Transcript What is MRI - University of Waterloo

What is Magnetic Resonance Imaging
and How Does it Work ?
By Aneta Chmielewski
What is MRI ?
Magnetic resonance imaging (MRI) is an
imaging technique used primarily in
medical settings to produce high quality
images of the inside of the human body.
 More specifically the images of protons in
water. Hydrogen is used because it is
plentiful in the human body and has the
highest NMR signal of any atomic species.
Based on the principles of Nuclear
Magnetic Resonance
A Brief History of MRI
 Felix Bloch and Edward Purcell were awarded the Nobel
Prize in 1952, for discovering the magnetic resonance
phenomenon independently in 1946.
 1950-1970: NMR was developed and used for chemical
and physical molecular analysis (uniform magnetic field).
 In 1973 the x-ray-based computerized tomography (CT)
was introduced by Hounsfield
 Early 1970s Paul Lauterbur and Peter Mansfield
utilization of gradients in magnetic field.
 1975 Richard Ernst proposed magnetic resonance
imaging using phase and frequency encoding, and the
Fourier Transform
 In 1993 functional MRI (fMRI) was developed. This
technique allows the mapping of the function of the
various regions of the human brain.
Nuclear Magnetic Resonance
 All atoms consist of outer shells of negatively charged particles
called electrons buzzing around in diffuse clouds, and a dense
central portion called the nucleus.
 Some of these nuclei behave like small bar magnets and when
placed in a powerful magnetic field about half line up in the direction
of the magnetic field and about half line up in the opposite direction.
The nuclei in opposing directions will cancel each other out but a few
out of a million will not.
 By providing energy in the form of radio waves these tiny magnets
can be caused to change orientation, to resonate absorbing energy
at a resonance frequency that depends directly on the strength of the
magnetic field.
The frequency of this precession is
described by the Larmor frequency.
ωo = -γHo
Resonance Frequency
The frequency of the radiation necessary for absorption of
energy depends on three things:
 First, it is characteristic of the type of nucleus (e.g., 1H or
13C).
 Second, the frequency depends on chemical
environment of the nucleus. For example, the methyl and
hydroxyl protons of methanol absorb at different
frequencies, and amide protons of two different
tryptophan residues in a native protein absorb at
different frequencies since they are in different chemical
environments.
 The NMR frequency also depends on spatial location in
the magnetic field if that field is not uniform everywhere.
NMR to MRI
 At first the magnetic resonance technique was used mainly as a research tool for
determining the structures of molecules. It relied on using very uniform magnetic
fields, so that every part of the sample was exposed to the same field.
 It was more than twenty-five years after the original discovery of the magnetic
resonance phenomenon that the possibility of using non-uniform magnetic fields
which varied in a know way through out the sample to produce images was first
realised.
 This results in each distinct part of the sample experiencing its own unique magnetic
field which is characteristic of its position. Nuclei each at a different position will have
different characteristic resonance frequencies.
 So, detecting the resonance frequencies of the nuclei becomes equivalent to
detecting where they are in the sample, and detecting the size of the signal tells you
how many nuclei there are at that position. With information relating number of
atomic nuclei with position in the sample it is possible using computer programs to
reconstruct a detailed three-dimensional image of the whole sample which can then
be examined on a monitor screen as cross-sectional slices in any direction.
 The implementation of these ideas independently by Lauterbur in New York State
University at Stonybrook and Mansfield and Grannell at Nottingham University
signalled the beginning of a new imaging method.
So how would this work in the patient..
 When a patient is subject to a
magnetic field (located straight down
the center of the tube the patient is
placed into) the H atoms in his/her
body will line up in the direction of
either his/her head or feet.
 The vast majority of the H+ will cancel
each other out, but a couple out of a
million will not.
 When an RF pulse specific to only H is
applied to a specific part of the body
being examined, the protons not
cancelled out will absorb the energy
required to make them spin or precess
in a different direction a specific
frequency called the Larmour
frequency.
