Spinal CSF flow in response to forced thoracic and abdominal respiration.

BACKGROUND: Respiration-induced pressure changes represent a powerful driving force of CSF dynamics as previously demonstrated using flow-sensitive real-time magnetic resonance imaging (MRI). The purpose of the present study was to elucidate the sensitivity of CSF flow along the spinal canal to forced thoracic versus abdominal respiration. METHODS: Eighteen subjects without known illness were studied using real-time phase-contrast flow MRI at 3 T in the aqueduct and along the spinal canal at levels C3, Th1, Th8 and L3. Subjects performed a protocol of forced breathing comprising four cycles of 2.5 s inspiration and 2.5 s expiration. RESULTS: The quantitative results for spinal CSF flow rates and volumes confirm previous findings of an upward movement during forced inspiration and reversed downward flow during subsequent exhalation-for both breathing types. However, the effects were more pronounced for abdominal than for thoracic breathing, in particular at spinal levels Th8 and L3. In general, CSF net flow volumes were very similar for both breathing conditions pointing upwards in all locations. CONCLUSIONS: Spinal CSF dynamics are sensitive to varying respiratory performances. The different CSF flow volumes in response to deep thoracic versus abdominal breathing reflect instantaneous adjustments of intrathoracic and intraabdominal pressure, respectively. Real-time MRI access to CSF flow in response to defined respiration patterns will be of clinical importance for patients with disturbed CSF circulation like hydrocephalus, pseudotumor cerebri and others.

Diffusion-weighted MRI of the prostate without susceptibility artifacts: Undersampled multi-shot turbo-STEAM with rotated radial trajectories.

The aim of this study was to develop and evaluate a clinically feasible approach to diffusion-weighted (DW) MRI of the prostate without susceptibility-induced artifacts. The proposed method relies on an undersampled multi-shot DW turbo-STEAM sequence with rotated radial trajectories and a multi-step inverse reconstruction with denoised multi-shot phase maps. The total acquisition time was below 6 min for a resolution of 1.4 × 1.4 × 3.5 mm3 and six directions at b = 600 s mm-2 . Studies of eight healthy subjects and two patients with prostate cancer were performed at 3 T employing an 18-channel body-array coil and elements of the spine coil. The method was compared with conventional DW echo-planar imaging (EPI) of the prostate. The results confirm that DW STEAM MRI avoids geometric distortions and false image intensities, which were present for both single-shot EPI (ssEPI) and readout-segmented EPI, particularly near the intestinal wall of the prostate. Quantitative accuracy of the apparent diffusion coefficient (ADC) was validated with use of a numerical phantom providing ground truth. ADC values in the central prostate gland of healthy subjects were consistent with those measured using ssEPI and with literature data. Preliminary results for patients with prostate cancer revealed a correct anatomical localization of lesions with respect to T2 -weighted MRI in both mean DW STEAM images and ADC maps. In summary, DW STEAM MRI of the prostate offers clinically relevant advantages for the diagnosis of prostate cancer compared with state-of-the-art EPI-based approaches. The method warrants extended clinical trials.

Multi-slice real-time MRI of temporomandibular joint dynamics.

OBJECTIVES: The purpose of this work was to improve the clinical versatility of high-speed real-time MRI studies of temporomandibular joint (TMJ) dynamics by simultaneous recordings of multiple MRI movies in different sections. METHODS: Real-time MRI at 3 T was realized using highly undersampled radial FLASH acquisitions and image reconstruction by regularized nonlinear inversion (NLINV). Multi-slice real-time MRI of two, three or four slices at 0.75 mm resolution and 6 to 8 mm thickness was accomplished at 50.0 ms, 33.3 ms or 25.5 ms temporal resolution, respectively, yielding simultaneous movies at 2 × 10, 3 × 10 or 4 × 10 frames per second in a frame-interleaved acquisition mode. Real-time MRI movies were evaluated by three blinded raters for visibility of the anterior and posterior border of disc, shape of the disk body and condyle head as well as movement of the disc and condyle (1 = excellent, 5 = no visibility). RESULTS: Effective delineation of the disk atop the mandibular condyle was achieved by T1-weighted images with opposed-phase water-fat contrast. Compared to 8 mm sections, multi-slice recordings with 6 mm thickness provided sharper delineation of relevant structures as confirmed by inter-rater evaluation. Respective dual-slice and triple-slice recordings of a single TMJ as well as dual-slice recordings of both joints (one slice per TMJ) received the highest visibility ratings of ≤ 2 corresponding to high confidence in diagnostic content. CONCLUSIONS: The improved access to TMJ dynamics by multi-slice real-time MRI will contribute to more effective treatment of temporomandibular disorders.

