nuevas tecnologias en la localizacion de nervios

8/9/2019 Nuevas Tecnologias en La Localizacion de Nervios

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New technologies in nerve location

N. M. Bedforth

Honorary Special Lecturer, University of Nottingham & Consultant Anaesthetist, Department of Anaesthesia,

Nottingham University Hospitals NHS Trust, UK

Summary

Regional anaesthesia is undergoing a renaissance, perhaps assisted by the introduction of (and

enthusiasm for) ultrasound-guided regional anaesthesia into clinical practice. This article summa-

rises the technology and principles of ultrasound imaging in anaesthesia and describes the devel-

opment of three-dimensional ultrasound imaging, considering whether this new technology has an

application in regional anaesthesia.

........................................................................................................

Correspondence to: Dr Nigel M. BedforthE-mail: [email protected]

Development of ultrasound

The first description of ultrasound probably dates back to

1794 when Lazzaro Spallanzani demonstrated that bats

navigate in the dark using echo reflection from high

frequency sound waves inaudible to the human ear [1]. In

1880, Pierre Curie and his brother Jacques demonstrated

that quartz crystals such as the Rochelle salt (sodium

potassium tartrate tetrahydrate) produce an electrical

potential in response to mechanical pressure. The

following year, the physicist Gabrielle Lippman mathe-

matically deduced that this property, later termed the

piezoelectric effect, was reciprocal, and this was imme-

diately verified by the Curie brothers. These discoveries

made possible both the future production and reception

of high frequency sound waves and the development of

modern day ultrasound imaging.

Advances were then made around the First World War

period with the development of sound navigation and

ranging (SONAR), unfortunately too late for HMS

Titanic, which sank in 1912. Industry later developed

ultrasonic metal flaw detectors; these were the forerun-

ners of medical diagnostic ultrasound machines firstdemonstrated by workers such as Ludwig in the late

1940s in animals and humans. Work then began in many

medical specialities with the development of B-mode

scanning in the late 1950s, which produces the two-

dimensional images we are used to seeing today. Ultra-

sound was already well established in many medical

specialities when the possibility of ultrasound assisted

peripheral nerve blockade came to light in 1978. La

Grange et al. [2] used Doppler ultrasound to assist a series

of supraclavicular brachial plexus block placements.

Kapral et al. [3] described ultrasound-guided supraclavic-

ular blockade in 1994. Since then, there has been a year-

on-year increase in the number of related publications

and a similar growth in clinical practice. Developments in

microprocessor technology have facilitated equipment

miniaturisation and this, combined with improved probe

quality, has enabled production of small (laptop computer

sized) ultrasound machines capable of producing the

quality and resolution required to image neural structures.

Studies have shown that ultrasound, when compared to

traditional landmark and nerve stimulation techniques,

can reduce the local anaesthetic (LA) volume used,

decrease block onset time, increase block duration and

increase comfort during block placement. Safety may also

be improved by direct observation of the needles

interaction with nerves, vessels and other sensitive

structures. For these reasons, ultrasound has been pro-

posed as the gold standard for regional anaesthesia [4]

and has been recommended for both elective and

emergency central venous cannulation [5]. Regardingthe current quality of image production of ultrasound in

anaesthesia, the early days of imagining have now been

firmly replaced by imaging.

Development of three- and four-dimensional

ultrasound imaging

Two-dimensional ultrasound imaging was already well

established when Tom Brown developed his multiplanar

scanner in 1973. This device produced a pair of stereo-

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scopic images that, when viewed through a prism stereo-

scope, allowed the operator to construct a three-dimen-

sional image. The device was not a commercial success and

was discontinued, but the technology and potential of

three-dimensional ultrasound had been revealed.

The development of three-dimensional ultrasound

during the 1980s was driven mainly by cardiology,

obstetrics and gynaecology [6]. These early images were

produced by manually scanning a two-dimensional probe

across a volume of interest to gather a series of two-

dimensional ultrasound image slices. These slices were

then stacked by the machine and compounded together

to form a three-dimensional volume. The positional

information from the probe could be made available to

the analysing computer by having the scan head mounted

on a mechanical arm; this meant that the early devices

were fairly cumbersome. Advances in miniaturisation,

computer processor power, probe technology (described

below) and also the software for image processing haveled to huge improvements in image quality.