 The RF pulses are applied through a
coil, designed for different parts of the
body and conform to the contour of the
body.
In the patient..
 By switching three small gradient
magnets (18-27mT) on and off a
variable magnetic field is formed.
 The large magnet immerses the
patient in a stable and very intense
magnetic field.
 By altering the gradient magnets, we
can choose exactly which specific area
of the body we want to analyze in
slices.
 When the RF pulse is turned off, the H
protons begin to slowly return to their
natural alignment within the magnetic
field and release their excess stored
energy.
 The released energy, gives off a signal
that the coil now picks up and sends to
the computer system.
 The mathematical data is converted
through the use of a Fourier transform,
into a picture that we can put on film.
Electronics and Computer
Processing
Stable Magnets For MRI
 Commonly in the range of 0.5 to 2.0 T
Three types of magnets used:
a) Resistive – many windings or coils of wire wrapped
around a cylinder through which an electric current is
passed, generating a magnetic field. Good for field of
less than 0.3T, require large amount of money to
operate.
b) Permanent - Very heavy, weight is proportional to
magnetic field strength.
c) Superconducting – most commonly used. Similar to
resistive magnet but continually bathed in He at -452.4
°C, insulated by a vacuum. The cold temperature
decreases the resistance in the wires, allowing for easy
generation of 0.5-2.0 tesla fields. Very expensive.
Visualization
 Most imaging modalities such as CT
and X-ray scan use injectable
contrasts or dyes for certain
procedures. These agents work by
blocking the X-ray photons from
passing through the area where they
are located and reaching the X-ray
film. This results in differing levels of
density on the X-ray/CT film.
 MRI contrast works by altering the
local magnetic field in the tissue being
examined.
 Normal and abnormal tissue will
respond differently to this slight
alteration, giving different signals.
 These varied signals are transferred
to the images, allowing us to visualize
many different types of tissue
abnormalities and disease processes
better than we could without the
contrast.
Advantages and Disadvantages of MRI
 Advantages:
- do not use ionizing radiation
- very low incidence of side effects
- Ability to image any plane: axial, sagitall, coronally
- Ideal for orthopaedic and neurological applications.
 Disadvantages:
- Lower sensitivity then CT and X-Ray scans.
- Many people who can not be scanned by MRI because they have metal in their body or
are too big to be scanned or are claustrophobic.
- Make a tremendous amount of noise. The stronger the main field, the louder the gradient
noise.
- MRI scans require patients to hold still from 20 to 90 minutes or more. Very slight
movement can cause very distorted images that will have to be repeated.
- Orphopedic hardware (screws, plates, artificial joints) in the area of a scan can cause
severe distortions on the images. The hardware causes a significant alteration in the main
magnetic field.
- MRI systems are very expensive to purchase.
Current and Future developments of MRI
 Functional MRI (FMRI) is a technique that has recently
been introduced to obtain functional information from the
central nervous system. FMRI detects subtle increases in
blood flow associated with activation of parts of the brain.
FMRI may be useful for preoperative neurosurgical
planning, epilepsy evaluation, and "mapping" of the brain.
 Looking at Hydrogen atoms in fat.
 Detection of other atoms: recently, excellent MRI images
of the airways in human lungs have been obtained by
detecting inert gases such as helium or xenon inhaled by
the patient.
 Improvements in strength of superconducting magnets at
lower costs.
 A less claustrophobic design, such that the patient does
not have to lie on the magnet bore.
 MRI for pregnant patients.
References
Gould, T.A. (2004) How MRI works.
January 24, 2004.
www.howstuffworks.com/mri.htm
Hoole, P.R.P. Electromagnetic Imaging in
Science and Medicine with wavelet
applications. Wit Press: Boston, 2000.
Hornak, J.P. (2004) The Basics of MRI.
January, 24, 2004.
www.cis.rit.edu/htbooks/mri/