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

QUIPSS II with thin-slice TI1 periodic saturation: a method for improving accuracy of quantitative perfusion imaging using pulsed arterial spin labeling

Quantitative imaging of perfusion using a single subtraction, second version (QUIPSS II) is a pulsed arterial spin labeling (ASL) technique for improving the quantitation of perfusion imaging by minimizing two major systematic errors: the variable transit delay from the distal edge of the tagged region to the imaging slices, and the contamination by intravascular signal from tagged blood that flows through the imaging slices. However, residual errors remain due to incomplete saturation of spins over the slab-shaped tagged region by the QUIPSS II saturation pulse, and spatial mismatch of the distal edge of the saturation and inversion slice profiles. By replacing the original QUIPSS II saturation pulse with a train of thin-slice periodic saturation pulses applied at the distal end of the tagged region, the accuracy of perfusion quantitation is improved. Results of single and multislice studies are reported.

Reperfusion in a gerbil model of forebrain ischemia using serial magnetic resonance FAIR perfusion imaging

BACKGROUND AND PURPOSE

Existing methods for the quantitative measurement of the changing cerebral blood flow (CBF) during reperfusion suffer from poor spatial or temporal resolution. The aim of this study was to implement a recently developed MRI technique for quantitative perfusion imaging in a gerbil model of reperfusion. Flow-sensitive alternating inversion recovery (FAIR) is a noninvasive procedure that uses blood water as an endogenous tracer.

METHODS

Bilateral forebrain ischemia of 4 minutes' duration was induced in gerbils (n=8). A modified version of FAIR with improved time efficiency was used to provide CBF maps with a time resolution of 2.8 minutes after recirculation had been initiated. Quantitative diffusion imaging was also performed at intervals during the reperfusion period.

RESULTS

On initiating recirculation after the transient period of ischemia, the FAIR measurements demonstrated either a symmetrical, bilateral pattern of flow impairment (n=4) or an immediate side-to-side difference that became apparent with respect to the cerebral hemispheres in the imaged slice (n=4). The flow in each hemisphere displayed a pattern of recovery close to the preocclusion level or, alternatively, returned to a lower level before displaying a delayed hypoperfusion and a subsequent slow recovery. The diffusion measurements during this latter response suggested the development of cell swelling during the reperfusion phase in the striatum.

CONCLUSIONS

The CBF during the reperfusion period was monitored with a high time resolution, noninvasive method. This study demonstrates the utility of MRI techniques in following blood flow changes and their pathophysiological consequences.

Glial neoplasms: dynamic contrast-enhanced T2*-weighted MR imaging

PURPOSE

To evaluate the role of T2*-weighted echo-planar perfusion imaging by using a first-pass gadopentetate dimeglumine technique to determine the association of magnetic resonance (MR) imaging-derived cerebral blood volume (CBV) maps with histopathologic grading of astrocytomas and to improve the accuracy of targeting of stereotactic biopsy.

MATERIALS AND METHODS

MR imaging was performed in 29 patients by using a first-pass gadopentetate dimeglumine T2*-weighted echo-planar perfusion sequence followed by conventional imaging. The perfusion data were processed to obtain a color map of relative regional CBV. This information formed the basis for targeting the stereotactic biopsy. Relative CBV values were computed with a nondiffusible tracer model. The relative CBV of lesions was expressed as a percentage of the relative CBV of normal white matter. The maximum relative CBV of each lesion was correlated with the histopathologic grading of astrocytomas obtained from samples from stereotactic biopsy or volumetric resection.

RESULTS

The maximum relative CBV in high-grade astrocytomas (n = 26) varied from 1.73 to 13.7, with a mean of 5.07 +/- 2.79 (+/- SD), and in the low-grade cohort (n = 3) varied from 0.92 to 2.19, with a mean of 1.44 +/- 0.68. This difference in relative CBV was statistically significant (P < .001; Student t test).

CONCLUSION

Echo-planar perfusion imaging is useful in the preoperative assessment of tumor grade and in providing diagnostic information not available with conventional MR imaging. The areas of perfusion abnormality are invaluable in the precise targeting of the stereotactic biopsy.

MR perfusion imaging in human brain using the UNFAIR technique. Un-inverted flow-sensitive alternating inversion recovery

Pulsed arterial spin labeling magnetic resonance techniques have been developed recently to estimate cerebral blood flow (CBF). Flow-sensitive alternating inversion recovery (FAIR) is one such technique that has been implemented successfully in humans. Un-inverted FAIR (UNFAIR) is an alternative technique in which the flow-sensitive image is acquired following inversion of all spins outside the slice of interest, and the control image is acquired without any spin labeling. This approach is potentially more efficient than FAIR since the UNFAIR control image is entirely flow independent and need only be acquired once. Here, we describe implementation of the sequence on a clinical 1.5 T magnetic resonance system. Both FAIR and UNFAIR perfusion-weighted images were obtained from six normal volunteers. Wash-in/wash-out curves measured in cortical gray and white matter were practically identical for the two techniques, as predicted by our model.

Perfusion imaging using FAIR with a short predelay

It is shown theoretically that the flow-sensitive alternating inversion recovery (FAIR) signal intensity difference for flow quantification is independent of the length of the predelay between repeated measurements when assuming complete labeling in between scans. Theory also predicts that flows quantified using the concomitant T1 difference increase significantly with decreasing predelay, because the biexponential relaxation behavior after nonselective inversion is fitted as a monoexponential. The new equations include the effect of the unequal relaxation times of water in tissue (T1) and arterial blood (T1a). While this effect is significant for the signal-difference approach, it is negligible for the T1-difference procedure when using maximum inversion-recovery times shorter than 4 sec. Experiments on cat brain using the FAIR excluding radiation damping (FAIRER) pulse sequence at three different predelays (0.8, 2, and 5 sec) confirm the theoretical predictions.

