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| Objectives |
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| At the completion of this chapter, the student will be able to: |
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- Discuss the Modes of Imaging
- Explain 2D Echocardiography
- Explain A/B/M Mode Echocardiography
- Discuss Color Flow Doppler
- Explain Pulse Wave Doppler
- Discuss Continuous Wave Doppler
- Describe Tissue Doppler Interrogation
- Discuss Color Tissue Doppler
- Explain Color M-Mode Doppler
- Discuss Color Kinesis
- Describe Harmonic Imaging
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| Modes |
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| Different modes
of echocardiographic scanning include: |
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- A-Mode
- B-Mode
- M-Mode
- 2D Mode
- Continuous Wave Doppler
- Pulse Wave Doppler
- Color Flow Doppler
- Color M-Mode
- Tissue Doppler Imaging
- Color Tissue Doppler
- Color Kinesis
- Harmonic Imaging
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| 2D Mode, Continuous
Wave Doppler, Pulse Wave
Doppler, and Color Flow Doppler are the most common modes used in examination
of a patient. |
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| A Mode/B
Mode/M Mode |
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A mode or Amplitude Mode displays the amplitude, as spikes, of reflected signals . The B mode displays the amplitude as brightness of the reflected objects. The higher the amplitude, the brighter the point. The M mode or Motion mode is the B mode displayed over time on a scrolling paper or a screen. The scrolling rate of the paper is 50-100 mm/sec. With these modes only a single line of sight is available. Pathology is easily missed or difficult to interpret because of the compression of a 3D moving object onto a 2 dimensional view. The sampling rate of the M-mode scan is very high, around 2000 cycles/sec whereas 2D mode is around 60 cycles/sec. This high sampling rate (2000 cycles/sec) of M Mode compared to 2D echo (30-60 cycles/sec) is preferable for measuring timing of events or viewing very rapidly moving objects.
 M-mode is useful for scanning very rapidly moving structures (i.e. vegetations). In figure 2.1.1 three specular reflectors have returned a signal of different amplitudes. The depth of the specular reflectors and the amplitude of the reflections are displayed. In figure 2.1.2, the amplitudes have been converted to a brightness. The brightness is proportional to the amplitude and plotted against the depth. In figure 2.1.3, the same specular reflectors are displayed against time. |
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Image
2.1.1 A Mode |
Image
2.1.2 B Mode |
Image
2.1.3 M Mode |
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| M-Mode |
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| In M-Mode scanning, a single beam is manually directed to various structures in the heart. M-Mode scanning has a very rapid rate, so it is able to discern rapidly moving structures that 2D Mode scanning may either miss or allow misinterpretation. Structures that are oscillating rapidly or moving rapidly are best viewed under M-Mode. Making quick measurements is easier under M-Mode scanning. LVOT and LVEDD measurements can be quickly performed with M-Mode. Since M-Mode is a single beam that is not scanning a sector, the anatomy is more difficult to interpret and unusual structures may lead to interpretation errors. Also, it suffers from one-dimensional interpretation. M-Mode does not provide information about spatial relationships with other structures in the heart, especially in the same cycle. |
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| 2D
Mode |
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2D Mode echocardiography
produces images that are similar to anatomical sections. 2D
Mode is B-Mode that is spatially arranged in depth and width.
Since 2D echocardiography has been introduced, the
interest in echocardiography has dramatically increased.
In
2D Mode the beam sweeps across the sector scan's field of
view. The number of scan lines correlates with the resolution.
A high number of scan lines has a higher resolution than a
low number of scan lines. If the number of scan lines is increased
then the time to scan the whole field of view, a frame, will
increase.
As the time to scan a whole frame increases the
frame rate decreases. If the frame rate is decreased, some
timing events may be missed or motion of some structures may
be jerky or aliased. The preferable frame rate is 30 frames
per second or 33 msec/frame and 128 scan lines per frame at
20 cm.
Most scans in 2D mode are black and white(gray scale).
The machine can be set to show the 2D mode
in a color. An advantage of showing 2D echocardiography
scans in color is the increased resolution
that the human eye visualizes because of a
large density of cones in the retina, compared
to the density of the rods. Color selection
is a personal preference. Compare the scans
of the black and white scan and the scan in
sepia color. Color scans should allow you
to have a clearer visual experience. |
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Some echocardiography machines have the ability to follow speckles in the 2D scan. Speckle Tracking Echocardiography (STE) is a 2D mode that can compute strain and strain rate of the myocardium. This advanced technology is able to show the myocardial function in a graphic format rather than relying on visualizaiton of wall motion by 2D echocardiography. Also, Speckle Tracking Echocardiography can calculate not only the amount of motion (excursion) but the rate of motion (rate) of the myocardium.
