Medical physics

You will be able to

• Calculate the specific acoustic impedance Z = ρc of a medium and the intensity reflection coefficient (Z₂ − Z₁)² / (Z₂ + Z₁)² at a boundary.
• Explain why a coupling gel is needed between the transducer and the skin.
• Determine the depth of a boundary from the time of its echo on an A-scan, using depth = ct / 2.

You will be able to

• Use I = I₀e^(−μx) to calculate the intensity of a parallel X-ray beam after it passes through a thickness x of a medium.
• Determine the half-value thickness x½ = ln 2 / μ of a material.
• Explain how the accelerating p.d. of the tube affects the contrast of an image, and how the focal spot size affects its sharpness.

You will be able to

• Explain why the annihilation of a positron and an electron produces two gamma photons travelling in opposite directions.
• Calculate the energy of each annihilation photon from E = mₑc².
• Describe how detecting photon pairs in coincidence, and the difference in their arrival times, locates the tracer in the body.