However, once these shifted frequencies are determined, the machine ignores the frequency information instead of using only the amplitude information from these waves. Using a pulsed interrogation over a sample volume, the system determines a range of echoes with shifted frequencies due to flow. To do this, the transducer first obtains Doppler information identically to that in color Doppler. Power Doppler overlays information about the energy of returning Doppler signal instead of frequency onto a B-mode image. Although some quantitative information can be gained by strategic use of color Doppler, it is generally useful for qualitative information about a given sample volume. In the case of cardiac imaging, it is used to qualify regurgitant and turbulent flow. This shade is generally lighter for faster-moving objects and darker for slower-moving objects.Ĭolor Doppler is useful to interrogate organs for the presence or absence of blood flow and quickly investigate large areas for turbulent flow. (Figure 1) Additionally, the machine will calculate a color shade based on the mean frequency shift of an interrogated pixel, which represents the mean velocity of flow in that area. This is traditionally assigned as red for flow towards the transducer or blue for flow away from the transducer. The machine can assign different colors depending on the direction of flow, also known as phase shift. The machine then uses the frequency and amplitude of the returning echoes from pulsed interrogation of that region to display information regarding flow in the given area. In this case, a sample volume is placed over an area of interest. In the case of ultrasound, depending on the function selected, the machine can use the frequency shift to calculate changes in velocity, which are then displayed as different shades of color or plotted out in a graphical form.Ĭolor Doppler overlays information about the velocity of moving objects over a typical B-Mode image. In the case of audible sounds, such as an ambulance siren or ice cream truck music, this frequency shift is perceived as a change in pitch as the source of sound travels in relation to one’s ears. Theta = angle between flow and direction of the transmitted pulseīecause each of these elements is known except for V, the equation can be solved for the velocity of the moving reflector. This is referred to as the Doppler frequency shift and can be calculated by: As it returns to the transducer, the frequency will be altered based on the speed and direction of the traveling red blood cell. This object then reflects a portion of this back as an echo. In the case of ultrasound, the transducer transmits sound waves at a given frequency to a moving object, such as a red blood cell. In cases where the source is moving away from the listening device, the frequency will be shifted lower, and in cases where the source is moving towards the listening device, the frequency is shifted higher. The Doppler effect states that when a sonic source is moving towards or away from a stationary listening device, the relative frequency heard by the device will be shifted according to the velocity of the source. Doppler ultrasonography analyzes the frequency of the returning echo to determine relative motion. Gray-scale or Brightness Mode (B-Mode) imaging utilizes the amplitude of reflected echoes to plot information into a 2-d image. Similarly, the amplitude of the waves returning to the transducer from a particular object informs the brightness of that object as displayed by the machine. Because the average propagation velocity of sound waves in soft tissue is 1540 m/s, the machine uses the time it takes sound waves to return to the transducer to determine the depth of objects. The machine is then able to process this information into an image. This, in turn, causes an electric current which is relayed back to the machine. As the sound waves return, they interact with the crystals in the ultrasound probe, causing vibration and deformation. Higher frequencies experience greater attenuation in tissues and therefore cannot penetrate as deeply as lower frequencies.Īs the sound wave travels through tissue, some echoes are reflected towards the transducer. The loss of energy associated with this is referred to as attenuation. At these interfaces, sound can either be reflected, scattered, refracted, or absorbed. As sound waves propagate through tissue, they encounter various tissue interfaces. As a comparison, audible sound to humans ranges from 20 Hz to 20 kHz. Diagnostic sonography generally uses sound waves with a 1 to 20 MHz frequency band. These crystals vibrate and generate sound waves with specific frequencies when exposed to an electric current. Sound waves in diagnostic sonography are typically generated by piezoelectric crystals in ultrasound probes.
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