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The principle of ultrasound: Difference between revisions
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An example of a moving object in cardiac ultrasound is red blood cells. Typical values for Doppler shift is 20 Hz to 20 kHz, thus comparing to the fundamental frequency, the Doppler shift is small. Since it “rides” on top of the much larger frequency (i.e., 5 MHz), the process of extracting this data is termed demodulation. Doppler shift = (2 x reflector speed x incident frequency x cosine (angle)) / propagation speed. There are two important concepts that must be emphasized. First, the Doppler shift is highly angle dependent. Since cosine (90) = 0 and cosine (0) = 1, then the most true velocity will be measured when the ultrasound beam is parallel to the axis of motion of the reflector. At perpendicular axis, the measured shift should be 0, however usually some velocity would be measured since not all red blood cells would be moving at 90 degree angle. The other concept is the direction of the motion of the reflector. When the reflector is moving away from the source of the ultrasound, the shift is negative, and when the reflector is moving towards the source of ultrasound the shift is positive. | An example of a moving object in cardiac ultrasound is red blood cells. Typical values for Doppler shift is 20 Hz to 20 kHz, thus comparing to the fundamental frequency, the Doppler shift is small. Since it “rides” on top of the much larger frequency (i.e., 5 MHz), the process of extracting this data is termed demodulation. Doppler shift = (2 x reflector speed x incident frequency x cosine (angle)) / propagation speed. There are two important concepts that must be emphasized. First, the Doppler shift is highly angle dependent. Since cosine (90) = 0 and cosine (0) = 1, then the most true velocity will be measured when the ultrasound beam is parallel to the axis of motion of the reflector. At perpendicular axis, the measured shift should be 0, however usually some velocity would be measured since not all red blood cells would be moving at 90 degree angle. | ||
[[File:PhysicsUltrasound_Fig33.svg|thumb|left|200px| Fig. 33]] | |||
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The other concept is the direction of the motion of the reflector. When the reflector is moving away from the source of the ultrasound, the shift is negative, and when the reflector is moving towards the source of ultrasound the shift is positive. | |||
Continuous wave (CW) Doppler required 2 separate crystals, one that constantly transmits, and one that constantly receives data. There is no damping using this mode of imaging. One can measure very high velocities (i.e., velocities of aortic stenosis or mitral regurgitation). The advantage of CW is high sensitivity and ease of detecting very small Doppler shifts. The disadvantage of CW is the fact that echos arise from the entire length of the beam and they overlap between transmit and receive beams. Thus one cannot determine where in the body the highest velocity is coming from – range ambiguity. | Continuous wave (CW) Doppler required 2 separate crystals, one that constantly transmits, and one that constantly receives data. There is no damping using this mode of imaging. One can measure very high velocities (i.e., velocities of aortic stenosis or mitral regurgitation). The advantage of CW is high sensitivity and ease of detecting very small Doppler shifts. The disadvantage of CW is the fact that echos arise from the entire length of the beam and they overlap between transmit and receive beams. Thus one cannot determine where in the body the highest velocity is coming from – range ambiguity. | ||