Magnetic Resonance Imaging(MRI)

by Alex Kiss

Introduction

• 1946: MRI science was developed independently by Felix Bloch and Edward Purcell

• 1952: both awarded Nobel Prize• By the late seventies the name was

changed from Nuclear Magnetic Resonance (NMR) to MRI because “nuclear” carried a negative connotation

• 1977: First MRI exam on a human

Basic Purpose

• Best type of Imaging Modality, especially in brain scans

• Used in the diagnosis of many injuries and conditions, exam can be tailored to answer the particular medical question asked

• Creates a detailed view inside human body by mapping tissues point by point (a point can be a 0.5 mm cube)

Structure

• Usually has an outer box (2m high x 2m wide x 3m long)

• Patient lies inside a large hollow cylinder• In the cylinder is several kilometers of wire

wrapped around in a coil• When current is passed through the wire,

a magnetic field (0.5 – 2.0T) is generated, especially in the center (bore) of the cylinder

A fully loaded pallet jack has been sucked into the bore of the MRI

system

Magnets

• Three types of magnets are used:• Resistive: already mentioned, require up to

50kW to maintain due to the high resistance of the wires

• Permanent: need no electricity, extremely heavy (many tons)

• Superconducting: most commonly used, same as resistive except wires are soaked in -452.4°F liquid helium to lower resistance to zero

Protons

• Hydrogen nuclei (single protons) have a strong tendency to line up with the direction of the magnetic field because of their large magnetic moments (spin)

• Some line up toward head, some toward feet of patient; 2 protons with opposite spin pair up to cancel each other out

• Only a couple hydrogen nuclei out of a million are not canceled out

Radio Frequency (RF)

• A radio frequency pulse specific to hydrogen is applied from a coil toward the area of body being examined

• Each unmatched proton absorbs the energy of a photon and undergoes a transition from the lower energy state to a higher energy state, effectively switching the spin and alignment of the proton in the magnetic field

• A single proton with its magnetic moment vector (spin) symbolized by magnetic poles

• When an external magnetic field is activated the spin vector of the proton aligns itself with the external field

• When the proton is hit with exactly the right amount of energy (equal to the difference in energy states, E=h*f, where f is the resonance frequency), the spin of the proton flips to that of a high energy state

Imaging

• When the radio frequency is turned off, the protons slowly return to their original alignment within the magnetic field and release their excess stored energy

• The signal is picked up by the coil and sent to the computer system

• This mathematical data is converted to a picture

Used to Diagnose or Evaluate:• Multiple Sclerosis• Tumors• Infections of the brain, spine, and joints• Torn ligaments• Shoulder injuries• Tendonitis• Strokes• Masses in the soft tissue• Bone Tumors, Cysts, and Bulging or Herniated

Disks

Knee

Lower Back

Head