Early changes in water diffusion, perfusion, T1, and T2 during focal cerebral ischemia in the rat studied at 8.5 T

Time Course Changes of Synthetic Relaxation Time During Neoadjuvant Chemotherapy in Breast Cancer: The Optimal Parameter for Treatment Response Evaluation

BACKGROUND

Synthetic MRI (syMRI) has enabled quantification of multiple relaxation parameters (T1/T2 relaxation time [T1/T2], proton density [PD]), and their longitudinal change during neoadjuvant chemotherapy (NAC) promises to be valuable parameters for treatment response evaluation in breast cancer.

PURPOSE

To investigate the time course changes of syMRI parameters during NAC and evaluate their value as predictors for pathological complete response (pCR) in breast cancer.

STUDY TYPE

Retrospective, longitudinal.

POPULATION

A total of 129 women (median age, 50 years; range, 28-69 years) with locally advanced breast cancer who underwent NAC; all performed multiple conventional breast MRI examinations with added syMRI during NAC.

FIELD STRENGTH/SEQUENCE

A 3.0 T, T1-weighted dynamic contrast enhanced and syMRI acquired by a multiple-dynamic, multiple-echo sequence.

ASSESSMENT

Breast MRI was set at four time-points: baseline, after one cycle, after three or four cycles of NAC and preoperation. SyMRI parameters and tumor diameters were measured and their changes from baseline were calculated. All parameters were compared between pCR and non-pCR. Interaction between syMRI parameters and clinicopathological features was analyzed.

STATISTICAL TESTS

Mann-Whitney U tests, random effects model of repeated measurement, receiver operating characteristic (ROC) analysis, interaction analysis.

RESULTS

Median synthetic T1/T2/PD and tumor diameter generally decreased throughout NAC. Absolute T1 at early-NAC, T1, and PD at mid-NAC were significantly lower in the pCR group. After early-NAC, the T1 change was significantly higher in the pCR (median ± IQR, 18.17 ± 11.33) than the non-pCR group (median ± IQR, 10.90 ± 10.03), with the highest area under the ROC curves (AUC) of 0.769 (95% CI, 0.684-0.838). Interaction analysis showed that histological grade III patients had higher odds ratio (OR) (OR = 1.206) compared to grade II patients (OR = 1.067).

DATA CONCLUSION

Synthetic T1 changes after one cycle of NAC maybe useful for early evaluating NAC response in breast cancer during whole treatment cycles. However, its discriminative ability is significantly affected by histological grade.

EVIDENCE LEVEL

4 TECHNICAL EFFICACY: Stage 2.

Contrast-free MRI quantitative parameters for early prediction of pathological response to neoadjuvant chemotherapy in breast cancer

OBJECTIVES

To assess early changes in synthetic relaxometry after neoadjuvant chemotherapy (NAC) for breast cancer and establish a model with contrast-free quantitative parameters for early prediction of pathological response.

METHODS

From March 2019 to January 2021, breast MRI were performed for a primary cohort of women with breast cancer before (n = 102) and after the first (n = 93) and second (n = 90) cycle of NAC. Tumor size, synthetic relaxometry (T1/T2 relaxation time [T1/T2], proton density), and ADC were obtained, and the changes after treatment were calculated. Prediction models were established by multivariate logistic regression; evaluated with discrimination, calibration, and clinical application; and compared with Delong tests, net reclassification (NRI), and integrated discrimination index (IDI). External validation was performed from February to June 2021 with an independent cohort of 35 patients.

RESULTS

In the primary cohort, all parameters changed after early treatment. Synthetic relaxometry decreased to a greater degree in major histologic responders (MHR, Miller-Payne G4-5) compared with non-MHR (Miller-Payne G1-3). A model combining ADC after treatment, changes in T1 and tumor size, and cancer subtype achieved the highest AUC after the first (primary/validation cohort, 0.83/0.82) and second cycles (primary/validation cohort, 0.85/0.84). No difference of AUC (p ≥ 0.27), NRI (p ≥ 0.31), and IDI (p ≥ 0.32) was found between models with different cycles and size-measured sequences. Model calibration and decision curves demonstrated a good fitness and clinical benefit, respectively.

CONCLUSIONS

Early reduction in synthetic relaxometry indicated pathological response to NAC. Contrast-free T1 and ADC combined with size and cancer subtype predicted effectively pathological response after one NAC cycle.

KEY POINTS

• Synthetic MRI relaxometry changed after early neoadjuvant chemotherapy, which demonstrated pathological response for mass-like breast cancers. • Contrast-free quantitative parameters including T1 relaxation time and apparent diffusion coefficient, combined with tumor size and cancer subtype, stratified major histologic responders. • A contrast-free model predicted an early pathological response after the first treatment cycle of neoadjuvant chemotherapy.

Matching Quantitative MRI Parameters with Histological Features of Treatment-Naïve IDH Wild-Type Glioma

Quantitative MRI allows to probe tissue properties by measuring relaxation times and may thus detect subtle changes in tissue composition. In this work we analyzed different relaxation times (T1, T2, T2* and T2') and histological features in 321 samples that were acquired from 25 patients with newly diagnosed IDH wild-type glioma. Quantitative relaxation times before intravenous application of gadolinium-based contrast agent (GBCA), T1 relaxation time after GBCA as well as the relative difference between T1 relaxation times pre-to-post GBCA (T1rel) were compared with histopathologic features such as the presence of tumor cells, cell and vessel density, endogenous markers for hypoxia and cell proliferation. Image-guided stereotactic biopsy allowed for the attribution of each tissue specimen to its corresponding position in the respective relaxation time map. Compared to normal tissue, T1 and T2 relaxation times and T1rel were prolonged in samples containing tumor cells. The presence of vascular proliferates was associated with higher T1rel values. Immunopositivity for lactate dehydrogenase A (LDHA) involved slightly longer T1 relaxation times. However, low T2' values, suggesting high amounts of deoxyhemoglobin, were found in samples with elevated vessel densities, but not in samples with increased immunopositivity for LDHA. Taken together, some of our observations were consistent with previous findings but the correlation of quantitative MRI and histologic parameters did not confirm all our pathophysiology-based assumptions.

The diagnostic performance of quantitative mapping in breast cancer patients: a preliminary study using synthetic MRI

Background

Previous studies have indicated that quantitative MRI (qMR) is beneficial for diagnosis of breast cancer. As a novel qMR technology, synthetic MRI (syMRI) may be advantageous by offering simultaneous generation of T1 and T2 mapping in one scan within a few minutes and without concern to the deposition of the gadolinium contrast agent in cell nucleus. In this study, the potential of quantitative mapping derived from Synthetic MRI (SyMRI) to diagnose breast cancer was investigated.

Methods

From April 2018 to May 2019, a total of 87 patients with suspicious breast lesions underwent both conventional and SyMRI before treatment. The quantitative metrics derived from SyMRI, including T1 and T2 values, were measured in breast lesions. The diagnostic performance of SyMRI was evaluated with unpaired Student’s t-tests, receiver operating characteristic curve analysis and multivariate logistic regression analysis. The AUCs of quantitative values were compared using Delong test.

Results

Among 77 patients who met the inclusion criteria, 48 were diagnosed with histopathological confirmed breast cancers, and the rest had benign lesions. The breast cancers showed significantly higher T1 (1611.61 ± 215.88 ms) values and lower T2 (80.93 ± 7.51 ms) values than benign lesions. The area under the ROC curve (AUC) values were 0.931 (95% CI: 0.874–0.989) and 0.883 (95% CI: 0.810–0.956) for T1 and T2 maps, respectively, in diagnostic discrimination between breast cancers and benign lesions. A slightly increased AUC of 0.978 (95% CI: 0.915–0.993) was achieved by combining those two relaxation-based quantitative metrics.