Determination of relative CMRO2 from CBF and BOLD changes: significant increase of oxygen consumption rate during visual stimulation

The blood oxygenation level-dependent (BOLD) effect in functional magnetic resonance imaging depends on at least partial uncoupling between cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) changes. By measuring CBF and BOLD simultaneously, the relative change in CMRO2 can be estimated during neural activity using a reference condition obtained with known CMRO2 change. In this work, nine subjects were studied at a magnetic field of 1.5 T; each subject underwent inhalation of a 5% carbon dioxide gas mixture as a reference and two visual stimulation studies. Relative CBF and BOLD signal changes were measured simultaneously using the flow-sensitive alternating inversion recovery (FAIR) technique. During hypercapnia established by an end-tidal CO2 increase of 1.46 kPa, CBF in the visual cortex increased by 47.3 +/- 17.3% (mean +/- SD; n = 9), and deltaR2* was -0.478 +/- 0.147 sec(-1), which corresponds to BOLD signal change of 2.4 +/- 0.7% with a gradient echo time of 50 msec. During black/white visual stimulation reversing at 8 Hz, regional CBF increase in the visual cortex was 43.6 +/- 9.4% (n = 18), and deltaR2* was -0.114 +/- 0.086 sec(-1), corresponding to a BOLD signal change of 0.6 +/- 0.4%. Assuming that CMRO2 does not change during hypercapnia and that hemodynamic responses during hypercapnia and neural stimulation are similar, relative CMRO2 change was determined using BOLD biophysical models. The average CMRO2 change in the visual cortex ranged from 15.6 +/- 8.1% (n = 18) with significant cerebral blood volume (CBV) contribution to 29.6 +/- 18.8% without significant CBV contribution. A weak positive correlation between CBF and CMRO2 changes was observed, suggesting the CMRO2 increase is proportional to the CBF increase.

Differentiating cortical areas related to pain perception from stimulus identification: temporal analysis of fMRI activity

In a recent functional magnetic resonance imaging study (fMRI), we reported the cortical areas activated in a thermal painful task and compared the extent of overlap between this cortical network and those activated during a vibrotactile task and a motor task. In the present study we examine the temporal properties of the cortical activations for all three tasks and use linear systems identification techniques to functionally differentiate the cortical regions identified in the painful thermal task. Cortical activity was examined in the contralateral middle third of the brain of 10 right-handed subjects, using echo-planar imaging and a surface coil. In another eight subjects the temporal properties of the thermal task were examined psychophysically. The fMRI impulse response function was estimated from the cortical activations in the vibrotactile and motor tasks and shown to correspond to earlier reports. Given the fMRI impulse response function and the time courses for the thermal stimulus and the associated pain ratings, predictor functions were generated. The correlation between these predictor functions and cortical activations in the painful thermal task indicated a gradual transition of information processing anteroposteriorly in the parietal cortex. Within this region, activity in the anterior areas more closely reflected thermal stimulus parameters, whereas activity more posteriorly was better related to the temporal properties of pain perception. Insular cortex at the level of the anterior commissure was the region best related to the thermal stimulus, and Brodmann's area 5/7 was the region best related to the pain perception. The functional implications of these observations are discussed.

Multislice perfusion and perfusion territory imaging in humans with separate label and image coils

An arterial spin labeling technique using separate RF labeling and imaging coils was used to obtain multislice perfusion images of the human brain at 3 T. Continuous RF irradiation at a peak power of 0.3 W was applied to the carotid arteries to adiabatically invert spins. Labeling was achieved without producing magnetization transfer effects since the B1 field of the labeling coil did not extend into the imaging region or couple significant power into the imaging coil. Eliminating magnetization transfer allowed the acquisition of multislice perfusion images of arbitrary orientation. Combining surface coil labeling with a reduced RF duty cycle permitted significantly lower SAR than single coil approaches. The technique was also found to allow selective labeling of blood in either carotid, providing an assessment of the artery's perfusion territory. In normal subjects, these territories were well-defined and localized to the ipsilateral hemisphere.

Stimulus-dependent BOLD and perfusion dynamics in human V1

Blood oxygenation level-dependent (BOLD) fMRI signals often exhibit pronounced over- or undershoot upon changes in stimulation state. Current models postulate that this is due to the delayed onset or decay of perfusion-dependent attenuating responses such as increased cerebral blood volume or oxygen consumption, which are presumed to lag behind the rapid adjustment of blood flow rate to a new steady-state level. If this view is correct, then BOLD overshoot amplitudes in a specific tissue volume should be correlated with steady-state increases in perfusion, independent of stimulus type. To test this prediction, we simultaneously recorded BOLD and relative perfusion signals in primary visual cortex while inducing graded perfusion increases with three types of visual stimulus. Two of these, a diffuse chromatic stimulus with no luminance variation and a very high spatial frequency luminance grating, did not produce detectable BOLD overshoot (or undershoot) when an equal mean luminance baseline was used. Radial checkerboard stimuli, however, caused pronounced over/undershoot of both BOLD and perfusion signals even when temporal mean luminance was held constant and stimulus contrast was adjusted to produce the same steady-state blood flow increases evoked by the other stimuli. Transient amplitudes were relatively invariant in spite of large changes in steady-state response, demonstrating nonlinear BOLD and perfusion step responses in human V1. These findings suggest that, rather than a purely tissue-specific biomechanical or metabolic phenomenon, BOLD overshoot and undershoot represent transient features in the perfusion signal whose effects may be amplified by slowly evolving blood volume changes.

Exploring brain function with magnetic resonance imaging

Since its invention in the early 1990s, functional magnetic resonance imaging (fMRI) has rapidly assumed a leading role among the techniques used to localize brain activity. The spatial and temporal resolution provided by state-of-the-art MR technology and its non-invasive character, which allows multiple studies of the same subject, are some of the main advantages of fMRI over the other functional neuroimaging modalities that are based on changes in blood flow and cortical metabolism. This paper describes the basic principles and methodology of fMRI and some aspects of its application to functional activation studies. Attention is focused on the physiology of the blood oxygenation level-dependent (BOLD) contrast mechanism and on the acquisition of functional time-series with echo planar imaging (EPI). We also provide an introduction to the current strategies for the correction of signal artefacts and other image processing techniques. In order to convey an idea of the numerous applications of fMRI, we will review some of the recent results in the fields of cognitive and sensorimotor psychology and physiology.

