User guide for mastering MRI (magnetic resonance) physics for radiology residents and medical physicists

MRI Physics Review  Part 1 (AV's notes)
     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

1.     Larmor Equation: f₀ = 42.58 x B₀

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 (M_xy) equal to M_z? When all the spins are in phase.

                                              ii.     Which component (M_xy or M_z) gives rise to NMR signal? M_xy (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: M_z (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 M_z/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.     M_xy (t) = M_o 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 M_xy/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.