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Questions and Answers
What is the relationship between wavelength and frequency in a given medium for sound waves?
How is the amplitude of a sound wave defined?
What does the period (T) of a wave represent?
What physical phenomena describe the fluctuations in density and pressure as a sound wave passes through a medium?
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Which of the following parameters is NOT used to describe sound waves?
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How is the speed of sound propagation (c) in a medium primarily determined?
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In the context of sound waves, what does the maximum change of pressure refer to?
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What is defined as the number of vibrations per second in the context of sound waves?
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What does the reflection coefficient (reflexivity, R) represent?
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What is the primary reason for ultrasonic shadowing at the soft tissue/bone interface?
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How is the phenomenon of beam bending or refraction described?
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What can reduce reflections when ultrasound is used on soft tissues?
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What is the formula for the Doppler-shift when the ultrasound beam is parallel to the blood flow direction?
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Which of the following statements about Doppler-shift in ultrasound is true?
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Why are weak echoes sufficient for visualization in ultrasound imaging?
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What is a consequence of completely reflecting sound at the air/soft tissue interface?
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What color represents blood flow towards the transducer in the Colour-Doppler technique?
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What phenomenon causes the ultrasound wave to exert pressure on objects?
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What effect does significant refraction have in ultrasound diagnostics?
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What is a primary consequence of high-frequency ultrasound waves in a medium?
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What role do acoustic lenses play in ultrasound?
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Which statement correctly describes blood cells when forming echoes in ultrasound?
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Why is the measurement of Doppler-shift convenient in ultrasound applications?
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What occurs when the ultrasound beam is incident perpendicular to the direction of blood flow?
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What is the primary function of the damping unit behind the piezoelectric plate in a transducer?
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How does Time Gain Compensation affect signals from deeper tissues?
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What characteristic of the piezoelectric plate is ideal for generating ultrasound pulses?
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What is the ideal relationship between the acoustic impedance of the couplant layer and the acoustic impedances of the piezoelectric crystal and body tissues?
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Which function do transducers perform in ultrasound technology?
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What is the primary method used to generate a 2D image by moving the scanner in one line?
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What role does the couplant layer play in ultrasound transmission?
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What is the speed of ultrasound propagation in soft tissues?
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What is one advantage of using sector scanning over linear scanning?
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Which of the following is a typical imaging artefact associated with ultrasound images?
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In pulse-echo methods, what is the primary use of the same transducer?
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How is axial resolution defined in ultrasound imaging?
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What is a significant factor that improves axial resolution in ultrasound imaging?
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Which ultrasound technique involves inserting the scanner into body cavities?
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What does the term spatial resolution refer to in ultrasound imaging?
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What causes a mirror-image artefact in ultrasound imaging?
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Study Notes
Transverse Waves
- Transverse waves can also be formed in solids and at the surface of liquids.
Waves: Physical Parameters
- The vibration of particles in longitudinal waves is parallel to the direction of wave propagation.
- The vibration of particles in transverse waves is perpendicular to the direction of wave propagation.
- Wavelength (λ) is the distance between nearest points with identical values of particle displacement, pressure, or density.
- Frequency (f) is the number of vibrations per second.
- The speed of sound propagation (c) is determined by the composition of the medium.
- Wavelength and frequency are inversely proportional to each other and their product is constant, and equal to the speed of sound propagation: (c = λf).
- Period (T) is the time for one complete vibration (1/f).
- The amplitude of the vibration (A) is the maximal displacement of particles from their resting state.
Sound Wave Propagation
- Sound waves travel through an elastic medium as a series of compressions and rarefactions.
- This means that both the density and pressure fluctuate compared to their steady-state values.
- Sound waves can be described using the pressure difference (Δpt,x) as a periodic function of time (t) and position (x).
- In harmonic oscillations, Δpt,x can be represented by a sine function.
Sound Waves: Frequency and Reflection
- Sound waves can be classified by their frequency (pitch).
- The extent of reflection is described by the reflection coefficient (R), which is the ratio of reflected (JR) and incoming (J0) sound intensities.
- The reflexivity of an interface is determined by the change in acoustic impedance (Z) across the interface.
- The angle of incidence equals the angle of reflection, and in perpendicular incidence, the reflected beam returns to the source.
Acoustic Impedance and Reflection
- Acoustic impedances of soft tissues (e.g., liver, kidney, fat) are close, resulting in weak reflections.
- However, these weak echoes may be sufficient for visualization in ultrasound imaging.
- A large difference in acoustic impedance, such as in soft tissue/bone interface, results in strong reflection.
