A-mode, or “amplitude-mode,” provided one of the original evaluations of the human body using sound. A-mode ultrasound included an oscilloscope display for returning amplitude information and the traditional image did not exist. The peak amplitude information on the horizontal axis provided information regarding the strength or “loudness” of the wave, while the vertical axis provided reflector distance information from the transducer (Figure 3-8A). A-mode imaging is used in ophthalmology when measuring precise distances to the retina for calculating intraocular lens implantation. A-mode is generally not used in emergency ultrasound.
B-mode, or “brightness-mode,” converts the amplitude of the returning echo into a grayscale image allowing better correlation with anatomical structures. This “2D” tomographic slice is what most emergency ultrasound exams use and is referred to as B-mode imaging (Figure 3-8A).
M-mode, or “motion-mode,” permits a simultaneous display of the 2D B-mode image and a characteristic waveform (Figure 3-8B). This waveform depicts the motion or deflection of the tissue relative to the transducer on the vertical axis and represents time or changes in the cardiac cycle on the horizontal axis. M-mode technology can be of value in the emergency and acute care setting during pregnancy examinations and permits measurement and documentation of fetal cardiac activity. It can also be useful to demonstrate timing of events during changes in the cardiac cycle such as identifying right ventricular diastolic collapse secondary to cardiac tamponade. Similarly, documentation of lung sliding can also be achieved using M-mode.
Doppler is presented in a few different forms. Doppler technology relies on the interpretation of the “frequency shift” that exists between the transmitted and received Doppler signal, while the anatomy (blood within the vessel) is moving as it is imaged. For example, as a train whistle is engaged, the pedestrian at the crossing will experience an increase in the pitch (Doppler shift) of the whistle as the train approaches and a decrease in the pitch as the train continues to move away.
Doppler ultrasound technology makes use of this “frequency shift.” Sources (e.g., RBC) moving toward the receiver (transducer) produce a higher reflected frequency, while sources moving away produce a lower reflected frequency, allowing the system to display flow direction and velocity. The angle of interrogation of blood flow is a prominent factor in the quality and accuracy of the Doppler signal.
Spectral Doppler provides a characteristic waveform and allows a quantitative assessment for blood flow analysis consisting of continuous wave or pulsed wave technologies. Pulsed wave Doppler produces short bursts of sound. It uses the same crystal to generate and receive the signal. By the transducer listening at specific intervals, it can control precisely where the reflected sound is coming from and thus has “range gate resolution.” The limitation of pulsed wave techniques is that it can only display certain maximum or peak velocities (known as the Nyquist limit) before the signal will alias and become nonquantifiable. The maximum velocity it can display is limited by the transducer's Doppler frequency and the depth of the moving target. Aliasing or wraparound occurs if the velocity is too high to display. The peaks are cut off and displayed below baseline. Continuous wave Doppler uses different crystals to send and receive signals. One crystal constantly sends signals while another receives the reflected signal. Therefore, a live B-mode image is not seen on the viewing screen. Continuous wave Doppler has no depth or range gate resolution but it does not alias and can quantify much higher velocities.
Color Doppler utilizes the pulse-echo principle to generate color images. The color image is superimposed on the 2D image. The red and blue displays provide information regarding direction and mean velocity of flow. It cannot display instantaneous peak velocities and is not truly quantifiable. The color at the top of the display represents flow toward the transducer, and color at the bottom of the display represents flow away from the transducer. It is sensitive to transducer position (Figure 3-9).
Color Doppler of portal vein. Red circle emphasizes color key for direction and/or mean velocity of flow.
Whether using pulsed wave Doppler or color Doppler imaging, if the sound pulse strikes the blood vessel or blood flow at an angle over 60 degrees, it can result in inaccurate velocity readings and may even suggest that there is no flow when in fact it is due to poor Doppler angle.
Power Doppler images are based on amplitude or strength of the motion. Color maps in power Doppler are represented by one continuous color. It provides better sensitivity for slow flow or low blood volume states like ovarian or testicular torsion. It accomplishes this because it compares or averages several frames storing the accumulated flow over a number of cardiac cycles. This does take additional processing time and slows the acquisition frame rate. It is less angle dependent, but more sensitive to motion artifacts.
Each form of Doppler consists of benefits and limitations; however, its operation appears deceptively simple. A comprehensive discussion of Doppler physics and velocity measurements is beyond the scope of this chapter.