thoracic ultrasound: physics and general principles
TRANSCRIPT
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Thoracic Ultrasound: Physics and General Principles
Pen Li, MD, FRCPC
Interventional Pulmonology
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Acknowledgements
• Thank you Brian Buchanan for helping out with the development of this presentation
• Thank you Ashley Gillson for organizing the course
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Objectives
1. Learn basic ultrasound physics
2. Probe selection
3. Orientation
4. Knobology
5. Image optimization
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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”
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Physics
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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
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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)
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Other technologies: IQ probe
Micromachines act as drums to produce Ultrasound
Process data through a semiconductor chip
Permits portability with low price range
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Physics of ultrasound: Interactions with tissue
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Physics of ultrasound: Sound wave physicsHigher frequency
1. Higher resolution2. Reduced penetration
Lower frequency
1. Lower resolution2. Increased penetration
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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
ns
du
cer
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Physics of ultrasound: Acoustic impedence
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Physics of ultrasound: Air causes scatter
• Air causes high scatter of ultrasound waves
• Ultrasound gel is used as a coupling agent
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Echogenicity terminology
Isoechoic
Hyperechoic
Hypoechoic
Anechoic
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Common ultrasound artifacts
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Artifact: Reverberation – Long path (A-lines)
Pleural line
A - line
A - line
A - line
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Artifact: Reverberation – Short path (B-lines)
More B lines = less air, and more lung density
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Artifact: Reverberation – Short path (B-lines)
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Artifact: Acoustic shadowing
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Artifact: Posterior acoustic enhancement
Acoustic enhancement(by time gain compensation)
• May mask free fluid
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Artifact: Mirror Images
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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
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Ultrasound technique: Transducer probes
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Ultrasound technique: Probe orientation
(conventionally left)
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Freeze
IMAGING TYPES
AUTO-GAIN
TIME GAIN COMPENSATION
DEPTH
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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
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Adjustments: Time gain compensation
FAR FIELD
NEAR FIELD
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Adjustments: Depth, the “zoom knob”
Center the area of interest
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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
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M mode
Normal Seashore sign Barcode sign (pneumothorax)
Waves
Sand
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Thanks for listening!
• Understanding physics helps to understand your image
• Play with the knobs and buttons on the machine