Ultrasound And MRI Flashcards
What kind of wave is sound?
Longitudinal
i.e. particle motion and direction of travel are parallel
Particle motion is perpendicular to the wave direction for transverse waves
What is the frequency range of ultrasound equipment?
Audible: f =20 Hz - 20 kHz
Ultrasound: f = >20 kHz
Medical Ultrasound: f = 2-15 MHz
Requires systems that oscillate at a very high frequency
How does the medical ultrasound crystal enable its function?
Piezoceramic quartz material with an electrical current through it.
Current causes quartz signal to change shape then vibrate
This gives echoes where the US bounces off tissue with different densities.
What interactions does US have with tissues?
Reflection
Scattering
Refraction
Absorption
What can be seen on an US image?
Speckled grey represents layers of fat
Black areas represent where the US has had no interaction, eg amniotic fluid
White shows air pockets OR tissue, hence you cant see a cyst for example behind an air pocket - requires imaging at different angles
Why is using a moving (Doppler) wave source useful?
Measuring the changing frequency can allow you to estimate the blood flow through a vessel.
These different different frequencies can be converted to colour to show the different speeds/directions of blood
Useful when looking at blood flow through a stenosis
What are the advantages of US?
Portable
Inexpensive
No ionising radiation
Non-invasive
What is the use of MRI?
Gives us information on structure and function
Excellent soft tissue contrast
What are the principles of MRI?
Depends abundance of H ions in body
Single proton in H ion has a nuclear spin, this spinning charge causes a magnetic field which forms a magnetic dipole.
When a patient is put in an external applied magnetic field (MRI) all their proton spins align creating a net magnetism in the Bo direction (z axis), this is the equilibrium state.
RF energy is transmitted into the patient inside the MRI, this excites the magnetisation, bringing all protons into phase (x axis).
When the RF energy is switched off the magnetisation returns to equilibrium (Bo, z axis). This can occur by two processes:
- longitudinal recovery (z component): THIS IS T1 RECOVERY
protons return to the low energy state
- transverse decay (x-y component): THIS IS T2 DECAY
Protons going out of phase
Both relaxation processes emit RF energy with is detected by a scanner
How does MRI differentiate tissues?
T1 and T2 relaxation times are tissue dependent
T1 & T2 (water) > T1 & T2 (fat)
Can manipulate these relaxation times to improve tissue contrast
What is T1 weighting?
T1 (water) > T1 (fat) weight it so that more spins perform longitudinal recovery to the z phase for water than for fat
Makes fat brighter than water
What is T2 weighting?
T2 (water) > T2 (fat) weight it so that more spins perform transverse decay for water than for fat = OPPOSITE TO PREV
Makes water brighter than fat
How is MRI angiography performed?
GADOLINIUM CONTRAST AGENT
Can also be performed dynamic to see how the contrast agent flows through vessels
How is contrast enhancement utilised in tumour vessels?
Tumour vessels are much Lemaire than healthy ones so the contrast agent leaks out giving a brighter signal
Over time this concentration can be plotted, showing the leaker areas, can therefore show whether cancerous areas are improving
How is functional MRI used?
Results are compared between a patient ‘doing something’ and not
