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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. |