Magnetic Resonance Imaging

Astounding

The MRI magnet and patient table

The operator’s console

Most MR magnets areSuperconductors

Liquid helium is the primary cryogen

Boiling Point @ 1 atm:-452.1°F (-268.9°C, 4oK)

Once ramped up, MR magnets remain on all the time

Housed in an 18 wheeler, mobile MR machines use permanent or resistance coil magnets.

Damadian’s apparatus

1972 patent did not describe imaging

Lauterbur built the first imaging machine, but Damadian made The first for human imaging.

Hydrogen: 55-60% of the body is water

2/3 of water is hydrogen

An MRI scan of the head and brain Created from imaginghydrogen

A spinning, charged particle, such as a hydrogen proton,has a magnetic north and south pole.

It will align to an external magnetic field like a compass needle aligns to the earth’s magnetosphere

North

South

Magnetic poles of H are randomly oriented inthe human body

Aligned to the MRI magnet roughly half spin up, and half spin down

Opposing spins canceleach other’s force

Unpaired spinningprotons create a net magnetizationvector

The protons that are used tocreate the MR signal are the unpaired ones

In a sample of 20 million protons,about 7 are usable.

Hydrogen protons in thepatient’s body do not alignfully parallel to the magneticfield. They precess (wobble).

The rate of precession iscalled the larmor frequency.

It is 42.5 MHz at one tesla

RF energy

42.5 MHz is in the range of radio frequencies

A H proton will absorb RFwhen it has the same frequency. It resonates.

90o flip

180o flip

RF energy

When the RF is turned off,the proton returns to alignmentwith the MRI magnet. It relaxes.

As it does it gives off the energy it absorbed from theRF in a process called freeInduction decay (FID)

The net magnetization vectorscans coils of wire, inducinga flow of electrical currentwhich is the MR signal.

Z

X

YGradient coils vary the magnetic fieldstrength thoughout the x, y, z planescausing every H proton to precess ata different frequency

42.5 42.6 42.7 42.8 42.943.0 43.1 43.2 43.3 43.443.5 43.6 43.7 43.8 43.944.0 44.1 44.2 44.3 44.444.5 44.6 44.7 44.8 44.9

43.7 MHz

The net magnetization vectorscans coils of wire, inducinga flow of electrical currentwhich is the MR signal.

QD Head Speeder

Surface Coils

http://en.wikipedia.org/wiki/Image:Mra1.jpg

Functional MRI fMRI

Is that the truth, the whole truth, and nothing but the truth?

Computed Tomography (CT)

Magnetic Resonance Imaging (MRI) Cardiovascular Interventional

Technology (CIT)

Radiologic Science

Advanced Registry Modalities

Mammography

www.arrt.org