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The principle of ultrasound: Difference between revisions
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'''Second Harmonic''' is an important concept that is used today for image production. The basis for this is that fact that as ultrasound travels through tissue, it has a non-linear behavior and some of its energy is converted to frequency that is doubled (or second harmonic) from the initial frequency that is used (or fundamental frequency). There are several parameters that make second harmonic imaging preferential. Since it is produced by the tissue, the deeper the target the more second harmonic frequency is returned. As the ultrasound beam travels through tissue, new frequencies appear that can be interrogated. Second harmonic data gets less distortion, thus it produces better picture. Also, the second harmonic is strongest in the center of the beam, thus it has less side lobe artifacts. At the chest wall the fundamental frequency gets the worst hit due to issues that we have discussed (reflection, attenuation) – if one can eliminate the fundamental frequency data then these artifacts will not be processed. One concept of eliminating fundamental frequency data is called pulse inversion technology. The transducer sends out 2 fundamental frequency pulses of the same amplitude but of different phase. As these pulses are reflected back to the transducer, because of the different phase they cancel each other out (destructive interference) and what is left is the second harmonic frequency data which is selectively amplified and used to generate an image. | '''Second Harmonic''' is an important concept that is used today for image production. The basis for this is that fact that as ultrasound travels through tissue, it has a non-linear behavior and some of its energy is converted to frequency that is doubled (or second harmonic) from the initial frequency that is used (or fundamental frequency). | ||
[[File:PhysicsUltrasound_Fig30.svg|thumb|left|400px| Fig. 30]] | |||
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There are several parameters that make second harmonic imaging preferential. Since it is produced by the tissue, the deeper the target the more second harmonic frequency is returned. As the ultrasound beam travels through tissue, new frequencies appear that can be interrogated. Second harmonic data gets less distortion, thus it produces better picture. Also, the second harmonic is strongest in the center of the beam, thus it has less side lobe artifacts. At the chest wall the fundamental frequency gets the worst hit due to issues that we have discussed (reflection, attenuation) – if one can eliminate the fundamental frequency data then these artifacts will not be processed. One concept of eliminating fundamental frequency data is called pulse inversion technology. The transducer sends out 2 fundamental frequency pulses of the same amplitude but of different phase. As these pulses are reflected back to the transducer, because of the different phase they cancel each other out (destructive interference) and what is left is the second harmonic frequency data which is selectively amplified and used to generate an image. | |||
'''Doppler Effect''' is change in frequency of sound as a result of motion between the source of ultrasound and the receiver. Greater velocity creates a larger shift in ultrasound frequency. 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. | '''Doppler Effect''' is change in frequency of sound as a result of motion between the source of ultrasound and the receiver. Greater velocity creates a larger shift in ultrasound frequency. 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. | ||