Multi-gradient echo with susceptibility inhomogeneity compensation (MGESIC): demonstration of fMRI in the olfactory cortex at 3.0 T

Differential functional change in olfactory bulb and olfactory eloquent areas in Parkinson's disease

Olfactory dysfunction, or hyposmia, frequently occurs as a prodromal symptom and ongoing sign of Parkinson's disease. Functional MRI is a powerful tool for studying functional changes in the olfactory brain regions in patients with Parkinson's disease. However, existing studies show inconsistent results and no study has measured olfactory functional MRI abnormalities in the human olfactory bulb directly. This is mainly due to the well-known susceptibility artefacts in conventional functional MRI images that affect several key olfactory-eloquent brain regions, and especially the olfactory bulb. In this study, olfactory functional MRI was performed using a recently developed functional MRI approach that can minimize susceptibility artefacts and measure robust functional MRI signals in the human olfactory bulb during olfactory stimulation. Experiments were performed on high magnetic field (7 T) in 24 early (<5 years of parkinsonian symptoms) Parkinson's disease patients and 31 matched healthy controls. Our data showed increased functional MRI signal changes (ΔS/S) in the olfactory bulb in patients with early Parkinson's disease, which correlated with behavioural olfactory measures. Temporally, functional MRI signals in the olfactory bulb returned to the pre-stimulus state earlier after reaching peak amplitude in patients with early Parkinson's disease, implicating a faster olfactory habituation effect. The piriform cortex showed reduced numbers of activated voxels in patients with early Parkinson's disease, which correlated with behavioural olfactory assessment. Several secondary olfactory regions including the orbitofrontal cortex, temporal pole and amygdala exhibited reduced numbers of activated voxels and increased functional MRI signal changes in patients with early Parkinson's disease. Our data also showed that functional MRI results are highly dependent on voxel selection in the functional analysis. In summary, we demonstrate differential spatial and temporal characteristics of olfactory functional MRI signals between the primary and secondary olfactory regions in patients with early Parkinson's disease. These results may assist the development of novel quantitative biomarkers (especially in the early stages of Parkinson's disease) to track and predict disease progression, as well as potential treatment targets for early intervention.

Measurement of neuro-energetics and neurotransmission in the rat olfactory bulb using 1H and 1H-[13C] NMR spectroscopy

The olfactory bulb (OB) plays a fundamental role in the sense of smell and has been implicated in several pathologies, including Alzheimer's disease. Despite its importance, high metabolic activity and unique laminar architecture, the OB is not frequently studied using MRS methods, likely due to the small size and challenging location. Here we present a detailed metabolic characterization of OB metabolism, in terms of both static metabolite concentrations using 1H MRS and metabolic fluxes associated with neuro-energetics and neurotransmission by tracing the dynamic 13C flow from intravenously administered [1,6-13C2]-glucose, [2-13C]-glucose and [2-13C]-acetate to downstream metabolites, including [4-13C]-glutamate, [4-13C]-glutamine and [2-13C]-GABA. The unique laminar architecture and associated metabolism of the OB, distinctly different from that of the cerebral cortex, is characterized by elevated GABA and glutamine levels, as well as increased GABAergic and astroglial energy metabolism and neurotransmission. The results show that, despite the technical challenges, high-quality 1H and 1H-[13C] MR spectra can be obtained from the rat OB in vivo. The derived metabolite concentrations and metabolic rates demonstrate a unique metabolic profile for the OB. The metabolic model provides a solid basis for future OB studies on functional activation or pathological conditions.

What Does the Human Olfactory System Do, and How Does It Do It?

Historically, the human sense of smell has been regarded as the odd stepchild of the senses, especially compared to the sensory bravado of seeing, touching, and hearing. The idea that the human olfaction has little to contribute to our experience of the world is commonplace, though with the emergence of COVID-19 there has rather been a sea change in this understanding. An ever increasing body of work has convincingly highlighted the keen capabilities of the human nose and the sophistication of the human olfactory system. Here, we provide a concise overview of the neuroscience of human olfaction spanning the last 10-15 years, with focus on the peripheral and central mechanisms that underlie how odor information is processed, packaged, parceled, predicted, and perturbed to serve odor-guided behaviors. We conclude by offering some guideposts for harnessing the next decade of olfactory research in all its shapes and forms.

Inhalation-modulated detection of olfactory BOLD responses in the human brain

Introduction

In contrast to other sensory domains, detection of primary olfactory processes using functional magnetic resonance imaging has proven to be notably challenging with conventional block designs. This difficulty arises from significant habituation and hemodynamic responses in olfactory areas that do not appear to align with extended boxcar functions convolved with a generic hemodynamic response model. Consequently, some researchers have advocated for a transition to event-related designs, despite their known lower detection power compared to block designs.

Methods

Here, we conducted a block design experiment with 16s of continuous odorant stimulation alternating with 16s of continuous odorless air stimulation in 33 healthy participants. We compared four statistical analyses that relied either on standard block designs (SBD1-2) or on block designs that were modulated by the participants' individual breathing patterns (MBD1-2).

Results

We found that such modulated block designs were comparatively more powerful than standard block designs, despite having a substantially lower design efficiency. Using whole-brain effect size maps, we observed that the right insular and medial aspects of the left piriform cortex exhibited a preference for a breathing-modulated analysis approach.

Discussion

Research in olfaction that necessitates designs with longer-lasting blocks, such as those employed in the investigation of state-dependent processing, will benefit from the breathing-modulated analyses outlined in this study.

