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Rad T 265 MRI Lecture

Rad T 265 MRI Lecture. No Magnetic Field. =. No Net Magnetization. Protons align with a magnetic field…. In a magnetic field, protons can take either high - or low- energy states.

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Rad T 265 MRI Lecture

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  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

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

  14. 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

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

  16. 10,000 gauss = 1 tesla Earth’s magnetic field is 0.5 g Gauss

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

  18. 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

  19. 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

  20. 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  37. 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

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

  39. 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

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

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

  42. T1 relaxation Spin lattice Longitudinal TR Controls

  43. T2 Spin spin Transverse relaxation - dephasing TE Controls

  44. 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

  45. 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

  46. Increasing TR increases SNR Provides more relaxation Affecting SNR • Decreasing TE increases SNR • Less dephasing occurs

  47. 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

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

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