Ultrasound Physics and Imaging Principles

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Questions and Answers

What is the relationship between the wavelength (λ) and the frequency (f) of an ultrasound wave?

  • λ is directly proportional to f
  • The relationship between λ and f is complex and cannot be easily described
  • λ is inversely proportional to f (correct)
  • λ is independent of f

Which of the following is NOT a major type of interaction between an ultrasound wave and soft tissue?

  • Attenuation
  • Polarization (correct)
  • Scattering
  • Reflection

What is the primary cause of attenuation in soft tissue?

  • Absorption of the wave's energy (correct)
  • Scattering of the wave
  • Refraction of the wave
  • Reflection of the wave

How does the propagation speed (c) of an ultrasound wave in soft tissue affect its wavelength (λ) for a given frequency (f)?

<p>λ is directly proportional to c (B)</p> Signup and view all the answers

What does the equation c = f ⨉ λ represent?

<p>The relationship between the propagation speed and the wavelength of an ultrasound wave. (B)</p> Signup and view all the answers

What is the relationship between the attenuation of an ultrasound wave and its frequency?

<p>Attenuation increases as frequency increases (A)</p> Signup and view all the answers

Which of the following is the most accurate statement regarding the image resolution of an ultrasound?

<p>Higher frequency ultrasound results in higher image resolution. (C)</p> Signup and view all the answers

Why is it not possible to keep increasing the frequency of ultrasound to achieve ever better resolution?

<p>Higher frequency ultrasound waves are more likely to be absorbed by tissue, limiting their penetration depth. (D)</p> Signup and view all the answers

What happens to the reflected ultrasound when the incident angle is not 90 degrees?

<p>The reflected ultrasound is not detected by the transducer. (D)</p> Signup and view all the answers

What is the primary difference between scattering and reflection in ultrasound imaging?

<p>Scattering produces weaker echoes than reflection. (C)</p> Signup and view all the answers

What is the main source of echo information in a typical ultrasound image?

<p>Scattering from soft tissue (D)</p> Signup and view all the answers

What does a reflection coefficient of 0.01 indicate?

<p>1% of the ultrasound energy is reflected. (A)</p> Signup and view all the answers

What is the significance of the acoustic impedance of two tissues being equal?

<p>No energy is reflected. (C)</p> Signup and view all the answers

When z1 and z2 are very different, what happens to the reflection coefficient?

<p>It approaches one. (D)</p> Signup and view all the answers

In the context of ultrasound imaging, what is 'perpendicular incidence'?

<p>The incident angle is 90 degrees. (B)</p> Signup and view all the answers

Which of the following accurately describes the relationship between acoustic impedance and the speed of sound in a medium?

<p>Acoustic impedance is directly proportional to the speed of sound. (A)</p> Signup and view all the answers

What is the frequency range of diagnostic ultrasound?

<p>2 MHz to 20 MHz (B)</p> Signup and view all the answers

Why are high frequencies used in diagnostic ultrasound?

<p>To increase image resolution (A)</p> Signup and view all the answers

What is the principle behind the generation of ultrasound waves?

<p>The transducer vibrates against the skin surface at the ultrasound frequency (D)</p> Signup and view all the answers

What happens to the tissue when the ultrasound transducer moves towards the body?

<p>The tissues are compressed, leading to increased pressure. (B)</p> Signup and view all the answers

What happens to the tissue when the ultrasound transducer moves away from the body?

<p>The tissues are decompressed, leading to decreased pressure (A)</p> Signup and view all the answers

Which of the following factors determines the amount of energy in an ultrasound wave?

<p>Amplitude (D)</p> Signup and view all the answers

Which of the following parameters describes the time taken for one complete cycle of pressure oscillation in an ultrasound wave?

<p>Period (C)</p> Signup and view all the answers

What is the relationship between the amplitude and energy of an ultrasound wave?