Perfusion magnetic resonance imaging with continuous arterial spin labeling: methods and clinical applications in the central nervous system

Several methods are now available for measuring cerebral perfusion and related hemodynamic parameters using magnetic resonance imaging (MRI). One class of techniques utilizes electromagnetically labeled arterial blood water as a noninvasive diffusible tracer for blood flow measurements. The electromagnetically labeled tracer has a decay rate of T1, which is sufficiently long to allow perfusion of the tissue and microvasculature to be detected. Alternatively, electromagnetic arterial spin labeling (ASL) may be used to obtain qualitative perfusion contrast for detecting changes in blood flow, similar to the use of susceptibility contrast in blood oxygenation level dependent functional MRI (BOLD fMRI) to detect functional activation in the brain. The ability to obtain blood flow maps using a non-invasive and widely available modality such as MRI should greatly enhance the utility of blood flow measurement as a means of gaining further insight into the broad range of hemodynamically related physiology and pathophysiology. This article describes the biophysical considerations pertaining to the generation of quantitative blood flow maps using a particular form of ASL in which arterial blood water is continuously labeled, termed continuous arterial spin labeling (CASL). Technical advances permit multislice perfusion imaging using CASL with reduced sensitivity to motion and transit time effects. Interpretable cerebral perfusion images can now be reliably obtained in a variety of clinical settings including acute stroke, chronic cerebrovascular disease, degenerative diseases and epilepsy. Over the past several years, the technical and theoretical foundations of CASL perfusion MRI techniques have evolved from feasibility studies into practical usage. Currently existing methodologies are sufficient to make reliable and clinically relevant observations which complement structural assessment using MRI. Future technical improvements should further reduce the acquisition times for CASL perfusion MRI, while increasing the slice coverage, resolution and stability of the images. These techniques have a broad range of potential applications in clinical and basic research of brain physiology, as well as in other organs.

Delivery of imaging agents into brain

Delivery of diagnostic agents to the central nervous system (CNS) poses several challenges as a result of the special features of CNS blood vessels and tissue fluids. Diffusion barriers exist between blood and neural tissue, in the endothelium of parenchymal vessels (blood-brain barrier, BBB), and in the epithelia of the choroid plexuses and arachnoid membrane (blood-CSF barriers), which severely restrict penetration of several diagnostic imaging agents. The anatomy of large vessels can be imaged using bolus injection of X-ray contrast agents to identify sites of malformation or occlusion, and blood flow measured using MRI and CT, while new techniques permit analysis of capillary perfusion and blood volume. Absolute quantities can be derived, although relative measures in different CNS regions may be as useful in diagnosis. Local blood flow, blood volume, and their ratio (mean transit time) can be measured with high speed tomographic imaging using MRI and CT. Intravascular contrast agents for MRI are based on high magnetic susceptibility agents such as gadolinium, dysprosium and iron. Steady-state imaging using agents that cross the BBB including (123)I- and (99m)Tc-labelled lipophilic agents with SPECT, gives a 'snapshot' of perfusion at the time of injection. Cerebral perfusion can also be measured with PET, using H(2)(15)O, (11)C- or (15)O-butanol, and (18)F-fluoromethane, and cerebral blood volume measured with C(15)O. Recent advances in MRI permit the non-invasive 'labelling' of endogenous water protons in flowing blood, with subsequent detection as a measure of blood flow. Imaging the BBB most commonly involves detecting disruptions of the barrier, allowing contrast agents to leak out of the vascular system. Gd-DTPA is useful in imaging leaky vessels as in some cerebral tumors, while the shortening of T(1) by MR contrast agents can be used to detect more subtle changes in BBB permeability to water as in cerebral ischemia. Techniques for imaging the dynamic activity of the brain parenchyma mainly involve PET, using a variety of radiopharmaceuticals to image glucose transport and metabolism, neurotransmitter binding and uptake, protein synthesis and DNA dynamics. PET methods permit detailed analysis of regional function by comparing resting and task-related images, important in improving understanding of both normal and pathological brain function.

Perfusion imaging of the human lung using flow-sensitive alternating inversion recovery with an extra radiofrequency pulse (FAIRER)

Pulmonary perfusion is an important parameter in the evaluation of lung diseases such as pulmonary embolism. A noninvasive MR perfusion imaging technique of the lung is presented in which magnetically labeled blood water is used as an endogenous, freely diffusible tracer. The perfusion imaging technique is an arterial spin tagging method called Flow sensitive Alternating Inversion Recovery with an Extra Radiofrequency pulse (FAIRER). Seven healthy human volunteers were studied. High-resolution perfusion-weighted images with negligible artifacts were acquired within a single breathhold. Different patterns of signal enhancement were observed between the pulmonary vessels and parenchyma, which persists up to TI = 1400 ms. The T1s of blood and lung parenchyma were determined to be 1.46s and 1.35 s, respectively.

Implementation of quantitative FAIR perfusion imaging with a short repetition time in time-course studies

Flow-sensitive alternating inversion recovery (FAIR) is a pulsed arterial spin labeling magnetic resonance imaging method for perfusion quantification. In its standard implementation for quantification with full longitudinal relaxation between acquisitions, its use in time-course investigations of rapidly changing flow values is limited. The time efficiency can be improved by decreasing the repetition time but quantification becomes problematic. This situation is further complicated if a whole-body radiofrequency transmit coil is not used since fresh blood spins will flow in from outside the coil. To alleviate these problems, the use of global pre-saturation is proposed. The resulting expression for the flow signal depends on the relationship between the imaging parameters and the coil inflow time and can be significantly simplified under certain combinations of these parameters. With this implementation of FAIR, quantitative flow maps of gerbil brains were obtained with a 3 minute time resolution in a study of the effects of reperfusion. The pre-occlusion flow measurements were in good agreement with values obtained by the standard FAIR implementation and by other techniques, but the low values following occlusion were underestimated due to the increased transit times.

A direct comparison between whole-brain PET and BOLD fMRI measurements of single-subject activation response

We present the results of a direct comparison of single-subject activation using identical tasks for both functional PET and fMRI whole-brain studies. We examined the most commonly employed methods for each modality. For fMRI this is the blood oxygenation level-dependent (BOLD) contrast method with echo-planar imaging. In PET single-subject activation studies are based on the development of high sensitivity 3D imaging of regional cerebral blood flow from multiple [15O]water injections. The identical activation paradigm of a visually cued sequential finger opposition was used for PET and fMRI. For both modalities the entire brain volume difference images were smoothed to the same final resolution and the peak t value within the primary sensory/motor (PSM) area was then identified. All contiguous voxels in the PSM above a predetermined threshold of statistical significance were determined. Finally, the difference-weighted centroid location was calculated for the PSM region for each modality. These studies showed a very similar pattern of activation, with the volume of activation greater in fMRI and higher levels of statistical significance. The centroids of activation, however, differed by 9 +/- 3 mm between the modalities, with the fMRI centroid location dorsal to that for PET. These results were stable across all processing options including differing levels of image smoothing and thresholds of statistical significance. These results are consistent with the hypothesis that draining veins contribute a substantial signal for fMRI activation studies and indicate caution for the interpretation of BOLD fMRI images with activation sites near draining veins.

