Peripheral Nerve Blocks: A Color Atlas, 3rd Edition

32. Principles of Sonography

Paul Bigeleisen

The frequency of medical ultrasound ranges between 2 MHz and 13 MHz. The average wave length in this band is about 1 mm. This limits the resolution to structures that are larger than 1 mm. Most nerves of interest range in size from 2 mm to 10 mm. Veins and arteries of interest are typically 3 mm to 15 mm.

Many factors contribute to the quality and resolution of the ultrasound image. In general, higher frequency probes generate higher resolution images. Unfortunately, high frequency ultrasound waves (8 MHz to 13 MHz) are rapidly attenuated in tissue so that high frequency probes are best suited for structures less than 5 cm deep to the skin.

The ultrasound beam may be refracted as it passes through tissue. When this occurs, a nerve or other organ may appear at a different anatomical location than its actual site. This is the same phenomenon responsible for the apparent bending of your forearm when you place it in a bucket of water (Fig. 32-1). Fat globules below the skin, in the muscle and around nerves are about 1 mm in diameter. These globules serve as diffraction sites for the incident and reflected ultrasound beam and cause a speckled appearance in the image (Fig. 32-2). Fat is also extremely efficient at absorbing ultrasound so that little of the beam is returned to the receiver. For these reasons, obese patients can be very difficult to image.

The image formed of a nerve on ultrasound is very sensitive to the angle of incidence of the beam relative to the nerve. Sometimes changing the angle of incidence by only a few degrees can bring the nerve into focus. This phenomenon is thought to be caused by diffraction of the type described above.

Modern platforms allow the user to adjust the brightness (gain) of the entire image or more superficial (near field) and deep (far field) structures. Increasing the gain makes the entire image whiter. Increasing the gain too much creates a snowy background whereby all of the structures become indistinguishable. In general, the gain should be set so that most of the background is black and only the structures of interest, such as nerves and vessels, are easily seen. Modern machines also allow the user to adjust the contrast. The formal term for contrast is dynamic range compression. Increasing the dynamic range compression makes the white images whiter and the black images blacker. This may bring the edges of anatomic structure into better view. Decreasing the dynamic range compression makes everything look grey.

Arteries can usually be distinguished by their pulsatile nature. Veins can be distinguished by their compressibility. Pressing on the skin with the probe will usually cause the vein to collapse. Color flow Doppler imaging can also be used to identify and distinguish arteries and veins. By convention, blood flowing toward the probe is colored red. Blood flowing away from the probe is colored blue. Blood flowing perpendicular to the probe remains black. Velocity gates can be set to measure the flow velocity. High velocities are usually arteries. Low velocities are usually veins.

Figure 32-1. Ultrasound beam refracting as it passes through tissue.

Figure 32-2. Fat globules below the skin serve as diffraction sites for the incident and reflected ultrasound beam and cause a speckled appearance in the image.

Transducer elements can be arranged in linear or curved arrays (Fig. 32-3). Linear arrays create rectangular images and are most useful for superficial structures. Curved arrays create wedge-shaped images and are most useful for deeper structures. Because the beam disperses in a curved array, its resolution is usually lower than a linear array. A phased array retains the elements in a straight line. But the elements fire in sequence creating a phase delay between each element. The net result is a wedge-shaped image from a set of linear transducers. Because this signal is averaged, its resolution is also lower than a standard linear array.

Figure 32-3. Transducer elements can be arranged in linear or curved arrays.

Most probes have transducers that emit the highest amplitude of their ultrasound wave at a specific fundamental frequency. Harmonics of this frequency are also emitted at lower amplitudes. By listening for the echo at these higher harmonic frequencies, image resolution can be enhanced. Because the harmonics are very low amplitude, only transducers that have sufficient power output can be used for harmonic imaging. This type of image enhancement is referred to as tissue harmonic imaging.

