Acute cerebral ischemia in rats studied by Carr-Purcell spin-echo magnetic resonance imaging: assessment of blood oxygenation level-dependent and tissue effects on the transverse relaxation

Tissue characterization using R1rho dispersion imaging at low locking fields

Measurements of the variations of spin-locking relaxation rates (R_1ρ) with locking field amplitude allow the derivation of quantitative parameters that describe different dynamic processes, such as slow molecular motions, chemical exchange and diffusion. In some samples, changes in R_1ρ values between locking frequency 0 and 200 Hz may be dominated mainly by diffusion of water in intrinsic field gradients, while those at higher locking fields are due to exchange processes. The exchange and diffusion effects act independently of each other, as confirmed by simulation and experimentally. In tissues, the relevant intrinsic field gradients may arise from the magnetic inhomogeneities caused by microvascular blood so that R_1ρ dispersion over weak locking field amplitudes (≤ 200 Hz) is affected by changes in capillary density and geometry. Here we first review the theoretical and experimental background to the interpretation of R_1ρ dispersions caused by intrinsic magnetic susceptibility variations within the tissue. We then provide new empirical results of R_1ρ dispersion imaging of the human brain and skeletal muscle at low locking field amplitudes for the first time and identify potential applications of R_1ρ dispersion imaging in clinical studies.

Multiparametric magnetic resonance imaging of acute experimental brain ischaemia

Ischaemia is a condition in which blood flow either drops to zero or proceeds at severely decreased levels that cannot supply sufficient oxidizable substrates to maintain energy metabolism in vivo. Brain, a highly oxidative organ, is particularly susceptible to ischaemia. Ischaemia leads to loss of consciousness in seconds and, if prolonged, permanent tissue damage is inevitable. Ischaemia primarily results in a collapse of cerebral energy state, followed by a series of subtle changes in anaerobic metabolism, ion and water homeostasis that eventually initiate destructive internal and external processes in brain tissue. (31)P and (1)H NMR spectroscopy were initially used to evaluate anaerobic metabolism in brain. However, since the early 1990s (1)H Magnetic Resonance Imaging (MRI), exploiting the nuclear magnetism of tissue water, has become the key method for assessment of ischaemic brain tissue. This article summarises multi-parametric (1)H MRI work that has exploited diffusion, relaxation and magnetisation transfer as 'contrasts' to image ischaemic brain in preclinical models for the first few hours, with a view to assessing evolution of ischaemia and tissue viability in a non-invasive manner.

Neuroprotection afforded by antagonists of endothelin-1 receptors in experimental stroke

Endothelin-1 (ET-1) is involved on the development of cerebral edema in acute ischemic stroke. As edema is a therapeutic target in cerebral ischemia, our aim was to study the effect of antagonists for ET-1 receptors (Clazosentan® and BQ-788, specific antagonists for receptors A and B, respectively) on the development of edema, infarct volume and sensorial-motor deficits in rats subjected to ischemia by occlusion of the middle cerebral artery (MCAO). We used Wistar rats (280-320 g) submitted to ischemia by intraluminal transient (90 min) MCAO. After ischemia, rats were randomized into 4 groups (n = 6) treated with; 1) control group (saline), 2) Clazosentan® group (10 mg/kg iv), 3) BQ-788 group (3 mg/kg iv), and 4) combined treatment (Clazosentan® 10 mg/kg plus BQ-788 3 mg/kg iv). We observed that rats treated with Clazosentan® showed a reduction of edema, measured by MRI, at 72 h (hours) and at day 7 (both p < 0.0001), and a decrease in the serum levels of ET-1 at 72 h (p < 0.0001) and at day 7 (p = 0.009). The combined treatment also induced a reduction of edema at 24 h (p = 0.004), 72 h (p < 0.0001) and at day 7 (p < 0.0001), a reduction on infarct volume, measured by MRI, at 24 and 72 h, and at day 7 (all p < 0.01), and a better sensorimotor recovery at 24 and 72 h, and at day 7 (all p < 0.01). Moreover, Clazosentan® induced a decrease in AQP4 expression, while BQ-788 induced an increase in AQP9 expression. These results suggest that antagonists for ET-1 receptors may be a good therapeutic target for cerebral ischemia.

