Page 75 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

52 BIOMECHANICS OF THE HUMAN BODY

Like any other experimental method, determination of blood flow by radioactive microspheres is sub-
ject to many sources of error, including individual variation among the sample population, the counting
accuracy of the total microspheres in the tissue sample by the gamma counter, and the effect of arteri-
ovenous shunting or the migration of the microspheres. Despite all the limitations of this method, the
microsphere technique of blood flow determination has become the most powerful method available
today and has been used to evaluate the accuracy of other techniques of blood flow measurement.
Doppler Ultrasound. Doppler ultrasound has been widely used to provide qualitative measure-
ments of the average flow velocity in large to medium-size vessels if the vessel diameter is known.
These include the extracranial circulation and peripheral limb vessels. It is also used in an assess-
ment of mapped occlusive disease of the lower extremities. The frequency used for Doppler ultra-
sound is typically between 1 and 15 MHz. The basis of this method is the Doppler shift, which is the
observed difference in frequency between sound waves that are transmitted from simple piezoelec-
tric transducers and those that are received back when both transmitter and receiver are in relative
motion. The average frequency shift of the Doppler spectrum is proportional to the average particu-
late velocity over the cross-sectional area of the sample. When used to measure blood flow, the trans-
ducers are stationary and motion is imparted by the flowing blood cells. In this event, red cell
velocity V is described by the relationship
δF/F = (2V/C) cosθ or V =δF/F (C/2cos) (2.23)

where δF = frequency change of the emitted wave
C = mean propagation velocity of ultrasound within tissues (about 1540 m/s)
θ= angle between the ultrasound beam and the flow velocity

The frequency shift is usually in the audible range and can be detected by an audible pitch variation
or can be plotted graphically.
Attenuation of ultrasound increases nearly linearly with frequency in many types of tissue, causing
high frequencies to be attenuated more strongly than low frequencies. The depth of penetration of the
signal also depends on the density of the fluid; hence sampling of the velocity profile could be inaccu-
rate in situations where this can vary. Determination of absolute flow/tissue mass with this technique
has limited potential, since vessel diameter is not accurately measured and volume flow is not
recorded. It is not possible, using currently available systems, to accurately measure the angle made by
the ultrasonic beam and the velocity vector. Thus, Doppler flow measurements are semiquantitative.

Laser Doppler Flowmetry. Laser Doppler flowmetry (LDF) offers the potential to measure flow in
small regional volumes continuously and with repetitive accuracy. It is ideally suited to measure sur-
face flow on skin or mucosa or following surgical exposure. LDF couples the Doppler principle in
detecting the frequency shift of laser light imparted by moving red blood vessels in the blood stream.
Incident light is carried to the tissue by fiber optic cables, where it is scattered by the moving red
blood cells. By sampling all reflected light, the device can calculate flux of red blood cells within
the sample volume. Depending on the light frequency, laser light penetrates tissue to a depth of less
than approximately 3 mm.
The output from LDF is measured not in easily interpretable units of flow but rather in hertz. It
would be ideal to define a single calibration factor that could be used in all tissues to convert laser
output to flow in absolute units. Unfortunately, the calibration to determine an absolute flow is
limited by the lack of a comparable standard and the lack of preset controlled conditions. This may
be due to varying tissue optical properties affected by tissue density (Obeid et al., 1990). Further,
LDF signals can be affected by movement of the probe relative to the tissue.
Despite its limitations, LDF continues to find widespread applications in areas of clinical
research because of its small probe size, high spatial and temporal resolution, and entire lack of tis-
sue contact if required. It has been suggested that LDF is well suited for comparisons of relative
changes in blood flow during different experimental conditions (Smits et al., 1986). It is especially
valuable to provide a direct measurement of cutaneous blood flow. In patients with Raynaud’s