Respiration and the watershed of spinal CSF flow in humans.

The dynamics of human CSF in brain and upper spinal canal are regulated by inspiration and connected to the venous system through associated pressure changes. Upward CSF flow into the head during inspiration counterbalances venous flow out of the brain. Here, we investigated CSF motion along the spinal canal by real-time phase-contrast flow MRI at high spatial and temporal resolution. Results reveal a watershed of spinal CSF dynamics which divides flow behavior at about the level of the heart. While forced inspiration prompts upward surge of CSF flow volumes in the entire spinal canal, ensuing expiration leads to pronounced downward CSF flow, but only in the lower canal. The resulting pattern of net flow volumes during forced respiration yields upward CSF motion in the upper and downward flow in the lower spinal canal. These observations most likely reflect closely coupled CSF and venous systems as both large caval veins and their anastomosing vertebral plexus react to respiration-induced pressure changes.

Diagnosis of disk displacement using real-time MRI: Clinical report of two patients.

The clinical application of real-time magnetic resonance imaging (MRI) for the diagnosis of temporomandibular joint disk displacement (DD) with and without reduction is presented. In 2 patients with presumed DD, real-time MRI at 15 frames per second was performed during the natural opening and closing of the mouth. In one patient unilateral DD with reduction and in the other patient bilateral DD without reduction were observed. In contrast with conventional static MRI, real-time MRI moving images of temporomandibular joint DD offer comprehensive information about the dynamics of all involved structures, which in turn promises more reliable diagnoses. Real-time MRI is more rapid, more reliable, more informative, and less stressful for patients with temporomandibular disorders (TMDs).

Identification of the Upward Movement of Human CSF In Vivo and its Relation to the Brain Venous System.

CSF flux is involved in the pathophysiology of neurodegenerative diseases and cognitive impairment after traumatic brain injury, all hallmarked by the accumulation of cellular metabolic waste. Its effective disposal via various CSF routes has been demonstrated in animal models. In contrast, the CSF dynamics in humans are still poorly understood. Using novel real-time MRI, forced inspiration has been identified recently as a main driving force of CSF flow in the human brain. Exploiting technical advances toward real-time phase-contrast MRI, the current work analyzed directions, velocities, and volumes of human CSF flow within the brain aqueduct as part of the internal ventricular system and in the spinal canal during respiratory cycles. A consistent upward CSF movement toward the brain in response to forced inspiration was seen in all subjects at the aqueduct, in 11/12 subjects at thoracic level 2, and in 4/12 subjects at thoracic level 5. Concomitant analyses of CSF dynamics and cerebral venous blood flow, that is, in epidural veins at cervical level 3, uniquely demonstrated CSF and venous flow to be closely communicating cerebral fluid systems in which inspiration-induced downward flow of venous blood due to reduced intrathoracic pressure is counterbalanced by an upward movement of CSF. The results extend our understanding of human CSF flux and open important clinical implications, including concepts for drug delivery and new classifications and therapeutic options for various forms of hydrocephalus and idiopathic intracranial hypertension.SIGNIFICANCE STATEMENT Effective disposal of brain cellular waste products via CSF has been demonstrated repeatedly in animal models. However, CSF dynamics in humans are still poorly understood. A novel quantitative real-time MRI technique yielded in vivo CSF flow directions, velocities, and volumes in the human brain and upper spinal canal. CSF moved upward toward the head in response to forced inspiration. Concomitant analysis of brain venous blood flow indicated that CSF and venous flux act as closely communicating systems. The finding of a human CSF-venous network with upward CSF net movement opens new clinical concepts for drug delivery and new classifications and therapeutic options for various forms of hydrocephalus and ideopathic intracranial hypertension.