Automated slice-specific z-shimming for functional magnetic resonance imaging of the human spinal cord

Functional magnetic resonance imaging (fMRI) of the human spinal cord faces many challenges, such as signal loss due to local magnetic field inhomogeneities. This issue can be addressed with slice-specific z-shimming, which compensates for the dephasing effect of the inhomogeneities using a slice-specific gradient pulse. Here, we aim to address outstanding issues regarding this technique by evaluating its effects on several aspects that are directly relevant for spinal fMRI and by developing two automated procedures in order to improve upon the time-consuming and subjective nature of manual selection of z-shims: one procedure finds the z-shim that maximizes signal intensity in each slice of an EPI reference-scan and the other finds the through-slice field inhomogeneity for each EPI-slice in field map data and calculates the required compensation gradient moment. We demonstrate that the beneficial effects of z-shimming are apparent across different echo times, hold true for both the dorsal and ventral horn, and are also apparent in the temporal signal-to-noise ratio (tSNR) of EPI time-series data. Both of our automated approaches were faster than the manual approach, lead to significant improvements in gray matter tSNR compared to no z-shimming and resulted in beneficial effects that were stable across time. While the field-map-based approach performed slightly worse than the manual approach, the EPI-based approach performed as well as the manual one and was furthermore validated on an external corticospinal data-set (N > 100). Together, automated z-shimming may improve the data quality of future spinal fMRI studies and lead to increased reproducibility in longitudinal studies.

Functional Activities Detected in the Olfactory Bulb and Associated Olfactory Regions in the Human Brain Using T2-Prepared BOLD Functional MRI at 7T

Olfaction is a fundamental sense that plays a vital role in daily life in humans, and can be altered in neuropsychiatric and neurodegenerative diseases. Blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) using conventional echo-planar-imaging (EPI) based sequences can be challenging in brain regions important for olfactory processing, such as the olfactory bulb (OB) and orbitofrontal cortex, mainly due to the signal dropout and distortion artifacts caused by large susceptibility effects from the sinonasal cavity and temporal bone. To date, few studies have demonstrated successful fMRI in the OB in humans. T2-prepared (T2prep) BOLD fMRI is an alternative approach developed especially for performing fMRI in regions affected by large susceptibility artifacts. The purpose of this technical study is to evaluate T2prep BOLD fMRI for olfactory functional experiments in humans. Olfactory fMRI scans were performed on 7T in 14 healthy participants. T2prep BOLD showed greater sensitivity than GRE EPI BOLD in the OB, orbitofrontal cortex and the temporal pole. Functional activation was detected using T2prep BOLD in the OB and associated olfactory regions. Habituation effects and a bi-phasic pattern of fMRI signal changes during olfactory stimulation were observed in all regions. Both positively and negatively activated regions were observed during olfactory stimulation. These signal characteristics are generally consistent with literature and showed a good intra-subject reproducibility comparable to previous human BOLD fMRI studies. In conclusion, the methodology demonstrated in this study holds promise for future olfactory fMRI studies in the OB and other brain regions that suffer from large susceptibility artifacts.

Olfactory fMRI: Implications of Stimulation Length and Repetition Time

Studying olfaction with functional magnetic resonance imaging (fMRI) poses various methodological challenges. This study aimed to investigate the effects of stimulation length and repetition time (TR) on the activation pattern of 4 olfactory brain regions: the anterior and the posterior piriform cortex, the orbitofrontal cortex, and the insula. Twenty-two healthy participants with normal olfaction were examined with fMRI, with 2 stimulation lengths (6 s and 15 s) and 2 TRs (0.901 s and 1.34 s). Data were analyzed using General Linear Model (GLM), Tensorial Independent Component Analysis (TICA), and by plotting the event-related time course of brain activation in the 4 olfactory regions of interest. The statistical analysis of the time courses revealed that short TR was associated with more pronounced signal increase and short stimulation was associated with shorter time to peak signal. Additionally, both long stimulation and short TR were associated with oscillatory time courses, whereas both short stimulation and short TR resulted in more typical time courses. GLM analysis showed that the combination of short stimulation and short TR could result in visually larger activation within these olfactory areas. TICA validated that the tested paradigm was spatially and temporally associated with a functionally connected network that included all 4 olfactory regions. In conclusion, the combination of short stimulation and short TR is associated with higher signal increase and shorter time to peak, making it more amenable to standard GLM-type analyses than long stimulation and long TR, and it should, thus, be preferable for olfactory fMRI.

Imaging Cerebral Activity in Amyotrophic Lateral Sclerosis

Advances in neuroimaging, complementing histopathological insights, have established a multi-system involvement of cerebral networks beyond the traditional neuromuscular pathological view of amyotrophic lateral sclerosis (ALS). The development of effective disease-modifying therapy remains a priority and this will be facilitated by improved biomarkers of motor system integrity against which to assess the efficacy of candidate drugs. Functional MRI (FMRI) is an established measure of both cerebral activity and connectivity, but there is an increasing recognition of neuronal oscillations in facilitating long-distance communication across the cortical surface. Such dynamic synchronization vastly expands the connectivity foundations defined by traditional neuronal architecture. This review considers the unique pathogenic insights afforded by the capture of cerebral disease activity in ALS using FMRI and encephalography.

Language Mapping Using T2-Prepared BOLD Functional MRI in the Presence of Large Susceptibility Artifacts-Initial Results in Patients With Brain Tumor and Epilepsy

At present, presurgical functional mapping is the most prevalent clinical application of functional magnetic resonance imaging (fMRI). Signal dropouts and distortions caused by susceptibility effects in the current standard echo planar imaging (EPI)-based fMRI images are well-known problems and pose a major hurdle for the application of fMRI in several brain regions, many of which are related to language mapping in presurgical planning. Such artifacts are particularly problematic in patients with previous surgical resection cavities, craniotomy hardware, hemorrhage, and vascular malformation. A recently developed T2-prepared (T2prep) fMRI approach showed negligible distortion and dropouts in the entire brain even in the presence of large susceptibility effects. Here, we present initial results comparing T2prep- and multiband EPI-fMRI scans for presurgical language mapping using a sentence completion task in patients with brain tumor and epilepsy. In all patients scanned, T2prep-fMRI showed minimal image artifacts (distortion and dropout) and greater functional sensitivity than EPI-fMRI around the lesions containing blood products and in air-filled cavities. This enhanced sensitivity in T2prep-fMRI was also evidenced by the fact that functional activation during the sentence completion task was detected with T2prep-fMRI but not with EPI-fMRI in the affected areas with the same statistical threshold, whereas cerebrovascular reactivity during a breath-hold task was preserved in these same regions, implying intact neurovascular coupling in these patients. Although further investigations are required to validate these findings with invasive methods such as direct cortical stimulation mapping as the gold standard, this approach provides an alternative method for performing fMRI in brain regions with large susceptibility effects.

