Ultrasound physics and knobology

Ultrasound physics and Knobology

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Why Bother learning Physics

• To better understand the equipment and use its full Potential.

• Absolutely essential to acquire and interpret images accurately.

• Identify the importance of artifacts and differentiate from real structures.

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Acoustic Waves

• Sound is a mechanical wave that is transmitted by pressure waves through a medium

• The frequency of these waves is the number of vibrations that particles in the medium make per second

• Humans can detect sound with frequencies in the reange of 20-20000Hz. ULTRASOUND - More than 20000Hz

Acoustic Velocity (c) = frequency * Wavelength

Basic Nomenclature

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Acoustic Velocity in Ultrasound

Soft Tissue = 1540m/sec1540mm/msec

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Intensity and Power

Internsity is the rate at which energy is transmitted by the sound wave over a small area. It signifies particle oscillation. Frquency, wavelength and acoustic velocity is not affected by the change in intensityPower (W) is measure of the total energy transmitted per unit time (intensity) summed over the entire cross sectional area of the beamW = Intensity * Area (Watts)

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The different ways the sound beam interacts with the tissues is the foundation of ultrasound.

Interactions of Acoustic waves with tissue

• Reflection
• Scatter
• Refraction
• Diffraction
• Interference
• Absorption

Reflection

Sound reflects off a surface at an angle equal to the incedence angle.Interfaces where the sound interacts with the smooth surface at a 90degree are called specular reflectors

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What determines Reflection - Acoustic Impedence

Acoustic Impedance: The resistance to the propagation of ultrasound waves through tissues. Each tissue type has a unique acoustic impedance. Acoustic impedance is the product of the density and speed of sound in the tissue.If the acoustic impedences of two media are similar, sound will travel easilly between them. think of two billiard balls.If AI differ some portion of sound energy is reflected. Think of billiard ball hitting a truck

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What determines reflection

Percent reflectivity%R= { Z2-Z1/Z2+Z1 } * 100Independent from direction of travel%Transmisssion = 100 - %RSoft tissue- Bone - %R = 43 & %T = 57Air - soft tissue - %R =99.9%thats why we use gel

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US Reflection

Scattering

Gives tissues their unique echotexture

Fewer Echoes reflected back to transducer because there are many different angles of reflection

Refraction

Diffraction

Diffraction is the phenomenon b y which the beam spreads out as it moves farther from a point source.This increase as the source of the beam gets smaller (4-1 phased array probe)This affects the lateral resolution of the beam

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Interference

Constructive interferenceDestructive interferenceWe use Constructive interference in phased array transducers and dopplers.

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Absorption

Absorption is a process by which the sound energy is dissipated in the medium usually as heat.Therapeutic use.Think of friction

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Factors affecting absorption

• Frequency : imagine rubbing hands together
• Viscosity : Molecules have impedence to kinetic motion
• Relaxation time : more energy is required to get particles moving in opposite direction

Attenuation

• Attenuation is the overall effect of scatter and absorption.
• This is one determinate of the depth of beam penetration.
• While we cannot measure every interction of the beam with each particle it hits, we can make the assumption that penetrance is inversely related to the frequency of the beam.

Attenuation coefficients

Soft tissue Attenuation Coefficient

Gallstone shadowing: Posterior acoustic shadowing helps properly identify gallstones within the gallbladder and can help distinguish stones from other structures such as polyps.

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Gain and Time Gain compensation

Gain is a uniform amplification of the ultrasonic signal that is returning to the transducer after it travels through the tissue.TGC- Signal gain is increased as time passes from the emitted wave pulse. This correction makes equally echogenic tissues look the same even if they are located in different depths.

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Dynamic Range Compression

The term dynamic range is used to describe the overall range of echo intensities received by the ultrasound machine.It is defined as the ratio of the strongest to the weakest echo and is generally measured in decibels.the dynamic range of the machine’s display (and our eyes) is much more limited than the dynamic range of the echoes.The machine must compress the dynamic range of the echo signal so it matches the dynamic range of the display.

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Focus

Focal zone = Best lateral resolutionAdjust Focus to the level of the point of interestMost modern machines can have multiple focal zonesIncrease number of focal zone -> decreased frame ratef

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Long path reverbration artifacts - Alines

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Short path reverberations - B lines

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Short path reverberation artifact- Comet tail

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Generation of Ultrasound Waves

Piezoelectric effect = the ability of a material to exert mecahnical strain in response to an electric field and vice versa.Example Lead Zirconate titanate

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Pulse repetition frequency

PRF is the rate of pulses per secondMachine sets it up according to the depth set by the operator and acoustic velocity in the medium. Example = for 10cm, PRF max ~ 7700Hz

Modes of Transmission

A: Amplitude: not used anymore.B: Brightness, most commonly used. M: Motion, often used for cardiac. D: Doppler; "Colour" and "Pulse Wave" functions.

