Ultrasound for Regional Anesthesia
Ultrasound for Regional Anesthesia





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Regional Anesthesia > Ultrasound Equipment
Transducer
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Transducer Selection
Transducer characteristics, such as frequency and shape, determine ultrasound image quality. The transducer frequencies used for peripheral nerve blocks range from 3-15 MHz. Linear and curvilinear (or curved) transducers are most useful for nerve imaging to provide high resolution images. Sector phased array transducers may also be used but the images are more "grainy".

Modern transducers are broad bandwidth transducers that are designed to generate more than one frequency. For example, a L 5-12 MHz transducer can generate waves ranging in frequency from 5-12 MHz. With broad bandwidth transducers, the operator can select the examination frequency to match the target requirement. The resonance frequency is the one frequency at which the piezoelectric transducer is most efficient in converting electrical energy to acoustic energy and vice versa. The resonance frequency is determined by the thickness of the piezoelectric element.
For superficial structures (e.g. nerves in the interscalene, supraclavicular and axillary regions), it is ideal to use high frequency transducers greater than or equal to 7 MHz. Transducers in the range of 10-15 MHz are preferred but depth of penetration is often limited to 2-3 cm below the skin surface.

For visualization of deeper structures (e.g. in the infraclavicular and popliteal regions), it may be necessary to use a lower frequency transducer (less than or equal to 7 MHz) because it offers ultrasound penetration of 4-5 cm or more below the skin surface. However, the image resolution is often inferior to that obtained with a higher frequency transducer.

Linear transducers less than or equal to 5 cm wide are available for high frequency transducers. Smaller transducers, i.e., transducers with smaller footprints are useful for detailed scanning where the patient's anatomy prohibits the use of bulkier transducers (e.g., the supraclavicular region where there is limited access). Curved transducers are best suited for scanning whenever a wide field of view is required.

It is important to remember that:
high frequency = high spatial resolution but limited depth of penetration
low frequency = greater depth of penetration but lower spatial resolution

Examples of SonoSites Transducers:

Examples of Philips Transducers:
(7-4 MHz Transducer)

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Frequency and Image Resolution
It is best to select the highest frequency transducer possible for the required depth of penetration.

A. The Use of a Higher Frequency Transducer

A higher frequency transducer (10-12 MHz) provides the best image resolution for superficial structures.

The Brachial Plexus in the Interscalene Groove (1-2 cm from the skin surface)

12 MHz Transducer
8 MHz Transducer
Note that the texture of the anterior scalene muscle (ASM) and middle scalene muscle (MSM) is less clearly defined with the 8 MHz transducer compared to the 12 MHz transducer.
Arrowheads = nerve roots

B. The Use of a Lower Frequency Transducer

A lower frequency transducer (less than 7 MHz) is required to image deep structures. Higher frequency transducers (10-12 MHz) have a limited depth of penetration (less than 3-4 cm deep).

The Brachial Plexus in the Infraclavicular Region (5-6 cm from the skin surface)
The sonograms are captured with a linear 3-12 MHz transducer. The anatomical structures at 5-6 cm deep are not clearly visualized when the transducer is set at 12 MHz. The structures (AA = axillary artery; Arrowheads = nerves) appear brighter and more clearly defined with the 3 MHz setting. The focus for both images is set at 5-6 cm depth.

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Curvature and Field of View
The curved transducer provides a wider field of view.

Popliteal Sciatic Nerve Imaging (7 MHz transducer)
Curved transducer
N = sciatic nerve      PA = popliteal artery
Linear transducer
N = sciatic nerve      PA = popliteal artery

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Curvature and Image Resolution
Curved transducers often generate lower frequency waves than linear transducers thus provide images of lower resolution.

N = sciatic nerve
N = sciatic nerve      PA = popliteal artery

Ultrasound for Regional Anesthesia