BOLD fMRI is hampered by dropout of signal in the orbitofrontal and parietal brain regions due to magnetic field gradients near air-tissue interfaces. This work reports the use of spiral-in trajectories that begin at the edge of k-space and end at the origin, and spiral in/out trajectories in which a spiral-in readout is followed by a conventional spiral-out trajectory. The spiral-in trajectory reduces the dropout and increases the BOLD contrast. The spiral-in and spiral-out images can be combined in several ways to simultaneously achieve increased signal-tonoise ratio (SNR) and reduced dropout artifacts. Activation experiments employing an olfaction task demonstrate significantly increased activation volumes due to reduced dropout, and overall increased SNR in all regions. The most widely used form of fMRI exploits BOLD contrast (1,2) to produce maps of neuronal activation. When the transverse magnetization decay is exponential, changes in BOLD contrast are maximized if the echo time (TE) is made equal to the susceptibility-mediated transverse relaxation time constant, T* 2 . In uniform brain the T* 2 for gray matter is about 50 ms at 3T (3,4); thus, in sensitizing the acquisition to BOLD changes from the microscopic gradients surrounding capillaries, the acquisition is also made exquisitely sensitive to intravoxel dephasing resulting from macroscopic field gradients established near air-tissue interfaces. These susceptibility-induced field gradients (SFGs) cause severe dropout of signal in the frontal orbital and lateral parietal brain regions due to the difference in magnetic susceptibility of tissue and air (ϳ -8 ppm). These dropouts can limit the applicability of fMRI for many cognitive experiments.Several methods have been proposed to reduce the effect of SFGs. One class of techniques corrects for dropouts caused when SFGs shift the center of excitation k-space (k z direction), by applying compensation gradients in the slice-selection direction to refocus the dephased spins (5,6). 3D compensation schemes were introduced by Yang et al. (7,8) in which multiple echoes and Fourier inversion are used to create compensated images, and by Glover (9), who used extended coverage of k z -space with windowed reconstruction to provide efficiency improvements in gathering the compensated images. A related method simply decreases the slice thickness and averages adjacent slices (10,11). However, each of these methods suffers from prolonged scan time and loss of SNR efficiency. Another class of methods uses tailored RF pulses to compensate the dephasing during excitation (12)(13)(14). The design of these pulses is complex and ideally must be tailored for each subject, and their effectiveness is reduced by gradient system limitations that cause the pulses to be lengthy. In addition, all of these compensation methods are effective only for SFGs in the slice-select direction, and they provide no mitigation for intravoxel dephasing caused by in-plane gradients.Spiral methods have several advantages over other techniques for ...

Quantitative spinal cord (SC) magnetic resonance imaging (MRI) is fraught with challenges, among which is the lack of standardized imaging protocols. Here we present a prospectively harmonized quantitative MRI protocol, which we refer to as the spine generic protocol, for the three main 3T MRI vendors: GE, Philips and Siemens. The protocol provides valuable metrics for assessing SC macrostructural and microstructural integrity: T1-weighted and T2-weighted imaging for SC cross-sectional area (CSA) computation, multi-echo gradient echo for gray matter CSA, as well as magnetization transfer and diffusion weighted imaging for assessing white matter microstructure. The spine generic protocol was used to acquire data across 42 centers in 260 healthy subjects, as detailed in the companion paper [REF-DATA]. The spine generic protocol is open-access and its latest version can be found at: https://spinalcordmri.org/protocols. The protocol will serve as a valuable starting point for researchers and clinicians implementing new SC imaging initiatives. Note to the reviewer/editor/publisher: the companion paper is referred to as [REF-DATA]6/52 121 122dealing with cervical myelopathy and MS populations. Applications of the MethodThe proposed protocol is not geared towards a specific disease and it is suitable for imaging WM pathology (demyelination and Wallerian degeneration via axon/myelin-sensitive 122 https://mssociety.ca/about-ms-research/about-our-research-program/research-we-fund/canadian-prospect ive-cohort-study-to-understand-progression-in-ms-canproco 121 https://www.wingsforlife.com/us/research/imaging-spinal-cord-injury-and-assessing-its-predictive-value-th e-inspired-study-2675/ 9/52

Purpose Simultaneous brain and spinal cord functional MRI is emerging as a new tool to study the central nervous system but is challenging. Poor B0 homogeneity and small size of the spinal cord are principal obstacles to this nascent technology. Here we extend a dynamic shimming approach, first posed by Finsterbusch, by shimming per slice for both the brain and spinal cord. Methods We shim dynamically by a simple and fast optimization of linear field gradients and frequency offset separately for each slice in order to minimize off‐resonance for both the brain and spinal cord. Simultaneous acquisition of brain and spinal cord fMRI is achieved with high spatial resolution in the spinal cord by means of an echo‐planar RF pulse for reduced FOV. Brain slice acquisition is full FOV. Results T2*‐weighted images of brain and spinal cord are acquired with high clarity and minimal observable image artifacts. Fist‐clenching fMRI experiments reveal task‐consistent activation in motor cortices, cerebellum, and C6‐T1 spinal segments. Conclusions High quality functional results are obtained for a sensory‐motor task. Consistent activation in both the brain and spinal cord is observed at individual levels, not only at group level. Because reduced FOV excitation is applicable to any spinal cord section, future continuation of these methods holds great potential.

