Estimation of water extraction fractions in rat brain using magnetic resonance measurement of perfusion with arterial spin labeling

Water exchange across the blood-brain barrier and epilepsy: Review on pathophysiology and neuroimaging

The blood-brain barrier (BBB) is a barrier protecting the brain and a milieu of continuous exchanges between blood and brain. There is emerging evidence that the BBB plays a major role in epileptogenesis and drug-resistant epilepsy, through several mechanisms, such as water homeostasis dysregulation, overexpression of drug transporters, and inflammation. Studies have shown abnormal water homeostasis in epileptic tissue and altered aquaporin-4 water channel expression in animal epilepsy models. This review focuses on abnormal water exchange in epilepsy and describes recent non-invasive MRI methods of quantifying water exchange. PLAIN LANGUAGE SUMMARY: Abnormal exchange between blood and brain contribute to seizures and epilepsy. The authors describe why correct water balance is necessary for healthy brain functioning and how it is impacted in epilepsy. This review also presents recent MRI methods to measure water exchange in human brain. These measures would improve our understanding of factors leading to seizures.

Blood-brain barrier water exchange measurements using contrast-enhanced ASL

A technique for quantifying regional blood-brain barrier (BBB) water exchange rates using contrast-enhanced arterial spin labelling (CE-ASL) is presented and evaluated in simulations and in vivo. The two-compartment ASL model describes the water exchange rate from blood to tissue,k b , but to estimatek b in practice it is necessary to separate the intra- and extravascular signals. This is challenging in standard ASL data owing to the small difference inT 1 values. Here, a gadolinium-based contrast agent is used to increase thisT 1 difference and enable the signal components to be disentangled. The optimal post-contrast bloodT 1 (T 1 , b post ) at 3 T was determined in a sensitivity analysis, and the accuracy and precision of the method quantified using Monte Carlo simulations. Proof-of-concept data were acquired in six healthy volunteers (five female, age range 24-46 years). The sensitivity analysis identified the optimalT 1 , b post at 3 T as 0.8 s. Simulations showed thatk b could be estimated in individual cortical regions with a relative error ϵ < 1 % and coefficient of variation CoV = 30 %; however, a high dependence on bloodT 1 was also observed. In volunteer data, mean parameter values in grey matter were: arterial transit timet A = 1 . 15 ± 0 . 49  s, cerebral blood flow f = 58 . 0 ± 14 . 3  mL blood/min/100 mL tissue and water exchange ratek b = 2 . 32 ± 2 . 49 s-1 . CE-ASL can provide regional BBB water exchange rate estimates; however, the clinical utility of the technique is dependent on the achievable accuracy of measuredT 1 values.

Non-contrast estimate of blood-brain barrier permeability in humans using arterial spin labeling and magnetization transfer at 7 T

Blood-brain barrier (BBB) dysfunction is associated with a number of central nervous system diseases. This study demonstrates the application of a novel noninvasive technique to measure the BBB permeability in the human brain at 7 T. The technique exploits the fact that, when tissue macromolecules are saturated by off-resonance RF pulse, the intravascular and the extravascular (tissue) water experience different magnetization transfer effects. This principle was combined with arterial spin labeling to distinguish between the intravascular and the tissue water, and was used to calculate perfusion, water extraction fraction (E), and BBB permeability surface area product for water (PS). Simultaneous coregistered magnetization transfer ratio maps were also generated that can provide valuable additional information. Eighteen healthy volunteers (seven females), age = 27 ± 11 years and weight = 65 ± 9 kg, participated in the study. Average perfusion was 67 ± 5 and 29 ± 4 ml/100 g/min (p < 0.05); and E was 0.921 ± 0.025 and 0.962 ± 0.015 (p < 0.05) in the gray matter (GM) and the white matter (WM), respectively. PS was higher in the GM (171 ± 20 ml/100 g/min) compared with the WM (95 ± 18 ml/100 g/min) (p < 0.05). The parameters exhibited good reliability with test re-test experiments. The sensitivity of this technique was demonstrated by 200 mg caffeine intake, which resulted in a decrease in the resting PS by ~31%.

Contributions of blood-brain barrier imaging to neurovascular unit pathophysiology of Alzheimer's disease and related dementias

The blood-brain barrier (BBB) plays important roles in the maintenance of brain homeostasis. Its main role includes three kinds of functions: (1) to protect the central nervous system from blood-borne toxins and pathogens; (2) to regulate the exchange of substances between the brain parenchyma and capillaries; and (3) to clear metabolic waste and other neurotoxic compounds from the central nervous system into meningeal lymphatics and systemic circulation. Physiologically, the BBB belongs to the glymphatic system and the intramural periarterial drainage pathway, both of which are involved in clearing interstitial solutes such as β-amyloid proteins. Thus, the BBB is believed to contribute to preventing the onset and progression for Alzheimer's disease. Measurements of BBB function are essential toward a better understanding of Alzheimer's pathophysiology to establish novel imaging biomarkers and open new avenues of interventions for Alzheimer's disease and related dementias. The visualization techniques for capillary, cerebrospinal, and interstitial fluid dynamics around the neurovascular unit in living human brains have been enthusiastically developed. The purpose of this review is to summarize recent BBB imaging developments using advanced magnetic resonance imaging technologies in relation to Alzheimer's disease and related dementias. First, we give an overview of the relationship between Alzheimer's pathophysiology and BBB dysfunction. Second, we provide a brief description about the principles of non-contrast agent-based and contrast agent-based BBB imaging methodologies. Third, we summarize previous studies that have reported the findings of each BBB imaging method in individuals with the Alzheimer's disease continuum. Fourth, we introduce a wide range of Alzheimer's pathophysiology in relation to BBB imaging technologies to advance our understanding of the fluid dynamics around the BBB in both clinical and preclinical settings. Finally, we discuss the challenges of BBB imaging techniques and suggest future directions toward clinically useful imaging biomarkers for Alzheimer's disease and related dementias.

Evidence of cerebral hypoperfusion consecutive to ultrasound-mediated blood-brain barrier opening in rats

PURPOSE

This work aims to explore the effect of Blood Brain Barrier (BBB) opening using ultrasound combined with microbubbles injection on cerebral blood flow in rats.

METHODS

Two groups of n = 5 rats were included in this study. The first group was used to investigate the impact of BBB opening on the Arterial Spin Labeling (ASL) signal, in particular on the arterial transit time (ATT). The second group was used to analyze the spatiotemporal evolution of the change in cerebral blood flow (CBF) over time following BBB opening and validate these results using DSC-MRI.

RESULTS

Using pCASL, a decrease in CBF of up to 29 . 6 ± 15 . 1 % $$ 29.6\pm 15.1\% $$ was observed in the target hemisphere, associated with an increase in arterial transit time. The latter was estimated to be 533 ± 121ms $$ 533\pm 12\mathrm{1ms} $$ in the BBB opening impacted regions against 409 ± 93ms $$ 409\pm 93\mathrm{ms} $$ in the contralateral hemisphere. The spatio-temporal analysis of CBF maps indicated a nonlocal hypoperfusion. DSC-MRI measurements were consistent with the obtained results.

