Flashcards · 9702 · A Level Topic 24

Medical physics (A Level) flashcards

17 cards on A Level Topic 24, subtopics 24.1 to 24.3. Pick a subtopic or study them all.

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How is ultrasound produced and detected?: By a piezo-electric transducer: an alternating p.d. makes the crystal vibrate, producing ultrasound; returning vibrations make the same crystal produce an alternating p.d.
Specific acoustic impedance: Z = ρc, the density of the medium × the speed of sound in it. Unit kg m⁻² s⁻¹.
Intensity reflection coefficient at a boundary: I_R ÷ I = (Z₂ − Z₁)² ÷ (Z₂ + Z₁)². The bigger the difference in acoustic impedance, the more is reflected.
Why is coupling gel used in ultrasound scanning?: Air and skin have very different acoustic impedances, so almost all the ultrasound would reflect at the skin. Gel has an impedance close to skin’s and displaces the air (impedance matching).
Soft tissue Z = 1.63 × 10⁶, bone Z = 6.40 × 10⁶ kg m⁻² s⁻¹. What fraction of the intensity is reflected?: (6.40 − 1.63)² ÷ (6.40 + 1.63)² = 22.75 ÷ 64.48 = 0.35, about 35%.
Attenuation of ultrasound, and how a pulse-echo scan gives diagnostic information: Intensity falls as I = I₀e^(−μx). A pulse is partly reflected at each boundary between tissues of different acoustic impedance; the echo times give the depths of the boundaries.

How are X-rays produced?: Electrons are accelerated through a large p.d. and strike a metal target. Most of their energy becomes heat (the anode is cooled); a very small fraction becomes X-rays.
Minimum X-ray wavelength for an accelerating p.d. V: λ_min = hc ÷ eV: an electron transfers all its kinetic energy eV to one photon. It depends only on V, not on the target.
Braking radiation (bremsstrahlung) in an X-ray spectrum: The continuous background, from electrons decelerating in the target. Its maximum photon energy equals eV.
Characteristic lines in an X-ray spectrum: Sharp peaks from electron transitions in the target atoms. Their positions depend only on the target material.
Attenuation of X-rays, and why bone shows up white: I = I₀e^(−μx). μ depends strongly on atomic number: bone (calcium) absorbs much more than soft tissue, giving contrast. Contrast media of high atomic number (barium, iodine) make soft tissue such as the gut visible.
How does CT scanning produce a 3D image?: X-ray images of the same section are taken from many angles and combined by computer into a 2D image of that section. This is repeated along an axis and the 2D sections are combined into a 3D image.

What tracer is used in PET scanning?: A substance labelled with a positron-emitting isotope (commonly fluorine-18 in a glucose compound), injected into the patient. Active tissue, such as a tumour, takes up more of it.
What happens to an emitted positron in PET?: It travels a short distance and annihilates with an electron, producing two identical gamma-ray photons travelling in opposite directions.
Why do the two annihilation photons travel in opposite directions?: Conservation of momentum: the initial momentum is (almost) zero, so the photons’ momenta must cancel.
Energy of each annihilation photon: E = mc² for the rest mass of the electron (and of the positron): 9.11 × 10⁻³¹ × (3.00 × 10⁸)² = 8.2 × 10⁻¹⁴ J, about 0.511 MeV.
How is the location of an annihilation found?: A ring of detectors records both photons. The line between the two detectors and the difference in arrival times locate the annihilation; a computer builds an image of tracer concentration.