3281Approach to the Patient with Neurologic Disease CHAPTER 422

Cutaneous Reflexes The plantar reflex is elicited by stroking, with

a noxious stimulus such as a tongue blade, the lateral surface of the

sole of the foot beginning near the heel and moving across the ball of

the foot to the great toe. The normal reflex consists of plantar flexion

of the toes. With upper motor neuron lesions above the S1 level of the

spinal cord, a paradoxical extension of the toe is observed, associated

with fanning and extension of the other toes (termed an extensor

plantar response, or Babinski sign). However, despite its popularity, the

reliability and validity of the Babinski sign for identifying upper motor

neuron weakness is limited—it is far more useful to rely on tests of

tone, strength, stretch reflexes, and coordination. Superficial abdominal reflexes are elicited by gently stroking the abdominal surface

near the umbilicus in a diagonal fashion with a sharp object (e.g., the

wooden end of a cotton-tipped swab) and observing the movement of

the umbilicus. Normally, the umbilicus will pull toward the stimulated

quadrant. With upper motor neuron lesions, these reflexes are absent.

They are most helpful when there is preservation of the upper (spinal

cord level T9) but not lower (T12) abdominal reflexes, indicating a

spinal lesion between T9 and T12, or when the response is asymmetric.

Other useful cutaneous reflexes include the cremasteric (ipsilateral elevation of the testicle following stroking of the medial thigh; mediated

by L1 and L2) and anal (contraction of the anal sphincter when the

perianal skin is scratched; mediated by S2, S3, S4) reflexes. It is particularly important to test for these reflexes in any patient with suspected

injury to the spinal cord or lumbosacral roots.

Primitive Reflexes With disease of the frontal lobe pathways,

several primitive reflexes not normally present in the adult may appear.

The suck response is elicited by lightly touching with a tongue blade the

center of the lips, and the root response the corner of the lips; the

patient will move the lips to suck or root in the direction of the stimulus. The grasp reflex is elicited by touching the palm between the

thumb and index finger with the examiner’s fingers; a positive response

is a forced grasp of the examiner’s hand. In many instances, stroking

the back of the hand will lead to its release. The palmomental response

is contraction of the mentalis muscle (chin) ipsilateral to a scratch

stimulus diagonally applied to the palm.

■ SENSORY EXAMINATION • The bare minimum: Ask whether the patient can feel light touch and

the temperature of a cool object in each distal extremity. Check double

simultaneous stimulation using light touch on the hands. Perform the

Romberg maneuver.

Evaluating sensation is usually the most unreliable part of the examination because it is subjective and is difficult to quantify. In the compliant

and discerning patient, the sensory examination can be extremely

helpful for the precise localization of a lesion. With patients who are

uncooperative or lack an understanding of the tests, it may be useless.

The examination should be focused on the suspected lesion. For example, in spinal cord, spinal root, or peripheral nerve abnormalities, all

major sensory modalities should be tested while looking for a pattern

consistent with a spinal level and dermatomal or nerve distribution. In

patients with lesions at or above the brainstem, screening the primary

sensory modalities in the distal extremities along with tests of “cortical”

sensation is usually sufficient.

The five primary sensory modalities—light touch, pain, temperature, vibration, and joint position—are tested in each limb. Light touch

is assessed by stimulating the skin with single, very gentle touches of

the examiner’s finger or a wisp of cotton. Pain is tested using a new pin,

and temperature is assessed using a metal object (e.g., tuning fork) that

has been immersed in cold and warm water. Vibration is tested using

a 128-Hz tuning fork applied to the distal phalanx of the great toe or

index finger just below the nail bed. By placing a finger on the opposite side of the joint being tested, the examiner compares the patient’s

threshold of vibration perception with his or her own. For joint position testing, the examiner grasps the digit or limb laterally and distal

to the joint being assessed; small 1- to 2-mm excursions can usually be

sensed. The Romberg maneuver is primarily a test of proprioception.

The patient is asked to stand with the feet as close together as necessary

to maintain balance while the eyes are open, and the eyes are then

closed. A loss of balance with the eyes closed is an abnormal response.

