Medical Physics A2

A piezoelectric crystal generates an e.m.f. when:

• A constant p.d. is applied

• It is heated

• Its shape changes

• Its density increases

Answer explanation

A piezoelectric crystal generates an e.m.f. when its shape changes due to mechanical stress, causing a displacement of charges within the crystal. This is the principle behind piezoelectricity, making 'its shape changes' the correct answer.

Ultrasound waves are generated in a transducer by:

• Magnetic induction

• Piezoelectric effect

• Photoelectric effect

• Thermionic emission

Answer explanation

Ultrasound waves are generated in a transducer by the piezoelectric effect, where certain materials produce an electric charge when mechanically stressed, allowing them to convert electrical energy into sound waves.

Which property makes piezoelectric crystals suitable for ultrasound?

• They are magnetic

• They fluoresce

• They change shape under p.d.

• They are low density

Answer explanation

Piezoelectric crystals are suitable for ultrasound because they change shape under an applied potential difference (p.d.), allowing them to convert electrical energy into mechanical vibrations and vice versa.

Ultrasound imaging relies on:

• Transmission of visible light

• Reflection of sound

• Scattering of gamma rays

• Refraction of light

Answer explanation

Ultrasound imaging relies on the reflection of sound waves. When sound waves are transmitted into the body, they bounce back upon hitting different tissues, allowing for the creation of images.

Specific acoustic impedance is defined as:

• Z = ρ/c

• Z = c/ρ

• Z = ρc

• Z = ρc²

Answer explanation

Specific acoustic impedance (Z) is defined as the product of density (ρ) and sound speed (c) in a medium, hence Z = ρc is the correct formula. The other options do not accurately represent this relationship.

In I = I₀e^(-μx), μ represents:

• Density

• Attenuation coefficient

• Acoustic impedance

• Absorption length

Answer explanation

In the equation I = I₀e^(-μx), μ is the attenuation coefficient, which quantifies how much the intensity of a wave decreases as it travels through a medium. It is not density, acoustic impedance, or absorption length.

The reflection coefficient at a boundary depends on:

• Frequency

• Acoustic impedances

• Crystal thickness

• Amplitude

Answer explanation

The reflection coefficient at a boundary is primarily determined by the acoustic impedances of the two media. It quantifies how much of an acoustic wave is reflected versus transmitted at the interface.

If Z₁ = Z₂ at a boundary, reflection coefficient is:

• 0

• 1

• 0.5

• Undefined

Answer explanation

When Z₁ equals Z₂ at a boundary, there is no impedance mismatch, leading to no reflection. Therefore, the reflection coefficient is 0, indicating that all incident energy is transmitted and none is reflected.

Ultrasound attenuation increases with:

• Lower frequency

• Higher frequency

• Lower impedance

• Smaller μ

Answer explanation

Ultrasound attenuation increases with higher frequency due to increased scattering and absorption in tissues. Lower frequencies attenuate less, making higher frequency the correct choice.

The intensity reflection coefficient is maximum when:

• Z₁ ≈ Z₂

• Z₁ ≫ Z₂

• Z₁ ≪ Z₂

• Z₁ = 0

Answer explanation

The intensity reflection coefficient is maximum when the impedance of the first medium (Z₁) is much greater than that of the second medium (Z₂), leading to significant reflection. Thus, the correct choice is Z₁ ≫ Z₂.