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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 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.