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

The effects of multi-echo fMRI combination and rapid T2*-mapping on offline and real-time BOLD sensitivity

A variety of strategies are used to combine multi-echo functional magnetic resonance imaging (fMRI) data, yet recent literature lacks a systematic comparison of the available options. Here we compare six different approaches derived from multi-echo data and evaluate their influences on BOLD sensitivity for offline and in particular real-time use cases: a single-echo time series (based on Echo 2), the real-time T₂*-mapped time series (T₂*FIT) and four combined time series (T₂*-weighted, tSNR-weighted, TE-weighted, and a new combination scheme termed T₂*FIT-weighted). We compare the influences of these six multi-echo derived time series on BOLD sensitivity using a healthy participant dataset (N = 28) with four task-based fMRI runs and two resting state runs. We show that the T₂*FIT-weighted combination yields the largest increase in temporal signal-to-noise ratio across task and resting state runs. We demonstrate additionally for all tasks that the T₂*FIT time series consistently yields the largest offline effect size measures and real-time region-of-interest based functional contrasts and temporal contrast-to-noise ratios. These improvements show the promising utility of multi-echo fMRI for studies employing real-time paradigms, while further work is advised to mitigate the decreased tSNR of the T₂*FIT time series. We recommend the use and continued exploration of T₂*FIT for offline task-based and real-time region-based fMRI analysis. Supporting information includes: a data repository (https://dataverse.nl/dataverse/rt-me-fmri), an interactive web-based application to explore the data (https://rt-me-fmri.herokuapp.com/), and further materials and code for reproducibility (https://github.com/jsheunis/rt-me-fMRI).

Simultaneous Measurement of Perfusion and T2* in Calf Muscle at 7T with Submaximal Exercise using Radial Acquisition

Impairments in oxygen delivery and consumption can lead to reduced muscle endurance and physical disability. Perfusion, a measure of microvascular blood flow, provides information on nutrient delivery. T₂* provides information about relative tissue oxygenation. Changes in these parameters following stress, such as exercise, can yield important information about imbalance between delivery and consumption. In this study, we implemented novel golden angle radial MRI acquisition technique to simultaneously quantify muscle perfusion and T₂* at 7T with improved temporal resolution, and demonstrated assessment of spatial and temporal changes in these parameters within calf muscles during recovery from plantar flexion exercise. Nine healthy subjects participated the studies. At rest, perfusion and T₂* in gastrocnemius muscle group within calf muscle were 5 ± 2 mL/100 g/min and 21.1 ± 3 ms respectively. Then the subjects performed plantar flexion exercise producing a torque of ~8ft-lb. Immediately after the exercise, perfusion was elevated to 79.3 ± 9 mL/100 g/min and T₂* was decreased by 6 ± 3%. The time constants for 50% perfusion and T₂* recovery were 54.1 ± 10 s and 68.5 ± 7 s respectively. These results demonstrate successful simultaneous quantification of perfusion and T₂* in skeletal muscle using the developed technique.

Impaired skeletal muscle microcirculation in systemic sclerosis

Introduction

Muscle symptoms in systemic sclerosis (SSc) may originate from altered skeletal muscle microcirculation, which can be investigated by means of blood oxygenation level dependent (BOLD) magnetic resonance imaging (MRI).

Methods

After ethics committee approval and written consent, 11 consecutive SSc patients (5 men, mean age 52.6 years, mean SSc disease duration 5.4 years) and 12 healthy volunteers (4 men, mean age 45.1 years) were included. Subjects with peripheral arterial occlusive disease were excluded. BOLD MRI was performed on calf muscles during cuff-induced ischemia and reactive hyperemia, using a 3-T whole-body scanner (Verio, Siemens, Erlangen, Germany) and fat-suppressed single-short multi-echo echo planar imaging (EPI) with four different effective echo times. Muscle BOLD signal time courses were obtained for gastrocnemius and soleus muscles: minimal hemoglobin oxygen saturation (T2*_min) and maximal T2* values (T2*_max), time to T2* peak (TTP), and slopes of oxygen normalization after T2* peaking.

