Reduced transit-time sensitivity in noninvasive magnetic resonance imaging of human cerebral blood flow

Functional MRI and its applications to the clinical neurosciences

Functional magnetic resonance imaging (fMRI) is an emerging methodology for studying regional brain function in vivo at relatively high spatial and temporal resolution. Because MRI methods are comparatively inexpensive and entirely noninvasive, fMRI has rapidly become one of the most popular approaches for brain mapping in cognitive and systems neuroscience. There has also been great interest in using fMRI to assist in clinical diagnosis and management, with promising demonstrations of feasibility in a number of applications. Both resting and task-specific regional brain activity can be measured, primarily utilizing alterations in regional cerebral blood flow (CBF) as a surrogate marker for neural function. This article reviews the biophysical and physiological bases of fMRI and its applications to the clinical neurosciences, with particular attention to potential challenges of fMRI under pathophysiological conditions. Carefully controlled prospective evaluation of clinical fMRI in its various potential applications will be required for fMRI to be validated as a clinically useful tool. Because the technology for fMRI is widely available, its impact could be substantial.

RF excitation profiles with FAIR: impact of truncation of the arterial input function on quantitative perfusion

This study investigates the impact of imaging coil length and consequent truncation of the arterial input function on the perfusion signal contrast obtained in the flow-sensitive alternating inversion recovery (FAIR) perfusion imaging measurement. We examined the difference in perfusion contrast achieved with head, head and neck, and body imaging coils based on the hypothesis that the standard head coil provides a truncated input function compared with that provided by the body coil and that this effect will be accentuated at long inversion times. The TI-dependent cerebral response of the FAIR sequence was examined at 1.5 T by varying the TI from 200 to 3500 msec with both the head and whole body coils (n = 5) as well as using a head and neck coil (n = 3). Difference signal intensity DeltaM and quantitative cerebral blood flow (CBF) were plotted against TI for each coil configuration. Despite a lower signal-to-noise ratio, relative CBF was significantly greater when measured with the body or head and neck coil compared with the standard head coil for longer inversion times (two-way ANOVA, P < or = 0.002). This effect is attributed to truncation of the arterial input function of labeled water by the standard head coil and the resultant inflow of unlabeled spins to the image slice during control image acquisition, resulting in overestimation of CBF. The results support the conclusion that the arterial input function depends on the anatomic extent of the inversion pulse in FAIR, particularly at longer mixing times (TI > 1200 msec at 1.5 T). Use of a head and neck coil ensures adequate inversion while preserving SNR that is lost in the body coil.

Acute changes in MRI diffusion, perfusion, T(1), and T(2) in a rat model of oligemia produced by partial occlusion of the middle cerebral artery

Oligemic regions, in which the cerebral blood flow is reduced without impaired energy metabolism, have the potential to evolve toward infarction and remain a target for therapy. The aim of this study was to investigate this oligemic region using various MRI parameters in a rat model of focal oligemia. This model has been designed specifically for remote-controlled occlusion from outside an MRI scanner. Wistar rats underwent remote partial MCAO using an undersize 0.2 mm nylon monofilament with a bullet-shaped tip. Cerebral blood flow (CBF(ASL)), using an arterial spin labeling technique, the apparent diffusion coefficient of water (ADC), and the relaxation times T(1) and T(2) were acquired using an 8.5 T vertical magnet. Following occlusion there was a decrease in CBF(ASL) to 35 +/- 5% of baseline throughout the middle cerebral artery territory. During the entire period of the study there were no observed changes in the ADC. On occlusion, T(2) rapidly decreased in both cortex and basal ganglia and then normalized to the preocclusion values. T(1) values rapidly increased (within approximately 7 min) on occlusion. In conclusion, this study demonstrates the feasibility of partially occluding the middle cerebral artery to produce a large area of oligemia within the MRI scanner. In this region of oligemic flow we detect a rapid increase in T(1) and decrease in T(2). These changes occur before the onset of vasogenic edema. We attribute the acute change in T(2) to increased amounts of deoxyhemoglobin; the mechanisms underlying the change in T(1) require further investigation.

