1. Rad T 265 MRI Lecture

2. No Magnetic Field = No Net Magnetization Protons align with a magnetic field…

3. In a magnetic field, protons can take either high- or low-energy states

4. The difference between the numbers of protons in the high-energy and low-energy states results in a net magnetization (M) and gives rise to the Larmor Equation.

5. MRI Equipment Main Magnet Shim Magnet Gradient Coil RF system

6. Main Magnetic Field ▪Typically oriented to the long axis of the patient ▪B o Increase B o Homogeneity Precessional frequency Chemical shift

7. Type of Magnets - Resistive ▪Require constant electrical current ▪Max field is less than 0.3T

8. Type of Magnets - Permanent ▪Can be built in a variety of shapes and configurations ▪Tend to be heavy ▪Lower field strengths ▪Made of aluminum, nickel, and cobalt - alnico

9. Type of Magnets - Superconductor ▪Most common ▪Lowest electrical costs ▪Highest field strength ▪Use cryogens Helium -450 F, -269 C, 4.2 K Nitrogen -320 F, -160 C, 77.3 K

10. Shim Coils Increase homogeneity ▪Passive ▸ Steel plates attached to the magnet ▪Active ▸ Electromagnets with an opposite polarity

11. Gradient Fields ▪Measured in mT/m ▪Rise time ▸ 1 ms for 0 to 10mT/m is good

12. Gradient Coils ▪Produce noise ▪They rattle in their mountings ▪Greatest stress is caused by obliques

14. Faraday’s Law Used for most MRI activities ▪Ramping the magnet ▪Shimming ▪RF field ▪Gradients ▪MR signal

15. RF Receiver Coil ▪Designed to detect transverse magnetization ▪Based on Faraday’s Law ▸ Variable magnetic fields produce an electric current in a loop of wire

16. RF Shielding ▪Copper is preferred ▸ Expensive ▪Aluminum can be used ▪Problems with the RF shield produce zipper artifacts

17. Gauss ▪10,000 gauss = 1 tesla ▪Earth’s magnetic field is 0.5 g

18. Larmor Equation ▪Precession frequency is based on B o ▪ For a 1 T magnetic the precessional frequency is 42.6 Mhz

19. RF ▪Needs to be perpendicular to B o ▪Needs to be at the precession frequency Spins are only in phase during RF pulses When the pulse ends dephasing begins immediately

20. Inherent Tissue Characteristics ▪T1, T2, PD, flow, motion ▪We can only demonstrate these not change them ▪Pulse sequences are used to maximize differences in tissue characteristics

21. Weighting T 1 T 1 weighted images have a short TE and TR Provide more anatomical info – better spatial resolution T 2 T 2 weighted images have a long TE and TR More pathologic info

22. MRI Contrast Agents ▪Types; Paramagnetic, Ferromagnetic ▪Administration ▪Reactions ▪Contraindications

23. Paramagnetics ▪Gadolinium ▸ Positive contrast ▸ Shortens T1 relaxation ▸ Appears brighter on the image ▸ Elimination half life 1 - 2 hrs

25. Ferromagnetics ▪Ferumoxides ▸ Negative contrast ▸ Shorten T2 relaxation ▸ Appears darker on the image

26. SAR Dependent on ▪Duration of RF ▸ Flip angle and strength ▪Frequency ▸ Pulse sequence and strength ▪Patient Mass ▸ Weight

27. SAR Limits Increase core temp 1 C ▪Whole body ▸ 0.4 W/kg ▪Head ▸ 3.2 W/kg ▪Small Volume ▸ 8.0 W/kg

28. Static Field Exposure ▪Whole body ▸ 3T ▸ Extremities ▸ 5T

29. High Field Exposure Possible effects ▪Magnetophosphenes ▪Nausea ▪Vertigo ▪Metallic taste

30. Fringe Field ▪Public is limited to 0.5 mT ▸ 0.5 mT = 5 gauss ▪No pacemakers beyond this line

31. Noise Limitations ▪Earplugs are necessary above 100 db ▪Remember noise is related to gradient activity ▪Gradients are rattling in their supports