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Doppler Modes
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| Doppler echocardiography is a section of echocardiography that measures
flow or motion rather than imaging cardiac structures.
A scan that shows the cardiac structure while
using Doppler is called a duplex scan. The
types of Doppler modes include continuous
wave Doppler (CWD), pulse
wave Doppler (PWD), Color Flow Doppler (CFD) and Tissue Doppler Imaging (TDI). CWD and PWD measure the velocity of the flow of the blood whereas TDI is measuring the velocity of myocardial structures. The modes are
based upon the Doppler effect and the Doppler
equation. Doppler modes also have their own
settings, although some of the settings may
overlap with 2D echo scans. Image 2.1.8, a duplex
scan, shows a pulse wave Doppler of the descending
aorta and a small view of the Doppler scan
line. The probe is emitting and receiving
ultrasound from the top of the sector scan. |
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| Doppler
Shift or Doppler Effect |
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| The Doppler effect or Doppler
shift is the shift in the transmitted frequency to the
received frequency. As objects (i.e. RBCs) move towards the
transmitting focus (echo probe) the transmitted frequency is
returned (scattered) back towards the echo probe at a higher
frequency. If the flow was away from the echo probe the reflected
frequency would be less than the transmitted frequency. The
velocity of the blood cells determines the amount of frequency shift. Therefore, if the transmitted frequency is known
and the scattered (received) frequency is known, then the velocity
can be calculated using the Doppler equation. The formula for
the change in frequency is: |
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Δf = Fs - Ft |
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Image
2.1.9 Doppler Shift Formula |
Doppler Shift Animation 2.1.10 |
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| Animation 2.1.10 shows red blood cells moving in
the blood vessel. As the ultrasound
wave reflects off of the moving red blood
cells, the amount of frequency shift is displayed
on the screen. The frequency
shift can be transformed into a velocity for
further calculations. |
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| The Δf
is the change in frequency, Fs is the received (scattered) frequency, and Ft is the transmitted frequency. Since the transmitted
frequency and the received frequency are known the
velocity can be calculated by the formula: |
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Image
2.1.11 Doppler Equation |
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Image2.1.12 Doppler Equation Image |
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| In the formula, v is the calculated velocity,
c is the speed of sound (1540 m/sec), Fs is the received frequency, Ft is the transmitted frequency, cos Θ is the angle of incidence of the Doppler signal,
and 2 is the factor of the distance of the ultrasound
wave to and from the object. Of note is Θ
the angle of incidence of the Doppler signal. Below
is the cosine of some common angles of incidence. |
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| Angle (Θ) |
Cos Θ |
Measured Velocity (example) |
| 0° |
1.00 |
1.00 m/sec |
| 10° |
0.98 |
0.98 m/sec |
| 20° |
0.94 |
0.94 m/sec |
| 60° |
0.50 |
0.50 m/sec |
| 90° |
0.00 |
0.00 m/sec |
| 180° |
1.00 |
-1.00 m/sec |
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Table 2.1.1 Cosine Table |
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Bad Doppler Angle Animation
2.1.13 |
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| In Animation 2.1.13 the angle of incidence is not
zero degrees. Therefore, the calculated velocities are
less than the calculated velocities in Animation 2.1.10. An
increasing angle will decrease the returned frequency difference
and calculated velocities. |
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| An error > 6% occurs for a Doppler angle > 20 degrees. |
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| If the angle of incidence is more than 20° off then an error greater than 6% will occur. In table 6.2, if the measured velocity is 1 m/sec with an angle of 0°, the velocity measurements decreases as the angle increases. In animation 2.1.13, the angle of interrogation by the Doppler is about 45°, which results in a velocity calculation significantly less than if the angle is near zero degrees (animation 2.1.10) Doppler is most accurate if the Doppler signal is parallel to the measured flow. 2D echocardiography, on the other hand, is most accurate if the imaged structure is perpendicular to the ultrasound beam. |
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| Doppler Angle |
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| As mentioned above, a 20° off angle will yield a 6% error so aligning the beam with the blood flow to be measured is important. Since the beam is measuring blood flow in a three dimensional environment, the Doppler angle can appear to be less than 20° off, but in reality, it is more than 20° off from another view. Therefore, interpretation of the whole clinical picture is required when making Doppler measurements. Usually Doppler measurements are quite accurate, but, if the measurement doesn't correlate with the clinical picture, try to rule out that the Doppler data is inaccurate data by taking multiple Doppler measurements of the same blood flow from different views. |
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Bad Doppler Angle Animation
Animation 2.1.14 |
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Doppler Angle
Image 2.1.15 |
Good Doppler Angle
Image 2.1.16 |
Bad Doppler Angle
Image 2.1.17 |
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