Transit time, trailing time, and cerebral blood flow during brain activation: measurement using multislice, pulsed spin-labeling perfusion imaging

Transit time and trailing time in pulsed spin-labeling perfusion imaging are likely to be modulated by local blood flow changes, such as those accompanying brain activation. The majority of transit/trailing time is due to the passage of the tagged blood bolus through the arteriole/capillary regions, because of lower blood flow velocity in these regions. Changes of transit/trailing time during activation could affect the quantification of CBF in functional neuroimaging studies, and are therefore important to characterize. In this work, the measurement of transit and trailing times and CBF during sensorimotor activation using multislice perfusion imaging with pulsed arterial spin-labeling is described. While CBF elevated dramatically ( thick similar80.7%) during the sensorimotor activation, sizable reductions of transit time ( thick similar0.11 sec) and trailing time ( thick similar0.26 sec) were observed. Transit and trailing times were dependent on the distances from the leading and trailing edges of the tagged blood bolus to the location of the imaging slices. The effects of transit/trailing time changes on CBF quantification during brain activation were analyzed by simulation studies. Significant errors can be caused in the estimation of CBF if such changes of transit/trailing time are not taken into account.

Turbo ASL: arterial spin labeling with higher SNR and temporal resolution

A modified pulsed arterial spin labeling (ASL) technique is introduced here that has both higher temporal resolution and higher SNR per unit time than existing ASL techniques. In this technique, the time TI between the application of the tag and image acquisition is longer than the repetition time TR, allowing for the use of greatly reduced TR values without a significant decrease in the amplitude of the ASL signal. This improves both the temporal resolution and the sensitivity of ASL for functional brain mapping.

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

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

H(2)(15)O PET validation of steady-state arterial spin tagging cerebral blood flow measurements in humans

Steady-state arterial spin tagging approaches can provide quantitative images of CBF, but have not been validated in humans. The work presented here compared CBF values measured using steady-state arterial spin tagging with CBF values measured in the same group of human subjects using the H(2)(15)O IV bolus PET method. Blood flow values determined by H(2)(15)O PET were corrected for the known effects of incomplete extraction of water across the blood brain barrier. For a cortical strip ROI, blood flow values determined using arterial spin tagging (64+/-12 cc/100 g/min) were not statistically different from corrected blood flow values determined using H(2)(15)O PET (67+/-13 cc/100 g/min). However, for a central white matter ROI, blood flow values determined using arterial spin tagging were significantly underestimated compared to corrected blood flow values determined using H(2)(15)O PET. This underestimation could be caused by an underestimation of the arterial transit time for white matter regions.

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

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

Is all perfusion-weighted magnetic resonance imaging for stroke equal? The temporal evolution of multiple hemodynamic parameters after focal ischemia in rats correlated with evidence of infarction

Although perfusion-weighted imaging techniques are increasingly used to study stroke, no particular hemodynamic variable has emerged as a standard marker for accumulated ischemic damage. To better characterize the hemodynamic signature of infarction. the authors have assessed the severity and temporal evolution of ischemic hemodynamics in a middle cerebral artery occlusion model in the rat. Cerebral blood flow (CBF) and total and microvascular cerebral blood volume (CBV) changes were measured with arterial spin labeling and steady-state susceptibility contrast magnetic resonance imaging (MRI), respectively, and analyzed in regions corresponding to infarcted and spared ipsilateral tissue, based on 2,3,5-triphenyltetrazolium chloride histology sections after 24 hours ischemia. Spin echo susceptibility contrast was used to measure microvascular-weighted CBV, which had a maximum sensitivity for vessels with radii between 4 and 30 microm. Serial measurements between 1 and 3 hours after occlusion showed no change in CBF (22 +/- 20% of contralateral, mean +/- SD) or in total CBV (78 +/- 13% of contralateral) in regions destined to infarct. However, microvascular CBV progressively declined from 72 +/- 5% to 64 +/- 11% (P < 0.01) during this same period. Microvascular CBV changes with time were entirely due to decreases in subcortical infarcted zones (from 73 +/- 9% to 57 +/- 14%. P < 0.001) without changes in the cortical infarcted territory. The hemodynamic variables showed differences in magnitude and temporal response, and these changes varied based on histologic outcome and brain architecture. Such factors should be considered when designing imaging studies for human stroke.