Conclusion

In conclusion, our preliminary study showed that the quantitative T1 and T2 values obtained by SyMRI could distinguish effectively between benign and malignant breast lesions, and T1 relaxation time showed the highest diagnostic efficiency. Furthermore, combining the two quantitative relaxation metrics further improved their diagnostic performance.

T2 and T2∗ mapping in ex situ porcine myocardium: myocardial intravariability, temporal stability and the effects of complete coronary occlusion

Diagnosis of ischaemia-related sudden cardiac death in the absence of microscopic and macroscopic ischaemic lesions remains a challenge for medical examiners. Medical imaging techniques increasingly provide support in post-mortem examinations by detecting and documenting internal findings prior to autopsy. Previous studies have characterised MR relaxation times to investigate post-mortem signs of myocardial infarction in forensic cohorts. In this prospective study based on an ex situ porcine heart model, we report fundamental findings related to intramyocardial variability and temporal stability of T₂ as well as the effects of permanent coronary occlusion on T₂ and T₂^∗ relaxation in post-mortem myocardium. The ex situ porcine hearts included in this study (n= 19) were examined in two groups (S_s, n= 11 and S_i, n= 8). All magnetic resonance imaging (MRI) examinations were performed ex situ, at room temperature and at 3 T. In the S_s group, T₂ mapping was performed on slaughterhouse porcine hearts at different post-mortem intervals (PMI) between 7 and 26 h. Regarding the intramyocardial variability, no statistically significant differences in T₂ were observed between myocardial segments (p= 0.167). Assessment of temporal stability indicated a weak negative correlation (r=- 0.21) between myocardial T₂ and PMI. In the S_i group, animals underwent ethanol-induced complete occlusion of the left anterior descending artery. T₂ and T₂^∗ mapping were performed within 3 h of death. Differences between the expected ischaemic and remote regions were statistically significant for T₂ (p= 0.007), however not for T₂^∗ (p= 0.062). Our results provide important information for future assessment of the diagnostic potential of quantitative MRI in the post-mortem detection of early acute myocardial infarction.

Determining T2 relaxation time and stroke onset relationship in ischaemic stroke within apparent diffusion coefficient-defined lesions. A user-independent method for quantifying the impact of stroke in the human brain

BACKGROUND AND OBJECTIVE

In hyperacute ischaemic stroke, T2 of cerebral water increases with time. Quantifying this change may be informative of the extent of tissue damage and onset time. Our objective was to develop a user-unbiased method to measure the effect of cerebral ischaemia on T2 to study stroke onset time-dependency in human acute stroke lesions.

METHODS

Six rats were subjected to permanent middle cerebral occlusion to induce focal ischaemia, and a consecutive cohort of acute stroke patients (n = 38) were recruited within 9 hours from symptom onset. T1-weighted structural, T2 relaxometry, and diffusion MRI for apparent diffusion coefficient (ADC) were acquired. Ischaemic lesions were defined as regions of lowered ADC. The median T2 difference (ΔT2) between lesion and contralateral non-ischaemic control region was determined by the newly-developed spherical reference method, and data compared to that obtained by the mirror reference method. Linear regressions and receiver operating characteristics (ROC) were compared between the two methods.

RESULTS

ΔT2 increases linearly in rat brain ischaemia by 1.9 ± 0.8 ms/h during the first 6 hours, as determined by the spherical reference method. In patients, ΔT2 linearly increases by 1.6 ± 1.4 and 1.9 ± 0.9 ms/h in the lesion, as determined by the mirror reference and spherical reference method, respectively. ROC analyses produced areas under the curve of 0.83 and 0.71 for the spherical and mirror reference methods, respectively.

CONCLUSIONS

Data from the spherical reference method showed that the median T2 increase in the ischaemic lesion is correlated with stroke onset time in a rat as well as in a human patient cohort, opening the possibility of using the approach as a timing tool in clinics.

Complete Restitution of the Ischemic Penumbra after Successful Thrombectomy : A Pilot Study Using Quantitative MRI

PURPOSE

Endovascular thrombectomy is highly effective in patients with proximal large artery occlusion but the relevance of reperfusion injury after recanalization is a matter of debate. The aim of this study was to investigate potential residual metabolic distress and microstructural tissue damage or edema after reperfusion using quantitative oxygen-sensitive T2' and T2-mapping in patients successfully treated by thrombectomy.

METHODS

Included in this study were 11 patients (mean age 70 ± 11.4 years) with acute ischemic stroke due to internal carotid artery and/or middle cerebral artery occlusion. Quantitative T2 and T2' (1/T2' = 1/T2* - 1/T2) were determined within the ischemic core and hypoperfused but salvaged tissue with delayed time-to-peak (TTP) in patients before and after successful thrombectomy and compared to a control region within the unaffected hemisphere.

RESULTS

Decreased T2' values within hypoperfused tissue before thrombectomy showed a normalization after recanalization (p < 0.01). In formerly hypoperfused but salvaged tissue, T2 values increased significantly after thrombectomy (p < 0.05) but did not differ from reference values in the control region. In salvaged tissue, increases of quantitative T2' and T2 to follow-up were more pronounced in areas with severe TTP delay.

CONCLUSION

After successful recanalization, T2' re-increased back to normal in formerly hypoperfused areas as a sign of prompt normalization of oxygen metabolism. Furthermore, quantitative T2 in the formerly hypoperfused tissue did not differ from reference values in unaffected tissue. These results indicate complete restitution of salvaged tissue after reperfusion and support the overall safety of endovascular thrombectomy with respect to microstructural tissue integrity.

Vascular and Tissue Changes of Magnetic Susceptibility in the Mouse Brain After Transient Cerebral Ischemia

Quantitative susceptibility mapping (QSM) has been recently introduced as a novel MRI post-processing technique of gradient recalled echo (GRE) data. QSM is useful in depicting both brain anatomy and for detecting abnormalities. Its utility in the context of ischemic stroke has, however, not been extensively characterized so far. In this study, we explored the potential of QSM to characterize vascular and tissue changes in the transient middle cerebral artery occlusion (tMCAO) mouse model of cerebral ischemia. We acquired GRE data of mice brains at different time points after tMCAO, from which we computed QSM and MR frequency maps, and compared these maps with diffusion imaging and multi-slice multi-echo imaging data acquired in the same animals. Prominent vessels with increased magnetic susceptibility were visible surrounding the lesion on both frequency and magnetic susceptibility maps at all time points (mostly visible at > 12 h after reperfusion). Immunohistochemistry revealed the presence of compressed capillaries and dilated larger vessels, suggesting that the appearance of prominent vessels after reestablishment of reperfusion may serve compensatory purposes. In addition, on both contrast maps, tissue regions of decreased magnetic susceptibility were observed at 24 and 48 h after reperfusion that were distinctly different from the lesions seen on maps of the apparent diffusion coefficient and T₂ relaxation time constant. Since QSM can be extracted as an add-on from GRE data and thus requires no additional acquisition time in the course of acute stroke MRI examination, it may provide unique and complementary information during the course of acute stroke MRI examinations.

Magnetic Resonance Imaging Protocol for Stroke Onset Time Estimation in Permanent Cerebral Ischemia

MRI provides a sensitive and specific imaging tool to detect acute ischemic stroke by means of a reduced diffusion coefficient of brain water. In a rat model of ischemic stroke, differences in quantitative T₁ and T₂ MRI relaxation times (qT₁ and qT₂) between the ischemic lesion (delineated by low diffusion) and the contralateral non-ischemic hemisphere increase with time from stroke onset. The time dependency of MRI relaxation time differences is heuristically described by a linear function and thus provides a simple estimate of stroke onset time. Additionally, the volumes of abnormal qT₁ and qT₂ within the ischemic lesion increase linearly with time providing a complementary method for stroke timing. A (semi)automated computer routine based on the quantified diffusion coefficient is presented to delineate acute ischemic stroke tissue in rat ischemia. This routine also determines hemispheric differences in qT₁ and qT₂ relaxation times and the location and volume of abnormal qT₁ and qT₂ voxels within the lesion. Uncertainties associated with onset time estimates of qT₁ and qT₂ MRI data vary from ± 25 min to ± 47 min for the first 5 hours of stroke. The most accurate onset time estimates can be obtained by quantifying the volume of overlapping abnormal qT₁ and qT₂ lesion volumes, termed 'V^overlap' (± 25 min) or by quantifying hemispheric differences in qT₂ relaxation times only (± 28 min). Overall, qT₂ derived parameters outperform those from qT₁. The current MRI protocol is tested in the hyperacute phase of a permanent focal ischemia model, which may not be applicable to transient focal brain ischemia.

