Quantification of relative cerebral blood flow change by flow-sensitive alternating inversion recovery (FAIR) technique: application to functional mapping

Assessment of hemodynamic response during focal neural activity in human using bolus tracking, arterial spin labeling and BOLD techniques

In this study, the hemodynamic response and changes in oxidative metabolism during functional activation were measured using three functional magnetic resonance imaging (fMRI) techniques: the blood oxygenation level-dependent (BOLD) technique, flow-sensitive alternating inversion recovery (FAIR), and bolus tracking (BT) of an MR contrast agent. With these three techniques we independently determined changes in BOLD signal, relative cerebral blood flow (rCBF), and cerebral blood volume (rCBV) associated with brain activation in eight healthy volunteers. In the motor cortex, the BOLD signal increased by 1.8 +/- 0.5%, rCBF by 36.3 +/- 8.2% (FAIR), and 35.1 +/- 8.6% (BT), and rCBV by 19.4 +/- 4.1% (BT) in response to simultaneous bilateral finger tapping. In the visual cortex, BOLD signal increased by 2.6 +/- 0.5%, rCBF by 38.5% +/- 7.6 (FAIR), and 36.9 +/- 8.8% (BT), and rCBV by 18.8 +/- 2.8% (BT) during flickering checkerboard stimulation. Comparing the experimentally measured rCBV with the calculated rCBV using Grubb's power-law relation, we conclude that the use of power-law relationship results in systematic underestimate of rCBV.

An approach to probe some neural systems interaction by functional MRI at neural time scale down to milliseconds

In this paper, we demonstrate an approach by which some evoked neuronal events can be probed by functional MRI (fMRI) signal with temporal resolution at the time scale of tens of milliseconds. The approach is based on the close relationship between neuronal electrical events and fMRI signal that is experimentally demonstrated in concurrent fMRI and electroencephalographic (EEG) studies conducted in a rat model with forepaw electrical stimulation. We observed a refractory period of neuronal origin in a two-stimuli paradigm: the first stimulation pulse suppressed the evoked activity in both EEG and fMRI signal responding to the subsequent stimulus for a period of several hundred milliseconds. When there was an apparent site-site interaction detected in the evoked EEG signal induced by two stimuli that were primarily targeted to activate two different sites in the brain, fMRI also displayed signal amplitude modulation because of the interactive event. With visual stimulation using two short pulses in the human brain, a similar refractory phenomenon was observed in activated fMRI signals in the primary visual cortex. In addition, for interstimulus intervals shorter than the known latency time of the evoked potential induced by the first stimulus ( approximately 100 ms) in the primary visual cortex of the human brain, the suppression was not present. Thus, by controlling the temporal relation of input tasks, it is possible to study temporal evolution of certain neural events at the time scale of their evoked electrical activity by noninvasive fMRI methodology.

A CBF-based event-related brain activation paradigm: characterization of impulse-response function and comparison to BOLD

A perfusion-based event-related functional MRI method for the study of brain activation is presented. In this method, cerebral blood flow (CBF) was measured using a recently developed multislice arterial spin-labeling (ASL) perfusion imaging method with rapid spiral scanning. Temporal resolution of the perfusion measurement was substantially improved by employing intertrial subtraction and stimulus-shifting schemes. Perfusion and blood oxygenation level-dependent (BOLD) signals were obtained simultaneously by subtracting or adding the control and labeled images, respectively, in the same data sets. The impulse response function (IRF) of perfusion during brain activation was characterized for multiple stimulus durations and compared to the simultaneously acquired BOLD response. The CBF response curve preceded the BOLD curve by 0.21 s in the rising phase and 0.64 s in the falling phase. Linear additivity of the CBF and BOLD responses was assessed with rapidly repeated stimulations within single trials, and departure from linearity was found in both responses, characterized as attenuated amplitude and delayed rising time. Event-related visual and sensorimotor activation experiments were successfully performed with the new perfusion technique.

Delay and dispersion effects in dynamic susceptibility contrast MRI: simulations using singular value decomposition

Dynamic susceptibility contrast (DSC) MRI is now increasingly used for measuring perfusion in many different applications. The quantification of DSC data requires the measurement of the arterial input function (AIF) and the deconvolution of the tissue concentration time curve. One of the most accepted deconvolution methods is the use of singular value decomposition (SVD). Simulations were performed to evaluate the effects on DSC quantification of the presence of delay and dispersion in the estimated AIF. Both delay and dispersion were found to introduce significant underestimation of cerebral blood flow (CBF) and overestimation of mean transit time (MTT). While the error introduced by the delay can be corrected by using the information of the arrival time of the bolus, the correction for the dispersion is less straightforward and requires a model for the vasculature.

Effect of restricted water exchange on cerebral blood flow values calculated with arterial spin tagging: a theoretical investigation

Arterial spin tagging techniques originally used the one-compartment Kety model to describe the dynamics of tagged water in the brain. The work presented here develops a more realistic model that includes the contribution of tagged water in the capillary bed and accounts for the finite time required for water to diffuse across the blood-brain barrier. The new model was used to evaluate potential errors in cerebral blood flow values calculated using the one-compartment Kety model. The results predict that if the one-compartment Kety model is used to analyze arterial spin tagging data the observed grey matter cerebral blood flow values should be relatively insensitive to restricted diffusion of water across the capillary bed. For instance, the observed grey matter cerebral blood flow should closely approximate the true cerebral blood flow and not the product of the extraction fraction and the cerebral blood flow. This prediction is in agreement with recent experimental arterial spin tagging results.

RF pulse concatenation for spatially selective inversion

It is shown that spatially selective inversion and saturation can be achieved by concatenation of RF pulses with lower flip angles. A concatenation rule which enables global doubling of the flip angle of any given excitation pulse applied to initial z magnetization is proposed. In this fashion, the selectivity of the single pulse is preserved, making the high selectivity achievable in the low flip-angle regime available for inversion and large flip-angle saturation purposes. The profile quality achievable with exemplary concatenated pulses is investigated in comparison with adiabatic inversion. It is verified that by using concatenated inversion in the transfer insensitive labeling technique (TILT), the MT artifact is suppressed. Copyright 2000 Academic Press.

