Download Module 6 - Magnetic Resonance Imaging

CIHR Strategic Training Program in Vascular Research
VASCPROG 560
Vascular Imaging Techniques
Module 6 - Magnetic Resonance Imaging (MRI)
CIHR Strategic Training Program
In Vascular Research
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1
Credits
Information in this module comes from
several sources, including lecture notes given
by Robarts Scientists as part of the
coursework in the Department of Biomedical
Engineering at the University of Western
Ontario, and the Radiology Residents’ Course
entitled “Physics of Diagnostic Imaging”,
given in partnership with the Department of
Diagnostic Radiology and Nuclear Medicine,
London Health Sciences Centre, Imaging
Research Laboratories, Robarts Research
Institute, St. Joseph’s Health Centre, and The
Lawson Research Institute, all in London,
Ontario.
Some of the figures in the module were
used with permission from “Figures from
the Essential Physics of Medical Imaging,
Second Edition. Jerrold T. Bushberg, J.
Anthony Seibert, Edwin M. Leidholdt Jr. &
John M. Boone. These are denoted by
the publisher’s mark, ©2003 Lippincott,
Williams & Wilkins.
Other sources of information were:
W. Huda & R. Slone. Review of Radiologic
Physics, 2nd edition. Lippincott Williams &
Wilkins. 2003. Chapter 12 – Magnetic
Resonance
I would like to thank Dr. Grace Parraga, and
Dr. Yi-Fen Yen for permission to reproduce
some of their lecture material.
Jackie Williams also wrote some of the
content and organized the module in its
present format for WebCT.
2
Introduction
Magnetic Resonance Imaging (MRI) is
another rather complex modality, and it is
difficult to write a concise module that will
help students understand the basics,
without going into too much detail.
Where possible, links to more detailed
websites have been included to assist
students who need more information, but
students whose current research does
not involve MRI can ignore them.
This module will give a basic overview of
the principles of MRI and how it is used.
For students who would like more in-depth
information, or for those who prefer an
interactive approach, IMAIOS offers a free
e-course on MRI physics that is excellent.
You will be required to register, but the
registration is free, The link is:
MRI step-by-step, interactive course on
magnetic resonance imaging
The module starts with a brief description
of MRI and what makes it such an
outstanding imaging technique. Then the
basic physics of how it works are
explained, followed by a description of
the hardware.
MR angiography is described in Module
8 – Angiographic Techniques.
Images from Functional MRI
3
What is Magnetic Resonance Imaging (MRI)?
Of all imaging techniques used in medicine,
MRI produces the clearest and most detailed
soft tissue contrast on stationary parts of the
body. MRI can be used on moving body parts,
such as the heart, but this requires advanced
techniques that will be explained later in the
module.
MRI is based on the principles of nuclear
magnetic resonance (NMR), which is one of
the spectroscopic techniques used to study the
properties of molecules. When the technology
became available for clinical use in hospitals in
the 1970s, the word nuclear was dropped from
the name, as it was thought this might frighten
patients, and the term magnetic resonance
imaging was adopted. Both terms are used in
this module, but they are synonymous.
Medical MRI is based on the fact that the
composition of the human body is more than
70% water, which means that a patient can
become magnetized by magnetizing their
water molecules.
During the magnetization process, the water
molecules absorb and then emit radio
waves. In contrast to x-ray imaging that
uses energy in the x-ray frequency of the
electromagnetic
spectrum,
magnetic
resonance imaging requires the absorption
and emission of energy in the radio
frequency range. This makes it a safe, nonionizing form of imaging.
The whole process of how the final MR
image is obtained is very complicated, and
involves advanced mathematics, of which
only a brief overview is given in this module.
For any students who would like another,
more advanced online source of MRI
information, there is an excellent website
written by Dr. Joseph P. Hornak from the
Rochester Institute of Technology at:
The Basics of MRI – Once at the site, click
on the image to enter the website.
4
Why Use MRI?
Compared to x ray and CT, MRI is noninvasive unless contrast agents are used.
It is well tolerated and has an excellent
safety record.
As the MR signal is generated from the
water content in tissues, solid tissues
such as bone are transparent, while soft
tissue delineation is excellent because of
the differences in water content in
different tissues.