A comparison study between the saturation-recovery-T1 and CASL MRI methods for quantitative CBF imaging

The saturation-recovery (SR)-T₁ MRI method for quantitatively imaging cerebral blood flow (CBF) change (ΔCBF) concurrently with the blood oxygenation level dependence (BOLD) alteration has been recently developed and validated by simultaneous measurement of relative CBF change using laser Doppler flowmetry (LDF) in rats at 9.4T. In this study, ΔCBF induced by mildly transient hypercapnia and measured by the SR-T₁ MRI method was rigorously compared with an established perfusion MRI method-continuous arterial spin labeling (CASL) approach in normal and preclinical middle cerebral artery occlusion (MCAo) rat models. The results show an excellent agreement between ΔCBF values measured with these two imaging methods. Moreover, the intrinsic longitudinal relaxation rate (R₁^int) was experimentally determined in vivo in normal rat brains at 9.4T by comparing two independent measures of the apparent longitudinal relaxation rate (R₁^app) and CBF measured by the CSAL approach across a wide range of perfusion. In turn, the R₁^int constant can be employed to calculate the CBF value based on the R₁^app measurement in healthy brain. This comparison study validates the fundamental relationship for linking brain tissue water R₁^app and cerebral perfusion, demonstrates the feasibility of imaging and quantifying both CBF and its change using the SR-T₁ MRI method in vivo.

Stroke onset time estimation from multispectral quantitative magnetic resonance imaging in a rat model of focal permanent cerebral ischemia

Background

Quantitative T2 relaxation magnetic resonance imaging allows estimation of stroke onset time.

Aims

We aimed to examine the accuracy of quantitative T1 and quantitative T2 relaxation times alone and in combination to provide estimates of stroke onset time in a rat model of permanent focal cerebral ischemia and map the spatial distribution of elevated quantitative T1 and quantitative T2 to assess tissue status.

Methods

Permanent middle cerebral artery occlusion was induced in Wistar rats. Animals were scanned at 9.4T for quantitative T1, quantitative T2, and Trace of Diffusion Tensor (Dav) up to 4 h post-middle cerebral artery occlusion. Time courses of differentials of quantitative T1 and quantitative T2 in ischemic and non-ischemic contralateral brain tissue (ΔT1, ΔT2) and volumes of tissue with elevated T1 and T2 relaxation times ( f1, f2) were determined. TTC staining was used to highlight permanent ischemic damage.

Results

ΔT1, ΔT2, f1, f2, and the volume of tissue with both elevated quantitative T1 and quantitative T2 (VOverlap) increased with time post-middle cerebral artery occlusion allowing stroke onset time to be estimated. VOverlap provided the most accurate estimate with an uncertainty of ±25 min. At all times-points regions with elevated relaxation times were smaller than areas with Dav defined ischemia.

Conclusions

Stroke onset time can be determined by quantitative T1 and quantitative T2 relaxation times and tissue volumes. Combining quantitative T1 and quantitative T2 provides the most accurate estimate and potentially identifies irreversibly damaged brain tissue.

A new NOE-mediated MT signal at around -1.6ppm for detecting ischemic stroke in rat brain

In the present work, we reported a new nuclear Overhauser enhancement (NOE)-mediated magnetization transfer (MT) signal at around -1.6ppm (NOE(-1.6)) in rat brain and investigated its application in the detection of acute ischemic stroke in rodent model. Using continuous wave (CW) MT sequence, the NOE(-1.6) is reliably detected in rat brain. The amplitude of this new NOE signal in rat brain was quantified using a 5-pool Lorentzian Z-spectral fitting method. Amplitudes of amide, amine, NOE at -3.5ppm (NOE(-3.5)), as well as NOE(-1.6) were mapped using this fitting method in rat brain. Several other conventional imaging parameters (R1, R2, apparent diffusion coefficient (ADC), and semi-solid pool size ratio (PSR)) were also measured. Our results show that NOE(-1.6), R1, R2, ADC, and APT signals from stroke lesion have significant changes at 0.5-1h after stroke. Compared with several other imaging parameters, NOE(-1.6) shows the strongest contrast differences between stroke and contralateral normal tissues and stays consistent over time until 2h after onset of stroke. Our results demonstrate that this new NOE(-1.6) signal in rat brain is a new potential contrast for assessment of acute stroke in vivo and might provide broad applications in the detection of other abnormal tissues.

T2 relaxation time post febrile status epilepticus predicts cognitive outcome

Evidence from animal models and patient data indicates that febrile status epilepticus (FSE) in early development can result in permanently diminished cognitive abilities. To understand the variability in cognitive outcome following FSE, we used MRI to measure dynamic brain metabolic responses to the induction of FSE in juvenile rats. We then compared these measurements to the ability to learn an active avoidance spatial task weeks later. T2 relaxation times were significantly lower in FSE rats that were task learners in comparison to FSE non-learners. While T2 time in whole brain held the greatest predictive power, T2 in hippocampus and basolateral amygdala were also excellent predictors. These signal differences in response to FSE indicate that rats that fail to meet metabolic and oxygen demand are more likely to develop spatial cognition deficits. Place cells from FSE non-learners had significantly larger firing fields and higher in-field firing rate than FSE learners and control animals and imply increased excitability in the pyramidal cells of FSE non-learners. These findings suggest a mechanistic cause for the spatial memory deficits in active avoidance and are relevant to other acute neurological insults in early development where cognitive outcome is a concern.

Simultaneous Imaging of CBF Change and BOLD with Saturation-Recovery-T1 Method

A neuroimaging technique based on the saturation-recovery (SR)-T₁ MRI method was applied for simultaneously imaging blood oxygenation level dependence (BOLD) contrast and cerebral blood flow change (ΔCBF), which is determined by CBF-sensitive T₁ relaxation rate change (ΔR₁^CBF). This technique was validated by quantitatively examining the relationships among ΔR₁^CBF, ΔCBF, BOLD and relative CBF change (rCBF), which was simultaneously measured by laser Doppler flowmetry under global ischemia and hypercapnia conditions, respectively, in the rat brain. It was found that during ischemia, BOLD decreased 23.1±2.8% in the cortical area; ΔR₁^CBF decreased 0.020±0.004s^-1 corresponding to a ΔCBF decrease of 1.07±0.24 ml/g/min and 89.5±1.8% CBF reduction (n=5), resulting in a baseline CBF value (=1.18 ml/g/min) consistent with the literature reports. The CBF change quantification based on temperature corrected ΔR₁^CBF had a better accuracy than apparent R₁ change (ΔR₁^app); nevertheless, ΔR₁^app without temperature correction still provides a good approximation for quantifying CBF change since perfusion dominates the evolution of the longitudinal relaxation rate (R₁^app). In contrast to the excellent consistency between ΔCBF and rCBF measured during and after ischemia, the BOLD change during the post-ischemia period was temporally disassociated with ΔCBF, indicating distinct CBF and BOLD responses. Similar results were also observed for the hypercapnia study. The overall results demonstrate that the SR-T₁ MRI method is effective for noninvasive and quantitative imaging of both ΔCBF and BOLD associated with physiological and/or pathological changes.

