Emergency us physics and instrumentation

Emergency Ultrasound Physics &

Instrumentation :

Basic & Technical Facts For The Beginner

Nik Ahmad Shaiffudin Nik HimEmergency Physician

Hospital Sultanah Nur Zahirahdrnikahmad@gmail.com

Presentation Outline

1. Introduction to basic ultrasound physics

2. Ultrasound Modes

3. Artifacts

4. Probes

5. Terminology

6. Your machine functions

7. Summary

WHAT DO WE UNDERSTAND ABOUT

ULTRASOUND PHYSICS?

Introduction……1. Basic Ultrasound physics

Bats navigate using ultrasound

Bats make high-pitched chirps which are too high for humans to

hear. This is called ultrasound

Like normal sound, ultrasound echoes off objects

The bat hears the echoes and works out what caused them

• Dolphins also navigate with ultrasound

• Submarines use a similar method called sonar

We can also use ultrasound to

look inside the body…

Sound…?Sound is a mechanical, longitudinal wave that travels

in a straight line

Sound requires a medium through which to travel

Cycle 1 Cycle = 1 repetitive periodic oscillation

Cycle

frequency1 cycle in 1 second = 1Hz

1 second

= 1 Hertz

What is Ultrasound?Ultrasound is a mechanical, longitudinal wave with a

frequency exceeding the upper limit of human

hearing, which is 20,000 Hz or 20 kHz.

Medical Ultrasound 2MHz to 16MHz

to the audible frequency range ~ 20Hz - 20kHzThe human ear can only respond to the audible

frequency range ~ 20Hz - 20kHz

ULTRASOUND – How is it produced?

Produced by passing an electrical current through a

piezoelectrical (material that expands and contracts

with current) crystal….. “Pulse Echo” principle

Naturally occurring -

quartz

Synthetic - Lead

zirconate titanate (PZT)

In ultrasound, the following events

happen:

1. The ultrasound machine transmits high-frequency(1 to 12 megahertz) sound pulses into the bodyusing a probe.

2. The sound waves travel into the body and hit aboundary between tissues (e.g. between fluid andsoft tissue, soft tissue and bone).

3. Some of the sound waves reflect back to theprobe, while some travel on further until theyreach another boundary and then reflect back tothe probe .

4. The reflected waves are detected by the probeand relayed to the machine.

5. The machine calculates the distance from theprobe to the tissue or organ (boundaries) using thespeed of sound in tissue (1540 m/s) and the timeof the each echo's return (usually on the order ofmillionths of a second).

6. The machine displays the distances and intensities

of the echoes on the screen, forming a two

dimensional image.

Velocity in tissue

Medium Velocity of US (m/sec)

Air 330

Fat 1450

Water 1480

Soft tissue 1540

Kidney 1560

Blood 1570

Muscle 1580

Bone 4080

Ultrasound Production

Transducer produces ultrasound pulses

These elements convert electrical energy into a mechanical

ultrasound wave

Reflected echoes return to the scanhead which converts the

ultrasound wave into an electrical signal

Piezoelectric Crystals

The thickness of the crystal determines the

frequency of the scanhead

Low Frequency

3 MHz

High Frequency

10 MHz

What determines how far ultrasound

waves can travel?

The FREQUENCY of the transducer

The HIGHER the frequency, the LESS it can penetrate

The LOWER the frequency, the DEEPER it can penetrate

Attenuation is directly related to frequency

Frequency vs. Resolution

The frequency also affects the QUALITY of the ultrasound image

The HIGHER the frequency, the BETTER the resolution

The LOWER the frequency, the LESS the resolution

A 12 MHz transducer has very good resolution, but cannot

penetrate very deep into the body

A 3 MHz transducer can penetrate deep into the body, but the

resolution is not as good as the 12 MHz

Low Frequency

3 MHz

High Frequency

12 MHz

2. Ultrasound Modes

A-mode: A-mode (amplitude mode) is the simplest type of ultrasound. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. Therapeutic ultrasound aimed at a specific tumoror calculus is also A-mode, to allow for pinpoint accurate focus of the destructive wave energy.

B-mode or 2D mode: In B-mode (brightness mode) ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen. More commonly known as 2D mode now.

C-mode: A C-mode image is formed in a plane normal to a B-mode image. A gate that selects data from a specific depth from an A-mode line is used; then the transducer is moved in the 2D plane to sample the entire region at this fixed depth. When the transducer traverses the area in a spiral, an area of 100 cm2 can be scanned in around 10 seconds.[10]

M-mode: In M-mode (motion mode) ultrasound, pulses are emitted in quick succession – each time, either an A-mode or B-mode image is taken. Over time, this is analogous to recording a video in ultrasound. As the organ boundaries that produce reflections move relative to the probe, this can be used to determine the velocity of specific organ structures.

Doppler mode: This mode makes use of the Doppler effect in measuring and visualizing blood flow

Color Doppler: Velocity information is presented as a color-coded overlay on top of a B-mode image

Continuous Doppler: Doppler information is sampled along a line through the body, and all velocities detected at each time point is presented (on a time line)

Pulsed wave (PW) Doppler: Doppler information is sampled from only a small sample volume (defined in 2D image), and presented on a timeline

Duplex: a common name for the simultaneous presentation of 2D and (usually) PW Doppler information. (Using modern ultrasound machines color Doppler is almost always also used, hence the alternative name Triplex.)

