Imagination at work

Basics of Ultrasound Physics

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Clinical Ultrasound

• Mode of medical imaging with wide array of clinical applications

• Clinical applications vary widely by market, by physician

• Clinical Ultrasound can be the primary imaging modality or used as an adjunct to other imaging tests

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Advantages of Ultrasound

• Portable

• Relatively inexpensive

• Non-invasive

• Non-ionizing radiation

• No magnets

• Relatively quick

• Dynamic and real-time

• Side-by-side imaging

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Disadvantages of Ultrasound

• Operator dependent

• Learning curve

• Cannot penetrate bone or air

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Advances in Ultrasound

• Software based systems

• Digitization

• Beam former technology changes

• New probes

• Miniaturization

How Does Ultrasound Work?

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Transducers

Any device that converts one form of energy into another.

Examples:

• A microphone converts mechanical energy waves or sound into electrical energy waves

• A light bulb converts electrical energy into light and heat

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Ultrasound Transducer

• An ultrasound transducer converts electrical energy into acoustic pulses (sound) and transmits them into the body

• The transducer then acts as the receiver of the reflected echoes and converts them into electrical signals which are processed as an image

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Pulsed Echo Ultrasound

• A few cycles of ultrasound are separated in time by gaps of no signal

• The transducer “sends” a pulse less than 1% of the time and “listens” for returning echoes more than 99% of the time

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Time

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Components of a Transducer (probe)

Wire Case Backing Material

PZT Matching Layer

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Ultrasound Transducer

The source of sound in medical ultrasound is the piezoelectric element (crystal)

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•

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NEVER heat sterilize an ultrasound transducer

Crystals are delicate, avoid dropping transducers especially on the face of the probe

NEVER use a transducer with a cracked housing or frayed/damaged cord

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Transmission and Reflection of Ultrasound

4080

1580

1550

1540

1560

1570

1450

1480

330

0 1000 2000 3000 4000 5000

Bone

Muscle

Liver

Soft…

Kidney

Blood

Fat

Water

Air

Speedm/s

Medium

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Types of Arrays

Sector Linear Convex

Small footprint, large field of view

High resolution near field

Max field of view & penetration

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Test Your Knowledge

Q What are three advantages of ultrasound over other imaging modalities?

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Test Your Knowledge

A What are three advantages of ultrasound over other imaging modalities?

• Portable

• Relatively inexpensive

• Non-invasive

• Non-ionizing radiation

• No magnets

• Relatively quick

• Dynamic and real-time

• Easy side-by-side imaging

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Test Your Knowledge

Q True or False? Transducers (probes) act as both transmitters and receivers.

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Test Your Knowledge

A True or False? Transducers (probes) act as both transmitters and receivers. True!

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Frequency

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Frequency

• The number of cycles that occur in one second.

• The unit of measurement for frequency is Hertz (Hz).

• Frequency affects the detail and contrast resolution of the image, as well as penetration of the ultrasound wave.

1 cycle

1s

1 second

(8 cycles/sec)= 8 Hz

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Frequency

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Frequency

Ultrasound has the same properties of audible sound, just at much higher frequencies.

Sub Sound < 20 Hz

Audible Sound 20 Hz – 20 kHz

Ultra Sound > 20 kHz

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Transducers are characterized by their frequency

• High frequency probes: greater resolution, less penetration

• Low frequency probes: more penetration, less resolution.

High Frequency

Low Frequency

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Frequency vs Penetration

It’s an age-old balancing act choosing the right probe with adequate depth penetration and exceptional image quality.

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Frequency and Depth

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Attenuation

High Energy

Low Energy

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Attenuation and Frequency

Attenuation is the weakening of the ultrasound wave as it passes through tissue and returns to the sound source.

• High frequency ultrasound pulses are attenuated faster than low frequency ultrasound pulses

• Low frequency ultrasound pulses are attenuated slower than high frequency ultrasound pulses

• Attenuation in soft tissue = 0.5 to 1 dB/cm/MHz

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Time Gain Compensation (TGC)

Helps compensate for attenuation

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Test your knowledge

The higher the frequency, the ____________ the resolution and the __________ the penetration.