18F-GE-180: a novel TSPO radiotracer compared to 11C-R-PK11195 in a preclinical model of stroke

PURPOSE

Neuroinflammation plays a critical role in various neuropathological conditions, and hence there is renewed interest in the translocator protein (TSPO) as a biomarker of microglial activation and macrophage infiltration in the brain. This is reflected in the large amount of research conducted seeking to replace the prototypical PET radiotracer (11)C-R-PK11195 with a TSPO ligand with higher performance. Here we report the in vivo preclinical investigation of the novel TSPO tracer (18)F-GE-180 in a rat model of stroke.

METHODS

Focal cerebral ischaemia was induced in Wistar rats by 60-min occlusion of the middle cerebral artery (MCAO). Brain damage was assessed 24 h after MCAO by T2 MRI. Rats were scanned with (11)C-R-PK11195 and (18)F-GE-180 5 or 6 days after MCAO. Specificity of binding was confirmed by injection of unlabelled R-PK11195 or GE-180 20 min after injection of (18)F-GE-180. In vivo data were confirmed by ex vivo immunohistochemistry for microglial (CD11b) and astrocytic biomarkers (GFAP).

RESULTS

(18)F-GE-180 uptake was 24 % higher in the core of the ischaemic lesion and 18 % lower in the contralateral healthy tissue than that of (11)C-R-PK11195 uptake (1.5 ± 0.2-fold higher signal to noise ratio). We confirmed this finding using the simplified reference tissue model (BPND = 3.5 ± 0.4 and 2.4 ± 0.5 for (18)F-GE-180 and (11)C-R-PK11195, respectively, with R 1 = 1). Injection of unlabelled R-PK11195 or GE-180 20 min after injection of (18)F-GE-180 significantly displaced (18)F-GE-180 (69 ± 5 % and 63 ± 4 %, respectively). Specificity of the binding was also confirmed by in vitro autoradiography, and the location and presence of activated microglia and infiltrated macrophages were confirmed by immunohistochemistry.

CONCLUSION

The in vivo binding characteristics of (18)F-GE-180 demonstrate a better signal to noise ratio than (11)C-R-PK11195 due to both a better signal in the lesion and lower nonspecific binding in healthy tissue. These results provide evidence that (18)F-GE-180 is a strong candidate to replace (11)C-R-PK11195.

A novel, noninvasive, predictive epilepsy biomarker with clinical potential

A significant proportion of temporal lobe epilepsy (TLE), a common, intractable brain disorder, arises in children with febrile status epilepticus (FSE). Preventative therapy development is hampered by our inability to identify early the FSE individuals who will develop TLE. In a naturalistic rat model of FSE, we used high-magnetic-field MRI and long-term video EEG to seek clinically relevant noninvasive markers of epileptogenesis and found that reduced amygdala T2 relaxation times in high-magnetic-field MRI hours after FSE predicted experimental TLE. Reduced T2 values likely represented paramagnetic susceptibility effects derived from increased unsaturated venous hemoglobin, suggesting augmented oxygen utilization after FSE termination. Indeed, T2 correlated with energy-demanding intracellular translocation of the injury-sensor high-mobility group box 1 (HMGB1), a trigger of inflammatory cascades implicated in epileptogenesis. Use of deoxyhemoglobin-sensitive MRI sequences enabled visualization of the predictive changes on lower-field, clinically relevant scanners. This novel MRI signature delineates the onset and suggests mechanisms of epileptogenesis that follow experimental FSE.

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.

Present status and future challenges of electroencephalography- and magnetic resonance imaging-based monitoring in preclinical models of focal cerebral ischemia

Animal models are useful tools for better understanding the mechanisms underlying neurological deterioration after an ischemic insult as well as subsequent evolution of changes and recovery of functions. In response to the updated requirements for preclinical investigations of stroke to include relevant functional measurement techniques and biomarker endpoints, we here review the state of knowledge on application of some translational electrophysiological and neuroimaging methods, and in particular, electroencephalography monitoring and magnetic resonance imaging in rodent models of ischemic stroke. This may lead to improvement of diagnostic methods and identification of new therapeutic targets, which would considerably advance the translational value of preclinical stroke research.

A review of current imaging methods used in stroke research

Stroke is a serious healthcare problem with high mortality and long-term disability. However, to date, our ability to prevent and cure stroke remains limited. One important goal in stroke research is to identify the extent and location of lesion for treatment. In addition, accurately differentiating salvageable tissue from infarct and evaluating therapeutic efficacies are indispensible. These objectives could potentially be met with the assistance of modern neuroimaging techniques. This paper reviews current imaging methods commonly used in ischemic stroke research. These methods include positron emission tomography, computed tomography, T1 MRI, T2 MRI, diffusion and perfusion MRI, diffusion tensor imaging, blood-brain barrier permeability MRI, pH-weighted MRI, and functional MRI.

Characterisation of endothelin-1-induced intrastriatal lesions within the juvenile and adult rat brain using MRI and 31P MRS

Improved non-invasive magnetic resonance (MR) characterisation of in vivo models of focal ischaemic insults such as transient ischaemic attack (TIA) and perinatal arterial ischaemic stroke (AIS) may assist diagnosis, outcome prediction and treatment design. The classic middle cerebral artery occlusion (MCAO) model of ischaemic stroke is well documented in MR studies but generates extensive and complex lesions involving an acute inflammatory response and de-occlusion that immediately restores circulation. By contrast, intrastriatal microinjection of the potent vasoconstrictor, endothelin-1 (ET-1), induces a focal, reversible and low-flow ischaemia in the absence of a typical inflammatory response, which gradually restores blood flow over several hours and may be more relevant to TIA and AIS pathology. This study presents the first comprehensive longitudinal MR characterisation of the real-time anatomical [T1-weighted (T1-w)/T2-weighted (T2-w)], pathophysiological [apparent diffusion coefficient (ADC), cerebral blood volume, gadolinium contrast imaging of blood-brain barrier (BBB) integrity] and metabolic [phosphorus magnetic resonance spectroscopy (31P MRS)] evolution of a purely ischaemic ET-1-induced lesion within the juvenile and adult rat brain. ET-1-induced cytotoxic oedema was visualised on T2-w magnetic resonance imaging (MRI), inconsistent with the conventional notion that it cannot be detected using anatomical MRI. There was no immunohistochemical evidence of an acute inflammatory response or loss of BBB integrity, thus excluding a vasogenic oedema contribution to the pathology. Maximal T2-w intensity correlated with the lowest ADC value in both age groups, re-emphasising the purely ischaemic nature of the lesion and the absence of vasogenic oedema. Furthermore, extensive acute T1-w hypointensity was observed in the presence of cytotoxic oedema-induced T2-w changes, whereas other authors have shown that increased T1 values following MCAO reflect vasogenic oedema. Intriguingly, the lesion border exhibited hyperintensity on T2-w and ADC MRI at later time points, and the former may be a consequence of phagocytosis-induced fatty droplet deposition by macrophages detected immunohistochemically. In spite of a chronically reduced ADC, typically associated with ischaemia-induced energy failure, a 31P MRS-detectable reduction in the phosphocreatine (PCr) to gamma adenosine triphosphate (γATP) ratio was not observed at any time point in either age group, suggesting dissociation of tissue water diffusion and metabolic changes within the ET-1-induced lesion.

