Advancing the Science of Ultrasound Guided Regional Anesthesia and Pain Medicine

Scanning Technique - Machine Setting

Optimizing Image Quality

Visualization of nerves with ultrasound depends on the operator's ability to properly:

1. locate the nerve (see Nerve Localization)
2. handle the transducer (see Transducer Movement)
3. maximize the ultrasound machine capability:

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)

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.

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

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

The Impact of DEPTH Setting on Image Quality

The figures above illustrate how the image of the median nerve in the forearm (arrowhead) gets smaller and smaller as the depth is increased from 2-6 cm. It is important to select the appropriate depth setting (e.g. 2 cm in this case) according to the target nerve location.

The Impact of GAIN Setting on Image Quality

The GAIN function compensates for attenuation (a reduction in sound amplitude) as sound travels deep into the body. The intensity of the returning signals can be amplified by the receiver upon arrival so that the displayed image is brighter and more visible on the screen. Gain can be adjusted for the near field, far field or the entire field (overall gain). Excessive increase in GAIN will add "noise" to the image.

An Illustration of Different Gain Settings

Figure A shows a proper gain settings.

Figure B shows under gain thus the overall image is very dark. The muscle layers are not well visualized.

Figure C shows excessive gain thus the overal image is very bright.

The Impact of FOCUS Setting on Image Quality

Image quality and beam focus is best at the focal zone. Most modern electronically steered transducers provide electronic focusing adjustable for depth. It is important to place the FOCUS at or slightly below the level of the target structure of interest.

Infraclavicular Region (8-12 MHz Range)

Figure A shows inappropriate focus setting at a superficial level (less than 2 cm) resulting in a dark image. The target structures (AA = axillary vessels and Arrowheads = cords) are 5-6 cm deep to the skin.

Figure B shows appropriate focus setting at 5-6 cm. The target structures (AA = axillary vessels and Arrowheads = cords) are 5-6 cm deep to the skin.

The Impact of COMPOUND IMAGING on Image Quality

COMPOUND IMAGING is a broad bandwidth technology that combines multiple coplanar images captured from different beam angles and from multiple ultrasound frequency spectra to form a single image in real time. Spatial compounding reduces speckle artifacts and improves contrast resolution.

No Compound Imaging
Compound Imaging

Median Nerve in the Forearm (12 MHz Transducer, 3 cm Depth)

No Compound Imaging
Compound Imaging

Supraclavicular Region (10 MHz Transducer, 3 cm Depth)

No Cross Beam (No Compound Imaging)
Cross Beam (Compound Imaging)

Color Doppler

Color Doppler is an instrument to characterize blood flow. The Doppler effect occurs when there is a moving source (blood flow of red blood cells, RBC) and a stationary listener (ultrasound transducer). There is an apparent change in the returning echoes due to the relative motion between the sound source and the receiver. If the source (RBC) is moving towards the receiver (transducer), the perceived frequency is HIGHER (display in RED) and when the source (RBC) is moving away from the receiver, the perceived frequency is LOWER than the actual (display in BLUE).

It is important to note that Color Doppler detection of flow and flow direction is worst when the transducer is perpendicular (90 degrees) to the vessel and best when the transducer is parallel (0 degrees) to the blood flow.

Transducer perpendicular to radial artery (no flow is detected)

Transducer aiming away from artery (flow in BLUE)

Transducer aiming towards artery (flow in RED)

For ultrasound guided regional anesthesia, Color Power Doppler (CPD) is useful for differentiating vascular from non vascular structures. CPD is more sensitive than Color Doppler in flow detection but does not indicate flow direction.

Transducer perpendicular to radial artery (weak flow is detected)

Transducer aiming towards or away from artery (strong flow is detected)

Share to Facebook Share to Twitter More...