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MAGNETIC RESONANCE
IMAGING OF THE BRAIN
John R. Hesselink, MD, FACR
Magnetic resonance (MR) is a dynamic and receiver coil to detect the returning radio signal,
flexible technology that allows one to tailor the 5) gradient coils to provide spatial localization of
imaging study to the anatomic part of interest the signal, and 6) a computer to reconstruct the
and to the disease process being studied. With radio signal into the final image.
its dependence on the more biologically variable The signal intensity on the MR image is
parameters of proton density, longitudinal determined by four basic parameters: 1) proton
relaxation time (T1), and transverse relaxation density, 2) T1 relaxation time, 3) T2 relaxation
time (T2), variable image contrast can be time, and 4) flow. Proton density is the
achieved by using different pulse sequences and concentration of protons in the tissue in the form
by changing the imaging parameters. Signal of water and macromolecules (proteins, fat, etc).
intensities on T1, T2, and proton density- The T1 and T2 relaxation times define the way
weighted images relate to specific tissue that the protons revert back to their resting states
characteristics. For example, the changing after the initial RF pulse. The most common
chemistry and physical structure of hematomas effect of flow is loss of signal from rapidly
over time directly affects the signal intensity on flowing arterial blood.
MR images, providing information about the age The contrast on the MR image can be
of the hemorrhage. Moreover, with MR's manipulated by changing the pulse sequence
multiplanar capability, the imaging plane can be parameters. A pulse sequence sets the specific
optimized for the anatomic area being studied, number, strength, and timing of the RF and
and the relationship of lesions to eloquent areas gradient pulses. The two most important
of the brain can be defined more accurately. parameters are the repetition time (TR) and the
Flow-sensitive pulse sequences and MR echo time (TE). The TR is the time between
angiography yield data about blood flow, as well consecutive 90 degree RF pulse. The TE is the
as displaying the vascular anatomy. Even brain time between the initial 90 degree RF pulse and
function can be investigated by having a subject
perform specific mental tasks and noting changes
in regional cerebral blood flow and oxygenation. MR Imaging
Finally, MR spectroscopy has enormous Basic Physical Principles
potential for providing information about the
biochemistry and metabolism of tissues. As an
imaging technology, MR has advanced 1. Radiofrequency pulse to perturb
considerably the past 10 years, but it continues to steady-state proton magnetization
evolve and new capabilities will likely be 2. Transient, small radio signal emitted
developed. 3. Spatial encoding with magnetic
field gradients
BASIC PRINCIPLES 4. Image map of MR signal strength
An MR system consists of the following
the echo.
components: 1) a large magnet to generate the
The most common pulse sequences are the
magnetic field, 2) shim coils to make the
T1-weighted and T2-weighted spin-echo
magnetic field as homogeneous as possible, 3) a
sequences. The T1-weighted sequence uses a
radiofrequency (RF) coil to transmit a radio
short TR and short TE (TR < 1000msec, TE <
signal into the body part being imaged, 4) a
30msec). The T2-weighted sequence uses a long
TR and long TE (TR > 2000msec, TE > 80msec). Spin-echo Pulse Sequence
The T2-weighted sequence is usually employed Dual Echo T2 -weighted
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 RF
is very helpful for evaluating periventricular TR
pathology, such as multiple sclerosis, because the
TE
hyperintense plaques are con-trasted against the TE
lower signal CSF. More recently, the FLAIR
(Fluid Attenuated Inversion Recovery) sequence Signal
has replaced the PD image. FLAIR images are 1st 2nd
T2-weighted with the CSF signal suppressed. echo echo
When reviewing an MR image, the easiest
way to determine which pulse sequence was weighted image. Next look at the signal intensity
used, or the "weighting" of the image, is to look of the brain structures.
On MR images of the brain, the primary
determinants of signal intensity and contrast are
Spin-echo Pulse Sequence the T1 and T2 relaxation times. The contrast is
Single Echo T1-weighted distinctly different on T1 and T2-weighted
images. Also, brain pathology has some
common signal characteristics.