Three-dimensional ultrasound imaging has now be-

come available on portable or hand-held machines,

which may make its use more feasible in the operating

theatre environment. The application of three-dimen-

sional ultrasound has been described in other specialties

[79], but there is little experience with it in anaesthesia,

for example during vascular access or regional anaesthe-

sia. However, the use of three-dimensional ultrasound

imaging has been described during vascular access [10]

and it has been used to demonstrate nerves and perform

spatial mapping of the brachial plexus [11]. Four-dimen-

sional ultrasound (live or real time three-dimensional

described below) has facilitated successful placement of a

popliteal catheter [12] and aided needle-guidance during

performance of a peripheral radial nerve block [13].

Why has three-dimensional ultrasound imaging not

been applied as widely in clinical practice as two-

dimensional ultrasound?

The reasons for this are multiple. Ultrasound imaging in

anaesthesia is a relatively new skill that many anaesthetists

are only beginning to practise. Two-dimensional ultra-

sound is a simpler imaging method and produces high

quality and easily understandable images. For three-dimensional imaging to gain popularity and even become

preferable to two-dimensional imaging in clinical practice,

it must also become easy to use and understand; the

equipment should be user-friendly (complexity of opera-

tion and acquisition speed) and the images simple to

interpret as wellas offeringthe userextrauseful information.

To date, many of these criteria are unmet, meaning that

two-dimensional imaging is dominant in clinical practice,

but this situation is changing rapidly with advancing

technology and better understanding of the potential

advantages that three-dimensional imaging offers [14].

Physics of ultrasound

Ultrasound consists of sound waves of a higher frequency

than the upper limit of the audible range (approximately

20 kHz); medical ultrasound operates at much higher

frequencies still, typically 218 MHz. Ultrasound is

produced by transducers that convert electrical energy

into mechanical energy (and vice versa) by the piezo-

electric effect. The crystal of the transducer oscillates in

response to an electrical stimulus to produce pressure, i.e.

sound, waves and when the transducer is in turn

subjected to sound waves by reflection from imaged

tissue, it will produce an electrical potential that may

then be converted into an image. Sound waves are

reflected at the interfaces between tissues with differing

sound wave conductive properties or acoustic resis-

tance; highly reflected signals lead to larger amplitudereflected sound waves that are represented by brightness

on the ultrasound image. This is why we term the

imaging B-mode (for brightness). The other types of

imaging are the older A-mode (amplitude) and M-mode

(movement). The ultrasound machine constructs an

image by calculating the distances from the transducer

of reflected objects using the speed of sound in tissue and

time taken for the emitted waves to return to the

transducer. The higher the frequency of the ultrasound,

the shorter the wavelength of the sound waves produced

and the better the resulting image resolution. Attenuation

of the incident sound waves is caused by scattering,

refraction and absorption by the tissues and leads to a

reduction in amplitude or intensity of the returning

signal. Unfortunately, attenuation is greater at higher

frequencies, and this means that selection of ultrasound

frequency in clinical practice is a balance between

obtaining the best resolution and being able to achieve

adequate penetration of the tissues to produce a reflected

signal of adequate amplitude.

Ultrasound imaging is subject to artefact, which

degrades the quality of the image produced. Advances

in probe technology (miniaturisation and broad-band)

and image processing, e.g. harmonic and cross-beam

imaging, have led to major advances in image qualityleading to increased interest in ultrasound as a substitute

to other forms of imaging.

Use of ultrasound in anaesthesia

Ultrasound images are presented as a single two-dimen-

sional plane producing an image slice of approximately

1 mm thickness of a variable and adjustable depth. Nerves

and vessels may be viewed in their short axis or long axis.

Needles can be guided towards targets whilst being

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viewed in-plane or out-of-plane with respect to the

ultrasound beam [15].

Safety of ultrasound-guided needle techniques

Little evidence exists regarding the safety of regional

anaesthesia when utilising ultrasound-guidance compared

with the other needle insertion techniques. This evidence

will be difficult to gather, as complications associated

regional anaesthesia are rare. Ultrasound has been demon-

strated to be a sensitive method for detecting intraneural

injection[16] anddirect observation of the tissue expansion

created by injected LA allows for early detection of

intravenous injection of LA. The use of ultrasound

decreases complication rates during central venous access,

but complication rates of up to 5% are still quoted [17].

