Temporal and regional changes during focal ischemia in rat brain studied by proton spectroscopic imaging and quantitative diffusion NMR imaging

The early development of focal ischemia after permanent occlusion of the right middle cerebral artery (MCA) was studied in six rats using interleaved measurements by diffusion-weighted NMR imaging (DWI) of water and two variants of proton spectroscopic imaging (SI), multiecho SI (TE: 136, 272, 408 ms) and short TE SI (TE: 20 ms). Measurements on a 4.7-T NMR imaging system were performed between the control phase and approximately 6 h postocclusion. In the center of the ischemic lesion of all rats, the apparent diffusion coefficient (ADC) decreased rapidly to 84.4 ± 4.2% (mean ± SD) of the control values approximately 2 min postocclusion. Approximately 6 h postocclusion, the ADC was reduced to 67.1 ± 5.9%. In contrast, large differences between the animals were observed for the temporal increase of lactate (Lac) in the ipsilateral hemisphere. The maximum Lac signal was reached in four rats after 0.5-1.5 h, and in two rats was not reached even after 6 h postocclusion. Six h postocclusion, SI spectra measured at a TE of 136 ms revealed a decrease in the CH³ signal of N-acetylaspartate (NAA) to 67 ± 13% of the control values. Differences were observed between the spatial regions of decreased NAA and increased Lac. In the lesions, a T2 relaxation time of Lac of 292 ± 40 ms, considering a J-cou-pling constant of 6.9 Hz, was measured. Furthermore, a prolongation of the T₂ of the CH₃ signal of creatine/phosphocre-atine (Cr/PCr) was observed in the lesion, from 163 ± 22 ms during control to 211 ± 41 ms approximately 6 h postocclusion. The experiments proved that DWI and proton SI are valuable tools to provide complementary information on processes associated with brain infarcts.     

Interleukin-1 exacerbates focal cerebral ischemia and reduces ischemic brain temperature in the rat.

The proinflammatory cytokine interleukin-1 (IL-1) is a key mediator of inflammation in cerebral ischemia, but its precise mechanisms of action remain elusive. Temperature is critical to outcome in brain injury and given the importance of IL-1 in pyrogenesis this has clear mechanistic implications. IL-1 exacerbates ischemia independently of core (rectal) temperature. However, it is temperature in the ischemic brain that influences outcome and rectal temperature is likely to be a poor surrogate marker. This study tested the hypothesis that IL-1 exacerbates cerebral ischemia by increasing ischemic brain temperature. Wistar rats undergoing transient middle cerebral artery occlusion received either 4 microg/kg IL-1 (n=9) or vehicle (n=10) intraperitoneally. NMR-generated maps of brain temperature, tissue perfusion, and the trace of the diffusion tensor were collected during occlusion, early reperfusion, and at 24 hr. IL-1 significantly increased ischemic damage at 24 hr by 35% but rectal temperature did not vary significantly between groups. However, ischemic brain was 1.7 degrees C cooler on reperfusion in IL-1-treated animals (vs. vehicle) and a corresponding reduction in cerebral blood flow was identified in the ischemic striatum. Contrary to the stated hypothesis, IL-1 reduced ischemic brain temperature during reperfusion and this may be due to a reduction in tissue perfusion.     

Three-dimensional 31P magnetic resonance spectroscopic imaging of regional high-energy phosphate metabolism in injured rat heart

The purpose of this study was to measure the spatially varying ³¹P MR signals in global and regional ischemic injury in the isolated, perfused rat heart. Chronic myocardial infarcts were induced by occluding the left anterior descending coronary artery eight weeks before the MR examination. The effects of acute global low-flow ischemia were observed by reducing the perfusate flow. Chemical shift imaging (CSI) with three spatial dimensions was used to obtain ³¹P spectra in 54-/μl voxels. Multislice ¹H imaging with magnetization transfer contrast enhancement provided anatomical information. In normal hearts (n = 8), a homogeneous distribution of high-energy phosphate metabolites (HEP) was found. In chronic myocardial infarction (n = 6), scar tissue contained negligible amounts of HEP, but their distribution in residual myocardium was uniform. The size of the infarcted area could be measured from the metabolic images; the correlation of infarct sizes determined by histology and ³¹P MR CSI was excellent (P < 0.006). In global low-flow ischemia (n = 8), changes of HEP showed substantial regional heterogeneity. Three-dimensional ³¹P MR CSI should yield new insights into the regionally distinct metabolic consequences of various forms of myocardial injury.     

