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Frederick W. Kremkau, PhD
Professor and Director
Program for Medical Ultrasound
Center for Applied Learning Wake Forest University School of Medicine
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Physicians, Sonographers and others who perform and/or interpret vascular ultrasound.
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Estimated Time for Completion: Approximately 1 hour
Date of Release and Review: November 5, 2012 and June 16, 2015
Expiration Date: June 16, 2018
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Sonography is a term combing the Latin for sound and Greek to write. Thus it is a term meaning writing with sound. In diagnostic medical sonography, short pulses of ultrasound are sent into the body by a device called a transducer (Fig 1).
Each pulse travels down a straight path, interacting with the biological structures and producing a stream of echoes that is received and displayed as a straight line of dots on the sonographic display. This line of dots is called a scan line. Approximately 100 to 250 scan lines combine to yield a gray-scale presentation of anatomy on the display of a sonographic instrument (Fig. 2).
3 can be generated in a fraction of a second. Thus, many images (called frames) can be generated per second yielding real-time sonography capable of following motion as it occurs. This motion can be the motion of the tissue (e.g. pulsating vessels walls) or the motion of the moving scan plane as the sonographer manually slides the transducer over the surface of the patient
Echo arrival time is used to determine the proper location in depth of each echo on each scan line. Echo strength is indicated by its brightness (gray level).
Sonographic displays are of two types. The rectangular image in Fig. 3 is good for superficial imaging with its wide field of view up close. The pie-slice-shaped, sector image is good for deep imaging, providing a wide field of view deep while requiring a small contact area on the surface (Fig. 4). Several 2D images can be acquired and combined into a 3D volume (Fig. 5). This stored volume can then be presented in multiple ways. Figure 6 gives an example.
Sound is a traveling (propagating) pressure variation. The number of complete variations (cycles) per second is called frequency. Frequency determines the pitch of the sound that humans hear.
Sound of frequency higher than detectable with human hearing is called ultrasound (the prefix ultra means "beyond"). The speed of sound travel is determined by the medium through which the sound is traveling, biological tissue in this case. The average speed in tissue is 1.54 mm/?s. This speed yields the round-trip travel-time rule of 13 ?s/cm. This means that if an echo arrives at the transducer 13 ?s after the pulse is emitted, the echo came from an echo-generating object 1-cm in depth from the transducer. Arrival times of 39 ?s, 65 ?s and 130 ?s correspond to depths of 3, 5 and 10 cm, respectively. For 10-cm depth imaging, an image containing 100 scan lines requires 130 × 100 = 13,000 ?s or 13 ms to acquire. Therefore, for those conditions, 1 ÷ (13 × 10-3) = 77 images per second can be generated. This is called the frame rate. Frame rate determines the ability to follow the temporal detail of moving objects which is called temporal resolution. Frame rate decreases with increasing imaging depth or increasing scan lines per frame and with multiple focuses.
The length and width of the ultrasound pulse determines the detail resolution of the image. Lateral resolution is in the direction perpendicular to the direction of sound travel and across scan lines. Focusing the pulse decreases the pulse width, improves lateral resolution (and detail resolution) and provides the best lateral resolution at the focus. Axial resolution is in the direction of sound travel and the scan lines. Increasing frequency shortens pulses and improves the axial resolution (and detail resolution), but decreases penetration. This is because attenuation (the weakening of sound as it travels) increases with frequency. Therefore, increasing frequency improves detail resolution but degrades penetration. This is an image quality/quantity tradeoff.
The transducer is held on the patient’s skin surface by the hand of the sonographer. It is composed of three primary parts (Fig. 7), elements, matching layers and damping layer. The elements are small rectangular slices of thin ceramic material. They respond to electrical pulses by vibrating, producing the outgoing ultrasound pulses.
