Basics of Ultrasound Physics - PAWS · Basics of Ultrasound Physics ... o In ultrasound, the...
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11/4/2015 1 { Basics of Ultrasound Physics Department of Anesthesiology and Perioperative Medicine Division of Cardiothoracic Anesthesiology and Critical Care Medical College of Georgia, Georgia Regents University Manuel Castresana, MD, FCCM, FACA o Sound is the vibration of a physical medium o In ultrasound, the transducer is a mechanical vibrator – the transducer creates tissue vibrations o The resulting tissue vibrations consist of areas of compression and rarefaction resembling a sine wave Vibrations
Transcript of Basics of Ultrasound Physics - PAWS · Basics of Ultrasound Physics ... o In ultrasound, the...
11/4/2015
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Basics of Ultrasound Physics
Department of Anesthesiology and Perioperative Medicine
Division of Cardiothoracic Anesthesiology and Critical Care Medical College of Georgia, Georgia Regents University
Manuel Castresana, MD, FCCM, FACA
o Sound is the vibration of a physical medium
o In ultrasound, the transducer is a mechanical vibrator – the transducer creates tissue vibrations
o The resulting tissue vibrations consist of areas of compression and rarefaction resembling a sine wave
Vibrations
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o US is sound with frequencies greater than those of the audible range for humans (20,000 Hz)
o Medical ultrasound waves have frequencies of 2–10 MHz
o Sound waves are mechanical waves created in the transducer by back and forth displacement
Physics
o US transducers send out sound waves then “listen” for returning echoes
o Amplitude of a sound wave represents its peak pressure
o Frequency and wavelength: Sound waves are characterized by their frequency (expressed in cycle per second Hz) and wavelength
o Propagation velocity determine by the medium (soft tissue 1,540 m/s)
Physics
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Sound Wave
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Wavelength
Velocity
0 .5 microseconds
Frequency = 4 cycles/0.5 = 8 MHz
o Acoustic impedance determines the amount of sound waves transmitted and reflected by tissues
o Reflection occurs when the ultrasound beam hits two tissues (areas) with different acoustic impedances
o Large differences in impedances inhibit useful information
Physics
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o The propagation of sound waves through the body is affected by the different tissues encountered
o It can result in signal
o Reflection
o Refraction (angle, acoustic impedance)
o Scattering (small or irregular structures)
o Attenuation (dispersion, absorption)
Interactions
Interactions of Sound and Tissue
Transesophageal Echocardiography. 2nd ed. Perrino and Reeves
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o Hyperechoic
o Structure reflects most sound waves
o Structure appears white on screen
Terms
o Anechoic
o Structure allows most sound waves through
o Structure appears black on screen
Terms
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o Lower frequency
Transducers
o Lower spatial resolution
o Deeper penetration
o Higher frequency
Transducers
o Better spatial resolution
o Reduced penetration
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o The higher the frequency, the better the resolution
o The better the resolution, the better your ability to distinguish objects from each other
Transducers
o High frequency US – provides a very detailed image of superficial structures to a depth of approximately of 5 cm
o Lower frequency US – capable of reaching into deeper structures but provides less detailed images
Transducers
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o Low 2-5 MHz o Image depth: Deep
o Attenuation: Low
o Image quality: Lower
o High 5-10 MHz o Image depth: Shallow
o Attenuation: High
o Image quality: Higher
Frequency
o Linear probe: 4-10 MHz, good near field images, poor penetration (vessels)
o Curved probe: 2-5 MHz, wider field of view (abdomen)
o Sector probe: 1.5-4 MHz, small footprint (cardiac)
Probe Selection
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o Transverse (short axis) – a cross-section, provides information about structures adjacent to the vessel of interest
o Longitudinal (long axis) – depicts structures anterior and posterior to the vessel of interest
Planes and Views
o 2D (two dimensional): Standard imaging mode
o M (motion) mode: Exchanges 2D image quality for high temporal resolution along a single line
o 3D: Uses ECG-gated acquisition of 2D images over a wide range of angles, reconstructs the image after acquiring the data
Modes: B mode
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o PW: Displays velocity spectrum at a point (limit by velocity)
o CW: Displays velocity spectrum at a line (not limited by high velocity)
o Color
o The strength of the Doppler signal is related to the velocity and the angle (zero)
Modes: Doppler
Doppler
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M Mode
Color Doppler
Notch of the transducer
should match the dot
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o Gain – Degree of amplification of the returning sound
o Increasing the gain increases the strength of the returning echoes and results in a lighter image
o Decreasing the gain does the opposite
Knobology
Knobology Too much gain
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Too little gain
Knobology
Optimal gain
Knobology
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o Depth
o Each frequency has a range of depth of penetration
o Decrease the depth to visualize superficial structures
o May need to increase the depth of penetration to visualize larger organs
Knobology
o Zoom
o Can place zoom box on a portion of a frozen image to enlarge that portion of the image
o May lose some resolution because pixels are enlarged
Knobology
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o Resolution
o Attenuation
o Propagation
o Doppler
Artifacts
o Acoustic shadowing: When US reaches an structure with very high acoustic impedance it will not penetrate any further
o Reverberation: If the US reaches two reflectors, it might reflect multiple times, like a candle standing between two mirrors
o Mirror image: Areas of high acoustic impedance may serve as an acoustic mirror deflecting the bean to the side
o Refraction artifact: Side by side with the anatomical structure
Common Artifacts
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Reverberation
Mirror Image
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o Dynamic guidance: Localization and insertion image guided, more precise, difficult to maintain sterility
o Static guidance: Localization and marking, cannulation is not image guided
US Guidance
o Pressure
o Alignment
o Rotation
o Tilt
Probe Adjustments
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o Choose the correct probe
o Identify landmarks
o Probe orientation
o Maintain for 2D 90 for Doppler 0° Adjust gain and depth (and focus)
o Make adjustments to the probe (PART)
Key Points
Thank You