“Cortical” sensation is mediated by the parietal lobes and represents

an integration of the primary sensory modalities; testing cortical sensation is only meaningful when primary sensation is intact. Double

simultaneous stimulation is especially useful as a screening test for

cortical function; with the patient’s eyes closed, the examiner lightly

touches one or both hands and asks the patient to identify the stimuli.

With a parietal lobe lesion, the patient may be unable to identify the

stimulus on the contralateral side when both hands are touched. Other

modalities relying on the parietal cortex include the discrimination of

two closely placed stimuli as separate (two-point discrimination), identification of an object by touch and manipulation alone (stereognosis),

and the identification of numbers or letters written on the skin surface

(graphesthesia).

■ COORDINATION EXAMINATION • The bare minimum: Observe the patient at rest and during spontaneous movements. Test rapid alternating movements of the hands and

feet and finger to nose.

Coordination refers to the orchestration and fluidity of movements.

Even simple acts require cooperation of agonist and antagonist muscles, maintenance of posture, and complex servomechanisms to control

the rate and range of movements. Part of this integration relies on

normal function of the cerebellar and basal ganglia systems. However,

coordination also requires intact muscle strength and kinesthetic and

proprioceptive information. Thus, if the examination has disclosed

abnormalities of the motor or sensory systems, the patient’s coordination should be assessed with these limitations in mind.

Rapid alternating movements in the upper limbs are tested separately on each side by having the patient make a fist, partially extend

the index finger, and then tap the index finger on the distal thumb

as quickly as possible. In the lower limb, the patient rapidly taps the

foot against the floor or the examiner’s hand. If these rapid alternating

movements are imprecise or vary in amplitude or rhythm, a cerebellar

lesion is suspected; if however they are slow compared with the other

side, a lesion of the pyramidal tract is most likely. Finger-to-nose

testing is primarily a test of cerebellar function; the patient is asked to

touch his or her index finger repetitively to the nose and then to the

examiner’s outstretched finger, which moves with each repetition. A

similar test in the lower extremity is to have the patient raise the leg and

touch the examiner’s finger with the great toe. Another coordination

test in the lower limbs is the heel-knee-shin maneuver; in the supine

position the patient is asked to slide the heel of each foot from the knee

down the shin of the other leg. For all these movements, the accuracy,

speed, and rhythm are noted.

■ GAIT EXAMINATION • The bare minimum: Observe the patient while walking normally, on

the heels and toes, and along a straight line.

Watching the patient walk is the most important part of the neurologic

examination. Normal gait requires that multiple systems—including

strength, sensation, and coordination—function in a highly integrated

fashion. Unexpected abnormalities may be detected that prompt the

examiner to return in more detail to other aspects of the examination.

The patient should be observed while walking and turning normally,

walking on the heels, walking on the toes, and walking heel-to-toe

along a straight line. The examination may reveal decreased arm swing

on one side (corticospinal tract disease), a stooped posture and shortstepped gait (parkinsonism), a broad-based unstable gait (ataxia), scissoring (spasticity), or a high-stepped, slapping gait (posterior column

or peripheral nerve disease), or the patient may appear to be stuck in

place (apraxia with frontal lobe disease).

NEUROLOGIC DIAGNOSIS

The clinical data obtained from the history and examination are interpreted to arrive at an anatomic localization that best explains the clinical findings (Table 422-2), to narrow the list of diagnostic possibilities,

3282 PART 13 Neurologic Disorders

TABLE 422-2 Findings Helpful for Localizations within the Nervous

System

SIGNS

Cerebrum Abnormal mental status or cognitive impairment

Seizures

Unilateral weaknessa

 and sensory abnormalities including

head and limbs

Visual field abnormalities

Movement abnormalities (e.g., diffuse incoordination,

tremor, chorea)

Brainstem Isolated cranial nerve abnormalities (single or multiple)

