2 magnetized materials and moving electric charges. Magnetic fieldsmagnetized materials and moving electric charges.
3 Electric inductionSimilarly a moving magnetic field can be used to create electric current (moving charge).
4 Or you could use an electric current to move a magnet… Electric inductionOr you could use an electric current to move a magnet…
5 Force and field directions Right hand ruleForce and field directions
6 Nuclear spinsProtons are positively charged atomic particles that spin about themselves because of thermal energy.
7 Magnetic momentμ (magnetic moment) = the torque (turning force) felt by a moving electrical charge as it is put in a magnet field.The size of a magnetic moment depends on how much electrical charge is moving and the strength of the magnetic field it is in.A Hydrogen proton has a constant electrical charge.
8 Spin alignmentEarth’s magnetic field is relatively small ( Tesla), so the spins happen in different directions and cancel out.
9 But when in a strong external magnetic field (e.g. 1.5 Tesla). Spin alignmentBut when in a strong external magnetic field (e.g. 1.5 Tesla).
10 Net magnetization (M)Sum of magnetic moments in a sample with a particular volume at a given time.
11 PrecessionHydrogen protons not only spin. They also precess around the axis of the magnetic field.Magnetic field directionTrue for all atoms with an odd number of protons
12 Precession speedTwo factors govern the speed of precession (Larmor frequency): magnetic field strength & gyromagnetic ratio Larmor frequency = Bo * /2π
13 Gyromagnetic ratioGyromagnetic ratio ( ) Magnetic moment / Angular momentum Combination of electromagnetic and mechanical forces. Angular momentum is dependant on the mass of the atom.
14 Different atoms have different gyromagnetic ratios: NucleusGyromagnetic ratio (γ)1H7Li13C67.26219F23Na70.76131P
15 Larmor Frequency at 1 Tesla Different atoms placed in the same magnetic field have different Larmor frequencies: “Tune in” to the Hydrogen frequency.NucleusLarmor Frequency at 1 Tesla1HMGHz7LiMGHz13CMGHz19FMGHz23NaMGHz31PMGHz
16 The hydrogen atoms are precessing around z (direction of B0) Longitudinal & transverse directionsThe hydrogen atoms are precessing around z (direction of B0)
17 Net magnetization is all pointing in the z direction Steady stateNet magnetization is all pointing in the z direction
18 Applying a perpendicular magnetic field “flips” the protons Excitation pulsesApplying a perpendicular magnetic field “flips” the protons
19 Excitation & Relaxation Excite the sample in a perpendicular direction and let it relax. Net magnetization of the sample changes as it relaxes, inducing current to move in a near by coil.Larmor frequency
20 Flip angleDefined by the strength of B1 pulse and how long it lasts (T) θ = *B1*T This is one of the parameters we set during a scan It defines how far we “flip” the protons…xyz<900 pulsexyz900 pulsexyz>900 pulsexyz1800 pulse
21 Changes in the direction of the sample’s net magnetization T1 and T2/T2*T1: relaxation in the longitudinal direction T2*: relaxation in the transverse planeChanges in the direction of the sample’s net magnetization
22 Realignment of net magnetization with main magnetic field direction Before excitationAt excitationRelaxationNet magnetization along the longitudinal direction
23 T1T1 = 63% recovery of original magnetization value M0
24 What influences T1?Has something to do with the surroundings of the excited atom. The excited hydrogen needs to “pass on” its energy to its surroundings (the lattice) in order to relax.Different tissues offer different surroundings and have different T1 relaxation times…We can also introduce external molecules to a particular tissue and change its relaxation time. These are called “contrast agents”…
25 Loss of net magnetization phase in the transverse plain T2*/T2Loss of net magnetization phase in the transverse plainBefore excitationAt excitationRelaxationNet magnetization in the transverse plain
26 T2/T2*T2 = 63% decay of magnetization in transverse plain
27 T2* = T2 - T2’Two main factors effect transverse relaxation: 1. Intrinsic (T2): spin-spin interactions. Mechanical and electromagnetic interactions Extrinsic (T2’): Magnetic field inhomogeneity. Local fluctuations in the strength of the magnetic field experienced by different spins.
