Resolving fiber crossing using advanced fast marching tractography based on diffusion tensor imaging.

Magnetic resonance diffusion tensor tractography is a powerful tool for the non-invasive depiction of the white matter architecture in the human brain. However, due to limitations in the underlying tensor model, the technique is often unable to reconstruct correct trajectories in heterogeneous fiber arrangements, such as axonal crossings. A novel tractography method based on fast marching (FM) is proposed which is capable of resolving fiber crossings and also permits trajectories to branch. It detects heterogeneous fiber arrangements by incorporating information from the entire diffusion tensor. The FM speed function is adapted to the local tensor characteristics, allowing in particular to maintain the front evolution direction in crossing situations. In addition, the FM's discretization error is reduced by increasing the number of considered possible front evolution directions. The performance of the technique is demonstrated in artificial data and in the healthy human brain. Comparisons with standard FM tractography and conventional line propagation algorithms show that, in the presence of interfering structures, the proposed method is more accurate in reconstructing trajectories. The in vivo results illustrate that the elucidated major white matter pathways are consistent with known anatomy and that multiple crossings and tract branching are handled correctly.

SENSE-DTI at 3 T.

While holding vast potential, diffusion tensor imaging (DTI) with single-excitation protocols still faces serious challenges. Limited spatial resolution, susceptibility to magnetic field inhomogeneity, and low signal-to-noise ratio (SNR) may be considered the most prominent limitations. It is demonstrated that all of these shortcomings can be effectively mitigated by the transition to parallel imaging technology and high magnetic field strength. Using the sensitivity encoding (SENSE) technique at 3 T, brain DTI was performed in nine healthy volunteers. Despite enhanced field inhomogeneity, parallel acquisition permitted both controlling geometric distortions and enhancing spatial resolution up to 0.8 mm in-plane. Heightened SNR requirements were met in part by high base sensitivity at 3 T. A further significant increase in SNR efficiency was accomplished by SENSE acquisition, exploiting enhanced encoding speed for echo time reduction. Based on the resulting image data, high-resolution tensor mapping is demonstrated.

Somatomotor functional MRI in a large congenital arachnoid cyst.

The immature human brain, when damaged, is able to reorganise functionally. We performed functional MRI during eight different movements in a patient found incidentally to have an extensive, frontal, congenital arachnoid cyst, looking at which neural substrates contribute to motor control. Significant changes from the normal pattern of activation were seen in cortical and cerebellar areas which could not be accounted for by the space-occupying effect of the cyst alone. These findings in this asymptomatic patient with a congenital anomaly demonstrate an alternative organisation of the central motor system, with a preservation of neurological function.

Identification of multiple nonprimary motor cortical areas with simple movements.

The human cortex reportedly contains at least five nonprimary motor areas: in the frontolateral convexity, the dorsal and ventral premotor cortex (PMd and PMv), and in the frontomesial wall, the presupplementary and supplementary motor areas (pre-SMA and SMA), and the rostral, dorsal and ventral cingulate areas (CMAr, CMAd, and CMAv). Activation of these regions in neuroimaging studies has been generally associated either with the performance of complex motor tasks or with reorganization occurring with motor recovery in the presence of pathology. Recent evidence from neuroimaging studies suggests that the same areas are activated with well controlled simple movements in healthy subjects providing support to the observation that their contribution may be more quantitative rather than exclusively specific to a certain aspect of motor behaviour. An important consequence of this observation is that activation of multiple nonprimary motor areas during simple motor tasks should not be considered unique to patients with upper or lower motoneuron lesions but rather as a normal physiological process.

An inverse problem approach to the correction of distortion in EPI images.

Magnetic resonance imaging using the echo planar imaging (EPI) technique is particularly sensitive to main (B0) field inhomogeneities. The primary effect is geometrical distortion in the phase encoding direction. In this paper, we present a method based on the conjugate gradient algorithm to correct for this geometrical distortion, by solving the EPI imaging equation. Two versions are presented: one that attempts to solve the full four-dimensional (4-D) imaging equation, and one that independently solves for each profile along the blip encoding direction. Results are presented for both phantom and in vivo brain EPI images and compared with other proposed correction methods.

Plasticity of the human motor cortex in patients with arteriovenous malformations: a functional MR imaging study.