Diffusion of metabolites in normal and ischemic rat brain measured by localized 1H MRS

The apparent diffusion coefficient (ADC) of choline-containing compounds (Cho), creatine and phosphocreatine (Cre), N-acetyl-aspartate (NAA), lactate, and water was measured in normal rat brain, and in the ischemic and contralateral region of rat brain approximately 3 and 24 h after induction of focal cerebral ischemia. After 3 h of ischemia, the ADC of Cre and NAA in the ischemic region had significantly decreased by 29% and 19%, respectively (P < 0.05). Lactate ADC was also obtained in the ischemic region. After 24 h of focal ischemia, no ADC values could be measured for NAA, Cre and Cho in the ischemic region because their concentrations had become too low. The ADCs of lactate and water in the ischemic volume were virtually identical at 3 and 24 h after occlusion. The experiments suggest that the ADC decrease of water after induction of ischemia is partly caused by changes in the diffusion characteristics of the intracellular compartment.     

In vivo 1H MR spectroscopic imaging and difusion weighted mri in experimental hydrocephalus

The severity and progression of ventricular enlargement, the occurrence of cerebral edema, and the localization of isch-emic metabolic changes were investigated in a rat model of hydrocephalus, using in vivo ¹H MR spectroscopic imaging (SI) and diffusion weighted MRI (DW MRI). Hydrocephalic rats were studied 1, 2, 4, and 8 weeks after injection of kaolin into the cisterna magna. Parametric images of the apparent diffusion coefficient (ADC) revealed a varying degree of ventricu-lomegaly in all rats, with different time courses of ventricular explansion. Extracellular white matter edema was observed during the early stages of hydrocephalus, most extensively in cases of progressive ventriculomegaly. In gray matter regions, ADC values were not changed, compared with controls. In case of fatal hydrocephalus, high lactate levels were observed throughout the whole brain. In all other rats, at all time points after kaolin injection, lactate was detected only in voxels containing cerebrospinal fluid. This suggests accumulation of lactate in the ventricles, and/or an ongoing peri ventricular production of lactate as a consequence of cerebral ischemia in experimental hydrocephalus.     

Quantitative proton spectroscopic imaging of the neurochemical profile in rat brain with microliter resolution at ultra-short echo times.

Proton spectroscopy allows the simultaneous quantification of a high number of metabolite concentrations termed the neurochemical profile. The spin echo full intensity acquired localization (SPECIAL) scheme with an echo time of 2.7 ms was used at 9.4T for excitation of a slab parallel to a home-built quadrature surface coil in conjunction with phase encoding in the two remaining spatial dimensions to yield an effective spatial resolution of 1.7 microL. The absolute concentrations of at least 10 metabolites were calculated from the spectra of individual voxels using LCModel analysis. The calculated concentrations were used for constructing quantitative metabolic maps of the neurochemical profile in normal and pathological rat brain. Summation of individual spectra was used to assess the neurochemical profile of unique brain regions, such as corpus callosum, in rat for the first time. Following focal ischemia in rat pups, imaging the neurochemical profile indicated increased choline groups in the ischemic core and increased glutamine in the penumbra, which is proposed to reflect glutamate excitotoxicity. We conclude that it is feasible to achieve a sensitivity that is sufficient for quantitative mapping of the neurochemical profile at microliter spatial resolution.     

31P-{1H} echo-planar spectroscopic imaging of the human brain in vivo.

