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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). 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. | |||
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. | ||
Pulsed wave (PW) Doppler requires only one crystal. It alternates between transmitting and receiving data. The transducer “listens” for the data at a certain time only, since the sampling volume is coming from the location that is selected by the sonographer (i.e., the velocity at the LVOT or at the tips of the mitral valve). This is called range resolution. The major disadvantage of PW Doppler is aliasing. In PW mode, the transducer has to sample a certain frequency at least twice to resolve it with certainty. This put a limit on the max velocity that it can resolve with accuracy. 2 x Doppler frequency (Nyquist) = PRF. If the velocity is greater than the sampling rate / 2, aliasing is produced. The following maneuvers can be performed to eliminate aliasing: change the Nyquist limit (change the scale), select a lower frequency transducer, select a view with a shallower sample volume. | '''Pulsed wave''' (PW) Doppler requires only one crystal. It alternates between transmitting and receiving data. The transducer “listens” for the data at a certain time only, since the sampling volume is coming from the location that is selected by the sonographer (i.e., the velocity at the LVOT or at the tips of the mitral valve). This is called range resolution. The major disadvantage of PW Doppler is aliasing. In PW mode, the transducer has to sample a certain frequency at least twice to resolve it with certainty. This put a limit on the max velocity that it can resolve with accuracy. 2 x Doppler frequency (Nyquist) = PRF. If the velocity is greater than the sampling rate / 2, aliasing is produced. The following maneuvers can be performed to eliminate aliasing: change the Nyquist limit (change the scale), select a lower frequency transducer, select a view with a shallower sample volume. | ||
Imaging and PW Doppler can be achieved with a single crystal transducer (both are created using pulsed ultrasound). With 2D imaging, one uses high frequencies and the incidence is usually at 90 degrees. With PW Doppler, one uses lower frequency and the incidence is usually at 0 degrees for optimal data. | Imaging and PW Doppler can be achieved with a single crystal transducer (both are created using pulsed ultrasound). With 2D imaging, one uses high frequencies and the incidence is usually at 90 degrees. With PW Doppler, one uses lower frequency and the incidence is usually at 0 degrees for optimal data. | ||
Color Flow Doppler uses pulsed Doppler technique. The velocity data is encoded in color, and it reports mean velocities. Since it is a pulsed Doppler technique, it is subject to range resolution and aliasing. Color data is extremely complex and consumes significant computational resources, thus several assumptions are made to speed up this process. Returned echo frequencies are compared to a predetermined threshold to decide whether this is a 2D image vs Doppler shift. Once the computer decides that the frequency is low enough to be a Doppler shift data, repetitive sampling determines the mean velocity and variance. Then a color is assigned using a color look-up table rather than doing a discrete Fourier transform for each data point. Velocities that move toward the transducer are encoded in red, velocities that move away are encoded in blue. One must remember that the color jets on echo are not equal to the regurgitant flow for a number of reasons. The regurgitant flow is a three dimensional structure with jet momentum being the primary determinant of jet size. This parameter is effected by the jet velocity as well as flow rate. Blood pressure will affect the velocity and thus the regurgitant flow. Chamber constraints will have an effect on the appearance of the color jet, especially eccentric jets. Lastly, the settings of the echo machine will have an effect on how the color flow jet appears on the screen. | Color Flow Doppler uses pulsed Doppler technique. The velocity data is encoded in color, and it reports mean velocities. Since it is a pulsed Doppler technique, it is subject to range resolution and aliasing. Color data is extremely complex and consumes significant computational resources, thus several assumptions are made to speed up this process. Returned echo frequencies are compared to a predetermined threshold to decide whether this is a 2D image vs Doppler shift. Once the computer decides that the frequency is low enough to be a Doppler shift data, repetitive sampling determines the mean velocity and variance. Then a color is assigned using a color look-up table rather than doing a discrete Fourier transform for each data point. Velocities that move toward the transducer are encoded in red, velocities that move away are encoded in blue. One must remember that the color jets on echo are not equal to the regurgitant flow for a number of reasons. The regurgitant flow is a three dimensional structure with jet momentum being the primary determinant of jet size. This parameter is effected by the jet velocity as well as flow rate. Blood pressure will affect the velocity and thus the regurgitant flow. Chamber constraints will have an effect on the appearance of the color jet, especially eccentric jets. Lastly, the settings of the echo machine will have an effect on how the color flow jet appears on the screen. | ||