Results

The vast majority of SSc patients lacked skeletal muscle atrophy, weakness or serum creatine kinase elevation. Nevertheless, more intense oxygen desaturation during ischemia was observed in calf muscles of SSc patients (mean T2*_min -15.0%), compared with controls (-9.1%, P = 0.02). SSc patients also had impaired oxygenation during hyperemia (median T2*_max 9.2% vs. 20.1%, respectively, P = 0.007). The slope of muscle oxygen normalization was significantly less steep and prolonged (TTP) in SSc patients (P<0.001 for both). Similar differences were found at a separate analysis of gastrocnemius and soleus muscles, with most pronounced impairment in the gastrocnemius.

Conclusions

BOLD MRI demonstrates a significant impairment of skeletal muscle microcirculation in SSc.

Skeletal muscle BOLD MRI: from underlying physiological concepts to its usefulness in clinical conditions

Blood oxygenation-level dependent (BOLD) MRI has gained particular attention in functional brain imaging studies, where it can be used to localize areas of brain activation with high temporal resolution. To a higher degree than in the brain, skeletal muscles show extensive but transient alterations of blood flow between resting and activation state. Thus, there has been interest in the application of the BOLD effect in studying the physiology of skeletal muscles (healthy and diseased) and its possible application to clinical practice. This review outlines the potential of skeletal muscle BOLD MRI as a diagnostic tool for the evaluation of physiological and pathological alterations in the peripheral limb perfusion, such as in peripheral arterial occlusive disease. Moreover, current knowledge is summarized regarding the complex mechanisms eliciting BOLD effect in skeletal muscle. We describe technical fundaments of the procedure that should be taken into account when performing skeletal muscle BOLD MRI, including the most often applied paradigms to provoke BOLD signal changes and key parameters of the resulting time courses. Possible confounding effects in muscle BOLD imaging studies, like age, muscle fiber type, training state, and drug effects are also reviewed in detail.

Clinical implications of skeletal muscle blood-oxygenation-level-dependent (BOLD) MRI

Blood-oxygenation-level-dependent (BOLD) contrast in magnetic resonance (MR) imaging of skeletal muscle mainly depends on changes of oxygen saturation in the microcirculation. In recent years, an increasing number of studies have evaluated the clinical relevance of skeletal muscle BOLD MR imaging in vascular diseases, such as peripheral arterial occlusive disease, diabetes mellitus, and chronic compartment syndrome. BOLD imaging combines the advantages of MR imaging, i.e., high spatial resolution, no exposure to ionizing radiation, with functional information of local microvascular perfusion. Due to intrinsic contrast provoked via changes in hemoglobin oxygen saturation, it is a safe and easy applicable procedure on standard whole-body MR devices. Therefore, BOLD MR imaging of skeletal muscle is a potential new diagnostic tool in the clinical evaluation of vascular, inflammatory, and muscular pathologies. Our review focuses on the current evidence concerning the use of BOLD MR imaging of skeletal muscle under pathological conditions and highlights ways for future clinical and scientific applications.

Blood oxygenation level-dependent (BOLD) MRI of human skeletal muscle at 1.5 and 3 T

PURPOSE

To evaluate the dependence of skeletal muscle blood oxygenation level-dependent (BOLD) effect and time course characteristics on magnetic field strength in healthy volunteers using an ischemia/reactive hyperemia paradigm.

MATERIALS AND METHODS

Two consecutive skeletal muscle BOLD magnetic resonance imaging (MRI) measurements in eight healthy volunteers were performed on 1.5 T and 3.0 T whole-body MRI scanners. For both measurements a fat-saturated multi-shot multiecho gradient-echo EPI sequence was applied. Temporary vascular occlusion was induced by suprasystolic cuff compression of the thigh. T2 time courses were obtained from two different calf muscles and characterized by typical curve parameters. Ischemia- and hyperemia-induced changes in R2 (ΔR2) were calculated for both muscles in each volunteer at the two field strengths.

RESULTS

Skeletal muscle BOLD changes are dependent on magnetic field strength as the ratio ΔR2(3.0 T)/ΔR2(1.5 T) was found to range between 1.6 and 2.2. Regarding time course characteristics, significantly higher relative T2 changes were found in both muscles at 3.0 T.