Echo-planar spectroscopic imaging (EPSI) is one of the fastest spectroscopic imaging (SI) methods. It has been applied to (1)H MR spectroscopy (MRS) studies of the human brain in vivo. However, to our knowledge, EPSI with detection of the (31)P nucleus to monitor phosphorus-containing neurometabolites has not yet been considered. In this work, eight different (31)P-{(1)H} EPSI sequence versions with spectral widths ranging from 313 Hz to 2.27 kHz were implemented on a clinical 1.5T whole-body MR tomograph. The sequence versions utilized the heteronuclear nuclear Overhauser effect (NOE) for (31)P signal enhancement. The sensitivity observed in experiments with model solutions was in good agreement with theoretical predictions. In vivo measurements performed on healthy volunteers (N = 16) demonstrated the feasibility of performing two-dimensional (2D) (31)P-{(1)H} EPSI in the human brain, and the technique enabled fast acquisition of well-resolved localized spectra.     

Out-and-in spiral spectroscopic imaging in rat brain at 7 T.

With standard spectroscopic imaging, high spatial resolution is achieved at the price of a large number of phase-encoding steps, leading to long acquisition times. Fast spatial encoding methods reduce the minimum total acquisition time. In this article, a k-space scanning scheme using a continuous series of growing and shrinking, or "out-and-in," spiral trajectories is implemented and the feasibility of spiral spectroscopic imaging for animal models at high B(0) field is demonstrated. This method was applied to rat brain at 7 T. With a voxel size of about 8.7 microl (as calculated from the point-spread function), a 30 x 30 matrix, and a spectral bandwidth of 11 kHz, the minimum scan time was 9 min 20 sec for a signal-to-noise ratio of 7.1 measured on the N-acetylaspartate peak.     

2D 1H spectroscopic imaging of the human brain at 4.1 T

A two-dimensional spectroscopic imaging sequence consisting of an inversion recovery pulse, a plane selective prefocused pulse, and a semiselective water suppression pulse has been used to create ¹H spectroscopic images of the human brain with nominal voxels of 0.5 cc. Due to the excellent lipid suppression provided by the inversion recovery pulse and subsequent delay, only planar volume selection is required enabling the entire brain within the slice to be imaged without contamination from extracerebral lipids in the brain voxels. The use of a semiselective refocusing pulse for water suppression permits any echo evolution time to be used, minimizing J-modulation and T₂ losses, while retaining full sensitivity in the lactate resonance. Using this sequence we have visualized the lactate elevation in the peri-infarct region about a 6-week-old stroke.     

Delayed changes in T1-weighted signal intensity in a rat model of 15-minute transient focal ischemia studied by magnetic resonance imaging/spectroscopy and synchrotron radiation X-ray fluorescence.

Previous studies have found that rats subjected to 15-min transient middle cerebral artery occlusion (MCAO) show neurodegeneration in the dorsolateral striatum only, and the resulting striatal lesion is associated with increased T1-weighted (T1W) signal intensity (SI) and decreased T2-weighted (T2W) SI at 2-8 weeks after the initial ischemia. It has been shown that the delayed increase in T1W SI in the ischemic region is associated with deposition of paramagnetic manganese ions. However, it has been suggested that other mechanisms, such as tissue calcification and lipid accumulation, also contribute to the relaxation time changes. To clarify this issue, we measured changes in relaxation times, lipid accumulation, and elemental distributions in the brain of rats subjected to 15-min MCAO using MRI, in vivo 1H MR spectroscopy (MRS), and synchrotron radiation X-ray fluorescence (SRXRF). The results show that a delayed (2 weeks after ischemia) increase in T1W SI in the ischemic striatum is associated with significant increases in manganese, calcium, and iron, but without evident accumulation of MRS-visible lipids or hydroxyapatite precipitation. It was also found that 15-min MCAO results in acutely reduced N-acetylaspartate (NAA)/creatine (Cr) ratio in the ipsilateral striatum, which recovers to the control level at 2 weeks after ischemia.     

1H spectroscopic imaging of human brain at 3 Tesla: Comparison of fast three‐dimensional magnetic resonance spectroscopic imaging techniques

Purpose

To investigate the signal‐to‐noise‐ratio (SNR) and data quality of time‐reduced three‐dimensional (3D) proton magnetic resonance spectroscopic imaging (¹H MRSI) techniques in the human brain at 3 Tesla.