Detection of the brain response during a cognitive task using perfusion-based event-related functional MRI

Event-related (ER) fMRI has evoked great interest due to the ability to depict the dynamic features of human brain function during various cognitive tasks. Thus far, all cognitive ER-fMRI studies have been based on blood oxygenation level-dependent (BOLD) contrast techniques. Compared with BOLD-based fMRI techniques, perfusion-based fMRI is able to localize the region of neuronal activity more accurately. This report demonstrates, for the first time, the detection of the brain response to a cognitive task using high temporal resolution perfusion-based ER-fMRI. An English verb generation task was used in this study. Results show that perfusion-based ER-fMRI accurately depicts the activation in Broca's area. Average changes in regional relative cerebral blood flow reached a maximum value of 30.7% at approximately 6.5 s after the start of stimulation and returned to 10% of the maximum value at approximately 12.8 s. Our results show that perfusion-based ER-fMRI is a useful tool for cognitive neuroscience studies, providing comparable temporal resolution and better localization of brain function than BOLD ER-fMRI.

The measurement of diffusion and perfusion in biological systems using magnetic resonance imaging

The aim of this review is to describe two recent developments in the use of magnetic resonance imaging (MRI) in the study of biological systems: diffusion and perfusion MRI. Diffusion MRI measures the molecular mobility of water in tissue, while perfusion MRI measures the rate at which blood is delivered to tissue. Therefore, both these techniques measure quantities which have direct physiological relevance. It is shown that diffusion in biological systems is a complex phenomenon, influenced directly by tissue microstructure, and that its measurement can provide a large amount of information about the organization of this structure in normal and diseased tissue. Perfusion reflects the delivery of essential nutrients to tissue, and so is directly related to its status. The concepts behind the techniques are explained, and the theoretical models that are used to convert MRI data to quantitative physical parameters are outlined. Examples of current applications of diffusion and perfusion MRI are given. In particular, the use of the techniques to study the pathophysiology of cerebral ischaemia/stroke is described. It is hoped that the biophysical insights provided by this approach will help to define the mechanisms of cell damage and allow evaluation of therapies aimed at reducing this damage.

Noise reduction in 3D perfusion imaging by attenuating the static signal in arterial spin tagging (ASSIST)

Phase-encoded multishot SPIRAL approaches were used to acquire true 3D cerebral blood flow images of the human head using arterial spin tagging approaches. Multiple-inversion background suppression techniques, which suppress phase noise due to interacquisition fluctuations in the static magnetic field, reduced the temporal standard deviation of true 3D delta M images acquired using arterial spin tagging approaches by approximately 50%. Background suppressed arterial spin tagging (ASSIST) approaches were used to obtain high-resolution isotropic true 3D cerebral blood flow images, and to obtain true 3D activation images during cognitive (working memory) tasks. Magn Reson Med 44:92-100, 2000. Published 2000 Wiley-Liss, Inc.

Comparison of simultaneously measured perfusion and BOLD signal increases during brain activation with T(1)-based tissue identification

Perfusion and blood oxygenation level-dependent (BOLD) signals were simultaneously measured during a finger-tapping task at 3T using QUIPSS II with thin-slice TI(1) periodic saturation, a modified pulsed arterial spin labeling technique that provides quantitative measurement of perfusion. Perfusion and BOLD signal changes due to motor activation were obtained and correlated with the T(1) values estimated from echo-planar imaging (EPI)-based T(1) maps on a voxel-by-voxel basis. The peak perfusion signal occurs in voxels with a T(1) of brain parenchyma while the peak BOLD signal occurs in voxels with a T(1) characteristic of blood and cerebrospinal fluid. The locations of the peak signals of functional BOLD and perfusion only partially overlap on the order of 40%. Perfusion activation maps will likely represent the sites of neuronal activity better than do BOLD activation maps. Magn Reson Med 44:137-143, 2000.

Quantitation of renal perfusion using arterial spin labeling with FAIR-UFLARE

Quantitative perfusion imaging of human kidneys was performed using arterial spin labeling MRI with a fast spin echo readout-sequence. Perfusion maps of centrally located single slices were obtained in axial and coronal orientations. In ten healthy volunteers, the mean value of perfusion was 213+/-55 mL/(100g min) with a range from 140 to 319 mL/(100g min). These results are in accordance with literature data, considering the fact that FAIR only measures the perfusion component normal to the imaging plane. Intra-individual reproducibility errors of +/-11% were smaller than the natural interindividual variability of renal perfusion (SD = +/- 25%). Perfusion in the cortex was approximately 3-4 times higher compared to the medulla. Considering the relatively high resolution of 2x2x10 mm3, the ability to quantify perfusion, and the lack of ionizing radiation and contrast media, this technique should prove useful in diagnosing renal pathologies that are associated with reductions in tissue perfusion.

A protocol for assessing subtraction errors of arterial spin-tagging perfusion techniques in human brain

A protocol for assessing signal contributions from static tissue (subtraction errors) in perfusion images acquired with arterial spin-labeling (ASL) techniques in human brain is proposed. The method exploits the reduction of blood T(1) caused by the clinically available paramagnetic contrast agent, gadopentetate dimeglumine (Gd-DTPA). The protocol is demonstrated clinically with multislice FAIR images acquired before, during, and after Gd-DTPA administration using a range of selective inversion widths. Perfusion images acquired postcontrast for selective inversion widths large enough (threshold) to avoid interaction with the imaging slice had signal intensities reduced to noise level, as opposed to subtraction errors manifested on images acquired using inversion widths below the threshold. The need for these experiments to be performed in vivo is further illustrated by comparison with phantom results. The protocol allows a one-time calibration of relevant ASL parameters (e.g., selective inversion widths) in vivo, which may otherwise cause subtraction errors. Magn Reson Med 43:896-900, 2000. Published 2000 Wiley-Liss, Inc.