Real-time MRI of the temporomandibular joint at 15 frames per second-A feasibility study.

The purpose of this study was to develop and evaluate a novel method for real-time MRI of TMJ function at high temporal resolution and with two different contrasts. Real-time MRI was based on undersampled radial fast low angle shot (FLASH) acquisitions with iterative image reconstruction by regularized nonlinear inversion. Real-time MRI movies with T1 contrast were obtained with use of a radiofrequency-spoiled FLASH sequence, while movies with T2/T1 contrast employed a gradient-refocused FLASH version. TMJ function was characterized in 40 randomly selected volunteers by sequential 20s acquisitions of both the right and left joint during voluntary opening and closing of the mouth (in a medial, central and lateral oblique sagittal section perpendicular to the long axis of the condylar head). All studies were performed on a commercial MRI system at 3T using the standard head coil, while online reconstruction was achieved with a bypass computer fully integrated into the MRI system. As a first result, real-time MRI studies of the right and left TMJ were successfully performed in all 40 subjects (80 joints) within a total examination time per subject of only 15min. Secondly, at an in-plane resolution of 0.75mm and 5mm section thickness, the achieved temporal resolution was 66.7ms per image or 15 frames per second. Thirdly, both T1-weighted and T2/T1-weighted real-time MRI movies provided information about TMJ function such as disc position, condyle mobility and disc-condyle relationship. While T1 contrast offers a better delineation of structures during rapid jaw movements, T2/T1 contrast was rated superior for characterizing the articular disc. In conclusion, the proposed real-time MRI method may become a robust and efficient tool for the clinical assessment of TMJ function.

High-resolution myocardial T1 mapping using single-shot inversion recovery fast low-angle shot MRI with radial undersampling and iterative reconstruction.

OBJECTIVE: To develop a novel method for rapid myocardial T1 mapping at high spatial resolution. METHODS: The proposed strategy represents a single-shot inversion recovery experiment triggered to early diastole during a brief breath-hold. The measurement combines an adiabatic inversion pulse with a real-time readout by highly undersampled radial FLASH, iterative image reconstruction and T1 fitting with automatic deletion of systolic frames. The method was implemented on a 3-T MRI system using a graphics processing unit-equipped bypass computer for online application. Validations employed a T1 reference phantom including analyses at simulated heart rates from 40 to 100 beats per minute. In vivo applications involved myocardial T1 mapping in short-axis views of healthy young volunteers. RESULTS: At 1-mm in-plane resolution and 6-mm section thickness, the inversion recovery measurement could be shortened to 3 s without compromising T1 quantitation. Phantom studies demonstrated T1 accuracy and high precision for values ranging from 300 to 1500 ms and up to a heart rate of 100 beats per minute. Similar results were obtained in vivo yielding septal T1 values of 1246 ± 24 ms (base), 1256 ± 33 ms (mid-ventricular) and 1288 ± 30 ms (apex), respectively (mean ± standard deviation, n = 6). CONCLUSION: Diastolic myocardial T1 mapping with use of single-shot inversion recovery FLASH offers high spatial resolution, T1 accuracy and precision, and practical robustness and speed. Advances in knowledge: The proposed method will be beneficial for clinical applications relying on native and post-contrast T1 quantitation.

Sensitivity of Diffusion-Weighted STEAM MRI and EPI-DWI to Infratentorial Ischemic Stroke.