CONCLUSION

This study provided strong evidence that BBB opening using microbubble intravenous injection induces a transient hypoperfusion. A spatiotemporal analysis of the hypoperfusion changes allows to establish some points of similarity with the cortical spreading depression phenomenon.

Pattern of Altered Magnetization Transfer Rate in Alzheimer's Disease

BACKGROUND

Biomarkers for Alzheimer's disease (AD) are crucial for early diagnosis and treatment monitoring once disease modifying therapies become available.

OBJECTIVE

This study aims to quantify the forward magnetization transfer rate (kfor) map from brain tissue water to macromolecular protons and use it to identify the brain regions with abnormal kfor in AD and AD progression.

METHODS

From the Cardiovascular Health Study (CHS) cognition study, magnetization transfer imaging (MTI) was acquired at baseline from 63 participants, including 20 normal controls (NC), 18 with mild cognitive impairment (MCI), and 25 AD subjects. Of those, 53 participants completed a follow-up MRI scan and were divided into four groups: 15 stable NC, 12 NC-to-MCI, 12 stable MCI, and 14 MCI/AD-to-AD subjects. kfor maps were compared across NC, MCI, and AD groups at baseline for the cross-sectional study and across four longitudinal groups for the longitudinal study.

RESULTS

We found a lower kfor in the frontal gray matter (GM), parietal GM, frontal corona radiata (CR) white matter (WM) tracts, frontal and parietal superior longitudinal fasciculus (SLF) WM tracts in AD relative to both NC and MCI. Further, we observed progressive decreases of kfor in the frontal GM, parietal GM, frontal and parietal CR WM tracts, and parietal SLF WM tracts in stable MCI. In the parietal GM, parietal CR WM tracts, and parietal SLF WM tracts, we found trend differences between MCI/AD-to-AD and stable NC.

CONCLUSION

Forward magnetization transfer rate is a promising biomarker for AD diagnosis and progression.

Optical and magnetic resonance imaging approaches for investigating the tumour microenvironment: state-of-the-art review and future trends

The tumour microenvironment (TME) strongly influences tumorigenesis and metastasis. Two of the most characterized properties of the TME are acidosis and hypoxia, both of which are considered hallmarks of tumours as well as critical factors in response to anticancer treatments. Currently, various imaging approaches exist to measure acidosis and hypoxia in the TME, including magnetic resonance imaging (MRI), positron emission tomography and optical imaging. In this review, we will focus on the latest fluorescent-based methods for optical sensing of cell metabolism and MRI as diagnostic imaging tools applied both in vitro and in vivo. The primary emphasis will be on describing the current and future uses of systems that can measure intra- and extra-cellular pH and oxygen changes at high spatial and temporal resolution. In addition, the suitability of these approaches for mapping tumour heterogeneity, and assessing response or failure to therapeutics will also be covered.

A Comprehensive View on MRI Techniques for Imaging Blood-Brain Barrier Integrity

The blood-brain barrier (BBB) is the interface between the blood and brain tissue, which regulates the maintenance of homeostasis within the brain. Impaired BBB integrity is increasingly associated with various neurological diseases. To gain a better understanding of the underlying processes involved in BBB breakdown, magnetic resonance imaging (MRI) techniques are highly suitable for noninvasive BBB assessment. Commonly used MRI techniques to assess BBB integrity are dynamic contrast-enhanced and dynamic susceptibility contrast MRI, both relying on leakage of gadolinium-based contrast agents. A number of conceptually different methods exist that target other aspects of the BBB. These alternative techniques make use of endogenous markers, such as water and glucose, as contrast media. A comprehensive overview of currently available MRI techniques to assess the BBB condition is provided from a scientific point of view, including potential applications in disease. Improvements that are required to make these techniques clinically more easily applicable will also be discussed.

Measuring water exchange across the blood-brain barrier using MRI

The blood-brain barrier (BBB) regulates the transfer of solutes and essential nutrients into the brain. Growing evidence supports BBB dysfunction in a range of acute and chronic brain diseases, justifying the need for novel research and clinical tools that can non-invasively detect, characterize, and quantify BBB dysfunction in-vivo. Many approaches already exist for measuring BBB dysfunction in man using positron emission tomography and magnetic resonance imaging (e.g. dynamic contrast-enhanced MRI measurements of gadolinium leakage). This review paper focusses on MRI measurements of water exchange across the BBB, which occurs through a wide range of pathways, and is likely to be a highly sensitive marker of BBB dysfunction. Key mathematical models and acquisition methods are discussed for the two main approaches: those that utilize contrast agents to enhance relaxation rate differences between the intravascular and extravascular compartments and so enhance the sensitivity of MRI signals to BBB water exchange, and those that utilize the dynamic properties of arterial spin labelling to first isolate signal from intravascular spins and then estimate the impact of water exchange on the evolving signal. Data from studies in healthy and pathological brain tissue are discussed, in addition to validation studies in rodents.

Detection of hemodynamic impairment on 15O gas PET using visual assessment of arterial spin-labeling MR imaging in patients with moyamoya disease

It is unclear whether the visual assessment of noninvasive arterial spin labeling magnetic resonance imaging (ASL) can identify instances of hemodynamic compromise including an elevated oxygen extraction fraction (OEF) measured by ¹⁵O-gas positron emission tomography (PET). Here we evaluated the relationship between a four-point visual assessment system referred to as 'ASL scores' using ASL with two postlabeling delays (PLDs; 1525 ms and 2525 ms) and some quantitative hemodynamic parameters measured by PET. We retrospectively evaluated the cases of 18 Japanese patients with moyamoya disease who underwent ASL and PET. We compared the patients' regional ASL scores on two ASL images to the regional values of PET parameters, and we observed a significant trend in accord with the presumed clinical severity among all PET parameters and ASL scores (p < .003). The ASL score of the long PLD (2525 ms) showed the highest specificity (98.5%) for elevated OEF. Our results suggest that hemodynamic impairment (including elevated OEF) in patients with moyamoya disease may be grossly assessed by a visual assessment of noninvasive ASL images, which can be easily obtained in clinical settings.

Noninvasive Evaluation of CBF and Perfusion Delay of Moyamoya Disease Using Arterial Spin-Labeling MRI with Multiple Postlabeling Delays: Comparison with 15O-Gas PET and DSC-MRI

BACKGROUND AND PURPOSE

Arterial spin-labeling MR imaging with multiple postlabeling delays has a potential to evaluate various hemodynamic parameters. To clarify whether arterial spin-labeling MR imaging can identify CBF and perfusion delay in patients with Moyamoya disease, we compared arterial spin-labeling, DSC, and ¹⁵O-gas PET in terms of their ability to identify these parameters.

MATERIALS AND METHODS

Eighteen patients with Moyamoya disease (5 men, 13 women; ages, 21-55 years) were retrospectively analyzed. CBF values of pulsed continuous arterial spin-labeling using 2 postlabeling delays (short arterial spin-labeling, 1525 ms; delayed arterial spin-labeling, 2525 ms) were compared with CBF values measured by ¹⁵O-gas PET. All plots were divided into 2 groups by the cutoff of time-based parameters (the time of the maximum observed concentration, TTP, MTT, delay of MTT to cerebellum, and disease severity [symptomatic or not]). The ratio of 2 arterial spin-labeling CBFs (delayed arterial spin-labeling CBF to short arterial spin-labeling CBF) was compared with time-based parameters: time of the maximum observed concentration, TTP, and MTT.