It is the best imaging modality for looking
at abnormalities in the brain where
differences in white and gray matter can
be clearly seen, and also gives excellent
images in other organs.
However, just because MR provides
clearer images doesn’t necessarily mean
it should be chosen over other imaging
modalities in every situation. MRI is very
expensive and other modalities may be
just as effective at correctly diagnosing a
patient, even if the images are not as
clear. In some cases (high resolution
skeletal imaging for example) CT remains
the preferred modality.
MR is also excellent at depicting the
vasculature and the heart, internal
organs,
breast,
pelvis,
and
the
musculoskeletal system.
As well as showing anatomy, MR can be
used to show the functional aspects of
tissues. It is most often used to look at
brain function by measuring the blood
oxygenation in response to different
stimuli. Then it is referred to as fMRI
(functional MRI).
5
Potential Risks and Contraindications
Although multiple studies have been performed, no significant permanent biological hazards
have been demonstrated as a result of exposure to patients from the magnetic fields or radiofrequency electromagnetic pulses used in magnetic resonance imaging. However, there can
be adverse effects on various medical devices implanted into patients and therefore all
patients must be carefully screened to determine if MR scanning can be safely performed.
Patients who cannot undergo an MRI should be able to be scanned using CT.
Potential risks and contraindications include the following:
1.
Cardiac pacemakers: Absolute contraindication. These patients cannot be scanned.
2.
Cerebral aneurysm clip: Unless there is documented proof that a non-ferromagnetic
clip was used, these patients cannot be scanned.
3.
Metal fragments in body (bullet, BB, shrapnel, etc.): Safe, unless in contact with
vital organ, such as heart, spinal cord, eye.
4.
Surgical clips: Safe.
5.
Pregnancy. While there are no known hazards, MRI is not proven to be safe during
pregnancy. If a pregnant woman must undergo an MRI, she may be asked to sign a
special consent form.
6.
Claustrophobic patients. Patients with claustrophobia will usually find it impossible
to go inside an MRI machine.
6
Basic Steps in Acquiring MR Images
The diagram below shows the steps in acquiring MR images in the necessary sequence.
Magnetization of Patient
Repeat
Hundreds of
Times
Patient Absorbs Radio Waves
Patients Emits Radio Waves
Formation of Image
Acquisition of Different Types of Images
Viewing, Filming, and Archiving of Images
7
The Concept of Slices
MRI uses computers to transform digital data into three dimensions, one slice at a time...
Just as in CT.
The slices are as thin as a few millimeters and can be generated from any part of the body
in any direction, giving an advantage over any other imaging modality. Neither the machine
nor the patient needs to move to produce images from different directions – this is all done
by manipulating the gradient magnets.
8
Where does the MR Signal Come From?
The original term, nuclear magnetic
resonance, was a very good one in
explaining the basis of the technique. It
is the ability of the nucleus of the atom
to resonate in the presence of a
magnetic field which makes MRI
possible.
The basic stages in NMR are:
1. A person is placed in a constant
magnetic field.
2. Then, another magnetic field that is
oscillating in the radiofrequency
(RF) range of the electromagnetic
spectrum is applied for a certain
length of time, which makes the
body’s nuclei resonate.
3. After the RF radiation is switched
off, the nuclei continue to resonate
and actually start to emit RF
radiation which can be detected as
an NMR signal.
There is a fundamental difference
between all the previous imaging
modalities that have been described and
MRI. In the other modalities there is an
interaction where the body attenuates or
reflects ultrasound waves or x rays,
whereas in MRI, the signal is not coming
from attenuation or reflection of the RF
radiation, but from the tissues
themselves that have been stimulated
by the RF radiation.
Only nuclei that have the property of
spin and which have an odd number of
protons and neutrons can be made to
resonate and are able to be used in
MRI. These nuclei possess intrinsic
magnetism (or magnetic moment) so
that each is a magnetic dipole and acts
like a tiny bar magnet.
9
Where does the MR Signal Come From?
MRI depends on the fact that hydrogen atoms can be magnetized. In fact, any nucleus that
has an odd number of protons and neutrons has a net magnetic moment, so other possible
candidates for MRI would be 19F, 23Na, and 31P, but as 1H is the most abundant isotope of
hydrogen, it is the one that is used.