“Crossed” weaknessa

 and sensory abnormalities of head

and limbs, e.g., weakness of right face and left arm and leg

Spinal cord Back pain or tenderness

Weaknessa

 and sensory abnormalities sparing the head

Mixed upper and lower motor neuron findings

Sensory level

Sphincter dysfunction

Spinal roots Radiating limb pain

Weaknessb

 or sensory abnormalities following root

distribution (see Figs. 25-2 and 25-3)

Loss of reflexes

Peripheral nerve Mid or distal limb pain

Weaknessb

 or sensory abnormalities following nerve

distribution (see Figs. 25-2 and 25-3)

“Stocking or glove” distribution of sensory loss

Loss of reflexes

Neuromuscular

junction

Bilateral weakness including face (ptosis, diplopia,

dysphagia) and proximal limbs

Increasing weakness with exertion

Sparing of sensation

Muscle Bilateral proximal or distal weakness

Sparing of sensation

a

Weakness along with other abnormalities having an “upper motor neuron” pattern,

i.e., spasticity, weakness of extensors > flexors in the upper extremity and flexors >

extensors in the lower extremity, and hyperreflexia.

b

Weakness along with other abnormalities having a “lower motor neuron” pattern,

i.e., flaccidity and hyporeflexia.

Numerous noninvasive imaging options are available to clinicians

evaluating patients with neurologic disorders. These include computed

tomography (CT) and magnetic resonance (MR) imaging (MRI),

plus their variations, including: CT angiography (CTA); perfusion

CT (pCT); dual-energy CT; MR angiography (MRA); MR vessel wall

imaging; functional MRI (fMRI); MR spectroscopy (MRS); MR neurography (MRN); diffusion-weighted MR imaging (DWI); diffusion

tensor MR imaging (DTI); susceptibility-weighted MR imaging (SWI);

arterial spin label imaging (ASL); and perfusion MRI (pMRI). Furthermore, a number of interventional neuroradiologic techniques have

matured including catheter embolization, stent retrieval thrombectomy, aneurysm coiling and stenting, as well as numerous techniques

for spine disorders, including CT myelography, fluoroscopy and

CT-guided spine interventional procedures for pain and oncology,

including radiofrequency and cold ablation and image-guided blood

patches. Multidetector CTA (MDCTA) and gadolinium-enhanced

MRA techniques have reduced the need for catheter-based 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 423-1).

In general, MRI is more sensitive than CT for the detection of

lesions affecting the peripheral and central nervous system (CNS).

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 is also useful in the detection

and characterization of encephalitis, abscess, Creutzfeldt-Jacob disease, cerebral tumors, and acute demyelinating lesions. CT, however, is

acquired more quickly, making it a pragmatic choice for uncooperative

patients, or those with suspected acute stroke, hemorrhage, and acute

intracranial or spinal trauma. CT is also more sensitive than MRI for

visualizing fine osseous detail and thus is appropriate for the initial

imaging evaluation of conductive hearing loss, and lesions affecting the

osseous skull and spine. MR may, however, add important diagnostic

information regarding bone marrow infiltrative processes that can be

difficult to detect on CT.

COMPUTED TOMOGRAPHY

■ TECHNIQUE

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

423 Neuroimaging in

Neurologic Disorders

William P. Dillon

and to select the laboratory tests most likely to be informative. The

laboratory assessment may include (1) serum electrolytes; complete

blood count; and renal, hepatic, endocrine, and immune studies; (2)

cerebrospinal fluid examination; (3) focused neuroimaging studies

(Chap. 423); or (4) electrophysiologic studies. The anatomic localization, mode of onset and course of illness, other medical data, and laboratory findings are then integrated to establish an etiologic diagnosis.

The neurologic examination may be normal even in patients with

a serious neurologic disease, such as seizures, chronic meningitis, or a

TIA. A comatose patient may arrive with no available history, and in

such cases, the approach is as described in Chap. 28. In other patients,

an inadequate history may be overcome by a succession of examinations from which the course of the illness can be inferred. In perplexing cases it is useful to remember that uncommon presentations of

common diseases are more likely than rare etiologies. Thus, even in

tertiary care settings, multiple strokes are usually due to emboli and not

vasculitis, and dementia with myoclonus is usually Alzheimer’s disease

and not a prion disorder or a paraneoplastic illness. Finally, the most

important task of a primary care physician faced with a patient who has

a new neurologic complaint is to assess the urgency of referral to a specialist. Here, the imperative is to rapidly identify patients likely to have

nervous system infections, acute strokes, and spinal cord compression

or other treatable mass lesions and arrange for immediate care.