28 Magnetic field inhomogeneities Examples of causes:Transition to air filled cavities (sinusoids)Paramagnetic materials like cavity fillingsMost importantly – Deoxygenated hemoglobin
29 What influences T2?Again, has to do with the molecular neighborhood affecting the amount and quality of spin-spin interactions.Different tissues will have different T2 relaxation times.The stronger the static magnetic field, the more interactions there are, quicker T2 decay.
30 We only have one measurement: MR signalWe only have one measurement:Measurement of the net magnetization in the transverse plain as the sample relaxes.Once T2* relaxation is completeProtons precess out of phase in the transverse plainNet magnetization in transverse plain = 0
31 TR and TETwo important scanning parameters: TR – repetition time between excitation pulses. TE – time between excitation pulse and data acquisition (“read out”). Creating scanning protocols with different TR and TE lengths will allow us to derive T1 and T2/T2* relaxation times.
32 Short TR = weaker MR signal on consecutive pulses. TR length & MR signal strengthShort TR = weaker MR signal on consecutive pulses.With short TRs relaxation in the longitudinal direction will not be complete. So there will be fewer relaxed protons to excite.
33 TE: when to measure MR signal We can measure the amplitude of net magnetization immediately after excitation or we can wait a bit.Longer TEs will allow more transverse relaxation to happen and the MR signal will be weaker.
34 Different image contrasts We can scan the brain using different pulse sequences by choosing particular TR and TE values to create images with different contrasts.TR length will determine how much time the sample has had to relax in the longitudinal direction.TE will determine how much time the sample has had to relax (loose phase) in the transverse plain.
35 Proton density contrast Measuring the amount of hydrogen in the voxels regardless of their T1 or T2 relaxation constants.This is done using a very long TR and very short TE
36 Higher intensity in voxels containing more hydrogen protons Proton densityHigher intensity in voxels containing more hydrogen protons
37 T1 contrastMeasuring how T1 relaxation differs between voxels. This is done using a medium TR and very short TEYou need to know when largest difference between the tissues will take place…
38 T1 contrastImages have high intensity in voxels with shorter T1 constants (faster relaxation/recovery = release of more energy)CSF: msGray matter: msWhite matter: msMuscle: msFat: ms
39 We can combine a T2 acquisition with proton density… T2/T2* contrastMeasuring how T2 relaxation differs between voxels. This is done using a long TR and medium TEWe can combine a T2 acquisition with proton density…
40 T2 contrastImages have high intensity in voxels with longer T2 constants (slower relaxation = more detectable energy)CSF: msGray Matter: 80 msWhite Matter: 60 msMuscle: 50 msFat: 50 ms
41 Same as T2 only smaller numbers (faster relaxation) T2* contrastSame as T2 only smaller numbers (faster relaxation)CSF: msGray Matter: 40 msWhite Matter: 30 msFat: 25 ms
42 T2* and BOLD fMRIT2* = T2 +T2’ T2: Spin-spin interactions T2’: field inhomogeneities Exposed iron (heme) molecules create local magnetic inhomogeneitiesBOLD – blood oxygen level dependantAssuming everything else stays constant during a scan one can measure BOLD changes across time…
43 T2* and BOLDMore deoxygenated blood = more inhomogeneity more inhomogeneity = faster relaxation (shorter T2*) Shorter T2* = weaker energy/signal (image intensity) So what would increased neural activity cause?
44 So what happened in particular time points of this scan? T2* and BOLDSo what happened in particular time points of this scan?
46 MR imagesSo far we’ve talked about a bunch of forces and energies changing in a sample across time… How can we differentiate locations in space and create an image?2004 Nobel prize in MedicinePaul LauterburPeter Mansfield
47 Spatial gradientsCreate magnetic fields in each direction (x,y,z) that move from stronger to weaker (hence gradient).
48 Spatial gradientsDifferent magnetic fields at different points in space. Hydrogen will precess at a different speed in each spatial location. By “tunning in” on the specific precession speed we can separate different spatial locations. Similarly to how we “tunned in” on hydrogen atoms…
50 Spatial gradientsLot’s of Fourier transforms. Work in k-space (a vectorial space that keeps track of the spin phase & frequency variation across magnet space). It’s possible to turn gradients on and off very quickly (ms). Image reconstruction Pulse sequences
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