Sources of error during ultrasound-guided needle techniques

Errors made during needle guidance include needle

overshoot, mainly due to imperfect imaging of theadvancing needle. Vascular puncture and intraneural

injection have been reported during ultrasound-guided

regional anaesthesia procedures [1820]. Those new to

ultrasound-guidance may have higher complication rates;

Sites et al. noted that novices regularly over-inserted

needles when attempting to target an olive using

ultrasound-guidance [21]. Error rates during ultrasound-

guided needle insertion will be minimised by adequate

training and experience. Training courses do exist; but to

date there has been no formal consensus on the need for

formal assessment of competency before clinical practice.

Errors made when imaging needles

The short-axis view (out-of-plane with respect to the

ultrasound beam) allows considerable freedom of needle

movement but only a cross-section of the needle (ideally

the tip) is seen. The commonest error is incorrectly

interpreting the shaft as the tip of the needle, leading to

over-insertion. The long-axis view (in-plane with

respect to the ultrasound beam) allows observation of

the full length of the needle. There is less freedom of

movement of the needle during insertion since move-

ment may only be in one plane to keep the shaft visible.

Common errors with this technique include incorrect

line-up, when the proximal shaft is seen within thenarrow ultrasound beam width, but the distal shaft and

needle tip has strayed laterally and is not seen, again

resulting in needle over-insertion. These errors may lead

to an increase in complication rate, with trauma to

sensitive structures beyond the target, e.g. nerves or

pleura. Errors in needle identification can also occur

during the in-plane approach when using steep needle

insertion angles with consequent reduction in clarity of

the needle image [22].

Errors made during imaging of nerves and vessels

Two-dimensional imaging allows either short-axis or

long-axis views of anatomy; observation of both of these

views can aid or confirm identification of a structure and

its surrounding relations. The probe needs to be rotated

through 90 C to acquire this information with a standard

two-dimensional probe. This may be challenging to the

inexperienced operator; for example, the internal jugular

vein may be correctly identified in long-axis before

needle insertion, but unintended medial movement of the

probe over the carotid artery during needle insertion, that

is not realised by the inexperienced operator, could lead

to subsequent carotid puncture.

Imaging during injection

Injected local anaesthetic is seen spreading around the

target nerve only in one plane (usually the short-axis

view); the probe needs to be rotated through 90 C to

view spread along the long-axis of the nerve to gain abetter impression of the spread pattern.

Potential advantages of three-dimensional ultrasound

Three-dimensional ultrasound imaging may help us

overcome some of these issues by providing the operator

with simultaneous multiple planes of view or by provid-

ing a representation of the whole region of interest. This

may give the operator an improved spatial awareness and

understanding of the anatomy and needle position.

Three-dimensional ultrasound datasets have already been

used to assess fluid injected into the dermis [23] and could

also be used to assess the quantity and spread pattern of

the injected LA.

Three- and four-dimensional image acquisition

A three-dimensional ultrasound image is created by the

capture of a dataset. The machine scans across a region of

interest gathering many two-dimensional slices, that are

then stacked together and displayed retrospectively as a

static image. The image may be a representation of the

whole volume or a selection of planes through the image

of varying angle and thickness (see below). The quality of

the resulting image depends, in part, upon the acquisition

speed. A slow acquisition speed yields more scanned slices

and volume datapoints (but requires a static object),producing a superior image that is then displayed

retrospectively. Faster acquisition speeds can produce a

continuously updating image of the newly acquired

volume, creating the impression of a moving structure,

with a consequent reduction of image quality. This live

imaging is variously termed live three-dimensional or

four-dimensional imaging, where time is the fourth

dimension. This can allow performance of needle-guid-

ance in real time [10].

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Three- and four-dimensional ultrasound machines

Some three- and four-dimensional ultrasound machines

operate at acquisition speeds of approximately four

volumes per second during live imaging. This means that

there are visible pauses between displayed frames on the

screen, producing a jerky image. Others display approx-

imately 16 volumes (or more) per second, which creates a

smoother perceived transition between images [24].

