Multislice imaging of quantitative cerebral perfusion with pulsed arterial spin labeling

Comparison of multislice and single-slice acquisitions for pulsed arterial spin labeling measurements of cerebral perfusion

Multislice Q2TIPS is a widely used pulsed arterial spin labeling (PASL) technique for efficient and accurate quantification of cerebral blood flow (CBF). Slices are typically acquired inferior to superior from a tagging plane. Superior slices show signal loss greater than the loss expected from blood T1 decay. In order to assess the reasons for this additional signal loss, three single-slice acquisition studies were compared to multislice acquisition (six slices) in healthy volunteers. In Study 1 (n=8), the tagging plane was fixed in location, and the inversion time (TI2) was 1500 ms for each slice. For Study 2 (n=12), the tagging plane was fixed as in Study 1; however, TI2 increased as slices were acquired further from the tagging plane. In Study 3 (n=9), the tagging plane was kept adjacent to the imaging slice, and TI2 was 1500 ms for every slice. Gray matter (GM) and white matter (WM) signal-to-noise ratio (SNR) and CBF were measured per slice. GM SNR from single-slice acquisitions was significantly higher at slices 4-6 in Study 2 and at slices 2-6 in Study 3 compared to multislice acquisitions. Signal loss in distal slices of multislice acquisitions can be attributed to the destruction of tagged bolus in addition to blood T1 decay. If limited brain coverage is acceptable, perfusion images with greater SNR are achievable with limited slices and placement of the tagging region immediately adjacent to the site of interest.

Non-invasive measurement of perfusion: a critical review of arterial spin labelling techniques

The non-invasive nature of arterial spin labelling (ASL) has opened a unique window into human brain function and perfusion physiology. High spatial and temporal resolution makes the technique very appealing not only for the diagnosis of vascular diseases, but also in basic neuroscience where the aim is to develop a more comprehensive picture of the physiological events accompanying neuronal activation. However, low signal-to-noise ratio and the complexity of flow quantification make ASL one of the more demanding disciplines within MRI. In this review, the theoretical background and main implementations of ASL are revisited. In particular, the perfusion quantification methods, including the problems and pitfalls involved, are thoroughly discussed in this article. Finally, a brief summary of applications is provided.

Removing the effects of CSF partial voluming on fitted CBF and arterial transit times using FAIR, a pulsed arterial spin labelling technique

FAIR, an arterial spin labelling technique, provides non-invasive, quantitative CBF values and arterial transit times deltat. This paper focuses on the negative impact of CSF partial voluming on FAIR results. To understand and solve this problem, we performed a theoretical analysis and a range of simulations. We then acquired FAIR data from a volunteer to illustrate our findings. We found that the determinant effect of CSF is a delayed zero-crossing during inversion recovery. The subtraction of magnitude inversion recovery data in FAIR generates erroneous negative data and distorted fit results: we simulated that for CSF percentages of 0-40%, CBF and deltat will be progressively overestimated by up to 50%. For higher CSF percentages the errors were found to increase steeply. We explored a straightforward solution: taking the magnitude of the FAIR data before fitting. This provided a remarkably strong antidote against the effects of CSF partial voluming: for CSF percentages of 0-40%, simulations now gave CBF values accurate within 1%, and deltat within 5%. The fit remained robust for high CSF fractions. Our analysis and simulations demonstrate that using magnitude FAIR data minimises the detrimental effects of CSF partial voluming. Data from a healthy volunteer illustrate these results.

Current and future applications of magnetic resonance imaging and spectroscopy of the brain in hepatic encephalopathy

Hepatic encephalopathy (HE) is a common neuro-psychiatric abnormality, which complicates the course of patients with liver disease and results from hepatocellular failure and/or portosystemic shunting. The manifestations of HE are widely variable and involve a spectrum from mild subclinical disturbance to deep coma. Research interest has focused on the role of circulating gut-derived toxins, particularly ammonia, the development of brain swelling and changes in cerebral neurotransmitter systems that lead to global CNS depression and disordered function. Until recently the direct investigation of cerebral function has been difficult in man. However, new magnetic resonance imaging (MRI) techniques provide a non-invasive means of assessment of changes in brain volume (coregistered MRI) and impaired brain function (fMRI), while proton magnetic resonance spectroscopy (¹H MRS) detects changes in brain biochemistry, including direct measurement of cerebral osmolytes, such as myoinositol, glutamate and glutamine which govern processes intrinsic to cellular homeostasis, including the accumulation of intracellular water. The concentrations of these intracellular osmolytes alter with hyperammonaemia. MRS-detected metabolite abnormalities correlate with the severity of neuropsychiatric impairment and since MR spectra return towards normal after treatment, the technique may be of use in objective patient monitoring and in assessing the effectiveness of various treatment regimens.

Arterial Spin Labeling: Benefits and Pitfalls of High Magnetic Field

Arterial spin labeling (ASL) techniques are MR imaging methods designed to measure the endogenous perfusion signal coming from arterial blood by manipulation of its magnetization. These methods are based on the subtraction of two consecutively acquired images: one acquired after preparation of the arterial blood magnetization upstream to the area of interest, and the second without any manipulation of its arterial magnetization. The subtraction of both images provides information on the perfusion of the tissue present in the slice of interest. Because ASL is a very low SNR technique, the shift from 1.5 T to 3.0 T should be regarded as a great way to increase signal-to-noise ratio (SNR). Furthermore, the concomitant increase in blood T(1) should improve the SNR of ASL further. Other effects related to poorer magnetic filed homogeneities and reduced T(2) relaxation times, however, will counterbalance both effects partially. In this article, the pros and cons of the use of ASL at high field are summarized, after a brief description of the major techniques used and their theoretical limitations. Finally, a summary of the few existing dedicated ASL perfusion techniques available are presented.

Why perfusion in neonates with congenital heart defects is negative — Technical issues related to pulsed arterial spin labeling

Pulsed arterial spin labeling (PASL) perfusion MRI has unique advantages for measuring cerebral blood flow (CBF) in the pediatric population. In neonates with congenital heart defects (CHDs), however, a considerable number of negative CBF values were observed in PASL perfusion images. A set of specific physiological and biophysical conditions were proposed as plausible explanations for this phenomenon, including small body size, low blood flow, prolonged tracer life time (blood T1) and the "shunt" between pulmonary and systemic circulations in CHD. An optimized PASL scheme with a restricted label volume was proposed, and experimental data demonstrated reduced spurious negative values and lower intersubject variability of perfusion measurements in neonates with CHD as compared to standard PASL sequences.