Neurovascular protection by targeting early blood-brain barrier disruption with neurotrophic factors after ischemia-reperfusion in rats*

The 'new penumbra' concept imbues the transition between injury and repair at the neurovascular unit with profound implications for selecting the appropriate type and timing of neuroprotective interventions. In this conceptual study, we investigated the protective effects of pigment epithelium-derived factor (PEDF) and compared them with the properties of epidermal growth factor (EGF) in a rat model of ischemia-reperfusion injury. We initiated a delayed intervention 3 hours after reperfusion using equimolar amounts of PEDF and EGF. These agents were then administered intravenously for 4 hours following reperfusion after 1 hour of focal ischemia. Magnetic resonance imaging indices were characterized, and imaging was performed at multiple time points post reperfusion. PEDF and EGF reduced lesion volumes at all time points as observed on T2-weighted images (T2-LVs). In addition PEDF selectively attenuated lesion volume expansion at 48 hours after reperfusion and persistently modulated blood-brain barrier (BBB) permeability at all time points. Intervention with peptides is suspected to cause edema formation at distant regions. The observed T2-LV reduction and BBB modulation by these trophic factors is probably mediated through a number of diverse mechanisms. A thorough evaluation of neurotrophins is still necessary to determine their time-dependent contributions against injury and their modulatory effects on repair after stroke.

Negative fluid-attenuated inversion recovery-based intravenous thrombolysis using recombinant tissue plasminogen activator in acute stroke patients with unknown onset time

BACKGROUND

Approximately 25% of acute stroke patients were excluded from intravenous thrombolysis using recombinant tissue plasminogen activator (IV-tPA) because of unknown onset time. Recent studies have shown that patients with unknown onset time would be able to receive IV-tPA when showing no ischemia on fluid-attenuated inversion recovery (negative FLAIR). The present study evaluated the safety and feasibility of IV-tPA in patients with unknown onset time and negative FLAIR compared to those with standard IV-tPA.

METHODS

Stroke patients with unknown onset time were prospectively enrolled. Only patients with an occlusion of the internal carotid artery (ICA) and/or middle cerebral artery (M1 and M2) with a Diffusion-Weighted Imaging-Alberta Stroke Program Early CT Score (DWI-ASPECTS) ≥5 were analyzed. IV-tPA was performed within 3 h from the 'first found abnormal time' if the patient showed negative FLAIR. Standard IV-tPA patients were extracted from our registry as controls after having been matched by age and occluded artery to the negative FLAIR (N-F) group.

RESULTS

Twenty patients in the N-F group and 60 in the control group were included. National Institutes of Health Stroke Scale (NIHSS) scores [median 18 (interquartile range 13-20) vs. 17 (12-20), p = 0.609] and DWI-ASPECTS [9 (7-9) vs. 8 (5-9), p = 0.213] were similar between the 2 groups. ICA occlusion was seen in 35%, M1 in 50%, and M2 in 15% in both groups. None of the N-F group and 1 (2%) of the control group experienced symptomatic intracerebral hemorrhage (p = 1.000). Recanalization within 1 h after IV-tPA was achieved in 6 (30%) patients in the N-F group and 24 (40%) in the control group (p = 0.595). Recanalization at 24 h after IV-tPA was seen in 13 (65%) patients in the N-F group and 43 (72%) in the control group (p = 0.584). At 7 days, 8 (40%) in the N-F group and 28 (47%) in the control group had a dramatic recovery (defined as a ≥10-point reduction in the total NIHSS score or a score of 0 or 1) (p = 0.796). At 3 months, a favorable outcome (modified Rankin scale score, 0-2) was seen in 47% in the N-F group and 33% in the control group (p = 0.365).

CONCLUSION

IV-tPA in negative FLAIR patients with unknown onset time appears safe and feasible.

Delayed administration of interleukin-1 receptor antagonist reduces ischemic brain damage and inflammation in comorbid rats

Many neuroprotective agents have been effective in experimental stroke, yet few have translated into clinical application. One reason for this may be failure to consider clinical comorbidities/risk factors in experimental models. We have shown that a naturally occurring interleukin-1 receptor antagonist (IL-1Ra) is protective against ischemic brain damage in healthy animals. However, protective effects of IL-1Ra have not been determined in comorbid animals. Thus, we tested whether IL-1Ra protects against brain injury induced by experimental ischemia in aged JCR-LA (corpulent) rats, which have clinically relevant risk factors. Male, aged, lean, and corpulent rats exposed to transient (90 minutes) occlusion of the middle cerebral artery (tMCAO) were administered two doses of IL-1Ra (25 mg/kg, subcutaneously) during reperfusion. Brain injury and neuroinflammatory changes were assessed 24 hours after tMCAO. Our results show that IL-1Ra administered at reperfusion significantly reduced infarct volume measured by magnetic resonance imaging (50%, primary outcome) and blood-brain barrier disruption in these comorbid animals. Interleukin-1Ra also reduced microglial activation, neutrophil infiltration, and cytokines levels in the brain. These data are the first to indicate that IL-1Ra protects against ischemic brain injury when administered via a clinically relevant route and time window in animals with multiple risk factors for stroke.

Multimodal MRI of nonhuman primate stroke

Stroke is the fourth leading cause of death. Despite decades of research, no neuroprotective drug has proven to be effective clinically. One widely accepted view to account for this negative outcome is that the rodent stroke model simply does not adequately reflect the complexity of human stroke. Recent failures of several high-profile neuroprotective drugs for stroke treatment in phase III clinical trials further underscore the importance of developing adequate animal models for stroke research. The brain organization and vascular circuitry of nonhuman primates (NHPs) are more homologous with humans than the widely used rodent for stroke modeling. The Stroke Therapy Academic Industry Roundtable, a national committee commissioned by the American Heart Association, recommended that clinically relevant NHP stroke models be established for developing and assessing neuroprotective drugs. The aim of this article is to review the challenges and applications of magnetic resonance imaging studies of NHP stroke models.

Magnetic resonance characterization of ischemic tissue metabolism

Magnetic resonance imaging (MRI) and spectroscopy (MRS) are versatile diagnostic techniques capable of characterizing the complex stroke pathophysiology, and hold great promise for guiding stroke treatment. Particularly, tissue viability and salvageability are closely associated with its metabolic status. Upon ischemia, ischemic tissue metabolism is disrupted including altered metabolism of glucose and oxygen, elevated lactate production/accumulation, tissue acidification and eventually, adenosine triphosphate (ATP) depletion and energy failure. Whereas metabolism impairment during ischemic stroke is complex, it may be monitored non-invasively with magnetic resonance (MR)-based techniques. Our current article provides a concise overview of stroke pathology, conventional and emerging imaging and spectroscopy techniques, and data analysis tools for characterizing ischemic tissue damage.

Investigation of protective effect of hydrogen-rich water against cisplatin-induced nephrotoxicity in rats using blood oxygenation level-dependent magnetic resonance imaging

PURPOSE

The aim of this study was to assess the mechanism of the protective effect of hydrogen-rich water (HW) against cisplatin (CP)-induced nephrotoxicity in rats using blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI).

MATERIALS AND METHODS

Apparent transverse relaxation time-weighted images (T2 WI) were acquired in 28 rats. The control group (n = 7) had free access to standard water (SW) and no CP injection. The CP group (n = 7) had free access to SW and was given a CP injection on day 0. The CP+HW group (n = 7) had free access to HW and had a CP injection. The HW group (n = 7) had free access to HW and no CP injection. The apparent transverse relaxation rate (R2) was estimated from T2 WI.

RESULTS

In the CP+HW group, the R2 value in the medulla normalized by the value of the day 0 was significantly greater than that in the CP group on days 4 and 7. The creatinine and blood urea nitrogen levels in the CP group were significantly higher than those in the control, CP+HW, and HW groups.

CONCLUSION

BOLD MRI may be useful for demonstrating the change in R2 in CP-induced nephrotoxicity in rats. The changes in the CP+HW group were suspected to be due to a reduction of cytotoxic oxygen radicals.

Arterial spin-labeling MR imaging in moyamoya disease compared with SPECT imaging

OBJECTIVE

Arterial spin-labeling (ASL) is a noninvasive magnetic resonance (MR) imaging method used to obtain brain perfusion information on various cerebrovascular diseases. We retrospectively compared the use of ASL-MRI and single-photon emission CT (SPECT) imaging to determine absolute cerebral blood flow (CBF) in moyamoya disease.