The spinning proton is like a gyroscope. With a gyroscope, the earth’s gravity tries to pull it
down, but its fast spin keeps it upright. Within a spinning nucleus, the interaction of the
magnetic moment of the nucleus, and the applied magnetic field, causes the nucleus behave
in much the same manner. Nuclei with even numbers of protons and neutrons have no net
magnetic moment.
However,
nuclei
with
unmatched
protons
and
neutrons (odd numbered
nuclei), such as 1H, have a
net magnetic moment, which
produces an overall nuclear
spin and with it a slight
positive charge. This induces
a weak
magnetic
field
because the nuclear spin acts
as a circular current. This
magnetization
of
water
molecules is what allows the
patient
to
become
magnetized.
10
Application of a Magnetic Field
Normally, nuclei spin randomly, pointing in all directions. In the presence of a magnetic field
(Bo), however, they align themselves both parallel (spin up) and anti-parallel (spin down) to the
magnetic field, with a preference for the parallel direction, as this requires less energy. Spin up
and spin down protons cancel each other, and it is only the very few excess protons in the spin
up direction that produce a tiny net magnetization.
Parallel direction = Low
Energy State
Anti-parallel direction
= High Energy State
Protons in their natural state
(No external magnetic field)
Protons in the presence of a large external magnetic
field (Bo). More protons are aligned in the low energy
state than the high energy state. So biological tissues
in a large magnetic field
have a small net
magnetization of unpaired hydrogen protons pointing
in the same direction as the magnet field.
11
The Boltzmann Distribution
So, to recap, the number of spins in the lower energy level is slightly greater than the number in
the higher energy level at room temperature. The entire nuclear magnetic resonance (NMR)
signal is generated by this tiny energy difference between the spins in the lower energy state
and the spins in the higher energy state.
This is called the BOLTZMANN distribution.
The Boltzmann distribution is dependent on the temperature and the different chemical
components in the environments of the organs (e.g. protons in the leg will be in a different
chemical environment than the brain). Most importantly, it depends on the field strength of the
magnet – the stronger the magnet the greater the energy difference between the protons in the
high versus low energy states. To give an idea of how tiny this energy difference is, the
difference in numbers between the two populations is approximately 1 in 10,000,000! These
exceedingly weak MR signals must be maximized to produce a signal-to-noise ratio (more on
this later).
The units of measurement of a magnetic field are Tesla and Gauss.
1T (Tesla) = 10,000 Gauss
The earth’s magnetic field is 0.5 Gauss (or 0.00005 T)
A fridge magnet is between 5-100 Gauss
MRI machines are named according to their field strength in Teslas e.g. the most common MRI
machines in routine clinical use are 1.5T magnets. Machines at 3 and 4T are considered high
field strength, although there are 11T machines available at the present time.
12
Precession and the Larmor Frequency
Precession
Besides the slight difference in the
population numbers, the magnetic field
causes the dipole moments to become
aligned at an angle to the magnetic field
(Bo). This is called precession and it is a
wobbling motion that occurs when spinning
objects are subjected to an external force.
This is the same motion as when a toy
spinning top slow downs and starts to
wobble (precess).
Spin frequency is defined as: Larmor
frequency (Hz) = magnetic field strength x
gyromagnetic constant
The rate of precession is very important in
MRI as the frequency increases as the
magnetic field strength increases. This is
referred to as the Larmor frequency. For
any given magnetic field strength, 1H will
precess with a certain Larmor frequency (fo)
and it gives rise to a cone-shaped movement
in the xyz plane pointed in the direction of
the magnetic field (see diagram on following
page).
B0 = the external magnetic field

f0 
B0
2
Where:
F0 = the Larmor frequency

2 = the gyromagnetic constant
The gyromagnetic constant is a specific
number for each different nuclear species.
This means that hydrogen nuclei under a
specified magnetic field will spin at a
predictable frequency. If the magnetic field
changes, the spin frequency changes.
13
Precession and the Larmor Frequency
This diagram shows the formula for calculating the Larmor frequency. To recap,
the Larmor frequency is the frequency of precession of the nucleus about the
vertical axis. The gyromagnetic ratio is a constant for a given nucleus, and is 42
MHz/Tesla for hydrogen, so 1H has a Larmor frequency of 21 MHz in a 0.5 Tesla
magnet, and 63 MHz at 1.5 Tesla.