A model system for perfusion quantification using FAIR

Flow-sensitive experiments (FAIR) have been performed on a tube-flow phantom in order to validate quantitative perfusion measurements on humans. A straight-forward correspondence between perfusion and bulk-flow is found. It is shown that the flow phantom model only holds when the slice profiles of the involved RF pulses are taken into account. A small flow-independent off-set may be present in the data. The off-set is explained by the model. Based on the correspondence between the phantom and the in vivo models, it is shown that the lowest flow values that could be measured in the phantom correspond to perfusion values lower than the cortical perfusion in the brain. Thus, the experimental accuracy and the computational methods for quantitative perfusion measurements in vivo can be validated by a tube-flow phantom.

Quantification of cerebral blood flow by bolus tracking and artery spin tagging methods

This study deals with perfusion quantification in healthy volunteers using two types of dynamic magnetic resonance imaging (MRI) methods. Absolute cerebral blood flow (CBF) measurements were performed in 11 subjects by applying both bolus tracking of exogenous contrast agent and non-invasive arterial spin labeling MRI techniques. Both methods produced CBF images with good tissue contrast and CBF values are in good agreement with literature data. The correlation between cerebral blood volume (CBV) and CBF is also discussed.

Cerebral perfusion and arterial transit time changes during task activation determined with continuous arterial spin labeling

Perfusion imaging by arterial spin labeling (ASL) can be highly sensitive to the transit time from the labeling site to the tissue. We report the results of a study designed to separate the transit time and perfusion contributions to activation in ASL images accompanying motor and visual stimulation. Fractional transit time decreases were found to be comparable to fractional perfusion increases and the transit time change was found to be the greatest contributor to ASL signal change in ASL sequences without delayed acquisition. The implications for activation imaging with ASL and the arterial control of flow are discussed.

Comparison of the temporal response in perfusion and BOLD-based event-related functional MRI

Event-related functional MRI (ER-fMRI) based on both blood oxygen level-dependent (BOLD) contrast and perfusion contrast has been recently developed to study human brain activation due to brief stimulation. In this report, both BOLD- and perfusion-based ER-fMRI were directly compared using repeated single-trial, short visual stimulation (1 sec) in six human volunteers. The results show that the cerebral blood flow change reached a maximum approximately 1 sec earlier than the BOLD signal change (4.2 +/- 0.2 sec vs. 5.1 +/- 0.2 sec after the stimulation, P < 0.05). The full width at half maximum of the hemodynamic response measured by perfusion was not significantly different from that measured with BOLD (5.1 +/- 0.6 sec vs. 5.9 +/- 0.6 sec). A positive linear correlation was found between the maximum perfusion and maximum BOLD signal changes (r = 0. 77, P = 0.07).

Spin echo entrapped perfusion image (SEEPAGE). A nonsubtraction method for direct imaging of perfusion

Arterial spin-labeled perfusion imaging is increasingly being applied to the study of the brain and other organs. To date, perfusion information has invariably been obtained by subtraction of images with and without spin-labeling of inflowing water. Due to the relatively small amount of blood which enters tissue over a typical inflow period (1-1.5 sec), subtraction errors due to image instability or, in certain circumstances, magnetization transfer effects, can lead to very significant amounts of artifactual image intensity. These problems are avoided in the nonsubtraction method described here. Initially, spins in the imaging slice are selectively saturated, leaving other spins unaffected. A subsequent spin-echo train traps these magnetizations irrespective of flow. Finally, an imaging module generates intensity only from those spins which have entered the imaging slice during the inflow period. A slight modification of the sequence facilitates validation by detecting any contaminating signal in a control image.

Cortical responses to thermal pain depend on stimulus size: a functional MRI study

Cortical activity patterns to thermal painful stimuli of two different sizes were examined in normal volunteers using functional magnetic resonance imaging (fMRI). Seven right-handed subjects were studied when the painful stimulus applied to the right hand fingers covered either 1,074-mm(2)-area large stimulator or 21-mm(2)-area small stimulator. Stimulus temperatures were adjusted to give rise to equivalent moderately painful ratings. fMRI signal increases and decreases were determined for the contralateral parietal and motor areas. When the overall activity in these regions was compared across subjects, increased fMRI activity was observed over more brain volume with the larger stimulator, whereas decreased fMRI activity was seen in more brain volume for the smaller stimulator. The individual subject and group-averaged activity patterns indicated regional specific differences in increased and decreased fMRI activity. The small stimulator resulted in decreased fMRI responses throughout the upper body representation in both primary somatosensory and motor cortices. In contrast, no decreased fMRI signals were seen in the secondary somatosensory cortex and in the insula. In another seven volunteers, the effects of the size of the thermal painful stimulus on vibrotactile thresholds were examined psychophysically. Painful stimuli were delivered to the fingers and vibrotactile thresholds were measured on the arm just distal to the elbow. Consistent with the fMRI results in the primary somatosensory cortex, painful thermal stimuli using the small stimulator increased vibrotactile thresholds on the forearm, whereas similarly painful stimuli using the large stimulator had no effect on forearm vibrotactile thresholds. These results are discussed in relation to the cortical dynamics for pain perception and in relation to the center-surround organization of cortical neurons.