A coupling gel is used to provide good acoustical contact of the transducer with the skin surface. The matching layers, in front of the elements, along with the coupling gel, facilitate the passage of the ultrasound pulses from the elements in the body and the passage of the returning echoes back into the elements. The coupling gel is inserted between the transducer and the patient’s skin. It removes air that would strongly reflect the ultrasound, inhibiting transmission into and out of the body. The returning echoes are converted into electric voltages that represent them in the instrument.
The sonographic instrument is organized into four primary parts (Fig. 8).
The transducer (T) is connected to the beam former. The beam former generates the electric voltage pulses that drive the transducer. The returning echo voltages from the transducer enter the beam former, are amplified and converted to digital form for processing in the signal processor. The signal processor converts the echo voltages to a simpler form and sends them on to the image processor where the scan lines form the image in image memory. Then the image is sent from memory to the flat-panel display for viewing.
In addition to anatomic imaging, ultrasound can provide blood flow information using the Doppler Effect. The Doppler Effect is a change in frequency caused by motion. The motion can be that of a sound source (e.g., a horn or siren on a vehicle sounding higher in pitch as it approaches and shifting to a lower pitch as passes), that of a listener/receiver or that of a reflector (which is a combination of receiver and source). Most applications of the Doppler Effect take advantage of the latter. In the case of diagnostic Doppler ultrasound, pulses of ultrasound that encounter flowing blood produce echoes of a differing frequency from what was transmitted. Returning echoes have a higher or lower frequency depending on whether the blood flow is approaching or receding from the transducer, respectively. The greater the flow speed, the greater the change in frequency. The difference between the transmitted and returning frequencies is called the Doppler shift. It is positive for approaching flow and negative for receding flow. For a given flow, the maximum Doppler shift occurs if the sound direction and flow are parallel. The angle between these two directions is called the Doppler angle (Fig. 9). The greater the Doppler angle, the smaller the Doppler shift. Because of this dependence, Doppler angle must be incorporated into Doppler ultrasound. The sonographer estimates the Doppler angle by placing a marker parallel to the assumed flow direction (usually parallel to the vessel walls). The instrument then calculates the flow speed from the Doppler shift and angle.
Figure 9. The Doppler angle is the angle between the ultrasound beam (shown in red) and the flow. The beam and flow are parallel here (zero Doppler angle), yielding the maximum Doppler shift. The graph shows that, as Doppler angle increases, Doppler shift decreases, as indicated by the shorter arrows. For example, at 60 degrees, the Doppler shift is half what it is at zero degrees.
Doppler shifts are presented on sonographic instruments three ways:
The color-Doppler presentation is usually observed first to get a global view and determine if there is any anatomic or flow abnormality. If the latter, then the spectral display is used to quantitatively evaluate the flow. A stenosis increases the flow speed and distal turbulent flow will often be associated with it. The abnormally high flow speed and distal turbulence (producing spectral broadening which is seen as an increase in the vertical spread of the spectral display) are observed on the spectral display. Both of these phenomena are seen in Figure 10 (lower).
In addition to evaluating flow conditions at the site of measurement, the spectral display can indicate downstream and upstream conditions. Figure 11 shows cases of high and low resistance to flow downstream. Figure 12 shows a spectral display for a case of proximal stenosis.
The previous discussion describes Doppler ultrasound working correctly. As in all diagnostic ultrasound, things can go wrong. These are called artifacts. The primary artifact in Doppler ultrasound, seen routinely, is called aliasing. In the example of Figure 13, it is seen that the systolic portions of the spectral display are chopped off and reappear incorrectly on the other side of the baseline.
Aliasing is corrected by shifting the baseline up or down, which in fact is electronic cutting-and-pasting to move the displaced portions back to where they should be (Fig. 14A), or increasing the vertical scale, which increases the pulsing rate to where aliasing is eliminated (Fig. 14B).
Sonography and Doppler ultrasound provide portable, relatively inexpensive, real-time anatomic imaging and flow detection and measurement means that are valuable in vascular diagnosis.
Reference 1. Kremkau, FW: Sonography Principles and Instruments, 8th Edition, Elsevier/Saunders, 2011.