Imagine yourself in a dark room where you have to spot a key. The rule is: you cannot bump into anything. Someone’s life depends on your finding it. In a similar manner, doctors have to locate tissues and organs inside a confusing maze of interconnected objects that constitute the human body. Medical diagnostic techniques function as the eyes and ears by providing clues in finding the key to successful treatment.
Mere decades ago there were few ways to observe internal organs besides directly cutting though someone. Then by the end of nineteenth century W. C. Röntgen developed the first X-ray image ever by X-ray imaging his wife’s hand. Rontgen sure got his image but his wife developed cancer due to hours of exposure to dangerous X-rays. Although imaging technologies have improved from simple x-rays to elegant imaging techniques, they remain associated with safety hazards such as mutagenic radiation.
Although scanning and imaging techniques include a wide variety of methods, almost all of them involve exposure of the patient to radiation. Different tissues in the patient’s body reflect the waves differentially and these reflected waves are then collected to develop the image. Some of these techniques also include injecting special isotopes (types of atoms of an element that vary in mass) in to the patient’s body which are helpful in developing the image because of their affinity for certain tissues and their response to waves or radiation projected on the patient. However, aside from being extremely harmful in prolonged exposure, most of the waves miss the details of soft tissues.
MRI solves just these issues by providing incredibly detailed images of internal body parts while working outside the body with no danger at all to the patient. An MRI machine basically consists of magnets, a radio wave transmitter and receiver coils as well as a powerful processor/computer system for processing the information collected by those radio waves. But how do these gadgets work out and provide detailed images of unseen tissues and organs?
The bulkiest part of the machine is a large magnet that produces a strong, stable magnetic field. To get an idea of how strong it is, consider that the Earth’s magnetic field is just 0.5 Gauss whereas these magnets in different MRI machines produce a magnetic field ranging from 5,000 to 20,000 Gauss. Besides the strong magnet there are three other, weaker magnets that are used to generate a much weaker gradient magnetic field instead of a strong stable one.
To economize, often superconductors are used instead of ordinary conductors because ordinary resistive conductors result in an enormous waste of electrical energy. To keep these super conductors cool for their superconductivity, usually liquid helium is used which is 269.1 below zero degrees Celsius (consider that -273 is the lowest possible temperature in whole universe). To insulate such an extreme temperature from the patient, a vacuum is used in insulation.
To understand how these magnets provide images of tissues and body organs, let’s have a look at the mechanics of it. Our body contains billions of hydrogen atoms. Each atom has a proton in its centre which spins like a ‘top’. Normally the axes are randomly oriented allowing the atoms to spin in random directions. During an MRI scan we are pushed to the centre or the ‘isocenter’ of the magnetic field which makes your hydrogen atoms (due to their strong magnetic moment) spin in either direction such that their axes are either pointing towards our head or feet. There is a 50% chance that each atom will point up or down. Therefore, for almost every atom that point up there is one pointing in the opposite direction. But startlingly, even out of a million atoms, many do not have a partner. And it is these outcasts that then make the magic possible.
Following this, a low frequency radio pulse is transmitted that can only be absorbed by the hydrogen atoms i.e. it makes these hydrogen atoms resonate. This makes the hydrogen atoms spin in a new direction at a new frequency - the Larmour frequency which is determined by the tissues being scanned and the strength of magnetic field. The three weaker gradient magnets are turned on and off a very fast rate and help alter the main strong magnetic field at certain places. This makes it possible to scan different parts of the. But when these magnets are turned on and off in the strong magnetic field of the larger magnet it tries to oppose shifts, producing a loud sound like a rapid hammering. Finally when this pulse is turned off the hydrogen atoms return to their previous alignment and release the energy they absorbed in the form of radio waves which are then picked by receiving coils.
The signals collected from each and every point of the area being scanned are then processed using mathematical formulas such as Fourier transform and a detailed 2-D or 3-D model is developed. To identify different tissues the MRI system makes use of a technique called ‘injectable contrast’. In this technique the local magnetic field is slightly altered. And different tissues respond differently. These responses show up as different shades of gray on the final image (a usual MRI can make up to 250 different shades). This contrast helps medical practitioners track where the tissues are showing the ‘normal’ colour.
MRI images can be developed for any part of the body in any direction without moving the patient’s body. It does however require the patient to stay still for about 20 to 90 minutes in order to obtain non-blurred images.
There are some safety limitations that have not been considered earlier. Due to the magnets, patients with metallic implants cannot have MRI scans because the strong magnets will pull the implant out of the body entirely! There is no known radiation hazard involved in MRI.
The future of MRI
Currently, research on the medical applications of MRI is involved in making it more patient-friendly by increasing the size of the cavity for the patient for instance. But today the use of MRI is not just limited to medical diagnosis. The technology is making its way in a wide variety of fields - even mummies and a 42,000-year old baby mammoth have been scanned for dating and historical analysis. Research is being carried out on the possibility of shrinking the size of MRI by increasing the processing capability and decreasing the size of the magnet (i.e. by decreasing the required strength of magnetic field and relying more on processing power). Either way, the key has never been so easy to locate before.