MATERIALS AND METHODS

CBF examinations using ASL-MRI on 3-T MRI and SPECT imagings with iodine-123-N-isopropyl-p-iodoamphetamine at resting (rest-IMP) and after acetazolamide challenge (ACZ-IMP) were performed on 12 patients with moyamoya disease (men, 5; women, 7; age range/average (year), 7-66/35.0). The CBF values determined by ASL-MRI (ASL-value), rest-IMP (rest-IMP-value), and ACZ-IMP (ACZ-IMP-value) of cerebral hemispheres (24 sides) were measured with normalized CBF maps created from data of those 3 perfusion imaging methods. Cerebrovascular reactivity (CVR) was calculated as follows: {(ACZ-IMP-value)-(rest-IMP-value)}/(rest-IMP-value)×100 (%). The ASL-value was compared with the rest-IMP-value, ACZ-IMP-value, and CVR.

RESULTS

The ASL-value, rest-IMP-value, ACZ-IMP-value, and CVR (average±standard deviation) were 26.6±14.8 (mL/100 g/min), 27.5±6.4 (mL/100 g/min), 37.1±13.2 (mL/100 g/min), and 35.9±44.3 (%), respectively. Significant relationships between the ASL-value versus (vs.) the rest-IMP-value (rs=0.500, p<0.05), the ASL-value vs. the ACZ-IMP-value (rs=0.863, p<0.01), and the ASL-value vs. the CVR (rs=0.699, p<0.01) were observed.

CONCLUSION

Although the ASL-value was lower than the rest-IMP-value, the significant relationship between the ASL-value and the rest-IMP-value may suggest that perfusion imaging by ASL-MRI could be used to recognize the condition of brain perfusion. In particular, the stronger correlation coefficient between the ASL-value and ACZ-IMP-value might suggest that perfusion imaging by ASL-MRI could show the potentially dangerous zone for ischemia.

Comprehensive small animal imaging strategies on a clinical 3 T dedicated head MR-scanner; adapted methods and sequence protocols in CNS pathologies

BACKGROUND

Small animal models of human diseases are an indispensable aspect of pre-clinical research. Being dynamic, most pathologies demand extensive longitudinal monitoring to understand disease mechanisms, drug efficacy and side effects. These considerations often demand the concomitant development of monitoring systems with sufficient temporal and spatial resolution.

METHODOLOGY AND RESULTS

This study attempts to configure and optimize a clinical 3 Tesla magnetic resonance scanner to facilitate imaging of small animal central nervous system pathologies. The hardware of the scanner was complemented by a custom-built, 4-channel phased array coil system. Extensive modification of standard sequence protocols was carried out based on tissue relaxometric calculations. Proton density differences between the gray and white matter of the rodent spinal cord along with transverse relaxation due to magnetic susceptibility differences at the cortex and striatum of both rats and mice demonstrated statistically significant differences. The employed parallel imaging reconstruction algorithms had distinct properties dependent on the sequence type and in the presence of the contrast agent. The attempt to morphologically phenotype a normal healthy rat brain in multiple planes delineated a number of anatomical regions, and all the clinically relevant sequels following acute cerebral ischemia could be adequately characterized. Changes in blood-brain-barrier permeability following ischemia-reperfusion were also apparent at a later time. Typical characteristics of intra-cerebral haemorrhage at acute and chronic stages were also visualized up to one month. Two models of rodent spinal cord injury were adequately characterized and closely mimicked the results of histological studies. In the employed rodent animal handling system a mouse model of glioblastoma was also studied with unequivocal results.

CONCLUSIONS

The implemented customizations including extensive sequence protocol modifications resulted in images of high diagnostic quality. These results prove that lack of dedicated animal scanners shouldn't discourage conventional small animal imaging studies.

MR angiography and arterial spin labelling

MRI offers the ability to visualise and measure blood flow in the human body non-invasively. MR angiography (MRA) provides images of the arterial blood vessels within the body and allows measurement of blood velocities along these arteries. Arterial spin labelling (ASL) is a method for measuring the perfusion of blood into tissue (i.e. blood flow at the capillary level). This provides a key indicator of nutrient supply to the tissue. In this chapter, we have described the technical basis and practical implementation of these methods, emphasising their non-invasive (no contrast agents required) and quantitative nature.

Experimental stroke research: the contributions of in vivo MRI

Stroke is a disease that develops from the very acute time point of first symptoms during the next several hours and further to a chronic time period of days or even weeks. During this evolution process, a whole series of pathophysiological events takes place. Therefore, the disease is characterized by a continuously changing pathophysiological pattern. In consequence, as the disease develops over time, different imaging modalities must be chosen to accurately describe the status of stroke. In the present chapter, we have divided the evolution of stroke into various dominant steps of the cascade of events, with corresponding time windows. Choice of MRI variables for depiction of the most important aspects during these time windows are presented and their information content is discussed for diagnosis and for investigations into a better understanding of the underlying mechanisms for the disease as well as the relevance of these imaging tools in success assessments for therapeutic strategies.

Signal intensity in T2' magnetic resonance imaging is related to brain glioma grade

OBJECTIVES

T2' values reflect the presence of deoxyhaemoglobin related to high local oxygen extraction. We assessed the feasibility of T2' imaging to display regions with high metabolic activity in brain gliomas.

METHODS

MRI was performed in 25 patients (12 female; median age 46 years; range 2-69) with brain gliomas with additional T2 and T2* sequences. T2' maps were derived from T2 and T2*. Dynamic susceptibility weighted contrast (DSC) perfusion was performed in 12/25 patients. Images were visually assessed by two readers and five ROIs were evaluated for each patient. Pearson correlation, Mann-Whitney and Kruskal-Wallis tests were applied for statistical analysis.

RESULTS

Three patients were not further evaluated because of artefacts. Mean values of high-grade (III-IV) gliomas showed significantly lower T2' values than low-grade (II) gliomas (p < 0.001). An inverse relationship was observed between rCBV and sqr (T2') (r = -0.463, p < 0.001). No correlation was observed between T2' and rCBV for grade II tumours (r = 0.038; p = 0.875).

CONCLUSIONS

High-grade tumours revealed lower T2' values, presumably because of higher oxygen consumption in proliferating tissue. Our results indicate that T2' imaging can be used as an alternative to DSC perfusion in the detection of subtle deviations in tumour metabolism.

Textures in magnetic resonance images of the ischemic rat brain treated with an anti-inflammatory agent

Computer-based analysis of textures in magnetic resonance images provides a higher sensitivity to textural changes that cannot be recognized by the naked human eye. Thus, there is a better potential for identifying pathophysiological processes at an earlier stage or of a different character than even a trained radiologist can find. In the present study, the potential of texture analysis for in vivo identification of the administering effect of an anti-inflammatory drug in cerebral stroke in rats was evaluated. Twenty-seven Wistar rats underwent middle cerebral artery occlusion resulting in local ischemic brain infarct. One group of rats received alpha-melanocyte stimulating hormone (alpha-MSH) and a control group received saline only. T2-weighted images, apparent diffusion maps, and T2 maps were recorded by MR. Texture features were calculated in the T2-weighted images and correlated to the apparent diffusion coefficient (ADC) and the T2 values. From an array of tested texture features three independent features were tested further. Two of which were found to provide a significant discriminative classification between the control and the alpha-MSH groups. Furthermore, the same two texture features were significantly correlated to the ADCs. Thus, quantification of texture features can be helpful in detecting the effects of stroke therapy.

