Some Brief Facts

Ultrasound is a name given to high-frequency sound waves; 20,000 cycles per second (20 kHz) or higher, inaudible to human ears, that can be transmitted in beams and can be used to scan tissues of the body. Transducers emit ultrasound waves of different frequencies. Frequencies used for nerve blocks usually range from 5 to 14 MHz.

The higher the frequency, the shorter the wavelength, the better the resolution.

However, higher frequencies are more readily scattered and absorbed and therefore have poorer penetration of the tissue. Thus, for imaging of superficial structures (e.g., axillary nerve block, interscalene nerve block, vascular access) a high frequency transducer should be used. Lower frequency transducers will have better penetration of the tissue but have poorer resolution and should be used for nerves that are embedded deeper in the tissue (e.g., popliteal, infraclavicular, sciatic nerve block).

This general rule is not written in stone, for example, you may be better off using a lower frequency when doing an interscalene nerve block in an obese patient and a high frequency could be used when doing a popliteal nerve block in a thin patient. In other words, the frequency should be adjusted according to the type of nerve block being performed and the patient’s body habitus.

The transducers both transmit and receive ultrasound beams. The reflected ultrasound beams are received by the transducer and are amplified in the scanner. Different tissues alter the waves in different ways. Some scatter, whereas others reflect the waves directly before they return to the transducer as echoes. Bone and gas significantly reflect and scatter the ultrasound waves, whereas fluids produce a weak echo. Strong echoes appear as bright dots or lines on the screen and weak echoes appear as dark areas. The echoes that return from deeper structures are weaker than those from superficial structures and need to be amplified, which is done by the time-gain-compensation amplifier. After picking the appropriate frequency and depth setting you may be able to improve your image prior to starting the nerve block by adjusting the gain. Another useful function of the scanner is Doppler ultrasound. It can be used to detect blood flow in peripheral vessels and help avoid vascular injury while advancing the needle toward the target nerve. For this reason we recommend using the Doppler function to identify vessels close to the nerve before beginning any nerve block.

The two types of transducers typically used for nerve blocks are linear array (Fig. 24-4) and microconvex (curvilinear) transducers (Fig. 24-5). The linear array transducer emits high frequency ultrasound beams as the name states in a linear array and produces scans of rectangular shape. These transducers have a larger footprint than the microconvex probes and are better for scanning superficial structures (e.g., axillary nerve block, interscalene nerve block).

Figure 24-4 Linear array ultrasound transducer with rectangular scan.

Figure 24-5 Microconvex ultrasound transducer with fan-shaped scan.

The microconvex transducer has a small footprint and produces scans that are fan-shaped. Thus, larger sections of deeper tissue can be scanned compared to the linear array transducer. The microconvex probes usually also emit lower frequency ultrasound waves. It is therefore used to scan deeper structures (e.g., popliteal nerve block, infraclavicular nerve block) or in cases where only a small space is available for scanning (due to its smaller footprint).

To successfully perform an ultrasound-guided nerve block, the nerve structures that are to be blocked and the structures that should be avoided, such as pleura and blood vessel, need to be clearly identified. The ultrasound probe is typically placed where the needle insertion point would be in a conventional block technique. The needle is then inserted several centimeters from the probe site and advanced in plane of the ultrasound beam (Fig. 24-6). This allows continuous visualization of the needle as a hyperechoic (white) line. This is sometimes challenging particularly when blocking nerves in deeper tissues. However, it is of paramount importance for safely performing the block to image the needle clearly, particularly the needle tip. The needle can easily veer out of plane while advancing, which can lead to significant misjudgment of the location of the needle tip (Fig. 24-7).

Figure 24-6 Demonstration of the in-line needle advancement technique using a linear array transducer and the “blue phantom gel model”. The needle is imaged in its entirety with the needle tip in close proximity to the nerve.

Figure 24-7 Using the in plane needle advancement technique as shown in Figure 24-6. The needle is placed intravascularly. A, intravascular position of the needle in the “blue phantom” model. B, same needle position as on the right, however, the ultrasound probe is placed out of plane. The virtual needle tip appears far away from the vessel. The real needle tip is, in fact, intravascular.

If this should occur we recommend pulling the needle back to just under the skin and then re-advancing it. Optimal imaging of the needle may also require moving the ultrasound transducer from side to side, tilting it, or rotating the probe using a slow and steady motion. In addition, the flatter the angle of approach, the more easily the needle can be visualized.⁷ It is sometimes helpful to image the needle at a flat angle before advancing the needle at the appropriate angle for the target nerve (Fig. 24-8).