Materials and Methods

Techniques that were investigated included ellipsoidal k‐space sampling, parallel imaging, and echo‐planar spectroscopic imaging (EPSI). The SNR values for N‐acetyl aspartate, choline, creatine, and lactate or lipid peaks were compared after correcting for effective spatial resolution and acquisition time in a phantom and in the brains of human volunteers. Other factors considered were linewidths, metabolite ratios, partial volume effects, and subcutaneous lipid contamination.

Results

In volunteers, the median normalized SNR for parallel imaging data decreased by 34–42%, but could be significantly improved using regularization. The normalized signal to noise loss in flyback EPSI data was 11–18%. The effective spatial resolutions of the traditional, ellipsoidal, sensitivity encoding (SENSE) sampling scheme, and EPSI data were 1.02, 2.43, 1.03, and 1.01 cm³, respectively. As expected, lipid contamination was variable between subjects but was highest for the SENSE data. Patient data obtained using the flyback EPSI method were of excellent quality.

Conclusion

Data from all ¹H 3D‐MRSI techniques were qualitatively acceptable, based upon SNR, linewidths, and metabolite ratios. The larger field of view obtained with the EPSI methods showed negligible lipid aliasing with acceptable SNR values in less than 9.5 min without compromising the point‐spread function. J. Magn. Reson. Imaging 2009;30:473–480. © 2009 Wiley‐Liss, Inc.     

Sodium mapping in focal cerebral ischemia in the rat by quantitative 23Na MRI

Purpose

To validate ²³Na twisted projection magnetic resonance imaging (MRI) as a quantitative technique to assess local brain sodium concentration ([Na⁺]_br) during rat focal ischemia every 5.3 minutes.

Materials and Methods

The MRI protocol included an ultrashort echo‐time (0.4 msec), a correction of radiofrequency (RF) inhomogeneities by B₁ mapping, and the use of 0–154 mM NaCl calibration standards. To compare MRI [Na⁺]_br values with those obtained by emission flame photometry in precision‐punched brain samples of about 0.5 mm³ size, MR images were aligned with a histological three‐dimensional reconstruction of the punched brain and regions of interest (ROIs) were placed precisely over the punch voids.

Results

The Bland–Altman analysis of [Na⁺]_br in normal and ischemic cortex and caudate putamen of seven rats quantitated by ²³Na MRI and flame photometry yielded a mean bias and limits of agreement (at ±1.96 SD) of 2% and 43% of average, respectively. A linear increase in [Na⁺]_br was observed between 1 and 6 hours after middle cerebral artery occlusion.

Conclusion

²³Na MRI provides accurate and reliable results within the whole range of [Na⁺]_br in ischemia with a temporal resolution of 5.3 minutes and precisely targeted submicroliter ROIs in selected brain structures. J. Magn. Reson. Imaging 2009;29:962–966. © 2009 Wiley‐Liss, Inc.     

1H and31P magnetic resonance spectroscopic imaging of white matter signal hyperintensity areas in elderly subjects

White matter signal hyperintensities (WMSH) are commonly seen on MRI of elderly subjects. The purpose of this study was to characterize metabolic changes in the white matter of elderly subjects with extensive WMSH. We used water-suppressed proton (¹H) magnetic resonance spectroscopic imaging (MRSI) to compare six subjects with extensive WMSH with eight age-matched elderly subjects with minimal or absent WMSH, and phosphorus (³¹P) MRSI to compare nine subjects with extensive WMSH and seven age-matched elderly subjects without extensive WMSH. Relative to region-matched tissue in elderly controls, extensive WMSH were associated with increased signal from choline-containing metabolites, no significant change of signal fromN-acetylaspartate, and a trend to a decreased phosphomonoester (PME) resonance. These findings suggest that WMSH may be associated with an alteration of brain myelin phospholipids in the absence of axonal damage. There were no differences in energy phosphates, consistent with lack of ongoing brain ischemia. Within the group with extensive WMSH, PME resonance measures were significantly lower in WMSH than in contralateral normal-appearing white matter. These results provide information on pathophysiology of WMSH and a basis for comparison with WMSH in Alzheimer's disease, vascular dementia, multiple sclerosis, and other diseases.     

