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29 Cards in this Set

  • Front
  • Back
Nuclear Magnetic Resonance which was later changed to a nicer sounding MRI
Bp (MRI)
the small magnetic field produced from the interaction between the proton's charge and it's spin
Why must MRI induce a magnetic field?
The protons in the atom will either allign paralle or anti-parallel with the induced field
Distinguish between spin up and spin down for protons in an applied magnetic field.
Spin up indicates low energy protons (parallel with the applied magnetic field) whereas spin down indicates high energy (induced magnetic field in an anti-parallel direction)
Bz (MRI)
the large external magnetic field applied in MRI
Why does MRI work only for elements with odd numbers of protons?
For elements with an even number of protons, the spin up and spin down cancel out creating no resulting magnetic field that can be detected.
Resonant Absorption
The precise frequency and energy of a photon required to excite a spin up proton to a spin down position.
Resonant Emission
The precise frequency and energy of a photon released when an excited spin down proton returns to a spin up position
Consider proton precession in regards to its spin.
Proton spin is not exactly parallel (or anti-parallel) to the applied magnetic field (Bz) but at a slight constant angle depending on quantum mechanics
Larmour Frequency (FL)
the number of times per second that BN precesses around Bz calculated using the Larmor Equation
the angled magnetic field of a proton which rotates about the applied magnetic field Bz
What does gamma stand for in the Larmor equation?
Gamma is the gyromagnetic ratio which varies with material. For protons, gamma = 42.5 MHz/T
What kind of photon is applied in MRI to flip proton spins
Radio waves (RF)
What sort of patter is seen in resonant emission as protons return to spin-up positions?
The protons don't all return at the same time but at slower and slower rates - an exponential decay
Longitudinal Magnetic Field - the resultant magnetic field of the individual protons in precession (BN) which is exactly parallel or anti-parallel to the applied magnetic field Bz
the left and right components cancel out
the transverse magnetic field of protons in precesseion; When the protons are in random phase, BT=0
BT can be observed only when the proton precessions are in phase (due to radio frequency)
How can you eliminate BL?
If the right number of radio photons (pulse) is selected, the number of of spin up and spin down protons can cancel each other out giving a BL of 0.
Why is BL so difficult to measure? What is done to overcome this?
BL is very small and also parallel to BZ. the radio waves put the proton precession in phase which creates BT (transverse field)
How is BT (transverse field) measured?
By placing metal coils near the tissue, BT will induce a detectable RF electrical current in the coils
State the effect of radio pulses on a nuclear magnetic field.
Reduction of BL by changing spin of protons.
Productino of BT by putting the protons in phase.
Relate between the Larmor frequency and BT.
BT precesses about the z-axis (BZ) at the Larmor frequency.
What happens when the radio waves are switched off? [2]
BL recovers as excited protons return to spin up and BT decays as precession falls out of phase. The two fields are independent of each other.
Spin-Lattice Relaxation
The recovery of BL characterised by the time constant T1
T1 (MRI)
the time required for BL to recover to 63% of its pre-pulse value. Applies to tissues rather than liquids. [varies from tissue to tissue from 0.3 to 2.0 seconds]
T0 (MRI)
the time which corresponds to when BL = 0; equal number of parallel and anti-parallel fields or
the time which corresponds to when BT is maximised (same phase) and the radio pulse is turned off
Why do protons go out of phase after radio waves are removed?
Extrinsic effects including extrenal magnetic fields.
Intrinsic effects including the magnetic fields of surrounding atoms
At what rate does BT decay when the RF pulse is switched off?
BT decays exponentially with the same half-life as the induced RF current decay.
Spin-Spin Relaxation
name for the decay of BT which is characterised by T2
T2 (MRI)
the time required for BT to decay to 37% of it's maximum value; associated with liquids