Ultrasound and Sound Waves in Medicine

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the primary aim of using ultrasound (US) in medicine?

To introduce the basic principles of diagnostic and therapeutic use of ultrasound.

Ultrasonography is considered a non-invasive medical imaging technique.

True

What types of sound waves can propagate in gases and liquids?

No sound waves

Longitudinal waves only (correct)

Both longitudinal and transverse waves

Transverse waves only

What is the frequency range for human hearing?

20 Hz to 20 kHz Signup and view all the answers

What is the approximate speed of sound propagation in soft tissues?

1540 m/s Signup and view all the answers

The phenomenon where sound intensity decreases due to interaction with small objects is called ______.

scattering Signup and view all the answers

What is acoustic impedance defined as?

The resistance of the medium against bringing into motion its particles. Signup and view all the answers

Increasing the frequency of ultrasound improves the intensity of the echo signal.

False Signup and view all the answers

What medical effect does higher intensity ultrasound have?

It can induce physical, chemical, or biological changes in tissues. Signup and view all the answers

The ratio of the angles of incidence and refraction is described by ______.

Snell’s Law Signup and view all the answers

Which phenomenon results in a decrease of sound intensity at the interface of two media?

All of the above Signup and view all the answers

How do piezoelectric materials generate ultrasound?

By mechanical deformation causing charge separation and producing voltage. Signup and view all the answers

What is the reciprocal effect of magnetostriction called?

Villari effect Signup and view all the answers

What is used for US generation?

Piezoelectric effect Signup and view all the answers

The same transducer is used for both generating US pulses and detecting echoes.

True Signup and view all the answers

What is the typical speed of US propagation in soft tissues?

1540 m/s Signup and view all the answers

What forms a two-dimensional B-mode image?

One-dimensional B-mode images Signup and view all the answers

What is the function of the damping unit behind the piezoelectric plate?

To absorb vibrations Signup and view all the answers

In ultrasound imaging, what is used to compensate for the gradual decrease of echo intensity?

Time Gain Compensation Signup and view all the answers

The Doppler-shift is zero when the US beam is perpendicular to the direction of blood flow.

True Signup and view all the answers

What is the ideal thickness of the couplant layer in ultrasound imaging?

One fourth of the emitted US wavelength Signup and view all the answers

The ability to distinguish two structures which lie along the axis of the US beam is known as ______ resolution.

axial Signup and view all the answers

HIFU stands for what in the context of ultrasound therapy?

High Intensity Focused Ultrasound Signup and view all the answers

Study Notes

Ultrasound and Its Applications

Ultrasound (US) is a valuable tool in both diagnostic and therapeutic medicine.
Diagnostic US, known as ultrasonography, utilizes low-intensity US waves to harmlessly image internal structures.
Therapeutic US employs high-intensity US to induce physical, chemical, or biological changes in targeted tissues.

Physical Properties of Sound

Sound is a mechanical wave propagating through elastic media.
This propagation occurs through the vibration of particles, not the movement of the particles themselves.
Sound requires a medium to travel; it does not travel in a vacuum.
Sound waves can be classified as either longitudinal or transverse, depending on the direction of particle vibration relative to wave propagation.
Longitudinal waves have particles vibrating parallel to the wave direction, while transverse waves have particles vibrating perpendicular to the direction.

Wave Parameters

Wavelength (λ) is the distance between two points with identical particle displacement or pressure.
Frequency (f) is the number of vibrations per second.
The speed of sound propagation (c) is determined by the medium's properties and is constant for a given medium.
The period (T) is the time required for a complete vibration cycle.
The amplitude (A) is the maximum displacement of a particle from rest.
Frequency and period are inversely related, expressed by the equation T = 1/f.

Sound Intensity

Sound intensity (J) is the energy per unit area per unit time, expressed in Watts per square meter (W/m2).
Sound intensity is proportional to the square of the amplitude of the vibrating particles.
In diagnostic ultrasound, low intensities are used to avoid tissue damage.
Therapeutic ultrasound employs higher intensities to induce targeted changes.

Attenuation of Sound

Sound attenuation occurs due to energy loss through friction between vibrating particles.
Attenuation is generally exponential, decreasing with distance traveled.
The attenuation coefficient (µ) accounts for both absorption and scattering.
Absorption is the conversion of sound energy into heat, and it increases with frequency.
Scattering occurs when sound waves interact with objects comparable in size to the wavelength, resulting in energy redirection.

Acoustic Impedance

Acoustic impedance (Z) is the resistance a medium offers to the movement of sound waves, defined as the ratio of pressure and particle velocity.
It's a key property determining sound transmission and reflection at interfaces between media.
Acoustic impedance is influenced by the medium's density (ρ) and compressibility (κ).

Reflection and Transmission of Sound

When a sound wave encounters an interface between two media with different acoustic impedances, it undergoes reflection and transmission.
Reflection is primarily determined by the difference in acoustic impedances.
The reflection coefficient (R) quantifies the amount of reflected sound intensity relative to the incoming intensity.
Reflection is stronger at interfaces with greater impedance differences.
Transmission is the passage of sound into the second medium, influenced by the angle of incidence and the ratio of sound speeds in the two media.

Ultrasound Generation

Transducers are essential for generating and detecting ultrasound.
Piezoelectric materials, like quartz and PZT, are commonly used due to their ability to convert mechanical deformation into electrical signals (direct piezoelectric effect) and vice versa (inverse piezoelectric effect).
By applying an alternating electric field to a piezoelectric material, we can induce vibrations at the desired frequency and generate ultrasound.
Other mechanisms for ultrasound generation include electrostriction, which uses the deformation of dielectric materials under electric fields, and magnetostriction, which utilizes the shape changes of ferromagnetic materials in magnetic fields.