1H NMR spectroscopic imaging of the mouse brain at 9.4 T

Purpose

To assess the feasibility of ¹H spectroscopic imaging (SI) in the mouse brain at 9.4 T, and investigate regional variations in brain metabolites.

Materials and Methods

A total of 21 SI studies were performed in CD‐1 mice to evaluate the basal ganglia (N = 5), hippocampus and thalamus (N = 11), and cerebellum (N = 5). We adjusted the B₀ homogeneity for each slice using a fully automated shim calculation method based on the B₀ map, which we measured using a multislice gradient‐echo sequence with multiple phase evolution delays. The SI employed a modified localization by adiabatic selective refocusing (LASER) sequence with TE/TR of 50/2000 msec, 24 × 24 encodes over a field of view (FOV) of 24 mm × 24 mm, 1 μL voxel resolution, and two averages, for a total acquisition time of 38 minutes.

Results

Sufficient shimming was achieved and high‐quality spectra were consistently obtained in each slice. N‐acetyl aspartate (NAA)/creatine (Cr) ratios in the basal ganglia and thalamus (0.86 ± 0.07, and 0.87 ± 0.07, respectively) were significantly higher than those in the hippocampus and cerebellum (0.76 ± 0.03 and 0.67 ± 0.07), which were also significantly different from each other.

Conclusion

¹H SI of the mouse brain is highly reproducible and allows differences in regional metabolite ratios to be easily visualized. J. Magn. Reson. Imaging 2006. © 2006 Wiley‐Liss, Inc.

     

Quantitative 1H spectroscopic imaging of human brain at 4.1 T using image segmentation

Metabolic differences in the content of N-acetylaspartate (NAA), creatine (CR), and choline (CH) in cerebral gray and white matter can complicate the interpretation of ¹H spectroscopic images. To account for these variations, the gray-and white-matter content of each voxel must be known. To provide these data, a T₁-based image segmentation scheme was implemented at 4.1 T. The tissue composition of each voxel was determined using the point-spread function of the spectroscopic imaging acquisition and the segmented anatomical image. Pure gray- and white-matter values for CR/NAA and CH/NAA, and the content of CR, CH, and NAA, were determined using a linear-regression analysis of 984 voxels acquired from 10 subjects using white-matter CR as an internal standard. This information was used to establish means and confidence intervals for CR/NAA and CH/NAA from a voxel of arbitrary tissue composition. Using a single-tailed t test, the extent and locations of the metabolic abnormalities (P < 0.05) in a patient with multiple sclerosis were identified.     

Early detection of cerebral infarction by31P spectroscopic imaging

Summary Recent advances in magnetic resonance spectroscopy permit noninvasive study of brain metabolism in vivo,³¹P spectroscopic imaging being the method for evaluation of localized phosphorous metabolism. Experimentally, an ischemic-hypoxic brain insult is characterized by depletion of high energy metabolites. These changes are seen immediately after an ischemic insult. We had the opportunity of carrying out³¹P spectroscopic imaging of hyperacute cerebral infarction, while MRI and CT were negative. Cerebral infarction of the middle cerebral artery territory was suggested by³¹P spectroscopic imaging, which was closely consistent with a later-developing region of low density on CT. In cerebral infarction, early detection of the lesion is a useful pointer to the patient's prognosis, making³¹P spectroscopic imaging a potential tool.     

Fast (13)C-glucose metabolite mapping in rat brain using (1)H echo planar spectroscopic imaging technique at 2T

For fast (13)C metabolite mapping in rat brains, (1)H-detected (13)C NMR spectroscopy using gradient-enhanced heteronuclear multiple-quantum coherence and (1)H echo planar spectroscopic imaging were combined. (13)C glucose and 3-/4-(13)C-Glu/Gln images of rat brain were successfully constructed with 35-minute temporal resolution under a 2T magnetic field. In the ischemic region of the suture middle cerebral artery occlusion model, glucose and Glu/Gln signals decreased and lactate signals appeared. J. Magn. Reson. Imaging 2001;13:787-791.     