In common with two-dimensional machines, those

with three-dimensional capability have been developed

on larger platforms such as the Philips iE22 (Philips

Medical Systems, Bothell, WA, USA) or GE Voluson 730

(GE Healthcare, Kretz, Zipf, Austria) but smaller hand-

held or at least laptop computer sized machines, for

example the GE Voluson, are now appearing.

Three-dimensional ultrasound probes

Three-dimensional ultrasound images can be produced

using standard linear array, mechanically-steered array ormatrix array probes. Standard linear array probes can be

manually scanned over a region of interest and the images

then retrospectively assembled by the machine into a

three-dimensional image. They may also be mounted on

mechanical arms to feed back positional information to

the machine. This process is fairly slow and the three-

dimensional images are produced retrospectively.

Mechanically-steered arrays produce still and real time

images. The scan head is held over a region of interest and

the array is moved mechanically within the probe back

and forth, thus scanning in two planes. Multiple two-

dimensional slices through the volume of interest are fed

back to the machine for assembly into the three-

dimensional image. These transducers may operate at

high ultrasound frequencies, thereby producing high-

resolution images, and may be of linear or curved types

depending on the application. The arrays can be moved

within the transducer in a number of ways. Arrays that are

moved linearly produce a rectangular image but have the

disadvantage of being bulky. The arrays can also be

pivoted (swung or tilted) back and forth through an angle

or rotated about an axis. These probes can be small, as are

those used in obstetric and gynaecological imaging or for

imaging the prostate, or larger for abdominal scanning.

The advantage is that the field of view is relatively wide asthe two-dimensional slices diverge away from the probe,

but the inherent disadvantage of this is that the resolution

of the three-dimensional image degrades with increasing

distance from the probe since the image has to be

reconstructed from slices that get progressively further

apart. During four-dimensional scanning, the display

frame rate is typically less than half that of current two-

dimensional systems, resulting in visible pauses between

displayed frames on the screen. The movement of the

array within the transducer head also creates heat and

image artefacts that need to be compensated for.

Matrix arrays scan and generate images simultaneously

in multiple planes. They contain many more elements

than a standard array (typically > 2000). The images are

generated electronically, so the scanning head is not

required to move. As a result, matrix array probes are

smaller and lighter, and therefore ergonomically superior

to the mechanically-steered arrays. The display frame rate

is typically three times greater than a mechanically-steered

array, producing a smoother image on-screen. Matrix

array probes were designed for echocardiography, and

operate at frequencies in the range 27 MHz, producing

lower resolution images but enabling deeper penetration

of tissue. They have a small probe-head (footprint),

facilitating transthoracic scanning between ribs, and

produce a truncated pyramid shaped image on the screen

as the ultrasound beam spreads out rather than passing

linearly from the probes surface, which would produce asmaller image. The mechanically-steered array and matrix

array probe types relay both the ultrasound signal and

positional information so that accurate reconstruction of

the volume of the scanned region can be carried out.

Recent developments in electronic matrix transducers

include capacitive micro-machined ultrasonic transducers

that combine silicon chip technology with novel silicon

membrane construction, produceing extremely light-

weight transducers that also produce less heat than

conventional transducers. These transducers could poten-

tially contain hundreds of thousands of elements [14].

Three- and four-dimensional multiplanar ultrasound imaging

Three- and four-dimensional multiplanar imaging is the

most similar type of three-dimensional display to conven-

tional two-dimensional imaging; it typically provides

simultaneous ultrasound images in up to three orthogonal

(perpendicular)planes (Fig. 1).The first twoof these views,

the transverse (Xor short-axis) and longitudinal (Yor long-

axis), can be obtained using conventional two-dimensional

ultrasound by rotating the probe through 90 C. The third

view, unobtainable using conventional two-dimensional

ultrasound, is parallel to the probes surface, and thus is

effectively looking down onto the structure being imaged.

This view has also been termed the Z-axis or coronal view,although the view obtained will differ from the true

anatomical coronal plane depending on where the probe is

placed on the body. We may therefore refer to this view as

the plan view due to the similarity with the architectural

term showing the layouts of buildings from the overhead

viewpoint. The point of intersection of the three planes is

often marked on each image by a marker dot; this can be

steered to differing positions and can therefore function as a

target during insertion of a needle [13].


