Magnetic Resonance Imaging Flashcards
1
Q
The magnet
A
- currents produce magnetic fields
- lower temperature - lower resistance - super conduction
- “ramped” up with a power supply, then the power supply can be removed
- current can be retained for many years
2
Q
magnetic field
A
- main magnet coils generate a strong magnetic field B - measured in Tesla
- most operate at 1.5T-3T
3
Q
Hydrogen Atom
A
- single proton and no neutron - strong net spin
- most abundant atom in the human body
- most signal in MR images comes from hydrogen atoms in water, fat and carbohydrates
4
Q
protons
A
- proton is constantly spinning
- spinning charge = magnetic field - magnetic moment - magnetic dipole moment (MDM)
5
Q
magnetic moments
A
- usually randomly orientated
- strong external magnetic field aligns them either with (parallel) or against (antiparallel) the external field
6
Q
Magnetization
A
- the preferred state of alignment is parallel to B - more aligned with B than against
- Net magnetization is small - depends on strength of B
7
Q
Net magnetization
A
- approximately 10,000,007 protons parallel, 10 million antiparallel - slight longitudnal magnetization
- this net magnetization becomes the source of MR image
8
Q
longitudinal magnetization
A
- in the direction of the z-axis, along B
- denoted by M
9
Q
precession
A
- a spinning top spins about its axis
- gravity attempts to pull the top so that it falls
- combined effect of gravity and spin causes it to precess
10
Q
nuclear precession
A
- if the spinning proton is placed in a strong magnetic field, the force from the magnetic field interacts either the spinning proton and results in precession
11
Q
Frequency of precession (ω)
A
- revolutions per second in MHz
- determined from the Larmor equation
- gyromagnetic ratio (ϒ) is characteristic of type of nuclei
12
Q
H protons Gyromagnetic ratio (ϒ)
A
42.6 MHz/T
13
Q
Transverse magnetization
A
- net magnetization is very small and is in direction of B0
- transverse magnetization is required
- radio frequency (RF) pulses
- Disturbance occurs through energy transfer from RF pulse (B1) to protons - RF transmit coil (body coil, head coil, knee coil)
- Only occurs when the RF pulse frequency = precessional frequency of the protons - Resonance
14
Q
RF pulse and transverse magnetization
A
- As energy is absorbed from RF pulse, net magnetization rotates away from longitudinal direction
- The amount of rotation (flip angle) depends on the strength and duration of the RF pulse - Strength and/or duration can be controlled to rotate to any angle
15
Q
Absorption of RF energy
A
- if the RF pulse rotates the net magnetization into the transverse plane, it is termed a 90 RF pulse
