The way magnetic resonance imaging (MRI) is generated is complicated and is much
harder to understand than plain radiography, CT and ultrasound. It has strong
underpinnings in physics which must be understood before any real sense of "how
it works" is gained. What follows is a very abbreviated or "broad strokes"
description of the process. For a more detailed information, please go to our
Plane Mathematics,
Cross-Sections, and
Physics of MRI pages.
A magnetic resonance system (the actual machine) consists of the following components:
| 1) |
A large magnet to generate the magnetic field. |
| 2) |
Shim coils to make the magnetic field as homogeneous
as possible. |
| 3) |
A radiofrequency (RF) coil to transmit a radio
signal into the body part being imaged. |
| 4) |
A receiver coil to detect the returning radio signals. |
| 5) |
Gradient coils to provide spatial localization of
the signals. |
| 6) |
A computer to reconstruct the radio signals into
the final image. |
The signal intensity on the MRI is determined by four basic parameters:
| 1) |
Proton density |
| 2) |
T1 relaxation time |
| 3) |
T2 relaxation time |
| 4) |
Flow |
The MRI Simplified
MRI uses a powerful magnetic field that makes the hydrogen protons in water
molecules, which comprise between 70% and 80% of the average human brain, line
up. This ubiquitous biological molecule has two protons, which by virtue of
their positive charge act as small magnets on a subatomic scale. Then once they
are all lined up from the magnetic field, they are then knocked out of line by
radio waves. When the radio waves are stopped, the protons relax back into line,
releasing resonance signals that are transmitted to a computer.
To have a better understanding or image of this, think of the hydrogen proton
as if it were a gyroscope or even the planet earth, spinning on its axis, with a
north-south pole. In this respect it behaves like a small bar magnet. Now under
normal conditions, these hydrogen proton "bar magnets" spin in the body with their
axes randomly aligned since nothing is pulling them magnetically in the same
direction. An example could be a group of children on a playground running around
with no sense of order.
When the body is placed in a strong magnetic field, such as an MRI scanner,
the protons' axes all line up. This uniform alignment creates a magnetic vector
oriented along the axis of the MRI scanner. Now think of those same children with
no order and how each one will line up if someone yells "cookie." How loud you yell
"cookie" depends on your strength and this is the same with different machines. Each
scanners come in different field strengths, usually between 0.5 and 1.5 tesla (T).
The strength of the magnetic field can be altered electronically from head to
toe using a series of gradient electric coils, and, by altering the local
magnetic field by these small increments, different slices of the body will
resonate as different frequencies are applied.
When additional energy, in the form of a radio wave, is added to the magnetic
field, the magnetic vector is deflected. The radio frequency (RF) wave that
causes the hydrogen nuclei to resonate is dependent on the element sought
(hydrogen) and the strength of the magnetic field.
When the RF source is switched off the magnetic vector returns to its resting
state, and this causes a signal, or radio wave to be emitted. It's this
signal which is used to create the MR images. Receiver coils are used around
the body part in question to act as aerials to improve the detection of the
emitted signal. The intensity of the received signal is then plotted on a
grey scale and cross sectional images are built up.
Multiple transmitted RF pulses can be used in sequence to emphasise particular
tissues or abnormalities. A different emphasis occurs because different tissues
relax at different rates when the transmitted RF pulse is switched off. The
time taken for the protons to fully relax is measured in two ways. The first
is the time taken for the magnetic vector to return to its resting state and
the second is the time needed for the axial spin to return to its resting
state. The first is called T1 relaxation, the second is called T2 relaxation.
A MR examination is thus made up of a series of pulse sequences. Different tissues
(such as fat and water) have different relaxation times and can be identified
separately. By using a "fat suppression" pulse sequence, for example, the signal
from fat will be removed, leaving only the signal from any abnormalities lying
within it.
Spatial encoding of the MRI signal is accomplished through the use of gradients
(smaller magnetic fields) which perturb the main magnetic field, and cause
hydrogen protons in different locations to precess (move) at slightly different
rates. The portion of the gradient coils and the associated current that is
perpendicular to the main magnetic field cause a force (Lorentz force) on the
coils. The gradients are turned on and off very quickly in this process causing
them to vibrate causing the majority of the noise associated with the MRI
environment. This still occurs even though they are embedded in an epoxy.
Parameter Weighting
Terms such as "T1-weighted" and "T2-weighted" are among the most overused and
least understood concepts in MR imaging. In the broadest sense, these terms
are used to communicate to other physicians the type of MR pulse sequence
employed to generate a series of images.
Most non-radiologists are often taught to look at the "color" of cerebrospinal
fluid (CSF) or other fluids to determine the type of "weighting" — dark CSF means
"T1-weighting" and bright CSF means "T2-weighting". Although this simple scheme
worked fine in the past, now consider the brain image from the commonly used
T2-FLAIR (fluid-attenuated inversion recovery) sequence. This sequence is known
to have strong sensitivity to T2 changes, but the CSF signal has been suppressed by
an inverting pulse and rendered black. This can lead to confusion for some since
parts of "weighting" image has similar coloring to a "FLAIR" image.
A fundamental misconception about "weighting", is that contrast in the image
is dominated by one specific tissue parameter to the exclusion of all others.
Another common misconception is that T1-weighted or T2-weighted images are
parameter "maps" whose pixel intensities are proportional to tissue T1 or T2
values.
It's alright to use terms like "T1-weighted" and "T2-weighted" as long as you
realize they are imprecise, are not parameter "maps", and that nearly all images
have mixed contributions from all the different tissue parameters.
