Sensors Technology Series Editor—in—Chief''s Preface
Preface
1 Biomedical Sensors： Temperature Sensor Technology
P.A.Kyriacou
2 Flow Sensors for Liquids
M.Mischi and J.A.Blom
3 Flow Sensors for Respiratory Gases
G.Kim Prisk
4 Biomedical Sensors oflonizing Radiation
Robert Speller， Alessandro Olivo， Silvia Pani， and Gary Royle
INDEX
內容試閱：
Another important advantage of PW over CW Doppler is the possibility ofdetermining the blood velocity profile.The delay between transmission and reception of a single burst is a direct indication of the depth from which the pulse has been reflected.Therefore, if the receiver decomposes the detected echoes into different time windows （referred to as gates, usually between six and thirry-rwo）, each window selects the part of the echoes that are reflected from a specific depth interval.The received bursts can be processed concurrently in different channels.The velocity profile is obtained by simply combining the output from each channel.This technique is referred to as multigate pulsed Doppler （Evans McDicken, 2000； Hedrick et al., 1995）.
2.2.3.5.Major Limitations
Both CW and PW Doppler present the problem of clutter noise.Clutter noise is mtroduced by the slow motion of tissue, which adds low-frequency components to the Doppler signal, for example, the slow expansion and contraction of vessel walls.In order to remove clutter noise, a high-pass filter is implemented in every system （Evans McDicken, 2000； Hedrick et al., 1995）.
Another major problem of Doppler ultrasound measurements is the dependency of the computed result on the angle a between the transducer axis and the blood flow.This angle is often unknown or only an estimate, therefore a substantial uncertainty is introduced into the measurement and thus into the estimated value of the absolute veloaty （and flow）.The only medical application where a can be estimated approximately is in the measurement oflaminar flow through a straight vessel.In this case, the angle can be estimated from the corresponding B-mode image of the same vessel.However, several methods have been developed to integrate a more accurate and quicker estimation of the angle a into the system （Jensen, 1996）.The current development of transducers capable ofspanning a three-dimensional （3D） space might open new possibilities for an accurate estimation of a （Ogura, Katakura, Okujima, 1997）.
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