Neuroimaging Assessment of Subarachnoid Hemorrhage - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 423

contrast. However, T2* images are susceptible to artifacts near
air-tissue interfaces and may obscure subarachnoid regions and
prevent detection of SAH.

1.3. Diffusion-
Weighted Imaging

Diffusion-weighted imaging (DWI) is often used in stroke as the
neuroimaging modality of choice due to its sensitivity to ongoing
tissue injury that is refl ected by decreased proton diffusion. However,
DWI and its quantitative parameter, the apparent diffusion coeffi -
cient (ADC), have not been routinely used in clinical or basic
research with regard to SAH. However, several clinical reports have
reported that use of DWI (or ADC) is useful for identifying isch-
emic tissue regions as a result of the SAH ( 9, 11-15 ). In virtually all
of these clinical reports, the authors demonstrate that the DWI
accurately reports the ischemic injury near the location of the SAH
injury or in proximity to vessels where the SAH injury occurred.
Weidauer et al. ( 12 ) reported cortical banding infarcts after
SAH that were visualized by DWI. Similarly, Sato et al. ( 13 )
demonstrated that aggressive clinical management of patients that
showed DWI or ADC abnormalities, irrespective of the relative
ADC (ratios) change, resulted in better outcomes for the patients.
Other studies have also reported the value of ADC in SAH. Liu
et al. ( 16 ) showed that ADC, but not T2-weighted imaging nor
DWI, could detect broad changes in frontal lobe white matter that
resolved over the course of 1 year.
Animal studies of SAH and DWI have been limited. Moseley
and colleagues ( 17, 18 ) in a series of reports detailed the use of
DWI to monitor the very early (within minutes) DWI and ADC
changes following induction of SAH. They reported that shortly
after SAH induction there was a pattern of diffusional changes
within the brain that appeared to refl ect the initial acute vasospasm
that then appeared to initiate spreading depression, a wave of
electrical depolarization, within the brain tissues. Others have
reported use of DWI and have showed changes on DWI but no
signifi cant conclusions were reached ( 19 ).

Fluid-attenuated inversion recovery imaging (FLAIR) is an excel-
lent MR sequence as it suppresses all fl uid signals within the brain
allowing improved visualization of SAH in the absence of cere-
brospinal fl uid (CSF) or edema (Figs. 1 and 2 ). FLAIR is a
T2-weighted sequence that has an inversion recovery pulse added
to null the signal from CSF, which is normally very bright on T2
images. SAH causes a hyperintense signal on T2-weighted images.
In a standard T2 image this hyperintensity would be hidden by
the bright CSF, thus, nulling the signal from CSF allows SAH to
be clearly visualized.
FLAIR was the fi rst MRI sequence to reliably detect acute SAH
( 20 ). Since then FLAIR has been shown to detect SAH in many
studies, particularly those comparing it to CT. In clinical settings,

1.4. Fluid-Attenuated
Inversion Recovery
Imaging