The lower the frequency, the __________ the resolution

and the ____________ the penetration.

Q

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Test your knowledge

A The higher the frequency, the greater the resolution and the less the penetration.

The lower the frequency, the less the resolution and the

greater the penetration.

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Test your knowledge

Q The weakening of the ultrasound wave as it passes through tissue and returns to the sound source is known as: A. Resolution B. Frequency C. Attenuation D. TGC

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Test your knowledge

A The weakening of the ultrasound wave as it passes through tissue and returns to the sound source is known as: A. Resolution B. Frequency C. Attenuation D. TGC

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Resolution

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Types of Resolution

Lateral and Axial

The minimum distance between two adjacent structures, or how close two structures can be to be visualized as two unique structures.

Detail

The minimum size of a structure that can be detected.

Contrast

Ability to appreciate subtle differences in grayscale.

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Resolution

Poor Resolution Good Resolution

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Resolution and Probe Position

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Resolution and Focus

The focus is the point of the ultrasound beam that is narrowest and provides the best imaging.

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Resolution and Focal Zones

Set/adjust the focus by setting/adjusting the focal zones.

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Characterized Tissue with Ultrasound

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Homogenous and Heterogenous

Homogenous: Uniform echoes

Heterogenous: Mix of different echoes (shades of gray)

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Anechoic

Without echoes

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Hyperechoic / Echogenic

• With echoes

• When used in comparison with other tissues/structures denotes a structure that has brighter echoes than surrounding structures

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Hypoechoic

Darker shade of gray than surrounding tissues/structures

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Test Your Knowledge

Why is Focus Important? Q

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Test Your Knowledge

Why is Focus Important? The focus is the point of the ultrasound beam that is narrowest and provides the best imaging.

A

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Test Your Knowledge

What does the term echogenicity mean? A. Ability to sharply and clearly define the extent or

shape of a structure on an image B. Describes the contrast of the image C. Relative term that describes the levels of gray D. The weakening of the sound beam

Q

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Test Your Knowledge

A What does the term echogenicity mean? A. Ability to sharply and clearly define the extent or

shape of a structure on an image B. Describes the contrast of the image C. Relative term that describes the levels of gray D. The weakening of the sound beam

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Artifacts

An error in imaging

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Artifacts

• Can help categorize tissue types

• Can help with a diagnosis

• Can be detrimental (can obscure anatomy)

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Shadowing

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Shadowing Characteristics

• Absence of reflectors on the image – too little information

• Caused by high attenuation or reflection of the sound beam

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Enhancement

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Enhancement

Overly bright reflectors

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Test Your Knowledge

True or False: All ultrasound artifacts are detrimental. Q

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Test Your Knowledge

True or False: All ultrasound artifacts are detrimental. False!

A

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Test Your Knowledge

What is the difference between hypoechoic, hyperechoic and anechoic?

Q

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Test Your Knowledge

What is the difference between hypoechoic, hyperechoic and anechoic? Hypoechoic = darker gray than surrounding tissues Hyperechoic= overly bright echoes Anechoic= no echoes/black

A

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Image Orientation

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Image Orientation

Long Sagittal Longitudinal Long Axis

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Image Orientation

Anterior

Posterior

Head Feet

Anterior

Posterior

Head Feet

Anterior

Sagittal/Long Axis Plane

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Image Orientation

Posterior

• Short

• Short Axis

• Axial

• Transverse

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Image Orientation

Anterior

Posterior

Right Left

Anterior

Posterior

Right Left

Transverse/Short Axis Plane

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Knobology

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ALARA Principle

• The ALARA Principle (as low as reasonably achievable)

• The potential benefits and risks of each examination should be considered

• Consider this principle when adjusting controls that affect the acoustic output and by considering length of scan time