MRI measurement of the functional blood flow changes in a large superficial vein draining the motor cortex

In this study, phase-contrast MR techniques are applied in order to measure the blood flow changes induced by a motor task in a large superficial vein draining the motor cortex. The measurements were applied to six healthy volunteers, in motor rest conditions and during performance of a motor task. The latter consisted of sequential finger-to-thumb opposition. The task was actually executed and mentally simulated. Significant blood flow increases were found when changing from from mental simulation to actual execution of the motor task (increases ranging between 1.6 and 10.3 ml/min, i.e. 9% and 45%, respectively) and from resting conditions to actual execution of the motor task (increases ranging between 1.7 and 14.0 ml/min, i.e. 32% and 72%, respectively).

MR perfusion imaging of pulmonary parenchyma using pulsed arterial spin labeling techniques: FAIRER and FAIR

Magnetic resonance imaging of pulmonary parenchyma perfusion using pulsed arterial spin labeling (ASL) techniques is presented. ASL uses magnetically labeled water as an endogenous, freely diffusible tracer. Presented are comparative results of ASL methods called Flow sensitive Alternating Inversion Recovery (FAIR), and FAIR with an Extra Radiofrequency pulse (FAIRER). Six healthy human volunteers were imaged. Perfusion-weighted images at different time delays, TI, were calculated from the subtraction of the control and tag images, which were acquired within a single breathhold. Detailed pulmonary structures can be visualized with negligible cardiac or respiratory motion artifacts. Different patterns of signal enhancement between the pulmonary vessels and parenchyma are shown in the perfusion images acquired at different TIs.

Simultaneous recording of evoked potentials and T2*-weighted MR images during somatosensory stimulation of rat

Somatosensory evoked potentials (SEP) and T2*-weighted nuclear magnetic resonance (NMR) images were recorded simultaneously during somatosensory stimulation of rat to investigate the relationship between electrical activation of the brain tissue and the signal intensity change in functional NMR imaging. Electrical forepaw stimulation was performed in Wistar rats anesthetized with alpha-chloralose. SEPs were recorded with calomel electrodes at stimulation frequencies of 1.5, 3, 4.5, and 6 Hz. At the same time, T2*-weighted imaging was performed, and the signal intensity increase during stimulation was correlated with the mean amplitude of the SEP. Both the stimulation-evoked signal intensity increase in T2*-weighted images and the amplitude of SEPs were dependent on the stimulation frequency, with the largest signals at a stimulation frequency of 1.5 Hz and decreasing activations with increasing frequencies. The feasibility of simultaneous, artifact-free recordings of T2*-weighted NMR images and of evoked potentials is proved. Furthermore, the study demonstrates-in the intact brain-the validity of functional magnetic resonance imaging for estimating the intensity of electrocortical activation.

Parameter estimation from Rician-distributed data sets using a maximum likelihood estimator: application to T1 and perfusion measurements

General expressions are presented to calculate the maximum likelihood (ML) estimator and corresponding Fisher matrix for Rician-distributed data sets. This estimator results in the most precise, unbiased estimations of T1 from magnitude data sets, even when low signal-to-noise ratios (<6) are present. By optimizing the sample point distributions for inversion-recovery experiments, a 32% increase in precision of the estimated T1 is obtained, compared with a linear sampling scheme. Perfusion rates are estimated from combined data sets of the slice- and nonslice-selective inversion-recovery experiments, as obtained with the flow-sensitive alternating inversion recovery (FAIR) technique. The ML estimator for the combined data set results in the most precise, unbiased estimations of the perfusion rate. Error analysis shows that very high signal-to-noise ratios are required for precise estimation of perfusion rates from FAIR experiments.

Transfer insensitive labeling technique (TILT): application to multislice functional perfusion imaging

Cerebral blood flow can be studied in a multislice mode with a recently proposed perfusion sequence using inversion of water spins as an endogenous tracer without magnetization transfer artifacts. The magnetization transfer insensitive labeling technique (TILT) has been used for mapping blood flow changes at a microvascular level under motor activation in a multislice mode. In TILT, perfusion mapping is achieved by subtraction of a perfusion-sensitized image from a control image. Perfusion weighting is accomplished by proximal blood labeling using two 90 degrees radiofrequency excitation pulses. For control preparation the labeling pulses are modified such that they have no net effect on blood water magnetization. The percentage of blood flow change, as well as its spatial extent, has been studied in single and multislice modes with varying delays between labeling and imaging. The average perfusion signal change due to activation was 36.9 +/- 9.1% in the single-slice experiments and 38.1 +/- 7.9% in the multislice experiments. The volume of activated brain areas amounted to 1.51 +/- 0.95 cm3 in the contralateral primary motor (M1) area, 0.90 +/- 0.72 cc in the ipsilateral M1 area, 1.27 +/- 0.39 cm3 in the contralateral and 1.42 +/- 0.75 cm3 in the ipsilateral premotor areas, and 0.71 +/- 0.19 cm3 in the supplementary motor area.

Measurement of human myocardial perfusion by double-gated flow alternating inversion recovery EPI

This paper presents a flow-sensitive alternating inversion recovery (FAIR) method for measuring human myocardial perfusion at 1.5 T. Slice-selective/non-selective IR images were collected using a double-gated IR echoplanar imaging sequence. Myocardial perfusion was calculated after T1 fitting and extrapolation of the mean signal difference SI(Sel - SI(NSel). The accuracy of the method was tested in a porcine model using graded intravenous adenosine dose challenge. Comparison with radiolabeled microsphere measurements showed a good correlation (r = 0.84; mean error = 20%, n = 6) over the range of flows tested (0.9-7 ml/g/min). Applied in humans, this method allowed for the measurement of resting myocardial flow (1.04+/-0.37 ml/g/min, n = 11). The noise in our human measurements (SE(flow) = 0.2 ml/g/min) appears to come primarily from residual respiratory motion. Although the current signal-to-noise ratio limits our ability to measure small fluctuations in resting flow accurately, the results indicate that this noninvasive method has great promise for the quantitative assessment of myocardial flow reserve in humans.