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

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

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

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

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

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

Altered diffusion and perfusion in hydrocephalic rat brain: a magnetic resonance imaging analysis

OBJECT

It can be inferred from data published in the literature that brain compression occurs in the early stages of acute hydrocephalus and that drainage of extracellular waste products is impaired. The authors hypothesized that compression of the cortex would alter water distribution and retard the diffusion of fluid in the hydrocephalic brain.

METHODS

Proton diffusion, blood perfusion, and T1 and T2 relaxation times were determined in adult rat brain by using magnetic resonance imaging prior to, and 1 and 8 days after induction of hydrocephalus by kaolin injection. Five anatomical regions of interest were studied. The striatum, dorsal cortex, and lateral cortex exhibited decreased T2 and apparent diffusion coefficient (ADC) values but no change in perfusion. Examination of white matter revealed an initial decrease in ADC followed by a significant increase. The T2 relaxation times increased and perfusion decreased progressively between 1 and 8 days after induction of hydrocephalus.

CONCLUSIONS

Acute experimental hydrocephalus causes compression of gray matter, perhaps associated with reduction in total water, which impairs diffusion of water in the tissue. White matter compression and hypoperfusion precede the development of edema. These findings have importance for understanding the neurochemical changes that occur in hydrocephalic brains.

Magnetic resonance perfusion imaging in acute ischemic stroke using continuous arterial spin labeling

BACKGROUND AND PURPOSE

Continuous arterial spin-labeled perfusion MRI (CASL-PI) uses electromagnetically labeled arterial blood water as a diffusible tracer to noninvasively measure cerebral blood flow (CBF). We hypothesized that CASL-PI could detect perfusion deficits and perfusion/diffusion mismatches and predict outcome in acute ischemic stroke.

METHODS

We studied 15 patients with acute ischemic stroke within 24 hours of symptom onset. With the use of a 6-minute imaging protocol, CASL-PI was measured at 1.5 T in 8-mm contiguous supratentorial slices with a 3.75-mm in-plane resolution. Diffusion-weighted images were also obtained. Visual inspection for perfusion deficits, perfusion/diffusion mismatches, and effects of delayed arterial transit was performed. CBF in predetermined vascular territories was quantified by transformation into Talairach space. Regional CBF values were correlated with National Institutes of Health Stroke Scale (NIHSS) score on admission and Rankin Scale (RS) score at 30 days.

RESULTS

Interpretable CASL-PI images were obtained in all patients. Perfusion deficits were consistent with symptoms and/or diffusion-weighted imaging abnormalities. Eleven patients had hypoperfusion, 3 had normal perfusion, and 1 had relative hyperperfusion. Perfusion/diffusion mismatches were present in 8 patients. Delayed arterial transit effect was present in 7 patients; serial imaging in 2 of them showed that the delayed arterial transit area did not succumb to infarction. CBF in the affected hemisphere correlated with NIHSS and RS scores (P=0.037 and P=0.003, Spearman rank correlation). The interhemispheric percent difference in middle cerebral artery CBF correlated with NIHSS and RS scores (P=0.007 and P=0.0002, respectively).

CONCLUSIONS

CASL-PI provides rapid noninvasive multislice imaging in acute ischemic stroke. It depicts perfusion deficits and perfusion/diffusion mismatches and quantifies regional CBF. CASL-PI CBF asymmetries correlate with severity and outcome. Delayed arterial transit effects may indicate collateral flow.

Noninvasive magnetic resonance imaging evaluation of cerebral blood flow with acetazolamide challenge in patients with cerebrovascular stenosis

To evaluate the utility of using magnetic resonance imaging (MRI) of cerebral blood flow (CBF) in conjunction with pharmacologic flow augmentation, the authors imaged 14 patients with ischemic symptoms referable to large artery cerebrovascular stenosis of the anterior circulation. CBF was measured by using continuous arterial spin labeling (CASL) both at rest and 10 minutes after 1 g intravenous acetazolamide on a commercial 1.5 Tesla scanner. Quantitative CBF images were calculated along with augmentation images showing the effects of acetazolamide. Interpretable studies were obtained from all patients. Based on the image data as well as a region of interest analysis of CBF changes in middle cerebral artery distributions, varying patterns of augmentation were observed that suggested differing mechanisms of ischemic symptomatology. The ability to obtain this information in conjunction with a structural MRI examination extends the diagnostic potential for MRI in cerebrovascular disease and allows the value of augmentation testing in clinical management to be assessed more widely. J. Magn. Reson. Imaging 1999;10:870-875.