No abstract

Dermatomal maps are a mainstay of clinical practice and provide information on the spatial distribution of the cutaneous innervation of spinal nerves. Dermatomal deficits can help isolate the level of spinal nerve root involvement in spinal conditions and guide clinicians in diagnosis and treatment. Dermatomal maps, however, have limitations, and the spatial distribution of spinal cord sensory activity in humans remains to be quantitatively assessed. Here we used spinal cord functional MRI to map and quantitatively compare the spatial distribution of sensory spinal cord activity during tactile stimulation of the left and right lateral shoulders (i.e. C5 dermatome) and dorsal third digits of the hands (i.e., C7 dermatome) in healthy humans (n = 24, age = 36.8 ± 11.8 years). Based on the central sites for processing of innocuous tactile sensory information, we hypothesized that the activity would be localized more to the ipsilateral dorsal spinal cord with the lateral shoulder stimulation activity being localized more superiorly than the dorsal third digit. The findings demonstrate lateralization of the activity with the left- and right-sided stimuli having more activation in the ipsilateral hemicord. Contradictory to our hypotheses, the activity for both stimulation sites was spread across the dorsal and ventral hemicords and did not demonstrate a clear superior-inferior localization. Instead, the activity for both stimuli had a broader than expected distribution, extending across the C5, C6, and C7 spinal cord segments. We highlight the complexity of the human spinal cord neuroanatomy and several sources of variability that may explain the observed patterns of activity. While the findings were not completely consistent with our a priori hypotheses, this study provides a foundation for continued work and is an important step towards developing normative quantitative spinal cord measures of sensory function, which may become useful objective MRI-based biomarkers of neurological injury and improve the management of spinal disorders.

In a companion paper by Cohen-Adad et al. we introduce the spine generic quantitative MRI protocol that provides valuable metrics for assessing spinal cord macrostructural and microstructural integrity. This protocol was used to acquire a single subject dataset across 19 centers and a multi-subject dataset across 42 centers (for a total of 260 participants), spanning the three main MRI manufacturers: GE, Philips and Siemens. Both datasets are publicly available via git-annex. Data were analysed using the Spinal Cord Toolbox to produce normative values as well as inter/intra-site and inter/intra-manufacturer statistics. Reproducibility for the spine generic protocol was high across sites and manufacturers, with an average inter-site coefficient of variation of less than 5% for all the metrics. Full documentation and results can be found at https://spine-generic.rtfd.io/. The datasets and analysis pipeline will help pave the way towards accessible and reproducible quantitative MRI in the spinal cord.

The conventional spiral-in/out trajectory samples k-space sufficiently in the spiral-in path and sufficiently in the spiral-out path to enable creation of separate images. We propose an "interleaved spiral-in/out" trajectory comprising a spiral-in path that gathers one half of the k-space data, and a complimentary spiral-out path that gathers the other half. The readout duration is thereby reduced by approximately half, offering two distinct advantages: reduction of signal dropout due to susceptibilityinduced field gradients (at the expense of signal-to-noise ratio [SNR]), and the ability to achieve higher spatial resolution when the readout duration is identical to the conventional method. Two reconstruction methods are described; both involve temporal filtering to remove aliasing artifacts. Empirically, interleaved spiral-in/out images are free from false activation resulting from signal pileup around the air/tissue interface, which is common in the conventional spiral-out method. The gradient-recalled echo (GRE) method is the most common technique for blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI). fMRI requires rapid imaging-sequences and long echo time (TE) to capture the hemodynamic response due to neural activity in brain. Usually a single-shot trajectory is employed to cover a desired k-space volume with each radio frequency (RF) excitation. A single-shot trajectory is used because it can generate an image much faster than conventional imaging techniques (e.g., spin warp), thereby helping to mitigate motion effects. But the readout duration of a singleshot trajectory can be long compared to T* 2 decay (e.g., with a typical commercial scanner, acquiring a 64 ϫ 64 image with 20-cm field of view [FOV] takes at least 25 ms with an echo-planar imaging trajectory). Long readout duration increases image susceptibility to pulsatile motion and to off-resonance. Long TE worsens the effect of field inhomogeneities, caused by susceptibility effects, which results in signal loss in regions near air/tissue interfaces (e.g., TE is usually set to T* 2 , which is 30 -40 ms at 3T) (1). These disadvantages lead to artifacts in images and hamper study of memory and attention involving regions near the ventral frontal, medial temporal, and inferior temporal lobes.Several techniques have been proposed in the past, to reduce susceptibility-induced signal dropout. They fall into two main categories:1. Inhomogeneity minimization (e.g., localized resistive shim coils (2) or diamagnetic passive shims (3), tailored RF excitation pulses (4,5), and Z-shim techniques (6 -8)). Localized resistive shim coils and local passive shims are effective only in small areas and may cause discomfort. Tailored RF pulses require individualized design per subject and may be too long, reducing signal-to-noise ratio (SNR) in uniform brain areas and reducing the scan efficiency. The Z-shim technique generally lengthens scan duration by sampling with several shim values. 2. Modification of readout trajectories to minim...

No abstract