MRI Lecture Notes

Overview

• Topics covered:
  • Intrinsic tissue and scan parameters
  • Extrinsic scan parameters
  • Pulse sequences
  • MRI math
  • Artifacts in MRI
• Structure of sessions:
  • Two sessions with a break in between
  • Encouraged to ask questions during the lecture

Instructor Background

• 40 years of radiology experience
• 30 years in MRI, 10 in CAT scan and special procedures
• 14 years teaching at community college
• 5 years as application specialist with GE Healthcare
• Lecturer for Pulse Radiology and Advanced Health
• Author of books on MRI technology

Intrinsic Tissue Parameters

• T1 and T2 Relaxation
  • T1 time: Recovery of 63% of net magnetization vector
  • T2 time: Decay to 37% of original value
  • Proton density (PD): Modern term for spin density; also known as hydrogen density
• Definitions
  • T1: Spin-lattice
  • T2: Spin-spin

Intrinsic Scan Parameters

• Parameters
  • TR (Repetition Time)
  • TE (Echo Time)
  • Flip Angle
  • Echo Train Length
  • TI (Inversion Time)
• TR and TE
  • TR controls T1 contrast
  • TE controls T2 contrast
• Flip Angle
  • Related to T1 in gradient echo

Image Contrast and Weighting

• T1 Weighted Image
  • Bright fat, dark fluid
• T2 Weighted Image
  • Bright fluid, dark fat
• Proton Density Image
  • High signal-to-noise ratio

Geometric Parameters

• Field of View (FOV)
  • Affects signal-to-noise and resolution
• Slice Thickness
  • Thicker slices increase signal-to-noise, decrease resolution
• Matrix and Pixel Size
  • Higher matrix numbers increase resolution

Scan Time and Calculations

• Basic formula: TR x Phase x NEX (Number of excitations)
• Adjust for echo train in fast spin echo
• Considerations for 2D and 3D sequences

Artifacts

• Motion Artifact
  • Occurs in phase direction
• RF Artifact (Zipper Artifact)
  • From external RF interference
• Aliasing (Wrap Around)
  • Tissue outside FOV wraps into image
• Chemical Shift Artifact
  • Occurs due to difference in processional frequencies between fat and water
• Magnetic Susceptibility
  • Differences in how materials are magnetized

Pulse Sequences

• Spin Echo
  • Conventional and fast spin echo
• Gradient Echo
  • No 180-degree pulse, sensitive to field inhomogeneities
• Inversion Recovery
  • STIR, FLAIR
  • Used to suppress specific tissues

Special Techniques

• Fat Saturation
  • Uses selective RF pulse targeting fat
• Time of Flight (TOF) MRA
  • Highlights blood flow
• Diffusion Weighted Imaging (DWI)
  • Detects ischemic areas based on diffusion differences

Practical Tips

• Practice answering registry questions
• Understand scan time formulas
• Manage artifacts through best practices
• Use the mock registry for preparation