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Source: Courtesy of Warner Bros

Chap.12 (3) Medical imaging systems: MRI. Science or black magic?. Source: Courtesy of Warner Bros. Principles of MRI. MRI. Source: Biomed resources. MRI. Source: MT Scott Diagnostic imaging. A brief recipe of MRI. Put the subject into a strong magnetic field

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Source: Courtesy of Warner Bros

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  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

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