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Introduction to the Physics of Ultrasound
Dr MS Abdullatif Critical care consultant
RCoA Tutor Stepping Hill hospital Exam Board RCoA
MBbCH, MSC, FRCA, FICM, BSE-TOE
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Introduction to the Physics of Ultrasound
• Explain the Basics of ultrasound and wave creation • Describe Factors that affect US image and discuss image
optimization techniques (Terminology & knobology) • Review applications of specific imaging modes • Choose appropriate transducers
Objectives
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Introduction to the Physics of Ultrasound
Basics of ultrasound and wave creation
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Introduction to the Physics of Ultrasound
What is Sound? • Sound is a mechanical, longitudinal wave that
travels in a straight line • Sound requires a medium through which to travel
Measured in Hertz (Hz) -Human Hearing 20 - 20,000 Hz -Ultrasound > 20,000 Hz -Diagnostic Ultrasound 2.5 to 10 MHz (this is what we use!)
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Introduction to the Physics of Ultrasound
How can we produce US Piezo-electricity
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Introduction to the Physics of Ultrasound
Frequency and wavelength
• 15 MHz
• O.1 mm
• 5 MHz
• 0.31 mm
C = f x λ λ = C / f
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Introduction to the Physics of Ultrasound
How does an ultrasound machine make an image ?
Distance = 1/2 X C (1540 m/s) X T
Distance = time
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Introduction to the Physics of Ultrasound
US Interaction with tissues
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Introduction to the Physics of Ultrasound
US Interaction with tissues
Reflection Back to Transducer Scatter In Multiple Directions Refraction Redirection or Bending Absorption Converted to Heat
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Introduction to the Physics of Ultrasound
Interactions of Ultrasound with Tissue
• Reflection – The ultrasound reflects off tissue and returns to
the scanhead- amount of reflection depends on differences in acoustic media
– The ultrasound image is formed from reflected echoes
Scanhead
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Introduction to the Physics of Ultrasound
• Transmission – Some of the ultrasound waves continue deeper into
the body – These waves will reflect from deeper tissue structures
Scanhead
Interactions of Ultrasound with Tissue
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Introduction to the Physics of Ultrasound
Reflection Vs Scattering
• Large and θ near to 90° • Smooth • Target size > λ
• Small • Rough surface • Target size < λ
Reflection Scattering
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Introduction to the Physics of Ultrasound
Scattering
RA
LA
Uneven surface Small target size
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Introduction to the Physics of Ultrasound
• Attenuation – The deeper the wave travels in the body, the weaker it
becomesdue to processes: reflection, absorption, scattering
– Air (lung)> bone > muscle > soft tissue >blood > water
Interactions of Ultrasound with Tissue
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Introduction to the Physics of Ultrasound
Factors affecting US image and image optimization techniques
(Terminology & knobology)
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Introduction to the Physics of Ultrasound
Image Quality and Θ
Chest wall
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Introduction to the Physics of Ultrasound
Media (tissue) Characteristics
• Density
• Stiffness
• Propagation Speeds • Impedance = density X propagation speed
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Introduction to the Physics of Ultrasound
Image Quality and Acoustic Impedance (Z)
Tissue AI (106 Raylas) Air 0.0004 Lung 0.18 Fat 1.34 Liver 1.65 Blood 1.65 Kidney 1.63 Muscle 1.71 Bone 7.8
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Introduction to the Physics of Ultrasound
Image Quality and Z
• Air AI 0.0004 • Tissue AI 1.34 • Air-tissue great AI
mismatch • Strong reflection and
little transmission • Gel
1.Acoustic coupling
02/02/16
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Introduction to the Physics of Ultrasound
Image Quality and Z
Hypoechoic Less echogenic than surrounding tissue
Hyperechoic More echogenic than surrounding tissue
Anechoic Absence of Echoes
Isoechoic Same echogenicity as
surrounding tissue
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Introduction to the Physics of Ultrasound
AI and Echogenicity
0.18
1.71 0.01 7.8
Echogenicity
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Introduction to the Physics of Ultrasound
Goal of an Ultrasound System The ultimate goal of any ultrasound system is to
make like tissues look alike and unlike tissues look different
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Introduction to the Physics of Ultrasound
What determines how far ultrasound waves can travel?