Pulsed arterial spin labeling parameter optimization for an elderly population

PURPOSE

To optimize pulsed arterial spin labeling (PASL) parameters for the elderly to take into account possible perfusion changes that occur in the brain with age.

MATERIALS AND METHODS

Healthy young (N = 14, age range = 21-27 years) and elderly (N = 12, age range = 61-67 years) subjects were scanned using Q2TIPS (QUIPSS II with thin-slice TI, periodic saturation) with varying inversion times (TI(2)) at 1.5T. The difference signal (DeltaM), transit time (deltat), and cerebral blood flow (CBF) were calculated in segmented gray matter (GM).

RESULTS

The young displayed more perfusion-weighted signal difference than the elderly at all TI(2)'s. The peak DeltaM occurred at TI(2) approximately 1300 msec and 1500 msec in the young and elderly groups, respectively. Qualitatively, intravascular signal was minimal in the younger group by TI(2) = 1500 msec, whereas a longer TI(2) of 1800 msec was needed to minimize this signal in the elderly. The transit time was approximately 100 msec longer in the elderly, and CBF was in the range of literature values.

CONCLUSION

For acquiring perfusion-weighted images with minimal intravascular signal and adequate tissue signal for PASL studies of cerebral perfusion in the elderly, a longer inversion time is advantageous.

Cerebral vascular response to hypercapnia: Determination with perfusion MRI at 1.5 and 3.0 Tesla using a pulsed arterial spin labeling technique

PURPOSE

To compare the quantification of cerebral blood flow (CBF) at 1.5 and 3.0 Tesla, under normo- and hypercapnia, and to determine the cerebral vascular response (CVR) of gray matter (GM) to hypercapnia, a pulsed arterial spin labeling technique was used. Additionally, to improve GM CBF quantification a high-resolution GM-mask was applied.

MATERIALS AND METHODS

CBF was determined with the QUIPSS II with thin slice TI1 periodic saturation (Q2TIPS) sequence at 1.5 and 3.0 Tesla in the same group of eight subjects, both under normocapnia and hypercapnia. Absolute GM-CBF maps were calculated using a GM-mask obtained from a high-resolution structural scan by segmentation. The CVR to hypercapnia was derived from the quantitative GM-CBF maps.

RESULTS

For both field strengths, the GM-CBF was significantly higher under hypercapnia compared to normocapnia. For both conditions, there was no significant difference of GM-CBF for 1.5 and 3.0 Tesla; the same applies to the CVR, which was 4.3 and 4.5%/mmHg at 1.5 and 3.0 Tesla, respectively.

CONCLUSION

The method presented allows for the quantification of CBF and CVR in GM at the common clinical field strengths of 1.5 and 3.0 Tesla and could therefore be a useful tool to study these parameters under physiological and pathophysiological conditions.

Towards quantification of blood-flow changes during cognitive task activation using perfusion-based fMRI

Multi-slice perfusion-based functional magnetic resonance imaging (p-fMRI) is demonstrated with a color-word Stroop task as an established cognitive paradigm. Continuous arterial spin labeling (CASL) of the blood in the left common carotid artery was applied for all repetitions of the functional run in a quasi-continuous fashion, i.e., it was interrupted only during image acquisition. For comparison, blood oxygen level dependent (BOLD) contrast was detected using conventional gradient-recalled echo (GE) echo planar imaging (EPI). Positive activations in BOLD imaging appeared in p-fMRI as negative signal changes corresponding to an enhanced transport of inverted water spins into the region of interest, i.e., increased cerebral blood flow (CBF). Regional differences between the localization of activations and the sensitivity of p-fMRI and BOLD-fMRI were observed as, for example, in the inferior frontal sulcus and in the intraparietal sulcus. Quantification of CBF changes during cognitive task activation was performed on a multi-subject basis and yielded CBF increases of the order of 20-30%.

Spatially-confined arterial spin-labeling with FAIR

PURPOSE

To investigate the effectiveness of slab-selective inversion in pulsed arterial spin labeling with body coil excitation as a means to reduce large vessel contamination of the perfusion signal.

MATERIALS AND METHODS

Studies were conducted by varying the tagging width in multislice flow-sensitive alternating inversion recovery (FAIR) in conjunction with body coil excitation on a Siemens Sonata whole-body 1.5-T scanner. The results of spatially-confined tagging were then compared with conventional nonselective tagging in the presence and absence of a bipolar gradient crusher pair in order to determine the effectiveness of suppressing vascular signal and to estimate the bolus width that reaches the capillary bed.

RESULTS

It is shown in five volunteers, ages 23-38 years, that depending on the average velocity of the arterial blood flow in the tagging region, a bolus of 6-8 cm in width reaches the capillary bed at a fixed inversion time TI of 1.4 seconds, while a bolus of 11.2-16.5 cm in width enters the imaging region. Further, noticeable velocity differences have been found among the participating subjects, with averages ranging from 10.1 to 13.9 cm/second.

CONCLUSION

The data suggest that it is advantageous to replace nonselective global tagging in FAIR perfusion imaging with body coil excitation by spatially-confined tagging to reduce undesired residual tagged blood in large vessels.

Quantitative assessment of the reproducibility of functional activation measured with BOLD and MR perfusion imaging: Implications for clinical trial design

BOLD contrast is the most commonly used functional MRI method for studies of brain activity. However, the underlying physiological processes giving rise to measured BOLD signal changes (which include contribution from changes in cerebral blood flow (CBF), cerebral blood volume (CBV) and cerebral metabolic rate of oxygen consumption (CMRO2)) vary substantially between sessions and subjects. To determine whether direct CBF measurement is a more reliable technique, we compared the localisation of activation and reproducibility of relative signal change measured by optimised BOLD versus CBF measured using the arterial spin labelling (ASL) technique. Data were collected within the primary sensorimotor cortex in normal healthy controls performing a simple finger-tapping task over three imaging sessions (two on same day and one on a different day). The displacement between the foci of BOLD and CBF activation was less than the linear dimension of one voxel (2.4 mm), however, BOLD activation was significantly closer to the nearest draining vein compared to CBF activation (P=0.030). For the relative signal change measurement, we found that CBF has a lower inter-subject variation than BOLD (P<0.05), enabling a smaller sample size for any given effect size, although the intra-subject variation across sessions for CBF was not significantly different from BOLD. BOLD imaging provides the optimal contrast for exploratory brain activation mapping, however, for a single time-point group study, CBF has reduced variance. In addition, the reduction of variance over time using CBF measurements (non-significant) suggests it could potentially provide a more useful approach when assessing longitudinal activation changes.

