Perfusion imaging by a flow-sensitive alternating inversion recovery (FAIR) technique: application to functional brain imaging

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.

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).

In vivo intermolecular double-quantum imaging on a clinical 1.5 T MR scanner

A novel MRI method based on the intermolecular double-quantum coherence (DQC) for soft tissues is described. DQC images of human brain were obtained for the first time on a whole-body 1.5 T scanner. The combination of quantum and classical formalisms was used to characterize multiple-quantum coherences, and to aid in the design of a DQC imaging sequence. The theoretical analysis suggests that signals from the intermolecular DQCs have higher sensitivity than those from the zero-quantum coherence (ZQC) for human brain, and the sensitivity increases with increased field strength. The DQC signal may provide a new form of contrast for MRI.

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.

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.

Gender differences in cerebral blood flow and oxygenation response during focal physiologic neural activity

Using functional magnetic resonance imaging techniques CBF and oxygenation changes were measured during sustained checkerboard stimulation in 38 right-handed healthy volunteers (18 men and 20 women). The average blood oxygenation level dependent (BOLD) contrast technique signal intensity change was 1.67 +/- 0.6% in the group of male volunteers and 2.15 +/- 0.6% in the group of female volunteers (P < .05). Baseline regional CBF (rCBF) values in activated gray matter areas within the visual cortex were 57 +/- 10 mL x 100 g(-1) x min(-1) in women and 50 +/- 12 mL x 100 g(-1) x min(-1) in men, respectively (P = .09). Despite a broad overlap between both groups the rCBF increase was significantly higher in women compared to men (33 +/- 5 mL x 100 g(-1) x min(-1) versus 28 +/- 4 mL x 100 g(-1) x min(-1), P < .01). The increase of rCBF was not correlated with the baseline rCBF (mL x 100 g(-1) x min(-1)) (r(s) = 0.01, P = .9). Moreover, changes of rCBF were not correlated with changes in BOLD signal intensities (r(s) = 0.1, P = .7). Enhanced rCBF response in women during visual stimulation could be related to gender differences in visual physiology or may reflect gender differences in the vascular response to focal neuronal activation. Gender differences must be considered when interpreting the results of functional magnetic resonance imaging studies.

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.

Assessment of cerebral oxidative metabolism with breath holding and fMRI

Carbon dioxide inhalation can be used to map changes in cerebral metabolic rate of oxygen (CMRO(2)) during neuronal activation with functional MRI (fMRI). A hypercapnic stress also can be achieved with a simple breath-holding test. Using this test as means of manipulating cerebral blood flow (CBF) independent of CMRO(2), we assessed changes in CMRO(2) during visual stimulation. With this task, CBF increased by 61 +/- 7%, whereas CMRO(2) changed by 2.43 +/- 4.97%. These results are in good agreement with previous positron emission tomographic (PET) data, indicating that changes in oxidative metabolism during focal neuronal activity can potentially be determined with the breath-holding test. This test could easily be performed during a routine MRI examination. Magn Reson Med 42:608-611, 1999.

Comparison of matched BOLD and FAIR 4.0T-fMRI with [15O]water PET brain volumes

Valid comparisons of functional activation volumes from fMRI and PET require accurate registration, matched spatial resolution, and if possible matched noise. We coregistered 4.0T-fMRI and PET volumes, using a series of linear and nonlinear transformations applied to the PET volumes. Because of the limited number of fMRI slices that were available, PET volumes were transformed to the fMRI space. Since 4.0T-fMRI and 4.0T-MRI volumes have significant spatial distortion due to magnet inhomogeneities, high resolution 1.5T-MRI volumes were nonlinearly transformed to 4.0T-MRI volumes as part of the transformation chain. The smoothing effects of these registration transformations were measured, in order to match the spatial resolution of the coregistered fMRI and PET volumes. Spatial resolution of the transformed PET volumes in the fMRI space was degraded by up to 60% due to the transformation process. Due to both the image acquisition characteristics and the coregistration process, the transformed PET volumes had a spatial resolution that was lower than that of tMRI. Therefore, significant smoothing of fMRI volumes was necessary to match their spatial resolution with that of the transformed PET volumes. Matching the spatial resolution of the fMRI volumes to those of the transformed PET volumes was achieved by matching the shape of their point spread functions. In order to do this, Gaussian kernels were employed to smooth the fMRI volumes. We were unable to simultaneously match the resolution and noise of fMRI and PET signals in the motor cortex. Activation maps derived from transformed PET and smoothed fMRI volumes were compared. Contralateral motor cortex was active in all modalities but there were large variations in the size of the activated region and its signal to noise ratio across BOLD, FAIR, and PET images within each subject. Nevertheless, the relative CBF changes measured by FAIR were consistent with those determined by PET.

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.