Designing hyperbolic secant excitation pulses to reduce signal dropout in gradient-echo echo-planar imaging

PURPOSE

To design hyperbolic secant (HS) excitation pulses to reduce signal dropout in the orbitofrontal and inferior temporal regions in gradient-echo echo-planar imaging (GE-EPI) for functional MRI (fMRI) applications.

METHODS

An algorithm based on Bloch simulations optimizes the HS pulse parameters needed to give the desired signal response across the range of susceptibility gradients observed in the human head (approximately ±250 μT·m(-1) ). The impact of the HS pulse on the signal, temporal signal-to-noise ratio, blood oxygen level-dependent (BOLD) sensitivity, and ability to detect resting state BOLD signal changes was assessed in six healthy male volunteers at 3T.

RESULTS

The optimized HS pulse (μ = 4.25, β = 3040 Hz, A0 = 12.3 μT, Δf = 4598 Hz) had a near uniform signal response for through-plane susceptibility gradients in the range ±250 μT·m(-1) . Signal, temporal signal-to-noise ratio, BOLD sensitivity, and the detectability of resting state networks were all partially recovered in the orbitofrontal and inferior temporal regions; however, there were signal losses of up to 50% in regions of homogeneous field (and signal loss from in-plane susceptibility gradients remained).

CONCLUSION

The HS pulse reduced signal dropout and could be used to acquire task and resting state fMRI data without loss of spatial coverage or temporal resolution.

Optimizing the magnetization-prepared rapid gradient-echo (MP-RAGE) sequence

The three-dimension (3D) magnetization-prepared rapid gradient-echo (MP-RAGE) sequence is one of the most popular sequences for structural brain imaging in clinical and research settings. The sequence captures high tissue contrast and provides high spatial resolution with whole brain coverage in a short scan time. In this paper, we first computed the optimal k-space sampling by optimizing the contrast of simulated images acquired with the MP-RAGE sequence at 3.0 Tesla using computer simulations. Because the software of our scanner has only limited settings for k-space sampling, we then determined the optimal k-space sampling for settings that can be realized on our scanner. Subsequently we optimized several major imaging parameters to maximize normal brain tissue contrasts under the optimal k-space sampling. The optimal parameters are flip angle of 12°, effective inversion time within 900 to 1100 ms, and delay time of 0 ms. In vivo experiments showed that the quality of images acquired with our optimal protocol was significantly higher than that of images obtained using recommended protocols in prior publications. The optimization of k-spacing sampling and imaging parameters significantly improved the quality and detection sensitivity of brain images acquired with MP-RAGE.

A comparison of dual gradient-echo and spin-echo fMRI of the inferior temporal lobe

Magnetic susceptibility differences at tissue interfaces lead to signal loss in conventional gradient-echo (GE) EPI. This poses a problem for fMRI in language and memory paradigms, which activate the most affected regions. Two methods proposed to overcome this are spin-echo EPI and dual GE EPI, where two EPI read-outs are serially collected at a short and longer echo time. The spin-echo method applies a refocusing pulse to recover dephased MR signal due to static field inhomogeneities, but is known to have a relatively low blood oxygenation level dependant (BOLD) sensitivity. In comparison, GE has superior BOLD sensitivity, and by employing an additional shorter echo, in a dual GE sequence, it can reduce signal loss due to spin dephasing. We directly compared dual GE and spin-echo fMRI during a semantic categorization task, which has been shown to activate the inferior temporal region-a region known to be affected by magnetic susceptibility. A whole brain analysis showed that the dual GE resulted in significantly higher activation within the left inferior temporal fusiform (ITF) cortex, compared to spin-echo. The inferior frontal gyrus (IFG) was activated for dual GE, but not spin-echo. Regions of interest analysis was carried out on the anterior and posterior ITF, left and right IFG, and part of the cerebellum. Dual GE outperformed spin-echo in the anterior and posterior ITF and bilateral IFG regions, whilst being equal in the cerebellum. Hence, dual GE should be the method of choice for fMRI studies of inferior temporal regions.

Networks involved in olfaction and their dynamics using independent component analysis and unified structural equation modeling

The study of human olfaction is complicated by the myriad of processing demands in conscious perceptual and emotional experiences of odors. Combining functional magnetic resonance imaging with convergent multivariate network analyses, we examined the spatiotemporal behavior of olfactory-generated blood-oxygenated-level-dependent signal in healthy adults. The experimental functional magnetic resonance imaging (fMRI) paradigm was found to offset the limitations of olfactory habituation effects and permitted the identification of five functional networks. Analysis delineated separable neuronal circuits that were spatially centered in the primary olfactory cortex, striatum, dorsolateral prefrontal cortex, rostral prefrontal cortex/anterior cingulate, and parietal-occipital junction. We hypothesize that these functional networks subserve primary perceptual, affective/motivational, and higher order olfactory-related cognitive processes. Results provided direct evidence for the existence of parallel networks with top-down modulation for olfactory processing and clearly distinguished brain activations that were sniffing-related versus odor-related. A comprehensive neurocognitive model for olfaction is presented that may be applied to broader translational studies of olfactory function, aging, and neurological disease.

