The principal modes of ultrasound in echocardiography are:
This is the default mode that comes on when any ultrasound / echo machine is turned on. It is a 2 dimensional cross sectional view of the underlying structures and is made up of numerous B-mode (brightness mode) scan lines. This is the most intuitive of all modes to understand. The field of view is the portion of the organs or tissues that are intersected by the scanning plane. Depending on the probe used, the shape of this field could be a sector - commonly seen with Echo and abdominal ultrasound probes or rectangular or trapezoid - seen with superficial or vascular probes.
Multiple images of the field or frames are generated every second on the screen, giving an illusion of movement. A frame rate of at least 20 frames per second is needed to give a realistic illusion of motion.
On a grey scale, high reflectivity (bone) is white; low reflectivity (muscle) is grey and no reflection (water) is black. Deeper structures are displayed on the lower part of the screen and superficial structures on the upper part.
The main uses for 2-D mode are to measure cardiac chamber dimensions, assess valvular structure & function, estimate global & segmental ventricular systolic function, and improve accuracy of interpretation of Doppler modalities.
While this mode is useful to accurately represent the 2- dimensional structure of the underlying tissues, it does not resolve rapid movements well and may misrepresent 3-dimensional nature of structures.
If you would like some more information about 2D, the section on 2-Dimensional imaging at http://folk.ntnu.no/stoylen/strainrate/Ultrasound/ is very informative with excellent graphical explanations.
This represents movement of structures over time. Initially a 2-D image is acquired and a single scan line is placed along the area of interest. The M-mode will then show how the structures intersected by that line move toward or away from the probe over time.
The M-mode has good temporal resolution, so it is useful in detecting and recording rapid movements. We can also correlate and time events with ECG or respiratory pressure waveforms traced alongside the M-mode tracings. The M-mode is commonly used for measuring chamber dimensions and calculating fractional shortening and ejection fraction.
Colour flow Doppler imaging (CFI)
In this mode, the velocity and direction of blood flows are depicted in a color map superimposed on the 2-D image.
It uses pulse wave Doppler signals to derive this image. This is usually done with lower frequency ultrasound waves and hence the resolution of the 2-D image deteriorates in this mode. As it takes many pulses in each scan line to derive the color image, the frame rate is reduced compared to 2-D mode. Reducing the depth and size of the color box and reducing the scanning sector width can compensate for this.
Although it can be changed, by convention, blood flowing away from the probe is depicted in blue and that flowing toward the probe in red. (BART: blue away, red toward). Blood flowing perpendicular to the scanning plane will appear black. Areas of turbulent flow may be depicted in green or white.
Color flow imaging tells us about intra-cardiac blood flows in relation to the anatomy. Hence it is useful in visualizing and semi-quantitatively assessing regurgitant jets and other abnormal flows. It can also be used to guide the accurate placement of the cursor for pulse and continuous wave Dopplers.
In addition to the poor 2D resolution, the reduced frame rate also reduces temporal resolution. Estimates of velocity and direction of blood flow are not as accurate as in Continuous wave or pulse wave Dopplers.
Pulse wave Doppler (PWD)
This is a pulsed Doppler technique in which the Doppler signal arising from a specific position in the scanned tissue is analyzed to depict velocity and direction of flow.
The transducer crystal transmits the ultrasound and receives it after a preset delay. This allows it to precisely localize the site of origin of a velocity signal. For this, a cursor or 'sample volume' is placed over the 2-D image at the region of interest.
The PWD also gives us information about the nature of flow-laminar or turbulent. In laminar flows, since most of the RBCs are traveling at the same velocity, the Doppler waveform has a thick white edge but is black within. In turbulent flow - e.g. across a stenotic valve, there is a wide distribution of RBC velocities and the Doppler signal appears filled-in. This is known as spectral broadening.
A key disadvantage with PWD is the inability to measure high velocities accurately. High velocities result in a phenomenon called 'aliasing'. This causes the velocity waveform to wrap around both sides of the baseline. Direction and velocity information cannot be interpreted for an aliased waveform.
Aliasing usually sets in when the Doppler shift being measured exceeds one-half of the pulse repetition frequency (PRF). At usual settings, this is seen to happen above a velocity of 2m/sec.
To minimize the possibility of aliasing, the following can be done:
Steps 3 and 4 may not be possible on all machines. If the velocities are still too high, use continuous wave Doppler.
PWD is used to analyze Mitral and Tricuspid inflow patterns, measure velocities of flow at the left ventricular outflow tract (LVOT) and pulmonary and hepatic venous flow patterns.
Continuous Wave Doppler (CWD)
In this mode, a part of the transducer is continuously transmitting and a part of the transducer is continuously receiving the Doppler signal along a single line that is placed on the 2-D image. This method gives very good resolution of high velocities, but it does not give any information about the location of the signal, which may originate anywhere along the preset line of the ultrasound beam. As it measures velocities along the entire line, there will be a range of RBC velocities and the Doppler waveform is normally filled-in in contrast to the PWD.
CWD is used to measure velocities of Tricuspid, Pulmonary, Mitral and Aortic regurgitation, and velocity of systolic flow through the aortic valve.
This mode is similar to the Pulsed Wave Doppler except that it is used to measure velocities of tissue movement, which are much lower than blood velocities. The cursor or sample volume is placed on the 2-D image over the tissue of interest and the Doppler waveforms are acquired. The machine filters out the high velocities and displays a waveform that is very similar in appearance to the PWD waveform.
Tissue Doppler is used to measure tricuspid and mitral annulus velocities to assess RV systolic function and estimate LA pressure.
Tissue Harmonic Imaging
In this modality, the transducer looks for reflected echoes at twice the frequency of that which was emitted. This results in darker cavities and brighter walls leading to better endocardial definition, better resolution even at greater depths and reduced near field clutter.
It is better to leave this mode on at all times throughout the echocardiographic examination. Other modes can be used concomitantly with this.