OBJECTIVES: To assess the sensitivity of stimulated echo acquisition mode diffusion weighted imaging (STEAM-DWI) to ischemic stroke in comparison to echo-planar imaging diffusion weighted imaging (EPI-DWI) in the infratentorial compartment. METHODS: Fifty-seven patients presenting with clinical features of infratentorial stroke underwent STEAM-DWI, high-resolution EPI-DWI (HR-DWI, 2.5 mm slice thickness) and low-resolution EPI-DWI (LR-DWI, 5 mm slice thickness). Four readers assessed the presence of ischemic lesions and artifacts. Agreement between sequences and interobserver agreement on the presence of ischemia were calculated. The sensitivities of the DWI sequences were calculated in 45 patients with a confirmed diagnosis of infratentorial stroke. RESULTS: Median time from symptom onset to imaging was 24 hours. STEAM-DWI agreed with LR-DWI in 89.5% of cases (kappa = 0.72, p<0.0001) and with HR-DWI in 89.5% of cases (kappa = 0.68, p<0.0001). STEAM-DWI showed fewer intraparenchymal artifacts (1/57) than HR-DWI (44/57) and LR-DWI (41/57). Ischemia was visible in 87% of cases for LR-DWI, 93% of cases for HR-DWI, and 89% of cases for STEAM-DWI. Interobserver agreement was good for STEAM-DWI (kappa = 0.62, p<0.0001). CONCLUSIONS: Compared to the best currently available MR sequence for detecting ischemia (HR-DWI), STEAM-DWI shows fewer artifacts and a similar sensitivity to infratentorial stroke.

Real-time magnetic resonance imaging of deep venous flow during muscular exercise-preliminary experience.

BACKGROUND: The accurate assessment of peripheral venous flow is important for the early diagnosis and treatment of disorders such as deep-vein thrombosis (DVT) which is a major cause of post-thrombotic syndrome or even death due to pulmonary embolism. The aim of this work is to quantitatively determine blood flow in deep veins during rest and muscular exercise using a novel real-time magnetic resonance imaging (MRI) method for velocity-encoded phase-contrast (PC) MRI at high spatiotemporal resolution. METHODS: Real-time PC MRI of eight healthy volunteers and one patient was performed at 3 Tesla (Prisma fit, Siemens, Erlangen, Germany) using a flexible 16-channel receive coil (Variety, NORAS, Hoechberg, Germany). Acquisitions were based on a highly undersampled radial FLASH sequence with image reconstruction by regularized nonlinear inversion at 0.5×0.5×6 mm3 spatial resolution and 100 ms temporal resolution. Flow was assessed in two cross-sections of the lower leg at the level of the calf muscle and knee using a protocol of 10 s rest, 20 s flexion and extension of the foot, and 10 s rest. Quantitative analyses included through-plane flow in the right posterior tibial, right peroneal and popliteal vein (PC maps) as well as signal intensity changes due to flow and muscle movements (corresponding magnitude images). RESULTS: Real-time PC MRI successfully monitored the dynamics of venous flow at high spatiotemporal resolution and clearly demonstrated increased flow in deep veins in response to flexion and extension of the foot. In normal subjects, the maximum velocity (averaged across vessel lumen) during exercise was 9.4±5.7 cm·s-1 for the right peroneal vein, 8.5±4.6 cm·s-1 for the right posterior tibial vein and 17.8±5.8 cm·s-1 for the popliteal vein. The integrated flow volume per exercise (20 s) was 1.9, 1.6 and 50 mL (mean across subjects) for right peroneal, right posterior tibial and popliteal vein, respectively. A patient with DVT presented with peak flow velocities of only about 2 cm·s-1 during exercise and less than 1 cm·s-1 during rest. CONCLUSIONS: Real-time PC MRI emerges as a new tool for quantifying the dynamics of muscle-induced flow in deep veins. The method provides both signal intensity changes and velocity information for the assessment of blood flow and muscle movements. It now warrants extended clinical trials to patients with suspected thrombosis.

Inspiration is the major regulator of human CSF flow.

The mechanisms behind CSF flow in humans are still not fully known. CSF circulates from its primary production sites at the choroid plexus through the brain ventricles to reach the outer surface of the brain in the subarachnoid spaces from where it drains into venous bloodstream and cervical lymphatics. According to a recent concept of brain fluid transport, established in rodents, CSF from the brain surface also enters the brain tissue along para-arterial routes and exits through paravenous spaces again into subarachnoid compartments. This unidirectional flow is mainly driven by arterial pulsation. To investigate how CSF flow is regulated in humans, we applied a novel real-time magnetic resonance imaging technique at high spatial (0.75 mm) and temporal (50 ms) resolution in healthy human subjects. We observed significant CSF flow exclusively with inspiration. In particular, during forced breathing, high CSF flow was elicited during every inspiration, whereas breath holding suppressed it. Only a minor flow component could be ascribed to cardiac pulsation. The present results unambiguously identify inspiration as the most important driving force for CSF flow in humans. Inspiratory thoracic pressure reduction is expected to directly modulate the hydrostatic pressure conditions for the low-resistance paravenous, venous, and lymphatic clearance routes of CSF. Furthermore, the experimental approach opens new clinical opportunities to study the pathophysiology of various forms of hydrocephalus and to design therapeutic strategies in relation to CSF flow alterations.

