generated from the interaction with moving structures such as blood. Reflection is created when the waves travelling in the medium interact with a second medium with different acoustic characteristics [42].
Part of the original wave is reflected backwards, while a portion continues to diffuse through the second medium. This principle is used in ultrasound imaging techniques, but is rather discarded in ultrasound Doppler flowmetry, which processes the shift in frequency for assessing blood flow [42]. Similarly to laser Doppler flowmetry, the change in frequency in the detected sound wave is proportional to the velocity of the moving object and their relationship is described by Equation 3.3 [42].
∆f = fd− fe= 2fevcos θ
c (3.3)
Where ∆f is the shift in frequency, fdis the detected shifted frequency, feis the original emitted frequency, v is the velocity of the moving object, θ is the angle between the sensor and the direction of motion, and c is the speed of sound in the medium [42]. The Doppler effect of ultrasound waves for motion of blood in a vessel is also graphically depicted in Figure 3.5a. Differently from light beams, ultrasound waves are able to maintain their directionality. Therefore, changes in frequency detected yield the direc-tion of modirec-tion, with a positive shift if the object is moving towards the sensor and a negative shift in case the object is moving away [42]. Thus, the technique is able to provide information on blood flow moving towards the sensor (forward flow) and flow moving away from the sensor (reverse flow) [42]. In ultrasound Doppler flowmetry, the Doppler shift is then used to infer a flow signal (spectral Doppler) which will be proportional to the velocity and direction of blood’s motion. Figure 3.5b shows an example of ultrasound Doppler readings (spectral Doppler) acquired from the radial artery of a healthy subject. If the angle θ at which the sensor is held (with respect to the direction of blood flow) is known, the blood velocity can also be estimated from
3.4. Ultrasound Doppler Flowmetry
Figure 3.5: Doppler shift of ultrasound wave propagating in tissue and ultrasound Doppler measurement from the radial artery of a healthy subject. (a) A sensor, inclined with an angle θ, emits ultrasound waves into a tissue. The emitted ultrasound waves impinge on the moving blood in the vessel and, after experiencing Doppler shift, the reflected waves return to the sensor. (b) Ultrasound Doppler measurements acquired from the radial artery of a healthy subject. The forward flow (black trace) generates a positive Doppler shift, while a smaller reverse flow (grey trace) is noticed from the negative Doppler shift. The sensor was held opposite to the direction of blood movement as in (a).
Ultrasound Doppler flowmetry sensors have limited dimensions and they can be easily held by the examiner. The sensors usually comprise of both emitting and detecting elements. Ultrasound waves are generated and detected by plates composed of a piezo-electric material. Piezopiezo-electric materials produce mechanical vibrations proportional to the intensity and frequency of the voltage at which they are supplied and, inversely, generate a proportional voltage when mechanically strained [43]. The detecting element transforms the reflected waves into voltages in order to be processed by the instrumen-tation [42,43]. Emission and detection can be performed by the same piezoelectric plate in pulsed waves ultrasound Doppler flowmetry [42].
Ultrasound Doppler is widely used for the measurement of blood flow and velocity in arteries and veins [42], as well as for the diagnosis of vascular pathologies [42]. Hand-held and implantable Doppler sensors are often used to monitor flaps perfusion [36,44], whereas transcranial Doppler systems can be used to monitor cerebrovascular blood flow [45].