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MAGNETIC RESONANCE

VIEWS: 12 PAGES: 8

									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
     RF
                                                     common signal characteristics.
                         TR
                                                              Recognizing the MR Image
              TE


   Signal                                                  T2-weighted image
                   1st          2nd                              CSF bright
               echo            echo                                                            matter
                                                                 Gray matter brighter than white
                                                           PD -weighted image
at the cerebro-spinal fluid (CSF). If the CSF is                 CSF gray
bright (high signal), then it must be a T2-                                                    matter
                                                                 Gray matter brighter than white
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 T 2 and Long T 1.
             High signal on T2WI
             Low signal on T1WI                                       AxialT2-weighted images
    Except fat and subacute blood, which have short T 1.             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,
                         T2W I        PD/FLAIR   T1W I
                                                            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

1.   Mugler JP III: Basic principles, in Edelman, Hesselink, Zlatkin & Crues, eds., Clinical Magnetic
     Resonance Imaging, 3rd edition, Saunders-Elsevier, Philadelphia, 2006, pp 23-57.

2.   Hesselink JR, Healy ME, Press GA, Brahme FJ: Benefits of Gd-DTPA for MR imaging of intracranial
     abnormalities. JCAT 12:266-274, 1988.

3.   Warach S. Stroke neuroimaging. Stroke 34:345-7, 2003.

4.   Simon JH: Neuroimaging of multiple sclerosis. Neuroimag Clin North Am 3:229-246, 1993.

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.

11. Rivera PP, Willinsky RA, Porter PJ: Intracranial cavernous malformations. Neuroimag Clin N Am
    13:27-40, 2003.

								
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