Thoracic Ultrasound: Physics and General Principles

Pen Li, MD, FRCPC

Interventional Pulmonology

Acknowledgements

• Thank you Brian Buchanan for helping out with the development of this presentation

• Thank you Ashley Gillson for organizing the course

Objectives

1. Learn basic ultrasound physics

2. Probe selection

3. Orientation

4. Knobology

5. Image optimization

Why learn thoracic ultrasound

• No radiation

• Fast bedside imaging

• Inexpensive

• Better for pleural disease than chest x-rays and often times CT if characterizing pleural fluid

• Standard of care for thoracic procedures

• “A picture is worth a thousand words”

Physics

Physics of ultrasound: Producing ultrasound

• “Ultrasound” waves (~2-15MHz) are soundwaves with frequencies higher than the human upper audible limit (~2-20 kHz)

• Electrical pulses deforms/excites the piezoelectric crystals

• Ultrasound vibrations are produced by piezoelectric crystals in the transducer probe

Physics of ultrasound: Producing ultrasound

• Ultrasound waves interact with the medium and then some reflect back

• Echo waves reflected back to the transducer crystals

• Data is translated by machine into an image

• Longer time for waves to return to transducer = increased depth (deeper)

Other technologies: IQ probe

Micromachines act as drums to produce Ultrasound

Process data through a semiconductor chip

Permits portability with low price range

Physics of ultrasound: Interactions with tissue

Physics of ultrasound: Sound wave physicsHigher frequency

1. Higher resolution2. Reduced penetration

Lower frequency

1. Lower resolution2. Increased penetration

Physics of ultrasound: Acoustic impedence

• 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 change/difference of the AI = more signal reflected

• works both ways (send and receive directions)

Medium 1 Medium 2 Medium 3Tra

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Physics of ultrasound: Acoustic impedence

Physics of ultrasound: Air causes scatter

• Air causes high scatter of ultrasound waves

• Ultrasound gel is used as a coupling agent

Echogenicity terminology

Isoechoic

Hyperechoic

Hypoechoic

Anechoic

Common ultrasound artifacts

Artifact: Reverberation – Long path (A-lines)

Pleural line

A - line

A - line

A - line

Artifact: Reverberation – Short path (B-lines)

More B lines = less air, and more lung density

Artifact: Reverberation – Short path (B-lines)

Artifact: Acoustic shadowing

Artifact: Posterior acoustic enhancement

Acoustic enhancement(by time gain compensation)

• May mask free fluid

Artifact: Mirror Images

Ultrasound technique:

• Beam comes out as a 1mm slice

• Image produced is “2D”

• Be sure to scan the general area to better understand all the adjacent structures

• Don’t be afraid to move it around

Ultrasound technique: Transducer probes

Ultrasound technique: Probe orientation

(conventionally left)

Freeze

IMAGING TYPES

AUTO-GAIN

TIME GAIN COMPENSATION

DEPTH

Adjustments: Gain, the “brightness knob”• Degree of amplification of the returning sound

• Increasing the gain, increases the strength of the returning echoes and results in a lighter image

Adjustments: Time gain compensation

FAR FIELD

NEAR FIELD

Adjustments: Depth, the “zoom knob”

Center the area of interest

Ultrasound modes

• B mode (brightness mode, 2D mode): • The default mode• “brightness” depends on the intensity/amplitude of the

received signals

• M mode (motion mode)• Displays time motion display of ultrasound wave along a

chosen line

• Color mode• Assess flow• Red = towards transducer• Blue = away from transducer

M mode

Normal Seashore sign Barcode sign (pneumothorax)

Waves

Sand

Thanks for listening!

• Understanding physics helps to understand your image

• Play with the knobs and buttons on the machine