Acknowledgment

The editors acknowledge the contributions of Joseph B. Martin to earlier

editions of this chapter.

■ FURTHER READING

Brazis P et al: Localization in Clinical Neurology, 7th ed. Philadelphia,

Lippincott William & Wilkins, 2016.

Campbell WW, Barohn RJ: DeJong’s The Neurological Examination,

8th ed. Philadelphia, Lippincott William & Wilkins, 2019.

GBD 2019 Diseases and Injuries Collaborators: Global burden of

369 diseases and injuries in 204 countries and territories, 1990–2019:

A systematic analysis for the Global Burden of Disease Study 2019.

Lancet. 396:1204, 2020.

O’Brien M: Aids to the Examination of the Peripheral Nervous System,

5th ed. Edinburgh, WB Saunders, 2010.

3283 Neuroimaging in Neurologic Disorders CHAPTER 423

be reformatted into various slice thicknesses and planes. Advantages

of MDCT include shorter scan times and thus reduced patient and

organ motion, and the ability to acquire images dynamically during

the infusion of intravenous (IV) contrast, the basis of CTA and CT

perfusion (Figs. 423-1B and C). CTA is displayed in three dimensions to yield angiogram-like images (Figs. 423-1C, 423-2E and F,

and see Fig. 420-3).

IV iodinated contrast is used to identify vascular structures and

to detect defects in the blood-brain barrier (BBB) that are caused by

tumors, infarcts, and infections. In the normal CNS, only vessels and

structures lacking a BBB (e.g., the pituitary gland, choroid plexus,

and dura) enhance after contrast administration. While helpful in

characterizing mass lesions as well as essential for the acquisition of

CTA studies, the decision to use contrast material should always be

considered carefully as it carries a small risk of allergic reaction and

adds additional expense.

■ INDICATIONS

CT is the primary study of choice in the evaluation of an acute change

in mental status, focal neurologic findings, acute trauma to the brain and

spine, suspected subarachnoid hemorrhage, and conductive hearing

loss (Table 423-1). CT often is complementary to MR in the evaluation of the skull base, orbit, and osseous structures of the spine. In the

spine, CT is useful in evaluating patients with osseous spinal stenosis

and spondylosis, but MRI is often preferred in those with neurologic

deficits. CT is often acquired following intrathecal contrast injection to

evaluate for spinal and intracranial cerebrospinal fluid (CSF) fistula, as

well as the spinal subarachnoid space (CT myelography) in failed back

surgery syndromes.

■ COMPLICATIONS

CT is safe, fast, and reliable. Radiation exposure depends on the dose

used but is normally 2–5 mSv (millisievert) for a routine brain CT

study. For all patients, especially children, it is important to use as low a

radiation dose as possible for diagnostic purposes. Where feasible, MR

or ultrasound is preferred. With the advent of MDCT, CTA, and CT

perfusion, the benefit must be weighed against the increased radiation

doses associated with these techniques. Advances in postprocessing

software now permit acceptable diagnostic CT scans at 30–40% lower

radiation doses.

The most frequent complications are those associated with use of

IV contrast agents. While two broad categories of contrast media, ionic

and nonionic, are in use, ionic agents have been largely replaced by

safer nonionic compounds.