Perfusion analysis using dynamic arterial spin labeling (DASL)

A variety of magnetic resonance (MR) techniques have proved useful to quantify perfusion using endogenous water as a blood flow tracer. Assuming that water is a freely diffusable tracer, the model used for these techniques predicts that the quantitation of perfusion is based on three parameters, all of which can depend on blood flow. These are the longitudinal tissue relaxation time, the transit time from point of labeling to tissue, and the difference in tissue MR signal between an appropriate control and the labeled state. To measure these three parameters in parallel, a dynamic arterial spin labeling (DASL) technique is introduced based on the analysis of the tissue response to a periodic time varying degree of arterial spin labeling, called here the labeling function (LF). The LF frequency can be modulated to overdetermine parameters necessary to define the system. MR schemes are proposed to measure the tissue response to different LF frequencies efficiently. Sprague-Dawley rats were studied by DASL, using various frequencies for the LF and various arterial pCO2 levels. During data processing, the periodic behavior of the tissue response to the LF allowed for frequency filtering of periodic changes in signal intensity unrelated to perfusion and arterial spin labeling. Measures of transit time, tissue longitudinal relaxation time, and perfusion agreed well over a range of LF frequencies and with previous results. DASL shows potential for more accurately quantifying perfusion as well as measuring transit times associated with arterial spin labeling techniques.

Functional MRI of human brain during breath holding by BOLD and FAIR techniques

BOLD (blood oxygenation level-dependent) and FAIR (flow-sensitive alternating inversion recovery) imaging techniques were used to investigate the oxygenation and hemodynamic responses of human brain during repeated challenges of breath holding and prolonged single breath holding. The effects of different breathing techniques on BOLD and FAIR image contrasts were carefully examined. With a periodic breath-holding paradigm of 30 s, global changes in gray matter were observable both in T*2-weighted and FAIR images. T*2-weighted images showed 1-4% relative signal intensity increases, while FAIR images demonstrated relative cerebral blood flow (CBF) increase up to 30-70%. The activated pixels depicted in FAIR images were about three times less than those seen in T*2-weighted images. With prolonged breath holding, it was observed that signal intensities in T*2-weighted and FAIR images were dependent on the breathing techniques used. Breath holding after expiration gave rise to immediate signal intensity increases in T*2-weighted and FAIR images, whereas breath holding performed after deep inspiration signals showed a biphasic change both in flow and T*2-weighted. T*2-weighted and FAIR signals showed a transient decrease before rising above the baseline level.

Rapid and continuous monitoring of cerebral perfusion by magnetic resonance line scan assessment with arterial spin tagging

A new approach is presented for rapid and continuous monitoring of cerebral perfusion which is based upon line-scan MR column imaging with arterial spin tagging (AST) of endogenous water. Spin tagging of arterial water protons is accomplished using adiabatic fast passage inversion, followed by acquisition of the perfusion sensitive MR signal from a column placed at the desired level through the brain using line scan localization techniques. A perfusion sensitive line scan is followed by a non-perfusion sensitive line scan, and perfusion is calculated pixel-by-pixel from the intensity difference of the two lines. Continuous perfusion measurements are reported with temporal resolution of 10 s in pixels of volume 0.027 cm3 or less. Examples of the methodology are given during hypercapnic challenge induced with carbon dioxide, and during an ischemic event induced by reversible middle cerebral artery occlusion. The method is also used to characterize the signal response as a function of arterial inversion time and post inversion acquisition delay. These methods permit rapid and continuous monitoring of cerebral perfusion with high spatial resolution, and can be interleaved with MR measurements of diffusion and T1 to follow the progression of cerebral events during physiological or pharmacological intervention.

Correction for eddy current induced Bo shifts in diffusion-weighted echo-planar imaging

The importance of diffusion-weighted MRI in the assessment of acute stroke is well-recognized, and quantitative maps of the apparent diffusion coefficient (ADC) are now widely used. Echoplanar imaging provides a robust method of acquiring diffusion-weighted images free of motion artifact. However, initial experience with clinical MRI systems indicates that calculation of artifact-free ADC maps from a series of echo-planar diffusion-weighted images is not necessarily straight-forward. One of the problems is that frequency shifts resulting from eddy currents can cause misregistration of base diffusion-weighted images. In this study, an on-line correction method that overcomes this problem is described, and phantom and human images that demonstrate the validity of the technique are presented. The method uses a non-phase-encoded reference scan to correct the phase of each echo in the echo train, and can provide ADC maps that are free of misregistration artifacts, without the need for off-line postprocessing.

Extraslice spin tagging (EST) magnetic resonance imaging for the determination of perfusion

A new magnetic resonance technique to measure perfusion is described in detail. The means by which this is done is to invert all the spins in the radiofrequency RF coil with a non-spatially selective pulse and immediately re-invert the spins in the imaging plane. The net effect is that the spins in the imaging plane experience minimal perturbation of their magnetization while the spins outside the plane (extraslice) are inverted, or tagged. Tagged spins that flow into the imaging plane before image data are acquired decrease the signal intensity in the imaging plane when compared with an image in which the inflowing spins are not tagged. This decrease in signal can be used to calculate the number of spins that have flowed into the imaging plane, i.e., can be used to calculate the perfusion in mL x 100 g(tissue)(- 1)x min(-1). The extraslice spin tagging (EST) magnetization preparation period was coupled with a fast imaging sequence to obtain perfusion maps for normal volunteers.

Using flow relaxography to elucidate flow relaxivity

We have investigated the theoretical and experimental linear dependence of the reciprocal of the apparent longitudinal relaxation time [(T*1)-1] of the NMR signal from spins in a flowing fluid on the volume flow rate, Fv, the so-called inflow effect. We refer to the coefficient of this dependence as the longitudinal flow relaxivity, r1F. A very simple model predicts that, under a range of conditions pertinent to modern flow studies and perfusion imaging experiments, r1F is controlled by the volume of the fluid in which the magnetization is perturbed by pulsed RF inversion or saturation, not the detection volume, and that it can be approximated as the reciprocal of half of the inversion volume. Phantom sample experiments, using a new, quantitative approach that we call flow relaxography, confirm the general predictions of this simple model. There are two intriguing implications of these findings for general NMR flow studies as well as for medical applications. It should be possible to vary the value of r1F by simply (noninvasively) adjusting the inversion slice thickness, and thus measure the value of (blood 1H2O, for example) Fv in a vessel without changing Fv, from the resultant varying T*1 values. Also, it should be possible to extrapolate to the intrinsic T1 value of the fluid signal (as if it were stationary), without altering or stopping the flow. Again, these are quite successful in phantom sample studies. Imaging versions of the flow relaxographic experiments are also possible. The twin goals of flow studies in medical MRI are the quantitative discrimination of the signals from flowing and nonflowing spins, and the accurate measurement of the flow rate of the former.

EPISTAR MRI: multislice mapping of cerebral blood flow

A method is described for multislice EPISTAR that perfectly compensates magnetization transfer effects. Inflowing arterial spins are labeled with a 360 degrees adiabatic pulse. Two control tags are applied sequentially at the same location as the labeling pulse, each with a 180 degrees adiabatic pulse so the total RF irradiation, frequency shift, and bandwidth of the labeling and control pulses are identical. Therefore, magnetization transfer effects are the same as for the labeling pulse and cancel with image subtraction for all slices. The method also eliminates tagging of venous spins and concern about asymmetric magnetization transfer effects.