• The FREQUENCY of the scanhead – The HIGHER the frequency, the LESS it can penetrate – The LOWER the frequency, the DEEPER it can penetrate – Attenuation is directly related to frequency
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Introduction to the Physics of Ultrasound
Frequency vs. Resolution • The frequency also affects the QUALITY of the
ultrasound image – The HIGHER the frequency, the BETTER the
resolution – The LOWER the frequency, the LESS the resolution
Frequency choice is a trade off between resolution and penetration
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Introduction to the Physics of Ultrasound
Adjusting the Frequency
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Introduction to the Physics of Ultrasound
Image optimization (Depth)
Too shallow Too deep Just right
Set the depth to the minimum required to see all structures
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Introduction to the Physics of Ultrasound
Depth
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Introduction to the Physics of Ultrasound
Image optimization (Gain)
• Too little • Too much • Just Right
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Introduction to the Physics of Ultrasound
Ultrasound Gain
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Introduction to the Physics of Ultrasound
Image Optimization (Zoom)
02/02/16
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Introduction to the Physics of Ultrasound
Caliper
AV diameter Bladder Volume
02/02/16
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Introduction to the Physics of Ultrasound
Applications of specific imaging modes
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Introduction to the Physics of Ultrasound
US Modes
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Introduction to the Physics of Ultrasound
Image formation B and 2D Modes
• The strength or amplitude (brightness) of each reflected wave is represented by a dot
• The position of the dot represents the depth from which the returning echo was received
• These dots are combined to form a complete image
2D phased array
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Introduction to the Physics of Ultrasound
Sectors
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Introduction to the Physics of Ultrasound
MM
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Introduction to the Physics of Ultrasound
Color Doppler
• Pixels assigned color based on mean velocity of the object
• Displays direction of blood flow
Direction of flow
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Introduction to the Physics of Ultrasound
Color Doppler is angle dependent. Therefore there is little or no flow at perpendicular angles. Remember BART – Blue Away – Red Towards when red bar is on top.
Towards Transducer NO FLOW Away from Transducer
Color Doppler
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Introduction to the Physics of Ultrasound
Color Doppler
Color Doppler is angle dependent
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Introduction to the Physics of Ultrasound
Probes
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Introduction to the Physics of Ultrasound
What is a scanhead? • Contains piezoelectric elements/crystals which
produce the ultrasound pulses • This element converts electrical energy into a
mechanical ultrasound wave
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Introduction to the Physics of Ultrasound
Anatomy of the Scan Head
Backing - dampens sound after pulse is generated Covering - protects transducer face Crystals - converts energy – transmits / receives sound Matching Layer - assists in sound transmission
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Introduction to the Physics of Ultrasound
Human Hair
Single Crystal
Microscopic view of scanhead
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Introduction to the Physics of Ultrasound
Scanhead Crystals • The thickness of the crystal determines the
frequency of the scanhead
Low Frequency 3 MHz
High Frequency 10 MHz
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Introduction to the Physics of Ultrasound
Frequency vs. Resolution
Frequency choice is a trade off between resolution and penetration
Format Footprint (mm) Frequency (MHz)
Linear L 38 13-6
Curved Linear C 60 5 - 2
Phased P 21 5 - 1
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Introduction to the Physics of Ultrasound
Position of Reflected Echoes
• Display screen divided into a matrix of PIXELS (picture elements)
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Introduction to the Physics of Ultrasound
Reflected Echos • Strong Reflections = White dots
– Diaphragm, gallstones, bone • Weaker Reflections = Grey dots
– Most solid organs, thick fluid • No Reflections = Black dots
– Fluid within a cyst, urine, blood
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Introduction to the Physics of Ultrasound
Transducer Orientation
Markings are located on one side of transducer only and correspond to orientation marker on screen
vertical protrusion
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Introduction to the Physics of Ultrasound Be aware that you are looking at a 1mm slice
• Think of a credit card coming out of the end!
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Introduction to the Physics of Ultrasound +Probes
• Uses: – Vascular access – Nerve Blocks – 6-12 MHz
02/02/16
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Introduction to the Physics of Ultrasound +Probes
• Uses: – Abdominal – Obs and Gynae – 2-5 MHz
02/02/16
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Introduction to the Physics of Ultrasound +Phased Array
• TTE • Smaller footprint • 1-5 MHZ
02/02/16
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Introduction to the Physics of Ultrasound
Questions