Frequency dependence of the functional MRI response after electrical median nerve stimulation

Localizing sensorimotor areas with high resolution functional MRI is of considerable interest for a wide range of medical applications from the preoperative planning of neurosurgical interventions to determining the course of neuroplastic reorganisation after brain lesions. We examined the effect of the stimulation frequency on the blood oxygen level dependent (BOLD) fMRI response and on perfusion weighted fMRI using electrical median nerve stimulation at 5, 15, 40, and 100 Hz. BOLD fMRI was performed using a single shot gradient echo EPI sequence to acquire 15 contiguous slices. For the qualitative flow sensitive studies, a single slice inversion recovery prepared spin echo echoplanar sequence (IR-SE EPI) was used. In the primary sensorimotor cortex, a linear increase of the fMRI-BOLD response, affecting both the number of activated pixels and the amplitude of the signal changes, was seen with increasing stimulation frequencies. The qualitative in-flow sensitive studies, using the IR-SE EPI sequence, indicate that the tissue perfusion also increases over the same range of frequencies. This implicates that larger fMRI responses can be obtained if electrical median nerve stimulation is performed at higher frequencies. The results are compared with electrophysiological data, which show a decrease of the early somatosensory evoked potentials at higher frequencies.

Detection of regional pulmonary perfusion deficit of the occluded lung using arterial spin labeling in magnetic resonance imaging

Detection of regional perfusion deficit in the lung has been demonstrated using an arterial spin labeling technique called flow-sensitive alternating inversion recovery with an extra radiofrequency pulse (FAIRER). A pulmonary artery was occluded using a nondetachable balloon catheter to simulate an acute pulmonary embolism in 3 of 10 rabbits. Inflating the balloon occludes the artery, and deflating the balloon allows for reperfusion. Perfusion imaging was performed pre-occlusion, during occlusion, and after reperfusion. Signal enhancement due to perfusion of the pulmonary parenchyma was observed in the perfusion images with negligible artifacts. The perfusion deficit of the pulmonary parenchyma was detected distal to the site of occlusion in all three rabbits. Return of the pulmonary parenchymal perfusion was observed after reperfusion. Magnetic resonance imaging using FAIRER can detect signal loss due to absence of perfusion caused by pulmonary embolism.

Cerebral perfusion MRI with arterial spin labeling technique at 0.5 Tesla

PURPOSE

Our aim was to evaluate the feasibility of cerebral perfusion MRI using an arterial spin labeling technique at 0.5 T.

METHOD

We performed perfusion imaging with a flow-sensitive alternating inversion recovery (FAIR) sequence in a total of 37 patients with cerebral infarction.

RESULTS

FAIR perfusion images demonstrated areas of pathological perfusion corresponding (13 patients) or not corresponding (15 patients) to the infarcted area on MR images. Among 19 patients in whom comparison between FAIR perfusion imaging and regional cerebral blood flow single photon emission CT was available, the two studies correlated well in 15 patients.

CONCLUSION

Our results indicate that the FAIR technique allows reliable cerebral perfusion imaging at 0.5 T.

EEG-Linked functional magnetic resonance imaging in epilepsy and cognitive neurophysiology

The ability to trigger functional magnetic resonance imaging (fMRI) acquisitions related to the occurrence of EEG-based physiologic transients has changed the field of fMRI into a more dynamically based technique. By knowing the temporal relationship between focal increases in neuronal firing rates and the provoked focal increase in blood flow, investigators are able to maximize the fMR-linked images that show where the activity originates. Our mastery of recording EEG inside the bore of a MR scanner has also allowed us to develop cognitive paradigms that record not only the fMR BOLD images, but also the evoked potentials (EPs). The EPs can subsequently be subjected to localization paradigms that can be compared to the localization seen on the BOLD images. These two techniques will most probably be complimentary. BOLD responses are dependent on a focal increase in metabolic demand while the EPs may or may not be related to energy demand increases. Additionally, recording EPs require that the source or sources of that potential come from an area that is able to generate far-field potentials. These potentials are related to the laminar organization of the neuronal population generating that potential. As best we know the BOLD response does not depend on any inherent laminar neuronal organization. Therefore, by merging these two recording methods, it is likely that we will gain a more detailed understanding of not only the areas involved in certain physiologic events, e.g. focal epilepsy or cognitive processing, but also on the sequencing of the activation of the various participating regions.

Simultaneous detection of changes in perfusion and BOLD contrast

A new functional magnetic resonance imaging (fMRI) technique for simultaneous detection (SIDE) of changes in perfusion and blood oxygenation level dependent (BOLD) contrast is described. Perfusion contrast is generated by using magnetically labeled endogenous water proton spins as a freely diffusible tracer. A single slice-selective inversion pulse is combined with dual echo echo-planar imaging to generate a spin-echo (SE) image sensitive to changes in perfusion and a gradient-echo (GE) image sensitive to changes in both perfusion and BOLD contrast. The SIDE technique was applied to detect functional changes induced by a visual search task. A theoretical analysis is provided to calculate quantitative maps of changes in cerebral blood flow (DeltaCBF) and effective transverse relaxation time (DeltaT(2)*) from the corresponding signal changes in the SE and GE images. Since SE an GE images are generated from the same longitudinal magnetization, no errors due to spatial or temporal mismatch can arise in the quantification of DeltaCBF and DeltaT(2)*.

Accuracy and limitation of functional magnetic resonance imaging for identification of the central sulcus: comparison with magnetoencephalography in patients with brain tumors

The aim of the present study was to clarify the accuracy and limitation of functional magnetic resonance imaging (fMRI) for the identification of the central sulcus affected by brain tumors. Twelve normal volunteers and 11 patients with intracranial tumors adjacent to the central sulcus underwent fMRI and magnetoencephalography (MEG). Three patients were evaluated again after surgery. fMRI was performed with a 1.5 Tesla scanner during repetitive opening and closing of each hand. Cross-correlation function was used to identify activation areas, and the central sulcus was defined as the nearest sulcus to the highest activation spots that were determined by elevating correlation coefficient threshold. Somatosensory-evoked fields were measured using a whole head MEG system. The central sulcus was defined as the nearest sulcus to the N20m for the median nerve stimulus. fMRI and MEG coincided in defining the central sulcus in all 24 hemispheres of volunteers and all 10 examined nonaffected hemispheres of patients. The fMRI-defined central sulcus coincided with the MEG-defined central sulcus in nine (82%) but did not in two (18%) affected hemispheres of patients. The preoperative mismatch disappeared after surgery in one of the two patients. The present study indicates that fMRI successfully defined the central sulcus in most of the patients with brain tumors. However, in a few cases, fMRI was not reliable probably due to venous flow changes by tumor compression and/or compensational activity by brain tissues surrounding the primary sensorimotor cortex. For precise functional assessment of the brain affected by intracranial tumors, combination of fMRI and MEG will be recommended.

