elementary ugra - · pdf filethe transducer •these crystals ... this block needle...

ELEMENTARY UGRAChristian R. Falyar, CRNA, DNAP

Department of Nurse Anesthesia

Virginia Commonwealth University

Objectives

• Discuss the physics of sound

• Explain how sound tissue interaction creates echoes

• Review how ultrasound uses different strength echoes generate an image

• Describe why nerves appear differently at various locations on the body

• Name the basic components and functions of an ultrasound system

• Identify factors that can improve the success of an UGRA procedure

• Review complications of UGRA and strategies that can used to reduce/prevent them

What is sound?

• Sound is a form of mechanical energy.

• Sound travels at different speeds through different

substances, or media.

• As sound travels through media, it can be reflected, refracted,

scattered, reverberated and ultimately attenuated.

• Frequency is determined by the number of cycles

(compressions and rarefactions) that occur in a second

• Wavelength is inverse to the frequency (i.e. the higher the

frequency the shorter the wavelength)

• Amplitude describes the energy of the wave, and is not

related to frequency

• Ultrasound used in in diagnostic imaging involves frequencies

greater than 1MHz.

What is sound?

Propagation velocities

What is sound?

Rarefactions (low pressure)

Compressions (high pressure)

Thoughts on frequency...

High frequency

• More cycles occur per

second

• Images are higher

resolution

• Greater attenuation of the

sound waves limits the

depth at which tissue can

be imaged

Low frequency

• Fewer cycles occur per

second

• Greater tissue penetration

but lower resolution

• Less attenuation allows

for imaging of deeper

structures

Thoughts on frequency…

High frequency Low frequency

Generation of sound waves

• A wave is generated when an ultrasound system

applies an electrical field to crystals located on

the transducer

• These crystals convert the electrical energy to

mechanical energy; this is known as the

piezoelectric effect

• The sum of the waves is an ultrasound beam

• These beams are emitted in pulses, with each

beam being two to three cycles in length

Generation of sound waves

piezoelectric crystals

outer housing

alternating current to create potential difference

backing material

power supply

acoustic insulator

matching layer

AlternatingCurrent

Sound wave propertiesTr

ansd

uce

r

Fresnel Zone(Near Zone)

Fraunhofer Zone(Far Zone)

Focal Zone

A

B

Sound/Tissue interaction

• The echoes interpreted by

ultrasound result from the

different acoustic

impedances of tissues

• The amount of reflection,

refraction, scattering, and

attenuation is dependent

on the degree of difference

Acoustic impedance

Sound/Tissue interaction

Reflection Refraction Scattering Attenuation

Incident Wave

Reflected Wave

Impedance Z1

Impedance Z2

Incident

WaveReflected

Wave

Refracted

Wave

Impedance Z1

Impedance Z2

Transducer

Incident

WaveIncident

Wave

Attenuated

Wave

Reflection

Reflection

Specular Diffuse

Refraction

Incident Wave

Reflected Wave

RefractedWave

Rayleigh scattering

• Rayleigh scattering occurs at interfaces involving structures of small dimensions (such as a red blood cell).

• This creates a relatively uniform average amplitude in all directions.

Rayleigh scattering

• Who/What is Mickey?

Attenuation

• The decreasing intensity of a sound wave as it passes through tissue.

• In medical ultrasound, attenuation is the result of the interaction of sound with tissue.

• The attenuation coefficient is the relation of attenuation to distance, and depends on the tissues traversed and the frequency of the ultrasound wave.

• Higher frequency waves are more severely attenuated than lower frequency waves.

Attenuation

Attenuation

What creates my picture?

• It’s all in the math!!!

• The computer assigns a

color in a grey scale based

on the strength of returning

echoes to form the image.

• This process repeats itself

hundreds of times a

second resulting in real-

time imaging.

What do tissues look like?