Real-time magnetic resonance imaging of cardiac function and flow-recent progress.

Cardiac structure, function and flow are most commonly studied by ultrasound, X-ray and magnetic resonance imaging (MRI) techniques. However, cardiovascular MRI is hitherto limited to electrocardiogram (ECG)-synchronized acquisitions and therefore often results in compromised quality for patients with arrhythmias or inabilities to comply with requested protocols-especially with breath-holding. Recent advances in the development of novel real-time MRI techniques now offer dynamic imaging of the heart and major vessels with high spatial and temporal resolution, so that examinations may be performed without the need for ECG synchronization and during free breathing. This article provides an overview of technical achievements, physiological validations, preliminary patient studies and translational aspects for a future clinical scenario of cardiovascular MRI in real time.

Real-time flow MRI of the aorta at a resolution of 40 msec.

PURPOSE: To evaluate a novel real-time phase-contrast magnetic resonance imaging (MRI) technique for the assessment of through-plane flow in the ascending aorta. MATERIALS AND METHODS: Real-time MRI was based on a radial fast low-angle shot (FLASH) sequence with about 30-fold undersampling and image reconstruction by regularized nonlinear inversion. Phase-contrast maps were obtained from two (interleaved or sequential) acquisitions with and without a bipolar velocity-encoding gradient. Blood flow in the ascending aorta was studied in 10 healthy volunteers at 3 T by both real-time MRI (15 sec during free breathing) and electrocardiogram (ECG)-synchronized cine MRI (with and without breath holding). Flow velocities and stroke volumes were evaluated using standard postprocessing software. RESULTS: The total acquisition time for a pair of phase-contrast images was 40.0 msec (TR/TE = 2.86/1.93 msec, 10° flip angle, 7 spokes per image) for a nominal in-plane resolution of 1.3 mm and a section thickness of 6 mm. Quantitative evaluations of spatially averaged flow velocities and stroke volumes were comparable for real-time and cine methods when real-time MRI data were averaged across heartbeats. For individual heartbeats real-time phase-contrast MRI resulted in higher peak velocities for values above 120 cm s(-1). CONCLUSION: Real-time phase-contrast MRI of blood flow in the human aorta yields functional parameters for individual heartbeats. When averaged across heartbeats real-time flow velocities and stroke volumes are comparable to values obtained by conventional cine MRI.

Real-time phase-contrast MRI of cardiovascular blood flow using undersampled radial fast low-angle shot and nonlinear inverse reconstruction.

Velocity-encoded phase-contrast MRI of cardiovascular blood flow commonly relies on electrocardiogram-synchronized cine acquisitions of multiple heartbeats to quantitatively determine the flow of an averaged cardiac cycle. Here, we present a new method for real-time phase-contrast MRI that combines flow-encoding gradients with highly undersampled radial fast low-angle shot acquisitions and phase-sensitive image reconstructions by regularized nonlinear inversion. Apart from calibration studies using steady and pulsatile flow, preliminary in vivo applications focused on through-plane flow in the ascending aorta of healthy subjects. With bipolar velocity-encoding gradients of alternating polarity that overlap the slice-refocusing gradient, the method yields flow-encoded images with an in-plane resolution of 1.8 mm, section thickness of 6 mm and measuring time at 3 T of 24 ms (TR/TE = 3.44/2.76 ms; flip angle, 10º; seven radial spokes per image). Accordingly, phase-contrast maps and corresponding velocity profiles achieve a temporal resolution of 48 ms. The evaluated peak velocities, stroke volumes, flow rates and respective variances over at least 20 consecutive heartbeats are in general agreement with literature data.

Spatially encoded phase-contrast MRI-3D MRI movies of 1D and 2D structures at millisecond resolution.

