Ultrasound Equipment - Transducer
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:
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 (< 7 MHz) is required to image deep structures. Higher frequency
transducers (10-12 MHz) have a limited depth of penetration (< 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
PA = popliteal artery
N = sciatic nerve
PA = popliteal artery
Retrieved on 12-10-2010 from www.usra.ca