Neuroimaging Assessment of Subarachnoid Hemorrhage - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 422

Fig. 2. ( a ) CT arrow indicating blood in the ventricles from SAH. ( b ) SW image ( c ) SWI phase image. Both SWI images clearly
depict the blood in the ventricle much more clearly and more completely than the CT. Images courtesy of Zhen Wu.

from decreasing accuracy with time. Accuracies as high as 95%
within the fi rst 24 h have been reported then falling to 90% after
1 day, 74-85% after 3-5 days, 50% after 1 week, 3% after 2 weeks,
and almost 0% after 3 weeks ( 5, 6 ). Other features/issues of CT
imaging include the decreased ability to visualize small bleeds
within the brain and in patients with anemic blood this ability to
visualize the intracranial blood further decreases. Clinically, many
physicians are likely to order a lumbar puncture if a CT scan is
negative to defi nitively rule out SAH. Finally, rare instances of
infl ammatory and tumor manifestations can result in false diagnosis
of SAH ( 1 ). However, new CT technologies, such as multidetector
CT confi gurations, may reduce some of these false positives. There
have been few CT studies in animal models of SAH.

1.2. PD, T1-, T2-, and
T2*-Weighted Imaging

One advantage of MRI is that there are numerous types of contrast
(e.g., T1, T2, diffusion, spectroscopy, etc.) that are available to
assist in the diagnosis of SAH. Standard T1- and T2-weighted
images typically have very poor SAH contrast ( 7 ); however, other
MR sequences can be very sensitive and specifi c for SAH ( 8 ).
Proton density MRI was found to have similar fi ndings in patients
to that on CT at early time points (<6 or <12 h post-SAH). T1 was
also reported to change but with less accuracy and only at >12 h
( 9, 10 ). Traditional T2-weighted imaging has also been useful for
the evaluation of SAH but primarily within the context of monitor-
ing edema, although if the region of extravasated blood is suffi -
ciently large, T2 can visualize these changes.
T2*-weighted imaging (T2*WI) can be reliably used to iden-
tify regions containing blood products. T2* images are also very
sensitive to SAH, having 94% sensitivity in the acute stage (<4 days)
and 100% sensitivity in the subacute sage (>4 days). This is compa-
rable to CT in the acute, and superior in the subacute periods ( 7 ).
The iron in deoxygenated hemoglobin and blood breakdown
products cause strong T2* shortening which gives excellent SAH