BACKGROUND AND PURPOSE: The capacity of the human brain to recover from damage has been explained on the basis of plasticity, according to which remaining areas assume functions that would normally have been performed by the damaged brain. Patients with cerebral arteriovenous malformations (AVMs) involving primary motor areas may present without significant neurologic deficits. We used functional MR imaging to investigate the organization of cortical motor areas in patients with AVMs. METHODS: Cortical motor hand and foot representations were mapped in nine right-handed patients harboring AVMs occupying the hand (n = 6) or foot (n = 3) region of the primary motor cortex (M1). None of the patients exhibited motor deficits. Simple movements of the hand and foot were performed. In eight patients, both right and left extremities were tested; in one patient, only the hand contralateral to the AVM was examined. Localization of activation in the affected hemisphere was compared with that in the unaffected hemisphere and evaluated with respect to the normal M1 somatotopic organization shown in earlier functional MR imaging investigations. RESULTS: Cortical activation showed three patterns: 1) functional displacement within the affected M1 independent of the structural distortion induced by the AVM (n = 4), 2) presence of activation within the unaffected M1 ipsilateral to the moving extremity without activation in the affected M1 (n = 3), and 3) prominent activation in nonprimary motor areas without activation in either the affected or unaffected M1 (n = 2). CONCLUSION: Preliminary evidence suggests that brain AVMs lead to reorganization within the somatotopic representation in M1 and to occasional abnormal expansion into nonprimary motor areas.

PRESTO-SENSE: an ultrafast whole-brain fMRI technique.

A new ultrafast MR imaging method is proposed and tested, which enables whole-brain fMRI with a true temporal resolution of 1 sec. The method combines a 3D PRESTO pulse sequence with the concept of sensitivity-encoding with multiple receiver coils (SENSE). The so-called PRESTO-SENSE technique is demonstrated on a set of functional block-type motor and visual experiments and compared with conventional functional imaging techniques, such as PRESTO and EPI. Comparable image quality and activation areas are found with all sequences. The noise characteristics of the proposed method are analyzed in detail and their implications for ultrafast fMRI studies are discussed. Magn Reson Med 43:779-786, 2000.

Beyond re-membering: phantom sensations of congenitally absent limbs.

Phantom limbs are traditionally conceptualized as the phenomenal persistence of a body part after deafferentation. Previous clinical observations of subjects with phantoms of congenitally absent limbs are not compatible with this view, but, in the absence of experimental work, the neural basis of such "aplasic phantoms" has remained enigmatic. In this paper, we report a series of behavioral, imaging, and neurophysiological experiments with a university-educated woman born without forearms and legs, who experiences vivid phantom sensations of all four limbs. Visuokinesthetic integration of tachistoscopically presented drawings of hands and feet indicated an intact somatic representation of these body parts. Functional magnetic resonance imaging of phantom hand movements showed no activation of primary sensorimotor areas, but of premotor and parietal cortex bilaterally. Movements of the existing upper arms produced activation expanding into the hand territories deprived of afferences and efferences. Transcranial magnetic stimulation of the sensorimotor cortex consistently elicited phantom sensations in the contralateral fingers and hand. In addition, premotor and parietal stimulation evoked similar phantom sensations, albeit in the absence of motor evoked potentials in the stump. These data indicate that body parts that have never been physically developed can be represented in sensory and motor cortical areas. Both genetic and epigenetic factors, such as the habitual observation of other people moving their limbs, may contribute to the conscious experience of aplasic phantoms.

Investigation of BOLD signal dependence on cerebral blood flow and oxygen consumption: the deoxyhemoglobin dilution model.

The relationship between blood oxygenation level-dependent (BOLD) MRI signals, cerebral blood flow (CBF), and oxygen consumption (CMR(O2)) in the physiological steady state was investigated. A quantitative model, based on flow-dependent dilution of metabolically generated deoxyhemoglobin, was validated by measuring BOLD signals and relative CBF simultaneously in the primary visual cortex (V1) of human subjects (N = 12) during graded hypercapnia at different levels of visual stimulation. BOLD and CBF responses to specific conditions were averaged across subjects and plotted as points in the BOLD-CBF plane, tracing out lines of constant CMR(O2). The quantitative deoxyhemoglobin dilution model could be fit to these measured iso-CMR(O2) contours without significant (P

Evaluation of gastric emptying and motility in diabetic gastroparesis with magnetic resonance imaging: effects of cisapride.