FAWSETS perfusion measurements in exercising skeletal muscle

Arterial spin labeling (ASL) techniques are now recognized as valid tools for providing accurate measurements of cerebral and cardiac perfusion. The labeling process used with most ASL techniques creates two problems, magnetization transfer (MT) effects and arterial transit time effects, that require compensation. The compensation process limits time resolution and hinders absolute quantification. MT effects are particularly problematic in skeletal muscle because they are large and change rapidly during exercise. The protocol presented here was developed specifically for quantification of perfusion in exercising skeletal muscle. The ASL technique that was implemented, FAWSETS, eliminates MT effects and arterial transit times. Localized, single-voxel perfusion measurements were acquired from rat hind limbs at rest, during ischemia and during three different levels of stimulated exercise. The results demonstrate sufficient sensitivity to determine the time constants for perfusion changes at onset of, and during recovery from, exercise and to distinguish the differences in the amplitude of the perfusion response to different levels of exercise. Additional measurements were conducted to demonstrate insensitivity to MT effects. The exercise protocol is easily adaptable to phosphorous magnetic resonance measurements, allowing the possibility to acquire local measurements of perfusion and metabolism from the same tissue in future experiments.

Validation and advantages of FAWSETS perfusion measurements in skeletal muscle

This work discusses the strengths, limitations and validity of a novel arterial spin labeling technique when used specifically to measure perfusion in limb skeletal muscle. The technique, flow-driven arterial water stimulation with elimination of tissue signal (FAWSETS), offers several advantages over existing arterial spin labeling techniques. The primary goal of this study was to determine the perfusion signal response to changes in net hind limb flow that were independently verifiable. The range of perfusate flow was relevant to skeletal muscle during mild to moderate exercise. Localized, single voxel measurements were acquired from a 5 mm-thick slice in the isolated perfused rat hind limb at variable net flow rates. The results show that the perfusion signal is linearly proportional to net hind limb flow with a correlation coefficient of 0.974 (p = 0.0013). FAWSETS is especially well suited for studies of skeletal muscle perfusion, where it eliminates the need to compensate for magnetization transfer and arterial transit time effects. A conceptual discussion of the basic principles underlying these advantages is presented.

Human cerebral blood flow and metabolism in acute insulin-induced hypoglycemia

How the human brain functions under conditions of acute hypoglycemia remains a complex question by virtue of the potential simultaneous shifts in processes of perfusion, metabolism, and changing demand. We examined this issue by measuring cerebral blood flow (CBF) and oxidative metabolism (CMRO2) in insulin-induced hypoglycemic (HG) and euglycemic (EG) conditions at rest and during motor activation in normal human subjects using magnetic resonance (MR). Experiments were performed on 12 subjects (9M, 3F). The protocol consisted of insulin-induced hypoglycemia (targeting a HG of 60 mg/dL) followed by euglycemia, or in reverse order, each phase lasting approximately 1.5 h. Euglycemia was performed with the same insulin infusion rate so as to match the hypoglycemic phase. Magnetic resonance data were acquired 30 mins after the target plasma glucose was achieved so as to minimize any acute effects. Although the depth of hypoglycemia achieved in the present study was relatively small, the present data found a significant increase in flow in motor cortex with mild hypoglycemia, from 56.4+/-13.6 mL/100 g min (euglycemia) to 64.3+/-7.6 mL/100 g min (hypoglycemia). Using the Renkin-Crone exponential model of oxygen extraction with MR models of susceptibility-based relaxation, analysis of the flow measurements, relaxation and BOLD data also implied that throughout the studies, metabolism and flow remained coupled. Elementary motor task activation was not associated with any consistent larger activated flows. Thus it remains that although mild hypoglycemia induced an increase in basal flow and metabolism, a similar increase was not seen in task activation.

In vivo functional NMR imaging of resistance artery control

Arterial spin labeling (ASL) in combination with NMR imaging is an in vivo technique that quantifies tissue perfusion in absolute values (ml blood x min(-1) x g tissue(-1)) with high temporal (1-10 s) and spatial (0.1-3 mm) resolution. It uses the arterial water spins as endogenous freely diffusible markers of perfusion and, hence, is a totally noninvasive method. The technique has been successfully applied to quantify baseline perfusion in many organs, including the heart, in humans and animals, and results were validated by comparison with gold standards, PET and microspheres, respectively. Because of the high sampling rate of perfusion with ASL and the possibility that measurements could be obtained without harm over indefinite periods of time, the technique has the potential for use in functional investigations of microcirculation regulation and resistance artery control in vivo. We describe examples of the use of ASL to this end. With use of specific technological developments, ASL determination of perfusion can be coupled with simultaneous acquisitions of (1)H and (31)P NMR spectroscopy data. These protocols offer new possibilities whereby the microcirculatory control of cell oxygenation and high-energy phosphate metabolism can be explored.

Simultaneous MRI acquisition of blood volume, blood flow, and blood oxygenation information during brain activation

Simultaneous acquisition of complementary functional hemodynamic indices reflecting different aspects of brain activity would be a valuable tool for functional brain-imaging studies offering enhanced detection power and improved data interpretation. As such, a new MRI technique is presented that is able to achieve concurrent acquisition of three hemodynamic images based primarily on the changes of cerebral blood volume, blood flow, and blood oxygenation, respectively, associated with brain activation. Specifically, an inversion recovery pulse sequence has been designed to measure VASO (vascular space occupancy), ASL (arterial spin labeling) perfusion, and BOLD (blood-oxygenation-level-dependent) signals in a single scan. The MR signal characteristics in this sequence were analyzed, and image parameters were optimized for the simultaneous acquisition of these functional images. The feasibility and efficacy of the new technique were assessed by brain activation experiments with visual stimulation paradigms. Experiments on healthy volunteers showed that this technique provided efficient image acquisition, and thus higher contrast-to-noise ratio per unit time, compared with conventional techniques collecting these functional images separately. In addition, it was demonstrated that the proposed technique was able to be utilized in event-related functional MRI experiments, with potential advantages of obtaining accurate transient information of the activation-induced hemodynamic responses.