Perfusion imaging using FOCI RF pulses

Pulsed arterial spin-tagging techniques for perfusion measurements (e.g., echo planar MR imaging and signal targeting with alternating radiofrequency (EPISTAR), flow-sensitive alternating inversion recovery (FAIR), quantitative imaging of perfusion using a single subtraction (QUIPPS), uninverted FAIR (UNFAIR)) generally use hyperbolic secant (HS) pulses for spin inversion. The performance of these techniques depends on the inversion efficiency, as well as the sharpness of the slice profiles. Frequency offset corrected inversion (FOCI) pulses, a recently proposed HS variant, can provide slice profiles with edges that can be up to 10 times sharper than those obtained with conventional HS pulses. In this communication, the implementation and application of the C-shape FOCI pulse for perfusion imaging in rat brain with the FAIR technique is summarized. Despite providing a more rectangular slice profile than a conventional HS pulse, it is demonstrated both theoretically and experimentally that the FAIR perfusion signal is not increased by using a FOCI tagging pulse. However, the use of a FOCI inversion pulse is shown to significantly minimize static signal subtraction errors that are common with conventional HS pulses. Finally, the suitability of the pulse for perfusion studies is demonstrated, in vivo, on rat brain.

Functional magnetic resonance imaging of regional cerebral blood oxygenation changes during breath holding

BACKGROUND AND PURPOSE

Recently, noninvasive MRI methods have been developed that are now capable of detecting and mapping regional hemodynamic responses to various stress tests, which involve the use of vasoactive substances such as acetazolamide or inhalation of carbon dioxide. The aim of this study was to assess regional cerebral blood oxygenation changes during breath holding at 1.5 T.

METHODS

In 6 healthy volunteers, T2*-weighted gradient echo images were acquired for a total dynamic scanning time of 10 minutes during alternating periods of breath holding and normal breathing at 40-second intervals after inspiration, at 30-second intervals after expiration, and at 18 seconds after expiration. To quantify the relative signal changes, 2.5-minute baseline image sampling with normal breathing was carried out.

RESULTS

Repeated challenges of breath holding of various durations induced an overall rise in blood oxygen level-dependent (BOLD) signal intensities. In general, BOLD signal intensity increases were greatest in gray matter and nonsignificant in white matter. Depending on the breath-holding duration and techniques, BOLD signal intensity increases of all activated pixels varied from 0.8% to 3.5%.

CONCLUSIONS

The present study demonstrates that cerebral blood oxygenation changes during breath holding can be detected by means of fMRI at 1.5 T. The breath-holding test, a short and noninvasive method to study cerebral hemodynamics with fMRI, could become a useful alternative to the acetazolamide or CO2 test.

Simultaneous oxygenation and perfusion imaging study of functional activity in primary visual cortex at different visual stimulation frequency: quantitative correlation between BOLD and CBF changes

A relationship between regional cerebral blood flow (CBF) and blood oxygenation level dependent (BOLD) changes in the primary visual cortex (V1) at varied visual stimulation frequency has been examined quantitatively using the multislice FAIR technique. A linear correlation in the common activation areas between functional BOLD and CBF maps was observed. This supports the hypothesis that the task-stimulated BOLD changes in microvasculature are correlated with the CBF changes that presumably reflect the degree of neuronal activity. The linear correlation coefficients for intrasubject comparisons are more significant than those for intersubject comparisons. This suggests that using intrasubject comparisons for quantitative studies of neuronal activity related to different task stimuli and task performances should be more reliable than using intersubject comparisons.

Radial echo-planar imaging

A new ultrafast magnetic resonance imaging pulse sequence named radial echo-planar imaging (rEPI) is introduced. The sequence is based on a modification of the echo-planar imaging (EPI) sequence to scan k-space radially, in an attempt to combine the speed of EPI with the benefits of radial sampling. Like in EPI, all the desired lines in k-space are scanned consecutively in opposite directions. The unique feature of this new sequence, however, is that the orientation of the readout gradient is incrementally rotated, so that all the echoes are refocused through the center of k-space. Therefore, rEPI data are acquired in a polar grid, and image reconstruction can be done either by means of filtered back-projection or by regridding the data to a Cartesian matrix followed by 2D Fourier transform. First results show that rEPI images can be acquired with the same speed and signal-to-noise ratio of EPI images. rEPI images are also shown to be less sensitive to off-resonance effects than EPI images. Further studies are underway to investigate the usefulness of rEPI for spectroscopic imaging and applications affected by motion.

FAIR excluding radiation damping (FAIRER)

It is shown that the flow-sensitive alternating inversion recovery (FAIR) technique is complicated by the effect of radiation damping, leading to problems in calibrating this method on phantoms and to inaccuracies in measured flows. A modified scheme called FAIRER (FAIR excluding radiation damping) is proposed, which suppresses the damping effects by employing very weak magnetic field gradients (0.06 G/cm) during the inversion recovery, spin-echo, and predelay periods. Results on phantoms and in vivo on cat brain are presented that demonstrate that FAIRER effectively solves these problems.

Continuous assessment of perfusion by tagging including volume and water extraction (CAPTIVE): a steady-state contrast agent technique for measuring blood flow, relative blood volume fraction, and the water extraction fraction

A new technique, CAPTIVE, that is a synthesis of arterial spin labeling (ASL) blood flow and steady-state susceptibility contrast relative blood volume imaging is described. Using a single injection of a novel, long half-life intravascular magnetopharmaceutical with a high tissue:blood susceptibility difference (deltachi) to deltaR1 ratio, changes in tissue transverse relaxivity (deltaR2 or deltaR2*) that arise from changes in blood volume were measured, while preserving the ability to measure blood flow using traditional T1-based ASL techniques. This modification permits the continuous measurement of both blood flow and blood volume. Also, because the contrast agent can be used to remove the signal from intravascular spins, it is possible to measure the first-pass water extraction fraction. Contrast-to-noise is easily traded off with repetition rate, allowing the use of non-EPI scanners and more flexible imaging paradigms. The basic theory of these measurements, several experimental scenarios, and validating results are presented. Specifically, the PaCO2-reactivity of microvascular and total relative cerebral blood volume (rCBV), cerebral blood flow (CBF), and the water extraction-flow product (EF) in rats with the new contrast agent MPEG-PL-DyDTPA is measured, and the values are concordant with those of previous literature. As an example of one possible application, continuous flow and volume measurements during transient focal ischemia are presented. It is believed that CAPTIVE imaging will yield a more complete picture of the hemodynamic state of an organ, and has further application for understanding the origins of the BOLD effect.