The declining infrastructure of the aging brain

Great effort has been dedicated to mapping the functional architecture of the brain in health and disease. The neural centers that support cognition and behavior are the "hubs" defining the salient geographic landmarks of the cerebral topography. Similar to urban cartography, however, the functionality of these hubs is critically dependent on the infrastructure permitting the transfer of relevant information from site to site, and this infrastructure is susceptible to deterioration. The groundwork of the brain lies in the form of the complexly organized myelinated nerve fibers responsible for the inter-regional transmission of electrical impulses among distinct neural areas. Damage to the myelin sheath and reduction in the total number of nerve fibers with aging are thought to result in a degradation in the efficiency of communication among neural regions and to contribute to the decline of function in older adults. This article describes selected studies that are relevant to understanding the deterioration in structural connectivity of the aging brain with a focus on potential consequences to functional network activity. First, the neural substrates of connectivity and techniques used in the study of connectivity are described with a focus on neuroimaging methodologies. This is followed with discussion of the negative effects of age on connective integrity, and the possible mechanisms and neural and cognitive consequences of this progressive disconnection. Given the potential for natural repair of certain elements of the connective network, understanding the basis of age-associated decline in connectivity could have important implications with regard to the amelioration of neural dysfunction and the restoration of the infrastructure necessary for optimal function in older adults.

Oxygen metabolism in ischemic stroke using magnetic resonance imaging

Detecting "at-risk" but potentially salvageable brain tissue, known as the ischemic penumbra, is of importance for identifying patients who may benefit from thrombolytic or other treatments beyond the currently FDA-approved short therapeutic window for tissue plasminogen activator. Since the magnetic resonance blood oxygenation level-dependent (BOLD) contrast may provide information concerning tissue oxygen metabolism, its utilization in ischemic stroke has been explored. The focus of this review is to provide an introduction of several BOLD-based methods, including susceptibility-weighted imaging, R2 BOLD, R2*, R2', MR_OEF, and MR_OMI approaches to assess cerebral oxygenation changes induced by ischemia. Specifically, we will review the underlying pathophysiological basis of the imaging approaches, followed by a brief introduction of BOLD contrast, and finally the applications of BOLD approaches in ischemic stroke. The advantages and disadvantages of each method are addressed. In summary, the BOLD-based methods are promising for imaging oxygenation in ischemic tissue. Future steps would include technical refinement and vigorous validation against another independent method, such as positron emission tomography.

R*(2) mapping in the presence of macroscopic B₀ field variations

R₂ mapping has important applications in MRI, including functional imaging, tracking of super-paramagnetic particles, and measurement of tissue iron levels. However, R₂ measurements can be confounded by several effects, particularly the presence of fat and macroscopic B₀ field variations. Fat introduces additional modulations in the signal. Macroscopic field variations introduce additional dephasing that results in accelerated signal decay. These effects produce systematic errors in the resulting R₂ maps and make the estimated R₂ values dependent on the acquisition parameters. In this study, we develop a complex-reconstruction, confounder-corrected R₂ mapping technique, which addresses the presence of fat and macroscopic field variations for both 2D and 3D acquisitions. This technique extends previous chemical shift-encoded methods for R₂, fat and water mapping by measuring and correcting for the effect of macroscopic field variations in the acquired signal. The proposed method is tested on several 2D and 3D phantom and in vivo liver, cardiac, and brain datasets.

Absolute oxygenation metabolism measurements using magnetic resonance imaging

Cerebral oxygen metabolism plays a critical role in maintaining normal function of the brain. It is the primary energy source to sustain neuronal functions. Abnormalities in oxygen metabolism occur in various neuro-pathologic conditions such as ischemic stroke, cerebral trauma, cancer, Alzheimer’s disease and shock. Therefore, the ability to quantitatively measure tissue oxygenation and oxygen metabolism is essential to the understanding of pathophysiology and treatment of various diseases. The focus of this review is to provide an introduction of various blood oxygenation level dependent (BOLD) contrast methods for absolute measurements of tissue oxygenation, including both magnitude and phase image based approaches. The advantages and disadvantages of each method are discussed.

Single, slice-specific z-shim gradient pulses improve T2*-weighted imaging of the spinal cord

T2*-weighted imaging of the spinal cord suffers from signal dropouts that hamper blood-oxygenation-level-dependent functional magnetic resonance imaging (fMRI). They are due to field inhomogeneities caused by the different magnetic susceptibilities of the vertebrae and the intervertebral disks that vary periodically along the cord and, thus, cannot be compensated appropriately with conventional (constant) shimming. In this study, a single, slice-specific gradient pulse ("z-shim") is applied in echo-planar imaging of axial sections in order to compensate for the corresponding through-slice signal dephasing without affecting the acquisition time, i.e. the temporal resolution. Based on a reference acquisition sampling a range of compensation moments, the value yielding the maximum signal amplitude within the spinal cord is determined for each slice. Severe N/2 ghosting for larger compensation moments is avoided by applying the gradient pulse after the corresponding reference echoes. Furthermore, first-order flow compensation in the slice direction of both the slice-selection and the z-shim gradient pulse considerably reduces signal fluctuations in the cerebro-spinal fluid surrounding the spinal cord, i.e. would minimize ringing artifacts in fMRI. Phantom and in vivo experiments show the necessity to use slice-specific compensation moments in the presence of local susceptibility differences. Measurements performed in a group of 24 healthy volunteers at 3T demonstrate that this approach improves T2*-weighted imaging of axial sections of the cervical spinal cord by (i) increasing the signal intensity (overall by about 20%) and (ii) reducing signal intensity variations along the cord (by about 80%). Thus, it may help to improve the feasibility and reliability of fMRI of the spinal cord.