This work demonstrates that the principles underlying phase-contrast MRI may be used to encode spatial rather than flow information along a perpendicular dimension, if this dimension contains an MRI-visible object at only one spatial location. In particular, the situation applies to 3D mapping of curved 2D structures which requires only two projection images with different spatial phase-encoding gradients. These phase-contrast gradients define the field of view and mean spin-density positions of the object in the perpendicular dimension by respective phase differences. When combined with highly undersampled radial fast low angle shot (FLASH) and image reconstruction by regularized nonlinear inversion, spatial phase-contrast MRI allows for dynamic 3D mapping of 2D structures in real time. First examples include 3D MRI movies of the acting human hand at a temporal resolution of 50 ms. With an even simpler technique, 3D maps of curved 1D structures may be obtained from only three acquisitions of a frequency-encoded MRI signal with two perpendicular phase encodings. Here, 3D MRI movies of a rapidly rotating banana were obtained at 5 ms resolution or 200 frames per second. In conclusion, spatial phase-contrast 3D MRI of 2D or 1D structures is respective two or four orders of magnitude faster than conventional 3D MRI.

Basal cerebral blood volume during the poststimulation undershoot in BOLD MRI of the human brain.

One of the characteristics of the blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) response to functional challenges of the brain is the poststimulation undershoot, which has been suggested to originate from a delayed recovery of either cerebral blood volume (CBV) or cerebral metabolic rate of oxygen to baseline. Using bolus-tracking MRI in humans, we recently showed that relative CBV rapidly normalizes after the end of stimulation. As this observation contradicts at least part of the blood-pool contrast agent studies performed in animals, we reinvestigated the CBV contribution by dynamic T1-weighted three-dimensional MRI (8 seconds temporal resolution) and Vasovist at 3 T (12 subjects). Initially, we determined the time constants of individual BOLD responses. After injection of Vasovist, CBV-related T1-weighted signal changes revealed a signal increase during visual stimulation (1.7% ± 0.4%), but no change relative to baseline in the poststimulation phase (0.2% ± 0.3%). This finding renders the specific nature of the contrast agent unlikely to be responsible for the discrepancy between human and animal studies. With the assumption of normalized cerebral blood flow after stimulus cessation, a normalized CBV lends support to the idea that the BOLD MRI undershoot reflects a prolonged elevation of oxidative metabolism.

Real-time MRI at a resolution of 20 ms.

The desire to visualize noninvasively physiological processes at high temporal resolution has been a driving force for the development of MRI since its inception in 1973. In this article, we describe a unique method for real-time MRI that reduces image acquisition times to only 20 ms. Although approaching the ultimate limit of MRI technology, the method yields high image quality in terms of spatial resolution, signal-to-noise ratio and the absence of artifacts. As proposed previously, a fast low-angle shot (FLASH) gradient-echo MRI technique (which allows for rapid and continuous image acquisitions) is combined with a radial encoding scheme (which offers motion robustness and moderate tolerance to data undersampling) and, most importantly, an iterative image reconstruction by regularized nonlinear inversion (which exploits the advantages of parallel imaging with multiple receiver coils). In this article, the extension of regularization and filtering to the temporal domain exploits consistencies in successive data acquisitions and thereby enhances the degree of radial undersampling in a hitherto unexpected manner by one order of magnitude. The results obtained for turbulent flow, human speech production and human heart function demonstrate considerable potential for real-time MRI studies of dynamic processes in a wide range of scientific and clinical settings.

Real-time cardiovascular magnetic resonance at high temporal resolution: radial FLASH with nonlinear inverse reconstruction.