RESULTS

The short arterial spin-labeling-CBF values were significantly correlated with the PET-CBF values (r = 0.63; P = .01). However, the short arterial spin-labeling-CBF value dropped in the regions with severe perfusion delay. The delayed arterial spin-labeling CBF overestimated PET-CBF regardless of the degree of perfusion delay. Delayed arterial spin-labeling CBF/short arterial spin-labeling CBF was well correlated with the time of the maximum observed concentration, TTP, and MTT (ρ = 0.71, 0.64, and 0.47, respectively).

CONCLUSIONS

Arterial spin-labeling using 2 postlabeling delays may detect PET-measured true CBF and perfusion delay in patients with Moyamoya disease. Provided its theoretic basis and limitations are considered, noninvasive arterial spin-labeling could be a useful alternative for evaluating the hemodynamics of Moyamoya disease.

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.

Impact of tissue T1 on perfusion measurement with arterial spin labeling

PURPOSE

Arterial spin labeling (ASL) may provide quantitative maps of cerebral blood flow (CBF). Because labeled water exchanges with tissue water, this study evaluates the influence of tissue T₁ on CBF quantification using ASL.

METHODS

To locally modify T₁ , a low dose of manganese (Mn) was intracerebrally injected in one hemisphere of 19 rats (cortex or striatum). Tissue T₁ and CBF were mapped using inversion recovery and continuous ASL experiments at 4.7T.

RESULTS

Mn reduced the tissue T₁ by more than 30% but had little impact on other tissue properties as assessed via dynamic susceptibility and diffusion MRI. Using a single-compartment model, the use of a single tissue T₁ value yielded a mean relative ipsilateral (Mn-injected) to contralateral (noninjected) CBF difference of -34% in cortex and -22% in striatum tissue. With a T₁ map, these values became -7% and +8%, respectively.

CONCLUSION

A low dose of Mn reduces the tissue T₁ without modifying CBF. Heterogeneous T₁ impacts the ASL estimate of CBF in a region-dependent way. In animals, and when T₁ modifications exceed the accuracy with which the tissue T₁ can be determined, an estimate of tissue T₁ should be obtained when quantifying CBF with an ASL technique. Magn Reson Med 77:1656-1664, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Mapping of arterial transit time by intravascular signal selection

The arterial transit time (δa ) is a potentially important physiological parameter which may provide valuable information for the characterization of cerebrovascular diseases. The present study shows that δa can be measured by arterial spin labeling (ASL) applied quasi-continuously in an amplitude-modulated fashion at the human neck. Imaging was performed using short repetition times and excitation flip angles of 90°, which resulted in the selection of an ASL signal of mostly intravascular origin. Model-independent estimates of δa were obtained directly from the temporal shift of the ASL time series. An extended two-compartment perfusion model was developed in order to simulate the basic features of the proposed method and to validate the evaluation procedure. Vascular structures found in human δa maps, such as the circle of Willis or cerebral border zones, hint at the sensitivity of the method to most sizes of arterial vessels. Group-averaged values of δa measured from the carotid bifurcation to the tissue of interest in selected regions of the human brain ranged from 925 ms in the insular cortex to 2000 ms in the thalamic region.

Noninvasive functional imaging of cerebral blood volume with vascular-space-occupancy (VASO) MRI

Functional MRI (fMRI) based on changes in cerebral blood volume (CBV) can probe directly vasodilatation and vasoconstriction during brain activation or physiologic challenges, and can provide important insights into the mechanism of blood oxygenation level-dependent (BOLD) signal changes. At present, the most widely used CBV fMRI technique in humans is called vascular-space-occupancy (VASO) MRI, and this article provides a technical review of this method. VASO MRI utilizes T1 differences between blood and tissue to distinguish between these two compartments within a voxel, and employs a blood-nulling inversion recovery sequence to yield an MR signal proportional to 1 - CBV. As such, vasodilatation will result in a VASO signal decrease and vasoconstriction will have the reverse effect. The VASO technique can be performed dynamically with a temporal resolution comparable with several other fMRI methods, such as BOLD or arterial spin labeling (ASL), and is particularly powerful when conducted in conjunction with these complementary techniques. The pulse sequence and imaging parameters of VASO can be optimized such that the signal change is predominantly of CBV origin, but careful considerations should be taken to minimize other contributions, such as those from the BOLD effect, cerebral blood flow (CBF) and cerebrospinal fluid (CSF). The sensitivity of the VASO technique is the primary disadvantage when compared with BOLD, but this technique is increasingly demonstrating its utility in neuroscientific and clinical applications.

Blood oxygenation level dependent, blood volume, and blood flow responses to carbogen and hypoxic hypoxia in 9L rat gliomas as measured by MRI

PURPOSE

To study vascular responsiveness to hypoxia and hypercarbia together with vessel size index (VSI) in a 9L rat glioma (n = 11) using multimodal MRI.

MATERIALS AND METHODS

VSI was determined using T2 and T2* MRI following AMI-227 contrast agent. Blood oxygenation level dependent (BOLD) signal response was determined using T2 EPI MRI, blood volume changes using AMI-227 and blood flow by means of continuous arterial spin labeling.

RESULTS

VSI in the cortex, tumor rim, and core of 2.2 ± 1.0, 18.2 ± 5.4, and 23.9 ± 14.7 μm, respectively, showing a larger average vessel size in glioma than in the brain parenchyma. BOLD and blood volume signal changes to hypoxia and hypercapnia were much more profound in the tumor rim than the core. Hypoxia led to rim BOLD signal change that was larger in amplitude and it attained the low value much faster than either core or brain cortex. The vasculature in the rim appears more responsive to respiratory challenges in terms of volume adaptation than the core. Blood flow values within the gliomas were much lower than in the contralateral brain. Neither hypercarbia nor hypoxia had an effect on the tumor blood flow.

CONCLUSION

Vascular responses of 9L gliomas to respiratory challenge, in particular hypoxia, are heterogeneous between the core and rim zones, potentially offering a means to classify and separate intratumor tissues with differing hemodynamic characteristics.

Estimation of single-kidney glomerular filtration rate without exogenous contrast agent

PURPOSE

Measurement of single-kidney filtration fraction and glomerular filtration rate (GFR) without exogenous contrast is clinically important to assess renal function and pathophysiology, especially for patients with comprised renal function. The objective of this study is to develop a novel MR-based tool for noninvasive quantification of renal function using conventional MR arterial spin labeling water as endogenous tracer.

THEORY AND METHODS

The regional differentiation of the arterial spin labeling water between the glomerular capsular space and the renal parenchyma was characterized and measured according to their MR relaxation properties (T1ρ or T2 ), and applied to the estimation of filtration fraction and single-kidney GFR. The proposed approach was tested to quantify GFR in healthy volunteers at baseline and after a protein-loading challenge.

RESULTS

Biexponential decay of the cortical arterial spin labeling water MR signal was observed. The major component decays the same as parenchyma water; the minor component decays much slower as expected from glomerular ultra-filtrates. The mean single-kidney GFR was estimated to be 49 ± 9 mL/min at baseline and increased by 28% after a protein-loading challenge.