Amplitude-modulated continuous arterial spin-labeling 3.0-T perfusion MR imaging with a single coil: feasibility study

Written informed consent was obtained prior to all human studies after the institutional review board approved the protocol. A continuous arterial spin-labeling technique with an amplitude-modulated control was implemented by using a single coil at 3.0 T. Adiabatic inversion efficiency at 3.0 T, comparable to that at 1.5 T, was achieved by reducing the amplitude of radiofrequency pulses and gradient strengths appropriately. The amplitude-modulated control provided a good match for the magnetization transfer effect of labeling pulses, allowing multisection perfusion magnetic resonance imaging of the whole brain. Comparison of multisection continuous and pulsed arterial spin-labeling methods at 3.0 T showed a 33% improvement in signal-to-noise ratio by using the former approach.

The effect of B1 field inhomogeneity and the nonselective inversion profile on the kinetics of FAIR-based perfusion MRI

Perfusion imaging with pulsed arterial spin labeling techniques, like flow-sensitive alternating inversion recovery (FAIR), may suffer from inflow of fresh, i.e., unlabeled, spins. Inflow of fresh spins is caused by the arrival of unlabeled spins in the image slice and can lead to underestimation of the perfusion if not taken into account. In this study it was shown that a decrease in B(1) field strength toward the edge of the transmit coil and the consequent reduction in the inversion efficiency leads to a narrowing of the arterial delivery function and a reduction in FAIR signal. Increasing the B(1) amplitude of the adiabatic inversion pulse from 2.3 to 5.7 times its minimum amplitude requirement resulted in an observed increase of 40 to 80% in the rat brain FAIR signal at inflow times longer than 0.65 s. For coils with limited dimensions and significant B(1) inhomogeneity over the perfusion labeling slab, the application of an excessively large B(1) amplitude in combination with adiabatic inversion is recommended to optimize the FAIR perfusion contrast.

Perfusion- and BOLD-based fMRI in the study of a human pathological model for task-related flow reductions

In the present work, an arteriovenous malformation was taken as a pathological model for studying task-related flow decreases during a motor task. Combined Blood Oxygen Level Dependent (BOLD)-perfusion experiments were applied in order to evaluate the relative sensitivity of these techniques to task-related reductions in cerebral blood flow (CBF). Results shows that, by matching the sensitivity of the methods (which exhibit a different contrast-to-noise ratio) in the primary motor cortex, the spatial extent of the regions of decreased perfusion signal is larger than those of the BOLD signal reduction. The above finding suggests that perfusion imaging, that already represents a gold standard method in the detection of vascular phenomena, may estimate task-related flow decreases in a functional time-series better than BOLD.

Perfusion imaging using arterial spin labeling

Arterial spin labeling is a magnetic resonance method for the measurement of cerebral blood flow. In its simplest form, the perfusion contrast in the images gathered by this technique comes from the subtraction of two successively acquired images: one with, and one without, proximal labeling of arterial water spins after a small delay time. Over the last decade, the method has moved from the experimental laboratory to the clinical environment. Furthermore, numerous improvements, ranging from new pulse sequence implementations to extensive theoretical studies, have broadened its reach and extended its potential applications. In this review, the multiple facets of this powerful yet difficult technique are discussed. Different implementations are compared, the theoretical background is summarized, and potential applications of various implementations in research as well as in the daily clinical routine are proposed. Finally, a summary of the new developments and emerging techniques in this field is provided.

Two analytical solutions for a model of pulsed arterial spin labeling with randomized blood arrival times

A fairly general theoretical model for pulsed arterial spin labeling perfusion methods has been available for some time but analytical solutions were derived for only a small number of arterial blood input functions. These mostly assumed a sudden and simultaneous arrival of the tagged blood into the imaged region. More general cases had to be handled numerically. We present analytical solutions for two more realistic arterial input functions. They both allow the arrival times of the molecules of tagged arterial blood to be statistically distributed. We consider cases of (1) a uniform distribution on a finite time interval and (2) a normal distribution characterized by its mean and standard deviation. These models are physiologically meaningful because the statistical nature of the arrival times reflects the distribution of velocities and path lengths that the blood water molecules undertake from the tagging region to the imaged region. The model parameters can be estimated from the measured dependency of the perfusion signal on the tag inversion time.

Whole-brain 3D perfusion MRI at 3.0 T using CASL with a separate labeling coil

A variety of continuous and pulsed arterial spin labeling (ASL) perfusion MRI techniques have been demonstrated in recent years. One of the reasons these methods are still not routinely used is the limited extent of the imaging region. Of the ASL methods proposed to date, continuous ASL (CASL) with a separate labeling coil is particularly attractive for whole-brain studies at high fields. This approach can provide an increased signal-to-noise ratio (SNR) in perfusion images because there are no magnetization transfer (MT) effects, and lessen concerns regarding RF power deposition at high field because it uses a local labeling coil. In this work, we demonstrate CASL whole-brain quantitative perfusion imaging at 3.0 T using a combination of strategies: 3D volume acquisition, background tissue signal suppression, and a separate labeling coil. The results show that this approach can be used to acquire perfusion images in all brain regions with good sensitivity. Further, it is shown that the method can be performed safely on humans without exceeding the current RF power deposition limits. The current method can be extended to higher fields, and further improved by the use of multiple receiver coils and parallel imaging techniques to reduce scan time or provide increased resolution.

Effect of vascular crushing on FAIR perfusion kinetics, using a BIR-4 pulse in a magnetization prepared FLASH sequence

Flow-sensitive alternating inversion recovery (FAIR) perfusion imaging suffers from high vascular signal, resulting in artifacts and overestimation of perfusion. With TurboFLASH acquisition, crushing of vascular signal by bipolar gradients after each excitation is difficult due to the requirement of an ultrashort repetition time. Therefore, insertion of a preparation phase in the FAIR sequence, after labeling and prior to TurboFLASH acquisition, is proposed. A segmented adiabatic BIR-4 pulse, interleaved with crusher gradients, was used for flow crushing. The effect of the crusher preparation is shown as a function of crusher strength for a flow phantom and in rat brain. Influence of crusher strength on the time-dependent FAIR signal from rat brain was also measured. Signal from flowing spins in a flow phantom and from arterial spins in rat brain was significantly suppressed. Image quality was improved and the overestimation of perfusion at short inflow times was eliminated.