Contrast nephropathy is rare. It may result from hemodynamic

changes, renal tubular obstruction and cell damage, or immunologic

reactions to contrast agents. A rise in serum creatinine of at least

44 μmol/L (0.5 mg/dL) within 48 h of contrast administration is often

used as a definition of contrast nephropathy, although there is no

accepted definition and other causes of acute renal failure must be

excluded. The prognosis is usually favorable, with serum creatinine

levels returning to baseline within 1–2 weeks. Risk factors for contrast

nephropathy include age (>80 years), preexisting renal disease (serum

creatinine exceeding 2 mg/dL), solitary kidney, diabetes mellitus, dehydration, paraproteinemia, concurrent use of nephrotoxic medication

or chemotherapeutic agents, and high contrast dose. Patients with

diabetes and those with mild renal failure should be well hydrated prior

to the administration of contrast agents; careful consideration should

be given to alternative imaging techniques such as MRI, noncontrast

CT, or ultrasound (US). Nonionic, low-osmolar media produce fewer

abnormalities in renal blood flow and less endothelial cell damage but

should still be used carefully in patients at risk for allergic reaction.

Estimated glomerular filtration rate (eGFR) is a more reliable indicator

of renal function compared to creatinine alone because it takes into

account age and sex. In one study, 15% of outpatients with a normal

serum creatinine had an estimated creatinine clearance of ≤50 mL/min

per 1.73 m2

 (normal is ≥90 mL/min per 1.73 m2

). The exact eGFR

threshold, below which withholding IV contrast should be considered, is controversial. The risk of contrast nephropathy is minimal in

TABLE 423-1 Guidelines for the Use of CT, Ultrasound, and MRI

CONDITION RECOMMENDED TECHNIQUE

Hemorrhage

Acute parenchymal CT, MR

Subacute/chronic MRI

Subarachnoid hemorrhage CT, CTA, lumbar puncture → angiography

Angiography > CTA, MRA

Chronic subarachnoid blood MR with SWI

Aneurysm

Ischemic infarction

Hemorrhagic infarction CT or MRI

Bland infarction MRI with diffusion > CT, CTA, angiography

Carotid or vertebral dissection MRI/MRA

Vertebral basilar insufficiency CTA, MRI/MRA

Carotid stenosis CTA, MRA > US

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

Trauma

Acute trauma CT

Shear injury/chronic hemorrhage MRI + susceptibility-weighted imaging

Headache/migraine CT/MRI

Seizure

 First time, no focal neurologic

deficits

MRI > CT

 With neurologic deficit, or

immunocompromised or cancer

CT, followed by MR

Partial complex/refractory MRI

Cranial neuropathy MRI with contrast

Meningeal disease MRI with contrast

Spine

Low-back pain

No neurologic deficits MRI or CT after >6 weeks

With focal deficits MRI > CT

Spinal stenosis MRI or CT

Cervical spondylosis MRI, CT, CT myelography

Infection MRI + contrast, CT

Myelopathy MRI + contrast

Arteriovenous malformation MRI + contrast, angiography

Abbreviations: CT, computed tomography; CTA, CT angiography; MRA, magnetic

resonance angiography; MRI, magnetic resonance imaging; SWI, susceptibilityweighted imaging.

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” (whiter) on the scan,

whereas soft-tissue structures that have poor attenuation of x-rays,

such as organs and air-filled cavities, are lower (blacker) 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 0.4-mm in-plane resolution at a speed of 0.3 s

per rotation; complete studies of the brain can be completed in 1–10 s.

Multidetector CT (MDCT) is now standard. Single or multiple (from

4 to 320) solid-state detectors positioned opposite to the x-ray source

result in multiple slices per revolution of the beam around the patient.

In helical mode, the table moves continuously through the rotating

x-ray beam, generating a continuous “helix” of information that can

3284 PART 13 Neurologic Disorders

patients with eGFR >30 mL/min per 1.73 m2

; however, the majority of

these patients will have only a temporary rise in creatinine. The risk of

dialysis after receiving contrast significantly increases in patients with

eGFR <30 mL/min per 1.73 m2

. At the current time, there is very little

evidence that IV iodinated contrast material is an independent risk

factor for acute kidney injury in patients with eGFR ≥30 mL/min per

1.73 m2

. The American College of Radiology suggests, if a threshold for

risk is used at all, an eGFR of 30 mL/min per 1.73 m2

 seems to have the

greatest level of evidence.

