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ULTRASOUND – THE BASICS

Published by Marion Hunter Modified over 8 years ago

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Basic Ultrasound Physics - PowerPoint PPT Presentation

Basic Ultrasound Physics

Sound is a mechanical wave that travels in a straight line ... properties of sound waves. velocity. frequency. wavelength. amplitude. atl internal & confidential ... – powerpoint ppt presentation.

• Sound is a mechanical wave that travels in a straight line
• Requires a medium through which to travel
• Ultrasound is a wave with a frequency exceeding the upper limit of human hearing
• greater than 20,000 Hz (hertz)
• The speed with which a soundwave travels through a medium
• Units of measure are distance/time
• The speed of sound is determined by the density and stiffness of the media in which it travels
• slowest in air/gasses
• fastest in solids
• Average speed of ultrasound in the body is 1540 m/sec
• The number of cycles occurring in one second of time (cycles per second)
• One cycle is represented in red
• Hertz 1 cycle in one second
• Kilohertz (kHz) 1,000 cycles per second (or 1,000 Hertz)
• Megahertz (MHz) 1,000,000 cycles per second (or 1,000,000 Hertz)
• ultrasound imaging frequency range is 2-12 MHz
• Length of space over which one cycle occurs (distance)
• Given a constant velocity, as frequency increases wavelength decreases/shortens
• Common ultrasound frequencies and wavelengths
• 2.25 MHz 0.60 microns
• 5.0 MHz 0.31 microns
• 10.0 MHz 0.15 microns
• The strength/intensity of the soundwave at any given point in time
• Represented by the height of the wave
• Amplitude/intensity decreases with increasing depth
• Pulse-Echo Method
• Ultrasound scanhead produces pulses of ultrasound waves
• These waves travel within the body and interact with various organs
• The reflected waves return to the scanhead and are processed by the ultrasound machine
• An image which represents these reflections is formed on the monitor
• Transmission
• Attenuation
• Reflection occurs at a boundary/interface between two adjacent tissues
• The difference in acoustic impedence (z) between the two tissues causes reflection of the sound wave
• z density x velocity
• The greater the difference in acoustic impedence between two adjacent tissues, the greater the reflection
• If there is no difference in acoustic impedence, there is no reflection
• Reflection from a smooth tissue interface(specular) causes the soundwave to return to the scanhead
• The ultrasound image is formed from reflected echoes
• Redirection of the soundwave in several directions
• Caused by interaction with a very small reflector or a very rough interface
• Only a portion of the soundwave returns to the scanhead
• Not all of the soundwave is reflected, therefore some of the wave continues deeper into the body
• These waves will reflect from deeper tissue structures
• The deeper the wave travels in the body, the weaker it becomes
• The amplitude/strength of the wave decreases with increasing depth
• The ultimate goal of any ultrasound system is to make like tissues look alike and unlike tissues look different
• Resolving capability of the system
• axial/lateral resolution
• spatial resolution
• contrast resolution
• temporal resolution
• Beamformation
• send and receive
• Processing Power
• ability to capture, preserve and display the information
• Axial Resolution
• specifies how close together two objects can be along the axis of the beam, yet still be detected as two separate objects
• wavelength affects axial resolution
• Lateral Resolution
• the ability to resolve two adjacent objects that are perpendicular to the beam axis as separate objects
• beamwidth affects lateral resolution
• Spatial Resolution
• also called Detail Resolution
• the combination of AXIAL and LATERAL resolution
• some customers may use this term
• Contrast Resolution
• the ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects
• Temporal Resolution
• the ability to distinguish very rapid events in sequence
• also known as frame rate
• Matching Layer
• has acoustic impedance between that of tissue and the piezoelectric elements
• reduces the reflection of ultrasound at the scanhead surface
• Piezoelectric Elements
• produce a voltage when deformed by an applied pressure
• quartz, ceramics, man-made material
• Damping Material
• reduces ringing of the element
• helps to produce very short pulses
• The piezoelectric element/crystal produces the ultrasound pulses
• Electrical pulses applied to the crystal cause it to expand and contract
• This produces the transmitted ultrasound pulses
• The frequency of the scanhead is determined by the thickness of the crystals
• Thinner elements produce HIGHER frequencies
• Thicker elements produce LOWER frequencies
• The frequency also affects the quality of the image
• the higher the frequency, the shorter the wavelength
• the shorter the wavelength, the better the axial resolution
• Therefore, higher frequency scanheads produce better image resolution
• The HIGHER the frequency, the LESS it can penetrate into the body
• The LOWER the frequency, the DEEPER the penetration
• Bandwidth is the range of frequencies emitted by the scanhead
• Each crystal emits a spectrum of frequencies
• A broadband scanhead is one which uses the entire frequency bandwidth to form the image
• A narrowband scanhead uses only a portion of the frequency range to form the image
• Reflected echoes return to the scanhead where the piezoelectric elements convert the ultrasound wave back into an electrical signal
• The electrical signal is then processed by the ultrasound system
• The BEAMFORMER is the ultrasound engine
• It coordinates and processes all the signals to and from the scanhead elements
• It is the main component responsible for image formation
• The strength or amplitude of each reflected wave is represented by a dot
• The position of the dot represents the depth from which the returning echo was received
• The brightness of the dot represents the strength of the returning echo
• These dots are combined to form a complete image
• Display screen divided into a matrix of PIXELS (picture elements)
• How does the system know the depth of the reflection?
• The system calculates how long it takes for the echo to return to the scanhead
• The velocity in tissue is assumed constant at 1540m/sec
• Velocity Distance x Time
• 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
• DOPPLER is used to hear and measure blood flow
• COLOR or CPA (Color Power Angio) is added to visualize blood flow
• M-mode uses a graphic representation to measure the movement of heart structures