14
Resonance (Excitation)
Resonance in magnetic resonance imaging
refers to the same phenomenon that allows
a guitar string to make a note when
plucked. The plucking makes the guitar
string vibrate, which then finds a frequency
called its resonant frequency. Different
resonant frequencies lead to different
notes. In MRI, a magnetic force is used to
stimulate the vibration of nuclei.
So once in the main magnet, although the
dipole moments are precessing at the
same frequency, they are not necessarily in
phase (i.e. aligned) – they are all pointing in
different directions. To be of any use on
MR imaging they must be brought in phase.
This is achieved by applying a radio
frequency (RF) electromagnetic wave
whose frequency is equal to the Larmor
frequency. It is usually applied for fairly
short duration and is referred to as an
RF pulse.
Phase of Precessing Nuclei
Before the RF pulse all of
the magnet moments are
precessing out of phase.
The net magnetic moment
is static and vertical.
net magnetization
After the RF Pulse
All of the magnet moments
are precessing in phase.
The net magnetic moment
is now rotating and has a
component in both the x-y
plane and in the z-direction
15
Excitation (Resonance)
To recap, when atomic nuclei are exposed to a RF wave, they absorb energy from it and
become excited. This phenomenon is referred to as resonance. The RF pulse is applied
perpendicular to the external magnetic field (z-axis), this forces the magnetization vector
out of alignment and to rotate towards the x-y plane. Immediately after the RF pulse all the
nuclei are pointing in the same direction and have the same precession angle (i.e. they are
in phase).
An RF pulse is named after the size of the precession angle it produces – the larger the
pulse the larger the angle. The two most common pulses are the 90o and the 180o pulse.
When the RF wave stops, the atoms relax back to the equilibrium state and release
absorbed energy to the environment as RF wave emissions. The emitted RF waves can
then be detected by sensors.
The component of the net magnetization vector that runs parallel with the magnetic field is
called the longitudinal magnetization.
The component that runs perpendicular to the magnetic field is called the transverse
magnetization.
***It is this component in the horizontal plane that produces
the NMR signal.
16
Adding an RF Pulse
Protons are aligned in the
direction of the external
magnetic field
RF pulse at the Larmor
frequency keeps up with
the precessing
magnetization, which
eventually forces it down to
90 degrees to the external
magnetic field.
The tip angle is
dependent on the
amplitude and duration of
the RF pulse
17
Generation of the MR Signal from Patient
18
After the RF Pulse – T1 and T2 Relaxation
net magnetization
The following pages will describe both T1 and T2 relaxation. These are important concepts
in MRI as they help provide the processes determining the contrast in MR images.
19
T1 Relaxation (aka Longitudinal or Spin-Lattice Relaxation)
T1 is the time constant for the z-component of the magnetization generated by the excited nuclei
to return to equilibrium. Remember that the application of the RF pulse forces the nuclei to
precess in phase and drags them down to the transverse plane – at this point the NMR signal can
be detected. When the pulse stops, the T1 relaxation phase starts as the nuclei start to recover
the longitudinal component (see diagram on previous page). T1 is exponential and is defined as
the time taken for the z-component of the magnetization to return to 63% of its equilibrium value
(see diagram below). T1 is a time constant and not the time taken for full longitudinal recovery.
During this time the NMR signal being emitted by the patient is losing intensity.
20
T1 Relaxation (aka Longitudinal or Spin-Lattice Relaxation)
To recap, when the RF pulse stops, the
excited nuclei return from the high energy
state to the low energy state, losing that
energy to the surrounding nuclei. The
interaction between these nuclei and the
surrounding molecular lattice structure and
T1 was originally called “spin-lattice
relaxation”. T1 relaxation is also known as
longitudinal relaxation, as it is a return to the
longitudinal axis. The nuclei bump into the
other nuclei in the surrounding lattice, so the
size and speed of the surrounding nuclei
affect the rate of T1 (i.e. length of T1 time).
Different body tissues contain different sizes
of molecules and it is these differences that
cause differences in T1 times and hence
create contrast between tissues. Very small
molecules rotate too quickly, leading to a
small number of potential resonant
frequencies, and large macromolecules
rotate so slowly that no frequencies
comparable to the Larmor frequency exist.
