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| Amplitude. The amplitude of reflected ultrasound is displayed on an oscilloscope screen. It is just of historical importance. The A-mode is now used only in ophthalmology. |
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| Motion. It reflects a motion of the heart structures over time. Nowadays, the integration of 2D and M-mode images is possible. Due to its excellent temporal resolution (high sampling rate), M-mode is extremely valuable for accurate evaluation of rapid movements. |
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| Brightness. This is now the essential imaging modality in the diagnostic ultrasound. An amplitude of the reflected ultrasound signals is converted into a gray scale image. Owing to the wide gray scale (most of the ultrasound machines use 256 shades of gray) even very small differences in echogenicity are possible to visualize. |
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| Doppler. This imaging mode is based on the Doppler effect, ie. change in frequency (Doppler shift) caused by the reciprocal movement of the sound generator and the observer. Diagnostic ultrasound uses the change in frequency of ultrasound signal backscattered from red blood cells. The frequency of the reflected ultrasound wave increases or decreases according to the direction of blood flow in relation to the transducer. |
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Definition
| Doppler Duplex technique is based on the simultaneous B-mode real-time and Doppler imaging. A gray scale display serves for the localization of flow measurement site. Both spectral Doppler techniques (Continuous-Wave and Pulsed) can be used in Doppler Duplex imaging. |
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Definition
| This imaging mode requires two piezoelectric crystals. One continuously transmits and the other one receives the Doppler signals along the scan-line. CWD is very useful in high velocity signals recording. It defines blood flow direction but has no value in Doppler signal source identification. |
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Definition
| The same piezoelectric crystal transmits and receives the ultrasound signal. The reflected signal returns to the transducer after a definite delay that is known as pulse repetition frequency (PRF). PWD allows measurement from a small, specific blood volume (depth of interest), which is defined by a sample volume. It provides both blood flow direction and precise determination of of Doppler signal source. The main limitation is failure to display the high velocity signals. The maximum detectable frequency shift (the Nyquist limit) is determined by the value of half the PRF. After the velocity exceeds the limit, aliasing occurs. |
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Definition
| Color Doppler imaging is based on the PWD assumption. However, in Color Doppler imaging, multiple sample volume are evaluated along each scanning line. Velocities are displayed using a color scale. Velocities toward the transducer are red, and velocities away from the transducer are blue. |
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Definition
| Power Doppler is a variant of Color Doppler technique which displays the magnitude of the Doppler signal rather than the Doppler frequency. It is often used to icrease sensitivity to low flows and velocities. Power Doppler does not display flow direction or different velocities. |
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