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

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

Introduction to the Physics of Ultrasound

Basics of ultrasound and wave creation

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!)

Introduction to the Physics of Ultrasound

How can we produce US Piezo-electricity

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

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

Introduction to the Physics of Ultrasound

US Interaction with tissues

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

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

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

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

Introduction to the Physics of Ultrasound

Scattering

RA

LA

Uneven surface Small target size

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

Introduction to the Physics of Ultrasound

Factors affecting US image and image optimization techniques

(Terminology & knobology)

Introduction to the Physics of Ultrasound

Image Quality and Θ

Chest wall

Introduction to the Physics of Ultrasound

Media (tissue) Characteristics

•  Density

•  Stiffness

•  Propagation Speeds •  Impedance = density X propagation speed

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

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

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

Introduction to the Physics of Ultrasound

AI and Echogenicity

0.18

1.71 0.01 7.8

Echogenicity

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

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

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

Introduction to the Physics of Ultrasound

Adjusting the Frequency

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

Introduction to the Physics of Ultrasound

Depth

Introduction to the Physics of Ultrasound

Image optimization (Gain)

•  Too little •  Too much •  Just Right

Introduction to the Physics of Ultrasound

Ultrasound Gain

Introduction to the Physics of Ultrasound

Image Optimization (Zoom)

02/02/16

Introduction to the Physics of Ultrasound

Caliper

AV diameter Bladder Volume

02/02/16

Introduction to the Physics of Ultrasound

Applications of specific imaging modes

Introduction to the Physics of Ultrasound

US Modes

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

Introduction to the Physics of Ultrasound

Sectors

Introduction to the Physics of Ultrasound

MM

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

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

Introduction to the Physics of Ultrasound

Color Doppler

Color Doppler is angle dependent

Introduction to the Physics of Ultrasound

Probes

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

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

Introduction to the Physics of Ultrasound

Human Hair

Single Crystal

Microscopic view of scanhead

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

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

Introduction to the Physics of Ultrasound

Position of Reflected Echoes

•  Display screen divided into a matrix of PIXELS (picture elements)

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

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

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!

Introduction to the Physics of Ultrasound +Probes

•  Uses: –  Vascular access –  Nerve Blocks –  6-12 MHz

02/02/16

Introduction to the Physics of Ultrasound +Probes

•  Uses: –  Abdominal –  Obs and Gynae –  2-5 MHz

02/02/16

Introduction to the Physics of Ultrasound +Phased Array

•  TTE •  Smaller footprint •  1-5 MHZ

02/02/16

Introduction to the Physics of Ultrasound

Questions