Overview of functional magnetic resonance imaging

Blood Oxygen Level Dependent (BOLD) functional magnetic resonance imaging (fMRI) depicts changes in deoxyhemoglobin concentration consequent to task-induced or spontaneous modulation of neural metabolism. Since its inception in 1990, this method has been widely employed in thousands of studies of cognition for clinical applications such as surgical planning, for monitoring treatment outcomes, and as a biomarker in pharmacologic and training programs. More recently, attention is turning to the use of pattern classification and other statistical methods to draw increasingly complex inferences about cognitive brain states from fMRI data. This article reviews the methods, challenges, and future of fMRI.

Rapid 3D radial multi-echo functional magnetic resonance imaging

Functional magnetic resonance imaging with readouts at multiple echo times is useful for optimizing sensitivity across a range of tissue T2* values as well as for quantifying T2*. With single-shot acquisitions, both the minimum TE value and the number of TEs which it is possible to collect within a single TR are limited by the long echo-planar imaging readout duration (20-40 ms). In the present work, a multi-shot 3D radial acquisition which allows rapid whole-brain imaging at a range of echo times is proposed. The proposed 3D k-space coverage is implemented via a series of rotations of a single 2D interleaf. Data can be reconstructed at a variety of temporal resolutions from a single dataset, allowing for a flexible tradeoff between temporal resolution and BOLD contrast to noise ratio. It is demonstrated that whole-brain images at 5 echo times (TEs from 10 to 46 ms) can be acquired at a temporal rate as rapid as 400 ms/volume (3.75 mm isotropic resolution). Activation maps for a simultaneous motor/visual task consistent across multiple acceleration factors are obtained. Weighted combination of the echoes results in Z-scores that are significantly (p=0.016) higher than those resulting from any of the individual echo time images.

Improved shimming for fMRI specifically optimizing the local BOLD sensitivity

In functional MRI, magnetic field inhomogeneities due to air-tissue susceptibility differences may lead to severe signal dropouts and geometric distortions in echo-planar images. Therefore, the inhomogeneities in the field are routinely minimized by shimming prior to imaging. However in fMRI, the Blood Oxygen Level Dependent (BOLD) effect is the measure of interest, so the BOLD sensitivity (BS) should be optimized rather than the magnetic field homogeneity. The analytical expression for an estimate of the BOLD sensitivity has been recently developed, allowing for the computation of BOLD sensitivity maps from echo-planar images and field maps. This report describes a novel shimming procedure that optimizes the local BOLD sensitivity over a region of interest. The method is applied in vivo and compared to a standard global shimming procedure. A breath-holding experiment was carried out and demonstrated that the BS-based shimming significantly improved the detection of activation in a target region of interest, the medial orbitofrontal cortex.

Temperature mapping near the surface of ultrasound transducers using susceptibility- compensated magnetic resonance imaging

Magnetic resonance imaging (MRI)-based temperature mapping very close to the surface of an ultrasound transducer is not possible due to the large magnetic susceptibility- induced image artifacts that arise from the materials used in transducer construction. Here, it is shown in phantoms that "susceptibility-compensated" MRI sequences can be used to measure thermal increases approximately 1 mm from the surface of a 4-element cymbal array transducer, which has been used widely for noninvasive transdermal drug delivery. The estimated temperatures agree well with those measured using thermocouples.

Feasibility and safety of longitudinal magnetic resonance imaging in a rodent model with intracortical microwire implants

The purpose of this communication is to investigate (1) the feasibility of carrying out longitudinal magnetic resonance imaging (MRI) studies in animals with implanted microwire electrodes adapted for MRI compatibility, (2) the effect of MRI studies on the quality of neurophysiological recordings, (3) the use of MRI to study the extent and recovery of tissue damage due to electrode insertion and (4) histological tissue damage due to MRI. There was no evidence of chronic neural damage caused by repeated MRI by any of the measures used nor any statistical difference in the quality of the electrophysiological recordings between animals that had undergone MRI scans and those that had not.

A single-scan T2* mapping method based on two gradient-echo images with compensation for macroscopic field inhomogeneity

T2*-weighted imaging (T2*WI) and quantitative T2* mapping with conventional gradient-echo acquisition are often hindered by severe signal loss induced by macroscopic field inhomogeneity. Various z-shimming approaches have been developed for T2*WI/T2* mapping in which the effects of macroscopic field inhomogeneity are suppressed while the sensitivity of T2*-related signal intensity to alterations in the microscopic susceptibility is maintained. However, this is often done at the cost of significantly increased imaging time. In this work, a fast T2* mapping method with compensation for macroscopic field inhomogeneity was developed. A proton density-weighted image and a composite T2*-weighted image, both of which were essentially free from macroscopic field inhomogeneity-induced signal loss, were used for the T2* calculation. The composite T2*-weighted image was reconstructed from a number of gradient-echo images acquired with successively incremented z-shimming compensation. Because acquisition of the two images and z-shimming compensation were realized in a single scan, the total acquisition time for obtaining a T2* map with the proposed method is the same as the time taken for a conventional multiecho gradient-echo imaging sequence without compensation. The performance and efficiency of the proposed method were demonstrated and evaluated at 4.7 T.

Current trends and challenges in MRI acquisitions to investigate brain function

Functional magnetic resonance imaging (fMRI) studies using the blood oxygenation level dependent (BOLD) response have become a widely used tool for noninvasive assessment of functional organization of the brain. Yet the technique is still fairly new, with many significant challenges remaining. Capitalizing on additional contrast mechanisms available with MRI, several other functional imaging techniques have been developed that potentially provide improved quantification or specificity of neuronal function. This article reviews the challenges and the current state of the art in MRI-based methods of imaging cognitive function.