Correlating tissue outcome with quantitative multiparametric MRI of acute cerebral ischemia in rats

Predicting tissue outcome remains a challenge for stroke magnetic resonance imaging (MRI). In this study, we have acquired multiparametric MRI data sets (including absolute T(1), T(2), diffusion, T(1rho) using continuous wave and adiabatic pulse approaches, cerebral blood flow (CBF), and amide proton transfer ratio (APTR) images) during and after 65 mins of middle cerebral artery occlusion (MCAo) in rats. The MRI scans were repeated 24 h after MCAo, when the animals were killed for quantitative histology. Magnetic resonance imaging parameters acquired at three acute time points were correlated with regionally matching cell count at 24 h. The results emphasize differences in the temporal profile of individual MRI contrasts during MCAo and especially during early reperfusion, and suggest that complementary information from CBF and tissue damage can be obtained with appropriate MRI contrasts. The data show that by using three to four MRI parameters, sensitive to both hemodynamic changes and different aspects of parenchymal changes, the fate of the tissue can be predicted with increased correlation compared with single-parameter techniques. Combined multiparametric MRI data and multiparametric analysis may provide an excellent tool for preclinical testing of new treatments and also has the potential to facilitate decision-making in the management of acute stroke patients.

Cerebral ischemia-reperfusion injury in rats--a 3 T MRI study on biphasic blood-brain barrier opening and the dynamics of edema formation

Serial magnetic resonance imaging (MRI) was performed to investigate the temporal and spatial relationship between the biphasic nature of blood-brain barrier (BBB) opening and, in parallel, edema formation after ischemia-reperfusion (I/R) injury in rats. T(2)-weighted imaging combined with T(2)-relaxometry, mainly for edema assessment, was performed at 1 h after ischemia, after reperfusion, and at 4, 24 and 48 h after reperfusion. T(1)-weighted imaging was performed before and after gadolinium contrast at the last three time points to assess BBB integrity. The biphasic course of BBB opening with a significant reduction in BBB permeability at 24 h after reperfusion, associated with a progressive expansion of leaky BBB volume, was accompanied by a peak ipsilateral edema formation. In addition, at 4 h after reperfusion, edema formation could also be detected at the contralateral striatum as determined by the elevated T(2)-values that persisted to varying degrees, indicative of widespread effects of I/R injury. The observations of this study may indicate a dynamic temporal shift in the mechanisms responsible for biphasic BBB permeability changes, with complex relations to edema formation. Stroke therapy aimed at vasogenic edema and drug delivery for neuroprotection may also be guided according to the functional status of the BBB, and these findings have to be confirmed in human stroke.

Quantified T1 as an adjunct to apparent diffusion coefficient for early infarct detection: a high-field magnetic resonance study in a rat stroke model

BACKGROUND

Thrombolytic treatment for acute stroke has focused attention on accurate identification of injured vs. salvageable brain tissue, particularly if reperfusion occurs. However, our knowledge of differences in acute magnetic resonance imaging changes between transient and permanent ischemia and how they reflect permanently damaged tissue remain incomplete.

AIMS AND/OR HYPOTHESIS

Magnetic resonance imaging characteristics vary widely following ischemia and, at acute times, T1, T2 or apparent diffusion coefficient quantification may differentiate viable tissue from that destined to infarct.

METHODS

High-resolution magnetic resonance imaging was performed at 9.4 T following permanent or transient (90 min) middle cerebral artery occlusion in spontaneously hypertensive male rats or Wistar rats. Within 30 min, quantified maps of the apparent diffusion coefficient, T1, and T2 were performed and measures determined for sequences in the infarct and compared with that in the contralateral region. Lesion area for each magnetic resonance imaging sequence (T1, T2, apparent diffusion coefficient, and perfusion maps) was delineated for different time points using quantitative threshold measures and compared with final histological damage.

RESULTS

Early extensive changes in T1 following both transient and permanent middle cerebral artery occlusion provided a sensitive early indicator of the final infarct area. Following reperfusion, small but measurable early T2 changes indicative of early development of vasogenic edema occurred in the transient but not permanent groups. In transient middle cerebral artery occlusion, at 70 min apparent diffusion coefficient decreased (P<0.001) and then pseudonormalized at 150 min. In permanent middle cerebral artery occlusion, apparent diffusion coefficient declined over time. Lesion area detected using T1 maps exceeded that with T2 and apparent diffusion coefficient at 70 and 150 min in both groups (P<0.001).

CONCLUSIONS

The results indicate that, independent of reperfusion, quantified T1 is superior for detecting early ischemic changes that are not necessarily detected with T2 or apparent diffusion coefficient.

T2' imaging indicates decreased tissue metabolism in frontal white matter of MS patients

BACKGROUND AND PURPOSE

T2'-Magnetic resonance imaging (MRI) allows estimation of oxygen metabolism in normal appearing white and gray matter (NAWM and NAGM) and is sensitive to local iron deposition. We hypothesized that T2' imaging is feasible in routine use and reveals differences between MS patients and healthy subjects.

METHODS

T2- and T2*-weighted images were acquired in 23 MS patients (Mean age: 36.8, range: 23-58 years) and 23 age-matched healthy subjects. Quantitative T2- and T2*-values were determined in six regions of interest (ROIs).

RESULTS

The T2' values in thalamus and caudate nucleus were significantly lower in MS patients than in healthy subjects (139 ms vs 157 ms, P < 0.001 and 97 ms vs 115 ms, P < 0.01). The NAWM in the frontal lobe revealed significant higher T2' values than in healthy subjects (217 ms vs 170 ms, P < 0.001). The subcortical NAWM revealed significant lower T2' values than in healthy subjects (174 ms vs 187 ms, P < 0.028).

CONCLUSION

T2' values differed significantly between MS patients and healthy subjects. The reduced T2' values in the basal ganglia are presumably related to higher iron concentration whereas the increased T2' in frontal NAWM most probably reflects reduced tissue metabolism. T2' imaging is feasible for routine-use and promising for monitoring therapy effects.

In vivo virtual histology of mouse embryogenesis by ultrasound biomicroscopy and magnetic resonance imaging

Feasibility of magnetic resonance imaging (MRI) and ultrasound biomicroscopy (UBM) for sequential in vivo study of mouse embryo development between Days 6.5 and 13.5 of pregnancy was assessed in a first experiment. A second trial, based on the results of the first, determined the accuracy of UBM for imaging morphogenesis from implantation to the late embryo stage (Days 4.5 to 15.5). MRI allowed imaging of the entire uterus and all gestational sacs and embryos inside whilst the small scanning range of UBM precluded accurate counting of fetuses; however, its high resolution identified the decidual reaction at implantation sites from Day 4.5. At later stages, it was possible to assess key morphogenetic processes such as differentiation of the placenta, the cephalic region, the thoracic and abdominal organs, the skeletal system and the limbs, and dynamic structures such as the cardiovascular system. Thus, both techniques are reliable for in utero imaging of mouse embryo development. MRI may be more appropriate for studying embryo lethality and intrauterine growth retardation, because the entire uterus can be viewed. UBM may be more suitable for studies of cellular components of organs and tissues and assessment of haemodynamic changes in the circulatory system.

Quantitative assessment of water pools by T 1 rho and T 2 rho MRI in acute cerebral ischemia of the rat

The rotating frame longitudinal relaxation magnetic resonance imaging (MRI) contrast, T(1 rho), obtained with on-resonance continuous wave (CW) spin-lock field is a sensitive indicator of tissue changes associated with hyperacute stroke. Here, the rotating frame relaxation concept was extended by acquiring both T(1 rho) and transverse rotating frame (T(2 rho)) MRI data using both CW and adiabatic hyperbolic secant (HSn; n=1, 4, or 8) pulses in a rat stroke model of middle cerebral artery occlusion. The results show differences in the sensitivity of spin-lock T(1 rho) and T(2 rho) MRI to detect hyperacute ischemia. The most sensitive techniques were CW-T(1 rho) and T(1 rho) using HS4 or HS8 pulses. Fitting a two-pool exchange model to the T(1 rho) and T(2 rho) MRI data acquired from the infarcting brain indicated time-dependent increase in free water fraction, decrease in the correlation time of water fraction associated with macromolecules, and increase in the exchange correlation time. These findings are consistent with known pathology in acute stroke, including vasogenic edema, destructive processes, and tissue acidification. Our results show that the sensitivity of the spin-lock MRI contrast in vivo can be modified using different spin-lock preparation blocks, and that physicochemical models of the rotating frame relaxation may provide insight into progression of ischemia in vivo.