Probing ischemic tissue fate with BOLD fMRI of brief oxygen challenge

It has been recently shown that at-risk tissue exhibits exaggerated T(2)⁎-weighted MRI signal increases during transient oxygen challenge (OC), suggesting that the tissue is still metabolically active. This study further characterized the effects of transient OC on T(2)⁎-weighted MRI in permanent focal stroke rats (N=8) using additional quantitative measures. The major findings were: i) the ischemic core cluster showed no significant response, whereas the mismatch cluster showed markedly higher percent changes relative to normal tissue in the acute phase. ii) Many of the mismatch pixels showed exaggerated OC responses which became hyperintense on T(2)-weighted MRI at 24h. The area with exaggerated OC responses was larger than the mismatch, suggesting that some tissue with reduced diffusion were potentially at risk. iii) Basal T(2)⁎-weighted intensities on the perfusion-diffusion contourplot were high in normal tissue and low in the core, with a sharp transition in the mismatch. iv) OC-induced changes on the perfusion-diffusion contourplot dropped as perfusion and diffusion values fell below their respective viability thresholds. v) Basal T(1) increased slightly in the ischemic core (P<0.05). OC decreased T(1) in normal (P<0.05) but not in mismatch and core pixels. vi) OC decreased CBF in normal (P<0.05) but not in mismatch and core pixels. T(2)⁎-weighted MRI of OC has the potential to offer unique clinically relevant data.

Spatiotemporal characteristics of postischemic hyperperfusion with respect to changes in T1, T2, diffusion, angiography, and blood-brain barrier permeability

The spatiotemporal dynamics of postischemic hyperperfusion (HP) remains incompletely understood. Diffusion, perfusion, T2, T1, angiographic, dynamic susceptibility-contrast magnetic resonance imaging (MRI) and magnetic resonance angiography were acquired longitudinally at multiple time points up to 7 days after stroke in rats subjected to 30-, 60-, and 90-minutes middle cerebral artery occlusion (MCAO). The spatiotemporal dynamics of postischemic HP was analyzed and compared with T1, T2 and blood-brain barrier (BBB) changes. No early HP within 3 hours after recanalization was observed. Late (12 hours) HP was present in all animals of the 30-minute MCAO group (N=20), half of the animals in the 60-minute MCAO group (N=8), and absent in the 90-minute MCAO group (N=9). Dynamic susceptibility-contrast MRI and magnetic resonance angiography corroborated HP. Hyperperfusion preceded T2 increase in some animals, but HP and T2 changes temporally coincided in others. T2 peaked first at 24 hours whereas HP peaked at 48 hours after occlusion, and HP resolved by day 7 in most animals at which point the arteries became tortuous. Pixel-by-pixel tracking analysis showed that tissue did not infarct (migrated from core or mismatch at 30 minutes to normal at 48 hours) showed normal cerebral blood flow (CBF), whereas infarct tissue (migrated from core or mismatch at 30 minutes to infarct at 48 hours) showed exaggerated CBF, indicating that HP was associated with poor outcome.

Comparison of T2 and LASER T2dagger image contrast in a rat model of acute cerebral ischemia

Previous studies have shown that T2(dagger)-weighted magnetic resonance images acquired using localization by adiabatic selective refocusing (LASER) can provide early tissue contrast following ischemia, possibly due to alterations in microscopic susceptibility within the tissue. The purpose of this study was to make a direct in vivo comparison of T2-, T2(dagger)- and diffusion-weighted image contrast during acute ischemia. Acute middle cerebral artery (MCA) occlusion was attempted in 14 rats using a modified Tamura approach incorporating electrocoagulation of the left MCA. T2(dagger)-weighted LASER images (Echo Time [TE]=108 ms), T2-weighted Carr-Purcell-Meiboom-Gill (CPMG) images (TE=110 ms) and diffusion-weighted images (b value=105 s/mm(2)) were acquired at 4 T within 1.5 h of ischemia onset. Tissue contrast in the MCA territory was quantified for histologically verified ischemic tissue (n=6) and in sham controls (n=4). T2(dagger)-weighted LASER images demonstrated greater contrast compared to the T2-weighted CPMG images, and more focal contrast compared to the diffusion-weighted images, suggesting different contrast mechanisms were involved.

A comparison between blood oxygenation level-dependent and cerebral blood volume contrast in the rat cerebral and cerebellar somatosensoric cortex during electrical paw stimulation

PURPOSE

To implement and optimize cerebral blood volume (CBV)-weighted functional magnetic resonance imaging (fMRI) in the rat cerebral and cerebellar cortex during electrical paw stimulation.