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.

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.

Relationship between cerebral blood flow changes during visual stimulation and baseline flow levels investigated with functional MRI

Using fMRI, the relationship between regional cerebral blood flow (rCBF) changes during visual stimulation and the prevailing baseline global and regional flow levels was evaluated in 22 volunteers. The absolute increase in rCBF was not correlated with baseline rCBF values (r = 0.01, p = 0.8); however, the percentage change in rCBF showed a negative correlation (r=-0.78, p<0.001). Both absolute and relative changes in rCBF were independent of baseline global CBF values. These results indicate that caution should be exercised when comparing relative flow changes during focal brain activation, especially in functional neuroimaging studies dealing with altered baseline flow values.

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.

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.

High-resolution, multiple gradient-echo functional MRI at 1.5 T

A multiple gradient echo, high resolution imaging method is proposed to better visualize different sources of activation in functional magnetic resonance imaging (fMRI) experiments. Eight echoes are collected from 30 ms to 205 ms with an echo spacing of 25 ms. All echoes show significant activation, but each echo reveals its own pattern of activation. From this variability, it appears that large vessel contributions can be separated from small vessel contributions using a fuzzy cluster analysis across echo times. The results demonstrate the importance of a multiple gradient echo data acquisition approach in localizing various vascular contributions to brain activation in fMRI.

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.

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).

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.

Crossed cerebellar diaschisis: assessment with dynamic susceptibility contrast MR imaging

The authors investigated the feasibility of using T2-weighted, half-Fourier rapid acquisition with relaxation enhancement, or RARE, dynamic susceptibility contrast magnetic resonance (MR) imaging to depict crossed cerebellar diaschisis. In 10 patients after unilateral supratentorial stroke, crossed cerebellar diaschisis was demonstrated in the relative regional cerebellar blood volume maps obtained with MR imaging. Cerebellar blood volume values for the nonaffected cerebellar hemisphere were significantly larger than those for the affected side (P = .0003).

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.

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.

Physiological basis for BOLD MR signal changes due to neuronal stimulation: separation of blood volume and magnetic susceptibility effects

An NMR method is applied for separating blood volume and magnetic susceptibility effects in response to neuronal stimulation in a rat model. The method uses high susceptibility contrast agents to enhance blood volume induced signal changes. In the absence of exogenous agent, the dominant source of signal change on neuronal activation is associated with the signal increase from the blood oxygen level dependent (BOLD) effect. The relative negative contribution of blood volume changes to BOLD changes is maximally estimated to be 34%. The blood volume changes associated with median nerve stimulation (7 Hz) in the motor cortex are 26+/-7% and the corresponding blood susceptibility changes are 0.021+/-0.006 ppm. These methods can be applied to enhance the sensitivity of fMRI signal response and provide accurate quantitative measures of blood volume response to stimulation.

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.

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.

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.

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.

Functional imaging by I0- and T2*-parameter mapping using multi-image EPI

A new functional imaging method has been developed and used to measure T2*- and initial intensity (I0)-parameter maps of multiple slices during activation of the cortex with high temporal resolution. Multiple echo-planar images (EPIs) are read out after a single excitation of the spin system, leading to several images with increasing effective echo times. Changes induced by primary visual stimulation in T2* and I0 were measured in eight subjects. Although stimulation-induced increases in I0 only occurred at short repetition times, T2* increased from 57.3 to 60.9 ms on average. The method combines the high stability of a single shot EPI experiment with the high information content of a multiecho acquisition. In addition, stimulation-induced changes in inflow can be easily separated from true blood oxygenation level dependent (BOLD) signal changes.

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.

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.

Quantitative imaging of perfusion using a single subtraction (QUIPSS and QUIPSS II)

In the pulsed arterial spin labeling (ASL) techniques EPISTAR, PICORE, and FAIR, subtraction of two images in which inflowing blood is first tagged and then not tagged yields a qualitative map of perfusion. An important reason this map is not quantitative is that there is a spatially varying delay in the transit of blood from the tagging region to the imaging slice that cannot be measured from a single subtraction. We introduce here two modifications of pulsed ASL (QUIPSS and QUIPSS II) that avoid this problem by applying additional saturation pulses to control the time duration of the tagged bolus, rendering the technique relatively insensitive to transit delays and improving the quantitation of perfusion.

