Central Nervous System

New imaging modalities have often been developed for and evaluated in the head region before their introduction to clinical body imaging. The reasons for this are straightforward: the head is relatively small (at least in most of us), and—on a larger scale—there is no significant biological motion that can give us so much trouble when imaging heart, lungs, and abdomen. Yet a lot can go wrong in the head ... and since without brains life just isn't the same, advances in head and brain imaging are of prime interest to all of us. Consequently, the first computed tomography (CT) and magnetic resonance imaging (MRI) scanners were used exclusively for brain imaging. MRI made its earliest and biggest advances in the diagnosis of central nervous system (CNS) diseases because the composition of the brain parenchyma could be depicted with much greater clarity than ever before or with any other imaging modality. On these grounds, the relevance of head CT examinations tends to decrease in the elective setting for diagnosis of subtle CNS abnormalities. CT remains the modality of primary choice in acute CNS problems, in trauma, and—of course—if MRI is contraindicated or not available (Table 11.1). Since the advent of CT perfusion imaging and CT angiography of neck vessels and circle of Willis, both modalities offer strong diagnostic tools for the diagnosis ofacute stroke and their use depends mainly on their availability and local physicians' preference. Some contrast media studies of the spinal canal (myelographies) are still performed.

Because very subtle lesions may have an enormous clinical impact in the CNS, mostly depending on their location,

Table 11.1 Suggestions for diagnostic modalities in CNS imaging1

Clinical problem

Investigation

Comment

Congenital disorders

MRI

The definitive examination for all malformations that avoids ionizing radiation. Sedation is usually required for young children.

3D-CT

3D-CT may be needed for bone anomalies. Sedation is usually required for young children.

US

Consider in neonates.

Acute stroke

CT/CTA/CT-PI

Readily available in most institutions. Immediate CT adequately assesses most cases and shows hemorrhage; useful in determining the cause, site, and appropriate primary treatment and secondary prevention. Contrast enhanced CTA/CT-PI may show perfusion defect immediately.

MRI/MRA/DW imaging

Should be considered in young patients; more sensitive in early infarction and for posterior fossa lesions. May show previous hemorrhages. "Round-the-clock" availability is limited in most hospitals. Advanced techniques permit very detailed and sensitive work-up of acute stroke.

US carotids

Should only be performed in (1) stroke patients after full recovery in whom carotid surgery is contemplated for secondary prevention; (2) evolving stroke where dissection or acute occlusive embolus is suspected; (3) young patients with stroke.

Transient ischemic attack CT/CTA of aortic arch, May be normal. Can detect established infarction and hemorrhage (TIA) carotids, and circle of and exclude disease processes that can mimic stroke syndromes,

Willis such as glioma, extracerebral hemorrhage, and cerebritis. Contrast-enhanced advanced vascular imaging techniques (CTA) comprehensively assess extracranial and intracranial vasculature.

CT, computed tomography; 3D-CT, three-dimensional CT; CTA, CTangiography; CT-PI, CT perfusion imaging; DW, diffusion weighted; ENT, ear, nose, and throat; MRI, magnetic resonance imaging; NM, nuclear medicine; PET, positron emission tomography; rCBF, regional cerebral blood flow; SPECT, single-photon emission computed tomography; US, ultrasound; XR, radiography. "

Table 11.1 Suggestions for diagnostic modalities in CNS imaging1 (Continued)

Clinical problem

Investigation

Comment

US carotids

To assess suitability for carotid endarterectomy or angioplasty. Angiography, MRA, and CTA are alternatives to show vessels.

Demyelinating and other diffuse white-matter disease

MRI

Most sensitive and specific investigation for establishing a diagnosis of multiple sclerosis; may be negative in up to 25% of those with established multiple sclerosis.

Space-occupying lesion

MRI

More sensitive for early tumors, in resolving exact location (treatment planning); superior modality in the assessment of posterior fossa and for vascular lesions. MRI may miss calcification.

CT

Often sufficient in supratentorial lesions.

Headache: acute, severe; subarachnoid hemorrhage (SAH)

CT

Clinical history is critical. Classic migraine and cluster headaches are usually diagnosed without CT. SAH-related headache typically occurs within seconds, rarely in minutes, and hardly ever over more than 5 minutes. CT provides adequate assessment in most cases of SAH and other types of intracranial hemorrhage and associated hydrocephalus.

