MAGNETIC RESONANCE
IMAGING OF THE BRAIN
John R. Hesselink, MD, FACR
Magnetic resonance (MR) is a dynamic and
flexible technology that allows one to tailor the
imaging study to the anatomic part of interest
and to the disease process being studied. With
its dependence on the more biologically variable
parameters of proton density, longitudinal
relaxation time (T1), and transverse relaxation
time (T2), variable image contrast can be
achieved by using different pulse sequences and
by changing the imaging parameters. Signal
intensities on T1, T2, and proton densityweighted images relate to specific tissue
characteristics.
For example, the changing
chemistry and physical structure of hematomas
over time directly affects the signal intensity on
MR images, providing information about the age
of the hemorrhage.
Moreover, with MR's
multiplanar capability, the imaging plane can be
optimized for the anatomic area being studied,
and the relationship of lesions to eloquent areas
of the brain can be defined more accurately.
Flow-sensitive pulse sequences and MR
angiography yield data about blood flow, as well
as displaying the vascular anatomy. Even brain
function can be investigated by having a subject
perform specific mental tasks and noting changes
in regional cerebral blood flow and oxygenation.
Finally, MR spectroscopy has enormous
potential for providing information about the
biochemistry and metabolism of tissues. As an
imaging technology, MR has advanced
considerably the past 10 years, but it continues to
evolve and new capabilities will likely be
developed.
BASIC PRINCIPLES
An MR system 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 signal,
5) gradient coils to provide spatial localization of
the signal, and 6) a computer to reconstruct the
radio signal into the final image.
The signal intensity on the MR image is
determined by four basic parameters: 1) proton
density, 2) T1 relaxation time, 3) T2 relaxation
time, and 4) flow.
Proton density is the
concentration of protons in the tissue in the form
of water and macromolecules (proteins, fat, etc).
The T1 and T2 relaxation times define the way
that the protons revert back to their resting states
after the initial RF pulse. The most common
effect of flow is loss of signal from rapidly
flowing arterial blood.
The contrast on the MR image can be
manipulated by changing the pulse sequence
parameters. A pulse sequence sets the specific
number, strength, and timing of the RF and
gradient pulses.
The two most important
parameters are the repetition time (TR) and the
echo time (TE). The TR is the time between
consecutive 90 degree RF pulse. The TE is the
time between the initial 90 degree RF pulse and
MR Imaging
Basic Physical Principles
1. Radiofrequency pulse to perturb
steady-state proton magnetization
2. Transient, small radio signal emitted
3. Spatial encoding with magnetic
field gradients
4. Image map of MR signal strength
the echo.
The most common pulse sequences are the
T1-weighted and T2-weighted spin-echo
sequences. The T1-weighted sequence uses a
short TR and short TE (TR < 1000msec, TE <
30msec). The T2-weighted sequence uses a long
TR and long TE (TR > 2000msec, TE > 80msec).
The T2-weighted sequence is usually employed
as a dual echo sequence. The first or shorter
echo (TE < 30msec) is proton density (PD)
weighted or a mixture of T1 and T2. This image
is very helpful for evaluating periventricular
pathology, such as multiple sclerosis, because the
hyperintense plaques are con-trasted against the
lower signal CSF. More recently, the FLAIR
(Fluid Attenuated Inversion Recovery) sequence
has replaced the PD image. FLAIR images are
T2-weighted with the CSF signal suppressed.
When reviewing an MR image, the easiest
way to determine which pulse sequence was
used, or the "weighting" of the image, is to look
Spin-echo Pulse Sequence
Single Echo T1-weighted
RF
Spin -echo Pulse Sequence
Dual Echo T2-weighted
RF
TR
TE
TE
Signal
1st
echo
2nd
echo
weighted image. Next look at the signal intensity
of the brain structures.
On MR images of the brain, the primary
determinants of signal intensity and contrast are
the T1 and T2 relaxation times. The contrast is
distinctly different on T1 and T2-weighted
images.
Also, brain pathology has some
common signal characteristics.