In vivo quantitative mapping of cardiac perfusion in rats using a noninvasive MR spin-labeling method

Measurement of myocardial perfusion is important for the functional assessment of heart in vivo. Our approach is based on the modification of the longitudinal relaxation time T1 induced by magnetic spin labeling of endogenous water protons. Labeling is performed by selectively inverting the magnetization within the detection slice, and longitudinal relaxation is measured using a fast gradient echo MRI technique. As a result of blood flow, nonexcited spins enter the detection slice, which leads to an acceleration of the relaxation rate. Incorporating this phenomenon in a mathematical model that describes tissue as two compartments yields a simple expression that allows the quantification of perfusion from a slice-selective and a global inversion recovery experiment. This model takes into account the difference between T1 in blood and T1 in tissue. Our purpose was to evaluate the feasibility and reproducibility of this technique to map quantitatively myocardial perfusion in vivo in rats. Quantitative maps of myocardial blood flow were obtained from nine rats, and the reproducibility of the technique was evaluated by repeating the whole perfusion experiment four times. Evaluation of regions of interest within the myocardium yielded a mean perfusion value of 3.6 +/- .5 ml x min(-1) x g(-1) over all animals, which is in good agreement with previously reported literature values.

Simultaneous glutamate and perfusion fMRI responses to regional brain stimulation

Functional magnetic resonance imaging (fMRI) rests on the assumption that regional brain activity is closely coupled to regional cerebral blood flow (rCBF) in vivo. To test the degree of coupling, cortical brain activity was locally stimulated in rats by reversed microdialysis infusion of picrotoxinin, alphagamma-aminobutyric acid-A antagonist. Before and during the first 30 minutes of infusion, simultaneous fMRI (rCBF) and neurochemical (interstitial glutamate concentration) measures of brain activity were highly correlated (r = 0.83). After 30 minutes of picrotoxinin-induced stimulation, glutamate levels decreased but rCBF remained elevated, suggesting that additional factors modulate the relationship between neuronal neurotransmitters and hemodynamics at these later stages.

Velocity sensitivity of slice-selective excitation

The purpose of this study was to investigate how flow affects slice-selective excitation, particularly for radiofrequency (rf) pulses optimized for slice-selective excitation of stationary material. Simulation methods were used to calculate the slice profiles for material flowing at different velocities, using optimal flow compensation when appropriate. Four rf pulses of very different shapes were used in the simulation study: a 90 degrees linear-phase Shinnar-LeRoux pulse; a 90 degrees self-refocusing pulse; a minimum-phase Shinnar-LeRoux inversion pulse; and a SPINCALC inversion pulse. Slice profiles from simulations with a laminar flow model were compared with experimental studies for two different rf pulses using a clinical magnetic resonance imaging (MRI) system. We found that, for a given rf pulse, the effect of flow on slice-selective excitation depends on the product of the selection gradient amplitude, the component of velocity in the slice selection direction, and the square of the rf pulse duration. The shapes of the slice profiles from the Shinnar-LeRoux pulses were relatively insensitive to velocity. However, the slice profiles from the self-refocusing pulse and the SPINCALC pulse were significantly degraded by velocity. Experimental slice profiles showed excellent agreement with simulation. In conclusion, our study demonstrates that slice-selective excitation can be significantly degraded by flow depending on the velocity, the gradient amplitude, and characteristics of the rf excitation pulse used. The results can aid in the design of rf pulses for slice-selective excitation of flowing material.

A theoretical and experimental comparison of continuous and pulsed arterial spin labeling techniques for quantitative perfusion imaging

Under ideal conditions, continuous arterial spin labeling (ASL) techniques are higher in SNR than pulsed ASL techniques by a factor of e. Presented here is a direct theoretical and experimental comparison of continuous ASL and pulsed ASL, using versions of both that are amenable to multislice imaging and insensitive to variations in transit times (continuous ASL with a delay before imaging, and QUIPSS II (Quantitative Imaging of Perfusion Using a Single Subtraction-second version)). Perfusion image quality for comparable imaging time was nearly identical for both single-slice and multislice imaging. The measured raw signal was approximately 25% higher with continuous ASL, but the SNR per unit time was identical.

Simultaneous measurement of perfusion and oxygenation changes using a multiple gradient-echo sequence: application to human muscle study

We have developed a magnetic resonance imaging (MRI) technique based on a multiple gradient-echo sequence designed to probe perfusion and oxygenation simultaneously within skeletal muscle. Processing of the images acquired at successive echo times (TEs) generates two functional maps: one of the signal intensity (SI) extrapolated to zero echo time, which is sensitive to perfusion; and a second one of R2*, which reflects oxygenation. An advantage of the processing procedure lies in the selection of tissue of interest through the profile of T2* decay, leading to automatic rejection of pixels containing small vessels. This allows a more specific assessment of tissue perfusion and oxygenation. This technique was demonstrated successfully during post-ischemic reactive hyperemia in human calf. A perfusion peak of 123 mL x 100 g(-)1 x min(-1) was measured immediately after ischemia, whereas R2* value showed an 11.5% decrease at the same time, essentially reflecting blood oxygenation changes. Differences in the time courses of reperfusion and re-oxygenation were observed, oxygenation presenting a slower recovery. The mechanisms responsible for such a differential dynamic response are discussed.

A general kinetic model for quantitative perfusion imaging with arterial spin labeling

Recently, several implementations of arterial spin labeling (ASL) techniques have been developed for producing MRI images sensitive to local tissue perfusion. For quantitation of perfusion, both pulsed and continuous labeling methods potentially suffer from a number of systematic errors. In this study, a general kinetic model for the ASL signal is described that can be used to assess these errors. With appropriate assumptions, the general model reduces to models that have been used previously to analyze ASL data, but the general model also provides a way to analyze the errors that result if these assumptions are not accurate. The model was used for an initial assessment of systematic errors due to the effects of variable transit delays from the tagging band to the imaging voxel, the effects of capillary/tissue exchange of water on the relaxation of the tag, and the effects of incomplete water extraction. In preliminary experiments with a human subject, the model provided a good description of pulsed ASL data during a simple sensorimotor activation task.

