Magnets, Magnetism and Magnetic Fields
a.
Spectroscopy – study of interaction between
matter and radiated energy
b.
NMR – initially used to perform spectroscopy
i. Which
nucleus is imaged with MRI? Hydrogen
ii. Why?
Hydrogen does not have a 1:1 of protons: neutrons
iii. At
1.0T, what is the number of excess protons in the low energy state?
3ppm
iv. Define
Precession – torque perpendicular to the applied field that unaligned protons
experience
2.
What is the Larmor frequency at 1.5T? 3T? 64MHz,
128MHz
c.
Magnetism – property of a material causing it to
respond to an applied magnetic field
i. Permanent
magnets – ferromagnetic substances
ii. Electromagnets
– magnetic field based on electron movement
iii. Strength of magnetic field is measured in
the SI unit Tesla
1.
Conversion of Tesla to Gauss: 1 Tesla =
10,000 Gauss
2.
Safe strength for pacemakers? 5 Gauss
3.
Strength of earth’s magnetic field? 0.5 Gauss
d.
Two Configurations of Magnets used clinically
i. Solid
Core: permanent magnets, “open MRI”
1.
What is the field strength of Open MRI? ~0.5 Tesla
a.
Why is a large room required for Open MRI? The 5 Gauss line of permanent magnets
extends further in solid core magnets than in air à
Not actively shielded
ii. Air
Core: superconducting magnets
1.
Patients enclosed on all sides.
2.
High field strength – 1.5T, 3T+
e.
Superconducting Magnet Design (Identify A-F)
i.
ii. A.
Liquid Helium D. Shim Coil
iii. B.
Super Conducting Coil E. RF
Coil
iv. C.
Vacuum Insulation F. Gradient Coil
f.
Define Susceptibility: Degree to which a material perturbs the
surrounding magnetic field
g.
What are the Three Types of Susceptibility?
i. Ferromagnetic
– Fe, Ni, or Co à
significantly affect the local magnetic field
ii. Diamagnetic
– have slightly negative susceptibility (oppose magnetic field)
iii. Paramagnetic
– Deoxyhemoglobin, Gadolinium à
have positive susceptibility (enhance magnetic field)
2.
Magnetic Resonance Signal and Magnetization of
Tissue
a.
What are the two types of magnetization?
i. Longitudinal
Magnetization – component of the magnetic vector parallel to B0
ii. Transverse
Magnetization – component of magnetic field transverse to B0
b.
RF Burst – applied to a tissue voxel and
absorbed when it is tuned to the processional frequency of the spins (Larmor
frequency)
i. When
the longitudinal magnetization is 0, when is the transverse magnetization (Mxy)
equal to Mz? When all the spins
are in phase.
ii. Which
component (Mxy or Mz) gives rise to NMR signal? Mxy
(transverse)
iii. Define
the Flip Angle – the angle between
the net magnetization vector after B1 (RF) pulse and the net
magnetization vector at equilibrium.
1.
What 2 factors determine the flip angle? Amplitude, Duration of RF pulse
2.
Advantage of small flip angle? Short
scan time
3.
Advantage of 90˚ flip angle? High signal
4.
Advantage of 180˚ flip angle? Tissue
contrast
c.
Return to Equilibrium
i. T1
– spin-lattice relaxation à
recovery of longitudinal component
1.
Occurs as anti-parallel protons return to ground
state, releasing signal
2.
Occurs exponentially: Mz (t) = M0
(1 - e -t/T1) – remember that
T2 is recovery and thus will increase as time (t) increases. A larger T1 decay
constant means recovery takes longer, that at the same time (t), tissue with a
larger T1 recovery constant will have recovered less signal.
3.
Is T1 dependent on magnetic field strength? YES
a.
How? T1
increases as the field strength increases.
4.
In examples, calculate the percentage of initial
longitudinal magnetization recovery in n
seconds à Mz/M0 = 1 – e-t/T1
5.
Length of T1 (T1 > T2 for all tissue types at clinical field strength)
a.
Long à
i. Large
stationary molecules – low frequency tumbling / little Larmor frequency overlap
ii. Small
aqueous molecules – broad range tumbling / small Larmor frequency overlap
b.
Short à
medium molecules and viscous fluids (proteins/fats) – tumbling and vibration
results in significant Larmor frequency overlap
c.
Typical T1
values range 200 ms to 2000 ms
ii. T2
– spin-spin relaxation à
free induction decay
1.
De-phasing occurs at different rates in
different tissues due to slight differences in local magnetic field
2.
Length of T2 (T2 < T1 for all tissue types at clinical field strength)
a.
Short à
larger, more bound molecules
b.
Long à
smaller, free molecules
c.
Typical T2
values range 40 ms – 200 ms
3.
Calculate remaining T2 signal with this equation
a.
Mxy (t) = Mo e-t/T2 - remember that T2 is
decay and thus will decrease as time (t) increases. A larger T2 decay constant
means decay takes longer, that at the same time (t), tissue with a larger T2
decay constant will retain more signal.
b.
In examples, calculate the percentage of
transverse magnetization remaining after n
seconds à Mxy/M0 = e-t/T2
4.
Is T2 dependent on magnetic field strength? NO
iii. T2*
- includes the effect of extrinsic
inhomogeneity in local magnetic field (not due to tissue type)
1.
Is T2* every greater than T2? NO
Helpful tips on the physics of mri scans. Advanced radiology notes (online book) for residents studying boards, practicing radiologists, and radiology techs.
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