1 / 34

Source: Courtesy of Warner Bros

Learn the principles of MRI, from setting a strong magnetic field to converting RF data into images. Understand the role of nuclear spin, RF excitation, relaxation properties, and more.

jdecaro
Download Presentation

Source: Courtesy of Warner Bros

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. Chap.12 (3) Medical imaging systems: MRI Science or black magic? Source: Courtesy of Warner Bros

  2. Principles of MRI

  3. MRI Source: Biomed resources

  4. MRI Source: MT Scott Diagnostic imaging

  5. A brief recipe of MRI • Put the subject into a strong magnetic field • Pass radiowaves through the subject • Turn of the radiowaves • Recieve radiowaves coming back from the subject • Convert the measured RF-data to an image

  6. Elements contributing to a MRI • The quantitative properties of the nuclear spin • The radiofrequency (RF) exitation properties • Relaxationproperties of the tissue • Magnetic field strength and gradients • Thte timing of the gradients, RF-pulses and signal detection

  7. Prerequisites for depicted nucleus • A nucleus that is to be pictured must have both: • Spin • Charge Nucleus with even protonnumbers cannot be used because the spin will cancel each other

  8. Single-proton • A single proton has a charge on the surface which is sufficient to form a small current-loop and generates a magnetic momentum µ • The proton has also a mass that creates an angle-moment J due to the spin

  9. Hydrogenatoms • The hydrogenatom is the only large element in the body able to be depicted with MRI. (C, O and N have all even numbers in the proton number). • Hydrogen is everywhere in the body, primarily combined to water = All MRI are in fact a picture of hydrogen

  10. J m r v Angle momentum J = m=mvr

  11. Magnetic momentum µ A I The magnetic momentum vector µ=IA

  12. Precession og relaxation

  13. Vector direction • The magnetic momentum and the angle momentum vector is aligned to the spin-axis. µ=γJ Where γ is the gyromagnetic ratio, constant for a given nucleus

  14. Proton interaction with magnetism • Loaded particles spinning is constructing their own little magnetic field. - Will line up in the same direction as an external magnetic field Spinning particles with a mass have an angle momentum • The angle momentum works as a gyroscope and counteracts changes of the spin direction

  15. Ref:www.simplyphysics.com

  16. Larmour frequency The energy difference between the two alignment states depends on the nucleus •  E = 2 z Bo  Eh • /2 known as Larmor frequency /2= 42.57 MHz / Tesla for proton Ref: James Voyvodic

  17. Resonance frequencies of common nuclei Note: Resonance at 1.5T = Larmor frequency X 1.5 Ref: James Voyvodic

  18. Electromagnetic Radiation Energy X-Ray, CT MRI

  19. Magnetization • Sum of all contributions from each nucleus • Large magnetic fields create a big magnetization M • Temperature dependency • To be able to measure the magnetization, we will have to disturb it • The quantity of energy supplied (durability for the RF-pulse at the resonance frequency) will decide how far the nuclei will be pushed away from B

  20. Radiofrequency field • RF fields are used to manipulate the magnetization for a specific atom in a specific position • The hydrogen nucleus is tuned to a certain RF-frequecy • Eksternal RF-waves can be sent into the subject in order to disturb the hydrogen nucleus • Disturbed hydrogen nuclei will generate RF-signals with the same frequency – which can later be detected

  21. To record an MRI signal • Needs a receive coil tuned in to the same RF-requency as the excitasjonscoil • Measure net magnetization • The signal oscillates at the resonansfrequency when the net magnetization vector rotates in the room • Signalamplitude will be weakened when the netto magnetization returns to the B-direction

  22. MRI scanner

  23. Important MRI equations • Larmorequation: ω=γB • Relationship between parallell / antiparallell protones : Nn/Ne = ehν/kT =1+410-6 represents net magnetization at room temperature and 1 Tesla

  24. T1 recording

  25. T2 recording

  26. MR images T1 and T2 contrast

  27. 3D picture construction ω = γB

  28. T1, T2 and proton-density

  29. Vertical main field Source: Oulun Yliopisto

  30. Extremity MRI

  31. Interventional MRI

  32. Adv/disadv MRI • Adv: • No harmful radiation • Soft tissue imaging • High resolution images of T1 or T2 preferences • Disadv: • Expensive, large installation with superconducting magnets++ • Very strong magnetic field • Claustrophobic • Not for frozen tissue

More Related