Perfusion-based event-related functional MRI

Perfusion-based event-related functional MRI was performed by measuring flow-sensitive alternating inversion recovery (FAIR) signal changes during repeated single-trial, short visual stimulation (250 msec). In the visual cortex activation area, the blood flow increases immediately after the stimulus, reaches the maximum 4 sec later with a perfusion-sensitized signal change of 16. 1 +/- 2.6 %, and then decreases to baseline approximately 11 sec after the stimuli. As it is a more direct reflection of the hemodynamic response, perfusion-based event-related functional MRI techniques may be more useful for human cognitive function studies, compared with blood oxygenation level-dependent (BOLD)-based event-related functional MRI techniques. Magn Reson Med 42:1011-1013, 1999.

Multislice perfusion imaging in human brain using the C-FOCI inversion pulse: comparison with hyperbolic secant

Perfusion studies based on pulsed arterial spin labeling have primarily applied hyperbolic secant (HS) pulses for spin inversion. To optimize perfusion sensitivity, it is highly desirable to implement the HS pulse with the same slice width as the width of the imaging pulse. Unfortunately, this approach causes interactions between the slice profiles and manifests as residual signal from static tissue in the resultant perfusion image. This problem is currently overcome by increasing the selective HS width relative to the imaging slice width. However, this solution increases the time for the labeled blood to reach the imaging slice (transit time), causing loss of perfusion sensitivity as a result of T(1) relaxation effects. In this study, we demonstrate that the preceding problems can be largely overcome by use of the C-shaped frequency offset corrected inversion (FOCI) pulse [Ordidge et al., Magn Reson Med 1996;36:562]. The implementation of this pulse for multislice perfusion imaging on the cerebrum is presented, showing substantial improvement in slice definition in vivo compared with the HS pulse. The sharper FOCI profile is shown to reduce the physical gap (or "safety margin") between the inversion and imaging slabs, resulting in a significant increase in perfusion signal without residual contamination from static tissue. The mean +/- SE (n = 6) gray matter perfusion-weighted signal (DeltaM/M(o)) without the application of vascular signal suppression gradients were 1.19 +/- 0. 10% (HS-flow-sensitive alternating inversion recovery [FAIR]), and 1. 51 +/- 0.11% for the FOCI-FAIR sequence. The corresponding values with vascular signal suppression were 0.64 +/- 0.14%, and 0.91 +/- 0. 08% using the HS- and FOCI-FAIR sequences, respectively. Compared with the HS-based data, the FOCI-FAIR results correspond to an average increase in perfusion signal of up to between 26%-30%. Magn Reson Med 42:1098-1105, 1999.

Effect of transit times on quantification of cerebral blood flow by the FAIR T(1)-difference approach

The effect of finite transit times for the tagging bolus is known to be a significant error source for perfusion quantification using the flow-sensitive alternating inversion recovery (FAIR) technique. It is shown that, in the presence of transit times, both the slice-selective (SS) and nonselective (NS) inversion recovery experiments actually consist of an NS period followed by an SS period. This mixed process can be described using a newly defined time constant called the "switching time," which separates the two periods. Calculations predict that finite transit times always lead to decreased flow values in the signal-intensity-difference approach, but that the measured flows in the T(1)-difference approach may be decreased or increased. This theory well explains our recent experimental flow results on cat brain as a function of predelay. The results show the signal-intensity-difference method is superior over the T(1)-difference approach in terms of convenience and ease of quantification. Magn Reson Med 42:890-894, 1999.

A FAIR study of motor cortex activation under normo- and hypercapnia induced by breath challenge

In this study an arterial spin-tagging technique based on flow-sensitive alternating inversion recovery (FAIR) with single-shot spiral data acquisition was used to study how the basal cerebral blood flow (CBF) elevated by breath holding affects the regional cerebral blood flow (rCBF) response to focal brain activation in the motor cortex. Six subjects were examined using three types of activation studies. These were (a) bilateral finger tapping paced at 4 Hz under normal breathing, (b) repeated expiration breath holding of 30 s, and (c) simultaneous breath holding and finger tapping. It was found that in five of six subjects the prevailing CBF level adjusted by breath challenge and the increase in rCBF in motor cortex associated with bilateral finger tapping were completely additive. This finding from FAIR-based functional magnetic resonance imaging is in accordance with that reported from published positron emission tomography studies. The results indicate that in the majority of the subjects examined the regulatory mechanisms for vasodilatory reaction to CO(2) and rCBF response to neural activation in motor cortex region are independent.

Continuous saturation EPI with diffusion weighting at 3.0 T

This paper presents a steady-state method of arterial spin labelling using continuous saturation in conjunction with echo-planar imaging (EPI), which has been implemented at 3 T. The continuous saturation technique has the advantage of having high sensitivity compared to transient labelling techniques, when long repetition times are used. It is also easy to implement and requires minimal data to be acquired for quantitation. Like other arterial spin labelling techniques, continuous saturation is potentially prone to overestimation of perfusion rates due to the effect of tagged blood in vessels within the image slice. Using a simple model of the vasculature, the degree of diffusion weighting required to suppress the arterial signal has been determined, with the results indicating that a value of 2 s/mm2 is adequate. Histogram analysis of the experimental data has been used to evaluate the effect of diffusion weighting. Using a b-value of 2 s/mm2, the mean perfusion-related signal change in grey matter on continuous saturation was found to be 1.5 +/- 0.2%, yielding a mean perfusion rate of 87 +/- 9 ml/100 g/min. Brain activation studies using the diffusion weighted continuous saturation technique gave a mean increase in perfusion of 36 +/- 12% in activated motor cortex.