• Nerves – in cross section appear as round “honeycomb” structures

• Tendons – appear similar to nerves, but become flat and disappear when followed proximally along an extremity

• Vascular Structures – typically appear as anechoic circular structures in cross-section. Will appear tubular in longitudinal view

• Fat – appears hypoechoic with streaks of irregular hyperechoic lines

• Fascia – appears linear hyperechoic structures marking tissue boundries

• Muscle – appears feather-like in longitudinal view; appears as a “starry night in cross-section, more hypoechoic than nerves

• Pleura and Air – pleura appears hyperechoic, with the lung appearing hypoechoic underneath

• Cysts – similar appearance to vascular structures, cysts will also appear as hypoechoic in longitudinal view

• Bone – appear as hyperechoic linear structures with hypoechoic regions underneath (shadowing)

Not all nerves look the same…

Tibial and Common PeronealNerves

C5 C6 C7 Roots of the Brachial Plexus

Nerves vs. Tendons

• Nerves and tendons can

both appear as hyper-

echoic circles in the

periphery.

• They can be differentiated

by following their course

proximally along the

extremity


• Adipose tissue appears

hypoechoic with streaks of

irregular hyperechoic lines

• It is the most superficial

layer imaged


• Arteries, veins, and cysts

all appear as anechoic

structures

• Arteries are pulsatile, while

veins can be easily

compressed

• Arteries and veins will

appear as tube-like

structures in long view,

cysts will remain round


• Muscle appears

heterogeneous on

ultrasound because of the

varying acoustic

impedances between the

cell structures, the water

content within the cells,

and the intertwined fascia


• Lung tissue appears as a

thin hyperechoic line with

“tails” (areas of

reverberation) beneath it


• Bone is a significant

specular reflector, creating

a hyperechoic area with

shadowing beneath it

Doppler

• Animals use Doppler in

nature and it has been

adapted by man for many

purposes

• Doppler is dependent on

the angle of insonation

• Either the sender or

receiver must be moving

• Doppler is used to create

an image

Artifacts

• An artifact is any phenomenon that affects the

acquisition or interpretation of an ultrasound

image

• Artifacts can occur because of properties within

the tissue itself, or created by the anesthetist

• The most commonly seen artifacts are air

artifact, shadow artifact, acoustic

enhancement, and reverberation

Artifacts

• Air artifact occurs when

the transducer does not

fully contact the skin

• This is commonly seen

when imaging smaller

anatomical structures

• Applying sufficient gel to

the transducer and

applying even pressure will

correct this

Artifacts

• Shadow artifact (red

arrows) results from the

severe amount of

attenuation when an

ultrasound wave comes in

contact with bone or other

tissue with a high

attenuation coefficient

Artifacts

• Acoustic enhancement

occurs when a beam

passes through tissue with

low acoustic impedance

into tissue with a much

higher impedance, causing

it to look more echogenic

than it actually is

Artifacts

• Reverberation occurs

sound reflects off of strong

specular reflectors such as

this block needle creating

an illusion there are

“multiple” needles below

the actual one

Scanning basics

• Most ultrasound systems have the same basic

features. It is important to understand what each

function does prior to performing any procedure.

These include:

• B-Mode imaging

• Depth

• Contrast adjustment (gain)

• Color-flow Doppler

• Image recording capability (still images and video)

B-Mode imaging

• B-Mode is the default

image on all ultrasound

systems

• Pressing the 2D button will

reset the system to the

default mode, regardless

of the systems current

configuration

B-Mode imaging

B-Mode imaging

Cross-Section/Short-Axis Longitudinal/Long-Axis

Depth

• Depth determines how far

into the tissues echoes are

interpreted

• Increasing the depth,

decreases resolution

• In general, the structure of

interest is kept in the

center of the screen

Depth – Median nerve

Proper depth Too Deep

Gain

• Gain compensates for

attenuation of the

ultrasound wave

• It amplifies RETURNING

echoes

• Gain can be adjusted

either in the near field, far

field, or overall image

Gain – Just right

Gain

Too little gain Proper gain

Gain

Too much gain Proper gain

Time Gain Compensation

(TGC)

