Presentation on theme: "FMRI Methods Lecture2 – MRI Physics. magnetized materials and moving electric charges. Magnetic fields."— Presentation transcript:
fMRI Methods Lecture2 – MRI Physics
magnetized materials and moving electric charges. Magnetic fields
Similarly a moving magnetic field can be used to create electric current (moving charge). Electric induction
Or you could use an electric current to move a magnet… Electric induction
Force and field directions Right hand rule
Protons are positively charged atomic particles that spin about themselves because of thermal energy. Nuclear spins
μ (magnetic moment) = the torque (turning force) felt by a moving electrical charge as it is put in a magnet field. Magnetic moment 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.
Earths magnetic field is relatively small ( Tesla), so the spins happen in different directions and cancel out. Spin alignment
But when in a strong external magnetic field (e.g. 1.5 Tesla). Spin alignment
Sum of magnetic moments in a sample with a particular volume at a given time. Net magnetization (M)
Hydrogen protons not only spin. They also precess around the axis of the magnetic field. Precession Magnetic field direction True for all atoms with an odd number of protons
Two factors govern the speed of precession (Larmor frequency): magnetic field strength & gyromagnetic ratio Larmor frequency = B o * /2π Precession speed
Gyromagnetic ratio ( ) Magnetic moment / Angular momentum Combination of electromagnetic and mechanical forces. Angular momentum is dependant on the mass of the atom. Gyromagnetic ratio
Different atoms have different gyromagnetic ratios: Gyromagnetic ratio NucleusGyromagnetic ratio (γ) 1H1H Li C F Na P
Different atoms placed in the same magnetic field have different Larmor frequencies: Tune in to the Hydrogen frequency. Larmor frequency NucleusLarmor Frequency at 1 Tesla 1H1H MGHz 7 Li MGHz 13 C MGHz 19 F MGHz 23 Na MGHz 31 P MGHz
The hydrogen atoms are precessing around z (direction of B 0 ) Longitudinal & transverse directions
Net magnetization is all pointing in the z direction Steady state
Applying a perpendicular magnetic field flips the protons Excitation pulses
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. Excitation & Relaxation Larmor frequency
Defined by the strength of B 1 pulse and how long it lasts (T) θ = *B 1 *T This is one of the parameters we set during a scan It defines how far we flip the protons… Flip angle x y z x y z 90 0 pulse x y z x y z pulse x y z x y z <90 0 pulse x y z x y z >90 0 pulse
T1: relaxation in the longitudinal direction T2*: relaxation in the transverse plane T1 and T2/T2* Changes in the direction of the samples net magnetization
Realignment of net magnetization with main magnetic field direction T1 Before excitationAt excitationRelaxation Net magnetization along the longitudinal direction
T1 T1 = 63% recovery of original magnetization value M 0
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…
Loss of net magnetization phase in the transverse plain T2*/T2 Before excitationAt excitationRelaxation Net magnetization in the transverse plain
T2/T2* T2 = 63% decay of magnetization in transverse plain
Two main factors effect transverse relaxation: 1. Intrinsic (T2): spin-spin interactions. Mechanical and electromagnetic interactions. 2. Extrinsic (T2): Magnetic field inhomogeneity. Local fluctuations in the strength of the magnetic field experienced by different spins. T2* = T2 - T2
T2 Magnetic field inhomogeneities Examples of causes: Transition to air filled cavities (sinusoids) Paramagnetic materials like cavity fillings Most importantly – Deoxygenated hemoglobin
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.
MR signal We only have one measurement: Measurement of the net magnetization in the transverse plain as the sample relaxes. Once T2* relaxation is complete Protons precess out of phase in the transverse plain Net magnetization in transverse plain = 0
Two 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. TR and TE
Short TR = weaker MR signal on consecutive pulses. TR length & MR signal strength With short TRs relaxation in the longitudinal direction will not be complete. So there will be fewer relaxed protons to excite.
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.
We can scan the brain using different pulse sequences by choosing particular TR and TE values to create images with different contrasts. Different image 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.
Measuring the amount of hydrogen in the voxels regardless of their T1 or T2 relaxation constants. Proton density contrast This is done using a very long TR and very short TE
Higher intensity in voxels containing more hydrogen protons Proton density
Measuring how T1 relaxation differs between voxels. This is done using a medium TR and very short TE T1 contrast You need to know when largest difference between the tissues will take place…
Images have high intensity in voxels with shorter T1 constants (faster relaxation/recovery = release of more energy) T1 contrast CSF: 1800 ms Gray matter: 650 ms White matter: 500 ms Muscle: 400 ms Fat: 200 ms
Measuring how T2 relaxation differs between voxels. This is done using a long TR and medium TE T2/T2* contrast We can combine a T2 acquisition with proton density…
Images have high intensity in voxels with longer T2 constants (slower relaxation = more detectable energy) T2 contrast CSF: 200 ms Gray Matter: 80 ms White Matter:60 ms Muscle:50 ms Fat:50 ms
Same as T2 only smaller numbers (faster relaxation) T2* contrast CSF: 100 ms Gray Matter: 40 ms White Matter:30 ms Fat:25 ms
T2* = T2 +T2 T2: Spin-spin interactions T2: field inhomogeneities Exposed iron (heme) molecules create local magnetic inhomogeneities T2* and BOLD fMRI BOLD – blood oxygen level dependant Assuming everything else stays constant during a scan one can measure BOLD changes across time…
More deoxygenated blood = more inhomogeneity more inhomogeneity = faster relaxation (shorter T2*) Shorter T2* = weaker energy/signal (image intensity) So what would increased neural activity cause? T2* and BOLD
So what happened in particular time points of this scan? T2* and BOLD
So far weve 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? MR images Paul Lauterbur Peter Mansfield 2004 Nobel prize in Medicine
Create magnetic fields in each direction (x,y,z) that move from stronger to weaker (hence gradient). Spatial gradients
Different 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… Spatial gradients
64 MHz 65 MHz 66 MHz 63 MHz 62 MHz G (-) (+)
Lots of Fourier transforms. Work in k-space (a vectorial space that keeps track of the spin phase & frequency variation across magnet space). Its possible to turn gradients on and off very quickly (ms). Image reconstruction Pulse sequences Spatial gradients
Main static field Extremely large electric charge spinning on a helium cooled (-271 o c) super conducting coil. Earths magnetic field microtesla. MRI magnets suitable for scanning humans T.
Main coils The bulk of the structure contains the coils generating the static magnetic field and the gradient magnetic fields.
RF coil Transmit and receive RF coils located close to the sample do the actual excitation and read out.
Read Chapters 3-5 of Huettel et. al. Explain how a spin-echo pulse does the magic of separating T2 relaxation from T2* relaxation. You can include figures/drawings if you like. Homework!