Investigation of BOLD signal dependence on cerebral blood flow and oxygen consumption: the deoxyhemoglobin dilution model

The relationship between blood oxygenation level-dependent (BOLD) MRI signals, cerebral blood flow (CBF), and oxygen consumption (CMR(O2)) in the physiological steady state was investigated. A quantitative model, based on flow-dependent dilution of metabolically generated deoxyhemoglobin, was validated by measuring BOLD signals and relative CBF simultaneously in the primary visual cortex (V1) of human subjects (N = 12) during graded hypercapnia at different levels of visual stimulation. BOLD and CBF responses to specific conditions were averaged across subjects and plotted as points in the BOLD-CBF plane, tracing out lines of constant CMR(O2). The quantitative deoxyhemoglobin dilution model could be fit to these measured iso-CMR(O2) contours without significant (P </= 0.05) residual error and yielded MRI-based CMR(O2) measurements that were in agreement with PET results for equivalent stimuli. BOLD and CBF data acquired during graded visual stimulation were then substituted into the model with constant parameters varied over plausible ranges. Relative changes in CBF and CMR(O2) appeared to be coupled in an approximate ratio of approximately 2:1 for all realistic parameter settings. Magn Reson Med 42:849-863, 1999.

A comparative fMRI study of cortical representations for thermal painful, vibrotactile, and motor performance tasks

Cortical activity due to a thermal painful stimulus applied to the right hand was studied in the middle third of the contralateral brain and compared to activations for vibrotactile and motor tasks using the same body part, in nine normal subjects. Cortical activity was demonstrated utilizing multislice echo-planar functional magnetic resonance imaging (fMRI) and a surface coil. The cortical activity was analyzed based upon individual subject activity maps and on group-averaged activity maps. The results show significant differences in activations across the three tasks and the cortical areas studied. The study indicates that fMRI enables examination of cortical networks subserving pain perception at an anatomical detail not available with other brain imaging techniques and shows that this cortical network underlying pain perception shares components with the networks underlying touch perception and motor execution. However, the thermal pain perception network also has components that are unique to this perception. The uniquely activated areas were in the secondary somatosensory region, insula, and posterior cingulate cortex. The posterior cingulate cortex activity was in a region that, in the monkey, receives nociceptive inputs from posterior thalamic medial and lateral nuclei that in turn are targets for spinothalamic terminations. Discrete subdivisions of the primary somatosensory and motor cortical areas were also activated in the thermal pain task, showing region-dependent differences in the extent of overlap with the other two tasks. Within the primary motor cortex, a hand region was preferentially active in the task in which the stimulus was painful heat. In the primary somatosensory cortex most activity in the painful heat task was localized to area 1, where the motor and vibratory task activities were also coincident. The study also indicates that the functional connectivity across multiple cortical regions reorganizes dynamically with each task.

Simultaneous monitoring of dynamic changes in cerebral blood flow and oxygenation during sustained activation of the human visual cortex

Functional neuroimaging was used to investigate the effect of cerebral blood flow (CBF) adjustments on the blood oxygenation level dependent (BOLD) signal during visual stimulation. Temporal responses from both oxygenation- and perfusion-sensitized MRI revealed almost identical features during onset and ongoing activation, i.e. an activation-induced signal rise, and a gradual signal decrease during prolonged activation (overshoot). However, the post-stimulus responses exhibited a pronounced BOLD signal drop below prestimulus baseline (undershoot), but a rather rapid normalisation of the related CBF signal. Thus, an activation-induced initial BOLD signal rise and a gradual signal decrease reflect a coarse upregulation of CBF, which is followed by fine-tuning adjustments of flow. Regulations of other involved physiological parameters, including blood volume and oxidative metabolism give rise to a negative post-stimulus BOLD signal response.

Perfusion-corrected mapping of cardiac regional blood volume in rats in vivo

Measurement of regional blood volume (RBV) in the myocardium in vivo is important for the assessment of tissue viability and function. The method in this work is based on the acquisition of a T(1) map before and after intravascular contrast agent application. It is known that this method is influenced by perfusion that causes an overestimation of RBV values. In order to solve this problem, the new method is proposed which acquires T(1) maps with slice selective inversion pulses. Due to blood flow nonexcited spins enter the detection slice, which leads to an acceleration of the relaxation time. A model that divides tissue into two compartments is adapted to slice selective inversion in order to derive a simple expression for perfusion-corrected RBV. The aim of the study is to demonstrate the feasibility and accuracy of this technique for quantification of RBV in rat myocardium in vivo. RBV maps were obtained for five rats, and the reproducibility was determined by repeating the experiment several times. A mean RBV value of 12.8 +/- 0.7% (v/v) over all animals was obtained in the myocardium. The results were compared with RBV maps obtained with perfusion-sensitive RBV imaging in the same five rats and with first-pass RBV studies. In order to demonstrate the strength of the new method the vasodilator adenosine was administered and alterations in microcirculation were imaged. Magn Reson Med 42:500-506, 1999.

Simultaneous noninvasive determination of regional myocardial perfusion and oxygen content in rabbits: toward direct measurement of myocardial oxygen consumption at MR imaging

PURPOSE

To determine whether myocardial arterial perfusion and oxygen concentration can be quantified simultaneously from the same images by using spin labeling and the blood oxygenation level-dependent (BOLD) effect with fast spin-echo (SE) imaging.