Effect on BOLD sensitivity due to susceptibility-induced echo time shift in spiral-in based functional MRI

Susceptibility artifacts induced by the magnetic field inhomogeneity exist near the air/tissue interfaces at the ventral brain in functional magnetic resonance imaging (fMRI). These susceptibility artifacts will cause geometric distortions and signal loss in reconstructed images. Additionally, the in-plane susceptibility gradients will cause a shift in effective echo time, and therefore influence the blood-oxygen-level dependent (BOLD) sensitivity since it is proportional to effective echo time. In this work, we examine the effective echo time shift and the change of the BOLD sensitivity based on susceptibility gradients. The analysis results show that there are regions, such as the orbitofrontal cortex, that suffer from significant loss of BOLD sensitivity using spiral-in trajectory in BOLD fMRI.

Phase gradient mapping as an aid in the analysis of object-induced and system-related phase perturbations in MRI

In this note we wish to demonstrate the utility of phase gradient mapping (PGM) as an aid in the analysis and characterization of object-induced and system-related macroscopic phase perturbations in MRI. To achieve this goal, phase gradient maps and, if applicable, field gradient maps were derived from standard phase images via a forward difference operator taking into account phase wraps. By way of phantom experiments, PGM was shown to provide reliable phase and field gradient information, even in regions with multiple phase wraps. Phase gradient mapping was further shown to allow positive identification of local phase and field perturbations and global discrimination between positive and negative local susceptibility deviations. The suitability of PGM for in vivo studies was demonstrated by a 3D brain examination of a healthy volunteer.

Single-shot dual-z-shimmed sensitivity-encoded spiral-in/out imaging for functional MRI with reduced susceptibility artifacts

Blood oxygenation level-dependent (BOLD) functional MRI (fMRI) can be severely hampered by signal loss due to susceptibility-induced static magnetic field (B(0)) inhomogeneities near air/tissue interfaces. A single-shot spiral-in/out sequence with a z-shim gradient embedded between the two acquisitions was previously proposed to efficiently recover the signal. However, despite promising results, this technique had several limitations, which are addressed here as follows. First, by adding a second z-shim gradient before the spiral-in acquisition and optimizing both z-shim gradients slice-by-slice, a significantly more uniform signal recovery can be achieved. Second, by acquiring a B(0) map, the optimal z-shim gradients can be directly, efficiently, and accurately determined for each subject. Third, by complementing the z-shimming approach with sensitivity encoding (SENSE), the in-plane spatial resolution can be increased and, hence, susceptibility artifacts further reduced, while maintaining a high temporal resolution for fMRI applications. These advantages are demonstrated in human functional studies.

Volume parcellation for improved dynamic shimming

INTRODUCTION

The need for a homogeneous magnetic field in magnetic resonance imaging is well established, especially at high static magnetic field strengths where susceptibility-induced image distortions and signal losses become excessively large. Dynamic shim updating, where the optimal set of shim currents is applied for each slice during a multi-slice acquisition, has been shown to improve magnetic field homogeneity to a greater extent than conventional global shimming.

METHODS

Here, in an initial feasibility study, we show via simulation that improved efficacy of shimming can be achieved by using the novel parcellated dynamic shimming method.

RESULTS

The results of these simulations indicate that parcellated dynamic shimming based on just linear shim terms can perform approximately as well as slice-based dynamic shimming with up to third-order shim terms.

CONCLUSIONS

This work shows that the effective magnetic field inhomogeneity can be further reduced if shimming and image data acquisition are sequentially performed over a series of compact, cuboidal sub-volumes rather than planes. Further work is needed to develop an imaging approach that can be used for the optimal implementation of parcellated dynamic shimming.

Reducing susceptibility artifacts in fMRI using volume-selectivez-shim compensation

Susceptibility-induced magnetic field gradients (SFGs) can result in severe signal loss in the orbitofrontal cortex (OFC) in gradient-echo-based functional MRI (fMRI) studies. Although conventional z-shim techniques can effectively recover the MRI signal in this region, the substantial penalty in imaging time hampers their use in routine fMRI studies. A modified z-shim technique with high imaging efficiency is presented in this study. In this technique, z-shim compensations are applied only to a selective volume where the susceptibility artifact is severe. The results of an fMRI study (N=6) demonstrate the feasibility of detecting the OFC activation with z-shim in whole-brain fMRI studies at a temporal resolution of 2 s.

White-marker imaging—Separating magnetic susceptibility effects from partial volume effects

By applying dephasing gradients, local magnetic field inhomogeneitiescan selectively visualized with positive contrast, such as those created by magnetically labeled cells. This is known as "white-marker imaging." In white-marker imaging, subvoxel signal variations are also visualized as a result of partial volume (PV) effects and may compromise the identification of magnetic structures (e.g., magnetically-labeled cells). This study presents the theory and proof-of-principle experiments of a strategy to eliminate PV effects during white-marker imaging. The strategy employs the asymmetry of the signal response curves for non-PV effects as a function of externally applied gradients. In the case of PV effects, subtraction of the symmetrical signal responses eliminates their contribution. In vitro experimental images were made using a spherical phantom with cylindrical elements. In vivo images of the brain were obtained at a location that included air cavities (susceptibility effects) and the circle of Willis (PV effect). The results show that PV effects were eliminated in the in vitro experiments and were virtually absent under in vivo conditions.

Dual-echo spiral in/in acquisition method for reducing magnetic susceptibility artifacts in blood-oxygen-level-dependent functional magnetic resonance imaging

MRI signal dropout in gradient recalled echo acquisitions limits the capability of blood-oxygen-level-dependent functional magnetic resonance imaging (fMRI) to study activation tasks that involve the orbitofrontal, temporal, and basal areas of the brain where significant macroscopic magnetic susceptibility differences exist. Among the various approaches aimed to address this issue, the acquisition method based on spiral in/out trajectories is one of the most time-efficient and effective techniques. In this study, we extended further the spiral in/out approach into 3D acquisition and compared the effectiveness of the different spiral in/out trajectory combinations in reducing signal dropout. The activation results from whole brain fMRI studies using complex finger tapping and breath-holding tasks demonstrate that the acquisition method based on dual-echo spiral in/in (DSPIN) trajectories is the most favorable. The DSPIN acquisition method has the following advantages: (1) It reduces most effectively signal dropout in the brain where magnetic susceptibility inhomogeneity is problematic and significantly improves the sensitivity to detect functional activations in those regions. (2) It significantly improves SNR in the whole brain by dual echo averaging without compromising functional contrast. (3) There is no reduction in time-efficiency and spatial resolution.