Arterial transit time imaging with flow encoding arterial spin tagging (FEAST)

Arterial spin labeling (ASL) perfusion imaging provides direct and absolute measurement of cerebral blood flow (CBF). Arterial transit time is a related physiological parameter reflecting the duration for the labeled spins to reach the brain region of interest. Most of the existing ASL approaches to assess arterial transit time rely on multiple measurements at various postlabeling delay times, and thus are vulnerable to motion artifact as well as computational error. We describe the use of flow encoding arterial spin tagging (FEAST) technique to measure tissue transit time, which can be derived from the ratio between the ASL signals measured with and without appropriate bipolar gradients. In the present study, we provided a theoretical framework and carried out an experimental validation during steady-state imaging. The global mean tissue transit time was approximately 1100 and 1400 ms for two conditions of bipolar gradients with specific encoding velocity (Venc) of 29 and 8 mm/sec, respectively. The mean tissue transit time measured within cerebral vascular territories was shortest in the deep middle cerebral artery (MCA) territory. Application of the FEAST technique in two patients with cerebrovascular disease demonstrated prolonged tissue transit times in the affected vascular territories which were consistent with results from other MR imaging modalities.

Empirical analyses of null-hypothesis perfusion FMRI data at 1.5 and 4 T

Functional magnetic resonance imaging (fMRI) based on arterial spin labeling (ASL) perfusion contrast is an emergent methodology for visualizing brain function both at rest and during task performance. Because of the typical pairwise subtraction approach in generating perfusion images, ASL contrast manifests different noise properties and offers potential advantages for some experimental designs as compared with blood oxygenation-level-dependent (BOLD) contrast. We studied the noise properties and statistical power of ASL contrast, with a focus on temporal autocorrelation and spatial coherence, at both 1.5- and 4.0-T field strengths. Perfusion fMRI time series were found to be roughly independent in time, and voxelwise statistical analysis assuming independence of observations yielded false-positive rates compatible with theoretical values using appropriate analysis methods. Unlike BOLD fMRI data, perfusion data were not found to have spatial coherence that varied across temporal frequency. This finding has implications for the application of spatial smoothing to perfusion data. It was also found that the spatial coherence of the ASL data is greater at high magnetic field than low field, and including the global signal as a covariate in the general linear model improves the central tendency of test statistic as well as reduces the noise level in perfusion fMRI, especially at high magnetic field.

Improved spatial localization based on flow-moment-nulled and intra-voxel incoherent motion-weighted fMRI

Functional MRI signal based on the blood oxygenation level-dependent contrast can reveal brain vascular activities secondary to neuronal activation. It could, however, arise from vascular compartments of all sizes, and in particular, be largely influenced by contributions of large vein origins that are distant from the neuronal activities. Alternative contrasts can be generated based on the cerebral blood flow or volume changes that would provide complementary information to help achieve more accurate localization to the small vessel origins. Recent reports also indicated that apparent diffusion coefficient-based contrast using intravoxel incoherent motion (IVIM) weighting could be used to efficiently detect synchronized signal changes with the functional activities. It was found that this contrast has significant arterial contribution where flow changes are more dominant. In this study, a refined approach was proposed that incorporated the flow-moment-nulling (FMN) strategy to study signal changes from the brain activation. The results were then compared with those from conventional IVIM- and BOLD-weighted acquisitions. It was shown that the activated region using the new acquisition strategy had smaller spatial extent, which was contained within the activated areas from the other two methods. Based on the known characteristics of the conventional IVIM and BOLD contrasts, it was inferred that the FMN-IVIM acquisition had improved selective sensitivity towards smaller vessels where volume changes were prevalent. Therefore, such an acquisition method may provide more specific spatial localization closely coupled to the true neuronal activities.

Functional perfusion imaging using continuous arterial spin labeling with separate labeling and imaging coils at 3 T

Functional perfusion imaging with a separate labeling coil located above the common carotid artery was demonstrated in human volunteers at 3 T. A helmet resonator and a spin-echo echo-planar imaging (EPI) sequence were used for imaging, and a circular surface coil of 6 cm i.d. was employed for labeling. The subjects performed a finger-tapping task. Signal differences between the condition of finger tapping and the resting state were between -0.5% and -1.1 % among the subjects. The imaging protocol included a long post-label delay (PLD) to reduce transit time effects. Labeling was applied for all repetitions of the functional run to reduce the sampling interval.

Dietary caffeine consumption and withdrawal: confounding variables in quantitative cerebral perfusion studies?

PURPOSE

To evaluate the effects of dietary caffeine intake and withdrawal on cerebral blood flow (CBF), as determined from a randomized, blinded, placebo-controlled study.

MATERIALS AND METHODS

Twenty adults (16 men, four women; age range, 24-64 years) categorized as low (mean, 41 mg/d) or high (mean, 648 mg/d) caffeine users underwent quantitative flow-sensitive alternating inversion-recovery perfusion magnetic resonance (MR) imaging twice: 90 minutes after a dose of caffeine (250 mg) on one day and after a dose of placebo on another day (randomized counterbalanced design). Doses were preceded by 30 hours of caffeine abstinence to induce withdrawal in high caffeine users. Quantitative CBF maps were gray matter (GM)-white matter (WM) segmented and subjected to region-of-interest analysis to obtain mean CBF in WM, anterior circulation GM (AGM), and posterior circulation GM (PGM). By using two-way repeated-measures analysis of variance, regional CBF data were tested for within-subject differences between caffeine and placebo and for between-subject differences related to dietary caffeine habits. Linear regression was used to determine whether dietary caffeine use predicts CBF or CBF response to caffeine.

RESULTS

Caffeine reduced CBF (P < or =.05) by 23% (AGM, PGM) and 18% (WM) in all subjects. Postplacebo (withdrawal) CBF in high caffeine users exceeded that in low users (P < or =.05) by 31% (AGM) and 32% (WM) (PGM, not significant). Mean postcaffeine CBF reduction in AGM was 26% in high users versus 19% in low users (P < or =.05; PGM and WM, not significant). Increasing caffeine consumption predicted higher CBF (P < or =.05) in all regions: r = 0.79 (AGM), 0.57 (PGM), and 0.76 (WM). Dietary caffeine use did not predict CBF response to caffeine.

CONCLUSION

Dietary caffeine consumption and withdrawal are potential confounding variables in cerebral perfusion and functional MR imaging.

Effects of hematocrit and oxygen saturation level on blood spin-lattice relaxation

In the present study blood T(1) was determined as a function of hematocrit and oxygen saturation. T(1) showed a significant linear dependency on both of these parameters. In addition, oxygen dissolved in blood plasma in hyperoxygenated blood resulted in relaxation enhancement, comparable in size to that due to the change in oxygenation state of hemoglobin. As blood T(1) is a key factor for quantification of flow with arterial spin labeling methods, the influence of T(1) variation in the physiological range of hematocrit and oxygen saturation to flow determination is discussed.