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Three- and four-dimensional volumetric ultrasound imaging

Here the rendered (or reconstructed) image is a three-

dimensional volume representation of the scanned region

of interest displayed on the two-dimensional screen. The

image may be displayed as a thick slice, cube or truncated

pyramid (depending on the type of probe used). This

rendered image can be displayed as a surface image or a

transparent image. The visibility of structures within the

displayed volume is dependent on the difference in their

ability to conduct sound waves (or acoustic impedance).

Solid structures surrounded by fluid provide an excellent

reflective interface for ultrasound and high quality images

of the surface of the structure can be reconstructed. This

mode has been used widely in obstetric practice to

produce extremely detailed images of the fetus in utero.

Penetration of the fetus to display bone structure and

vascular structure has also been demonstrated. Doppler

flow imaging can also be carried out during real time,

three-dimensional imaging to display blood flow. The

ability to produce these rendered images has evolved from

computer graphics engineering. Accurate shape and

volume measurements can also be taken of solid structures.

Three- and four-dimensional imaging for regional anaesthesia

and needle guidance

Three-dimensional imaging can produce quality images

of neural structures (see below). Figure 2 is a three-

dimensional volumetric image of a lumbar vertebra taken

between the spinous processes and showing the epidural

space. This image is taken from a three-dimensional

dataset (retrospective analysis). If images of this quality can

be replicated during real time imaging, they may proveuseful for ultrasound-guided access to the epidural and

subarachnoid space and other neural structures.

Real time volumetric (four-dimensional) imaging

allows the operator potentially to guide a needle to an

exact point within the volume of interest displayed in real

time. The problem of needle visibility may be overcome

as the whole volume of the image is displayed and the

needle is continuously imaged within that volume.

Figure 3 is a screen shot during four-dimensional ultra-

sound imaging of internal jugular vein cannulation

utilising a matrix array probe. Although the picture

quality during real time volumetric imaging makes

demonstration of nerves difficult, improvements in res-

olution are awaited, which could to distinguish neural

structures better. However, live volumetric imaging has

already been used to place a popliteal sciatic nerve

catheter successfully [12].

Reduction of the size of the volume displayed

improves the visibility of the structures contained within;

the display can produce a slice of the volume of a

selectable thickness. Figure 4 depicts a median nerve

block in the forearm using a type of thick-slice four-

dimensional imaging termed volume contrast imaging in

C plane or VCI C (GE Healthcare). The transverse view

shows only the proximal portion of the in-plane needle,but the thick slice in the plan view (unobtainable using

conventional two-dimensional imaging) shows the nerve

and the entire needle shaft that has been passed under-

neath and beyond the nerve. Interestingly, the spread of

LA is also seen along the long axis of the nerve. These

types of imaging may therefore prove useful in aiding

ultrasound needle-guidance techniques by improving the

visibility of nerves, needles and related structures, and

thereby reducing needling errors.

(a)

(c)

(b)

Figure 1 Orthogonal planes displayed during multiplanarimaging a) Transverse view; b) longitudinal view; c) coronal orplain view.

Figure 2 Volumetric image of a lumbar vertebra showing theepidural space.


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Multiplanar imaging provides multiple simultaneous

views of the structures under examination and therefore

provides us with more information about the anatomy we

are observing and the needle during advancement towards

a target. Figure 5 shows a multiplanar view of the ulnar

nerve in the forearm. The ulnar artery is seen lying

adjacent to the nerve in the transverse and plan views.The marker dot (point of intersection of the three

displayed planes) has been placed over the nerve allowing

us to view the nerve in the multiple planes. Multiplanar

imaging has been used to obtain views of nerves and to

place peripheral nerve blocks [13].

Perhaps more complex needle guidance will benefit

from this type of imaging; axillary brachial plexus block

requires individual blocks of the terminal nerves as they

lie around the axillary artery. The operator also needs to

avoid intravascular injection in the presence of multiple

axillary veins. Figure 6 depicts a multiplanar view of the

axillary brachial plexus. The marker dot is positioned over

the radial nerve, making it visible in all three planes. The

radial nerve is seen lying beneath the ulnar nerve in the

longitudinal view (anatomically postero laterally in this

case). Views of the needle, nerves and vessels in multipleplanes or in a volumetric image may assist in the accuracy

of movement and positioning of the needle tip.