TR
Recognizing the MR Image
TE
 T2-weighted image
Signal
1st
echo
2nd
echo
CSF bright
Gray matter brighter than white matter
 PD -weighted image
at the cerebro-spinal fluid (CSF). If the CSF is
bright (high signal), then it must be a T2weighted imaged. If the CSF is dark, it is a T1-
CSF gray
Gray matter brighter than white matter
 T1-weighted image
CSF dark
White matter brighter than gray matter
the anatomy from CT. The other scan parameters
include a 256 x 256 matrix, 1 NEX, 22 cm FOV
and 5 mm slice thickness for a scan time of less
than 4 minutes and a voxel size of 5 x 0.86 x 0.86
mm. A 2.5 mm interslice gap prevents RF
interference between slices.1
MR Image Contrast
 T2-weighted image
Short T2 = low signal
Long T2 = high signal
 T1-weighted image
Short T1 = high signal
Long T2 = low signal
Brain Screening Protocol
 Most brain pathology has long T2 and Long T1.
High signal on T2WI
Low signal on T1WI
 Except fat and subacute blood, which have short T1.
High signal on T1WI
Axial T2-weighted images
Axial FLAIR images
If normal:
Stop
If abnormal:
Pathologic lesions can be separated into four
major groups by their specific signal
characteristics on the three basic images: T2weighted, proton density-weighted (PD)/FLAIR,
and T1-weighted.
MR Signal Intensities
T2WI
PD/FLAIR
T1WI
Solid mass
Bright
Bright
Dark
Cyst
Bright
Dark
Dark
Subacute blood
Bright
Bright
Bright
Acute & chronic
blood
Dark
Dark
Gray
Fat
Dark
Bright
Bright
Since studies have shown that T2-weighted
images are most sensitive for detecting brain
pathology, patients with suspected intracranial
disease should be screened with T2-weighted
spin-echo and FLAIR images. The axial plane is
commonly used because of our familiarity with
T1-weighted images
Gd-DTPA enhancement
If an abnormality is found, additional scans
help characterize the lesion. Noncontrast T1weighted images are needed only if the
preliminary scans suggest hemorrhage, lipoma,
or dermoid. Otherwise, contrast-enhanced scans
are recommended. Gadolinium-based contrast
agents for MR are paramagnetic and have
demonstrated excellent biologic tolerance.
Caution is advised in patients with decreased
renal function because several cases of
gadolinium-related nephrogenic systemic fibrosis
have been reported. It is injected intravenously
at a dose rate of 0.1 mmol/kg. The gadolinium
contrast agents do not cross the intact blood-brain
barrier (BBB). If the BBB is disrupted by a
disease process, the contrast agent diffuses into
the interstitial space and shortens the T1
relaxation time of the tissue, resulting in
increased signal intensity on T1-weighted
images. The scans should be acquired between 3
and 30 minutes postinjection for optimal results.
Contrast enhancement is especially helpful
for extra-axial tumors because they tend to be
isointense to brain on plain scans, but it also
identifies areas of BBB breakdown associated
with
intra-axial
lesions.
Gadolinium
enhancement is essential for detecting
leptomeningeal inflammatory and neo-plastic
processes. Contrast scans are obtained routinely
in patients with symptoms of pituitary adenoma
(elevated prolactin, growth hormone, and so
forth) or acoustic neuroma (sensorineural hearing
loss). To screen for brain metastases in patients
with a known primary, contrast-enhanced T1weighted scans alone are probably sufficient.2
Gadolinium does not enhance rapidlyflowing blood. If vascular structures are not
adequately seen on plain scan, the positive
contrast provided by gradient-echo techniques or
MR angiography may be helpful to confirm or
disprove a suspected carotid occlusion or
cerebral aneurysm, to evaluate the integrity of
the venous sinuses, and to assess the vascularity
of lesions. Gradient-echo imaging also enhances
the magnetic susceptibility effects of acute and
chronic hemorrhage, making them easily
observable, even on low and mid-field MR
systems.
Although the axial plane is the
primary plane for imaging the brain, the
multiplanar capability of MR allows one to select
the optimal plane to visualize the anatomy of
interest. Coronal views are good for parasagittal
lesions near the vertex and lesions immediately
above or below the lateral ventricles (corpus
callosum or thalamus), temporal lobes, sella, and
internal auditory canals. The coronal plane can
be used as the primary plane of imaging in
patients with temporal lobe seizures. Sagittal
views are useful for midline lesions (sella, third
ventricle, corpus callosum, pineal region), and
for the brain stem and cerebellar vermis.
CLINICAL INDICATIONS
As imaging techniques of the brain, MR and
CT are both competitive and complimentary. In
general, CT performs better in cases of trauma
and emergent situations. It provides better bone
detail and has high sensitivity for acute
hemorrhage. Support equipment and personnel
can be brought directly into the scan room. CT
scanning is fast. Single scans can be done in 1
second, so that even with uncooperative patients,
adequate scans usually can be obtained. CT is far
more sensitive than MR for subarachnoid
hemorrhage. CT is also more sensitive for
detecting intracranial calcifications.