Pulsed arterial spin labeling using TurboFLASH with suppression of intravascular signal

Accurate quantification of perfusion with the ADC techniques requires the suppression of the majority of the intravascular signal. This is normally achieved with the use of diffusion gradients. The TurboFLASH sequence with its ultrashort repetition times is not readily amenable to this scheme. This report demonstrates the implementation of a modified TurboFLASH sequence for FAIR imaging. Intravascular suppression is achieved with a modified preparation period that includes a driven equilibrium Fourier transform (DEFT) combination of 90 degrees-180 degrees-90 degrees hard RF pulses subsequent to the inversion delay. These pulses rotate the perfusion-prepared magnetization into the transverse plane where it can experience the suitably placed diffusion gradients before being returned to the longitudinal direction by the second 90 degrees pulse. A value of b = 20-30 s/mm(2) was thereby found to suppress the majority of the intravascular signal. For single-slice perfusion imaging, quantification is only slightly modified. The technique can be readily extended to multislice acquisition if the evolving flow signal after the DEFT preparation is considered. An advantage of the modified preparation scheme is evident in the multislice FAIR images by the preservation of the sign of the magnetization difference.

Improved perfusion-weighted MRI by a novel double inversion with proximal labeling of both tagged and control acquisitions

A novel pulsed arterial spin labeling (PASL) technique for multislice perfusion-weighted imaging is proposed that compensates for magnetization transfer (MT) effects without sacrificing tag efficiency, and balances transient magnetic field effects (eddy currents) induced by pulsed field gradients. Improved compensation for MT is demonstrated using a phantom. Improvement in perfusion measurement was compared to other PASL techniques by acquiring perfusion images from 13 healthy volunteers (nine women and four men; age range 29-64 years; mean age 45 +/- 14 years) and second-order image texture analysis. The main improvements with the new method were significantly higher image contrast, higher mean signal intensity, and better signal uniformity across slices. In conclusion, this new PASL method should provide improved accuracy in measuring brain perfusion.

TurboFLASH FAIR imaging with optimized inversion and imaging profiles

Optimal implementation of pulsed arterial spin labeling (PASL) methods such as flow-sensitive alternating inversion recovery (FAIR), require the minimization of interactions between the inversion and imaging slabs. For FAIR, the inversion:imaging slice thickness ratio (STR) is usually at least 3:1 in order to fully contain the extent of the imaging slice. The resulting gap exacerbates the transit time. So far, efforts to minimize the STR have concentrated on the inversion profile. However, the imaging profile remains a limiting factor especially for rapid sequences such as turbo fast low-angle shot (TurboFLASH) which uses short pulses. This study reports the implementation of a TurboFLASH sequence with optimized inversion and imaging profiles. Slice-selection is achieved with a preparation module incorporating a pair of identical adiabatic frequency offset corrected inversion (FOCI) pulses. The optimum radiofrequency (RF) and gradient scheme for this pulse combination is described, and the relaxation characteristics of the slice-selection scheme are investigated. Phantom experiments demonstrate a reduction in the STR to approximately 1.13:1. Implementation in an animal model is described, and the benefit of the improved profile in probing the sensitivity of the flow signal to tagging geometry is demonstrated. Sensitivity to transit time effects can be minimized with this sequence, and ASL methodologies can be better explored as a result of the improved conformance with the ideal of square slice profiles.

FMRI for Monitoring Dynamic Changes in Tissue Oxygenation/Blood Flow: Potential Applications for Tumor Response to Carbogen Treatment

The ability to differentiate between well-oxygenated and poorly-oxygenated tumors may play an important role in selecting an optimal therapeutic regime for tumor treatment of the individual patient. We present preliminary results in the development of a dynamic functional MRI method for mapping tissue oxygenation and blood flow distribution in humans simultaneously. We applied interleaved Blood Oxygenation Level Dependent (BOLD) and Flow-sensitive Alternating Inversion Recovery (FAIR) sequences to detect signals as a subject is inspiring gases of varying oxygen concentration. The method allows quantitation of the spatial distribution and time course of the important physiological functions that are easily registered with high resolution anatomic MR images. It may be used to critically evaluate the efficacy of varying durations of carbogen breathing in tumor patients, and allow a quantitative evaluation of the roles of carbogen and other radiosensitizers as potential adjuncts to radiotherapy and drug therapies.

FAIR and dynamic susceptibility contrast-enhanced perfusion imaging in healthy subjects and stroke patients

PURPOSE

To compare dynamic susceptibility contrast-enhanced magnetic resonance imaging (DSC-MRI) and the flow-sensitive alternating inversion recovery (FAIR) technique for measuring brain perfusion.

MATERIALS AND METHODS

We investigated 12 patients with acute stroke, and 10 healthy volunteers with FAIR and DSC maps of regional cerebral blood volume (rCBV), mean transit time (MTT), and regional cerebral blood flow (rCBF).

RESULTS

In volunteers good gray/white-matter contrast was observed in FAIR, rCBF, and rCBV maps. Regions with high signal intensities in FAIR matched well with high values of rCBV and rCBF. In ischemic stroke patients a high correlation (r = 0.78) of the ipsi- to contralateral signal intensity ratios in FAIR and rCBF was observed in areas with perfusion abnormalities. In contrast, FAIR and rCBV (r = 0.50), and FAIR and MTT (r = -0.22) correlated only modestly. Furthermore, FAIR and rCBF demonstrated similar sizes of perfusion abnormality.

CONCLUSION

This study demonstrates for the first time that FAIR and rCBF depict similar relations of perfusion in ischemic stroke patients and healthy subjects.

Comparison of quantitative perfusion imaging using arterial spin labeling at 1.5 and 4.0 Tesla

High-field arterial spin labeling (ASL) perfusion MRI is appealing because it provides not only increased signal-to-noise ratio (SNR), but also advantages in terms of labeling due to the increased relaxation time T(1) of labeled blood. In the present study, we provide a theoretical framework for the dependence of the ASL signal on the static field strength, followed by experimental validation in which a multislice pulsed ASL (PASL) technique was carried out at 4T and compared with PASL and continuous ASL (CASL) techniques at 1.5T, both in the resting state and during motor activation. The resting-state data showed an SNR ratio of 2.3:1.4:1 in the gray matter and a contrast-to-noise ratio (CNR) of 2.7:1.1:1 between the gray and white matter for the difference perfusion images acquired using 4T PASL, 1.5T CASL, and 1.5T PASL, respectively. However, the functional data acquired using 4T PASL did not show significantly improved sensitivity to motor cortex activation compared with the 1.5T functional data, with reduced fractional perfusion signal change and increased intersubject variability. Possible reasons for these experimental results, including susceptibility effects and physiological noise, are discussed.

