Magnetic Resonance Imaging Flashcards
(57 cards)
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
16
Q
Effect of RF pulse
A
- overall loss of M0
- Transverse magnetization occurs - protons precess in phase in a transverse plane - Mxy
17
Q
Transverse magnetization
A
- receive coil measures amount of transverse magnetization - electromagnetic induction
- induced electric current digitized and recorded in the computer for reconstruction as an image
18
Q
When the RF pulse is switched off, what happens to transversal magnetization?
A
it will be lost
- fall back into Z
19
Q
what are the two ways relaxation occurs?
A
- transverse magnetization begins to disappear - transverse (T2) relaxtion
- Longitudinal magnetization stats to return to original value - T1 relaxation
20
Q
T2 relaxation
A
- when the transverse magnetism is completely in phase, MR signal is at a max
- rephrasing occur due to magnetic interaction between spins
21
Q
what is transverse decay?
A
spin-spin interaction
- rephrasing occurring due to magnetic interaction between spins
22
Q
what is T2?
A
rate of rephrasing for the protons associated in specific tissue
23
Q
spin-spin interaction
A
- as the spin loses coherence, Mxy decreases until it reaches zero
- the rate of decay is exponential and is called free induction decay
24
Q
Signal intensity in MRI
A
- low intensity = dark gray/black
- high intensity = white
- intermediate intensity = gray
25
what has short T2?
bone, calcium and metal dont take very long to lose 37%
26
what has long T2?
fat
27
What has very long T2?
water
28
White matter T2
short T2 and dephases rapidly
29
CSF T2
long T2 and dephases slowly
30
Grey matter T2
intermediate T2 and dephases intermediately
31
what is echo time (ET)?
time between the delivery of the Rf pulse and the receipt of the echo signal
32
T1 relaxation
- after a 90 RF pulse the longitudinal magnetization is zero
- Once the RF pulse is turned off, the magnetization begins to grow back in the longitudinal direction
- Energy released to lattice (spin-lattice interaction)
33
Time for T1 relaxation
- time it takes the M0 to grow back to 63% of its final value
- T1 also depends on the strength of the main magnetic field
34
Fat and protein T1 phase?
short T1
35
water T1 time
long T1
36
Bone/Calcium/Metal T1 time
Very long T1
37
White matter T1
very short T1 and relaxes rapidly
38
CSF T1
long T1 and relaxes slowly
39
Grey matter T1
intermediate T1 and relaxes at intermediate rate
40
Repetition time?
time between successive pulse sequences (TR)
41
T1 and T2
- T1 and T2 processes occur simultaneously and independently
- After a few seconds, most of the transverse magnetization is dephased and some of the longitudinal magnetization has grown back
42
TR and TE
- Imaging parameters influencing the MR signal include TE and TR
- Values for both parameters are chosen to influence the tissue weighting of the image
43
what does the choice of TR dictate?
the amount of magnetization each tissue begins its T2 decay with
44
what does a short TR followed by very short TE result in?
contrast determined by T1 properties of tissue
45
what does a very long TR do?
minimize T1 effects since all tissues have had time to recover between excitations
46
CSF on T1 vs T2
dark on T1 and bright on T2 weighted image
47
TR and TE of PD image
Very long TR (2000+ms ) followed by very short TE (20ms)
- Contrast attributed to signal intensities at TR
- Higher PD = brighter
47
Proton density (PD) weighted images
- Related to the number of hydrogen protons instead of the magnetic characteristics of the hydrogen nuclei
- PD weighted images result when the contribution of both T1 and T2 contrast is minimized
48
gradient coils
- Gradient coils lie between the main magnet and RF coils
- Each generates a magnetic field in the same direction as B0
- Combine with B0 to spatially localize anatomy of interest
- Magnetic field strength varies along the direction of the applied gradient field
- Spatial Encoding
49
what are the main purposes of RF coils?
1. transmit RF energy to the tissue of interest
2. receive the induced RF signal back from the tissue of interest
50
RF coils
- some have dual transmission functions, others only transmit or receive
- output signal received by the RF coil is digitized and to computer processor for reconstruction
51
electromagnetic disturbances
faraday cage used to block unwanted external radio waves
52
RF noise
caused due to RF interference from electronics in or near scanner room
- zipper artifact
53
MRI safety
- Main safety issue involves the attraction of ferromagnetic material towards the magnet
- B0 can also exert mechanical forces on ferromagnetic components within implanted medical devices - may rotate or dislodge
- Gradient fields can cause potential auditory damage- ear protection is common
- RF pulses may lead to local tissue heating - potentially serious with implanted devices
54
Gadolinium
- contrast agent
- shortens T1 - brighter signal on T1 weighted images
- very small adverse reaction rate - possible issues in patients with severe renal impairment
55
T1 versus T2 in bone
very long T1 but very short T2
56
T1 versus T2 in water
the same
