Physics And Instrumentation of

Ultrasound

WHAT DO YOU UNDERSTAND ABOUT ULTRASOUND ?

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…

• Ultrasound

– Cyclic sound pressure with a frequency greater than the upper limit of human hearing.

• Human Ear Audible Range Frequency?

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

Medical sonography (ultrasonography) Ultrasound-based diagnostic imaging

technique used to visualize muscles and internal organs, their size, structures and possible pathologies or lesions.

APPLICATIONS? ADVANTAGES & DISADVANTAGES?

Diagnostic applications• Cardiology• Gynaecology & Obstetrics • Ophthalmology• Abdomen• Urology- to determine, for example, the amount of fluid

retained in a patient's bladder. • Musculoskeletal - tendons, muscles, and nerves • Vascular - arteries and veins • Interventional biopsy - emptying fluids, intrauterine

transfusion

Therapeutic applications• Therapeutic applications use

ultrasound to bring heat or agitation into the body.

• Therefore much higher energies are used than in diagnostic ultrasound.

ULTRASOUND PHYSICS

Format

What is sound/ultrasound?How is ultrasound producedTransducers - propertiesEffect of Frequency Image Formation Interaction of ultrasound with tissueAcoustic impedance Image appearance

Sound?Sound is a mechanical, longitudinal wave that

travels in a straight line

Sound requires a medium through which to travel

CATEGORIES OF SOUND Infrasound (subsonic) below 20Hz Audible sound 20-20,000Hz Ultrasound above 20,000Hz Nondiagnostic medical applications

<1MHz Medical diagnostic ultrasound

>1MHz

In 1826 DanielColladon, a Swissphysicist, and CharlesSturm, a Frenchmathematician,accurately measured itsspeed in water. Using along tube to listenunderwater (as Leonardoda Vinci suggested in1490), they recorded howfast the sound of asubmerged bell traveledacross Lake Geneva.Their result--1,435meters persecond in water of 1.8degrees Celsius (35degrees Fahrenheit)--wasonly 3 meters per secondoff from the speedaccepted today.

Compression wave

Acoustic Variables• Period• Wavelength• Amplitude• Frequency• Velocity

Acoustic Variables

Acoustic Variables

Amplitude, A (m)The maximum displacement that occurs in an acoustic variable.

Why we use different frequency?

Basic Ultrasound PhysicsAmplitude

oscillations/sec = frequency - expressed in Hertz (Hz)

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

ULTRASOUND – How is it produced?

Produced by passing an electrical current through a piezoelectrical (material that expands and contracts with current) crystal

Human HairHuman Hair

Single Single CrystalCrystal

Microscopic view of scanheadMicroscopic view of scanhead

In ultrasound, the following events happen:

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

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

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

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

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

6. The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image.

Piezoelectric materialAC applied to a piezoelectric crystal

causes it to expand and contract – generating ultrasound, and vice versa

Naturally occurring - quartz

Synthetic - Lead zirconate titanate (PZT)

Ultrasound Production Transducer produces ultrasound pulses

(transmit 1% of the time) 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 CrystalsThe thickness of the crystal determines

the frequency of the scanhead

Low Frequency3 MHz

High Frequency10 MHz

Frequency vs. Resolution The frequency also affects the QUALITY of the ultrasound image The HIGHERHIGHER the frequency, the BETTERBETTER the

resolution The LOWERLOWER the frequency, the LESSLESS 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 Frequency3 MHz

High Frequency12 MHz

Broadband vs. Narrowband

Frequency

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Broadband vs. Narrowband

Nerve Visualisation: 5-10 MHz 6-13 MHz By altering the transmit frequencies one transducer

replaces several transducers View a range of superficial to deep structures without

changing transducers

Transducer Design

Size, design and frequencydepend upon theexamination

Image Formation

Electrical signal produces ‘dots’ on the screen

Brightness of the dots is proportional to the strength of the returning echoes

Location of the dots is determined by travel time. The velocity in tissue is assumed constant at 1540m/sec