T1-Weighted Scan Basics
T1-weighted image (also referred to as T1WI or "spin-lattice" relaxation time)
is one of the basic pulse sequences in MRI and demonstrates differences in
the T1 relaxation times of tissues. This differentiates anatomical structures
mainly on the basis of T1 values, for example the scanning parameters are set
(short TR/short TE) to minimize T2 relaxation effects.
A T1-weighted image relies upon the longitudinal relaxation of a tissue's net
magnetization vector (NMV). Basically, spins aligned in an external magnetic field
(Bo)
are put into the transverse plane by an RF pulse. They then slide back toward the
original equilibirum of the
Bo.
Not all tissues get back to equilibirum equally quickly, and a tissue's T1 reflects
the amount of time its protons' spins realign with the
Bo.
Fat quickly realigns its longitudinal magnetization with the
Bo,
and it therefore appears bright on a T1-weighted image. Conversely, water
has much slower longitudinal magnetization realignment after an RF pulse,
and therefore has less transverse magnetization after a RF pulse. Thus,
water has low signal and appears dark.
If T1-weighted images didn't have short repetition times (TRs), then all the
protons would recover their alignment with the main magnetic field and the
image would be uniformly intense. Selecting a TR shorter than the tissues'
recovery time allows one to differentiate them, such as with tissue contrast.
T1-weighted sequences provide the best contrast for paramagnetic contrast
agents such as gadolinium-containing compounds.
A Gadolinium (gd)-Enhanced T1-Weighted Scan Reveals Only New Lesions
These are areas where the disease that are currently active. Before the MRI, an
injection of gadolinium (gd) is administered. This will distinguish the active
lesions from the normal parts of the brain. When MS is active, gadolinium will
cross the blood-brain barrier and reveal areas of inflammation by becoming
lighter or brighter. These are called "enhancing lesions" because they are able
to be seen.
A gadolinium-enhanced T1-weighted MRI scan supplies information about current
disease activity by highlighting areas of breakdown in the blood-brain barrier
that indicate inflammation. The blood-brain barrier is a cell layer around blood
vessels in the brain and spinal cord that prevents substances from passing out
of the blood stream into the central nervous system (CNS). These areas of
inflammation appear as active lesions-meaning that they are new, or getting
bigger. T1-weighted images also show "black holes," which are thought to
indicate areas of permanent damage. T2-weighted MRI scans are used to provide
information about disease burden or lesion load or the total amount of lesion
area.
Over time, lesions on gd-enhancing MRI may grow or shrink, depending on how
active an exacerbation is. This type of MRI will not show older, inactive
lesions.
T2-Weighted Scans
T2-weighted scans don't pick up new lesions as well as T1 scans but rather show
older, inactive lesions. Regular T2 MRI's are important for tracking long-term
disease progression.
T2-weighted scans differentiate anatomical structures mainly on the basis of T2
values, for example the scanning parameters are set (long TR/long TE) to minimize
T1 relaxation effects.
Appearance of FLAIR, T1 & T2-Weighted Scans
FLAIR MRI is a heavily T2-weighted technique that dampens ventricular CSF signal.
This causes the highest signals on the sequence are from certain brain parenchymal
abnormalities, such as MS lesions, while the CSF appears black.
With T1 and T2 scans, the brain will appear to be lighter in the middle and have
darker shades surrounding it. Just the opposite will occur with FLAIR scans with
the dark areas toward the middle and lighter shades surrounding.
On a T1-weighted scans show tissues with high fat content (such as white matter)
appear bright and compartments filled with water (CSF) appears dark. This is good
for demonstrating anatomy.
On a T2-weighted scan compartments filled with water (such as CSF compartments)
appear bright and tissues with high fat content (such as white matter) appear dark.
This is good for demonstrating pathology since most, but not all, lesions (damaged
tissue) tend to develop edema and are associated with an increase in water content.
With the addition of an additional RF pulse and additional manipulation of the
magnetic gradients, a T2-weighted sequence can be converted to a FLAIR sequence,
in which free water is now dark, but edematous tissues remain bright. This sequence
in particular is currently the most sensitive way to evaluate the brain for
demyelinating diseases, such as MS.
Gd-enhanced T1 and T2-weighted MRIs can depict acute active MS lesions. These
appear as enhancing white matter lesions, and the presence of an enhancing
lesion has been shown to increase the specificity for MS.
|
|
|
|
T1-weighting causes the nerve connections of white matter to appear
white, and the congregations of neurons of gray matter to appear
gray, while CSF appears dark.
|
Enlarge by passing over or clicking
|
|
|
|
T2-weighting causes the nerve connections of white matter to appear
gray, and the congregations of neurons of gray matter to appear
white, while the CSF appears light.
|
Enlarge by passing over or clicking
|
|
|
|
FLAIR is a heavily T2-weighted technique causing the nerve
connections of white matter to appear gray, and the congregations of
neurons of gray matter to appear white, while the CSF appears dark.
|
Enlarge by passing over or clicking
|
The Brain and Related Structures in MRI
|
|
| Normal |
T1 |
T2 |
| Bone |
Dark black |
Dark black |
| Air |
Dark black |
Dark black |
| Muscle |
Dark gray |
Dark gray |
| White matter |
Light gray |
Dark gray |
| Gray matter |
Dark gray |
Light gray |
| Fat |
Dark black |
Very white |
|
|
|
|
Home
Printer-Friendly Version 
Top of Page