If contrast must be administered to a patient with an eGFR <30 mL/min

per 1.73 m2

, the patient should be well hydrated, and a reduction in

the dose of contrast should be considered. Use of other agents such as

bicarbonate and acetylcysteine may reduce the incidence of contrast

nephropathy.

Below are suggested guidelines for creatinine testing prior to contrast administration. If serum creatinine is not available, creatinine

testing should be performed IF the patient has ANY of the following

risk factors:

• Age >60 years

• History of “kidney disease” as an adult, including tumor and transplant

• Family history of kidney failure

• Diabetes mellitus treated with insulin or other prescribed medications

• Hypertension

• Paraproteinemia syndromes or diseases (e.g., myeloma)

• Collagen vascular disease (e.g., systemic lupus erythematosus [SLE],

scleroderma, rheumatoid arthritis)

• Solid-organ transplant recipient

If creatinine testing is required, a creatinine level within the prior

6 weeks is sufficient in most clinical settings.

Allergy Immediate reactions following IV contrast media occur

through several mechanisms. The most severe reactions are related

to allergic hypersensitivity (anaphylaxis) and range from mild hives

to bronchospasm and death. The pathogenesis of allergic hypersensitivity reactions is thought to include the release of mediators such

as histamine, antibody-antigen reactions, and complement activation.

Severe allergic reactions occur in ~0.04% of patients receiving nonionic

media, sixfold lower than with ionic media. Risk factors include a

history of prior contrast reaction (fivefold increased likelihood), food

and or drug allergies, and atopy (asthma and hay fever). The predictive value of specific allergies, such as those to shellfish, once thought

important, actually is now recognized to be unreliable. Nonetheless, in

patients with a history worrisome for potential allergic reaction, a noncontrast CT or MRI procedure should be considered as an alternative

to contrast administration. If iodinated contrast is absolutely required,

a nonionic agent should be used in conjunction with pretreatment

with glucocorticoids and antihistamines (Table 423-2); however, pretreatment does not guarantee safety. Patients with allergic reactions to

iodinated contrast material do not usually react to gadolinium-based

MR contrast material, although such reactions can occur. It would be

wise to pretreat patients with a prior allergic history to MR contrast

administration in a similar fashion. Subacute (>1 h after injection)

reactions are frequent and probably related to T cell–mediated immune

reactions. These are typically urticarial but can occasionally be more

severe. Drug provocation and skin testing may be required to determine both the culprit agent involved and a safe alternative.

Other side effects of CT contrast include a sensation of warmth

throughout the body and a metallic taste during IV administration.

Extravasation of contrast media, although rare, can be painful and lead

to compartment syndrome. When this occurs, immediate consultation with plastic surgery is indicated. Patients with significant cardiac

disease may be at increased risk for contrast reactions, and in these

patients, limits to the volume and osmolality of the contrast media

should be considered. Patients who may undergo systemic radioactive

iodine therapy for thyroid disease or cancer should not receive iodinated contrast media, if possible, because this will decrease the uptake

of the radioisotope into the tumor or thyroid (see the American College

of Radiology Manual on Contrast Media, 2021; https://www.acr.org/-/

media/ACR/Files/Clinical-Resources/Contrast_Media.pdf).

MAGNETIC RESONANCE IMAGING

■ TECHNIQUE

MRI is a complex interaction between hydrogen protons in biologic

tissues, a static magnetic field (the magnet), and energy in the form of

radiofrequency (Rf) waves of a specific frequency introduced by coils

placed next to the body part of interest. Images are made by computerized processing of resonance information received from protons (typically hydrogen) in the body. Field strength of the magnet is directly

related to signal-to-noise ratio. While 1.5 Tesla (T) and 3-T magnets

are now widely available and have distinct advantages in the brain and

musculoskeletal systems, even higher field magnets (7-T) and positron

emission tomography (PET)-MR machines promise increased resolution and anatomic-functional information on a variety of disorders.