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.

Cerebrovascular dynamics of autoregulation and hypoperfusion. An MRI study of CBF and changes in total and microvascular cerebral blood volume during hemorrhagic hypotension

BACKGROUND AND PURPOSE

To determine how cerebral blood flow (CBF), total and microvascular cerebral blood volume (CBV), and blood oxygenation level-dependent (BOLD) contrast change during autoregulation and hypotension using hemodynamic MRI.

METHODS

Using arterial spin labeling and steady-state susceptibility contrast, we measured CBF and changes in both total and microvascular CBV during hemorrhagic hypotension in the rat (n=9).

RESULTS

We observed CBF autoregulation for mean arterial blood pressure (MABP) between 50 and 140 mm Hg, at which average CBF was 1.27+/-0.44 mL. g(-1). min(-1) (mean+/-SD). During autoregulation, total and microvascular CBV changes were small and not significantly different from CBF changes. Consistent with this, no significant BOLD changes were observed. For MABP between 10 and 40 mm Hg, total CBV in the striatum increased slightly (+7+/-12%, P<0.05) whereas microvascular CBV decreased (-15+/-17%, P<0.01); on the cortical surface, total CBV increases were larger (+21+/-18%, P<0.01) and microvascular CBV was unchanged (3+/-22%, P>0.05). With severe hypotension, both total and microvascular CBV decreased significantly. Over the entire range of graded global hypoperfusion, there were increases in the CBV/CBF ratio.

CONCLUSIONS

Parenchymal CBV changes are smaller than those of previous reports but are consistent with the small arteriolar fraction of total blood volume. Such measurements allow a framework for understanding effective compensatory vasodilation during autoregulation and volume-flow relationships during hypoperfusion.

Early perfusion after controlled cortical impact in rats: quantification by arterial spin-labeled MRI and the influence of spin-lattice relaxation time heterogeneity

Early posttraumatic cerebral hypoperfusion is implicated in the evolution of secondary damage after experimental and clinical traumatic brain injury (TBI). This is the first report of cerebral blood flow (CBF) measurement by continuous arterial spin-labeled magnetic resonance imaging (MRI) early after TBI in rats using the controlled cortical impact (CCI) model. CCI reduced CBF globally at approximately 3 hr (versus normal), with 85% and 49% reductions in a contused cortical region and contralateral cortex, respectively. In contrast, a prior MRI study from this laboratory showed at 24 hr post trauma a focal CBF reduction restricted to the injury site. In vivo spin-lattice relaxation time (T(1obs)), which is used in CBF quantification, was spatially heterogeneous early after CCI, a time when edema is developing in injured brain tissue. At 4.7 T, T(1obs) values are increased 29% in the contusion (versus normal), consequently reducing CBF quantification to a similar degree. MRI should facilitate coupling posttraumatic CBF with long-term functional outcome. Magn Reson Med 42:673-681, 1999.

Altered diffusion and perfusion in hydrocephalic rat brain: a magnetic resonance imaging analysis

It can be inferred from data published in the literature that brain compression occurs in the early stages of acute hydrocephalus and that drainage of extracellular waste products is impaired. The authors hypothesized that compression of the cortical extracellular compartment will alter water distribution and retard the diffusion of fluid in the hydrocephalic brain.

Using magnetic resonance imaging proton diffusion, blood perfusion, and T1 and T2 relaxation times were determined in adult rat brain prior to, and 1 and 8 days following induction of hydrocephalus by using kaolin injection. Five anatomical regions of interest were studied. The striatum, dorsal cortex, and lateral cortex were shown to exhibit decreased T2 and apparent diffusion coefficient (ADC) values but no change in perfusion. Examination of white matter demonstrated an initial decrease in ADC followed by a significant increase. The T2 relaxation times increased and perfusion decreased progressively from 1 to 8 days.

