Imaging Modalities

 
Objectives
 
At the completion of this chapter, the student will be able to: 
 
  • 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
 
Modes
 
Different modes of echocardiographic scanning include:
 
  • 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
 
2D Mode, Continuous Wave Doppler, Pulse Wave Doppler, and Color Flow Doppler are the most common modes used in examination of a patient.
 
A Mode/B Mode/M Mode
 
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.

Image 2.1.1 A Mode
Image 2.1.2 B Mode
Image 2.1.3 M Mode
M-Mode
 
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
 
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.
 
 
Doppler Modes
 
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.
Image 2.1.8 Duplex Scan
 
Doppler Shift or Doppler Effect
 
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:
 
Δf = Fs - Ft
Doppler Shift
Image 2.1.9 Doppler Shift Formula
Doppler Shift Animation 2.1.10
 
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.
 
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:
Image 2.1.11 Doppler Equation
 
Image2.1.12 Doppler Equation Image
 
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.
 
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
    Table 2.1.1 Cosine Table
Bad Doppler Angle
  Bad Doppler Angle Animation 2.1.13
 
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.
 
  An error > 6% occurs for a Doppler angle > 20 degrees.
 
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.  
 
Doppler Angle
 
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.
Bad Doppler Angle Animation
Animation 2.1.14
 
Doppler Angle Doppler Angle Doppler Angle
Doppler Angle
Image 2.1.15
Good Doppler Angle
Image 2.1.16
Bad Doppler Angle
Image 2.1.17