MATERIALS AND METHODS

A T2-weighted fast SE pulse sequence was written to image isolated, arrested, blood-perfused rabbit hearts (n = 6) at 4.7 T. Perfusion images with intensity in units of milliliters per minute per gram that covered the entire left ventricle with 0.39 x 0.39 x 3.00-mm resolution were obtained in less than 15 minutes with a 32-fold reduction in imaging time from that of a previous study. Estimates of oxygen concentration were made from the same images acquired for calculation of perfusion images.

RESULTS

Estimates of regional myocardial oxygen content could be made from the perfusion images; this demonstrated the feasibility of three-dimensional calculation of regional oxygen consumption, which requires concomitant measurement of both oxygen content and flow. Fast SE imaging was shown to be as sensitive to hemoglobin desaturation as standard SE imaging. Perfusion abnormalities and oxygen deficits were easily identified and verified qualitatively with gadopentetate dimeglumine on both perfusion and BOLD images obtained after coronary arterial ligation.

CONCLUSION

T2-weighted fast SE imaging combined with perfusion-sensitive spin labeling can be used to measure myocardial arterial perfusion and oxygen concentration. This provides the groundwork for calculation of regional myocardial oxygen consumption.

Pseudo-continuous arterial spin labeling technique for measuring CBF dynamics with high temporal resolution

Cerebral blood flow (CBF) can be measured noninvasively with nuclear magnetic resonance (NMR) by using arterial water as an endogenous perfusion tracer. However, the arterial spin labeling (ASL) techniques suffer from poor temporal resolution due to the need to wait for the exchange of labeled arterial spins with tissue spins to produce contrast. In this work, a new ASL technique is introduced, which allows the measurement of CBF dynamics with high temporal and spatial resolution. This novel method was used in rats to determine the dynamics of CBF changes elicited by somatosensory stimulation with a temporal resolution of 108 ms. The onset time of the CBF response was 0.6 +/- 0.4 sec (mean +/- SD) after onset of stimulation (n = 10). The peak response was observed 4.4 +/- 3.7 sec (mean +/- SD) after stimulation began. These results are in excellent agreement with previous data obtained with invasive techniques, such as laser-Doppler flowmetry and hydrogen clearance, and suggest the appropriateness of this novel technique to probe CBF dynamics in functional and pathological studies with high temporal and spatial resolution. Magn Reson Med 42:425-429, 1999.

Linear coupling between cerebral blood flow and oxygen consumption in activated human cortex

The aim of this study was to test the hypothesis that, within a specific cortical unit, fractional changes in cerebral blood flow (CBF) and cerebral metabolic rate of oxygen consumption (CMR(O(2))) are coupled through an invariant relationship during physiological stimulation. This aim was achieved by simultaneously measuring relative changes in these quantities in human primary visual cortex (V1) during graded stimulation with patterns designed to selectively activate different populations of V1 neurons. Primary visual cortex was delineated individually in each subject by using phase-encoded retinotopic mapping. Flow-sensitive alternating inversion recovery MRI, in conjunction with blood oxygenation-sensitive MRI and hypercapnic calibration, was used to monitor CBF and CMR(O(2)). The stimuli used included (i) diffuse isoluminant chromatic displays; (ii) high spatial-frequency achromatic luminance gratings; and (iii) radial checkerboard patterns containing both color and luminance contrast modulated at different temporal rates. Perfusion responses to each pattern were graded by varying luminance and/or color modulation amplitudes. For all stimulus types, fractional changes in blood flow and oxygen uptake were found to be linearly coupled in a consistent ratio of approximately 2:1. The most potent stimulus produced CBF and CMR(O(2)) increases of 48 +/- 5% and 25 +/- 4%, respectively, with no evidence of a plateau for oxygen consumption. Estimation of aerobic ATP yields from the observed CMR(O(2)) increases and comparison with the maximum possible anaerobic ATP contribution indicate that elevated energy demands during brain activation are met largely through oxidative metabolism.

Origin of the signal undershoot in BOLD studies of the visual cortex

The nature of the signal undershoot observed in the inter-stimulation intervals in fMRI studies using a block paradigm consisting of alternating periods of visual stimulation and rest was investigated using the following single shot EPI sequences: gradient echo (GRE), spin echo (SE), spin echo with additional flow dephasing gradients and inversion recovery (IR) prepared SE. Both the GRE and SE sequences showed a significant signal undershoot during the inter-stimulation intervals. DeltaR(2)*/DeltaR(2) ratios of 3.7 +/- 0.9 and 3.1 +/- 0.7 were measured in the stimulation and inter-stimulation periods, respectively, with the latter being lower than that which would be consistent with a pure extra-vascular effect arising from an elevated venous blood volume and a normal deoxyhaemoglobin content post-stimulation. The addition of dephasing gradients to the SE sequence in order to attenuate the signal from spins flowing within larger vessels produced a four-fold reduction in the number of activated pixels but had little effect on the time intensity profile. Our interpretation of these results is that both extra- and intra-vascular BOLD effects are present in the inter-stimulation intervals and the lack of any effect of the dephasing gradient on the time-intensity profile indicates that the intra-vascular component probably occurs mainly in smaller vessels, such as venules, which are affected relatively little by the relatively weak dephasing gradient (b = 29 s/mm(2)) used in this study. For the IR-SE sequence the DeltaR(2) measured during the inter-stimulation intervals was similar to that seen with the SE BOLD sequence and thus was consistent with a residual BOLD effect, implying that perfusion changes in the capillary vessels did not contribute significantly to the signal undershoot.

Dynamic imaging of perfusion in human skeletal muscle during exercise with arterial spin labeling

MR images acquired by using an arterial spin-labeling technique showed spatial and temporal variations of perfusion in the skeletal muscle of exercising humans. Perfusion measurements made during plantar flexion exercise in normal volunteers were consistent with those obtained by traditional techniques reported in the literature. Spatial heterogeneity of perfusion values clearly delineated the various muscle groups within the lower leg. These results are interpreted in terms of a quantitative model for the perfusion signal in muscle. This method can provide a useful tool in the study of muscle physiology. Magn Reson Med 42:258-267, 1999. Published 1999 Wiley-Liss, Inc.