Simultaneous perfusion and BOLD imaging using reverse spiral scanning at 3T: characterization of functional contrast and susceptibility artifacts

Reverse spiral scanning with arterial spin-labeling was developed at 3T to simultaneously detect perfusion and BOLD signals in the brain by subtracting or adding the control and labeled images, respectively, in the same dataset. BOLD contrast was improved with the longer effective echo time achieved in the reverse spiral scan compared to conventional forward spiral scans. Susceptibility artifacts near air-tissue interfaces in the brain were substantially reduced in the reverse spiral images due to their early data acquisition time and, hence, less signal attenuation. Brain activation experiments with the reverse spiral scan were performed on normal subjects and were compared to forward spiral imaging in the same subjects. The experiments demonstrated that reverse spiral imaging was able to detect perfusion and BOLD signals simultaneously and reliably, even in the brain regions with severe susceptibility-induced local gradients, while forward spiral scans were either not optimal for detecting the two functional signals at the same time or were vulnerable to susceptibility artifacts.

Test-retest reproducibility of quantitative CBF measurements using FAIR perfusion MRI and acetazolamide challenge

The reproducibility of quantitative cerebral blood flow (CBF) measurements using MRI with arterial spin labeling and acetazolamide challenge was assessed in 12 normal subjects, each undergoing the identical experimental procedure on two separate days. CBF was measured on a 1.5T scanner using a flow-sensitive alternating inversion recovery (FAIR) pulse sequence, performed both at baseline and 12 min after intravenous administration of acetazolamide. T(1) was measured in conjunction with the FAIR scan in order to calculate quantitative CBF. The CBF maps were segmented to separate gray matter (GM) from white matter (WM) for region-of-interest (ROI) analyses. Post- acetazolamide CBF values (ml/100 g/min, mean +/- SD) of 87.5 +/- 12.5 (GM) and 46.1 +/- 10.8 (WM) represented percent increases of 37.7% +/- 24.4% (GM) and 40.1% +/- 24.4% (WM). Day-to-day differences in baseline CBF were -1.7 +/- 6.9 (GM) and -1.4 +/- 4.7 (WM) or, relative to the mean CBF over both days for each subject, -2.5% +/- 11.7% (GM) and -3.8% +/- 13.6% (WM) Day- to-day differences in absolute post-ACZ CBF increase were -2.5 +/- 6.8 (GM) and 2.7 +/- 9.4 (WM) or, relative to the mean CBF increase over both days for each subject, -4.7% +/- 13.3% (GM) and 9.1% +/- 26.2% (WM). Thus, FAIR- based CBF measurements show satisfactory reproducibility from day to day, but with sufficient variation to warrant caution in interpreting longitudinal data. The hemispheric asymmetry of baseline CBF and post-acetazolamide CBF increases varied within a narrower range and should be sensitive to small changes related to disease or treatment.

Pulsed arterial spin labeling: comparison of multisection baseline and functional MR imaging perfusion signal at 1.5 and 3.0 T: initial results in six subjects

Flow-alternating inversion-recovery magnetic resonance imaging was performed at 3.0 T to measure cerebral perfusion during rest and motor activation in six healthy adult volunteers. Results were compared with those at 1.5 T. The mean signal-to-noise ratio for both gray matter and white matter perfusion measured with and without vascular suppression at 3.0 T was significantly (P <.01) higher (n = 6) than that at 1.5 T. Brain perfusion activation maps collected during a motor task showed a substantially larger number of activated pixels (>80%) at 3.0 T, with activation in the supplementary motor area in the 3.0-T data that was not present on 1.5-T perfusion maps.

Changes of cerebral blood flow, oxygenation, and oxidative metabolism during graded motor activation

In the present studies fMRI and a hypercapnic calibration procedure were used to monitor simultaneous changes in cerebral blood flow (CBF), cerebral blood oxygenation, and cerebral metabolic rate of oxygen (CMRO(2)) during activation in the sensorimotor cortex. In the first set of experiments seven volunteers performed bilateral, self-paced finger tapping and in the second set of experiments six volunteers performed bilateral finger tapping with six different frequencies (0.5-3 Hz). During the latter task relative CBF and BOLD signal intensity changes varied linearly as a function of stimulus frequency. In good agreement with recent PET and fMRI data increases in CMRO(2) were smaller than the corresponding changes in CBF during self-paced finger tapping and at all levels of graded motor activation. At a single level of activation and during graded activation there was a positive linear relationship between CBF and CMRO(2) with ratios of approximately 3:1. Comparable proportionality constants have been found in the visual cortex and primary sensory cortex, indicating similarities between the relationship of CBF and CMRO(2) in various cortical regions.

FAIR exempting separate T (1) measurement (FAIREST): a novel technique for online quantitative perfusion imaging and multi-contrast fMRI

A new pulse sequence, dubbed FAIR exempting separate T(1) measurement (FAIREST) in which a slice-selective saturation recovery acquisition is added in addition to the standard FAIR (flow-sensitive alternating inversion recovery) scheme, was developed for quantitative perfusion imaging and multi-contrast fMRI. The technique allows for clean separation between and thus simultaneous assessment of BOLD and perfusion effects, whereas quantitative cerebral blood flow (CBF) and tissue T(1) values are monitored online. Online CBF maps were obtained using the FAIREST technique and the measured CBF values were consistent with the off-line CBF maps obtained from using the FAIR technique in combination with a separate sequence for T(1) measurement. Finger tapping activation studies were carried out to demonstrate the applicability of the FAIREST technique in a typical fMRI setting for multi-contrast fMRI. The relative CBF and BOLD changes induced by finger-tapping were 75.1 +/- 18.3 and 1.8 +/- 0.4%, respectively, and the relative oxygen consumption rate change was 2.5 +/- 7.7%. The results from correlation of the T(1) maps with the activation images on a pixel-by-pixel basis show that the mean T(1) value of the CBF activation pixels is close to the T(1) of gray matter while the mean T(1) value of the BOLD activation pixels is close to the T(1) range of blood and cerebrospinal fluid.

