Ultra Physics

7/27/2019 Ultra Physics

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Ultrasound Physics

Sound is a mechanical, longitudinalwave that travels in a straight line

Sound requires a medium through which

to travelUltrasound is a mechanical, longitudinal

wave with a frequency exceeding the

upper limit of human hearing, which is20,000 Hz or 20 kHz.

Medical Ultrasound 2MHz to 16MHz


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Basic Ultrasound PhysicsAmplitude

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


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ULTRASOUND How is it produced?Produced by passing an electrical current

through a piezoelectrical (material thatexpands and contracts with current)

crystal


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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


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Frequency vs. Resolution The frequency also affects the QUALITY of the

ultrasound image The HIGHERHIGHERthe frequency, the BETTERBETTERthe

resolution

The LOWERLOWERthe frequency, the LESSLESS the resolution

A 12 MHz transducer has very good resolution, butcannot 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 MHzHigh Frequency

12 MHz


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Image Formation

Electrical signal produces dots on the screen

Brightness of the dots is proportional to thestrength 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


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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 AIs greater of the AI = more signal reflected

works both ways (send and receive directions)

Medium 1 Medium 2 Medium 3Transducer

Interactions of Ultrasound with

Tissue


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Greater the AI, greater the returned signal largest difference is solid-gas interface we dont like gas or air we dont like bone for the same reason

GEL!!

Sound is attenuated as it goes deeper into the body

Interaction of Ultrasound

with Tissue


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Tissue

Reflection Refraction

Transmission

Attenuation


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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


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Refraction

Incident

reflective

refraction

Angle of incidence = angle of reflection

Scattered

echoes


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Transmission

Some of the ultrasound waves continue deeper into

the body

These waves will reflect from deeper tissue

structures

transducertransducer


Tissue


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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


Tissue


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Reflected Echos Strong Reflections = White dots

Diaphragm, tendons, bone

Hyperechoic


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Weaker Reflections =Grey dots

Most solid organs,

thick fluid isoechoic

Reflected Echos


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Reflected Echos

No Reflections = Black dots

Fluid within a cyst, urine, blood

Hypoechoic or echofree
http://www.terarecon.com/gallery/images/us_ijv_tx_bmode.jpg


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What determines how far ultrasound waves can

travel?

The FREQUENCY of the transducer

The HIGHERthe frequency, the LESS it can

penetrate

The LOWERthe frequency, the DEEPERit can

penetrate Attenuation is directly related to frequency


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Ultrasound Beam Profile

Beam comes out as a slice

Beam Profile

Approx. 1 mm thick

Depth displayed user controlled Image produced is 2D

tomographic slice

assumes no thickness You control the aim

1mm


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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


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Types of Resolution Temporal Resolution

the ability to accurately locate the

position of moving structures at particularinstants in time

also known as frame rate


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Types of Resolution Contrast Resolution

the ability to resolve two adjacent objects

of similar intensity/reflective propertiesas separate objects - dependant on the

dynamic range


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Liver metastases


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Ultrasound Applications

Visualisation Tool:

Nerves, soft tissue masses

Vessels - assessment of position, size,

patency

Ultrasound Guided Procedures in realtime dynamic imaging; central venous

access, nerve blocks


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Imaging

Know your anatomy Skin, muscle,

tendons, nerves and vessels

Recognise normal appearances

compare sides!


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Epidermis

Loose connective tissue and subcutaneous fat

is hypoechoic

Muscle interface

Muscle fibres interface

Bone

Skin, subcutaneous tissue


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Transverse scan Internal Jugular Vein and

Common Carotid Artery


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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.


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www.upei.ca/~vetrad

Acoustic Impedance

The velocity of sound in a tissue and tissuedensity = determine acoustic impedance

Most soft tissues = 1400-1600m/sec Bone = 4080, Air = 330

Sound will not penetrate gets reflected orabsorbed

Travel time dot depth


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www.upei.ca/~vetrad

Attenuation

Absorption = energy is captured by the

tissue then converted to heat

Reflection = occurs at interfaces betweentissues of different acoustic properties

Scattering = beam hits irregular interface

beam gets scattered


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www.upei.ca/~vetrad

Ultrasound Terminology

Never use dense, opaque, lucent

AnechoicNo returning echoes= black (acellular fluid)

Echogenic Regarding fluid--some shade of grey d/t returning

echoes

Relative terms

Comparison to normal echogenicity of the same organor other structure

Hypoechoic, isoechoic, hyperechoic Spleen should be hyperechoic to liver

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