MATERIALS AND METHODS

fMRI of the cerebral and cerebellar cortex was performed during electrical paw stimulation on a 7-T MRI system (MRRS, Guilford, UK) comparing the blood oxygenation level-dependent (BOLD) and CBV-weighted contrast with different ultrasmall particles of iron oxide (USPIO) contrast doses (NC100150, 30 mg Fe/mL; Amersham Health, Oslo, Norway) and different TE.

RESULTS

Doses of 15 and 20 mg/kg USPIO at TE = T*2 or TE = 14 msec almost doubled the contrast-to-noise ratio (CNR) of the activated areas in the cerebral cortex without affecting the overall signal-to-noise ratio (SNR) or the incidence of activation (100%). In the cerebellum the SNR decreased significantly with an increasing contrast dose. At a dose of 15 mg/kg, the CNR was slightly smaller than the CNR measured in the BOLD images, but the activation incidence seemed to be doubled. At 20 mg/kg, the CNR was slightly increased, but the activation incidence was lower. At both contrast doses the venous artifacts disappeared.

CONCLUSION

A USPIO contrast dose of 20 mg/kg proved to be beneficial for fMRI in the rat, even though it affected the CNR and SNR in the cerebral and the cerebellar cortex differentially.

Temporal profile of T2-weighted MRI distinguishes between pannecrosis and selective neuronal death after transient focal cerebral ischemia in the rat

Transient middle cerebral artery occlusion (MCAO) by an intraluminal thread leads to primarily subcortical infarctions with little sensorimotor impairment in the Wistar rat strain. We investigated the course of infarct development in this lesion type for 10 weeks using magnetic resonance imaging (MRI) along with histological characterization. MCAO was induced in male Wistar rats (260 to 300 g) for 60 mins. Animals received follow-up T1- and T2-weighted MRI from day 1 until week 10. Separate groups of animals were analyzed histologically after 2, 6, and 10 weeks. Histology included immunohistochemistry for neuronal and astrocytic markers as well as hematoxylin eosin and luxol fast blue-cresyl violet staining. In contrast to lesions involving the cortex, exclusively subcortical infarctions were characterized by a complete resolution of initially increased T1 and T2 relaxation times by 10 weeks. Between 2 and 10 weeks, neuronal death and gliosis as well as a dense inflammatory infiltrate were evident in these lesions, without damage to fiber tracts or development of cystic cavities. Exclusively subcortical lesions in Wistar rats are characterized by normalization of T1 and T2 relaxation times, which might, however, not be mistaken for tissue recovery. Despite this MRI normalization, selective neuronal death and gliosis develop. Although MRI at individual time points might therefore be ambiguous, the temporal profile of relaxation time changes over the chronic time period allows discrimination of the lesion development into selective neuronal death or pannecrosis.

Exchange-influencedT2ρcontrast in human brain images measured with adiabatic radio frequency pulses

Transverse relaxation in the rotating frame (T(2rho)) is the dominant relaxation mechanism during an adiabatic Carr-Purcell (CP) spin-echo pulse sequence when no delays are used between pulses in the CP train. The exchange-induced and dipolar interaction contributions (T(2rho,ex) and T(2rho,dd)) depend on the modulation functions of the adiabatic pulses used. In this work adiabatic pulses having different modulation functions were utilized to generate T(2rho) contrast in images of the human occipital lobe at magnetic field of 4 T. T(2rho) time constants were measured using an adiabatic CP pulse sequence followed by an imaging readout. For these measurements, adiabatic full passage pulses of the hyperbolic secant HSn (n = 1 or 4) family having significantly different amplitude-and frequency-modulation functions were used with no time delays between pulses. A dynamic averaging (DA) mechanism (e.g., chemical exchange and diffusion in the locally different magnetic susceptibilities) alone was insufficient to fully describe differences in brain tissue water proton T(2rho) time constants. Measurements of the apparent relaxation time constants (T(2) (dagger)) of brain tissue water as a function of the time between centers of pulses (tau(cp)) at 4 and 7 T permitted separation of the DA contribution from that of dipolar relaxation. The methods presented assess T(2rho) relaxation influenced by DA in tissue and provide a means to generate T(2rho) contrast in MRI.