CONCLUSION

The present study shows an approximately linear field strength dependence of ΔR2 in the skeletal muscle in response to ischemia and reactive hyperemia. Using higher magnetic fields is advisable for future BOLD imaging studies of peripheral limb pathologies.

fMRI of the brainstem using dual-echo EPI

The brainstem is the part of the human brain that plays a pivotal role in the maintenance of many critical body functions. Due to the elevated level of cardiogenic noise, few fMRI studies have investigated the brainstem so far. Cardiac-gated echo-planar imaging with acquisition of two echoes per excitation (dual-echo EPI) is one method that significantly reduces cardiogenic noise and, thus, allows for fMRI measurements of the brainstem. As information on optimal preprocessing approaches for brainstem-fMRI data is still scarce, the goal of this study was to compare different combinations of normalization and smoothing procedures as implemented in standard fMRI software packages and to identify the combinations yielding optimal results for dual-echo EPI. 21 healthy subjects were measured while executing a simple motor paradigm to activate the facial and trigeminal motor nucleus in the brainstem. After motion correction and calculation of T(2)*-maps the data were preprocessed with 24 combinations of standard normalization (SPM classic, SPM unified, FSL, ABC) and smoothing procedures (pre-/post-smoothing with 3mm-, 4.5mm- and 6mm-kernel) before undergoing first- and second-level statistical analysis. Activation results were compared for first-level and second-level statistics using two anatomically defined regions of interest. Five methods were found to be sensitive for activation of both nuclei. These included FSL normalization with 3mm and 4.5mm pre-smoothing as well as 3mm post-smoothing, SPM unified normalization with 3mm pre-smoothing and ABC normalization with 4.5mm pre-smoothing. All these methods can be recommended for normalization and smoothing when analyzing fMRI data of the brainstem acquired by cardiac-gated dual-echo EPI.

Calf muscles at blood oxygen level-dependent MR imaging: aging effects at postocclusive reactive hyperemia

PURPOSE

To prospectively investigate age-related changes in muscle reperfusion by using blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging of the calf in young and elderly healthy volunteers during postocclusive reactive hyperemia.

MATERIALS AND METHODS

Institutional review board approval and informed consent were obtained. Eleven healthy elderly (mean age, 64.0 years +/- 6.4 [standard deviation]; six men, five women) and 17 healthy young volunteers (mean age, 30.3 years +/- 6.5; seven men, 10 women) underwent muscle BOLD MR imaging of the calf. A fat-suppressed T2*-weighted single-shot multiecho echo-planar imaging sequence was used. Temporary vascular occlusion was induced with suprasystolic cuff compression of the thigh. T2* time courses of the muscle BOLD MR signal intensity were obtained from four calf muscles and were characterized by the following curve parameters: hyperemia peak value, time to peak, and T2* end value after 360 seconds of hyperemia. Differences in these parameters between the two cohorts were assessed by using a Student t test.

RESULTS

Considerably lower T2* maxima were observed in the elderly group during hyperemia (P < .005), with a mean hyperemia peak value of 13.1% +/- 3.0 compared with 18.9% +/- 4.8 in young healthy adults. Peaking occurred earlier in the elderly group (P < .05), with a mean time to peak of 32.2 seconds +/- 10.6 compared with 43.1 seconds +/- 10.7 in young adults. Furthermore, the elderly group had a significantly slower decrease of the muscle BOLD signal after the hyperemia peak (P < .001), which led to a higher end value of 8.6% +/- 3.0 compared with 2.6% +/- 2.1 in the young group.

CONCLUSION

BOLD MR imaging results of the calf demonstrated statistically significant age-dependent differences in the rate, intensity, and recovery of the postocclusive muscle BOLD signal.

BOLD functional MRI in disease and pharmacological studies: room for improvement?