Medium-sized molecules provide the
shortest T1 times, so fat produces a short
T1 time, while water and most proteins
produce long T1 times. T1 times range from
0.1 to 1 second in soft tissues and from 1 to
4 seconds in aqueous tissues and water.
T1 times increase with magnetic field
strength.
The RF pulse must be applied many times
with relaxation and signal measurement
occurring after each application. The
number of repetitions is in multiples of 64,
usually 128 or 256. A minimum of 128-256
signal samples are needed to form an
image.
Contrast in the MR images occurs because
a high signal will show as a white area in
the image, whereas no signal will appear as
black. Intermediate amounts of signal
appear in the image as different shades of
gray.
21
Free Induction Decay (FID) and T2
Immediately after a 90o RF pulse stops, the now transverse magnetization vector rotates at
the Larmor frequency in the x-y (horizontal) plane perpendicular to the external magnetic
field. It sends out an NMR signal called the free induction decay (FID) signal. Free refers to
the fact that it is no longer under the influence of the RF pulse.
The FID signals are detected by a receiver coil, then digitized, stored in the computer and
then transformed into MR images using a Fourier transform analysis.
After the RF pulse is switched off, the protons start to lose phase gradually (decay) until
they return to their original state (dephasing). It is this decay rate that is referred to as T2.
T2 decreases with increasing molecular size and decreased molecular mobility.
Large molecules in the body and solids have short T2 times whereas liquids have long T2
times.
22
FID Signals from the Patient
TE and TR are explained in the next pages.
23
Pulse Sequences
The NMR signal is composed of four
separate components
– The amplitude (size of the signal)
– The frequency
- The phase
- The duration of resonance
Of course, a single NMR signal from one proton doesn’t provide much information, but a
whole sequence of RF pulses can be used to manipulate the amplitude of the NMR signal,
which eventually becomes translated into an intensity value on the final image. The larger the
amplitude, the brighter the intensity from that part of the body. It is the difference in amplitude
of the NMR signals in different body regions that produce the contrast in the final image,
amplitude is often referred to as intensity.
There are many different pulse sequences that are preset on MRI machines that can
manipulate the quality of the final image. Different sequences are used for different body parts
to enhance the features that will help with diagnosis. The different pulse sequences will not be
described here, but there is a good description of the most common ones at:
MR Pulse Sequences
There are certain parameters that are manipulated in pulse sequences, which will be
explained in the next few pages. For example, the simplest (but not the most commonly used)
pulse sequence is called saturation recovery. It involves repeated 90o RF pulses with the NMR
signal being measured after every pulse. The repetition time (time between pulses) is
called TR.
24
TR
If TR is long (about 3-4 the length of T1) then the nuclei have time to return to equilibrium
before the next RF pulse. The amplitude of the NMR signal after each pulse, therefore only
contains proton density information.
If TR is shorter (say equal to T1) then the nuclei do not have time to return to equilibrium (to
their full vertical position) before the next RF pulse.
25
TE (Time to Echo)
The next parameter to understand is TE (time to echo), which is the time between the 90o RF
pulse and when the echo signal is sampled. The larger the TE, the larger the T2 contribution in
the echo.
26
MRI Interactive Program on the Web
All that has been presented in the module so far is shown in an interactive form
in a demonstration program found at the following website:
http://www.simplyphysics.com/MRIntro.html
(This takes you to the introductory page – click on the link called Go to MRI
Introduction)
They say a picture is worth a thousand words and to actually see the
movements made by the protons is probably worth more than any number
of words. This program gives an excellent and clear description of the
physics that should help cement the information. I highly recommend
students to visit this site.
27
Instrumentation
There are three basic types of magnets used in MRI systems:
Resistive Magnets have many coils of wire wrapped around a cylinder through which an
electric current is passed (a solenoid). When connected to an electricity supply this
generates a magnetic field. These magnets are lower in cost to construct than a
superconducting magnet, but require huge amounts of electricity (up to 50 kilowatts) to
operate because of the natural resistance in the wire. These magnets are generally too
expensive to run in any but very low field MR machines (below 0.35T).
