Magnetic Resonance Imaging

1. Magnetic Resonance Imaging Mary Holleboom ENGR 302 May 7, 2002

2. Outline • Overview & History • Imaging Principles • Advanced Techniques • Imaging Hardware • Safety • Applications

3. Use strong magnetic field to scan an object and produce an image No radiation Utilize magnetism of internal particles Overview & History • 1973 – Back projection imaging technique • 1975 – Phase and frequency encoding, Fourier Transform • 1977 – Imaging of the whole body, Echo-planar imaging: real-time movie imaging • 1993 – Functional MRI

4. Spin Protons, Neutrons, Electrons +/- 1/2 Particle behaves like a magnet in presence of magnetic field Grouped in packets - create magnetization vector • Fourier transform • Most common technique used today • Sequence of applied gradients Imaging Principles

5. RF pulse Slice selection gradient pulse Phase encoding gradient pulse Frequency encoding gradient pulse Signal recorded Process repeated 128 – 256 times Signal Fourier Transformed in 2 directions Frequency encoding direction Phase encoding direction Intensities of data peaks converted into intensities of pixels Tomographic image Fourier Transform

6. Volume (3-D) Imaging Group of slices (volume) used instead of one slice at a time Flow Imaging (MR angiography) Image blood flowing through arteries & veins Velocity of blood flow determined by intensity of image Echo Planar Imaging (functional MRI) Imaging relates body function or thought to specific locations in the brain Tomographic images produced at video rates Advanced Techniques

7. Magnets Imaging Hardware • Coils • Inductive & capacitive elements allow it to resonate • Gradient coils • Create gradients in equilibrium magnetic fields • Room temperature coils • RF coils • Transmit magnetic field • Receive RF signal from spins • Various coils for specific applications • Surface • Bird cage • Saddle • Phased-array • Litz • Superconducting • Strongest • Electromagnet • Current flows in wire coil to create a magnetic field • Nearly zero resistance in wire at temperature close to absolute zero • Cooling achieved with liquid helium or nitrogen • Resistive • Electromagnet • Cooled by air • Greater resistance • Weaker magnetic field • Permanent • Solid magnetic material • Weakest magnetic field • Used for open MR scanners • Arranged in any position • No need for patient to be surrounded by magnet

8. Patients No biological hazards from magnetic fields yet discovered Most pregnant women prohibited from undergoing MR imaging to prevent possible damage to fetus Most people with metal implants prohibited Pacemakers - malfunction due to magnet Cerebral aneurysm clip - magnet could move clip Most orthopedic implants safe because they are firmly embedded in bone RF coil failure can severely burn patient Equipment Extremely powerful magnets No ferromagnetic objects allowed near scanner Example Bucket pulled off ground and into magnet Fully loaded pallet jacket pulled into bore Fix problem Pull object off magnet Use forceful device such as a winch Turn off magnet completely Safety

9. Brain Tumors Aneurysms Blood clots Spine Individual vertebrae Knees Shoulders Hips Prevent strokes Diagnose Multiple Sclerosis Applications

10. QUESTIONS ?