In the past decade the use of blood oxygen level-dependent (BOLD) fMRI to investigate the effect of diseases and pharmacological agents on brain activity has increased greatly. BOLD fMRI does not measure neural activity directly, but relies on a cascade of physiological events linking neural activity to the generation of MRI signal. However, most of the disease and pharmacological studies performed so far have interpreted changes in BOLD fMRI as "brain activation," ignoring the potential confounds that can arise through drug- or disease-induced modulation of events downstream of the neural activity. This issue is especially serious in diseases (like multiple sclerosis, brain tumours and stroke) and drugs (like anaesthetics or those with a vascular action) that are known to influence these physiological events. Here we provide evidence that, to extract meaningful information on brain activity in patient and pharmacological BOLD fMRI studies, it is important to identify, characterise and possibly correct these influences that potentially confound the results. We suggest a series of experimental measures to improve the interpretability of BOLD fMRI studies. We have ranked these according to their potential information and current practical feasibility. First-line, necessary improvements consist of (1) the inclusion of one or more control tasks, and (2) the recording of physiological parameters during scanning and subsequent correction of possible between-group differences. Second-line, highly recommended important aim to make the results of a patient or drug BOLD study more interpretable and include the assessment of (1) baseline brain perfusion, (2) vascular reactivity, (3) the inclusion of stimulus-related perfusion fMRI and (4) the recording of electrophysiological responses to the stimulus of interest. Finally, third-line, desirable improvements consist of the inclusion of (1) simultaneous EEG-fMRI, (2) cerebral blood volume and (3) rate of metabolic oxygen consumption measurements and, when relevant, (4) animal studies investigating signalling between neural cells and blood vessels.

Theoretical optimization of multi-echo fMRI data acquisition

Simulations are used to optimize multi-echo fMRI data acquisition for detection of BOLD signal changes in this study. Optimal sequence design (echo times and sampling period (receiver bandwidth)) and the variation in sensitivity between tissues with different baseline T*(2) are investigated, taking into account the effects of physiological noise and non-exponential signal decay. In the case of a single echo, for normally distributed, uncorrelated noise, the results indicate that the sampling period should be made as long as possible (so as to produce an acceptable level of image distortion), up to a maximum sampling period of 3T*(2), (i.e. optimum TE = 1.5T*(2)). Combining the signal from multiple echoes using weighted summation improves the contrast-to-noise ratio (CNR), at a reduced optimum echo interval. If the BOLD effect causes a constant change in relaxation rate, DeltaR*(2), independent of the tissue R*(2), then a multi-echo acquisition causes considerable variation in sensitivity to BOLD signal changes with tissue T*(2), so that if the sequence is optimized for a target tissue T*(2) it will be more sensitive to BOLD signal changes in tissues with shorter T*(2) values. Fitting for DeltaR*(2) reduces the CNR, and when using this approach, the shortest echo time interval should be used, down to a limit of about 0.3T*(2), and as many echoes as possible within the constraints of TR or hardware limitations should be collected. It is also shown that the optimal sequence will remain optimum or close to optimum irrespective of whether there are physiological noise contributions.

Bayesian inference of hemodynamic changes in functional arterial spin labeling data

The study of brain function using MRI relies on acquisition techniques that are sensitive to different aspects of the hemodynamic response contiguous to areas of neuronal activity. For this purpose different contrasts such as arterial spin labeling (ASL) and blood oxygenation level dependent (BOLD) functional MRI techniques have been developed to investigate cerebral blood flow (CBF) and blood oxygenation, respectively. Analysis of such data typically proceeds by separate, linear modeling of the appropriate CBF or BOLD time courses. In this work an approach is developed that provides simultaneous inference on hemodynamic changes via a nonlinear physiological model of ASL data acquired at multiple echo times. Importantly, this includes a significant contribution by changes in the static magnetization, M, to the ASL signal. Inference is carried out in a Bayesian framework. This is able to extract, from dual-echo ASL data, probabilistic estimates of percentage changes of CBF, R(2) (*), and the static magnetization, M. This approach provides increased sensitivity in inferring CBF changes and reduced contamination in inferring BOLD changes when compared with general linear model approaches on single-echo ASL data. We also consider how the static magnetization, M, might be related to changes in CBV by assuming the same mechanism for water exchange as in vascular space occupancy.

Blood Oxygenation Level–Dependent Magnetic Resonance Imaging of the Skeletal Muscle in Patients With Peripheral Arterial Occlusive Disease

Background— Blood oxygenation level–dependent (BOLD) magnetic resonance imaging (MRI) has been used to measure T2* changes in skeletal muscle tissue of healthy volunteers. The BOLD effect is assumed to primarily reflect changes in blood oxygenation at the tissue level. We compared the calf muscle BOLD response of patients with peripheral arterial occlusive disease (PAOD) to that of an age-matched non-PAOD group during postischemic reactive hyperemia.