Pulse inversion mode: In this mode two successive pulses with opposite sign are emitted and then subtracted from each other. This implies that any linearly responding constituent will disappear while gases with non-linear compressibility stands out.

Harmonic mode: In this mode a deep penetrating fundamental frequency is emitted into the body and a harmonic overtone is detected. In this way depth penetration can be gained with improved lateral resolution

2. Ultrasound Modes

M-mode: In M-mode (motion mode) ultrasound, pulses

are emitted in quick succession – each time, either an

A-mode or B-mode image is taken. Over time, this is

analogous to recording a video in ultrasound.

Doppler mode: This mode makes use of the Doppler

effect in measuring and visualizing blood flow

Shadowing

Posterior enhancement

Edge shadowing

Comet tail

Mirror Imaging

3. ArtifactsAttenuation artifact

Miscellaneous artifact Ring down

Side lobe

Echogenic :

When ultrasound waves pass

through solids (bones – stone)

all waves are reflected and

appears as white color with

posterior shadow .

Shadowing

It means the reflection of waves , and this depends on the

material which is penetrated by US.

• Echo free :

When ultrasound waves

pass through fluids (

ascites- simple cyst- blood

vessels) no reflection

occurs and these areas

appears as black areas

with posterior enhancement

. Posterior Enhancement & Mirrored Side

Posterior Enhancement, Side Lobe and Mirror Image

Mirror-image artifacts

Feldman M K et al. Radiographics 2009;29:1179-1189

US beam bounces between structure and deeper strong reflector

e.g. diaphragm. This means probe receives signals as if from same

object on other side of reflector.

Edge artifactEdge Artifact

Reverberations artifacts

Ultrasound echoes being

repeatedly reflected

between two highly

reflective interfaces

Comet Tail

Ring-down

Feldman M K et al. Radiographics 2009;29:1179-1189

Ring of bubbles with fluid trapped centrally. Fluid vibrations detected as

strong signal and displayed as line behind true source.

4. Probes Linear

Large convex

Sector

Intracavity ( Microconvex)

Probe typesSector Linear array Curved array

5. Common Terminology

Image Interpretation

Anechoic / Echolucent - Complete absent

of returning sound ( area is black)

Hypoechoic – Structures has very few

echoes and appears darker than

surrounding tissue

Hyperechoic/ Echogenic – Structure

appears brighter than surrounding tissues

No Reflections = Black dots

Fluid within a cyst, urine, blood

Anechoic / Echolucent - Complete absent of

returning sound ( area is black)

Hypoechoic – Structures has very few echoes and

appears darker than surrounding tissue

Weaker Reflections =

Grey dots

Most solid organs,

thick fluid – „isoechoic‟

Hyperechoic/ Echogenic – Structure appears

brighter than surrounding tissues

Strong Reflections = White dots

Diaphragm, tendons, bone

• Acoustic impedance (AI) is dependent on the density of the material

in which sound is propagated

- the greater the impedance the denser the material.

• Reflections comes from the interface of different AI‟s

• greater of the AI = more signal reflected

• works both ways (send and receive directions)

Medium 1 Medium 2 Medium 3

Tra

nsd

uc

er

Interactions of Ultrasound with Tissue

Sound is attenuated by tissue

More tissue to penetrate = more attenuation of

signal

Compensate by adjusting gain based on depthnear field / far field

AKA: TGC

Gain controls

receiver gain only

does NOT change power output

Increase gain = brighter

Decrease gain = darker

Use of Gain

GainMin

Max

Near field Far field

Attenuation

Time-gain compensation (TGC)

Pro

cess

ed

Ori

gin

al

Balanced Gain

Gain settings are important to obtaining adequate

images.

balanced

bad near fieldbad far field

Goal of an Ultrasound System

The ultimate goal of any ultrasound

system is to make like tissues look the

same and unlike tissues look different

Liver metastases

Epidermis

Loose connective tissue and subcutaneous fat

is hypoechoic

Muscle interface

Muscle fibres interface

Bone

Skin, subcutaneous tissue

Transverse scan – Internal Jugular Vein and

Common Carotid Artery

Image Acquisition / Probe position

Tranverse plane/ axial plane/ cross

section separates superior from

inferior

Sagittal plane – Oriented

perpendicular to the ground

separating left from right.

Coronal plane – Frontal plane,

separates anterior from posterior

Oblique Plane – The probe is oriented

neither parallel to nor at the right

angles from coronal, sagittal or

tranverse plane.

6. Your machine function

Your machine function

Summary

Know your anatomy – Skin, muscle, tendons, nerves

and vessels

Recognise normal appearances – compare sides!

Resolution determines image clarity

Frequency & wavelength are inversely proportional

Attenuation & frequency are inversely related

Display mode chosen determines how image is

registered

Diagnostic Medical Ultrasound is safe!

Conclusions

1. Imaging tool – Must have the knowledge to

understand how the image is formed

2. Dynamic technique

3. Acquisition and interpretation dependant upon the

skills of the operator.