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

Magnetic Resonance Imaging

Mary HolleboomENGR 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

QUESTIONS

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