Methods and Results— PAOD patients (n=17) with symptoms of intermittent calf claudication and an age-matched non-PAOD group (n=11) underwent T2*-weighted single-shot multiecho planar imaging on a whole-body magnetic resonance scanner at 1.5 T. Muscle BOLD MRI of the calf was performed during reactive hyperemia provoked by a cuff-compression paradigm. T2* maps were generated with an automated fitting procedure. Maximal T2* change (ΔT2* max ) and time to peak to reach ΔT2* max for gastrocnemius, soleus, tibial anterior, and peroneal muscle were evaluated. Compared with the non-PAOD group, patients revealed significantly lower ΔT2* max -values, with a mean of 7.3±5.3% versus 13.1±5.6% ( P <0.001), and significantly delayed time-to-peak values, with a mean of 109.3±79.3 versus 32.2±13.3 seconds ( P <0.001).

Conclusions— T2* time courses of the muscle BOLD MRI signal during postocclusive reactive hyperemia revealed statistically significant differences in the key parameters (ΔT2* max ; time to peak) in PAOD patients compared with age-matched non-PAOD controls.

Strategies for improving the detection of fMRI activation in trigeminal pathways with cardiac gating

Functional magnetic resonance imaging (fMRI) has become a powerful tool for studying the normal and diseased human brain. The application of fMRI in detecting neuronal signals in the trigeminal system, however, has been hindered by low detection sensitivity due to activation artifacts caused by cardiac pulse-induced brain and brainstem movement. A variety of cardiac gating techniques have been proposed to overcome this issue, typically by phase locking the sampling to a particular time point during each cardiac cycle. We sought to compare different cardiac gating strategies for trigeminal system fMRI. In the present study, we used tactile stimuli to elicit brainstem and thalamus activation and compared the fMRI results obtained without cardiac gating and with three different cardiac gating strategies: single-echo with TR of 3 or 9 heartbeats (HBs) and dual-echo T2*-mapping EPI (TR = 2 HBs, TE = 21/55 ms). The dual-echo T2* mapping and the single-echo with TR of 2 and 3 HBs cardiac-gated fMRI techniques both increased detection rate of fMRI activation in brainstem. Activation in the brainstem and the thalamus was best detected by cardiac-gated dual-echo EPI.

The role of fMRI in drug discovery

Pharmacological functional (phMRI) studies are making a significant contribution to our understanding of drug-effects on brain systems. Pharmacological fMRI has an additional contribution to make in the translation of disease models and candidate compounds from preclinical to clinical investigation and in the early clinical stages of drug development. Here it can demonstrate a proof-of-concept of drug action in a small human cohort and thus contribute substantially to decision-making in drug development. We review the methods underlying pharmacological fMRI studies and the links that can be made between animal and human investigations. We discuss the potential fMRI markers of drug effect, experimental designs and caveats in interpreting hemodynamic fMRI data as reflective of changes in neuronal activity. Although there are no current published examples of fMRI applied to novel compounds, we illustrate the potential of fMRI across a range of applications and with specific reference to processing of pain in the human brain and pharmacological analgesia. Pharmacological fMRI is developing to meet the neuroscientific challenges. Electrophysiological methods can be used to corroborate the drug effects measured hemodynamically with fMRI. In future, pharmacological fMRI is likely to extend to examinations of the spinal cord and into pharmacogenetics to relate genetic polymorphisms to differential responses of the brain to drugs.

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.

EEG-fMRI of focal epileptic spikes: analysis with multiple haemodynamic functions and comparison with gadolinium-enhanced MR angiograms

Combined EEG-fMRI has recently been used to explore the BOLD responses to interictal epileptiform discharges. This study examines whether misspecification of the form of the haemodynamic response function (HRF) results in significant fMRI responses being missed in the statistical analysis. EEG-fMRI data from 31 patients with focal epilepsy were analysed with four HRFs peaking from 3 to 9 sec after each interictal event, in addition to a standard HRF that peaked after 5.4 sec. In four patients, fMRI responses were correlated with gadolinium-enhanced MR angiograms and with EEG data from intracranial electrodes. In an attempt to understand the absence of BOLD responses in a significant group of patients, the degree of signal loss occurring as a result of magnetic field inhomogeneities was compared with the detected fMRI responses in ten patients with temporal lobe spikes. Using multiple HRFs resulted in an increased percentage of data sets with significant fMRI activations, from 45% when using the standard HRF alone, to 62.5%. The standard HRF was good at detecting positive BOLD responses, but less appropriate for negative BOLD responses, the majority of which were more accurately modelled by an HRF that peaked later than the standard. Co-registration of statistical maps with gadolinium-enhanced MRIs suggested that the detected fMRI responses were not in general related to large veins. Signal loss in the temporal lobes seemed to be an important factor in 7 of 12 patients who did not show fMRI activations with any of the HRFs.

