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MRI has established itself as an important clinical tool in the diagnosis of central nervous system diseases. While MRI indications in the body are expanding, evaluation of CNS diseases remains the most common application of MRI today. The complete imaging evaluation of patients with CNS neoplasms includes detection, characterization, and determination of tumor extent (or metastatic spread). Also, secondary effects such as hydrocephalus, compartmental herniation or vascular compromise can be discerned. The role of MRI in CNS inflammatory disease or infection is also well established.
MRI techniques are well suited to the CNS setting. Compared to CT, MRI offers superior contrast resolution and does not involve ionizing radiation. MRI is not compromised by adjacent osseous structures, making it particularly valuable in evaluating the posterior fossa and spine. In short, MRI provides greater sensitivity and specificity for the majority of CNS disease states compared to other radiologic modalities.
Why are contrast agents necessary given the good resolution of unenhanced MRI images?
- Relaxation characteristics of normal and pathologic tissues are not always different enough to produce obvious differences in signal intensity.
- Pathology which is sometimes occult on unenhanced images becomes obvious in the presence of contrast.
- Enhancement significantly increases MRI sensitivity.
- In addition to improving delineation between normal and abnormal tissues, the pattern of contrast enhancement can improve diagnostic specificity by facilitating characterization of the lesion(s) in question.
- Contrast can yield physiologic and functional information in addition to lesion delineation.
Considerable investigational work is being done in the area of MRI contrast agents, but at present the majority of products employed in CNS or body studies are all gadolinium compounds such as MAGNEVIST® (gadopentetate dimeglumine) injection produced by Bayer HealthCare Pharmaceuticals.
Common Indications
Contrast-enhanced MRI has numerous indications in the CNS including:
Brain
- CNS infections
- Noninfectious inflammatory disease (e.g., MS, etc.)
- Preoperative/pretreatment evaluation of brain tumors
- Postoperative evaluation of brain tumor therapy
- Meningeal disease
Spine
- Infection/inflammatory disease
- Primary tumors
- Drop metastases
- Initial evaluation of syrinx
- Postoperative evaluation of lumbar spine: disk vs. scar
Evaluation of Stroke
An important neuroradiological indication for MRI is evaluation of stroke via perfusion and diffusion imaging. These MRI techniques have become critical in the management of so-called "brain attack." Diffusion imaging can demonstrate the central effect of a stroke on the brain, whereas perfusion imaging visualizes the larger "second ring" delineating blood flow and blood volume. Diffusion and perfusion MRI are becoming increasingly critical in the management of incipient or acute stroke as neuroprotective agents are more widely studied.
Infection
Inflammatory disease can be especially difficult to detect along the ependyma or meninges, but contrast-enhanced MRI techniques can delineate and localize infection. Abscesses can be confirmed by visualization of capsule formation via contrast-enhanced MRI, a particularly important indication since attempts to drain a focal infection prior to completion of capsule formation are associated with considerable risk.
Brain Neoplasms
Brain NeoplasmsDetecting and monitoring brain neoplasms remains one of the chief indications for MRI in neuroradiology. MRI is used for initial diagnosis, to stage disease, to plan surgery, and to evaluate postinterventional results.
Primary brain tumors are generally well demonstrated on unenhanced MRI, but early parenchymal or subarachnoid metastatic disease usually requires enhancement for optimal diagnosis. Approximately half of all low-grade gliomas do not disrupt the blood brain barrier and therefore are not better enhanced with contrast agents. These can be well established with non-contrast MRI and hence primary intra-axial neoplasm generally does not require a contrast study. However, enhanced studies are particularly sensitive for the detection of small extra-axial neoplasms such as meningiomas and vestibular schwannomas.
Generally, if a primary brain tumor is discovered on an unenhanced study, a contrast-enhanced study will be performed prior to institution of therapy to help fully characterize and delineate the extent of the lesion and thereby help inform the therapeutic intervention.
Contrast studies are central to the detection of secondary brain neoplasms, e.g., metastases. Enhancing metastases can be detected even before significant edema or mass effects are present.
MRI is also a critical tool for post interventional monitoring, after the patient has undergone surgery, radiation, chemotherapy or any combination thereof. Changes in enhancement patterns in post-operative compared to pre-operative images provide information about residual or recurrent neoplasm or confirms if the therapy has successfully eradicated the lesion.
Indications in the Spine
Contrast-enhanced techniques delineate infections vs. malignancies in the spine. Evaluation of a syrinx cavity in the spinal cord is also performed with contrast techniques.
MRI is commonly employed in the evaluation of the lumbar spine pre- and postoperatively. After surgery for disk disease, significant fibrosis can occur in the lumbar spine. This scarring can mimic residual disk herniation. Enhanced MRI differentiates these two conditions reliably.
Based on the slower uptake into cartilage, negatively charged ionic contrast agents have been shown to further delineate disk and scar tissue.
The exact relationship between MRI findings and the clinical status of patients is not completely understood.
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