Principles and indications of MRI and radionuclide bone scintigraphy
19 questions2 source pages1 fact-check flags
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19 questions
Q1What are the components of an MRI scanner?▸
A superconducting electromagnet made of niobium-titanium, cooled by liquid helium
A radiofrequency coil system: RF synthesizer, power amplifier and transmitting coil
Gradient coils - determine the positions of protons in the scanning field
Q2How does an MRI scanner generate an image?▸
The superconducting electromagnet and radiofrequency coil impart energy on hydrogen protons, shifting them to a higher energy level by spinning and wobbling in phase
Upon removal of the RF, the released energy is converted into a signal represented in an image
Q3Why are hydrogen protons used for MRI?▸
Hydrogen protons are abundant in human tissue, in differing densities in different tissues
Each hydrogen proton has its own spin and wobble
Q4What happens to protons when a patient first enters the MRI scanner?▸
The vast majority align along the net longitudinal magnetization vector of the superconducting magnet
The spin is still out of phase
Q5What happens when the radiofrequency pulse is applied?▸
RF imparts energy to lower energy protons, flipping their axis to a higher energy state - half point one way, half the other - cancelling the longitudinal magnetization vector
The protons precess, so their spins synchronise in phase and the net magnetization vector becomes transverse/horizontal
Q6Describe T2 relaxation in MRI.▸
T2 (spin-spin) relaxation: spins previously in phase become out of phase - loss of transverse magnetization
Releases energy, converted into an image
Better for picking up pathology; H2O2 is high signal
Q7Describe T1 relaxation in MRI.▸
T1 (spin-lattice) relaxation: higher energy protons flip back to realign with the longitudinal magnetization vector of the superconducting magnet
Fat sensitive, used for anatomy
Q8What is TR and what contrast does it optimise?▸
TR = the rate at which the RF is applied
Short TR optimizes T1 contrast
Q9What is TE and what contrast does it optimise?▸
TE = the frequency taken to listen to the signal
Long TE optimizes T2 contrast
Q10What are STIR, FLAIR and MARS used for?▸
STIR (short tau inversion recovery) suppresses fat - useful for picking up fluid or oedema in fat-rich tissue (e.g. spinal trauma, PLC)
FLAIRsuppresses water - useful for subtle lesions within the spinal cord by nullifying CSF
MARS - listed in the notes
Q11How does gadolinium contrast work in MRI?▸
Taken up by tissue with high fluid content
In T1 fat suppression, the contrast area will show up
Q12What is a bone scan and how does it work?▸
A type of radionucleotide imaging
Use of a radioisotope attached to a ligand with affinity to a specific area of interest (e.g. bone), which emits radiation (gamma rays) when it disintegrates
Technetium-99m bound to phosphate gives a map of blood flow and osteoblast activity
Q13How is the emitted radiation detected in radionuclide imaging?▸
Detected by a scintillation camera with Na iodide
Q14What radioisotope-ligand pairs are used in nuclear imaging?▸
Technetium-99m -> methyl diphosphonate
Gallium -> transferrin
Indium-111 -> WCC
Q15What is the half-life and excretion route of technetium-99m, and how is it produced?▸
Molybdenum-99 decays into technetium-99m
Short half-life of 6 hrs
Excreted via the kidney
Q16What are the three phases of a bone scan?▸
Vascular phase (1-2 min) - arterial flow and hyperperfusion
Blood pool phase (3-5 min) - bone and soft tissue hyperaemia
Static phase (4 hrs) - bone activity
Q17What is SPECT?▸
SPECT = single photon emission CT
Gamma camera with CT component - multiplanar imaging increases resolution, decreases noise and increases localisation
Q18What is PET-CT and how does it work?▸
PET-CT = positron emission tomography CT
Exploits the increased metabolic rate of tumours, i.e. glucose consumption
Q19What do the note values T1 63% and T2 37% refer to?▸
T1 63%: recovery of the vertical (longitudinal) axis
T2 37%: loss of the horizontal (transverse) axis
Fact check
T2 relaxation is a 37% loss of the horizontal (transverse) magnetization — incorrect — After one T2 time constant the transverse magnetization has decayed TO 37% of its original value, i.e. it has lost 63%; T1 is the time for 63% recovery of longitudinal magnetization — source