Quantitative measurements of cerebral blood flow in rats using the FAIR technique: correlation with previous iodoantipyrine autoradiographic studies

Flow-sensitive alternating inversion recovery (FAIR) is a recently introduced MRI technique for assessment of perfusion that uses blood water as an endogenous contrast agent. To characterize the FAIR signal dependency on spin tagging time (inversion time (TI)) and to validate FAIR for cerebral blood flow (CBF) quantification, studies were conducted on the rat brain at 9.4 T using a conventional gradient-recalled echo sequence. The T1 of cerebral cortex and blood was found to be 1.9 and 2.2 s, respectively, and was used for CBF calculations. At short TIs (<0.8 s), the FAIR signal originates largely from vascular components with fast flows, resulting in an overestimation of CBF. For TI > 1.5 s, the CBF calculated from FAIR is independent of the spin tagging time, suggesting that the observed FAIR signal originates predominantly from tissue/capillary components. CBF values measured by FAIR with TI of 2.0 s were found to be in good agreement with those measured by the iodoantipyrine technique with autoradiography in rats under the same conditions of anesthesia and arterial pCO2. The measured pCO2 index on the parietal cortex using the FAIR technique was 6.07 ml/100 g/min per mmHg, which compares well with the pCO2 index measured by other techniques. The FAIR technique was also able to detect the regional reduction in CBF produced by middle cerebral artery occlusion in rats.

Quantitation of regional cerebral blood flow increases during motor activation: a steady-state arterial spin tagging study

Steady-state arterial spin tagging MRI approaches were used to quantitate regional cerebral blood flow increases during finger tapping tasks in seven normal subjects. Statistically significant increases in cerebral blood flow were observed in the contralateral primary sensorimotor cortex in all seven subjects and in the supplementary motor area in five subjects. The intrinsic spatial resolution of the cerebral blood flow images was approximately 4 mm. If no spatial filtering was applied, the average increase in cerebral blood flow in the activated primary sensorimotor cortex was 60 +/- 10 cc/100 g/min (91 +/- 32%). If the images were filtered to a spatial resolution of 15 mm, the average increase in cerebral blood flow in the activated primary sensorimotor cortex was 23 +/- 7 cc/100 g/min (42 +/- 15%), in agreement with previously reported 133Xe and PET results.

Comparison of blood oxygenation and cerebral blood flow effects in fMRI: estimation of relative oxygen consumption change

The most widely-used functional magnetic resonance imaging (fMRI) technique is based on the blood oxygenation level dependent (BOLD) effect, which requires at least partial uncoupling between cerebral blood flow (CBF) and oxygen consumption changes during increased mental activity. To compare BOLD and CBF effects during tasking, BOLD and flow-sensitive alternating inversion recovery (FAIR) images were acquired during visual stimulation with red goggles at a frequency of 8 Hz in an interleaved fashion. With the FAIR technique, absolute and relative CBF changes were determined. Relative oxygen consumption changes can be estimated using the BOLD and relative CBF changes. In gray matter areas in the visual cortex, absolute and relative CBF changes in humans during photic stimulation were 31 +/- 11 SD ml/100 g tissue/min and 43 +/- 16 SD % (n = 12), respectively, while the relative oxygen consumption change was close to zero. These findings agree extremely well with previous results using positron emission tomography. The BOLD signal change is not linearly correlated with the relative CBF increase across subjects and negatively correlates with the oxygen consumption change. Caution should be exercised when interpreting the BOLD percent change as a quantitative index of the CBF change, especially in inter-subject comparisons.

Functional magnetic resonance imaging of the human brain

The current technical and methodological status of functional magnetic resonance imaging (fMRI) is reviewed. The mechanisms underlying the effects of deoxyhemoglobin concentration and cerebral blood flow changes are discussed, and methods for monitoring these changes are described and compared. Methods for post-processing fMRI data are outlined. Potential problems and solutions related to vessels and motion are discussed in detail.

Multi-slice perfusion-based functional MRI using the FAIR technique: comparison of CBF and BOLD effects

Perfusion-weighted imaging techniques employing blood water protons as an endogenous tracer have poor temporal resolution because each image should be acquired with an adequate spin 'tagging' time. Thus, perfusion-based functional magnetic resonance imaging studies are typically performed on a single slice. To alleviate this problem, a multi-slice flow-sensitive alternating inversion recovery technique has been developed. Following a single inversion pulse and a delay time, multi-slice echo-planar images are acquired sequentially without any additional inter-image delay. Thus, the temporal resolution of multi-slice FAIR is almost identical to that of single slice techniques. The theoretical background for multi-slice FAIR is described in detail. The multi-slice FAIR technique has been successfully applied to obtain three-slice cerebral blood flow based functional images during motor tasks. The relative CBF change in the contralateral motor/sensory area during unilateral thumb-digit opposition is 45.0+/-12.2% (n=9), while the blood oxygenation level dependent signal change is 1.5+/-0.4 SD%. Relative changes of the oxygen consumption rate can be estimated from CBF and BOLD changes using FAIR. The BOLD signal change is not correlated with the relative CBF increase, and thus caution should be exercised when interpreting the BOLD change as a quantitative index of the CBF change, especially in inter-subject comparisons.