N.B.: A negative CT does not exclude SAH and, when clinically suspected, lumbar puncture should follow if there are no contraindications (e.g., obstructive hydrocephalus). Lumbar puncture may also be needed to exclude meningitis.

MRI

Better than CT for inflammatory causes. SPECT may be the most sensitive investigation for encephalitis and may provide evidence of circulatory derangement in migraine.

Headache: chronic

XR skull, sinus, cervical spine

Of little use in the absence of focal signs/symptoms.

CT/MRI

In the absence of focal neurological symptoms, imaging is not usually indicated. The following features significantly increase the odds of finding a major abnormality on CT/MRI:

■ Recent onset and rapidly increasing frequency and severity of headache

■ Headache on waking up patient from sleep

■ Associated dizziness, lack of coordination, tingling, and numbness.

Pituitary and juxtasellar problems

MRI

Urgent referral when vision is deteriorating.

CT

If MRI is not available or is contraindicated.

Posterior fossa signs

MRI

Much better than CT. CT images are often degraded by beam hardening artifacts.

Hydrocephalus; ?shunt function

CT

Adequate in most cases.

MRI

Sometimes necessary and may be more appropriate in children.

US

First choice for infants.

XR

Can demonstrate the whole valve system and assess continuity and configuration of ventriculoperitoneal shunt line if there is evidence of hydrocephalus on sectional imaging.

Middle or inner ear symptoms (including vertigo)

CT

Clinical evaluation of these symptoms requires ENT, neurological, or neurosurgical expertise.

CT, computed tomography; 3D-CT, three-dimensional CT; CTA, CTangiography; CT-PI, CT perfusion imaging; DW, diffusion weighted; ENT, ear, nose, and throat; MRI, magnetic resonance imaging; NM, nuclear medicine; PET, positron emission tomography; rCBF, regional cerebral blood flow; SPECT, single-photon emission computed tomography; US, ultrasound; XR, radiography. ►

Table 11.1 Suggestions for diagnostic modalities in CNS imaging1 (Continued)

Clinical problem

Investigation

Comment

Sensorineural deafness

MRI

Much better than CT, especially for acoustic schwannomas. In children, both MRI and CT may be needed in congenital and postinfective deafness.

Dementia and memory disorders; first onset psychosis

CT

Yield is low, even in younger patients, unless there are neurological signs and rapid progression. Over the age of 65, CTcan be reserved for patients with an onset within the last year or with an atypical presentation, rapid unexplained deterioration, unexplained focal neurological signs or symptoms, a recent head injury (preceding the onset of dementia), or urinary incontinence and/or gait ataxia early in illness.

MRI, SPECT

No proven clinical value, although they may be used in research.

Epilepsy

MRI

Multiplanar capability affords greater sensitivity and specificity for the identification of cortical lesions. Particularly valuable in partial epilepsy, e.g., temporal lobe epilepsy, if surgery is being considered.

CT

Following trauma; may complement MRI in the characterization of lesions, e.g., detection of small calcifications

PET/NM;SPECT/rCBF Ictal SPECT or interictal PET is useful in planning of epilepsy surgery when MRI is negative or results conflict with EEG or neurosurgical evidence. Regional cerebral blood flow (rCBF) agents are also of value.

1Modified after: RCR Working Party. Making the best use of a Department of Clinical Radiology. Guidelines For Doctors, 5th ed. London: The Royal College of Radiologists, 2003.

CT, computed tomography; 3D-CT, three-dimensional CT; CTA, CTangiography; CT-PI, CT perfusion imaging; DW, diffusion weighted; ENT, ear, nose, and throat; MRI, magnetic resonance imaging; NM, nuclear medicine; PET, positron emission tomography; rCBF, regional cerebral blood flow; SPECT, single-photon emission computed tomography; US, ultrasound; XR, radiography.

good anatomical/spatial resolution and superior contrast resolution are paramount. And you as the interpreting radiologist have to know the anatomy well!

f Imaging of the CNS is clinically extremely important and ¿ very sophisticated. The clinical neurological examination remains the first step for the clinician, however, and must not be neglected. Clinical signs and symptoms may in fact provide key information. Any imaging findings must be interpreted in their light; only the combination of clinical presentation and imaging allows for correct management decisions.

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