CT, MRI, US: How They Work

Question 1

How does MRI work?

Answer 1

1. H nuclei spin on their own axis
2. an external magnetic field (B0) is applied causing them to line up and spine in same direction
3. B0 also makes them spin at same frequency (precession)
4. RF pulse applied which tips the nuclei into the transverse plane and makes H nuclei have synchronised spinning (phase)
5. The RF pulse is removed:
   A. Nuclei relax back to original position releasing energy
   B. It also causes nuclei to go back to random desynchronised spinning

Question 2

How can you manipulate MRI images?

Answer 2

4 ways)

1. Proton density (good for normal anatomy/pathology)
2. fat saturation (suppresses high signal fatty tissue in T1)
3. STIR (short tau inversion recovery): even better at fat suppression (better visualisation of fluid
4. FLAIR –> suppresses CSF (small SAH)

Question 3

How does CT work?

Answer 3

Uses XRs (pass through tissues dif depending on their density)

Scanner has an XR tube and multiple detectors that rotate around the patient

XRs taken from many angles –> 1 rotation has 1000 profiles taken and is the equivelent to 1 slice

The profiles data are reconstructed into 1 slice (5-10mm CTH) which is a 2D image

Question 4

How does US work?

Answer 4

1. Uses high frequency sound waves (generated by the US probe)
2. The probe exerts pressure on particles when applied to body surface - temporarily shifting particles
3. ACOUSTIC IMPEDENCE = how easy an US wave can travel through a tissue/medium
4. as the wave travels through tissues it loses energy to them (attenuated). At the boundary between 2 tissues with different acoustic impedence is reached - some waves are reflected. The larger the difference - the more of the wave is reflected (e.g. gas + muscle have huge difference - 99% reflected)
5. the velocity of travel depends on the tissues density
6. The reflected waves are detected by a transducer –> the intensity of the reflected wave depends on distance travelled, tissue travelled through + nature of the object it was reflected from
7. Dense structures appear hyperechoic/bright w/ a posterior acoustic shadowing
8. Fluid appears hypoechoic/dark or black w/ a posterior acoustic enhancement

Question 5

What is the difference between high and low frequency US?

Answer 5

high frequency: shorter wavelength, ^ resolution, reduced penetration

Low frequency: longer wavelength, decreased resolution, better penetration

Question 6

What is acoustic impedence?

Answer 6

how easy an US wave can travel through a tissue/medium

Question 7

What is phase?

Answer 7

Synchronised spinning of H nuclei after RF pulse applied

Question 8

4 ways you can manipulate MRI?

Answer 8

Proton density
fat suppression
STIR (short tau inversion recovery)
FLAIR –> suppresses CSF

Question 9

What is procession?

Answer 9

when the H nuclei spin at same frequency (caused by EM field)

Question 10

What is acoustic impedance?

Answer 10

How easily the US wave travels through a medium/tissue

Question 11

What does the intensity of the reflected wave depend on in US?

Answer 11

distance travelled, medium SW travelled throiugh, the medium the SW was reflected from