CONCLUSION

We developed an arterial spin labeling-based MR imaging method that allows us to estimate renal filtration fraction and singe-kidney GFR without use of exogenous contrast.

Dose-dependent effect of isoflurane on regional cerebral blood flow in anesthetized macaque monkeys

The dose-dependent effect of isoflurane on regional CBF of cortical and subcortical structures in anesthetized macaque monkeys was investigated with the Continuous ASL MRI technique. High concentration of isoflurane resulted in global CBF increase and blood pressure decrease. Evident CBF change was observed in the subcortical structures. Specifically, CBF in thalamus and cerebellum was increased about 39% and 55% when isoflurane concentration was changed from 0.75% to 1.5%, respectively. Also, those regional CBF changes correlated linearly with isoflurane inspiratory concentrations, indicating impaired CBF autoregulation in these structures. In contrast, no obvious CBF changes were observed in anterior cingulated cortex, motor cortex, medial prefrontal cortex, and caudate. The results demonstrate that, under the 0.75-1.5% isoflurane maintenance doses, the CBF auto-regulation is well preserved in the cerebral cortical regions and caudate, but impaired in thalamus and cerebellum, indicating disturbed CBF-metabolism coupling and functional response in specific subcortical regions of anesthetized macaque monkeys.

Measuring biexponential transverse relaxation of the ASL signal at 9.4 T to estimate arterial oxygen saturation and the time of exchange of labeled blood water into cortical brain tissue

The transverse decay of the arterial spin labeling (ASL) signal was measured at four inflow times in the rat brain cortex at 9.4 T. Biexponential T2 decay was observed that appears to derive from different T2 values associated with labeled water in the intravasculature (IV) and extravascular (EV) compartments. A two compartment biexponential model was used to assess the relative contribution of the IV and EV compartments to the ASL signal, without assuming a value for T2 of labeled blood water in the vessels. This novel methodology was applied to estimate the exchange time of blood water into EV tissue space and the oxygen saturation of blood on the arterial side of the vasculature. The mean exchange time of labeled blood water was estimated to be 370±40 ms. The oxygen saturation of the arterial side of the vasculature was significantly less than 100% (∼85%), which may have implications for quantitative functional magnetic resonance imaging studies where the arterial oxygen saturation is frequently assumed to be 100%.

Noninvasive estimation of the arterial input function in positron emission tomography imaging of cerebral blood flow

Positron emission tomography (PET) with (15)O-labeled water can provide reliable measurement of cerebral blood flow (CBF). Quantification of CBF requires knowledge of the arterial input function (AIF), which is usually provided by arterial blood sampling. However, arterial sampling is invasive. Moreover, the blood generally is sampled at the wrist, which does not perfectly represent the AIF of the brain, because of the effects of delay and dispersion. We developed and validated a new noninvasive method to obtain the AIF directly by PET imaging of the internal carotid artery in a region of interest (ROI) defined by coregistered high-resolution magnetic resonance angiography. An ROI centered at the petrous portion of the internal carotid artery was defined, and the AIF was estimated simultaneously with whole brain blood flow. The image-derived AIF (IDAIF) method was validated against conventional arterial sampling. The IDAIF generated highly reproducible CBF estimations, generally in good agreement with the conventional technique.

Cerebral blood flow quantification in the rat: a direct comparison of arterial spin labeling MRI with radioactive microsphere PET

Background

Arterial spin labeling magnetic resonance imaging (ASL-MRI) has been recognised as a valuable method for non-invasive assessment of cerebral blood flow but validation studies regarding quantification accuracy by comparison against an accepted gold standard are scarce, especially in small animals. We have conducted the present study with the aim of comparing ASL flow-sensitive alternating inversion recovery (FAIR)-derived unidirectional water uptake (K₁) and ⁶⁸Ga/⁶⁴Cu microsphere (MS)-derived blood flow (f) in the rat brain.

Methods

In 15 animals, K₁and f were determined successively in dedicated small animal positron emission tomography and MR scanners. The Renkin-Crone model modified by a scaling factor was used for the quantification of f and K₁.

Results

Below about 1 mL/min/mL, we obtain an approximately linear relationship between f and K₁. At higher flow values, the limited permeability of water at the blood brain barrier becomes apparent. Within the accessed dynamic flow range (0.2 to 1.9 mL/min/mL), the data are adequately described by the Renkin-Crone model yielding a permeability surface area product of (1.53±0.46) mL/min/mL.

Conclusion

The ASL-FAIR technique is suitable for absolute blood flow quantification in the rat brain when using a one-compartment model including a suitable extraction correction for data evaluation.

Trial registration

24-9168.21-4/2004-1 (registered in Freistadt Sachsen, Landesdirektion Dresden)

Using manganese-enhanced MRI to understand BOLD

The 1990s were designated "The Decade of the Brain" by U.S. Congress, perhaps in great anticipation of the impact that functional neuroimaging techniques would have on advancing our understanding of how the brain is functionally organized. While it is impossible to overestimate the impact of functional MRI in neuroscience, many aspects of the blood oxygenation level-dependent (BOLD) contrast remain poorly understood, in great part due to the complex relationship between neural activity and hemodynamic changes. To better understand such relationship, it is important to probe neural activity independently. Manganese-enhanced MRI (MEMRI), when used to monitor neural activity, is a technique that uses the divalent manganese ion, Mn(2+), as a surrogate measure of calcium influx. A major advantage of using Mn(2+) as a functional marker is that the contrast obtained is directly related to the accumulation of the ion in excitable cells in an activity dependent manner. As such, the contrast in MEMRI is more directly related to neural activity then hemodynamic-based fMRI techniques. In the present work, the early conceptualization of MEMRI is reviewed, and the comparative experiments that have helped provide a better understanding of the spatial specificity of BOLD signal changes in the cortex is discussed.

Early development of arterial spin labeling to measure regional brain blood flow by MRI

Two major avenues of work converged in the late 1980's and early 1990's to give rise to brain perfusion MRI. The development of anatomical brain MRI quickly had as a major goal the generation of angiograms using tricks to label flowing blood in macroscopic vessels. These ideas were aimed at getting information about microcirculatory flow as well. Over the same time course the development of in vivo magnetic resonance spectroscopy had as its primary goal the assessment of tissue function and in particular, tissue energetics. For this the measurement of the delivery of water to tissue was critical for assessing tissue oxygenation and viability. The measurement of the washin/washout of "freely" diffusible tracers by spectroscopic based techniques pointed the way for quantitative approaches to measure regional blood flow by MRI. These two avenues came together in the development of arterial spin labeling (ASL) MRI techniques to measure regional cerebral blood flow. The early use of ASL to measure brain activation to help verify BOLD fMRI led to a rapid development of ASL based perfusion MRI. Today development and applications of regional brain blood flow measurements with ASL continues to be a major area of activity.