Reproducibility study of whole‐brain 1H spectroscopic imaging with automated quantification

A reproducibility study of proton MR spectroscopic imaging (¹H‐MRSI) of the human brain was conducted to evaluate the reliability of an automated 3D in vivo spectroscopic imaging acquisition and associated quantification algorithm. A PRESS‐based pulse sequence was implemented using dualband spectral‐spatial RF pulses designed to fully excite the singlet resonances of choline (Cho), creatine (Cre), and N‐acetyl aspartate (NAA) while simultaneously suppressing water and lipids; 1% of the water signal was left to be used as a reference signal for robust data processing, and additional lipid suppression was obtained using adiabatic inversion recovery. Spiral k‐space trajectories were used for fast spectral and spatial encoding yielding high‐quality spectra from 1 cc voxels throughout the brain with a 13‐min acquisition time. Data were acquired with an 8‐channel phased‐array coil and optimal signal‐to‐noise ratio (SNR) for the combined signals was achieved using a weighting based on the residual water signal. Automated quantification of the spectrum of each voxel was performed using LCModel. The complete study consisted of eight healthy adult subjects to assess intersubject variations and two subjects scanned six times each to assess intrasubject variations. The results demonstrate that reproducible whole‐brain ¹H‐MRSI data can be robustly obtained with the proposed methods. Magn Reson Med 60:542–547, 2008. © 2008 Wiley‐Liss, Inc.     

Dynamic changes of ADC, perfusion, and NMR relaxation parameters in transient focal ischemia of rat brain.

The potential of multiparametric MRI parameters for differentiating between reversibly and irreversibly damaged brain tissue was investigated in an experimental model of focal brain ischemia in the rat. The middle cerebral artery (MCA) was occluded by intraluminal suture insertion for 60 or 90 min, followed by 4.5 h of reperfusion. The apparent diffusion coefficient (ADC) of brain water, T(1) and T(2) relaxation times, and CBF(i), an MR-derived index of cerebral perfusion, were repeatedly measured and correlated with the outcome from the ischemic impact. A novel user-independent approach for segmentation of ADC maps into classes of increasing injury was introduced to define regions of interest (ROIs) in which these parameters were evaluated. MCA occlusion led to a graded decline of ADC, which corresponded with both the severity of flow reduction and an increase in T(1) and T(2) relaxation times. Removal of the suture led to a triphasic restitution of blood flow consisting of a fast initial rise, a secondary decline, and final normalization. Postischemic reperfusion led to a rise of ADC irrespective of the duration of ischemia. However, the quality of recovery declined with increasing severity of the ischemic impact. Throughout the observation time, T(1) and T(2) showed a continuous increase, the intensity of which correlated with the severity of ADC decline during ischemia. Particularly with longer ischemia time, elevated T(2) in combination with reduced ADC yielded a lower probability of recovery during recirculation, while intraischemic perfusion information contributed less to the prediction of outcome. In conclusion, the combination of MR parameters at the end of ischemia correlated with the probability of tissue recovery but did not permit reliable differentiation between reversibly and irreversibly damaged tissue.     

Fast three‐dimensional proton spectroscopic imaging of the human brain at 3 T by combining spectroscopic missing pulse steady‐state free precession and echo planar spectroscopic imaging

The combination of the principles of two fast spectroscopic imaging (SI) methods, spectroscopic missing pulse steady‐state free precession and echo planar SI (EPSI) is described as an approach toward fast 3D SI. This method, termed missing pulse steady‐state free precession echo planar SI, exhibits a considerably reduced minimum total measurement time T_min, allowing a higher temporal resolution, a larger spatial matrix size, and the use of k‐space weighted averaging and phase cycling, while maintaining all advantages of the original spectroscopic missing pulse steady‐state free precession sequence. The minor signal‐to‐noise ratio loss caused by using oscillating read gradients can be compensated by applying k‐space weighted averaging. The missing pulse steady‐state free precession echo planar SI sequence was implemented on a 3 T head scanner, tested on phantoms and applied to healthy volunteers. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.