32. Quench ▪Uncontrolled release of cryogens ▪Helium and nitrogen replace oxygen ▪Asphyxiation

33. Non Compatible Devices Absolute contraindications ▪Cardiac pacemakers ▪Internal defibrillators ▪Biostimulators ▪Implanted infusion pumps ▪Cochlear implants ▪Metallic orbital FB

34. Non Compatible Devices Continued Safe to image ▪Surgical hemostasis clips ▪Orthopedic prostheses ▪Dental work ▸ Except magnetic dentures ▪IUDs ▪Intra vascular coils

35. Wires ▪Important to remember that coiled wires will generate a current and that currents produce heat. ▪Faraday’s Law

36. MR Data Acquisition Fourier Transformation ▪Process that takes a complex signal and breaks it down into its component parts

37. Types of Pulse Sequences ▪SE, IR, STIR, GE ▪RARE, FLARE, FLAIR, FSE ▪EPI,

38. Spin Echo ▪Uses a 90 RF followed by a 180 RF ▪Traditionally the most popular sequence ▪Can provide T1 or T2 information

39. IR, STIR ▪Uses a 180 RF followed by a 90 RF and then a 180 RF ▪Provides heavy T1 weighting ▪Can be used to minimize signal by varying the TI time

40. Gradient Echo ▪Uses an initial RF pulse, usually less than 90 ▪Rephases the spins by using a gradient instead of other RF pulses

41. FSE ▪Uses ETL ▪ETL - obtain more than 1 echo per TR ▪Different from regular ME because second echo and beyond is used to fill the same k- space, not a new one

42. EPI ▪Similar to FSE ▪Difference is all the phase encoding steps are acquired during one TR

43. Length of sequence ▪SE, IR, traditional sequences ▪TR x NSA x #PE

44. TR Controls ▪T1 relaxation ▪Spin lattice ▪Longitudinal

45. TE Controls ▪T2 ▪Spin spin ▪Transverse relaxation - dephasing

46. Affecting SNR ▪Slice gap Increase slice gap, increase SNR, less cross-talk ▪Slice thickness Increase slice thickness, increase SNR, more anatomy per slice = more signal Also, increase partial volume and decrease resolution

47. Affecting SNR ▪FoV Increase FoV, increase SNR (more anatomy) Decrease resolution This is the same effect we discussed in CT ▪Increase matrix Increase resolution Decrease SNR, smaller pixels

48. Affecting SNR ▪Increasing TR increases SNR Provides more relaxation ▪Decreasing TE increases SNR Less dephasing occurs

49. Types of Suppression ▪STIR; short tau inversion, suppresses fat ▪FLAIR; suppresses fluids, long T1 values ▪Heavy T2; long TE and TR, maximizes T2 values ▪Spectral fat suppression; based on freq difference between fat and water

50. Gating Used to eliminate or minimize physiologic motion ▪Peripheral pulse ▪Respiratory ▪Cardiac ▪NOTE ALL INCREASE TR ▸ Or decrease slices

51. MR Angiography Two major techniques ▪Time of flight ▪Phase contrast

52. Time of Flight (ToF) Needs MIP ▪Uses slice entry phenomena ▪Can also be called ▸ Paradoxical enhancement ▸ Flow related enhancement ▪Evaluates vessel morphology

53. A. Safety issues surrounding static field.

54. RF pulse determines “flip angle”  Rotation determines amount of magnetization measured Field strength determines resolution  Increased magnetization leads to increased signal Pulse and Field Effects  Images adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htmhttp://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm Muscle Tissue Difference B 0 = 0.2 T B 0 = 1.5 T

55. Short TR Short TE Long TE Long TR T1 T2 PD