Measurement of absolute arterial cerebral blood volume in human brain without using a contrast agent

Arterial cerebral blood volume (CBV(a) ) is a vital indicator of tissue perfusion and vascular reactivity. We extended the recently developed inflow vascular-space-occupancy (iVASO) MRI technique, which uses spatially selective inversion to suppress the signal from blood flowing into a slice, with a control scan to measure absolute CBV(a) using cerebrospinal fluid (CSF) for signal normalization. Images were acquired at multiple blood nulling times to account for the heterogeneity of arterial transit times across the brain, from which both CBV(a) and arterial transit times were quantified. Arteriolar CBV(a) was determined separately by incorporating velocity-dependent bipolar crusher gradients. Gray matter (GM) CBV(a) values (n=11) were 2.04 ± 0.27 and 0.76 ± 0.17 ml blood/100 ml tissue without and with crusher gradients (b=1.8 s/mm(2) ), respectively. Arterial transit times were 671 ± 43 and 785 ± 69 ms, respectively. The arterial origin of the signal was validated by measuring its T(2) , which was within the arterial range. The proposed approach does not require exogenous contrast agent administration, and provides a non-invasive alternative to existing blood volume techniques for mapping absolute CBV(a) in studies of brain physiology and neurovascular diseases.

Arterial spin labeling and dynamic susceptibility contrast CBF MRI in postischemic hyperperfusion, hypercapnia, and after mannitol injection

Arterial spin labeling (ASL) and dynamic susceptibility contrast (DSC) magnetic resonance imaging (MRI) are widely used to image cerebral blood flow (CBF) in stroke. This study examined how changes in tissue spin-lattice relaxation-time constant (T(1)), blood-brain barrier (BBB) permeability, and transit time affect CBF quantification by ASL and DSC in postischemic hyperperfusion in the same animals. In Group I (n=6), embolic stroke rats imaged 48 hours after stroke showed regional hyperperfusion. In normal pixels, ASL- and DSC-CBF linearly correlated pixel-by-pixel. In hyperperfusion pixels, ASL-CBF was significantly higher than DSC-CBF pixel-by-pixel (by 25%). T(1) increased from 1.76±0.14 seconds in normal pixels to 1.93±0.17 seconds in hyperperfusion pixels. Arterial transit time decreased from 300 milliseconds in normal pixels to 200 milliseconds in hyperperfusion pixels. ΔR(2)(*) profiles showed contrast-agent leakages in the hyperperfusion regions. In Group II (n=3) in which hypercapnic inhalation was used to increase CBF without BBB disruption, CBF increased overall but ASL- and DSC-CBF remained linearly correlated. In Group III (n=3) in which mannitol was used to break the BBB, ASL-CBF was significantly higher than DSC-CBF. We concluded that in normal tissue, ASL and DSC provide comparable quantitative CBF, whereas in postischemic hyperperfusion, ASL-CBF and DSC-CBF differed significantly because ischemia-induced changes in T(1) and BBB permeability affected the two methods differently.

Striatal and cortical BOLD, blood flow, blood volume, oxygen consumption, and glucose consumption changes in noxious forepaw electrical stimulation

Recent reports showed noxious forepaw stimulation in rats evoked an unexpected sustained decrease in cerebral blood volume (CBV) in the bilateral striatum, whereas increases in spike activity and Fos-immunoreactive cells were observed. This study aimed to further evaluate the hemodynamic and metabolic needs in this model and the sources of negative functional magnetic resonance imaging (fMRI) signals by measuring blood oxygenation-level-dependent (BOLD), cerebral-blood-flow (CBF), CBV, and oxygen-consumption (i.e., cerebral metabolic rate of oxygen (CMRO(2))) changes using an 11.7-T MRI scanner, and glucose-consumption (i.e., cerebral metabolic rate of glucose (CMRglc)) changes using micro-positron emission tomography. In the contralateral somatosensory cortex, BOLD, CBF, CBV, CMRO(2) (n=7, P<0.05), and CMRglc (n=5, P<0.05) increased. In contrast, in the bilateral striatum, BOLD, CBF, and CBV decreased (P<0.05), CMRO(2) decreased slightly, although not significantly from baseline, and CMRglc was not statistically significant from baseline (P>0.05). These multimodal functional imaging findings corroborate the unexpected negative hemodynamic changes in the striatum during noxious forepaw stimulation, and support the hypothesis that striatal hemodynamic response is dominated by neurotransmitter-mediated vasoconstriction, overriding the stimulus-evoked fMRI signal increases commonly accompany elevated neuronal activity. Multimodal functional imaging approach offers a means to probe the unique attributes of the striatum, providing novel insights into the neurovascular coupling in the striatum. These findings may have strong implications in fMRI studies of pain.

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.

Characterizing the origin of the arterial spin labelling signal in MRI using a multiecho acquisition approach

Arterial spin labelling (ASL) can noninvasively isolate the MR signal from arterial blood water that has flowed into the brain. In gray matter, the labelled bolus is dispersed within three main compartments during image acquisition: the intravascular compartment; intracellular tissue space; and the extracellular tissue space. Changes in the relative volumes of the extracellular and intracellular tissue space are thought to occur in many pathologic conditions such as stroke and brain tumors. Accurate measurement of the distribution of the ASL signal within these three compartments will yield better understanding of the time course of blood delivery and exchange, and may have particular application in animal models of disease to investigate the relationship between the source of the ASL signal and pathology. In this study, we sample the transverse relaxation of the ASL perfusion weighted and control images acquired with and without vascular crusher gradients at a range of postlabelling delays and tagging durations, to estimate the tricompartmental distribution of labelled water in the rat cortex. Our results provide evidence for rapid exchange of labelled blood water into the intracellular space relative to the transit time through the vascular bed, and provide a more solid foundation for cerebral blood flow quantification using ASL techniques.

Longitudinal MRI studies in the isoflurane-anesthetized rat: long-term effects of a short hypoxic episode on regulation of cerebral blood flow as assessed by pulsed arterial spin labelling

MRI is a powerful tool for measuring cerebral blood flow (CBF) longitudinally. However, most animal studies require anesthesia, potentially interfering with normal physiology. Isoflurane anesthesia was used here to study CBF regulation during repetitive scanning in rats. MR perfusion images were acquired using FAIR (flow-sensitive alternating inversion recovery) arterial spin labeling, and absolute CBF was calculated. CBF changes in response to a hypoxic (12% O2) and hypercapnic (5% CO2) gas stimulus were monitored. Hypercapnia led to a robust increase in CBF compared with baseline (195.5+/-21.5 vs 123.6+/-17.9 ml/100 g/min), and hypoxia caused a smaller non-significant increase in mean CBF values (145.4+/-13.4 ml/100 g/min). Strikingly, when measurements were repeated 5 days later, CBF was dramatically reduced in hypoxia (93.2+/-8.1 ml/100 g/min) compared with the first imaging session. Without application of the hypoxic and hypercapnic gases during the first MRI, baseline CBF and CBF changes in response to hypoxia at the second MRI were similar to naive rats. Blood gas analyses revealed a slight reduction in arterial oxygenation during the second period of anesthesia compared with the first. These findings indicate that, in isoflurane-anesthetized rats, even a short hypoxic episode can have long-lasting effects on cerebrovascular regulation.

