Fig. 3.1
(a) Schematic of ultrasound transducer types: linear array, convex linear array, and phased array. (b) Linear and curved (convex linear) transducers (e.g., SonoSite MicroMaxx) are the most common types used for regional anesthesia. Each has different beam frequencies
Linear Array
Piezoelectric elements are arranged in parallel, forming a row of rectangular elements that produces a rectangular image.
High frequency with good resolution but shallow depth penetration.
There is a one-to-one ratio of coupling between the contact (footing) area and the image size (i.e., large coupling area) which can be a disadvantage of these transducers.
This array is widely used in the pediatric population owing to its good imaging quality in superficial structures of 3–4 cm in depth.
Convex (or Curved) Arrays
These are curved linear transducers with linear arrays shaped into convex curves, covering a large surface field of view.
The advantage is the larger image field produced with less coupling to the contact (footing) area.
The main disadvantage is the nonlinear line density in the image, which can make it slightly more difficult for the beginner to comprehend the image.
The C60 probe is a low-frequency transducer (5–2 MHz), giving good penetration but poor resolution in superficial structures. Therefore, it is seldom used in children.
The C11 probe is a medium-frequency (7–4 MHz) transducer which can be used for abdominal imaging in children for ilioinguinal/iliohypogastric nerve blocks.
Phased Array (12–2 MHz)
These transducers are similar to the linear array in that they contain a row of rectangular elements, but there are fewer elements in this transducer, and the elements are smaller.
The advantage with this transducer is the large field of view produced despite the small coupling area.
The disadvantages are the small near field (see below) and the nonlinear line density.
This transducer is generally dedicated to cardiac imaging or pediatric abdominal imaging.
Broadband Transducers
These transducers cover a range of bandwidths, such as 10–5 MHz or 14–8 MHz.
They have shorter pulse length, giving higher axial resolution and a wide bandwidth, which is important for harmonic imaging (at the expense of axial resolution).
They offer an excellent resolution of superficial structures in the upper limbs and good penetration depths in the lower limbs without having to change the transducer.
3.3 Sound Wave Properties in Tissue
In order to understand and interpret the image obtained by ultrasound scanning, it is important to grasp the basic principle of the propagation of ultrasound through tissues. Table 3.1 shows the summary of the practical aspects of ultrasound imaging.
Table 3.1
Practical aspects of ultrasound wave characteristics
Frequency (Hz) |
Number of periods per second |
High frequency provides high spatial resolution for superficial structures, but poor penetration; lower frequencies are required for deeper structures |
Wavelength (mm) |
Length of one cycle in one direction of propagation of the wave |
Velocity (m/s) |
Displacement of the wave per unit of time |
Different acoustic impedances (densities) of tissue determine the velocity of ultrasound waves |
Period (s) |
Time for one complete wave cycle |
Amplitude |
Strength of the wave, calculated by the square root of the wave energy |
Amplifier gain function adjusts the strength of weak echoes to improve the signal-to-noise ratio |
Attenuation |
Ultrasound wave amplitude decreases with time as it travels through tissue |
Time gain compensation (TGC) compensates for the attenuation of the wave by increasing the amplification factor of the signal as a function of time after the initial pulse; all interfaces are then uniform in signal regardless of their depth |
Field |
Characterizes the propagation of ultrasound energy within a medium |
Near field (Fresnel region) is the non-diverging portion of the beam adjacent to the transducer face; the length is a function of transducer frequency and diameter |
Far field (Fraunhofer region) is the diverging portion of the ultrasound beam with diminishing energy causing decreases in lateral resolution (or sharpness); less divergence occurs with high-frequency, large-diameter transducers |
Focused transducers can change the field interface, i.e., focal zone, being the area of greatest image resolution |
Reflection |
Each interface (from various acoustic impedances) within the tissue reflects sound waves back to the emitting transducer; good contour definition thus results between different tissues |
Fluids allow perfect sound transmission, with no echoes, and result in a black image; tissues attenuate and disperse sound waves, resulting in homogeneous or heterogeneous appearances |
3.3.1 Speed of Sound
The speed of ultrasound through tissue depends on the properties of the tissue.
Gases have the slowest propagation speed (e.g., air = 330 m/s); liquids have an intermediate speed (e.g., water = 1,480 m/s); solids have a high propagation speed (e.g., bone = 3,400 m/s).