<p>Higher amplitude means higher energy (C)</p> Signup and view all the answers

What is the primary cause of attenuation of ultrasound waves in the body?

<p>All of the above (D)</p> Signup and view all the answers

How is attenuation measured?

<p>In units of decibels (dB) (B)</p> Signup and view all the answers

What is the formula for calculating attenuation in decibels?

<p>attenuation = (α ⨉ L ⨉ f) dB (D)</p> Signup and view all the answers

What is the typical attenuation coefficient (α) for soft tissue?

<p>0.5 dB/cm/MHz (A)</p> Signup and view all the answers

If a 3 MHz ultrasound wave travels 20 cm through soft tissue, how much attenuation will it experience?

<p>30 dB (A)</p> Signup and view all the answers

What is the effect of increasing the ultrasound frequency on the total attenuation?

<p>Increased frequency will increase attenuation. (B)</p> Signup and view all the answers

What does a 60 dB attenuation mean in terms of the intensity of the ultrasound wave?

<p>The intensity is reduced by a factor of 1,000,000 (A)</p> Signup and view all the answers

Why is it necessary to consider attenuation when interpreting ultrasound images?

<p>Attenuation can make it difficult to distinguish between structures at different depths. (D)</p> Signup and view all the answers

What is the name given to the depth at which echoes become undetectable due to round path attenuation?

<p>Penetration depth (A)</p> Signup and view all the answers

If the maximum attenuation a machine can tolerate is 100 dB, and the attenuation coefficient of the tissue is 0.5 dB/cmMHz, what is the penetration depth at a frequency of 5 MHz?

<p>10 cm (C)</p> Signup and view all the answers

Which of the following is NOT a factor affecting the penetration depth of ultrasound?

<p>Velocity of sound in the tissue (D)</p> Signup and view all the answers

What is the relationship between the penetration depth and the frequency of the ultrasound?

<p>Penetration depth decreases as frequency increases. (D)</p> Signup and view all the answers

Which of the following best describes the mechanism by which ultrasound interacts with small structures within tissues?

<p>Scattering (C)</p> Signup and view all the answers

What is the significance of acoustic impedance in ultrasound imaging?

<p>It determines the amount of ultrasound that is reflected at a tissue boundary. (C)</p> Signup and view all the answers

Which of the following is an example of a tissue interface that would likely produce a strong reflection of ultrasound?

<p>The boundary between bone and soft tissue (C)</p> Signup and view all the answers

What is the main reason that echoes attenuated by 120 dB are not detectable by ultrasound machines?

<p>The echoes are too weak in amplitude. (B)</p> Signup and view all the answers

Which of the following factors could cause a change in the average brightness of speckle in an ultrasound image?

<p>The number of scatterers per unit volume of tissue (A), The size of the scatterers (B), The wavelength of the ultrasound beam (C)</p> Signup and view all the answers

Why is speckle considered a random phenomenon in ultrasound imaging?

<p>The random arrangement of scatterers within the tissue (B)</p> Signup and view all the answers

What is the relationship between the depth of tissue and the appearance of speckle in an ultrasound image?

<p>Speckle becomes coarser at greater depths. (D)</p> Signup and view all the answers

What is the primary cause of refraction of an ultrasound wave?

<p>The difference in propagation speed between two tissues (C)</p> Signup and view all the answers

What does the equation sin θi/c1 = sin θt/c~2 represent?

<p>Snell's Law for the refraction of ultrasound (B)</p> Signup and view all the answers

Which of the following scenarios would result in NO refraction of an ultrasound wave?

<p>The ultrasound wave travels from one tissue to another with the same propagation speed. (A)</p> Signup and view all the answers

How does refraction affect the accuracy of ultrasound images?

<p>Refraction can lead to distortions in the shape and position of structures in the image. (D)</p> Signup and view all the answers

What does Snells Law assume about the medium that the ultrasound beam is traveling through?