Quantification of cerebral blood flow in nonhuman primates using arterial spin labeling and a two-compartment model

Noninvasive absolute quantification of cerebral blood flow (CBF) with high spatial resolution is still a challenging task. Arterial spin labeling (ASL) is a promising magnetic resonance imaging (MRI) method for accurate perfusion quantification. However, modeling of ASL data is far from being standardized and has not been investigated in great detail. In this study, two-compartment modeling of monkey ASL data in three physiological conditions (baseline, sensory activated and globally elevated CBF) is reported. Absolute perfusion and arterial transit times were derived for gray matter (GM) and white matter (WM) separately. The uncertainties of the model's result were determined by Monte Carlo simulations. The fitted CBF values for GM were 133 ml/min/100 ml at baseline condition, 165 ml/min/100 ml during visual stimulation and 234 ml/min/100 ml for globally elevated CBF after intravenous injection of acetazolamide. The ratio of GM to WM CBF was 2.5 at baseline and was found to decrease to 1.6 after application of acetazolamide. The corresponding arterial transit times decreased from 742 to 607 ms in GM and from 985 to 875 ms in WM. Monte Carlo simulations showed that absolute CBF values can be determined with an error of 11-15%, while the arterial transit time values have a coefficient of variation of 25-31%. With an alternative acquisition scheme, the precision of the arterial transit times can be improved significantly. The CBF values in the occipital lobe of the monkey brain quantified with ASL are higher than previously reported in positron emission tomography studies.

When perfusion meets diffusion: in vivo measurement of water permeability in human brain

Quantification of water permeability can improve the accuracy of perfusion measurements obtained with arterial spin labeling (ASL) methods, and may provide clinically relevant information regarding the functional status of the microvasculature. The amount of labeled water in the vascular and tissue compartments in an ASL experiment can be estimated based on their distinct diffusion characteristics, and in turn, water permeability determined from the relative vascular and tissue contributions. In the present study, a hybrid magnetic resonance imaging technique was introduced by marrying a continuous ASL method with a twice-refocused spin-echo diffusion sequence. Series of diffusion-weighted ASL signals were acquired with systematically varied b values. The signals were modeled with fast and slow decaying components that were associated with the vascular and tissue compartments, respectively. The relative amount of labeled water in the tissue compartment increased from 61% to 74% and to 86% when the postlabeling delay time was increased from 0.8 to 1.2 and to 1.5 secs. With a b value of 50 secs/mm2, the capillary contribution (fast component) of the ASL signal could be effectively minimized. Using the single-pass approximation model, the water permeability of gray matter in the human brain was estimated based on the derived relative water fractions in the tissue and microvasculature. The potential for in vivo magnetic resonance mapping of water permeability was showed using two diffusion weighted ASL measurements with b=0 and 50 secs/mm2 in both healthy subjects and a case of brain tumor.

Functional uncoupling of hemodynamic from neuronal response by inhibition of neuronal nitric oxide synthase

The cerebrovascular coupling under neuronal nitric oxide synthase (nNOS) inhibition was investigated in alpha-chloralose anesthetized rats. Cerebral blood flow (CBF), cerebral blood volume (CBV), and blood oxygenation level dependent (BOLD) responses to electrical stimulation of the forepaw were measured before and after an intraperitoneal bolus of 7-nitroindazole (7-NI), an in vivo inhibitor of the neuronal isoform of nitric oxide synthase. Neuronal activity was measured by recording somatosensory-evoked potentials (SEPs) via intracranial electrodes. 7-Nitroindazole produced a significant attenuation of the activation-elicited CBF (P<10(-6)), CBV (P<10(-6)), and BOLD responses (P<10(-6)), without affecting the baseline perfusion level. The average DeltaCBF was nulled, while DeltaBOLD and DeltaCBV decreased to approximately 30% of their respective amplitudes before 7-NI administration. The average SEP amplitude decreased (P<10(-5)) to approximately 60% of its pretreatment value. These data describe a pharmacologically induced uncoupling between neuronal and hemodynamic responses to functional activation, and provide further support for the critical role of neuronally produced NO in the cerebrovascular coupling.

Quantitative regional cerebral blood flow MRI of animal model of attention-deficit/hyperactivity disorder

The spontaneously hypertensive rat (SHR) has been widely used as an animal model for attention-deficit/hyperactivity disorder (AD/HD), a developmental disorder that affects 3-5% of school-age children. Quantitative high-resolution (180 x 180 x 1500 microm) perfusion magnetic resonance imaging was performed to evaluate regional CBF in AD/HD rats (SHR, n=7) and control Wistar Kyoto rats (WKY, n=9) in the frontal cortex, motor cortex, sensory cortex, corpus callosum, hippocampus, thalamus, globus pallidus, caudoputamen and whole brain. The accuracy of repeated cerebral blood flow (CBF) measurements within animals in these brain regions ranged from 3% to 10% (7 repeated measures) and across animals ranged from 15% to 18% (n=7 rats), respectively, indicating highly accurate and reproducible CBF measurements. Regional CBF of the SHR were statistically different from those of the WKY rats in all structures analyzed (P<0.05) except for the caudate putamen (P=0.09) and the globus pallidus (P=0.12). Whole brain CBF of the SHR (1.5+/-0.2 ml/g/min, mean+/-S.D.) was approximately 25% higher than that of the WKY rats (1.2+/-0.2 ml/g/min), likely due to the hypertensive nature of the AD/HD rat model. Following normalization to eliminate global CBF differences, CBF in the medial prefrontal cortex, a structure thought to be the equivalent of the human dorsolateral prefrontal cortex and widely implicated in AD/HD, was found to be higher in SHR compared to WKY rats (P<0.05). The only other structure that was found to be statistically different after normalization is the corpus callosum (P<0.05). Since resting cerebral blood flow is intricately coupled to resting neural activity, these results suggest that there was abnormal resting neural activity in the medial prefrontal cortex and the corpus callosum between the control and AD/HD animals, consistent with the hyperactivity, impulsivity, inattention, and other AD/HD-like behaviors in this animal model.

What levels of precision are achievable for quantification of perfusion and capillary permeability surface area product using ASL?

We examine the use of arterial spin labeling (ASL) in normal brains of rats and humans to measure perfusion (F) and capillary permeability surface area product (PS) using a previously described two-compartment model. We investigate the experimental limits on F and PS quantification using simulations and experimental verification in rat brain at 9.4T. A sensitivity analysis on the two-compartment model is presented to estimate optimal experimental inversion times (TIs) for F and PS quantification and indicate how sensitive the model would be to changes in F and PS. We present the expected error on flow-sensitive alternating inversion recovery (FAIR)-based F and PS measurements and quantify the precision with which these parameters could be estimated at various signal-to-noise ratios (SNRs). Perfusion was measured in four rat brains using FAIR ASL, and we conclude that perfusion could be quantified with an acceptable level of precision using this technique. However, we found that to measure PS with even a 100% coefficient of variation (CV) would require an SNR increase of approximately 2 orders of magnitude over our acquired data. We conclude that with current MR capabilities and with the experimental approach used in this study, acceptable levels of precision in the measurement of PS are not possible.

Quantification of rodent cerebral blood flow (CBF) in normal and high flow states using pulsed arterial spin labeling magnetic resonance imaging

PURPOSE

To implement a pulsed arterial spin labeling (ASL) technique in rats that accounts for cerebral blood flow (CBF) quantification errors due to arterial transit times (dt)-the time that tagged blood takes to reach the imaging slice-and outflow of the tag.

MATERIALS AND METHODS

Wistar rats were subjected to air or 5% CO(2), and flow-sensitive alternating inversion-recovery (FAIR) perfusion images were acquired. For CBF calculation, we applied the double-subtraction strategy (Buxton et al., Magn Reson Med 1998;40:383-396), in which data collected at two inversion times (TIs) are combined.