Modern probes operate in pulsed mode. The transducer elements in the probe are used as both transmitters and receivers. The transducer elements emit a short ultrasound burst and then wait for the echo before emitting another ultrasound burst. This allows the probe to be smaller compared with continuous wave probes where there are separate emitters and transducers.

When a needle is inserted into tissue perpendicular to the ultrasound beam, it is a good reflector and it is easy to image. In some cases, it may be necessary to insert the needle nearly parallel to the beam in order to reach the targeted organ. In this case, most of the echo is lost and the needle image is much fainter. Ghosts on the deep side of the needle are cause by the needle vibrating itself (Fig. 32-4A). These reverberations return to the receiver later than the first volley of echoes. For this reason, they are seen as deeper in the tissue (Fig. 32-4B, 4C).

Above the collar bone, nerves are usually dark (hypoechoic) (Fig. 32-5A, B). Nerves located below the collar bone are usually white (hyperechoic) (Fig. 32-6A, B). The reasons for this dichotomy are not known, but it may be related to the depth of the nerves and the relative amounts of fat and stroma within the nerves themselves. On ultrasound cross section, nerves are round, hypo- or hyperechoic, reticulated structures. When imaged along their long axis, nerves appear as linear, hypo- or hyperechoic streaks, on ultrasound. Bones are hyperechoic and usually very bright white (Fig. 32-2). Arteries and veins are black unless color flow Doppler imaging is used (Fig. 32-6B).

Most nerves have some fascia around them. There is usually a potential space between the fascia and the epineurium. When a needle punctures the fascia, local anesthetic can usually be deposited between the fascia and nerve. This creates a black (hypoechoic) ring around the nerve. In some cases the fascia adheres to the epineurium or is missing. In that case, the needle may puncture the nerve and the nerve will swell as the local anesthetic is injected (Fig. 32-7A, B).

Tips

1.   Stimulation with the peripheral nerve stimulator is not necessary if the operator is certain of the nerve's identity on ultrasound. When the stimulator is used, it need only be used to confirm the target nerve, and can then be turned off. Because the local anesthetic is injected around the target nerve under ultrasound visualization, there is no need to “titrate” the current to the twitch.

2.   It is challenging to keep the needle perfectly parallel to the long axis of the transducer. Frequent fine adjustment of the transducer may be necessary, along with switching the line of site of the operator from the ultrasound unit screen to the site of needle insertion.

3.   As local anesthetic is injected, each increment should cause visible expansion of the tissues at the tip of the needle. This provides evidence that the tip of the needle is neither intravascular nor intraneural.

4.   If all of the local anesthetic solution seems to accumulate on only one side of the target nerve, the needle should be gently advanced or moved to another site around the nerve, to allow accumulation of the solution around the entire nerve (the “halo” effect). This optimizes block set-up time. It should be clear that each increment of subsequent local anesthetic injection causes distension of the tissues at the tip of the needle. It is not necessary to restimulate at each site, but the patient should be assessed for paresthesia or pain in the territory of the stimulated nerve.

Figure 32-4. Ghosts on the deep side of the needle cause reverberations to return to the receiver later than the first valley of echoes causing them to be seen deeper in the tissue.

Figure 32-5. Above the collar bone, nerves are usually dark (hypoechoic).

Figure 32-6. Nerves located below the collar bone are usually white (hyperechoic).

Figure 32-7. When a needle punctures the fascia, local anesthetic can usually be deposited between the fascia and nerve. This creates a black (hypoechoic) ring around the nerve.

5.   As with any block guided by the nerve stimulator, mild “pressure paresthesias” are sometimes evident during injection of local anesthetic in blocks carried out by ultrasound guidance. This should be differentiated from the severe pain of intrafascicular injection.

Suggested Reading

Jan J. Medical image processing, reconstruction and restoration. New York: Taylor and Francis, 2006.

Keegan BA. Understanding ultrasound. Kirkland: Advanced Medical Technologies, 2003.