Week 1 Flashcards
MRI scanner types
- permanent
- resistive electro
- superconducting
1 Tesla = ___ Gauss
10,000
Ultra-Low Field Tesla Strength
0.02 - 0.1 T
Low Field Tesla Strength
0.2 - 0.6 T
Mid Field Tesla Strength
1 - 1.5 T
High Field Tesla Strength
2 - 4 T
Ultra-High Field Tesla Strength
> 7 T
why do MRI need strong coolants
as temp decrease, metal resistance falls. when it drops to zero, electricity is conducted without energy loss as heat due to resistance
superconducting MR magnets typically use ___ wires
Nobium-Titanium, Tc = 9.4K
what is commonly used as a coolant / cryogen to lower temperature
liquid helium (4K)
HTS
high temperature superconductive
types of magnetic properties of materials
diamagnetic, paramagnetic, ferromagnetic
diamagnetic material properties
no intrinsic atomic magnetic moment but weakly repels magnetic field = small negative magnetic susceptibility
paramagnetic material properties
ions have unpaired electrons, resulting in a positive magnetic susceptibility less than ferromagnetic materials
MR Safe
item poses no hazard from exposure to MR environment
MR conditional
item with demonstrated safety in MR environment within defined conditions
MR unsafe
item poses unacceptable risks to patients within MR environment
how many preventive procedural zones
4
Zone I
freely accessible to general public
Zone II
screening & interviewing patients but patients may not be able to move freely throughout
Zone III
controlled area - access entirely under supervision of MR personnel
Zone IV
MR scanner room - highly restricted; non-MR personnel must be under immediate visual supervision of qualified MR personnel
MRI preventive procedures
- ferromagnetic detectors
- screening questionnaire
clinical MRI involves imaging of ___
hydrogen nuclei (protons)
NMV
net magnetization vector
resonance
tendency of system to absorb more energy when frequency of oscillations matches system’s natural frequency of vibration
excitation
when RF wave is applied at resonant frequency, protons will absorb energy & tip over
relaxation
when RF stops, they lose energy to surroundings and re-align to B0
at 1.5 T, resonant frequency of 1 H is at ___ MHz
63.87
T1 - longitudinal component Mz
time taken to lose energy to surroundings & realign to B0 (200ms - 2000ms)
T2 - transverse component Mxy
time taken to lose energy to other spins (10 - 100ms)
Unlike CT, MRI signal intensities are __
relative
T1 & T2 can be used to differentiate __
chemical compounds, tissue structures, healthy & diseased tissues
why are gadolinium based agents most commonly used
7 unpaired e- in 4f orbital = strong paramagnetic effect
gadolinium based agents affect ___ times
relaxation; mainly T1
larmor equation
f0 = y x B0
other mechanisms of contrast
PD, diffusion, BOLD, velocity
PD
proton density
T2 timing are shorter than T1 timing about __
10x
T1W contrast
- bright fat
- dark csf
- WM brighter than GM
PD contrast
- less bright Fat & CSF
- WM darker than GM
T2W contrast
- bright fat
- very bright CSF
- WM darker than GM
effect of fat & water on T2
fat = low T2
water = high T2
who is responsible for MRI safety
radiographer
why does MRI have excellent soft tissue contrast
due to tissues having different chemical structures
what contributes to nuclear moments in MRI
protons & neutrons
what does NMV represent
sum of nuclear magnetic moments aligned parallel to B0
MRI signal can be increased by __
increasing field strength, temperature and proton density
the magnitude of NMV tipped is dependent on __
frequency, duration & amplitude of RF pulse