Spatial localization is achieved by magnetic gradients surrounding the

main magnet, which impart slight changes in magnetic field throughout the imaging volume. Rf pulses transiently excite the energy state

of the hydrogen protons in the body. Rf is administered at a frequency

specific for the field strength of the magnet. The subsequent return to

equilibrium energy state (relaxation) of the hydrogen protons results

in a release of Rf energy (the echo), which is detected by the coils that

FIGURE 423-1 Computed tomography (CT) angiography (CTA) of ruptured anterior cerebral artery aneurysm in a patient presenting with acute headache. A. Noncontrast

CT demonstrates subarachnoid and intraventricular hemorrhage and mild obstructive hydrocephalus. B. Axial maximum-intensity projection from CTA demonstrates

enlargement of the anterior cerebral artery (arrow). C. Three-dimensional 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- to 1-mm helical CT scans performed during a rapid bolus infusion of

IV contrast medium.

A B C

3285 Neuroimaging in Neurologic Disorders CHAPTER 423

FIGURE 423-2 Acute left hemiparesis due to right middle cerebral artery occlusion. A. Axial noncontrast computed tomography (CT) scan demonstrates high density within

the right middle cerebral artery (arrow) associated with subtle low density involving the right putamen (arrowheads). B. Mean transit time CT perfusion parametric map

indicating prolonged mean transit time involving the right middle cerebral territory (arrows). C. Cerebral blood volume (CBV) map shows reduced CBV involving an area within

the defect shown in B, indicating a high likelihood of infarction (arrows). D. Axial maximum-intensity projection from a CT angiography (CTA) study through the circle of Willis

demonstrates an abrupt occlusion of the proximal right middle cerebral artery (arrow). E. Sagittal reformation through the right internal carotid artery demonstrates a lowdensity lipid-laden plaque (arrowheads) narrowing the lumen (black arrow). F. Three-dimensional surface-rendered CTA image demonstrates calcification and narrowing

of the right internal carotid artery (arrow), consistent with atherosclerotic disease. G. Coronal maximum-intensity projection from magnetic resonance angiography shows

right middle cerebral artery (MCA) occlusion (arrow). H. and I. Axial diffusion-weighted image (H) and apparent diffusion-coefficient image (I) document the presence of a

right middle cerebral artery infarction.

A B C

D E F

G H I

3286 PART 13 Neurologic Disorders

FIGURE 423-3 Cerebral abscess in a patient with fever and a right hemiparesis.

A. Coronal postcontrast T1-weighted image demonstrates a ring-enhancing mass

in the left frontal lobe. B. Axial diffusion-weighted image demonstrates restricted

diffusion (high signal intensity) within the lesion, which in this setting is highly

suggestive of cerebral abscess.

TABLE 423-3 Some Common Intensities on T1- and T2-Weighted MRI

Sequences

IMAGE TR TE

SIGNAL INTENSITY

CSF FAT BRAIN EDEMA

T1W Short Short Low High Low Low

T2W Long Long High High Medium High

FLAIR (T2) Long Long Low High Medium High

Abbreviations: CSF, cerebrospinal fluid; FLAIR, fluid-attenuated inversion recovery;

TE, interval between radiofrequency pulse and signal reception; TR, interval

between radiofrequency pulses; T1W and T2W, T1- and T2-weighted.

TABLE 423-2 Guidelines for Premedication of Patients with Prior

Contrast Allergy

12 h prior to examination:

Prednisone, 50 mg PO or methylprednisolone, 32 mg PO

2 h prior to examination:

 Prednisone, 50 mg PO or methylprednisolone, 32 mg PO and cimetidine,

300 mg PO or ranitidine, 150 mg PO

Immediately prior to examination:

Benadryl, 50 mg IV (alternatively, can be given PO 2 h prior to exam)

delivered the Rf pulses. Fourier analysis is used to transform the echo

into the information used to form an MR image. The MR image thus

consists of a map of the distribution of hydrogen protons, with signal

intensity imparted by both density of hydrogen protons and differences

in the relaxation times (see below) of hydrogen protons on different

molecules. Although clinical MRI currently makes use of the ubiquitous hydrogen proton, sodium and carbon imaging and spectroscopy

are also possible but have yet to be integrated into mainstream practice.

