DMS 104 Midterm Review: Ultrasound Imaging

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Questions and Answers

What is the result of the conversion of sound into heat?

Refraction

Diffraction

Reflection

Absorption (correct)

What is the unit used to quantify attenuation?

Joule

Decibel (correct)

Watt

Hertz

If the frequency of sound in soft tissue doubles, what happens to the attenuation?

It stays the same

It doubles (correct)

It triples

It halves

What is the approximate attenuation in soft tissue per centimetre for each MHz of frequency?

0.5 dB Signup and view all the answers

What happens to the attenuation when both frequency and path length are doubled in soft tissue?

It quadruples Signup and view all the answers

What is the dominant contributor to attenuation?

Absorption Signup and view all the answers

If an echo returns 130 µs after a pulse was emitted by a transducer in soft tissue, what is the depth of the echo-producing structure?

20 mm Signup and view all the answers

How does attenuation affect the penetration of the sound wave?

It limits the penetration Signup and view all the answers

What happens to the sound beam when it hits an interface and is reflected?

It goes back to the receiver and is picked up to form images on the screen Signup and view all the answers

If a reflector is 20 mm away from the transducer, how long does it take for sound to get back to the transducer?

It depends on the velocity of sound in the medium Signup and view all the answers

What is the unit of acoustic impedance?

Rayls Signup and view all the answers

What does it mean if there is no reflection at a boundary?

The medium impedances are equal Signup and view all the answers

What determines the refraction of sound as it crosses a boundary?

The difference in acoustic impedance between the two media Signup and view all the answers

What factors affect the setting of TGC?

All of the above Signup and view all the answers

What is Snell's law related to?

The relationship between angle of incidence/transmission and the velocity of sound of 2 media Signup and view all the answers

What is the primary function of contrast agents in CEUS?

To highlight particular isoechoic abnormal tissues Signup and view all the answers

What type of reflector is produced by the artery walls or the diaphragm?

Specular reflector Signup and view all the answers

What is the unit of amplitude in ultrasound?

All of the above Signup and view all the answers

What is scattering of sound?

The formation of an echo in multiple directions by rough surfaces Signup and view all the answers

What is the mechanism of speckle production in ultrasound images?

Constructive and destructive interference patterns Signup and view all the answers

What is the difference between TGC and overall gain?

TGC is used to adjust the gain for each depth, while overall gain adjusts the gain for the entire image Signup and view all the answers

What is the characteristic of contrast agents that allows them to perfuse tissues?

Small size Signup and view all the answers

What is the purpose of TGC slope?

To compensate for attenuation of the ultrasound signal Signup and view all the answers

How do contrast agents function in CEUS?

They produce microbubbles that increase the amplitude of the returning echoes Signup and view all the answers

Study Notes

Attenuation

Attenuation is the weakening of sound as it propagates through tissues
It limits the penetration and depth of the image
The dominant contributor to attenuation is absorption
Absorption is the conversion of sound into heat
The unit of attenuation is decibel (dB)
3 dB = 50% attenuation reduction, 10 dB = 90% attenuation reduction

Attenuation in Soft Tissues

Attenuation in soft tissues is approximately 0.5 dB/cm for each MHz of frequency
The attenuation coefficient in soft tissue increases as frequency increases
If frequency doubles, attenuation is doubled, and if path length is doubled, attenuation is also doubled
If both frequency and path length are doubled, attenuation is quadrupled

Bone and Soft Tissue

Attenuation in bone is higher than in soft tissue

Reflector and Sound Travel

If an echo returns 130 µs after a pulse was emitted, the echo-producing structure is located at a depth of approximately 10 mm
If a sound wave travels 26 µs to a reflector, the reflector is approximately 4 mm away
If a reflector is 10 mm away from the transducer, it takes approximately 26 µs for sound to get back to the transducer

Reflected Sound Beam

The reflected sound beam is known as an echo
When the perpendicular beam is reflected, it goes back to the receiver and is picked up to form images on the screen

Acoustic Impedance

Acoustic impedance is a measure of the resistance of a medium to the transmission of sound
Its symbol is "z", its unit is Rayls, and the equation is 𝑧 = c (m/s) × ρ(kg/m3)

Reflection and Refraction

If there is no reflection at a boundary, it means that all is transmission, and the medium impedances are equal
The reflective and transmission coefficients add up to 1
Oblique incidence is different from perpendicular incidence in that the angle is not perpendicular
Refraction is a change in the direction of sound as it crosses a boundary, determined by the different acoustic velocity/impedance in the different mediums
Snell's law tells us about the relationship between the angle of incidence/transmission and the velocity of sound of 2 media

Specular Reflector and Scattering

A specular reflector is a flat smooth boundary that reflects well, produced at the artery walls or the diaphragm
Scattering is the formation of echo in many directions by rough surfaces or heterogeneous media or very small objects
Scattering depends on the surface of the interface and the size of the object compared to the size of the wavelength

Time Gain Compensation (TGC)

TGC is needed to compensate for attenuation
The TGC slope depends on the attenuation coefficient
The operator needs to keep in mind the tissue composition, attenuation coefficient, transducer frequency, and path length when setting the TGC
TGC is different from overall gain
Factors affecting TGC setting include tissue composition, attenuation coefficient, and transducer frequency

Contrast-Enhanced Ultrasound (CEUS)

CEUS is contrast-enhanced ultrasound
Contrast agents are microbubble polymers introduced intravenously into the bloodstream to highlight particular isoechoic abnormal tissues (e.g. lesions)
Characteristics of contrast agents include stability, small size, different acoustic impedance, non-toxicity, and easy administration
Contrast agents function by increasing US amplitude, which can pop the bubbles and show the tissues and the rate at which they fill up again with the agent
Contrast agents are used to show perfusion

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Description

Review questions covering the basics of ultrasound imaging, including attenuation, absorption, and decibel units. Topics include how attenuation affects image quality and the dominant contributors to attenuation.