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Advanced soft-

ware that permits noise reduction may permit lower

radiation doses.

The most frequent complications are

associated with use of intravenous contrast agents.

Two

broad categories of contrast media, ionic and non-

ionic, are in use.

As a result, ionic

agents have been largely replaced by safer nonionic

compounds.

A.

Noncontrast CT demonstrates subarachnoid hemorrhage

and mild obstructive hydrocephalus.

B.

C.

A.

B.

C.

D.

E.

G.

H and I.

Field strength of the

magnet is directly related to signal-to-noise ratio.

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 mag-

net.

The echo is transformed

by Fourier analysis into the information used to form

an MR image.

The relaxation rate varies among

normal and pathologic tissues.

Two relaxation rates, T1 and T2, influence the

signal intensity of the image.

So-called

T1-weighted (T1W) images are produced by keeping

the TR and TE relatively short.

T2-weighted (T2W)

images are produced by using longer TR and TE times.

4-6B).

Many different MR pulse sequences exist, and each

can be obtained in various planes (Figs.

4-2, 4-3, 4-4).

4-6B).

4-5C).

MR images

can be generated in any plane without changing the

patient’s position.

Each sequence, however, must be

obtained separately and takes 1–10 min on average to

complete.

FIGURE 4-3

Cerebral abscess in a patient with fever and a right

hemiparesis.

A.

B.

Approximately 0.2 mL/kg body weight

is administered intravenously; the cost is ∼$60 per

dose.

4-3A) and areas of the brain that

normally are devoid of the BBB (pituitary, choroid

plexus).

Renal failure does not occur.

A and B.

C.

The suspected diagnosis of herpes sim-

plex encephalitis was confirmed by CSF PCR analysis.

Renal disease (including solitary kidney, renal trans-

plant, renal tumor)

2.

Age >60 years

3.

History of hypertension

4.

History of diabetes

5.

The incidence of NSF in patients with severe renal

dysfunction (GFR <30) varies from 0.19 to 4%.

A.

Noncontrast CT scan shows

one hyperdense lesion in the right hemisphere (arrow).

B.

T2-weighted fast spin echo image shows subtle low-intensity

lesions (arrows).

C.

Caution is advised for patients

with a GFR below 45.

The patient lies on a table

A B

C

that is moved into a long, narrow gap within the mag-

net.

Approximately 5% of the population experiences

severe claustrophobia in the MR environment.

This can

be reduced by mild sedation but remains a problem for

some.