Production and use of ultrasound
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Explain how a piezoelectric crystal is used to generate ultrasound.
A piezoelectric crystal changes shape when a p.d. is applied. An alternating voltage at the crystal's resonant frequency causes it to vibrate, producing ultrasound waves. The frequency of the applied voltage determines the frequency of the emitted ultrasound.
Explain how a piezoelectric crystal is used to detect ultrasound.
When an ultrasound wave strikes a piezoelectric crystal, it changes shape, generating an e.m.f. The magnitude of the e.m.f. is proportional to the intensity of the ultrasound wave, allowing for detection.
How is ultrasound used in medical imaging to obtain diagnostic information?
Pulses of ultrasound are emitted into the body. When these pulses encounter boundaries between tissues with different acoustic impedances, they are partially reflected. The reflected pulses are detected, and the time delay and intensity provide information about the depth and nature of the tissue boundaries.
Define specific acoustic impedance (Z) and provide its formula.
Specific acoustic impedance (Z) is a measure of a material's resistance to the propagation of sound. It is defined as Z = ρc, where ρ is the density of the medium and c is the speed of sound in the medium.
State the formula for the intensity reflection coefficient (IR / I0) at a boundary between two media and define the terms.
The intensity reflection coefficient is given by IR / I0 = (Z1 – Z2)² / (Z1 + Z2)², where IR is the intensity of the reflected wave, I0 is the intensity of the incident wave, and Z1 and Z2 are the specific acoustic impedances of the two media.
Describe what a high intensity reflection coefficient means at a tissue boundary in ultrasound imaging.
A high intensity reflection coefficient means that a large portion of the ultrasound pulse is reflected at the boundary between two tissues. This indicates a significant difference in the acoustic impedance of the tissues, which can help distinguish different anatomical structures.
State the formula for the attenuation of ultrasound intensity (I) as it travels through a medium.
The attenuation of ultrasound intensity is given by I = I0e–μx, where I is the intensity after traveling a distance x, I0 is the initial intensity, μ is the attenuation coefficient of the medium, and x is the distance traveled.
What does the attenuation coefficient (μ) represent in the context of ultrasound?
The attenuation coefficient (μ) represents the rate at which the intensity of an ultrasound wave decreases as it travels through a medium. A higher attenuation coefficient means that the ultrasound wave loses intensity more rapidly.
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