RF
TR
Recognizing the MR Image
TE
Signal T2-weighted image
1st 2nd CSF bright
echo echo Gray matter brighter than white matter
PD -weighted image
at the cerebro-spinal fluid (CSF). If the CSF is CSF gray
bright (high signal), then it must be a T2- Gray matter brighter than white matter
weighted imaged. If the CSF is dark, it is a T1- T1-weighted image
CSF dark
White matter brighter than gray matter
MR Image Contrast 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
T2-weighted image than 4 minutes and a voxel size of 5 x 0.86 x 0.86
Short T2 = low signal mm. A 2.5 mm interslice gap prevents RF
Long T2 = high signal
interference between slices.1
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 AxialT2-weighted images
Except fat and subacute blood, which have short T1. Axial FLAIR images
High signal on T1WI
If normal:
Stop
If abnormal:
T1-weighted images
Pathologic lesions can be separated into four Gd-DTPA enhancement
major groups by their specific signal
characteristics on the three basic images: T2-
weighted, proton density-weighted (PD)/FLAIR,
and T1-weighted. If an abnormality is found, additional scans
MR Signal Intensities help characterize the lesion. Noncontrast T1-
weighted images are needed only if the
preliminary scans suggest hemorrhage, lipoma,
T2WI PD/FLAIR T1WI
or dermoid. Otherwise, contrast-enhanced scans
Solid mass Bright Bright Dark are recommended. Gadolinium-based contrast
agents for MR are paramagnetic and have
Cyst Bright Dark Dark demonstrated excellent biologic tolerance.
Caution is advised in patients with decreased
Subacute blood Bright Bright Bright renal function because several cases of
Acute & chronic
gadolinium-related nephrogenic systemic fibrosis
Dark Dark Gray
blood have been reported. It is injected intravenously
Fat Dark Bright Bright 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
Since studies have shown that T2-weighted the interstitial space and shortens the T1
images are most sensitive for detecting brain relaxation time of the tissue, resulting in
pathology, patients with suspected intracranial increased signal intensity on T1-weighted
disease should be screened with T2-weighted images. The scans should be acquired between 3
spin-echo and FLAIR images. The axial plane is and 30 minutes postinjection for optimal results.
commonly used because of our familiarity with Contrast enhancement is especially helpful
for extra-axial tumors because they tend to be As imaging techniques of the brain, MR and
isointense to brain on plain scans, but it also CT are both competitive and complimentary. In
identifies areas of BBB breakdown associated general, CT performs better in cases of trauma
with intra-axial lesions. Gadolinium and emergent situations. It provides better bone
enhancement is essential for detecting detail and has high sensitivity for acute
leptomeningeal inflammatory and neo-plastic hemorrhage. Support equipment and personnel
processes. Contrast scans are obtained routinely can be brought directly into the scan room. CT
in patients with symptoms of pituitary adenoma scanning is fast. Single scans can be done in 1
(elevated prolactin, growth hormone, and so second, so that even with uncooperative patients,
forth) or acoustic neuroma (sensorineural hearing adequate scans usually can be obtained. CT is far
loss). To screen for brain metastases in patients more sensitive than MR for subarachnoid
with a known primary, contrast-enhanced T1- hemorrhage. CT is also more sensitive for
weighted scans alone are probably sufficient.2 detecting intracranial calcifications.
Gadolinium does not enhance rapidly- MR, on the other hand, functions best as an
flowing blood. If vascular structures are not elective outpatient procedure. Proper screening
adequately seen on plain scan, the positive of patients, equipment, and personnel for
contrast provided by gradient-echo techniques or ferromagnetic materials, pacemakers, etc. is
MR angiography may be helpful to confirm or mandatory to avoid possible catastrophe in the
disprove a suspected carotid occlusion or magnet room. If proper precautions are in place,
cerebral aneurysm, to evaluate the integrity of emergency studies can be done, but the set-up
the venous sinuses, and to assess the vascularity time is longer, and the imaging also requires
of lesions. Gradient-echo imaging also enhances more time. With conventional MR systems, most
the magnetic susceptibility effects of acute and pulse sequences take a minimum of 2 minutes.
chronic hemorrhage, making them easily At this time, echo-planar capability is not
observable, even on low and mid-field MR standard on most systems, but this advanced
systems. Although the axial plane is the technology can acquire sub-second MR scans.