Cocaine decreases cortical cerebral blood flow but does not obscure regional activation in functional magnetic resonance imaging in human subjects

The authors used functional magnetic resonance imaging (fMRI) to determine whether acute intravenous (i.v.) cocaine use would change global cerebral blood flow (CBF) or visual stimulation-induced functional activation. They used flow-sensitive alternating inversion recovery (FAIR) scan sequences to measure CBF and blood oxygen level-dependent (BOLD) sensitive T2* scan sequences during visual stimulation to measure neuronal activation before and after cocaine and saline infusions. Cocaine (0.6 mg/kg i.v. over 30 seconds) increased heart rate and mean blood pressure and decreased end tidal carbon dioxide (CO2). All measures returned to baseline by 2 hours, the interinfusion interval, and were unchanged by saline. Flow-sensitive alternating inversion recovery imaging demonstrated that cortical gray matter CBF was unchanged after saline infusion (-2.4 +/- 6.5%) but decreased (-14.1 +/- 8.5%) after cocaine infusion (n = 8, P < 0.01). No decreases were detected in white matter, nor were changes found comparing BOLD signal intensity in cortical gray matter immediately before cocaine infusion with that measured 10 minutes after infusion. Visual stimulation resulted in comparable BOLD signal increases in visual cortex in all conditions (before and after cocaine and saline infusion). Despite a small (14%) but significant decrease in global cortical gray matter CBF after acute cocaine infusion, specific regional increases in BOLD imaging, mediated by neurons, can be measured reliably.

Pulse sequences for steady-state saturation of flowing spins

It is useful to be able to suppress the NMR signal from spins in a flowing fluid, for example for "black-blood" visualization of blood vessels in vivo, for the suppression of flow artifacts, and for the estimation of tissue perfusion by continuous labeling of inflowing arterial spins. This work considers the flow of fluid through a region in which it is subjected to a train of saturation pulses. Computer simulations and in vitro measurements show that a train of equal-duration spoiler pulses produces less effective suppression than does a train of pulses of geometrically increasing duration. It is shown analytically that a long train of ideal equal-duration spoiler pulses converts initial magnetization (0, 0, M0) into a combination of longitudinal and transverse magnetization equal to 0. 29 (-M0, 0, M0) and is therefore unsatisfactory for continuous saturation.

On the characteristics of functional magnetic resonance imaging of the brain

In this review we discuss various recent topics that characterize functional magnetic resonance imaging (fMRI). These topics include a brief description of MRI image acquisition, how to cope with noise or signal fluctuation, the basis of fMRI signal changes, and the relation of MRI signal to neuronal events. Several observations of fMRI that show good correlation to the neurofunction are referred to. Temporal characteristics of fMRI signals and examples of how the feature of real time measurement is utilized are then described. The question of spatial resolution of fMRI, which must be dictated by the vascular structure serving the functional system, is discussed based on various fMRI observations. Finally, the advantage of fMRI mapping is shown in a few examples. Reviewing the vast number of recent fMRI application that have now been reported is beyond the scope of this article.

Magnetic resonance imaging of acute stroke

In the investigation of ischemic stroke, conventional structural magnetic resonance (MR) techniques (e.g., T1-weighted imaging, T2-weighted imaging, and proton density-weighted imaging) are valuable for the assessment of infarct extent and location beyond the first 12 to 24 hours after onset, and can be combined with MR angiography to noninvasively assess the intracranial and extracranial vasculature. However, during the critical first 6 to 12 hours, the probable period of greatest therapeutic opportunity, these methods do not adequately assess the extent and severity of ischemia. Recent developments in functional MR imaging are showing great promise for the detection of developing focal cerebral ischemic lesions within the first hours. These include (1) diffusion-weighted imaging, which provides physiologic information about the self-diffusion of water, thereby detecting one of the first elements in the pathophysiologic cascade leading to ischemic injury; and (2) perfusion imaging. The detection of acute intraparenchymal hemorrhagic stroke by susceptibility weighted MR has also been reported. In combination with MR angiography, these methods may allow the detection of the site, extent, mechanism, and tissue viability of acute stroke lesions in one imaging study. Imaging of cerebral metabolites with MR spectroscopy along with diffusion-weighted imaging and perfusion imaging may also provide new insights into ischemic stroke pathophysiology. In light of these advances in structural and functional MR, their potential uses in the study of the cerebral ischemic pathophysiology and in clinical practice are described, along with their advantages and limitations.

BASE imaging: a new spin labeling technique for measuring absolute perfusion changes

A new technique for magnetic resonance imaging of absolute perfusion changes that uses magnetically labeled tissue water proton spins as a freely diffusible tracer is described. It consists of unprepared basis (BA) images that serve as a reference and selective (SE) inversion prepared images that are sensitive to perfusion changes. In the present study, the BASE technique was applied to functional neuroimaging. BA and SE images were alternatingly and repeatedly acquired during periods of visual stimulation and control. Visual stimulation was achieved with an alternating black/white checkerboard operating at a frequency of 8 Hz. Maps of the absolute cerebral blood flow changes (deltaCBF) were calculated from the image intensities of the corresponding BA and SE images. The individual mean values of deltaCBF measured in five healthy volunteers ranged from 69 +/- 18 to 99 +/- 26 ml/min/100 g. Since the BASE technique does not require nonselective spin inversion, it can be used with small transmit/receive head coils (e.g., surface coils). In addition, the BASE technique is robust against a mismatch of the inversion and detection slice profiles.

Multislice imaging of quantitative cerebral perfusion with pulsed arterial spin labeling

A method is presented for multislice measurements of quantitative cerebral perfusion based on magnetic labeling of arterial spins. The method combines a pulsed arterial inversion, known as the FAIR (Flow-sensitive Alternating Inversion Recovery) experiment, with a fast spiral scan image acquisition. The short duration (22 ms) of the spiral data collection allows simultaneous measurement of up to 10 slices per labeling period, thus dramatically increasing efficiency compared to current single slice acquisition protocols. Investigation of labeling efficiency, suppression of unwanted signals from stationary as well as intraarterial spins, and the FAIR signal change as a function of inversion delay are presented. The assessment of quantitative cerebral blood flow (CBF) with the new technique is demonstrated and shown to require measurement of arterial transit time as well as suppression of intraarterial spin signals. CBF values measured on normal volunteers are consistent with results obtained from H2O15 positron emission tomography (PET) studies and other radioactive tracer approaches. In addition, the new method allows detection of activation-related perfusion changes in a finger-tapping experiment, with locations of activation corresponding well to those observed with blood oxygen level dependent (BOLD) fMRI.