ADC characterization of region‐specific response to cerebral perfusion deficit in rats by MRI at 9.4 T

Abstract

Region-specific cerebral blood flow (CBF) and apparent diffusion coefficient (ADC) of water in the rat brain were quantified in vivo by high-field MRI (9.4 T) for 6 -7 h after middle cerebral artery occlusion (MCAO). Upon occlusion, average CBF fell from about 1.5-2 ml/g/min to below 0.5 ml/g/min in cortical areas and the amygdala, and below 0. The apparent diffusion coefficient (ADC) of water, which is measurable noninvasively by MRI, has been proposed as a reliable parameter for early detection of brain ischemia (1-3). ADC was shown to be sensitive to acute ischemia within minutes of occurrence (4). Currently, the intraluminal thread model of middle cerebral artery occlusion (MCAO) in rats (5-7) is generally accepted as a standard model of focal cerebral ischemia. In this model, cerebral blood flow (CBF) measurements are usually carried out to determine adequate occlusion (in both permanent and transient MCAO models) and reperfusion (in the transient model) (8 -14). When describing quantitative changes in cerebral perfusion and ADC values, data are usually reported as averages over 1) very small local regions (of several pixels in size) inside the involved brain structures (9,11), or 2) the whole lesion region, with reference to the symmetric contralateral region or to the whole contralateral hemisphere (12,15).In perfusion and ADC studies with the MCAO model in rats, two issues deserve further elaboration. First, the detailed regional dynamics of ADC and CBF, averaged over specific macroscopic brain regions, remains for the most part unexplored. Second, variability in experimental approaches and animals used in the studies results in a certain diversity of experimental findings. Even more important is that the definition of occlusion based on perfusion measurements is not always sufficiently rigorous, and may lead to different cellular responses in the occluded area. To characterize ischemic damage more quantitatively, stringent criteria are needed. An ADC-based definition of occlusion appears to be more adequate, since it is associated with cellular response to ischemia. This feature also makes ADC a suitable candidate for clinical use.CBF measurements are usually performed by laser-Doppler flowmetry (8,14,16) or perfusion-weighted magnetic resonance imaging (MRI). Qualitative perfusion-weighted images may be produced using paramagnetic contrast agents and echo-planar imaging (EPI) (bolus-tracking), and relative CBF indexes may be obtained (11-13). An alternative technique has been developed that uses arterial spin tagging and thus does not need any exogenous contrast agents (17). The arterial spin tagging technique was shown to be suitable for noninvasive quantitative measurements of CBF (17)(18)(19). MRI at high magnetic field (7 T and above) ensures better sensitivity in perfusion measurements (20). The variability in magnetic fields, however, may contribute to lower consistency in results obtained, especially when the difference in magnetic relaxation times is concerned.Variability in animal responses to...

Differences between arterial occlusive and cortical photothrombosis stroke models with magnetic resonance imaging and microtubule-associated protein-2 immunoreactivity

“…Our values for CBF in CC and CB in the PT group were, however, not only higher than the corresponding values in the MCAT group but also higher compared with CBF in the nonischemic cortex from other MCAO studies [11,19,36]. This could be ascribed to the known phenomenon of mild CBF suppression in the homotopic contralateral cortex in the MCAO model due to presumed transcallosal diaschisis [37], as documented earlier [20].…”

“…Each model has been characterized using conventional histochemical and physiological techniques [11][12][13][14][15]. MR has been used to evaluate the cortical PT model [6,7,9,10,16] but not with respect to cerebral blood flow (CBF) or T1, whereas the MCAO model has been investigated more thoroughly with MR [17][18][19][20][21]. To our knowledge, except for the study of a particular diffusion property (dapparent restrictionT) in three different models of cerebral infarction [22], where the PT model was studied only from 24 h postinsult, and another study assessing the blood-brain barrier (BBB) disruption using contrast agents [23], no direct MR comparisons of MCAO and PT models have been published.…”

Faster Recovery of Cerebral Perfusion in SOD1-Overexpressed Rats After Cardiac Arrest and Resuscitation

“…The established methods, as detailed previously,9, 13–16 were used with the following parameters: imaging plane at the dorsal hippocampal level (plane 29 to 32 in Sawnson’s rat brain atlas17), plane offset for the arterial spin tagging = ± 1.4 cm, duration of tagging pulse = 0.8 s, recycle delay (TR) = 1 s, field-of-view (FOV) = 44 × 44 mm 2 , data matrix = 128 × 64, and acquisition time = 5 min for each pair of perfusion images. ADC maps were derived from pixel-by-pixel exponential fitting of five spin-echo images acquired with a pair of 5-ms Stejskal-Tanner18 diffusion-weighting gradients, applied in the readout direction on both sides of the 180° refocusing pulse to produce diffusion-weighting b factors of 0, 20, 81, 182, and 324 s/mm 2 .…”

Predictors of Apparent Diffusion Coefficient Normalization in Stroke Patients

“…The caudate putamen revealed a more pronounced ADC decrease (and tissue damage) than the substantia nigra and thalamus 25 or the cortex. 26 Although these reports argue against the recovery of deep brain structures, a recent study reported on ADC normalizations in subcortical gray matter ischemia. 27 We believe that our observation can be explained by the complex flow patterns in the basal ganglia.…”