RESULTS

The ASL signal fell off more rapidly than expected from TI = one second onward, due to outflow effects. Inversion times for CBF calculation were therefore chosen to be larger than the longest transit times, but short enough to avoid systematic errors caused by outflow of tagged blood. Using our method, we observed a marked regional variability in CBF and dt, and a region dependent response to hypercapnia.

CONCLUSION

Even when flow is accelerated, CBF can be accurately determined using pulsed ASL, as long as dt and outflow of the tag are accounted for.

An account of the discrepancy between MRI and PET cerebral blood flow measures. A high-field MRI investigation

There is controversy concerning the discrepancy between absolute cerebral blood flow (CBF) values measured using positron emission tomography (PET) and magnetic resonance imaging (MRI). To gain insight into this problem, the increased signal-to-noise ratio (SNR) and extended T(1) relaxation times of blood and tissue at 3.0 T were exploited to perform pulsed arterial spin labeling (PASL) MRI measurements as a function of spatial resolution and post-labeling delay. The results indicate that, when using post-labeling delays shorter than 1500 ms, MRI gray matter flow values may become as high as several times the correct CBF values owing to tissue signal contamination by remaining arterial blood water label. For delays above 1500 ms, regional PASL-based CBF values (n = 5; frontal gray matter: 48.8 +/- 3.3(SD) ml/100 g/min; occipital gray matter: 49.3 +/- 4.5 ml/100 g/min) comparable with PET-based measurements can be obtained by using spatial resolutions comparable with PET (5-7.5 mm in-plane). At very high resolution (2.5 x 2.5 x 3 mm(3)), gray matter CBF values were found to increase by 10-20%, a consequence attributed to reduction in partial volume effects with cerebrospinal fluid and white matter. The recent availability of MRI field strengths of 3.0 T and higher will facilitate the use of MRI-based CBF measurements in the clinic.

Modulatory role of cyclooxygenase-2 in cerebrovascular coupling

To investigate the role of cyclooxygenase-2 (COX-2) in the cerebrovascular coupling, hemodynamic and neuronal responses to forepaw stimulation were measured in alpha-chloralose-anesthetized rats (N = 18) before and after intravenous administration of Meloxicam (MEL), a preferential COX-2 inhibitor, and following a bolus of prostaglandin E(2) (PGE(2)), a prominent vasodilatatory product of COX-2 catalyzed metabolism of arachidonic acid. The cerebral blood flow (CBF) and blood-oxygenation-level-dependent (BOLD) response was quantified using continuous arterial spin labeling magnetic resonance imaging. Neuronal activity was measured by recording somatosensory-evoked potentials (SEPs) via intracranial electrodes. Both MEL and PGE(2) had a significant effect on the activation-elicited CBF (P < 10(-6)) and BOLD (P < 10(-6)) responses, without affecting the baseline perfusion. Meloxicam decreased brain COX enzymatic activity by 57 +/- 14% and decreased the stimulation-induced CBF response to 32 +/- 2% and BOLD to 46 +/- 1% of their respective pre-drug amplitudes. In turn, PGE(2) bolus resulted in a partial recovery of functional hyperemia, with the CBF response recovering to 52 +/- 3% and the BOLD response to 56 +/- 2% of their values prior to MEL administration. There was no concomitant decrease in either amplitudes or latencies of SEP components. These findings suggest a modulatory role of COX-2 products in the cerebrovascular coupling and provide evidence for existence of a functional metabolic buffer.

Imaging vascular physiology to monitor cancer treatment

The primary physiological function of the vasculature is to support perfusion, the nutritive flow of blood through the tissues. Vascular physiology can be studied non-invasively in human subjects using imaging methods such as positron emission tomography (PET), magnetic resonance imaging (MRI), X-ray computed tomography (CT), and Doppler ultrasound (DU). We describe the physiological rationale for imaging vascular physiology with these methods. We review the published data on repeatability. We review the literature on 'before-and-after' studies using these methods to monitor response to treatment in human subjects, in five broad clinical settings: (1) antiangiogenic agents, (2) vascular disruptive agents, (3) conventional cytotoxic drugs, (4) radiation treatment, and (5) agents affecting drug delivery. We argue that imaging of vascular physiology offers an attractive 'functional endpoint' for clinical trials of anticancer treatment. More conventional measures of tumour response, such as size criteria and the uptake of fluorodeoxyglucose, may be insensitive to therapeutically important changes in vascular function.

Quantification of cerebral perfusion using arterial spin labeling: two-compartment models

One of the advantages of arterial spin labeling (ASL) techniques over other techniques for measuring cerebral perfusion is that with ASL it is possible to achieve accurate quantification. This is particularly useful in the field of functional imaging, where accurate measurements of perfusion change can help untangle the complex physiological changes that occur following neuronal activation. However, the linearity of the perfusion estimate over a wide range of perfusion values may be more important than absolute values. For several years, single-compartment models have dominated the literature, and it has been assumed that the labeled water diffuses freely throughout the tissue voxel. However, recent work, as summarized in this review, has shown that this assumption is inaccurate and leads to an overestimation of perfusion at low perfusion rates, and an underestimation at high rates. The inclusion of restricted permeability of the capillary wall to water in a two-compartment model offers improved quantification.

Perfusion-based fMRI: insights from animal models

Modern functional neuroimaging techniques, including functional magnetic resonance imaging (fMRI), positron emission tomography (PET), and optical imaging of intrinsic signals (OIS), rely on a tight coupling between neural activity and cerebral blood flow (CBF) to visualize brain activity using CBF as a surrogate marker. Because CBF is a uniquely defined physiological parameter, fMRI techniques based on CBF contrast have the advantage of being specific to tissue signal change, and the potential to provide more direct and quantitative measures of brain activation than blood oxygenation level-dependent (BOLD)- or cerebral blood volume (CBV)-based techniques. The changes in CBF elicited by increased neural activity are an excellent index of the magnitude of electrical activity. Increases in CBF are more closely localized to the foci of increased electrical activity, and occur more promptly to the stimulus than BOLD- or CBV-based contrast. In addition, CBF-based fMRI is less affected by confounds from venous drainage common to BOLD. Animal studies of brain activation have yielded considerable insights into the advantages of CBF-based fMRI. Based on results provided by animal studies, CBF fMRI may offer a means of better assessing the magnitude, spatial extent, and temporal response of neural activity, and may be more specific to tissue state. These properties are expected to be particularly useful for longitudinal and quantitative fMRI studies.

Study design in fMRI: basic principles

There is a wide range of functional magnetic resonance imaging (fMRI) study designs available for the neuroscientist who wants to investigate cognition. In this manuscript we review some aspects of fMRI study design, including cognitive comparison strategies (factorial, parametric designs), and stimulus presentation possibilities (block, event-related, rapid event-related, mixed, and self-driven experiment designs) along with technical aspects, such as limitations of signal to noise ratio, spatial, and temporal resolution. We also discuss methods to deal with cases where scanning parameters become the limiting factor (parallel acquisitions, variable jittered designs, scanner acoustic noise strategies).