Acute experimental hydrocephalus causes compression of gray matter, perhaps associated with reduction in total water, which impairs diffusion of protons in the tissue. White matter compression and hypoperfusion precede the development of edema. These findings have importance for understanding the neurochemical changes that occur in hydrocephalic brains.

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

Steady-state arterial spin tagging approaches were used to construct multislice images of relative cerebral blood flow changes during finger-tapping tasks. Statistically significant increases in cerebral blood flow were observed in primary sensorimotor cortex in all seven subjects. The mean volume of the activated region in the contralateral primary sensorimotor cortex was 0.9 cm(3), and the mean increase in cerebral blood flow in the activated area was 54% +/- 11%. Although the extended spatial coverage is advantageous for activation studies, the intrinsic sensitivity of the multislice approach is smaller than the intrinsic sensitivity of the single-slice, arterial spin tagging approach. Magn Reson Med 42:404-407, 1999. Published 1999 Wiley-Liss, Inc.

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

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

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.

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.

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.

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.

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

The time evolution of water diffusion, perfusion, T1, and T2 is investigated at high magnetic field (8.5 T) following permanent middle cerebral artery occlusion in the rat. Cerebral blood flow maps were obtained using arterial spin tagging. Although the quantitative perfusion measurements in ischemic tissue still pose difficulties, the combined perfusion and diffusion data nevertheless distinguish between a "moderately affected area," with reduced perfusion but normal diffusion; and a "severely affected area," in which both perfusion and diffusion are significantly reduced. Two novel magnetic resonance imaging observations are reported, namely, a decrease in T2 and an increase in T1, both within the first few minutes of ischemia. The rapid initial decrease in T2 is believed to be associated with an increase in deoxyhemoglobin levels, while the initial increase in T1 may be related to several factors, such as flow effects, an alteration in tissue oxygenation, and changes in water environment.

A strategy to optimize the signal-to-noise ratio in one-coil arterial spin tagging perfusion imaging

The signal-to-noise ratio of the perfusion image (SNR(perfu)) in a spin-tagging experiment is shown to depend on both the degree of spin labeling (alpha) and the signal-to-noise ratio of the proton density images (SNRimage) used to calculate the perfusion image. When a single radiofrequency (RF) coil is used for both spin tagging and magnetic resonance (MR) imaging, magnetization transfer (MT) effects decrease SNRimage, and therefore SNRperfu, by an amount that depends on the strength B1 and offset deltaomega (determined by the gradient strength G(I) applied during spin tagging) of the labeling RF pulse. It is shown that by optimizing B1 and G(I), it is possible to reduce MT effects and thus increase SNRimage, while leaving alpha unchanged. As a result, SNRperfu, will be improved. An equation for calculating perfusion under general conditions of such reduced MT effects is derived and shown to give perfusion rates that are independent of the strength and offset of the labeling RF irradiation.

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.

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.

Technical solution for an interactive functional MR imaging examination: application to a physiologic interview and the study of cerebral physiology

Studies with functional magnetic resonance (MR) imaging produce large unprocessed raw data sets in minutes. The analysis usually requires transferring of the data to an off-line workstation, and this process frequently occurs after the subject has left the MR unit. The authors describe a hardware configuration and processing software that captures whole-brain raw data files as they are being produced from the MR unit. It then performs the reconstruction, registration, and statistical analysis, and displays the results in seconds after completion of the MR image acquisition.

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.

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.

Quantitation of regional cerebral blood flow increases in prefrontal cortex during a working memory task: a steady-state arterial spin-tagging study

Steady-state arterial spin-tagging MRI approaches were used to quantitate regional cerebral blood flow increases in prefrontal cortex during a working memory ("two-back") task in six normal subjects. Statistically significant increases in cerebral blood flow in prefrontal cortex were observed in all six subjects: the average increase in cerebral blood flow in activated prefrontal cortex regions was 22 +/- 5 cc/100 g/min (23 +/- 7%). The results demonstrate that spin-tagging approaches can be used to follow focal activation in prefrontal cortex during cognitive tasks.

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.

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.