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B Mode

Brightness Mode : This mode is utilized to detect static structures and appreciate anatomy. Amplitudes from a returning ultrasound wave are displayed as points with differingbrightness.The brightness of a point represents the strength of the return wave.Once all the echoes from the transmitted pulses have returned, a complete 2D Brightness-mode image is displayed

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M mode/Motion Mode

Motion-Mode: This mode is utilized to image rapidly moving structures such as cardiac valves or vessel walls. Unlike the brightness mode, the motion mode utilizes repeated emission of an ultrasound beam in a stationary location to gain information from moving structures at different time points.Information is displayed along a time axis which describes the motion of the structure at varyingtime points

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DOPPLER SONOGRAPHY

The Doppler effect states that a sound wave that is reflected from a moving object undergoes a change or shift in frequency. More specifically, if an object is moving towards the transducer the incident wave will have a lower frequency than the reflected sound wave and vice-versa for objects moving away from the transducer.In medical imaging, this effect can be harnessed to calculate blood velocity, utilizing sound waves that are reflected by the traveling red blood cells.

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Colour Doppler and Pulse wave doppler

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Linear transducers generally have a high frequency for better imaging of superficial structures and vessels.The piezoelectric crystals in linear transducers are arranged linearly. This transducer emits a rectangular-shaped beam and produces good near-field resolution images. Linear transducers for 2D imaging have a central frequency between 5 MHz and 13 MHz.These transducers have a wide footprint and keep the same field of view at the deep part.They are typically used for breast examinations, body fat and muscle measurement, thyroid evaluation, airway assessment and blood vessel visualization.

Types of Transducers - Linear Transducer

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Curvilinear Transducer

Widened footprint and lower frequency for transabdominal imaging and a widened field of view.They have piezoelectric crystals arranged in a curvilinear fashion which produces a convex-shaped ultrasound beam.Ideal for in-depth imaging, even when resolutions decrease with depth.Convex transducers used for 2D imaging has a wide footprint with a field view spread at the deep part. It has a central frequency between 2 MHz and 7.5 MHz.They are typically used for abdominal and organ examinations.

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Phased Array Transducers

These transducers have a piezoelectric crystal arrangement known as phased-array.They have a small footprint resulting in a narrow beam point with a widely spread field of view at the deep part. They operate at a low frequency between 1 MHz and 7.5 MHz with a low near field resolution.Several applications include abdominal, cardiac, and brain examinations.

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ENDOCAVITARY TRANSDUCERS

These transducers allow internal examinations of the patient. They are designed to enter and fit in specific body orifices like the vagina and rectum. They have a small footprint and operate at a frequency range between 3.5 MHz and 11.5 MHz.

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Transoesophageal Transducers

These transducers are designed for internal imaging and function similar to flexible endoscopes, with controls on the probe body that allow the technician to manipulate the transducer lens inside the patient.These transducers have a small footprint and are used for internal examinations. They can be used to get a better image of the heart and its surrounding tissues through the esophagus. These transducers operate between 3 MHz and 10 MHz.

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Butterfly hand held iQ+

KNOBOLOGY & machine care

WHY Bother?Best Possible imageMost accurate image

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EVERY MACHINE IS DIFFERENT

Standard Controls

• Transducer selection
• Preset Selection
• Frequency Selection
• Time gain compensation
• Gain

• Focal zone
• Depth
• Dynamic Range
• Tissue harmonics
• Auto optimise

Probe

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Probe marker

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How to Hold the Probe

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Which Preset

• Correct preset maximises many of the features you would otherwise need to change manually
• Dynamic range
• frequency
• focal zone
• depth
• framerate

What Frequency

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Other controls

GAIN+ TGC

Depth andDynamic Range

Focal Zone

Know your Keyboard

MACHINE CARE

Probe

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It’s very simple - you use it, you clean it.Use manufacturer recommended cleaning solutionAlcohol wipes can damage the probes

Cleaning and disinfection

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GROUND RULES

• Explain (focused scan, not formal)
• Enter patient data, document image (R / L etc)

• Save images
• Mark the patient as appropriate
• Clean the patient afterwards
• Clean the probe, cord & keyboard
• Document in the patient record
• Log scan in log book

QUESTIONS

END

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