MRI

Question 1

What are the advantages of MRI?

Answer 1

• no radiation
• multiplanar
• excellent tissue contrast and resolution
• functional can see cardiac and CSF motion

Question 2

What is the MRI hardware?

Answer 2

• RF (radio frequency) coil is what receives information from the patient
• B0 is the superconducting magnet (1.5-3T)
• Gradient could move the plane which the image is to be taken
• faraday cage is a copper lining of room to get rid of RF from other things which decrease image quality and increase artifacts

Question 3

Why is hydrogen used?

Answer 3

• most abundant atom in the body
• unpaired proton (positively charged)
• spinning protons act like little magnets

Question 4

What is Larmor frequency?

Answer 4

• rate at which the molecules are spinning
  W0= yB0

W0= precessional return home in Hz
y= gyromagnetic ratio of hydrogen 42.58M
B0= magnetic field strength

Question 5

What is a vector?

Answer 5

• a quantity having direction as well as magnitude, especially as determine the positron of one point in space relative to another
• net magnetisation as a vector

Question 6

What is RF excitation?

Answer 6

• RF pulse applied to the body at the Larmor frequency
• hydrogen gains energy and moves to a higher energy state
• vector moves from the z to y plain or longitudinal to transverse axis

Question 7

What is T1 relaxation?

Answer 7

• transverse magnetisation starts to relax back to longitudinal plane once RF excitation stops
• over time tissue relaxes back at different rates
• relaxation along z-axis

Question 8

What is T2 relaxation?

Answer 8

• decay or dephasing in transverse plane starts as soon as RF excitation is removed
• x-y axis

Question 9

What is signal?

Answer 9

• at a set time residual transverse magnetisation induces signal in coil
• signal in coil sent to computer
• raw data saved
• k space is mathematically converted through Fourier transform into an image

Question 10

What is the safety for MRI?

Answer 10

• the main magnetic field is always on
• it is invisible
• normal safe objects such as bobby pin become dangerous as they fly towards the machine

Question 11

Who should be screened?

Answer 11

• everyone (patients, friends and family, visitors, patient support personal, ancillary staff and visiting doctor or nurse)

Question 12

What can cause death in MRI?

Answer 12

• implanted defibrillators
• cochlear implants
• intra ocular foreign bodies
• ferromagnetic intra cranial aneurysm clips

Question 13

What are potential patient injuries from MRI?

Answer 13

• projectiles
• quench (shutting down machine)
• heating (specific absorption rate)
• acoustic noise
• nerve stimulation (muscle twitching)

Question 14

What is the signal to noise ratio?

Answer 14

• ratio of the amplitude of signal received by the coil to the amplitude of the noise
• signal is the voltage induced in the receiver coil
• TR (reception time) is from the start to next start of RF
• TE (echo time) is from the start to signal

Question 15

What are the MRI artifacts?

Answer 15

• aliasing or wrap
• zipper
• corduroy
• movement
• implants