Simultaneous blood oxygenation level-dependent and cerebral blood flow functional magnetic resonance imaging during forepaw stimulation in the rat

The blood oxygenation level-dependent (BOLD) contrast mechanism can be modeled as a complex interplay between CBF, cerebral blood volume (CBV), and CMRO2. Positive BOLD signal changes are presumably caused by CBF changes in excess of increases in CMRO2. Because this uncoupling between CBF and CMRO2 may not always be present, the magnitude of BOLD changes may not be a good index of CBF changes. In this study, the relation between BOLD and CBF was investigated further. Continuous arterial spin labeling was combined with a single-shot, multislice echo-planar imaging to enable simultaneous measurements of BOLD and CBF changes in a well-established model of functional brain activation, the electrical forepaw stimulation of alpha-chloralose-anesthetized rats. The paradigm consisted of two 18- to 30-second stimulation periods separated by a 1-minute resting interval. Stimulation parameters were optimized by laser Doppler flowmetry. For the same cross-correlation threshold, the BOLD and CBF active maps were centered within the size of one pixel (470 microm). However, the BOLD map was significantly larger than the CBF map. Measurements taken from 15 rats at 9.4 T using a 10-millisecond echo-time showed 3.7 +/- 1.7% BOLD and 125.67 +/- 81.7% CBF increases in the contralateral somatosensory cortex during the first stimulation, and 2.6 +/- 1.2% BOLD and 79.3 +/- 43.6% CBF increases during the second stimulation. The correlation coefficient between BOLD and CBF changes was 0.89. The overall temporal correlation coefficient between BOLD and CBF time-courses was 0.97. These results show that under the experimental conditions of the current study, the BOLD signal changes follow the changes in CBF.

Imaging of acute cerebral ischemia

Until recently, there was no efficacious treatment for acute cerebral ischemia. As a result, the role of neuroimaging and the radiologist was peripheral in the diagnosis and management of this disease. The demonstration of efficacy using thrombolysis has redefined this role, with the success of intervention becoming increasingly dependent on timely imaging and accurate interpretation. The potential benefits of intervention have only begun to be realized. In this State-of-the-Art review of imaging of acute stroke, the role of imaging in the current and future management of stroke is presented. The role of computed tomography is emphasized in that it is currently the most utilized technique, and its value has been demonstrated in prospective clinical trials. Magnetic resonance techniques are equally emphasized in that they have the potential to provide a single modality evaluation of tissue viability and vessel patency in an increasingly rapid evaluation.

Functional mapping in the human brain using high magnetic fields

An avidly pursued new dimension in magnetic resonance imaging (MRI) research is the acquisition of physiological and biochemical information non-invasively using the nuclear spins of the water molecules in the human body. In this trial, a recent and unique accomplishment was the introduction of the ability to map human brain function non-invasively. Today, functional images with subcentimetre resolution of the entire human brain can be generated in single subjects and in data acquisition times of several minutes using 1.5 tesla (T) MRI scanners that are often used in hospitals for clinical purposes. However, there have been accomplishments beyond this type of imaging using significantly higher magnetic fields such as 4 T. Efforts for developing high magnetic field human brain imaging and functional mapping using MRI (fMRI) were undertaken at about the same time. It has been demonstrated that high magnetic fields result in improved contrast and, more importantly, in elevated sensitivity to capillary level changes coupled to neuronal activity in the blood oxygenation level dependent (BOLD) contrast mechanism used in fMRI. These advantages have been used to generate, for example, high resolution functional maps of ocular dominance columns, retinotopy within the small lateral geniculate nucleus, true single-trial fMRI and early negative signal changes in the temporal evolution of the BOLD signal. So far these have not been duplicated or have been observed as significantly weaker effects at much lower field strengths. Some of these high-field advantages and accomplishments are reviewed in this paper.

In vivo determination of absolute cerebral blood volume using hemoglobin as a natural contrast agent: an MRI study using altered arterial carbon dioxide tension

The ability of the magnetic resonance imaging transverse relaxation time, R2 = 1/T2, to quantify cerebral blood volume (CBV) without the need for an exogenous contrast agent was studied in cats (n = 7) under pentobarbital anesthesia. This approach is possible because R2 is directly affected by changes in CBF, CBV, CMRO2, and hematocrit (Hct), a phenomena better known as the blood-oxygenation-level-dependent (BOLD) effect. Changes in CBF and CBV were accomplished by altering the carbon dioxide pressure, PaCO2, over a range from 20 to 140 mm Hg. For each PaCO2 value, R2 in gray and white matter were determined using MRI, and the whole-brain oxygen extraction ratio was obtained from arteriovenous differences (sagittal sinus catheter). Assuming a constant CMRO2, the microvascular CBV was obtained from an exact fit to the BOLD theory for the spin-echo effect. The resulting CBV values at normal PaCO2 and normalized to a common total hemoglobin concentration of 6.88 mmol/L were 42+/-18 microL/g (n = 7) and 29+/-19 microL/g (n = 5) for gray and white matter, respectively, in good agreement with the range of literature values published using independent methodologies. The present study confirms the validity of the spin-echo BOLD theory and, in addition, shows that blood volume can be quantified from the magnetic resonance imaging spin relaxation rate R2 using a regulated carbon dioxide experiment.

Measuring cerebral blood flow using magnetic resonance imaging techniques

Magnetic resonance imaging techniques measuring CBF have developed rapidly in the last decade, resulting in a wide range of available methods. The most successful approaches are based either on dynamic tracking of a bolus of a paramagnetic contrast agent (dynamic susceptibility contrast) or on arterial spin labeling. This review discusses their principles, possible pitfalls, and potential for absolute quantification and outlines clinical and neuroscientific applications.