primary plane for imaging the brain, the Due to its high sensitivity for brain water,
multiplanar capability of MR allows one to select MR is generally more sensitive for detecting
the optimal plane to visualize the anatomy of brain abnormalities during the early stages of
interest. Coronal views are good for parasagittal disease. For example, in cases of cerebral
lesions near the vertex and lesions immediately infarction,3 brain tumors or infections, the MR
above or below the lateral ventricles (corpus scan will become positive earlier than CT. When
callosum or thalamus), temporal lobes, sella, and early diagnosis is critical for favorable patient
internal auditory canals. The coronal plane can outcome, such as in suspected herpes
be used as the primary plane of imaging in encephalitis, MR is the imaging procedure of
patients with temporal lobe seizures. Sagittal choice. MR is exquisitely sensitive for white
views are useful for midline lesions (sella, third matter disease, such as multiple sclerosis,4
ventricle, corpus callosum, pineal region), and progressive multifocal leukoencephalopathy,
for the brain stem and cerebellar vermis. leuko-dystrophy, and post-infectious
encephalitis. Patients with obvious white matter
abnormalities on MR may have an entirely
CLINICAL INDICATIONS normal CT scan. Other clinical situations where
MR will disclose abnormalities earlier and more
definitively are temporal lobe epilepsy,5 can be imaged without intravenous contrast
nonhemorrhagic brain contusions and traumatic media. In cases of cryptic vascular
shear injuries.6 malformations and cavernous angiomas, where
In general, nonenhancing disease processes the angiogram and CT scan are often negative,
are much more apparent on MR than CT. When MR may reveal small deposits of hemosiderin
the blood-brain barrier is damaged, enhancement from prior small hemorrhages.11 Diffusion-
occurs with both gadolinium and iodinated weighted sequences are highly sensitive for
contrast agents on MR and CT, respectively. As restricted diffusion and cytotoxic edema
a rule, the degree of enhancement is greater on associated with acute cerebral infarction. By
MR scans. combining conventional MR images with
For evaluating posterior fossa disease, MR is diffusion and perfusion-weighted imaging and
preferable to CT. The CT images are invariable MR angiography, a complete workup of vascular
degraded by streaking artifacts from the bones at disease can be accomplished.
the skull base. In conjunction with gadolinium Along with the function of MR as a primary
enhancement, MR can reliably detect imaging procedure, there are indications for MR
intracanalicular acoustic neuromas and other as a secondary procedure after the pathology has
schwannomas arising along the cranial nerves already been demonstrated by CT. In patients
within the basal cisterns and foramina of the with solitary lesions on CT, in whom the
skull base. Similarly, MR has largely supplanted diagnosis of metastatic disease, abscess, or
CT for imaging the sella turcica and pituitary multiple sclerosis would be strengthened by
gland.7 finding additional lesions, MR may resolve the
The value of MR for defining congenital issue. Similarly, in a patient with brain
malformations is unquestioned. The multiplanar metastases in whom none of the lesions account
display of anatomy gives important information for the patient's signs or symptoms, MR can help
about the corpus callosum and posterior fossa evaluate the particular anatomic area of interest.
structures.8 The superior gray/white contrast A potential problem in both of these
allows accurate assessment of myelination. circumstances is the nonspecificity of white
The phenomenon of flow void within arteries matter hyperintensities, and contrast MR may be
on spin-echo images, the high sensitivity for necessary to clarify the situation.
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
References
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Resonance Imaging, 3rd edition, Saunders-Elsevier, Philadelphia, 2006, pp 23-57.
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abnormalities. JCAT 12:266-274, 1988.
3. Warach S. Stroke neuroimaging. Stroke 34:345-7, 2003.
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5. Bernal B, Altman, N: Evidence-based medicine: Neuroimaging of seizures. Neuroimaging Clinics N
Am, 2003; Vol. 13 Number 2 211-224
6. Hesselink JR, Dowd CF, Healy ME, et al: MR Imaging of Brain Contusions: A Comparative Study
with CT. AJNR 9:269-278, 1988.
7. 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.
8. Barkovich AJ: Pediatric Neuroimaging. 2nd ed., Raven Press, New York, 1995, pp. 177-276.
9. Mattle HP, Edelman RR, Schroth G, Kiefer F: Intracranial hemorrhage. in Edelman, Hesselink, Zlatkin
& Crues, eds., Clinical Magnetic Resonance Imaging, 3 rd edition, Saunders-Elsevier, Philadelphia,
2006, pp 1287-1345.
10. Korogi Y, Takahashi M, Mabuchi N, et al. Intracranial aneurysms: diagnostic accuracy of three-
dimensional, Fourier transform, time-of-flight MR angiography. Radiology 193:181, 1994.
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