- This leads to strong echo signals from bone surfaces and ultrasonic shadowing behind them.
- Air/soft tissue interface has practically complete reflection, requiring a connecting medium (e.g., gel or water) for ultrasound transmission.
- The acoustic impedance of the connecting medium is similar to the acoustic impedance of body tissues, facilitating ultrasound entry into the body.
Refraction
- At an interface, a sound wave entering a second medium may change direction, called "beam bending" or refraction.
- The amount of bending is determined by Snell's Law: the ratio of angles of incidence and refraction is equal to the ratio of sound speeds in the two media (c1/c2).
- Significant refraction requires relatively large changes in sound velocity.
- Refraction can contribute to imaging artifacts in ultrasound diagnostics.
- Acoustic lenses can be used to focus sound waves by utilizing refraction.
Ultrasound Imaging: Time, Intensity, and Echoes
- The time required for receiving an echo signal provides information about the distance of the echoing surface from the transducer.
- The intensities of the echo signals depend on the absorption coefficient of tissues and the distance of the echoing surface.
- US signals from deeper tissues are amplified more to compensate for the gradual decrease in echo intensity with depth (Time Gain Compensation).
Transducers
- US sources, known as transducers, convert one type of energy to another (electric energy↔mechanical energy).
- The actual sound-generating and detecting component of the transducers is a piezoelectric plate/crystal.
- The thickness of the piezoelectric plate is usually equal to half the wavelength of the US pulse.
- A damping unit absorbs vibrations traveling in the reverse direction and stops the vibrations of the piezoelectric plate after switching off the voltage, producing short US pulses.
Couplant Layer
- The couplant layer protects the piezoelectric plate from mechanical damage and helps transmit US pulses into the human body.
- Its ideal thickness is one-fourth of the emitted US wavelength.
Pulse-Echo Methods
- In pulse-echo methods, the same transducer generates and detects US pulses.
- 2D images are formed by combining multiple one-dimensional B-mode images while moving the transducer (scanning) on the body's surface.
Scanning Techniques
- Linear Scanning: A 2D image is generated by moving the scanner along a straight line on the body's surface.
- Sector Scanning: The angle of the scanner is changed. This is useful when US pulse transmission into body cavities is limited by surrounding bones (e.g., thoracic organs, neonatal cranial US).
- Intracavity Scanning: The scanner is introduced into body cavities (e.g., esophagus, rectum, vagina). This technique provides better US signals due to proximity to the structures.
- 3D Imaging: Multiple 2D images from different directions are combined to create 3D images.
Imaging Artifacts
- Imaging artifacts are reflections that do not represent real anatomical structures.
- Shadowing: A dark space behind highly reflective objects (e.g., kidney or gall bladder stones) reflects most of the US intensity, preventing echoes from distal structures.
- Mirror-Image Artifact (Image Duplication): The image of an object in front of a strongly reflecting surface (e.g., diaphragm) can be duplicated, as echoes from the diaphragm interact with the object again.
Spatial Resolution
- Spatial Resolution: The minimal distance between two points of an object that can be distinguished by their US images.
- Axial Resolution: Describes the ability to distinguish two structures along the axis of the US beam (~0.5 mm). Shorter pulses (higher frequencies) lead to better axial resolution.
Doppler-Shift
- US pulses are scattered by red blood cells.
- The frequency of the scattered US signal differs from the original signal due to blood cell movement relative to the transducer (Doppler-shift).
- The Doppler-shift is: fD=(2v/c)f, where v is the blood cell velocity and c is the speed of sound.
- If the US beam is not parallel to the blood flow, only the component of v parallel to the beam (2v × cosΘ) matters.
- Perpendicular incidence leads to zero Doppler-shift.
Doppler-Shift Applications
- Doppler-shift (fD) is typically in the audiofrequency range (20 Hz – 20 kHz), allowing operators to listen to it.
- By measuring fD and knowing c, the rate and direction of blood flow can be determined.
- Colour-Doppler technique displays this data as colours superimposed on 2D B-mode images (red for movement towards the transducer, blue for movement away).
US Primary Effects
- The primary effects of US (sound pressure, absorption, cavitation, and mechanical rubbing effect) are due to high-frequency, large-amplitude vibrations of the medium.
- Sound pressure, exerted by the US wave, is directly proportional to US intensity.
- Differences in particle sizes in the medium can lead to speed differences during sound propagation.
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Description
This quiz covers the properties and characteristics of transverse waves and sound wave propagation. It delves into physical parameters such as wavelength, frequency, amplitude, and speed of sound in different mediums. Test your understanding of how these concepts interrelate and their significance in wave mechanics.