3.3.2 Reflection
When an ultrasound wave arrives at the interface between two different types of tissue, it is partially reflected and partially transmitted.
The intensity of the echo depends on the acoustic impedance of the two tissues. Acoustic impedance is determined by the density and stiffness of the tissue.
The acoustic impedance of soft tissues ranges between 1.3 and 1.7 × 106 kg ∙ m/s; air = 430 kg ∙ m/s; bone = 6.47 × 106 kg ∙ m/s. The greater the difference in the acoustic impedance of two tissues, the more the reflection there is (e.g., soft tissue-air interface or soft tissue-bone interface). There will be little transmission beyond these boundaries; therefore, imaging beyond these boundaries is usually impossible.
The subsequent subsection of this chapter will cover the ultrasonographic appearance of different tissues in more detail.
3.3.3 Scattering
For interfaces whose dimensions are very small, such as blood vessels, reflections are known as scattering.
Scattered waves spread in all directions and distort the resulting image.
Different types of tissues cause different degrees of scattering; hence, it is more important as a diagnostic tool.
3.3.4 Resolution
Spatial resolution describes the ability to discriminate two adjacent objects on the display.
There are two types:
Axial resolution: the ability to distinguish objects that are located parallel to each other along the beam axis or at different depths. This depends on the pulse length, which is a function of the wavelength and the number of cycles in the pulse. The higher the frequency or shorter the pulse length, the better the axial resolution is produced. This is particularly important for the pediatric population, as their structures are smaller than in adults.
Lateral resolution: the ability to distinguish objects located beside each other. This depends on the beam width and the number of transducer elements per centimeter (line density).
3.3.5 Refraction
A sound wave meeting a boundary layer at an angle will change direction as it enters the next medium; this depends on the change of velocity of propagation.
If the velocity is greater in the first medium, refraction occurs toward the perpendicular and vice versa.
3.3.6 Absorption
Absorption is the main cause of energy loss as ultrasound propagates through the tissue.
When sound waves enter a body, friction causes kinetic energy to be converted to heat energy, and the energy lost cannot be used to construct an image.
Absorption depends on the tissue and the frequencies employed. High absorption occurs in the bone; hence, an acoustic shadow is cast. The higher the frequency, the greater the damping and the less the maximum depth of penetration.
3.4 Optimization of Image Quality: Knobology
3.4.1 Frequency
Selection of a transducer with a frequency appropriate to the depth of the target nerve being examined is the first step in obtaining a good ultrasound image.
With each transducer, there is a range of frequency, and a more precise frequency can be selected by choosing the type of examination to be performed, for example, the Resolution (or nerve examination), General, or Penetration (or abdomen examination) buttons on the ultrasound machine.
3.4.2 Depth
It is important to select the appropriate depth since too much depth results in a smaller target and too shallow produces an inadequate image which may exclude important neighboring structures necessary to perform a nerve block safely.
3.4.3 Gain
By increasing the gain, the amplitude of the echoes is increased, and the image will appear brighter.
This function serves to improve the signal-to-noise ratio such that weak echoes can be detected (Fig. 3.2).
Fig. 3.2
Gain adjustments and focal zone depth. Increasing the gain will increase the amplitude of echoes and improve the signal-to-noise ratio. Gain settings that are either too high (a) or too low (b) will result in artifactual images and poor resolution between structures. The focal zone (area of greatest image resolution) is adjusted by effectively reducing the beam diameter (compare c and d)
3.4.4 Time Gain Compensation
This function offers adjustment of gain in various image depths.
This is important since the amplitude of ultrasound decreases as it propagates the tissue; therefore, by adjusting gain at different depths of the image enables a uniform display of the signal throughout the image.
3.4.5 Focal Zone
There are three parts to the ultrasound energy as it propagates the tissue:
Near field (Fresnel region)
This is adjacent to the transducer surface and is a non-diverging portion of the ultrasound beam.
Its width is the same as that of the transducer.
Its length is a function of the transducer frequency and width.
Focal zone
At this point, the sound cone is at its narrowest and is at the end of the near field and before the far field.
This area has the best lateral resolution.
Some ultrasound machines allow the focal zone to be selected, and it should lie where the target nerve is.
Far field (Fraunhofer region)
This is where the ultrasound beam begins to diverge and diminish in energy since it is absorbed by the tissue.
Lateral resolution decreases as this diverging beam progresses.Related posts:
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