Pulse-Echo Methods in Ultrasound Imaging

These methods involve emitting short ultrasound pulses into the body and detecting the reflected echoes.
By analyzing the time of arrival and intensity of echoes, we can obtain information about the depth and characteristics of reflecting structures.
The short pulse duration is crucial for resolving closely spaced interfaces.
Commonly, the same transducer serves for both generation and reception of ultrasound.

Applications of Ultrasound

Ultrasonography uses low-intensity ultrasound for non-invasive imaging of internal structures, for instance, in fetal monitoring and diagnosis of various medical conditions.
Therapeutic ultrasound employs higher intensities for targeted treatment, such as tumor ablation and breaking up kidney stones.

Image Formation in Ultrasound

The intensity and temporal properties of the detected echoes are processed to generate an image.
The time delay between the emitted pulse and the received echo determines the distance of the reflecting surface.
The intensity of the echo reflects the acoustic impedance of the reflecting surface and the attenuation of the sound wave during its path through the medium.

Ultrasound Artifacts

Various artifacts can be generated during ultrasound imaging, including:
- Shadowing, due to high sound reflection from structures like bone, preventing echoes from deeper tissues.
- Refraction, caused by bending of the sound beam at interfaces, which can distort image geometry.
- Scattering, which can lead to echoes from unintended locations.

Conclusion

Ultrasound is a versatile tool in medicine, enabling both non-invasive imaging and targeted therapeutic procedures. Understanding the fundamental physical properties of sound, its interaction with tissues, and the principles behind ultrasound generation and detection is essential for interpreting ultrasound images and effectively applying this technology.

Ultrasound Imaging

Time Gain Compensation amplifies signals from deeper tissues to compensate for signal loss.
Transducers are devices that convert electric energy into mechanical energy and vice versa.
Piezoelectric plates inside transducers create sound waves. Their thickness is usually half the wavelength of the emitted ultrasound wave.
Damping units help to produce short ultrasound pulses by absorbing vibrations and stopping the piezoelectric plate's oscillation.
Couplant layer protects the piezoelectric plate, transmits ultrasound pulses into the body, and optimizes wave interference using a specific thickness.
Acoustic Impedance of the couplant layer is ideal when it equals the geometric mean of the acoustic impedances of the piezoelectric crystal and the body tissues.
Pulse-Echo Methods utilize the same transducer for both emitting and receiving ultrasound pulses.
Pulse Repetition Time (PRT) refers to the interval between the emissions of ultrasound pulses. Longer PRT allows for detection of echoes from deeper tissues.
A-mode (amplitude) imaging uses one-dimensional scanning to display echo signal intensities along a line representing arrival time.
B-mode (brightness) imaging displays echo signal intensities as brightness levels on a screen.
M-mode (motion) imaging generates a sequence of one-dimensional B-mode images over time to visualize movement of reflecting surfaces, such as heart valves.
Two-dimensional B-mode imaging creates cross-sectional images by combining multiple one-dimensional B-mode images acquired during scanning.
Linear scanning involves moving the transducer in a straight line to generate a 2D image.
Sector scanning changes the transducer's angle to enable imaging in limited access areas, like the thoracic organs or through the skull.
Intracavitary scanning places the transducer inside body cavities for better access and reduced signal attenuation.
3D imaging combines multiple 2D images from different angles to produce a three-dimensional representation.
Imaging artefacts are reflections that don't correspond to actual structures.
- Shadowing: A dark area behind a strongly reflective surface.
- Mirror-image artefact: Duplicate image of an object in front of a reflective surface.
Spatial resolution is the ability to distinguish between two points in an image.
- Axial resolution is the ability to distinguish objects along the ultrasound beam's axis.
  - Higher frequencies lead to better axial resolution due to shorter pulse lengths.
- Lateral resolution is determined by the beam width.
  - The near field provides better lateral resolution than the far field.
Doppler Phenomenon: Frequency shift occurs if the source and observer are in relative motion.
Doppler Shift (fD) is the difference between the observed and emitted frequencies.
Doppler Ultrasound utilizes the Doppler effect to measure blood flow velocity by analyzing frequency shifts in echoes scattered by red blood cells.
Colour Doppler Imaging displays blood flow direction and velocity as different colors superimposed on a B-mode image.
Primary effects of Ultrasound:
- Sound pressure: Exerted by the ultrasound wave, proportional to intensity.
- Mechanical rubbing effect (micro massage): Friction caused by different particle speeds in the medium.
- Absorption: Conversion of ultrasound energy into heat, causing temperature increase in the medium.
- Cavitation: Formation and collapse of bubbles in liquids due to pressure fluctuations, creating rapid temperature changes.
Secondary effects of Ultrasound:
- Dispergation: Dispersion of solid materials due to mechanical rubbing.
- Chemical effects: Chemical reactions induced by thermal effects.
- Biological effects: Cell damage caused by thermal and mechanical effects.
High Intensity Focused Ultrasound (HIFU) uses high-intensity ultrasound beams to generate heat and cavitation for tumor ablation.
Extracorporeal Shockwave Lithotripsy (ESWL) uses high-intensity shock waves to break kidney stones. The exact mechanism is still unknown, but cavitation and mechanical forces likely contribute.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz explores the key concepts of ultrasound and its applications in diagnostic and therapeutic medicine. It also covers the physical properties of sound, including its classification as mechanical waves and the parameters that describe wave behavior. Test your knowledge and understanding of these fundamental topics in physics and medical imaging.