Distance = Velocity Time

‘B’ mode

Image Formation

Interactions of Ultrasound with Tissue

Reflection Refraction Transmission Attenuation

Interactions of Ultrasound with Tissue

Reflection The ultrasound reflects off tissue and returns to

the transducer, the amount of reflection depends on differences in acoustic impedance

The ultrasound image is formed from reflected echoes

transducertransducer

Refraction

Incident

reflective

refraction

Angle of incidence = angle of reflection

Scattered

echoes

Interactions of Ultrasound with Tissue

Transmission Some of the ultrasound waves continue deeper

into the body

These waves will reflect from deeper tissue structures

transducertransducer

Interactions of Ultrasound with Tissue

Attenuation Defined - the deeper the wave travels in the

body, the weaker it becomes -3 processes: reflection, absorption, refraction

Air (lung)> bone > muscle > soft tissue >blood > water

• 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 3Tra

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r

Interactions of Ultrasound with Tissue

•Greater the AI, greater the returned signal• largest difference is solid-gas interface• we don’t like gas or air• we don’t like bone for the same reason GEL!!

•Sound is attenuated as it goes deeper into the body

Interaction of Ultrasound with Tissue

• Z (Rayls) = Density (kg/m³) x Speed (m/s)

• Incident beam has normal incidence 90 degree (perpendicular incidence) on the tissue interface, the magnitude of reflection can be calculated (IRC)

• α Z values

Attenuation & Gain

Sound is attenuated by tissueMore tissue to penetrate = more

attenuation of signalCompensate by adjusting gain based

on depth near field / far field AKA: TGC

Ultrasound Gain

Gain controls receiver gain only does NOT change power output think: stereo volume

Increase gain = brighter Decrease gain = darker

Balanced Gain Gain settings are important to obtaining

adequate images.

balancedbalanced

bad near fieldbad near fieldbad far fieldbad far field

Reflected Echo’s Strong Reflections = White dots

Diaphragm, tendons, bone

‘Hyperechoic’

Reflected Echo’s

Weaker Reflections = Grey dots

Most solid organs,

thick fluid – ‘isoechoic’

Reflected Echo’s No Reflections = Black dots

Fluid within a cyst, urine, blood‘Hypoechoic’ or echofree

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

Ultrasound Beam Depth• Need to image at proper depth• Can’t control depth of beam

• keeps going until attenuated• You can control the depth of displayed data

Ultrasound Beam Profile

Beam comes out as a sliceBeam Profile

Approx. 1 mm thick Depth displayed – user controlled

Image produced is “2D” tomographic slice assumes no thickness

You control the aim

1mm

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

Accomplishing this goal depends upon...

Resolving capability of the system axial/lateral resolution spatial resolution contrast resolution temporal resolution

Processing Power ability to capture, preserve and display the

information

Types of Resolution Axial Resolution

specifies how close together two objects can be along the axis of the beam, yet still be detected as two separate objects

frequency (wavelength) affects axial resolution – frequency resolution

Types of Resolution Lateral Resolution

the ability to resolve two adjacent objects that are perpendicular to the beam axis as separate objects

beamwidth affects lateral resolution

Types of Resolution Spatial Resolution

also called DetailDetail Resolution

the combination of AXIAL and LATERAL resolution - how closely two reflectors can be to one another while they can be identified as different reflectors

Types of Resolution Temporal Resolution

the ability to accurately locate the position of moving structures at particular instants in time

also known as frame rate

Types of Resolution Contrast Resolution

the ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects - dependant on the dynamic range

Liver metastases

Ultrasound ApplicationsVisualisation Tool:

Nerves, soft tissue masses

Vessels - assessment of position, size, patency

Ultrasound Guided Procedures in real time – dynamic imaging; central venous access, nerve blocks

Imaging

Know your anatomy – Skin, muscle, tendons, nerves and vessels

Recognise normal appearances – compare sides!

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

Summary

•Imaging tool – Must have the knowledge to understand how the image is formed

•Dynamic technique

•Acquisition and interpretation dependant upon the skills of the operator.