MR, on the other hand, functions best as an
elective outpatient procedure. Proper screening
of patients, equipment, and personnel for
ferromagnetic materials, pacemakers, etc. is
mandatory to avoid possible catastrophe in the
magnet room. If proper precautions are in place,
emergency studies can be done, but the set-up
time is longer, and the imaging also requires
more time. With conventional MR systems, most
pulse sequences take a minimum of 2 minutes.
At this time, echo-planar capability is not
standard on most systems, but this advanced
technology can acquire sub-second MR scans.
Due to its high sensitivity for brain water,
MR is generally more sensitive for detecting
brain abnormalities during the early stages of
disease. For example, in cases of cerebral
infarction,3 brain tumors or infections, the MR
scan will become positive earlier than CT. When
early diagnosis is critical for favorable patient
outcome, such as in suspected herpes
encephalitis, MR is the imaging procedure of
choice. MR is exquisitely sensitive for white
matter disease, such as multiple sclerosis,4
progressive multifocal leukoencephalopathy,
leuko-dystrophy,
and
post-infectious
encephalitis. Patients with obvious white matter
abnormalities on MR may have an entirely
normal CT scan. Other clinical situations where
MR will disclose abnormalities earlier and more
definitively are temporal lobe epilepsy,5
nonhemorrhagic brain contusions and traumatic
shear injuries.6
In general, nonenhancing disease processes
are much more apparent on MR than CT. When
the blood-brain barrier is damaged, enhancement
occurs with both gadolinium and iodinated
contrast agents on MR and CT, respectively. As
a rule, the degree of enhancement is greater on
MR scans.
For evaluating posterior fossa disease, MR is
preferable to CT. The CT images are invariable
degraded by streaking artifacts from the bones at
the skull base. In conjunction with gadolinium
enhancement, MR can reliably detect
intracanalicular acoustic neuromas and other
schwannomas arising along the cranial nerves
within the basal cisterns and foramina of the
skull base. Similarly, MR has largely supplanted
CT for imaging the sella turcica and pituitary
gland.7
The value of MR for defining congenital
malformations is unquestioned. The multiplanar
display of anatomy gives important information
about the corpus callosum and posterior fossa
structures.8 The superior gray/white contrast
allows accurate assessment of myelination.
The phenomenon of flow void within arteries
on spin-echo images, the high sensitivity for
hemorrhage and hemosiderin deposition,9 and the
capability of MR angiography give MR distinct
advantages over CT for imaging vascular
disease.
Vascular stenoses or occlusions,
aneurysms,10 and arterio-venous malformations
can be imaged without intravenous contrast
media.
In cases of cryptic vascular
malformations and cavernous angiomas, where
the angiogram and CT scan are often negative,
MR may reveal small deposits of hemosiderin
from prior small hemorrhages.11 Diffusionweighted sequences are highly sensitive for
restricted diffusion and cytotoxic edema
associated with acute cerebral infarction. By
combining conventional MR images with
diffusion and perfusion-weighted imaging and
MR angiography, a complete workup of vascular
disease can be accomplished.
Along with the function of MR as a primary
imaging procedure, there are indications for MR
as a secondary procedure after the pathology has
already been demonstrated by CT. In patients
with solitary lesions on CT, in whom the
diagnosis of metastatic disease, abscess, or
multiple sclerosis would be strengthened by
finding additional lesions, MR may resolve the
issue.
Similarly, in a patient with brain
metastases in whom none of the lesions account
for the patient's signs or symptoms, MR can help
evaluate the particular anatomic area of interest.
A potential problem in both of these
circumstances is the nonspecificity of white
matter hyperintensities, and contrast MR may be
necessary to clarify the situation.
References
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Am, 2003; Vol. 13 Number 2 211-224
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Hesselink JR, Dowd CF, Healy ME, et al: MR Imaging of Brain Contusions: A Comparative Study
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Hald JK, Brunberg JA, Chong BW: Pituitary gland and parasellar region, in Edelman, Hesselink,
Zlatkin & Crues, eds., Clinical Magnetic Resonance Imaging, 3rd edition, Saunders-Elsevier,
Philadelphia, 2006, pp 1181-1214.
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Barkovich AJ: Pediatric Neuroimaging. 2nd ed., Raven Press, New York, 1995, pp. 177-276.
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