Dephased MRI

In this work gradient dephasing is treated as a mechanism for manipulating contrast in otherwise conventional MR images. The paper provides a theoretical and experimental framework for this approach. It starts from the observation that dephasing gradients invoke a shift in k-space. From this it is inferred that the effects of in-plane and through-plane dephasing can be systematically explored in the context of any given imaging experiment by sampling k-space more widely and densely than dictated by the field of view (FOV) and the spatial resolution of the desired images. The oversampled k-space allows an ensemble of lower-resolution dephased images to be reconstructed in which the degree and direction of dephasing are determined by the off-center position of the reconstruction window. The efficacy of this approach is demonstrated for standard gradient-echo acquisitions in a phantom. The results indicate the potential of the proposed methodology for evaluating 3D image data and optimizing gradient dephasing in applications that rely on the exploitation of partial volume and susceptibility effects (e.g., tracking interventional devices and tracing magnetically labeled substances).

Blipped multi gradient-echo slice excitation profile imaging (bmGESEPI) for fastT2* measurements with macroscopicB0 inhomogeneity compensation

With the rapid development of human MRI at field strengths > or = 7 T, knowledge of T(2) (*) relaxation times at such field strengths is needed to optimize acquisition parameters and understand relaxation mechanisms in many applications. However, standard T(2) (*) measurements (e.g., using conventional multiecho gradient-echo (GE) sequences) are affected by macroscopic static magnetic field (B(0)) inhomogeneities, which are particularly severe at high field strength. The multi-GE slice excitation profile imaging (mGESEPI) method was developed for T(2) (*) measurements in the presence of macroscopic B(0) inhomogeneity, but it requires excessive acquisition times at field strengths > or = 7 T. In this paper a more efficient technique, named blipped mGESEPI (bmGESEPI), is proposed. To demonstrate its advantages, T(2) (*) maps were acquired using a conventional multiecho GE method, the mGESEPI method, and the bmGESEPI method in postmortem and in vivo human brains at 8 T.

BOLD contrast sensitivity enhancement and artifact reduction with multiecho EPI: Parallel-acquired inhomogeneity-desensitized fMRI

Functional MRI (fMRI) generally employs gradient-echo echo-planar imaging (GE-EPI) to measure blood oxygen level-dependent (BOLD) signal changes that result from changes in tissue relaxation time T(*) (2) between activation and rest. Since T(*) (2) strongly varies across the brain and BOLD contrast is maximal only where the echo time (TE) equals the local T(*) (2), imaging at a single TE is a compromise in terms of overall sensitivity. Furthermore, the long echo train makes EPI very sensitive to main field inhomogeneities, causing strong image distortion. A method is presented that uses accelerated parallel imaging to reduce image artifacts and acquire images at multiple TEs following a single excitation, with no need to increase TR. Sensitivity gains from the broadened T(*) (2) coverage are optimized by pixelwise weighted echo summation based on local T(*) (2) or contrast-to-noise ratio (CNR) measurements. The method was evaluated using an approach that allows differential BOLD CNR to be calculated without stimulation, as well as with a Stroop experiment. Results obtained at 3 T showed that BOLD sensitivity improved by 11% or more in all brain regions, with larger gains in areas typically affected by strong susceptibility artifacts. The use of parallel imaging markedly reduces image distortion, and hence the method should find widespread application in functional brain imaging.

Simultaneous perfusion and blood-oxygenation-level-dependent measurements using single-shot interleaved z-shim echo-planar imaging

Single-shot interleaved z-shim EPI (SSIZS-EPI) was extended to a simultaneous perfusion and blood-oxygenation-level-dependent (BOLD) imaging technique that reduces susceptibility-induced signal loss while preserving rapid image acquisition. Experiments on human brains showed that images acquired with this technique had improved signal-to-noise ratio in the inferior prefrontal, meso-, and lateral-temporal lobes compared with a conventional EPI. Perfusion maps obtained from the SSIZS-EPI images at resting state illustrated substantial signal recovery in these brain areas. Perfusion and BOLD images collected with a sensorimotor paradigm demonstrated the feasibility of the technique to simultaneously measure cerebral blood flow and blood oxygenation signals associated with brain activation. Functional experiments with a neuropsychiatric paradigm showed increased brain activities in the periamygdalar regions in both perfusion and BOLD maps, consistent with a previous H(2) (15)O PET study. The proposed technique, with its advantages of reducing susceptibility artifacts and fast scanning speed, would be useful for obtaining more reliable measurements of functional signals, particularly in the brain regions with field inhomogeneities.

Functional magnetic resonance imaging study of human olfaction and normal aging

BACKGROUND

The function of human olfaction declines with advancing age. An important question centers on whether functional alterations to olfactory brain structures accompany age-related behavioral changes. In the present study, we tested the hypothesis that aged adults have intact though reduced activity in the central olfactory system using functional magnetic resonance imaging (fMRI).

METHODS

University of Pennsylvania Smell Identification Test (UPSIT) was used to test the smell function of 11 young (23.9 +/- 1.6 years) and 8 aged (66.4 +/- 4.4 years) healthy participants. Then, the participants received fMRI at 3.0 T with lavender and spearmint as stimulants. After fMRI, the participants provided ratings for the odorants' intensity and pleasantness.