<p>It assumes that the medium is homogeneous and isotropic. (D)</p> Signup and view all the answers

Flashcards

Diagnostic Ultrasound

A medical imaging technique using high-frequency sound waves to visualize internal body structures.

Ultrasound Wave Frequencies

Ranges from 2 MHz to 20 MHz, significantly higher than audible sound frequencies.

Compression

Increase in pressure caused when the ultrasound transducer moves toward the tissue.

Rarefaction

Decrease in pressure that occurs when the ultrasound transducer moves away from the tissue.

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Amplitude (A)

Maximum change of pressure from its mean value, indicating energy content in the ultrasound wave.

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Period (T)

Time taken for one complete cycle of pressure oscillation in an ultrasound wave.

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Ultrasound Propagation

The travel of ultrasound waves through human tissues at a constant speed.

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Image Resolution

Quality of the image determined by the frequency of the ultrasound waves; higher frequency results in better resolution.

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Ultrasound frequency

The number of cycles per second of an ultrasound wave, represented as f.

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Wavelength (λ)

The physical length of a single cycle of an ultrasound wave, linked to image resolution.

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Propagation speed (c)

The speed at which an ultrasound wave travels through tissue, conventionally 1540 m/sec in soft tissue.

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Relationship of c, f, λ

The equation c = f × λ describes how speed, frequency, and wavelength are interrelated.

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Attenuation

The decrease in strength of ultrasound waves as they travel through tissue, mostly due to energy absorption.

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Resolution

The clarity of an ultrasound image, improved with higher frequencies resulting in smaller wavelengths.

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Limitations of frequency

There is a physical limit to the frequency that can be effectively used in clinical ultrasound imaging.

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Energy Loss Mechanism

Ultrasound loses energy through friction, reflection, and scattering.

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Attenuation Coefficient (α)

A specific measure of how much ultrasound intensity decreases per tissue type per distance and frequency.

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Decibels (dB)

A logarithmic unit used to express the ratio of two intensities, often in sound or ultrasound.

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Ultrasound Intensity Ratio

The ratio of initial intensity (I1) to final intensity (I2) calculated in dB.

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Distance Travelled by Ultrasound (L)

The total distance ultrasound travels in cm before attenuation occurs.

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Frequency (f)

The rate at which the ultrasound wave oscillates, measured in megahertz (MHz).

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Round Path Attenuation

Total attenuation as ultrasound travels to a depth and returns, doubling the initial attenuation effect.

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Depth of Penetration (P)

The depth beyond which echoes cannot be detected by the machine.

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Impact of Frequency on Penetration

As frequency increases, penetration depth decreases to maintain a constant product of P × f.

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Reflection

Ultrasound reflects off large, smooth surfaces like light on glass.

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Scattering

Ultrasound scatters off small structures like red blood cells within tissues.

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Acoustic Impedance (z)

The resistance of tissue to the passage of ultrasound, affecting reflection amount.

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Round Path Attenuation Formula

The formula, attenuation = α ⨉ (2P) ⨉ f, describes maximum tolerable attenuation by machines.

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Acoustic Impedance

A property of tissue calculated as z = ρ ⨉ c, where ρ is density and c is propagation speed.

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Reflection Coefficient (R)

A measure of the fraction of ultrasound energy reflected at an interface, calculated as R = (z1 - z2)^2 / (z1 + z2)^2.

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Reflection Coefficient Value

A value indicating the fraction of energy reflected; R=0.01 means 1% is reflected, 99% passes through.

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Perpendicular Incidence

A situation where ultrasound hits the interface at a 90° angle, leading to maximum reflection detection.

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Characteristics of Scattering

Scattered energy is weaker than reflected energy and displayed as low- to mid-level grey tones in ultrasound images.

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Echo Detection

Echoes from ultrasound can be missed if reflected energy does not return to the transducer, especially with non-perpendicular incidence.