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.

Parsing local signal evolution directly from a single-shot MRI signal: a new approach for fMRI

In this work a new single-shot MRI method, single-shot parameter assessment by retrieval from signal encoding (SS-PARSE), is introduced. This method abandons a fundamental simplifying assumption that is used in conventional MRI methods. Established MRI methods implicitly assume that the local intrinsic signal does not change its amplitude or phase during signal acquisition, even though these changes may be substantial, especially during the relatively long signals used in single-shot image acquisitions. SS-PARSE, on the other hand, acknowledges local decay and phase evolution, and models each signal datum as a sample from (k,t)-space rather than k-space. Because of this more accurate signal model, SS-PARSE promises improved performance in terms of accuracy and robustness, but requires more intensive reconstruction computations. The theoretical properties of the method are discussed, and simulation results are presented that demonstrate more robust and accurate measurements of relaxation rate changes associated with brain activation in functional MRI (fMRI), freedom from geometric errors due to off-resonance frequencies, and better tolerance of the large susceptibility gradients that occur naturally in parts of the brain. In addition, this technique has the potential to assess nonexponential relaxation behavior during a single-shot signal.

Functional magnetic resonance imaging: a review of methodological aspects and clinical applications

This paper gives an overview of the recent literature on methodological developments of functional magnetic resonance imaging (fMRI) and recent trends in clinical applications. With the recent introduction of high-field systems and methodological developments leading to more robust signal behavior, fMRI is in a transition state from a research modality for use by experts to a standard procedure with useful applications in patient management. Compared to the use in neuroscientific research, which is often based on BOLD techniques alone, the application in patients is distinguished by a multiparametric characterization of the brain using a combination of several techniques. Neuronal fiber tracking based on diffusion anisotropy measurements, in particular, has already turned out to provide relevant supplementary information to the BOLD-based cortical activation maps.

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.

Comparison of multi-echo spiral and echo planar imaging in functional MRI

Multi-echo spiral and echo-planar (EPI) imaging sequences were compared in functional imaging experiments at 3 Tesla. Both sequence types allow calculation of the effective transversal relaxation time T(2)* and the initial signal intensity I(0). These parameters can be used in evaluation of the functional signal with respect to inflow effects and other vascular sources. Prior to functional magnetic resonance imaging (fMRI) experiments T(2)* measurements in the human brain were performed with single- and multi-echo FLASH (fast low angle shot) and compared with EPI und spiral imaging sequences. These experiments resulted in T(2)* values ranging from 42.9 to 53.8 ms in a ROI including white and gray matter and CSF in a prefrontal brain region, and allowed validation of the quantitative results of the fast single-shot techniques. In functional experiments with motor stimulation mean absolute T(2)* increases during stimulation of 1.1 +/- 0.6 ms and 1.4 +/- 0.9 ms were found with multi-echo EPI and spiral imaging, respectively, averaged over the activated pixels. In addition, absolute T(2)* values and the size of activated areas obtained with both sequences are comparable. In these investigations spiral imaging allowed higher spatial resolution due to more efficient use of available gradient performance.

Motion correction of parametric fMRI data from multi-slice single-shot multi-echo acquisitions

Fast parametric imaging using multi-echo techniques has been proven to yield quantitative parameter maps with high stability for functional MRI (fMRI). Due to the different contrasts and signal-to-noise ratios (SNRs) in the various images, motion correction of the echo images or the resulting parameter maps is not a straightforward process. 3D motion correction of parametric imaging data has not yet been examined thoroughly. However, motion correction is an essential step in fMRI data processing. In this study several possible motion detection methods were tested and compared. Motion parameters can be estimated from the different echo images as well as from the parameter maps. The accuracy of the different methods was examined in simulations and in in vivo experiments. Motion parameters should be estimated from the I(0)-parameter maps and subsequently applied to the T(*)(2)-parameter maps.