RESULTS

The average UPSIT score of the aged adults was 34.1 +/- 1.5, which was significantly lower than that of the young adults (37.3 +/- 1.1) (p =.0004). Both age groups showed significant activation in major olfactory brain structures, including the primary olfactory cortex, entorhinal cortex, hippocampus and parahippocampal cortex, thalamus, hypothalamus, orbitofrontal cortex, and insular cortex and its extension into the inferior lateral frontal region. The aged adults showed less brain activity in olfactory structures (p =.022), consistent with lower ratings of odor intensity and UPSIT scores. Activation intensity in bilateral primary olfactory cortex areas and right insular cortex was also comparatively weaker (p <.019).

CONCLUSION

Results demonstrate that significant activation in aged adults can be observed in all the olfactory brain structures that are activated in young adults, but with lower activation volume and intensity. This finding provides a necessary baseline for further investigations in olfaction and aging.

Reduction of magnetic field inhomogeneity artifacts in echo planar imaging with SENSE and GESEPI at high field

Geometric distortion, signal-loss, and image-blurring artifacts in echo planar imaging (EPI) are caused by frequency shifts and T(2)(*) relaxation distortion of the MR signal along the k-space trajectory due to magnetic field inhomogeneities. The EPI geometric-distortion artifact associated with frequency shift can be reduced with parallel imaging techniques such as SENSE, while the signal-loss and blurring artifacts remain. The gradient-echo slice excitation profile imaging (GESEPI) method has been shown to be successful in restoring tissue T(2)(*) relaxation characteristics and is therefore effective in reducing signal-loss and image-blurring artifacts at a cost of increased acquisition time. The SENSE and GESEPI methods are complementary in artifact reduction. Combining these two techniques produces a method capable of reducing all three types of EPI artifacts while maintaining rapid acquisition time.

Computer simulation studies of the effects of dynamic shimming on susceptibility artifacts in EPI at high field

Dynamic shimming in multi-slice imaging aims to achieve optimal magnetic field homogeneity by updating the shim coil currents for each slice in real time. Dynamic shimming may reduce the signal loss and geometric distortion caused by magnetic susceptibility variations between tissues and is likely to be valuable for fast T2*-sensitive imaging techniques like EPI. A computer simulation of dynamic shimming using real image data has been developed to demonstrate the effectiveness of higher order dynamic shimming for echo planar imaging at high magnetic field, and to investigate the potential benefits of different orders of shim coil. Geometric distortions and signal intensities for different degrees of dynamic shimming were simulated and the results are compared with the images obtained with a conventional shimming technique. These results demonstrate the effectiveness, necessity and difficulty of high order dynamic shimming.

[Susceptibility weighted imaging. Theory and applications]

Susceptibility Weighted Imaging (SWI) is a new MR imaging technique using the BOLD effect (Blood Oxygen Level Dependent) and the differences of susceptibility between tissues. It is a 3D gradient echo, fully velocity compensated sequence. The echo time is chosen to maximize the signal cancellation in veins and a specific post-processing is applied using the phase images as a complementary source of contrast. It is very useful for the visualization of veins either normal or abnormal. It shows hemorrhage, even of small quantity, better than conventional gradient echo sequences. Its use is still limited by a long acquisition time and some remaining artifacts.

Limits of detection of SPIO at 3.0 T usingT2* relaxometry

T(2)* relaxometry for quantitative MR imaging is strongly hampered by large-scale field inhomogeneities, which lead to signal losses and an overestimation of the relaxation rate R(2)*. This is of particular importance for the sensitive detection of iron oxide contrast agent distributions. To derive an accurate measurement of T(2)*, a main field inhomogeneity correction is applied: the main field inhomogeneity is derived from multislice T(2)* relaxometry data and used as an initial value for an iterative optimization, by which the relaxation signal is corrected for each voxel. These corrected T(2)* maps show reduced influence of the local field variation and contain information about the local SPIO concentration. The method was tested on phantoms and the limit of detection of SPIO labeled cells using T(2)* relaxometry was estimated in volunteers to be 120 x 10(3) cells/mL (2.4 microg Fe/mL) in the brain and 385 x 10(3) cells/mL (8 microg Fe/mL) in the liver.

Simultaneous acquisition of gradient-echo and asymmetric spin-echo for single-shot z-shim: Z-SAGA

This article describes the Z-SAGA pulse sequence, a technique for recovering susceptibility losses in EPI images for neuroimaging applications. The pulse sequence is based on an asymmetric spin echo EPI sequence and acquires a gradient echo image and an asymmetric spin echo image in a single shot. For one of the images, a z-shim gradient pulse is applied to compensate for susceptibility-related field distortions. The two images are combined to form an image with reduced signal loss. This sequence is simple to implement and experimentally demonstrated to be effective for BOLD imaging.

Comparison of spiral-in/out and spiral-out BOLD fMRI at 1.5 and 3 T

Spiral-in/out functional magnetic resonance imaging (fMRI) methods acquire one image before the echo time (TE) and a second image after TE during each scan. Weighted combination of the two images provides a time series with reduced susceptibility dropout in frontal and medial temporal regions as well as increased signal-to-noise ratio (SNR) in regions of uniform cortex. In this study, task activation with the spiral-in/out method was compared to that with conventional spiral-out acquisitions at two field strengths (1.5 and 3.0 T) using episodic memory encoding, verbal working memory, and affective processing tasks in eight human volunteers. With the conventional spiral-out sequence, greater signal dropout is observed in lateral and medial prefrontal, amygdalar, and medial temporal regions at 3 T relative to 1.5 T, whereas such dropout at 3 T is reduced or mitigated with the spiral-in/out method. Similarly, activation volumes for frontal, amygdalar, and medial temporal regions are reduced for spiral-out acquisitions relative to spiral-in/out, and this difference is more apparent at 3 T than at 1.5 T. In addition, significant regionally specific increases in Z scores are obtained with the spiral-in/out sequence relative to spiral-out acquisitions at both field strengths. It is concluded the spiral-in/out sequence may provide significant advantages over conventional spiral methods, especially at 3 T.