BACKGROUND: Functional assessments of the heart by dynamic cardiovascular magnetic resonance (CMR) commonly rely on (i) electrocardiographic (ECG) gating yielding pseudo real-time cine representations, (ii) balanced gradient-echo sequences referred to as steady-state free precession (SSFP), and (iii) breath holding or respiratory gating. Problems may therefore be due to the need for a robust ECG signal, the occurrence of arrhythmia and beat to beat variations, technical instabilities (e.g., SSFP "banding" artefacts), and limited patient compliance and comfort. Here we describe a new approach providing true real-time CMR with image acquisition times as short as 20 to 30 ms or rates of 30 to 50 frames per second. METHODS: The approach relies on a previously developed real-time MR method, which combines a strongly undersampled radial FLASH CMR sequence with image reconstruction by regularized nonlinear inversion. While iterative reconstructions are currently performed offline due to limited computer speed, online monitoring during scanning is accomplished using gridding reconstructions with a sliding window at the same frame rate but with lower image quality. RESULTS: Scans of healthy young subjects were performed at 3 T without ECG gating and during free breathing. The resulting images yield T1 contrast (depending on flip angle) with an opposed-phase or in-phase condition for water and fat signals (depending on echo time). They completely avoid (i) susceptibility-induced artefacts due to the very short echo times, (ii) radiofrequency power limitations due to excitations with flip angles of 10 degrees or less, and (iii) the risk of peripheral nerve stimulation due to the use of normal gradient switching modes. For a section thickness of 8 mm, real-time images offer a spatial resolution and total acquisition time of 1.5 mm at 30 ms and 2.0 mm at 22 ms, respectively. CONCLUSIONS: Though awaiting thorough clinical evaluation, this work describes a robust and flexible acquisition and reconstruction technique for real-time CMR at the ultimate limit of this technology.

Efficacy of EMG-triggered electrical arm stimulation in chronic hemiparetic stroke patients.

PURPOSE: EMG-triggered electrostimulation (EMG-ES) may improve the motor performance of affected limbs of hemiparetic stroke patients even in the chronic stage. This study was designed to characterize cortical activation changes following intensified EMG-ES in chronic stroke patients and to identify predictors for successful rehabilitation depending on disease severity. METHODS: We studied 9 patients with severe residual hemiparesis, who underwent 8 weeks of daily task-orientated multi-channel EMG-ES of the paretic arm. Before and after treatment, arm function was evaluated clinically and cortical activation patterns were assessed with functional MRI (fMRI) and/or transcranial magnetic stimulation (TMS). RESULTS: As response to therapy, arm function improved in a subset of patients with more capacity in less affected subjects, but there was no significant gain for those with Box & Block test values below 4 at inception. The clinical improvement, if any, was accompanied by an ipsilesional increase in the sensorimotor cortex (SMC) activation area in fMRI and enhanced intracortical facilitation (ICF) as revealed by paired TMS. The SMC activation change in fMRI was predicted by the presence or absence of motor-evoked potentials (MEPs) on the affected side. CONCLUSIONS: The present findings support the notion that intensified EMG-ES may improve the arm function in individual chronic hemiparetic stroke patients but not in more severely impaired individuals. Functional improvements are paralleled by increased ipsilesional SMC activation and enhanced ICF supporting neuroplasticity as contributor to rehabilitation. The clinical score at inception and the presence of MEPs have the best predictive potential.

TTC post-processing is beneficial for functional MRI at low magnetic field: a comparative study at 1 T and 3 T.

This study aimed to broaden the diagnostic possibilities of low-field MRI systems (i) by examining the feasibility of functional MRI of human brain activation at 1 T, and (ii) by assessing its reliability in comparison with acquisitions at 3 T. Eight subjects were studied at 1 T and 3T using standard echo-planar-imaging sequences at 3-mm isotropic spatial resolution. Paradigms included silent word generation, sequential finger-to-thumb opposition, and passive finger movements. Image post-processing was carried out either with statistical parametric mapping (SPM5, single-subject and group analysis) or with a two-threshold correlation (TTC, single-subject analysis only) analysis. Single-subject analysis with SPM5 resulted in 3-5 times more activated pixels at 3 T than at 1 T in the examined Broca and sensorimotor regions. By comparison, the TTC single-subject analysis yielded the same amount of activated pixels at 3 T and 1 T. Moreover, this number was identical to that obtained with SPM at 3 T. The group analysis with SPM5 resulted in very similar numbers of activated pixels at both field strengths. The present findings suggest that a field strength of 1 T combined with adequate post-processing allows for reliable functional MRI studies of human brain activation. High-field advantages are therefore best invested in higher spatial resolution.