OBJECTIVE: The motor mechanisms that underlie both slow gastric emptying in diabetic gastroparesis and its acceleration by cisapride are poorly understood. We have recently shown that magnetic resonance imaging (MRI) allows concurrent evaluation of both gastric emptying and regional gastric motility. METHODS: Emptying and motility were measured in eight diabetic patients with previously demonstrated delayed gastric emptying using a rapid MRI technique during oral administration of cisapride and placebo. Studies were performed in a double blind fashion and each patient acted as his own control. Subjects were studied supine for 120 min in a 1.5 Tesla MRI scanner after ingestion of 500 ml of 10% Intralipid. Gastric emptying corrected for the volume of secretions was determined every 15 min using transaxial scans. Each transaxial scan was followed by 120 coronal scans at 1 s intervals. Coronal scans were angled to provide simultaneous imaging of the proximal and distal stomach. MRI studies were also performed in seven diabetic patients with normal emptying who served as disease controls. RESULTS: Emptying was slower in the gastroparetic patients (t(1/2): 124 +/- 10 min) compared to patients with normal emptying (81 +/- 9 min, p < 0.05). Cisapride accelerated gastric emptying (74 +/- 5 vs 124 +/- 10 min) in patients with gastroparesis. The contraction amplitudes in the proximal stomach of gastroparetic patients were increased during cisapride treatment (17.2% +/- 1.8% vs 13.2% +/- 0.6%; p < 0.02), whereas antral contraction frequency, amplitude, and velocity were unchanged. CONCLUSIONS: We conclude that cisapride-induced acceleration of liquid gastric emptying in diabetic gastroparesis does not appear to result from changes in antral contractility, but may be related to changes in proximal gastric tone or gastric outlet resistance.

Linear coupling between cerebral blood flow and oxygen consumption in activated human cortex.

The aim of this study was to test the hypothesis that, within a specific cortical unit, fractional changes in cerebral blood flow (CBF) and cerebral metabolic rate of oxygen consumption (CMR(O(2))) are coupled through an invariant relationship during physiological stimulation. This aim was achieved by simultaneously measuring relative changes in these quantities in human primary visual cortex (V1) during graded stimulation with patterns designed to selectively activate different populations of V1 neurons. Primary visual cortex was delineated individually in each subject by using phase-encoded retinotopic mapping. Flow-sensitive alternating inversion recovery MRI, in conjunction with blood oxygenation-sensitive MRI and hypercapnic calibration, was used to monitor CBF and CMR(O(2)). The stimuli used included (i) diffuse isoluminant chromatic displays; (ii) high spatial-frequency achromatic luminance gratings; and (iii) radial checkerboard patterns containing both color and luminance contrast modulated at different temporal rates. Perfusion responses to each pattern were graded by varying luminance and/or color modulation amplitudes. For all stimulus types, fractional changes in blood flow and oxygen uptake were found to be linearly coupled in a consistent ratio of approximately 2:1. The most potent stimulus produced CBF and CMR(O(2)) increases of 48 +/- 5% and 25 +/- 4%, respectively, with no evidence of a plateau for oxygen consumption. Estimation of aerobic ATP yields from the observed CMR(O(2)) increases and comparison with the maximum possible anaerobic ATP contribution indicate that elevated energy demands during brain activation are met largely through oxidative metabolism.

Stimulus-dependent BOLD and perfusion dynamics in human V1.

