Rad T 265 MRI Lecture

1. Rad T 265MRI 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. Main Magnet Shim Magnet Gradient Coil RF system MRI Equipment

6. Typically oriented to the long axis of the patient Bo Main Magnetic Field • Increase Bo • Homogeneity • Precessional frequency • Chemical shift

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

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

9. 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 Type of Magnets - Superconductor

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

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

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

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

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

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

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

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

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

20. T1, T2, PD, flow, motion We can only demonstrate these not change them Pulse sequences are used to maximize differences in tissue characteristics Inherent 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. Types; Paramagnetic, Ferromagnetic Administration Reactions Contraindications MRI Contrast Agents

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

39. 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 IR, STIR

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

41. 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 FSE

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

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

44. T1 relaxation Spin lattice Longitudinal TR Controls

45. T2 Spin spin Transverse relaxation - dephasing TE Controls

46. 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 Affecting SNR

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

48. Increasing TR increases SNR Provides more relaxation Affecting SNR • 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. Peripheral pulse Respiratory Cardiac NOTE ALL INCREASE TR Or decrease slices Gating Used to eliminate or minimize physiologic motion