A feasibility study on model-based evaluation of kidney perfusion measured by means of FAIR prepared true-FISP arterial spin labeling (ASL) on a 3-T MR scanner

RATIONALE AND OBJECTIVES

A feasibility study on measuring kidney perfusion by a contrast-free magnetic resonance (MR) imaging technique is presented.

MATERIALS AND METHODS

A flow-sensitive alternating inversion recovery (FAIR) prepared true fast imaging with steady-state precession (TrueFISP) arterial spin labeling sequence was used on a 3.0-T MR-scanner. The basis for quantification is a two-compartment exchange model proposed by Parkes that corrects for diverse assumptions in single-compartment standard models.

RESULTS

Eleven healthy volunteers (mean age, 42.3 years; range 24-55) were examined. The calculated mean renal blood flow values for the exchange model (109 +/- 5 [medulla] and 245 +/- 11 [cortex] ml/min - 100 g) are in good agreement with the literature. Most important, the two-compartment exchange model exhibits a stabilizing effect on the evaluation of perfusion values if the finite permeability of the vessel wall and the venous outflow (fast solution) are considered: the values for the one-compartment standard model were 93 +/- 18 (medulla) and 208 +/- 37 (cortex) ml/min - 100 g.

CONCLUSION

This improvement will increase the accuracy of contrast-free imaging of kidney perfusion in treatment renovascular disease.

Experimental comparison of four FAIR arterial spin labeling techniques for quantification of mouse cerebral blood flow at 4.7 T

Pulsed arterial spin labeling (ASL) is an attractive and robust method for quantification of rodent cerebral blood flow (CBF) in particular, although there is a need for sensitivity optimization. Look-Locker flow-sensitive alternating inversion recovery (FAIR) echo planar imaging (EPI) (LLFAIREPI) was expected to be a likely candidate for assessing sensitivity, although it has not yet been applied to rodents. In this study, the performance of two FAIR techniques and two Look-Locker FAIR techniques were compared in mouse brain at 4.7 T. FAIR-EPI (single inversion time, FAIREPI-1TI), FAIR-EPI (eight inversion times, FAIREPI-8TI), LLFAIREPI and Look-Locker FAIR gradient echo (LLFAIRGE) sequences were implemented with equal spatial resolution and equal FAIR preparation modules. Measurements were carried out sequentially on the brain in 10 healthy mice, and quantitative CBF maps were obtained after different acquisition times up to 23 min. All methods gave similar group variability in CBF. Especially at shorter acquisition times, LLFAIREPI gave lower relative variations in CBF within selected brain regions than the other techniques at the same acquisition time. The Look-Locker techniques, however, overestimated CBF compared with classical FAIR-EPI, which was attributed to bulk flow in arterioles and T(2) effects. The image quality with LLFAIREPI was less reproducible within the group. Both FAIREPI-1TI and LLFAIREPI appear to be good candidates for serial rapid measurement of CBF, but LLFAIREPI has the additional advantage that apparent T(1) can be measured simultaneously with CBF.

Effects of intra- and extracellular space properties on diffusion and T(2) relaxation in a tissue model

The purpose of this study was to investigate the effects of biophysical factors on the diffusion and the relaxation time T(2) independently. Certain properties of the extracellular and the intracellular space may change radically in pathological conditions resulting in water diffusion changes. A tissue model consisting of red blood cells was studied. The extra- and intracellular spaces were modified osmotically and by suspending medium concentration. Diffusion measurements were evaluated with regard to the effective medium theory. Neither the nature of the protein in the extracellular space nor an increased level of intracellular hydration caused a significant net water diffusion change in the cell suspension. The relaxation time T(2) exhibited very little dependence on the extracellular volume fraction or the concentration or the nature of the protein in the extracellular space. An increased level of intracellular hydration resulted in systematically larger T(2) values. It seems probable that increases in extracellular protein concentrations or in the extent of intracellular hydration do not play a significant role in the diffusion changes detected in pathological conditions. T(2) appears to depend on the level of hydration or the total water content but is seemingly less dependent of the concentration and the nature of the extracellular protein in our model solutions.

T2' imaging predicts infarct growth beyond the acute diffusion-weighted imaging lesion in acute stroke

PURPOSE

To show that measurement of the transverse relaxation time that characterizes signal loss caused by local susceptibilities (T2') is sensitive to an increased deoxyhemoglobin concentration in the brain, indicating tissue at risk for infarction.

MATERIALS AND METHODS

The study was approved by the local institutional review board; patients or their guardians provided informed consent. Magnetic resonance (MR) imaging was performed within 6 hours of symptom onset and again 1-11 days thereafter in 100 consecutive stroke patients, all of whom received intravenous thrombolytic therapy (mean age, 67 years). The MR imaging protocol included diffusion- and perfusion-weighted imaging for determination of apparent diffusion coefficient (ADC) and time to peak (TTP), along with quantitative T2 and T2* imaging. T2' maps were calculated and visually compared with ADC and TTP lesions by two independent observers.

RESULTS

A T2'>ADC mismatch was observed by reader 1 in 73 (73%) of 100 patients, and by reader 2 in 65 (65%) patients. Respective sensitivities of T2'>ADC and of TTP>ADC mismatches for later infarct growth were 0.87 and 0.98 for reader 1 and 0.78 and 0.98 for reader 2, with respective specificities of 0.42 and 0.04 for reader 1 and 0.46 and 0.04 for reader 2. The odds ratios for infarct growth in the presence of a T2'>ADC mismatch were 4.59 (reader 1 P = .002) and 3.10 (reader 2 P = .012), while the odds ratios for TTP>ADC mismatch were 2.22 (reader 1 P = .606) and 1.73 (reader 2 P > .999).

CONCLUSION

The presence of a T2'>ADC mismatch is a more specific predictor of infarct growth than is TTP>ADC mismatch and hence may be of clinical value in patient selection for acute stroke therapies in the future.

Blood-brain barrier permeability to manganese and to Gd-DOTA in a rat model of transient cerebral ischaemia

Loss of integrity of the blood-brain barrier (BBB) and brain swelling is a potentially lethal complication of reperfusion in human stroke. To assess the time course of BBB modifications, we performed angiography, diffusion-weighted imaging, T1-weighted (T1 W) imaging and T1 mapping, and monitored acute changes after middle cerebral artery occlusion and recanalization in rats (n = 27). The animals were grouped according to the duration of occlusion: 30 min (group A, n = 8), 1 h 30 min (group B, n = 9), and 2 h 30 min (group C, n = 10). For 17 animals (four in group A, six in group B, and seven in group C), MnCl2 and dimeglumine gadoterate (Gd-DOTA) were injected at 13 min and 34 min after recanalization, respectively. The 10 remaining animals (control groups) underwent the same acquisition protocols, but no contrast agents were injected. Cell damage was determined 1 h after recanalization on haematoxylin and eosin-stained sections. Our results indicate that in the middle cerebral artery occlusion model in the rat, changes in BBB permeability assessed by contrast agent extravasation occur within the first hour of reperfusion, even after an occlusion period not exceeding 30 min. No differences between BBB permeability to Gd-DOTA and Mn2+ were detected in our experimental conditions. The reduction in apparent diffusion coefficient during occlusion appears to be a good predictor of BBB modifications after reperfusion in this model.