Magnetic Resonance Imaging Mary Holleboom ENGR 302 May 7, 2002

Outline Overview & History Imaging Principles Advanced Techniques Imaging Hardware Safety Applications

Overview & History Use strong magnetic field to scan an object and produce an image No radiation Utilize magnetism of internal particles 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

Imaging Principles 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

Fourier Transform 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

Advanced Techniques 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

Imaging Hardware Magnets 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 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

Safety 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

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

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