Permanent Magnets produce a magnetic field that is always running at full strength, so it
costs nothing to maintain the field. Unfortunately, these magnets are extremely heavy and
are not feasible for any but very low field MR machines (less than 0.4T).
Superconducting magnets are by far the most commonly used. They are somewhat
similar to a resistive magnet in that they have coils of wire through which a constant
current of electricity is passed create the magnetic field. The important difference is that the
wire is continually bathed in liquid helium at 452.4 degrees below zero, which is kept in a
well insulated vacuum tube. The extremely low temperature causes the resistance in the
wire to drop to zero, reducing the electrical requirement for the system dramatically and
making it much more economical to operate. Superconductive systems are still very
expensive, but they can easily generate 0.5-tesla to 4.0-tesla fields, allowing for much
higher-quality imaging.
28
Magnet Assembly
Most magnets are long, narrow cylinders with other concentric
cylinders which perform different tasks inside them. The main
magnet has miles of wire wound around it and it is the electricity in
this wire that creates the magnetic field. Another type of magnet
found in every MRI system is called a gradient (coil) magnet. There
are usually three gradient magnets inside the MRI machine.
Magnet Bore
Shim Coils
Gradient Coils
RF Coils
RF coils create the B1 field
which
rotates
the
net
magnetization in a pulse
sequence. Three types; RF
pulse transmitter and receiver
coils, receiver only coils, and
transmitter only coils. MRI
machines have many different
coils designed for different parts
of the body according to size.
The
gradient
coils
produce the gradients in
the Bo magnetic field.
These are also magnets,
but with very, very low
strength compared to the
main magnetic field,
ranging from 180 gauss
to 270 gauss, or 18 to 27
millitesla. They produce
a variable field.
Shim Coils superimpose
small
corrective
field
differences on the main
field to improve the
magnetic field uniformity.
A more realistic diagram of
the Magnet assembly is
shown on the next page.
29
Superconducting Magnetic Resonance System
Diagram from W. Huda & R.
Slone. Review of Radiologic
Physics, 2nd edition. Lippincott
30
Williams & Wilkins. 2003.
Signal-to-Noise Ratio
If you hang around people working in the MRI
field long enough (usually less than half an
hour!) you will hear the term signal-to-noise
ratio (often written as S/N or SNR). SNR is a
measure of signal strength relative to
background noise. Noise is simply unwanted
electrical or electromagnetic energy that
degrades the quality of signals and data and it
occurs in both analog and digital signals. The
ratio is usually measured in decibels (dB).
Although SNR is an issue in other imaging
modalities, it is a much more important
concept in MRI as the signal is small to begin
with, and extraneous noise has a more serious
effect on the quality of the final image.
Obviously, a high signal-to-noise ratio in which
the signal predominates is the aim.
Many of the factors that affect the SNR can be
manipulated by the design of the MR
equipment and the methods used to acquire
images.
Adjusting the SNR is a tradeoff between the
contrast (which is the intensity difference
between two adjacent regions) and spatial
resolution. Images with low signal to noise
(i.e. a lot of noise) diminish the ability to see
low contrast structures.
There are many factors that affect SNR
and it is not important that you know
them all, or their action. Some of the
main factors that affect the SNR are:
The strength of the main magnet
The coil selection
The receiver bandwidth
The voxel size
The pulse sequence parameters
The relationship between these is shown in
the formula for SNR:
SNR  K  voxelsize 
measurements
bandwidth 31
RF Coils
RF pulses are usually applied through one
of the different transmitter coils designed
for different parts of the body that are
supplied with MRI machines. These coils
usually conform to the contour of the body
part being imaged, or are located as close
as possible to it during the exam.
The same coils can be used as receivers,
but sometimes the receiver is a separate
coil. Receiver coils detect the FID signals
coming from the patient. The proper
selection of RF coils can greatly increase
the SNR.
– Small RF coils maximize the SNR of
weak FID signals, but this is at the
expense of a smaller region that can
be imaged
– Quadrature coils can also improve
the SNR, as can Phased Array
coils.
Quadrature coils are actually made up of
two linear coils that are orthogonal to
each other. Each coil transmits a pulse,
but being orthogonal, they are 90o out of
phase with each other. Quadrature coils
usually have a birdcage design.
Phased Array coils use several surface
coils that overlap, but the overlap must be
at the correct geometry to be effective.
