A-Level Physics AQA 7408

Ultrasound imaging

Reflection and transmission characteristics of sound waves at tissue boundaries, acoustic impedance, Z, and attenuation.

Advantages and disadvantages of ultrasound imaging in comparison with alternatives including safety issues and resolution.

Piezoelectric devices

Principles of generation and detection of ultrasound pulses.

A-scans and B-scans.

Examples of applications.

Use of the equations $Z = ρc$ and $\dfrac{I_r}{I_i} = \bigg(\dfrac{Z_2 - Z_1}{Z_2 + Z_1}\bigg)^2$

Fibre optics and endoscopy

Properties of fibre optics and applications in medical physics; including total internal reflection at the core–cladding interface.

Physical principles of the optical system of a flexible endoscope; the use of coherent and non-coherent fibre bundles; examples of use for internal imaging and related advantages.

Magnetic resonance (MR) scanner

Basic principles of MR scanner:

• cross-section of patient scanned using magnetic fields
• protons initially aligned with spins parallel
• spinning hydrogen nuclei (protons) precess about the magnetic field lines of a superconducting magnet
• 'gradient' field coils used to scan cross-section
• short radio frequency (RF) pulses cause excitation and change of spin state in successive small regions
• protons excited during the scan emit RF signals as they de-excite
• RF signals detected and the resulting signals are processed by a computer to produce a visual image.

Students will not be asked about the production of magnetic fields used in an MR scanner, or about de-excitation relaxation times.