1 / 20

MRI

MRI. Magnetic Resonance. Principle first observed in 1946 Used for spectroscopy and imaging Imaging techniques are a form of tomography, where slices are ’cut’ and depict MRI utilizes signals from the body MRI is non-ionizing, operating in radiofrequency range, unlike CT, PET, SPECT

ona
Télécharger la présentation

MRI

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. MRI

  2. Magnetic Resonance Principle first observed in 1946 Used for spectroscopy and imaging Imaging techniques are a form of tomography, where slices are ’cut’ and depict MRI utilizes signals from the body MRI is non-ionizing, operating in radiofrequency range, unlike CT, PET, SPECT Resolution is not limited to radio wave lengths MRI is pricy

  3. Nuclearspin 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

  4. Nuclearspin I can be an intenger, half an intenger, or 0 If I is 0 there is nospin and nomagnetic moment The natural isotope 12C has quantum spin of 0 whereas13C has ½.

  5. Nuclei in a magneticfield – The classics Torque on the nuclei Torque makes the mucleiprecess chancing p ω0 is the Larmor frequency, the frequency that the nuclei precesses with

  6. Nucleiin a magneticfield – Quantum mechanics p is quantifiedallowed2I +1states Eg a proton 1H is allowedtwostatesordirections parallel to the field (spin up) antiparallel to the field (spindown)

  7. Manynucleiin a magneticfield An equlibriumbetweenspin up and spindownwillemearge A small excess of nuclei in the lowenergystate, N

  8. Back to the Larmor Frequency , the gyromagnetickonstant ‘material’ constant , Canbeaffected by chemicalbounds The magneticfieldmaybeinhomogeneous

  9. The Chemical shifteffect 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

  10. Bulk / Macroscopic / Sum magnetization Ns 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

  11. Excitation Adding a fieldB1perpendicular to B0 at Lamorfrequencywillexcite the system An ocillatingmagenticfield at 1 – 500 MHz is a Radio frequencywave B1 ~ 50 mT & B0 ~ 1-5T  is the flip angle A pertubation pulse is oftennamedafter the flip angle 90° pulse 180 ° pulse

  12. Excitation B1 is the envelopefunction The duration of the pulse  affects the flip angle  = B1 orifdifferent amplitudes areallowed

  13. Inducedcurrent In Eqlibrium Mz = M0 Mx = My = 0 ~ Mxy After perturbation

  14. FreeIndusctionDecay (FID) The Mxycomponentdecays 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

  15. Relaxation Mxy 0 : Spin-spin relaxation T2time to 36.7% of M0 Mz  M0 : Spin-lattice relaxation T1 time 63.2% of M0 Important for contrast in images

  16. Inversion recovery 180-TI-90-FID

  17. Invertionrecovery

  18. SpinEcho

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

  20. 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

More Related