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Jul 17, 2014

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Ultrasound. Ultrasound. What is Ultrasound Defined as sound w/ frequency > 20,000 cycles per sec Ultrasound travels thru materials Thermal & non-thermal properties. Ultrasound. Terminology Transducer (sound head) – converts electrical energy into sound energy

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• thicker tissues
• greater circulation cooler
• thermal effects
• carpal tunnel syndrome
• central nervous system tissue
• current resulting

Presentation Transcript

Ultrasound • What is Ultrasound • Defined as sound w/ frequency > 20,000 cycles per sec • Ultrasound travels thru materials • Thermal & non-thermal properties

Ultrasound • Terminology • Transducer (sound head) – converts electrical energy into sound energy • Power (W) – amount of sound energy per unit of time • Intensity (W/cm2) – amount of power per unit time

Ultrasound • Terminology • Continuous US • Pulsed US

Ultrasound • Terminology • Pulsed US • Duty cycles • 20% duty cycle – 20% on : 80% off • 50% duty cycle – 50% on : 50% off

Ultrasound • Terminology • Frequency – number of cycles per sec (Hz) • 1.0 MHz penetrates deeper tissues • 3.0 MHz or 3.3 MHz penetrates more superficial tissues • Phonophoresis – application of US with a topical drug

Generation of Ultrasound • Crystals in the sound head expand & contract in response to electrical current resulting in resonating ultrasound

Effects of Ultrasound • Thermal Effects • Tissues Affected

Effects of Ultrasound • Thermal Effects • Factors Affecting the Amount of Temperature Increase • Areas of increased collagen content achieve higher temps • Areas of greater circulation cooler faster • Thicker tissues heat slower

Effects of Ultrasound • Thermal Effects • Applying Other Physical Agents in Conjunction With Ultrasound • Hot pack prior to US • US & electrical stimulation • US & cryotherapy – used to limit/control thermal effects of US

Effects of Ultrasound • Nonthermal Effects • Increase intracellular Ca • Increase skin & membrane permeability (phonophoresis) • Increase macrophage response • Increase protein synthesis by fibroblasts

Clinical Applications of Ultrasound • Soft Tissue Shortening • Thermal effects • Pain Control • Thermal effects

Clinical Applications of Ultrasound • Dermal Ulcers • Thermal effects • Increased macrophage activity • Increased protein synthesis by fibroblasts

Clinical Applications of Ultrasound • Surgical Skin Incisions • Thermal effects • Increase macrophage response • Increase protein synthesis by fibroblasts

Clinical Applications of Ultrasound • Tendon Injuries • Thermal effects • Increase macrophage response • Increase protein synthesis by fibroblasts

Clinical Applications of Ultrasound

Clinical Applications of Ultrasound • Bone Fractures • Promotes healing • Simulates osteoblast activity • Increase intracellular Ca • Reabsorption of Calcium Deposits • Unknown how this occurs

Clinical Applications of Ultrasound • Carpal Tunnel Syndrome • Phonophoresis • Unit on phonophoresis in your book is required reading

Contraindications for the Use of Ultrasound • Malignant Tumor • Pregnancy • Central Nervous System Tissue • Joint Cement

Contraindications for the Use of Ultrasound • Plastic Components • Pacemaker • Thrombus • Eyes • Reproductive Organs

Precautions for the Applications of Ultrasound • Acute Inflammation • Epiphyseal Plates • Fractures (high frequency US) • Breast Implants

Adverse Effects of Ultrasound • Adverse effects are “rare” • Burn is the most common adverse effect

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