Malini Patel
An Evaluation of Magnetic Resonance Imaging
Intended Audience
The intended audience is a medical physics research group interested in undertaking a
project in MRI technology.
Executive Summary
 Magnetic Resonance Imaging (MRI) is a relatively new technique but has become
a well-established and robust tool in hospitals
 It is versatile and makes use of the property of nuclei known as ‘magnetic
moment’
 The advantages of MRI scanners outweigh the drawbacks
 There have been a number of significant advances in MRI and the technology
continues to develop.
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Introduction
MRI stands for ‘Magnetic Resonance Imaging’. It has only been in mainstream use since
the 1980’s. MRI Scanners are versatile and have also been very successful in the field of
diagnosis.
How they work
The core components of an MRI scanner are the magnets; namely the ‘primary magnet’
and the ‘gradient magnets’.
The primary magnet is used to produce a strong and uniform magnetic field. It
commonly varies in strength from 0.5 tesla to 3.0 tesla and so is extremely powerful.
Most MRI scanners use superconducting magnets (solenoids immersed in liquid helium).
The image quality of the scanner is dependant upon the strength of the magnet, and so
superconducting magnets ensure a better visual image.
The hydrogen atom plays an important part in the MRI process. All atoms produce a
small magnetic moment due to the negatively charged protons orbiting the positive
nucleus, as well as the spinning charge of the nucleus itself. Normally these moments
occur in random directions (fig.1) meaning that there is no fixed orientation and no
resultant magnetic field. However when nuclei are placed in a strong external magnetic
field their magnetic moments tend to align with the direction of the external field (fig.2).
The hydrogen atom is ideal for MRI scans firstly because it is abundant in the human
body and secondly because it has a large magnetic moment meaning it has a strong
tendency to align with an external field. The hydrogen nucleus consists of a single proton
with a spin quantum number I = ½. In this case according to the laws of quantum
mechanics two discrete energy levels (2I +1) are created; a higher energy level where the
magnetic moment is directed opposite to the external magnetic field, and a lower level
in which the moment is in the same direction as the magnetic field. In an MRI scanner,
the hydrogen atoms in a patient’s body will align to either their feet or head.
Gradient magnets, which are much smaller and weaker than the primary magnet, are
used to create a variable, local field. They allow the magnetic field to be altered very
precisely and help to create image ‘slices’ by focusing on a specific part of the body.
A coil is used to apply a pulse towards the area of the body being examined. These
pulses are of radio frequency and are specific to hydrogen. The energy transmitted by
the pulses acts to tip the hydrogen atoms out of alignment with the primary magnetic
field i.e. from a longitudinal plane to a transverse plane (fig.3), and also causes them to
precess with a particular frequency. This transition requires a pulse of a specific resonant
frequency called the Lamour frequency which is dependent on the type of tissue being
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examined and the strength of the primary magnet. The pulses are then stopped and the
atoms begin to return to their natural alignment. By doing so, they release their excess
energy (fig.4) which is picked up in the form of a signal by the coil and sent to a
computer. The data is processed using the Fourier Transformation and turned into an
image.
Figure 1
Figure 3
Figure 2
Figure 4
Advantages and Disadvantages
Major drawbacks of MRI scanners are that they are very expensive to purchase and also
require experienced technicians to operate them. Moreover they cause a large amount
of loud noise, and many patients may feel claustrophobic inside the scanner.
Furthermore some patients are restricted from having scans due to devices such a
pacemakers. The quality of the resultant image depends on the patient being still for
long amounts of time and the scans can be distorted very easily. From a medical
perspective the spatial resolution is not as good as other methods and in addition to this
MRI scans are not suited to the evaluation of calcium bone cortex and are less effective
at seeing air in the lungs. On the other hand, however a big advantage of MRI is that is
uses no ionizing radiation and so there is a low risk of side effects. Due to the gradient
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magnets scanners are able to produce an image in any plane without the patient having
to move. This allows for images of increased quality. MRI is also more effective at
detecting tissue abnormalities and helps us to visualise disease processes with increased
detail.
Improvements
There has been a continual advancement of MRI technology since its discovery.
Nowadays small scanners for examining different parts of the body are being developed.
These have the added advantages of being less costly, easy to use and are less
claustrophobic. Furthermore pacemakers that are safe to use in MRI are now
commercially available in Hong Kong, increasing the number of people who can be
diagnosed using the technology. The main progress has been in increasing the resolution
and power of scanners. For example, computer firm IBM have been able to produce
images that are 100,000,000 times finer than current scanners by developing a new
technique called Magnetic Resonance Force Microscopy (MRFM), which relies on
detecting very small magnetic forces. Through MRFM they have been able to view
biological objects on the nanometer scale which is useful when studying viruses. In the
future they hope to be able to study the structure of individual protein molecule. British
scientists have also increased the power of MRI and have used it to view breathing lungs
for the first time. The technique called hyperpolarisation increases the strength of MRI
signals by such large amounts that the images produced show details that could only
previously be attained by slicing a patient open. This advancement will be pivotal in
detecting early signs of emphysema and revealing obstructions caused by cystic fibrosis
and asthma.
Conclusion
MRI is a versatile and useful technique that has a significant impact on the world of
medical imaging. The future development of MRI is limitless and will only lead to
increased benefits for patients.
References
en.wikipedia.org/wiki/MRI
www.howstuffworks.com/mri.ht
Magnetization transfer in MRI: a review; 2001 Apr;14(2):57-64; Henkelman RM, Stanisz GJ,
Graham SJ
MRI: The Basics; Lippincott Williams & Wilkins, 2003; 2nd Edition; Ray H. Hashemi,