T1 and T2 Relaxation Times The rate of return to equilibrium

of perturbed protons is called the relaxation rate. The relaxation rate

varies among normal and pathologic tissues. The relaxation rate of a

hydrogen proton in a tissue is influenced by local interactions with

surrounding molecules and atomic neighbors. Two relaxation rates, T1

and T2, influence the signal intensity of the image. The T1 relaxation

time is the time, measured in milliseconds, for 63% of the hydrogen

protons to return to their normal equilibrium state, whereas the T2

relaxation is the time for 63% of the protons to become dephased

owing to interactions among nearby protons. The intensity and image

contrast of the signal within various tissues can be modulated by

altering acquisition parameters such as the interval between Rf pulses

(TR) and the time between the Rf pulse and the signal reception (TE).

T1-weighted (T1W) images are produced by keeping the TR and

TE relatively short, whereas using longer TR and TE times produces

T2-weighted (T2W) images. Fat and subacute hemorrhage have relatively shorter T1 relaxation rates and thus higher signal intensity

than brain on T1W images. Structures containing more water, such

as CSF and edema, have long T1 and T2 relaxation rates, resulting

in relatively lower signal intensity on T1W images and higher signal

intensity on T2W images (Table 423-3). Gray matter contains 10–15%

more water than white matter, which accounts for much of the intrinsic contrast between the two on MRI (Fig. 423-4A). T2W images are

more sensitive than T1W images to edema, demyelination, infarction,

and chronic hemorrhage, whereas T1W imaging is more sensitive to

subacute hemorrhage and fat-containing structures.

Many different MR pulse sequences exist, and each can be obtained

in various planes (Figs. 423-2, 423-3, and 423-4). The selection of

a proper protocol that will best answer a clinical question depends

on an accurate clinical history and indication for the examination.

Fluid-attenuated inversion recovery (FLAIR) is a very useful pulse

sequence that produces T2W images in which the normally high

signal intensity of CSF is suppressed (Fig. 423-4B). FLAIR images are

more sensitive than standard spin echo images for water-containing

lesions or edema, especially those close to CSF-filled cisterns and sulci.

Diffusion-weighted imaging is also routinely obtained in most brain

protocols. This sequence interrogates the microscopic motion of water,

which is restricted in areas of infarction, abscess, and some tumors.

SWI is a gradient echo sequence that is very sensitive to alterations in

local magnetic field generated by blood, calcium, and air. SWI is also

now routinely obtained and helps detect microhemorrhages, such as

is typical of amyloid angiopathy, hypertension, hemorrhagic metastases, traumatic brain injury, and thrombotic states (Fig. 423-5C). MR

images can be generated in any plane without changing the patient’s

position. Each sequence, however, is currently obtained separately and

takes 1–10 min on average to complete. Three-dimensional volumetric

imaging is also possible with MRI, resulting in a volume of data that

can be reformatted in any orientation to highlight certain disease processes. Perfusion techniques such as arterial spin labeling also provide

quantitative imaging information regarding cerebral blood flow.

MR Contrast Material The heavy-metal element gadolinium

forms the basis of all currently approved IV MR contrast agents. Gadolinium reduces the T1 and T2 relaxation times of nearby water protons

in the presence of a magnetic field, resulting in a high signal on T1W

images and a low signal on T2W images (the latter requires a sufficient

local concentration, usually in the form of an IV bolus). Unlike iodinated contrast agents, the effect of MR contrast agents depends on the

presence of local hydrogen protons on which it must act to achieve the

desired effect. There are nine different gadolinium agents approved

in the United States for use with MRI. These differ according to the

attached chelated moiety, which also affects the strength of chelation

of the otherwise toxic gadolinium element. The chelating carrier molecule for gadolinium can be classified by whether it is macrocyclic or

has linear geometry and whether it is ionic or nonionic. Macrocyclic

ligands (Group 2 agents) are considered more stable as the gadolinium

A

B