Quantification of cerebral arterial blood volume using arterial spin labeling with intravoxel incoherent motion-sensitive gradients

Quantification of cerebral arterial blood volume (CBVa) is important for understanding vascular regulation. To enable measurement of CBVa with diffusion-weighted (DW) arterial spin labeling (ASL), a theoretical framework was developed using the effects of intravoxel incoherent motion (IVIM). The pseudo-diffusion coefficient (D*) in the IVIM model was evaluated at 9.4 T in DW-ASL of rat brain under isoflurane anesthesia by variations of both post-labeling delay (w) and magnetization transfer ratio (MTR). D* and its volume fraction decreased at values of w>or=0.3 s, and the normalized apparent diffusion coefficient (ADC) increased with MTR, suggesting that D* is closely correlated with CBVa. Thus, the difference between ASL measurements with and without DW gradients is related to CBVa. The CBVa values measured by this approach were compared with values obtained using the modulation of tissue and vessel (MOTIVE) technique with ASL, which varies MT levels without changing spin labeling efficiency. CBVa values from both methods were highly correlated. The measured CBVa values were linearly correlated with cerebral blood flow (CBF) for a PaCO2 range of 25-50 mmHg; DeltaCBVa (ml/100 g)=0.007 (min-1)xDeltaCBF (ml/100 g/min). The DW-ASL approach is simple and easy to implement for human and animal CBVa studies.

Quantification of cerebral arterial blood volume and cerebral blood flow using MRI with modulation of tissue and vessel (MOTIVE) signals

Regional cerebral arterial blood volume (CBVa) and blood flow (CBF) can be quantitatively measured by modulation of tissue and vessel (MOTIVE) signals, enabling separation of tissue signal from blood. Tissue signal is selectively modulated using magnetization transfer (MT) effects. Blood signal is changed either by injection of a contrast agent or by arterial spin labeling (ASL). The measured blood volume represents CBVa because the contribution from venous blood was insignificant in our measurements. Both CBVa and CBF were quantified in isoflurane-anesthetized rats at 9.4T. CBVa obtained using a contrast agent was 1.1 +/- 0.5 and 1.3 +/- 0.6 ml/100 g tissue (N = 10) in the cortex and caudate putamen, respectively. The CBVa values determined from ASL data were 1.0 +/- 0.3 ml/100 g (N = 10) in both the cortex and the caudate putamen. The match between CBVa values determined by both methods validates the MOTIVE approach. In ASL measurements, the overestimation in calculated CBF values increased with MT saturation levels due to the decreasing contribution from tissue signals, which was confirmed by the elimination of blood with a contrast agent. Using the MOTIVE approach, accurate CBF values can also be obtained.

Procedure for minimizing stress for fMRI studies in conscious rats

Functional magnetic resonance imaging (fMRI) in conscious animals is evolving as a critical tool for neuroscientists. The present study explored the effectiveness of an acclimation procedure in minimizing the stress experienced by the animal as assessed by alterations in physiological parameters including heart rate, respiratory rate, and serum corticosterone levels. Results confirm that as the stress of the protocol is minimized, there is a significant decrease in head movements and enhancement in data quality. The feasibility of improving the quality of fMRI data acquired in alert rats by utilizing a relatively simple technique is presented.

Partial-volume effect on ischemic tissue-fate delineation using quantitative perfusion and diffusion imaging on a rat stroke model

Partial-volume effects (PVE) in stroke imaging could hinder proper delineation of normal, ischemic, and at-risk tissues. Cerebral-blood-flow (CBF) and apparent diffusion coefficient (ADC) were measured at high and low resolution (HR = 128 x 128, LR = 64 x 64) in focal ischemia in rats during the acute phase. The data were evaluated for PVE on ischemic tissue classification on a pixel-by-pixel basis and the misclassified pixels were quantified as ischemia progressed. The main drawbacks of high-resolution imaging are reduced temporal resolution and/or signal-to-noise ratio. The high- versus low-resolution scatterplots and histograms of pixels along the normal-abnormal boundaries in the ADC and CBF maps showed marked ischemia-related PVE. By comparison with the homologous regions in the contralateral normal hemisphere, the effect of increased noise and intrinsic tissue heterogeneity due to high resolution could be distinguished from ischemia-related PVE. Degrading the high-resolution (128 x 128) data to a 64 x 64 or 32 x 32 matrix increased the severity of PVE. Zero-filling of low-resolution (64 x 64) data to 128 x 128 also increased PVE. It was concluded that PVE: (1) misclassified substantial pixels along the normal-abnormal boundaries, (2) overestimated abnormal volumes at the expense of mostly "at-risk" and some "normal" tissues, (3) were more severe at the early time points postischemia, and (4) confounded the interpretation of the operationally defined ischemic penumbra.

Dynamic imaging of perfusion and oxygenation by functional magnetic resonance imaging

Cerebral blood flow can be measured with magnetic resonance imaging (MRI) by arterial spin labeling techniques, where magnetic labeling of flowing spins in arterial blood water functions as the endogenous tracer upon mixing with the unlabeled stationary spins of tissue water. The consequence is that the apparent longitudinal relaxation time (T1) of tissue water is attenuated. A modified functional MRI scheme for dynamic CBF measurement is proposed that depends on extraction of T1 weighting from the blood oxygenation level-dependent (BOLD) image contrast, because the functional MRI signal also has an intrinsic T1 weighting that can be altered by variations of the excitation flip angle. In the alpha-chloralose-anesthetized rat model at 7T, the authors show that the stimulation-induced BOLD signal change measured with two different flip angles can be combined to obtain a T1-weighted MRI signal, reflecting the magnitude of the CBF change, which can be deconvolved to obtain dynamic changes in CBF. The deconvolution of the T1-weighted MRI signal, which is a necessary step for accurate reflection of the dynamic changes in CBF, was made possible by a transfer function obtained from parallel laser-Doppler flowmetry experiments. For all stimulus durations (ranging from 4 to 32 seconds), the peak CBF response measured by MRI after the deconvolution was reached at 4.5 +/- 1.0 seconds, which is in good agreement with (present and prior) laser-Doppler measurements. Because the low flip angle data can also provide dynamic changes of the conventional BOLD image contrast, this method can be used for simultaneous imaging of CBF and BOLD dynamics.

Manganese-enhanced magnetic resonance imaging (MEMRI): methodological and practical considerations

Manganese-enhanced MRI (MEMRI) is being increasingly used for MRI in animals due to the unique T1 contrast that is sensitive to a number of biological processes. Three specific uses of MEMRI have been demonstrated: to visualize activity in the brain and the heart; to trace neuronal specific connections in the brain; and to enhance the brain cytoarchitecture after a systemic dose. Based on an ever-growing number of applications, MEMRI is proving useful as a new molecular imaging method to visualize functional neural circuits and anatomy as well as function in the brain in vivo. Paramount to the successful application of MEMRI is the ability to deliver Mn2+ to the site of interest at an appropriate dose and in a time-efficient manner. A major drawback to the use of Mn2+ as a contrast agent is its cellular toxicity. Therefore, it is critical to use as low a dose as possible. In the present work the different approaches to MEMRI are reviewed from a practical standpoint. Emphasis is given to the experimental methodology of how to achieve significant, yet safe, amounts of Mn2+ to the target areas of interest.