The images obtained during four-dimensional, i.e. real

time three-dimensional, imaging are generally of lower

quality than those obtained during conventional two-

dimensional imaging. The simultaneous transverse and

longitudinal plane displays give the operator enhanced

anatomical information on the structures being shown

and spread of any injected fluid. Figure 7 depicts a

Figure 4 Four-dimensional volume contrast imaging in the Cplane (VCI C) view of a median nerve block in forearm.

Figure 3 Live volumetric imaging of internal jugular veincannulation.

Figure 5 Multiplanar view of ulnar nerve in the forearm.

Figure 6 Multiplanar image of the axillary brachial plexus.


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cannulation of a vessel phantom using live multiplanar

imaging. The needle can be seen approaching the target

structure both as an in-plane and out-of-plane approach

with respect to the ultrasound beam. The coronal or plan

view produces a view at right angles or through the

standard transverse and longitudinal images at an adjust-

able depth (effectively parallel to the probe surface). Note

that if a needle is seen in the plan view and possibly not in

the other two views (due to poor alignment) it must have

already crossed the central plane of the target (depth of

plan view) and may need to be withdrawn. This gives the

operator the information that a set depth has been

exceeded by an inserted needle. This feature may proveuseful in avoiding needle overshoot [13].

Machines with three-dimensional capability can also

create a variety of other presentations of the collected data

sets. Extended views are created by moving a probe along

the region of interest; the machine then processes the

captured information into the extended image. Figure 8 is

an extended view of the sciatic nerve from the proximal

thigh to the popliteal fossa where the nerve is seen to

divide. Figure 9 is an extended view of the lumbar

transverse processes and associated muscles. Tomographicultrasound imaging (GE Healthcare) displays a number of

slices across a reference image. Figure 10 is a tomographic

ultrasound image of the axillary brachial plexus. The

central line on the longitudinal reference image (marked

with a star) represents the position of the central transverse

image. These views may become useful for performance

of regional anaesthesia if applied to live scanning by

increasing presented anatomical information and provid-

ing better imaging of the needle and LA spread pattern.

Figure 7 Live multiplanar ultrasound needle-guidance in avessel phantom.

Figure 8 Extended view of the sciatic nerve in the thigh.

Figure 9 Extended view of lumbar transverse processes.

Figure 10 Tomographic ultrasound image of the axillary bra-chial plexus.


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Potential advantages of three- four-dimensional ultrasound in

regional anaesthesia

There are few outcome data on live three-dimensional

(four-dimensional) ultrasound needle-guidance in regio-

nal anaesthesia, but there are reports of its use during

vascular access [10] and in various specialities (referenced

above). Early experience suggests potential for the

technique when applied to regional anaesthesia, but also

looks forward to improvements in the quality of live

imaging when compared with two-dimensional or

retrospective three-dimensional images. The potential

advantages are an improved spatial awareness and

demonstration of the anatomy, inserted needle and

injected fluid. This may reduce error rates in identifica-

tion of structures such as vessels, nerves and tendons.

Better needle observation may decrease the incidence of

needle overshoot that may, in turn, decrease intraneural

needle placement, and vessel or pleural puncture during

nerve blocks, and thereby the incidence of complicationsassociated with ultrasound-guided procedures. Improved

observation of injected fluid spread during regional

anaesthesia techniques may increase block success rates

or allow a decrease in the LA volume requirements.

Summary

Ultrasound-guided regional anaesthesia has become

widespread in clinical practice and there is substantial

evidence supporting its efficacy. Three-dimensional

ultrasound imaging is becoming more available and has

already been used in a number of specialties but, to date,

has found few routine indications. Little experience or

outcome data exist in anaesthesia. Further developments

in technology will improve live image quality and make

the probes smaller and easier to handle. In addition,

machines with three-dimensional capability will become

more affordable and available. We will then be able to

evaluate whether this exciting imaging modality can

improve the performance of regional anaesthesia.

Conflicts of interest

The authors experience with three-dimensional ultra-

sound has been assisted by equipment loans from GEHealthcare, Kretz, Zipf, Austria and Philips Medical

Systems, Bothell, WA, USA.

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