MRI

Magnetic Resonance 1.Principle first observed in Used for spectroscopy and imaging 3.Imaging techniques are a form of tomography, where slices are ’cut’ and depict 4.MRI utilizes signals from the body 5.MRI is non-ionizing, operating in radiofrequency range, unlike CT, PET, SPECT 6.Resolution is not limited to radio wave lengths 7.MRI is pricy 2

Nuclear spin A nucleui possesses a spin angular momentum, (p) Can be view as a rotation of the nuclei I is the quantum number of the spin. The spin gives raise to a magnetic moment: Where  is the gyromagnetic ratio 3

Nuclear spin I can be an intenger, half an intenger, or 0 If I is 0 there is no spin and no magnetic moment The natural isotope 12 C has quantum spin of 0 whereas 13 C has ½. 4

Nuclei in a magnetic field – The classics Torque on the nuclei Torque makes the muclei precess chancing p ω 0 is the Larmor frequency, the frequency that the nuclei precesses with 5

Nuclei in a magnetic field – Quantum mechanics p is quantified allowed 2I +1 states Eg a proton 1H is allowed two states or directions parallel to the field (spin up) antiparallel to the field (spin down) 6

Many nuclei in a magnetic field An equlibrium between spin up and spin down will emearge A small excess of nuclei in the low energy state,  N 7

Back to the Larmor Frequency , the gyromagnetic konstant ‘material’ constant , Can be affected by chemical bounds The magnetic field may be inhomogeneous 8

The Chemical shift effect Shielding electrons reduces the magnetic field ’seen’ by the nucleus The resonance frequency is also reduced  is the shielding constant  ~5e-6  depends on local chemical envionment Used for gaining knowledge about chemical structure; Spectroscopy 9

Bulk / Macroscopic / Sum magnetization N s is the number of atoms in a sample  i is the magnetic moment of the i-th atom M is always aligned to B in equilibrium M can be pertubed and will precess 10

Excitation Adding a field B 1 perpendicular to B 0 at Lamor frequency will excite the system An ocillating magentic field at 1 – 500 MHz is a Radio frequency wave B 1 ~ 50 mT & B 0 ~ 1-5T  is the flip angle A pertubation pulse is often named after the flip angle 90° pulse 180 ° pulse 11

Excitation B 1 is the envelope function The duration of the pulse  affects the flip angle  =  B 1  or if different amplitudes are allowed 12

Induced current In Eqlibrium M z = M 0 M x = M y = 0 ~ M xy After perturbation 13

Free Indusction Decay (FID) The M xy component decays to 0 The frequency is the peak The decay rate T2 is proportional to the width at half max Area under the envolope is the hight of the spectral amplitude 14

Relaxation M xy  0 : Spin-spin relaxation T2 time to 36.7% of M 0 M z  M 0 : Spin-lattice relaxation T1 time 63.2% of M 0 Important for contrast in images 15

Inversion recovery 180-TI-90-FID 16

Invertion recovery 17

Spin Echo 18

Magnetic Field Gradients G is the gradient of a magnetic field 19

Slice selection By applying a gradient G the resonance frequency becomes dependent on direction The bandwidth of the pulse determines the thickness of the slice 20