• TGC allows for control of

artifacts such as reflection,

shadowing and attenuation

• Increasing the TGC

increases the amplitude of

ultrasound wave at specific

levels of the image

TGC – No adjustment

TGC – With compensation

TGC - Dropout

Color-flow Doppler

• Color-flow allows you to

confirm the presence or

absence of vascular

structures

• Doppler is super-imposed

over the Bmode image

Color-flow Doppler

Image and video recording

• There are no current

guidelines as to what

should be documented

when performing an UGRA

procedure

• Image and video

recordings are valuable

tools for assessing block

performance

Images

Videos

Transducers

• The transducers used for UGRA are either linear

or curved linear array transducers

• Transducers are the link between the ultrasound

system and the tissue. They are complex and

play a large role in determining the resolution and

accuracy of an image

• Transducer selection is determined by the depth

of the structures to be imaged

Linear array

Curved array

Transducer handling

• Notch pointed to the anesthetist left or toward the patient’s head

• Hold the transducer flat against the patient’s skin for maximum contact

• Hold the transducer close to the skin surface, the ulnar aspect of the hand should rest on the patient

• Be gentle, but firm; sound waves penetrate tissue regardless of the pressure applied

Transducer handling

Proper Hand Position Poor Hand Position

Transducer manipulation

• In 1999, the American Institute for Ultrasound

Medicine (AIUM) established terminology to

describe transducer manipulation.

• These terms do not include specification of

direction (i.e. proximal, distal, clockwise)

• In general, a transducer can be manipulated in

five ways: sliding, tilting, rocking, rotating and

compressing.

Sliding

• To optimally display an

image, the transducer

must be manipulated

• This allows for smooth

scanning motion and

anatomy visualization

• Sliding allows the provider

to find the appropriate

level to perform the block

Tilting



must be manipulated


scanning motion and


• Tilting can be used to

correct air artifact and

increase contact of the

transducer

Tilting

Tilting the transducer to

correct air artifact Air artifact

Rocking



must be manipulated


scanning motion and


• Rocking improves nerve

image by accounting the

effect of anistropy

Rotating



must be manipulated


scanning motion and


• Rotating allows the

provider to optimize all

structures in the image on

the same plane

Compressing



must be manipulated


scanning motion and


• Compressing can be used

to decrease space from

the skin to the desired

structure being imaged

Transducer orientation

• Proper placement of the transducer in relation to

the patient’s position is key to obtaining a correct

image

• In general, the transducer should be oriented to

the anesthetist’s left in cross-section or toward

the patient’s head in longitudinal view

• When the transducer is not properly oriented, a

“mirror-image” will result

Transducer orientation

Body ergonomics

• Bed height at a

comfortable position

• U/S system in line with the

provider and patient

• Scanning arm resting in a

supported position

• Assistant (if required)

• Proper handling of

transducer

Body ergonomics

Poor ergonomics Good ergonomics

What now?

• You have the right patient, discussed the

proposed anesthetic technique, obtained

consent, verified the site, and gathered your

supplies

• Select the appropriate frequency transducer

• Prior to scanning, imagine how the image should

appear on the monitor

• Apply the principles of body ergonomics

• Apply sufficient gel to the transducer

• Jump in!

B-Mode imaging

• What am I seeing?!

• What can I do to make it

better?!

B-Mode imaging

Optimizing your image

• Use PLENTY of gel. Gel acts as a coupler

between the transducer and the skin, and

improves the image quality

• Ensure your transducer is initially perpendicular

and flat against the skin

• Optimize your depth so the structures you wish to

image are in the center of the screen

• Adjust your gain to make picture look uniform

Needle visualization

In-Plane Out-of-Plane

In-Plane/Long-Axis

Out-of-Plane/Short-Axis

Anatomy, anatomy, anatomy…

• Know your anatomy. Most nerves blocked using

regional anesthesia are in close proximity to

arteries, veins, or other vital organs (i.e. the

lungs)

• The key is being able to anticipate what you will

be seeing before you start scanning.

