Question 20N.3.HL.TZ0.18
| Date | November 2020 | Marks available | [Maximum mark: 6] | Reference code | 20N.3.HL.TZ0.18 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Explain, Identify, Outline | Question number | 18 | Adapted from | N/A |
Explain the cause of the radio-frequency emissions from a patient’s body during nuclear magnetic resonance (NMR) imaging.
[3]
use of strong magnetic field ✓
protons are aligned ✓
radio wave at «nuclear» resonant frequency flips «some of» them into higher energy state ✓
proton de-excites emitting energy at known «radio» wavelength/frequency/Larmor frequency ✓
«which can be located and detected»
Candidates proved to be familiar with the use of strong magnetic fields to produce proton alignment and its consequence on excitation de-excitation of protons by emission of radio frequencies (Larmor frequency).
Outline how a gradient field allows NMR to be used in medical resonance imaging.
[2]
mention of gradient field «added to the NMR uniform magnetic field» ✓
reference to «the total field that determines» the output «Larmor» frequency from the de-excitation ✓
different positions «in the body» give rise to different frequencies ✓
«and this can be mapped»
Most candidates continued successfully here by referring to a gradient field eventually leading to a mapping of the position of the protons from (a).
Identify one advantage of NMR over ultrasound in medical situations.
[1]
NMR higher resolution ✓
NMR less attenuation ✓
Accept the reverse argument
Higher resolution was the most popular answer in a high scoring question overall.
