The clinician caring for patients with neurologic symptoms is faced with myriad imaging options, including computed tomography (CT), CT angiography (CTA), perfusion CT (pCT), magnetic resonance imaging (MRI), MR angiography (MRA), functional MRI (fMRI), MR spectroscopy (MRS), MR neurography (MRN), diffusion and diffusion track imaging (DTI), susceptibility weighted MR imaging (SWI), and perfusion MRI (pMRI). In addition, an increasing number of interventional neuroradiologic techniques are available, including angiography catheter embolization, coiling, and stenting of vascular structures; and spine diagnostic and interventional techniques such as diskography, transforaminal and translaminar epidural and nerve root injections and blood patches. Recent developments such as multidetector CTA (MDCTA) and gadolinium-enhanced MRA, have narrowed the indications for conventional angiography, which is now reserved for patients in whom small-vessel detail is essential for diagnosis or for whom concurrent interventional therapy is planned (Table 368-1).
Table 368-1 Guidelines for the Use of CT, Ultrasound, and MRI
| Save Table
Table 368-1 Guidelines for the Use of CT, Ultrasound, and MRI
|Acute parenchymal||CT, MR|
|Subarachnoid hemorrhage||CT, CTA, lumbar puncture → angiography|
|Aneurysm||Angiography > CTA, MRA|
|Hemorrhagic infarction||CT or MRI|
|Bland infarction||MRI > CT, CTA, angiography|
|Carotid or vertebral dissection||MRI/MRA|
|Vertebral basilar insufficiency||CTA, MRI/MRA|
|Carotid stenosis||CTA > Doppler ultrasound, MRA|
|Suspected mass lesion|
|Neoplasm, primary or metastatic||MRI + contrast|
|Infection/abscess||MRI + contrast|
|Immunosuppressed with focal findings||MRI + contrast|
|Vascular malformation||MRI ± angiography|
|White matter disorders||MRI|
|Demyelinating disease||MRI ± contrast|
|Dementia||MRI > CT|
|Acute trauma||CT (noncontrast)|
|Shear injury/chronic hemorrhage||MRI + gradient echo imaging|
|Headache/migraine||CT (noncontrast) / MRI|
|First time, no focal neurologic deficits||?CT as screen ± contrast|
|Partial complex/refractory||MRI with coronal T2W imaging|
|Cranial neuropathy||MRI with contrast|
|Meningeal disease||MRI with contrast|
|Low back pain|
|No neurologic deficits||MRI or CT after 4 weeks|
|With focal deficits||MRI > CT|
|Spinal stenosis||MRI or CT|
|Cervical spondylosis||MRI or CT myelography|
|Infection||MRI + contrast, CT|
|Myelopathy||MRI + contrast|
|Arteriovenous malformation||MRI, angiography|
In general, MRI is more sensitive than CT for the detection of lesions affecting the central nervous system (CNS), particularly those of the spinal cord, cranial nerves, and posterior fossa structures. Diffusion MR, a sequence sensitive to the microscopic motion of water, is the most sensitive technique for detecting acute ischemic stroke of the brain or spinal cord, and it is also useful in the detection of encephalitis, abscesses, and prion diseases. CT, however, is quickly acquired and is widely available, making it a pragmatic choice for the initial evaluation of patients with acute changes in mental status, suspected acute stroke, hemorrhage, and intracranial or spinal trauma. CT is also more sensitive than MRI for visualizing fine osseous detail and is indicated in the initial evaluation of conductive hearing loss as well as lesions affecting the skull base and calvarium. MR may, however, add important diagnostic information regarding bone marrow infiltrative processes that are difficult to detect on CT.
The CT image is a cross-sectional representation of anatomy created by a computer-generated analysis of the attenuation of x-ray beams passed through a section of the body. As the x-ray beam, collimated to the desired slice width, rotates around the patient, it passes through selected regions in the body. X-rays that are not attenuated by body structures are detected by sensitive x-ray detectors aligned 180° from the x-ray tube. A computer calculates a “back projection” image from the 360° x-ray attenuation profile. Greater x-ray attenuation (e.g., as caused by bone), results in areas of high “density,” while soft tissue structures that have poor attenuation of x-rays such as organs and air-filled cavities, are lower in density. The resolution of an image depends on the radiation dose, the detector size, collimation (slice thickness), the field of view, and the matrix size of the display. A modern CT scanner is capable of obtaining sections as thin as 0.5–1 mm with submillimeter resolution at a speed of 0.3–1 s per rotation; complete studies of the brain can be completed in 2–10 s.
Multidetector CT (MDCT) is now standard in most radiology departments. Single or multiple (from 4 to 256) detectors positioned 180° to the x-ray source result in multiple slices per revolution of the beam around the patient. The table moves continuously through the rotating x-ray beam, generating a continuous “helix” of information that can be reformatted into various slice thicknesses and planes. Advantages of MDCT include shorter scan times, reduced patient and organ motion, and the ability to acquire images dynamically during the infusion of intravenous contrast that can be used to construct CT angiograms of vascular structures and CT perfusion images (Fig. 368-1B and C). CTA images are post-processed for display in three dimensions to yield angiogram-like images (Fig. 368-1C, 368-2 E and F, and see Fig. 370-4). CTA has proved useful in assessing the cervical and intracranial arterial and venous anatomy.
CT angiography (CTA) of ruptured anterior cerebral artery aneurysm in a patient presenting with acute headache. A. Noncontrast CT demonstrates subarachnoid hemorrhage and mild obstructive hydrocephalus. B. Axial maximum-intensity projection from CT angiography demonstrates enlargement of the anterior cerebral artery (arrow). C. 3D surface reconstruction using a workstation confirms the anterior cerebral aneurysm and demonstrates its orientation and relationship to nearby vessels (arrow). CTA image is produced by 0.5- 1-mm helical CT scans performed during a ...
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