It may be of interest to students that
Robarts scientists spun off a state-of-theart MR coil company, XLResonance, that
designs different MR coils for clinical and
research purposes.
32
Gradient Coils
Gradient coils are magnets used to produce a known variation in the constant magnetic
field of the main magnet. It is the gradient coils that perform the 'magic' which generates
information about position from NMR. They work by changing the magnetic field so that at
some points it is higher than at others. This provides a way of knowing where in the
sample the signal is coming from.
These coils are firmly attached to the inside of the scanner, but the huge magnetic forces
of the main magnet make the coils bang against their housing. It is this banging of the
gradient coils that makes the loud rhythmic noise when the MR is scanning.
MR systems have three gradient coils in the x, y, and z orientations, so that they can
provide a gradient field in any direction. The following images here and over the page show
what these gradients look like.
The x-gradient (gradient slopes in the x plane)
33
Gradient Magnetic Fields
Gradient magnetic fields (usually just referred to
as “gradients”) introduce a spatial variation in the
location. The larger gradient the more rapidly the
slope of the magnetic field varies. The gradient
magnetic fields all point in the same direction as
the main magnetic field (Bo), but the variation or
slope can be in any direction. The convention is
to apply gradient magnetic fields along the x, y or
z planes, but in principle they can be applied in
any direction.
The Z-Gradient
The Y-Gradient
34
Spatial Encoding
Spatial encoding is a crucial component of
MRI. It is the process in which the spatial
position of spins is encoded by applying a
temporary gradient field. The methods used
for spatial encoding are slice selection,
frequency encoding and phase encoding.
The "slice select" gradient is used to locate
the level of each plane. The "frequency" and
"phase encoding" gradients are used to
locate points of intensity within each plane.
These steps lead us to how the final
image is reconstructed from the raw data.
This uses a mathematical procedure
called the inverse Fourier Transform.
The next step is to understand the
concept of K-space and how it transforms
the raw data into the familiar MR image
with all its shades of gray.
For more information visit the following
website from the e-MRI, Magnetic
Resonance Imaging physics and technique
course on the web. You will need to register
to view the pages.
Spatial Encoding
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K-Space
The concept of k-space is one that even MR professionals have some difficulty grasping.
K-space is a map of all the waves of the actual image by a code that is spatial and
numerical. The k-space map corresponds to numbers that contain the inherent information
on the direction, strength (amplitude) and frequency of every wave in each slice that makes
up the total image.
The K-space map is divided into 4 quadrants with each quadrant being symmetrically the
same.
The numbers represent the raw data of the wave patterns that are present in any particular
slice. The "raw data" sampled during the echo tells the direction, strength (amplitude) and
frequency of the waves.
The frequency of a wave can be known by where it appears in one of four quadrants, so
the distance of the point from the centre of k-space gives the frequency. This means that
waves closer to the centre of k-space have a higher frequency.
The brightness of a point (the larger the number, the brighter the point) represents its
strength (amplitude).
Any point that is joined from the centre of K-space to any angle will show the direction of
the wave.
The centre 20% of the k-space map provides 90% of the contrast in the image.
This site is by Dave Higgins from the Department of Medical Physics at University of Leeds, UK
and has several multimedia elements that demonstrate how k-space works.
MRI Tutorials - K-Space
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Functional MRI (fMRI)
Functional MRI is a relatively new imaging modality that is used for brain mapping. The
technique allows visualization of the metabolic activity within the brain, rather than the
anatomy, making it similar to Positron Emission Tomography (PET). FMRI is performed on
a regular MR machine, but there are two tissue contrast mechanisms in MR imaging that
show functional activity. These are increases in blood flow and microvascular oxygenation.
The technique will not be described in any more detail in this module, but if any students
are interested in learning more there is an excellent website from the FMRIB Group at the
Oxford Centre for Functional Magnetic Resonance Imaging of the Brain, which is based at
the Radcliffe Infirmary, Oxford University, UK. at:
FMRIB - Brief Introduction to fMRI
For students who are really interested, one the best written sources is the book by scientists at
this centre called “Functional MRI: An Introduction to Methods”, Peter Jezzard, Paul M.
Mathews, & Stephen M. Smith (Eds).
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