Diffusion, perfusion, and T2 magnetic resonance imaging of anti-intercellular adhesion molecule 1 antibody treatment of transient middle cerebral artery occlusion in rat

The effect of anti-intercellular adhesion molecule-1 (anti-ICAM-1) antibody treatment of transient (2 h) middle cerebral artery (MCA) occlusion in the rat was measured using diffusion (DWI)-, T2 (T2I)- and perfusion (PWI)-weighted magnetic resonance imaging. Rats were treated upon reperfusion with an anti-ICAM-1 monoclonal antibody (n=11) or a control antibody (n=7). DWI, T2I and PWI were performed before, during, and after induction of focal cerebral ischemia from 1 h to 7 days. In both groups, the apparent diffusion coefficient of water (ADCw) and cerebral blood flow (CBF) values in the ischemic region significantly declined from the preischemic ADCw values (p<0. 05). The post ischemic increase in T2 of the control group was significantly higher at 48 h than in the anti-ICAM-1 treated group (p<0.05). CBF was not significantly different between the two groups. The temporal profiles of MRI cluster analysis, which combines ADCw and T2 maps into a single image, was significantly different between groups. These data suggest that the neuroprotective effect of anti-ICAM-1 antibody treatment is reflected in reductions of T2 and lesion growth during reperfusion and may not be associated with increased cerebral perfusion.

Functional MRI using steady-state arterial water labeling

Flow-sensitive functional MRI (fMRI) was performed using steady-state arterial water labeling (SS-AWL). Arterial water labeling was accomplished by flow induced adiabatic fast passage. The signal intensity of the visual cortex in arterial water labeled images decreased by approximately 1.4% during visual stimulation of the brain. Acquisition of arterial water unlabeled and labeled images allows measurement of relative cerebral blood flow increase during brain activation. During visual stimulation, cerebral blood flow in the visual cortex increased by 17 to 35% as measured by SS-AWL. Quantitation of brain activation in terms of a physiological parameter using SS-AWL will facilitate comparative fMRI studies under different conditions.

Phase insensitive preparation of single-shot RARE: application to diffusion imaging in humans

Although RARE and GRASE can produce single-shot images of excellent quality, their utility has been restricted because preparation of the magnetization with interesting contrast before imaging can cause severe artifacts. These artifacts relate to the strong sensitivity of multiple spin echo sequences to the phase of the prepared magnetization. Modifications of the RARE sequence to eliminate these artifacts are discussed, and an approach that eliminates the artifact producing signals from the very first echo is presented. The approach is applied to diffusion imaging of the human brain in normal volunteers and one patient.

Evidence for the exchange of arterial spin-labeled water with tissue water in rat brain from diffusion-sensitized measurements of perfusion

The extraction fraction of vascular water in rat brain is investigated by means of diffusion measurements of arterial spin labeled water at varying cerebral blood flow (CBF) values. The apparent diffusion coefficient (ADC) of the difference of the proton magnetization signal in the brain acquired with and without continuous arterial spin labeling is modeled to provide a measure of the amount of arterial water in tissue and vasculature and thus of the extraction fraction. The tissue and vascular portion of the arterial spin labeled water are differentiated based on their diffusion characteristics in a manner analogous to the intravoxel incoherent motion (IVIM) method. The amount of labeled arterial water that exchanges with tissue water is determined by estimating the fraction of the total signal that is associated with the slow-decaying component of a biexponential fit to the normalized difference signal between the magnetization of brain tissue acquired with and without arterial spin labeling. The results indicate that, at normal CBF (1.15 +/- 0.21 ml x g(-1) x min[-1]), about 90% of the arterial spin labeled water diffuses with an ADC of (1.21 +/- 0.37) x 10[-3] mm2 s[-1]), which is equal to tissue. At high CBF, an increasing fraction of the labeling water has a fast-pseudo-diffusion coefficient due to a decrease in water extraction fractions. The results also show that the contribution of vascular water to the measurement of perfusion by techniques that use endogenous water as a tracer can be efficiently eliminated by the use of diffusion sensitizing gradients with small effective b values (b approximately 20 s/mm2), enabling these techniques to monitor true changes in tissue perfusion.