Just how does a billion or more protons produce an MRI scan? Scanning the human body using Magnetic Resonance Imaging (MRI)
is actually a relatively recent
technology.
Back in 1980 the first clinical MRI machine was installed in Melbourne, Australia. Since then the complexity of the technology and amount of scans performed each year has escalated. Most Australians know of MRI scans because they are frequently used to image damaged footballers knees!
MRI is considered by many people to be the apex of medical imaging technology, it produces incredibly detailled images in the form of slices through the body in any plane (Coronal, Saggital, Axial or any combination, as in Oblique scans), MRI has no known side-effects and uses no X-Rays.
An MRI system relies on a strange property of the proton nucleus of all Hydrogen atoms, the so-called Spin. Hydrogen, is inside water, fat, muscle, nerves, basically anything fatty or watery (about 98% of the human body), contains an abundance hydrogen and therefore countless billions of Hydrogen protons. Protons within the Hydrogen nucleus in free space i.e. not subjected to a
magnetic field, tend to spin on their axis. albeit randomly. However, once a patient is moved into the intense static magnetic field, all of the hydrogen proton nuclei align with the field. Thankfully, it is impossible to "feel" this effect!
When the machine is scanning, a combination of Radio Frequency pulses and a constantly switching magnetic field (Gradient) causes the protons to leave their alignment and spiral down to a clock-like rotational movement. The speed of the spinning protons is determined by their position within the body being scanned, and the strength of the applied magnetic field and the gyromagnetic constant of hydrogen protons (I won't discuss this here.)
The most common system around today uses a field-strength of 1.5 Tesla-this causes the protons to spin around 63 million times per second! when the pulse/gradient is turned off, the protons relax back to their original alignment. It is this relaxation time, that is variable and therefore decides the brightness of the pixels in the scan. Other scans are produced by looking at the differences in rotational speed, or phase, of the individual protons.There are three main types of MRI scan a T1, T2 and Proton Density. Generally black and white/greyscale images are the most common scans produced. Newer technology uses colour-coded contour-mapped information that is dependent on function (Physiological) rather that just the purely anatomical information gleaned from the usual MRI scan.
All MRI machines rely on the exactness of their magnetic field. This is termed the homogeneity of the magnet, typical figures of homogeneity in good magnets is around 3 parts-per-million that means that there can only be an error of three millionths out of specification. When a magnet is installed a "shimming" of the magnet is performed, this takes up to 5 days to perfect in some systems! A hairpin stuck to the side of the magnet distorts the field lines to about 5 PPM! An image taken with that sort of error would be classified unusable. This is why as a patient you are asked to remove all magnetic material from your body before being scanned.