• Proper placement of your transducer will ensure

correct orientation of the picture

Anatomy, anatomy, anatomy…

Anistropy

• The term anistropy implies dependency on an

angle

• In ultrasound it is used to describe a change in

amplitude of received echoes when the angle of

insonation has changed.

Anistropy

Transducer Rocked Transducer at 90 degrees

Strategies to increase safety

• Ultrasound itself is a non-invasive modality that causes no complications to the patient

• Ultrasound-guided procedures introduce a needle and/or local anesthetic into a patient increasing the potential for complications

• Needle insertion should first be practiced using a phantom numerous times, with emphasis placed viewing the entire needle as it passes through the tissue

• Strategies such as wiggling, or hydro-location can be used to verify the location of the needle tip

Wiggling

• Tilt the needle to a more

superficial position,

reducing the angle of

insertion and bringing

angle closer to 90 degrees

• Wiggling from side-to-side

or slightly in and out until

the needle is seen

• Once the position is

determined, slight rotation

of the transducer will help

identify the tip

Hydro-location

• Injecting 1-2cc’s of fluid

through the block needle

will help determine the if

the part of the needle

visualized is the tip

• Using D5W will increase

nerve conduction if a nerve

stimulator is being used

Complications of UGRA

• Pneumothorax

• Hemothorax

• Vascular puncture

• Local Anesthetic toxicity (seizure, cardiac arrest)

• Venous thrombosis/Hematoma

• Dysrhythmias

• Neurologic complications (brachial plexus injury,

Horner syndrome)

• Parathesias

Risk factors

• Inexperience

• Large catheter size

• Morbid obesity (BMI > 30)

• Severe dehydration

• Coagulopathy

• Multiple attempts

Questions?

References

• Aldrich J E. Basic physics of ultrasound imaging. Crit Care Med. 2007;35(5 Suppl):S131-S137.

• Zagzebski JA. Physics and instrumentation in Doppler and B-mode ultrasonography. In: Zweibel WJ. Introduction to Vascular Ultrasonography. 4th ed. Philadelphia, PA: W.B. Saunders Company; 2000:17-43.

• Marhofer P, Frickey N. Ultrasonographic guidance in pediatric regional anesthesia part 1: Theoretical background. Paed Anaesth. 2006;16(10):1008-1018.

• Sites B D, Brull R, Chan V W, et al. Artifacts and pitfall errors associated with ultrasound-guided regional anesthesia. part I: understanding the basic principles of ultrasound physics and machine operations. Reg Anesth Pain Med2007;32(5):412-418.

• Falyar CR. Ultrasound in anesthesia: applying scientific principles to clinical practice. AANA J. 2010 Aug; 78(4):332-40.

• Kremkau F W. Doppler Ultrasound: Principles and Instruments. Philadelphia, PA: W.B. Saunders Company; 1990:5-51

References

• Taylor K J, Holland S. Doppler us. part i. basic principles, instrumentation, and pitfalls. Radiology. 1990; 174(2):297-307.

• Gray AT. Atlas of ultrasound-guided regional anesthesia. Philadelphia, PA. Saunders, Elsevier; 2010:45-67.

• Pollard BA, Chan VW. An introductory curriculum for ultrasound-guided regional anesthesia: a learner’s guide. Toronto. University of Toronto Press Inc.; 2009:23-28.

• Bigeleisen PE, ed, Orebaugh SL, Moayeri N, et al. Ultrasound-guided regional anesthesia ad pain medicine. Baltimore, MD. Lippincott Williams & Wilkins; 2010:26-33.

• Kossoff G. Basic physics and imaging characteristics of ultrasound. World J Surg. 2000; 24:134-142.

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