Determination of skeletal muscle perfusion using arterial spin labeling NMRI: validation by comparison with venous occlusion plethysmography

T(1)-based determination of perfusion was performed with the high temporal and spatial resolution that monitoring of exercise physiology requires. As no data were available on the validation of this approach in human muscles, T(1)-based NMRI of perfusion was compared to standard strain-gauge venous occlusion plethysmography performed simultaneously within a 4 T magnet. Two different situations were investigated in 21 healthy young volunteers: 1) a 5-min ischemia of the leg, or 2) a 2-3 min ischemic exercise consisting of a plantar flexion on an amagnetic ergometer. Leg perfusion was monitored over 5-15 min of the recovery phase, after the air-cuff arterial occlusion had been released. The interesting features of the sequence were the use of a saturation-recovery module for the introduction of a T(1) modulation and of single-shot spin echo for imaging. Spatial resolution was 1.7 x 2.0 mm and temporal resolution was 2 s. For data analysis, ROIs were traced on different muscles and perfusion was calculated from the differences in muscle signal intensity in successive images. To allow comparison with the global measurement of perfusion by plethysmography, the T(1)-based NMR measurements in exercising muscles were rescaled to the leg cross-section. The perfusion measurements obtained by plethysmography and NMRI were in close agreement with a correlation coefficient between 0.87 and 0.92. This indicates that pulsed arterial techniques provide determination of muscle perfusion not only with superior spatial and temporal resolution but also with exactitude.

Theoretical analysis of the effect of imperfect slice profiles on tagging schemes for pulsed arterial spin labeling MRI

Pulsed arterial spin labeling (ASL) techniques provide a noninvasive method of obtaining qualitative and quantitative perfusion images with MRI. ASL techniques employ inversion recovery and/or saturation recovery to induce perfusion weighting, and thus the performance of these techniques is dependent on the slice profiles of the inversion or saturation pulses. This article systematically examines through simulations the effects of slice profile imperfections on the perfusion signal for nine labeling schemes, including FAIR, FAIRER, and EST (UNFAIR). Each sequence is evaluated for quantitative accuracy, suppression of stationary signal, and magnitude of perfusion signal. Perfusion effects are modeled from a modified Bloch equation and experimentally determined slice profiles. The results show that FAIR, FAIRER, and EST have excellent tissue suppression. The magnitude of the perfusion signal is comparable for FAIR and FAIRER, with EST providing a slightly weaker signal. For quantitative measurements, all three methods underestimate the perfusion signal by more than 20%. Of the additional six ASL techniques examined, only one performed well in this model. This method, which combines inversion and saturation recovery, yields improved signal accuracy (<15% difference from the theoretical value) and tissue suppression similar to that of FAIR and its variants, but has only half the signal. Magn Reson Med 46:141-148, 2001.

Perfusion imaging using spin-labeling methods: contrast-to-noise comparison in functional MRI applications

In this study the performance of FLASH imaging with selective inversion preparation for functional perfusion studies was investigated. In addition to the absolute quantification of perfusion by measurement of the longitudinal relaxation times with global (T(1glob)) and selective (T(1sel)) inversion, the measurement of absolute (BASE) and relative (FAIR) perfusion increases by subtraction of appropriately weighted images was also considered. The subject averages of absolute perfusion obtained by the quantitative method were 70.7 +/- 4.0 ml/100g/min in gray matter, 10.2 +/- 3.4 ml/100g/min in white matter, and 89.0 +/- 3.1 ml/100g/min in visual cortex. These values, as well as the average increase of perfusion due to visual stimulation (44.4 +/- 3.7 ml/100g/min), agree well with respective data reported by PET and other MRI studies. However, for individual subjects the standard deviations over single ROIs inside the visual cortex lay around 100% which prevented the detection of significant activation. BASE and FAIR, on the other hand, were able to detect significant activation in single subjects. The measured average perfusion increases were 51.7 +/- 6.6 ml/100g/min and 56.5 +/- 13.8%, respectively. Magn Reson Med 46:172-182, 2001.

Separation and quantification of perfusion and BOLD effects by simultaneous acquisition of functional I(0)- and T2(*)-parameter maps

The nature of the coupling between neuronal activity and the hemodynamic response is the subject of intensive research. As a means to simultaneously measure parametric changes of T2(*), initial intensity (I(0)) and perfusion with high temporal resolution, a multi-image EPI technique with slice-selective inversion recovery (ssIR) for arterial spin labeling was developed and implemented. Comparative measurements with and without the preceding slice-selective inversion pulse were performed. I(0) and R2(*) changes induced by primary visual stimulation were separated. For ssIR-multi-image EPI the average change of I(0) over all 12 subjects was 3.4%, corresponding to a perfusion change of 40 ml/min/100 g, whereas only minor I(0) changes were observed without inversion. On average, the R2(*) of the activated pixels changed by -0.62 sec(-1) without inversion, while a significantly reduced average R2(*) change of -0.46 sec(-1) was calculated for ssIR-multi-image EPI due to a decreased BOLD effect contribution of the intravascular compartment.

Methodology of brain perfusion imaging

Numerous techniques have been proposed in the last 15 years to measure various perfusion-related parameters in the brain. In particular, two approaches have proven extremely successful: injection of paramagnetic contrast agents for measuring cerebral blood volumes (CBV) and arterial spin labeling (ASL) for measuring cerebral blood flows (CBF). This review presents the methodology of the different magnetic resonance imaging (MRI) techniques in use for CBV and CBF measurements and briefly discusses their limitations and potentials.

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.

Assessment of cerebrovascular reactivity with functional magnetic resonance imaging: comparison of CO(2) and breath holding

Cerebral blood flow (CBF) and oxygenation changes following both a simple breath holding test (BHT) and a CO(2) challenge can be detected with functional magnetic resonance imaging techniques. The BHT has the advantage of not requiring a source of CO(2) and acetazolamide and therefore it can easily be performed during a routine MR examination. In this study we compared global hemodynamic changes induced by breath holding and CO(2) inhalation with blood oxygenation level dependent (BOLD) and CBF sensitized fMRI techniques. During each vascular challenge BOLD and CBF signals were determined simultaneously with a combined BOLD and flow-sensitive alternating inversion recovery (FAIR) pulse sequence. There was a good correlation between the global BOLD signal intensity changes during breath holding and CO(2) inhalation supporting the notion that the BHT is equivalent to CO(2) inhalation in evaluating the hemodynamic reserve capacity with BOLD fMRI. In contrast, there was no correlation between relative CBF changes during both vascular challenges, which was probably due to the reduced temporal resolution of the combined BOLD and FAIR pulse sequence.