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Importance of Scattering

Most ultrasound imaging information comes from scattering in soft tissue rather than reflection between different tissues.

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Speckle

Granular echo texture in ultrasound images caused by random scatterers.

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Ultrasound Propagation Speed

The speed at which ultrasound waves travel through different tissues.

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Refraction of Ultrasound

Bending of ultrasound waves when passing through tissues with different propagation speeds.

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Snell's Law

Formula that calculates the angle of refraction between two media: sin(θi)/c1 = sin(θt)/c2.

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Incident Angle (θi)

The angle at which the ultrasound wave strikes the interface.

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Transmitted Angle (θt)

The angle at which the ultrasound wave exits into the second medium.

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Average Brightness of Speckle

The mean intensity of speckle in an ultrasound image that reflects tissue properties.

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Coarse vs Fine-grained Speckle

Texture of speckle varies with depth; finer near transducer, coarser deeper.

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Study Notes

Introduction to Diagnostic Ultrasound Technology

  • Course aims to provide a strong foundation in the physics of diagnostic ultrasound, explaining its clinical importance.
  • Course structure mirrors the textbook "The Physics and Technology of Diagnostic Ultrasound: A Practitioner's Guide (Second edition)."
  • The online course acts as an introduction, encouraging students to use the textbook for deeper understanding.
  • This module focuses on ultrasound wave properties, interaction with human tissues, and parallels textbook Chapter 2.

Ultrasound Interaction with Tissue

  • Ultrasound is high-frequency sound waves (2 MHz to 20 MHz), significantly greater than audible sound (20 Hz to 20 kHz).
  • Higher frequencies lead to better image resolution, enabling visualization of small structures.
  • Ultrasound waves are created by oscillating pressures.

Ultrasound Wave Propagation

  • Transducer oscillations generate ultrasound waves, which travel into tissues.
  • As the transducer moves towards the tissue, pressure increases (compression).
  • As the transducer moves away, pressure decreases (rarefaction).
  • These oscillating pressure changes constitute the ultrasound wave, traveling at a constant speed.

Key Ultrasound Wave Properties

  • Amplitude (A): Maximum pressure change from the mean value; determines energy level and tissue exposure.

  • Period (T): Time taken for one cycle, inversely related to frequency (f).

  • Wavelength (λ): Physical length of one cycle; related to resolution (shorter wavelength = better resolution).

  • Propagation Speed (c): Speed at which the ultrasound wave travels; approximately 1540 m/sec in normal soft tissue.

  • Relationship between speed, frequency, and wavelength (c = f X λ)

Attenuation

  • Attenuation describes the progressive weakening of an ultrasound wave as it travels through tissue.
  • Absorption is the primary cause of attenuation due to the friction of tissue particles.
  • Other factors include reflection and scattering which deflect energy. Defocusing spreads energy across a larger area.
  • Attenuation is calculated as a ratio of initial to final intensity in decibels (dB).
  • Attenuation=(x X L X f) dB, where a is the tissue's attenuation coefficient in dB/cm/MHz, L is travel distance in cm, and f is the frequency in MHz.
  • Tissue type, frequency, and distance traveled are factors in attenuation. Higher frequencies and longer travel distances mean higher attenuation.

Reflection and Scattering

  • Reflection: Interaction with large, smooth surfaces, akin to light reflecting from glass.

  • Scattering: Interaction with small structures (e.g., blood cells) resulting in energy distribution in all directions, generally weaker than reflected energy. Scattering is important in soft tissue imaging with grey tones in the image.

Refraction

  • Refraction is the bending of ultrasound waves as they pass between tissues with different propagation speeds. Total reflection can occur when the incidence angle exceeds a critical angle.

Speckle

  • Speckle refers to the granular echo texture caused by randomly positioned scatterers.
  • This random variation in echo amplitude affects image quality.
  • The structure of a speckle varies with depth and tissue characteristics.

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