Blood oxygenation level-dependent (BOLD) fMRI signals often exhibit pronounced over- or undershoot upon changes in stimulation state. Current models postulate that this is due to the delayed onset or decay of perfusion-dependent attenuating responses such as increased cerebral blood volume or oxygen consumption, which are presumed to lag behind the rapid adjustment of blood flow rate to a new steady-state level. If this view is correct, then BOLD overshoot amplitudes in a specific tissue volume should be correlated with steady-state increases in perfusion, independent of stimulus type. To test this prediction, we simultaneously recorded BOLD and relative perfusion signals in primary visual cortex while inducing graded perfusion increases with three types of visual stimulus. Two of these, a diffuse chromatic stimulus with no luminance variation and a very high spatial frequency luminance grating, did not produce detectable BOLD overshoot (or undershoot) when an equal mean luminance baseline was used. Radial checkerboard stimuli, however, caused pronounced over/undershoot of both BOLD and perfusion signals even when temporal mean luminance was held constant and stimulus contrast was adjusted to produce the same steady-state blood flow increases evoked by the other stimuli. Transient amplitudes were relatively invariant in spite of large changes in steady-state response, demonstrating nonlinear BOLD and perfusion step responses in human V1. These findings suggest that, rather than a purely tissue-specific biomechanical or metabolic phenomenon, BOLD overshoot and undershoot represent transient features in the perfusion signal whose effects may be amplified by slowly evolving blood volume changes.

Perfusion-based functional magnetic resonance imaging with single-shot RARE and GRASE acquisitions.

For perfusion-based functional magnetic resonance imaging, the previously introduced flow-sensitive alternating inversion recovery (FAIR) technique is combined with single-shot RARE (rapid acquisition with relaxation enhancement) and GRASE (gradient and spin echo) imaging sequences. The advantages of these sequences compared to commonly used echo-planar imaging (EPI) are an increased signal-to-noise ratio and the absence of distortions and artifacts due to magnetic field inhomogeneities. RARE- and GRASE-FAIR are applied to functional brain mapping studies in humans during visual stimulation. Results demonstrate that the presented techniques allow for perfusion maps with higher spatial resolution compared to EPI-FAIR. Relative regional cerebral blood flow change in the occipital cortex during visual stimulation was measured to be 41+/-4% (n = 5). The comparison of FAIR data obtained with RARE and GRASE techniques shows that RARE yields images with the higher signal-to-noise ratio. However, the GRASE technique features a shorter acquisition time and less RF power deposition and is thus better suited for multi-slice acquisitions.

Gastric emptying and motility: assessment with MR imaging--preliminary observations.

PURPOSE: To evaluate a magnetic resonance (MR) imaging method for simultaneous assessment of gastric emptying and motility. MATERIALS AND METHODS: Gastric emptying and motility were measured in nine volunteers after ingestion of a liquid meal. A specially designed MR imaging protocol was used that allowed simultaneous assessment of gastric emptying (spatial resolution, 1.5 mm; corrected for gastric secretion volume) and gastric motility (temporal resolution, 1.2 seconds; spatial resolution, 3.1 mm). To evaluate the ability to detect small changes in gastric motor activity with findings from this method, the influence of a prokinetic agent (loxiglumide) on gastric emptying and motility was tested in five volunteers. RESULTS: Each contraction could be individually visualized at MR imaging. Administration of loxiglumide resulted in decreased gastric half-emptying time (mean +/- 1 standard error of the mean, 88.1 minutes +/- 6.3 for the placebo and 39.1 minutes +/- 6.7 for loxiglumide) and increased gastric motility (contraction frequency, 2.26 contractions per minute +/- 0.15 for the placebo and 3.04 per minute +/- 0.04 for loxiglumide). CONCLUSION: MR imaging makes it feasible to study gastric emptying and gastric motility and to determine the influence of drugs on gastric motor activity.

Role of cholecystokinin in the regulation of liquid gastric emptying and gastric motility in humans: studies with the CCK antagonist loxiglumide.

BACKGROUND: Exogenous cholecystokinin (CCK) inhibits antral motility and slows gastric emptying (GE) but the effect of endogenous CCK on the gastric motor mechanisms responsible for GE remains unclear. METHODS: The effect of the CCK-A antagonist loxiglumide (LOX) on GE and motility was studied using magnetic resonance imaging in six healthy volunteers after ingestion of 500 ml Intralipid 10% (550 kcal). Subjects were studied in the supine position on two occasions during intravenous infusion of LOX (66 mumol/kg/h for 10 min followed by 22 mumol/kg/h) or placebo. GE was determined every 15 minutes using transaxial abdominal scans and motility was studied by means of 120 coronal scans, 1.2 seconds apart. For each coronal image the proximal and distal (antral) diameters were measured at a fixed point in the stomach to determine contraction frequency (ACF) and amplitude (AMP). RESULTS: GE was faster during LOX infusion than placebo (t1/2 31 (22) versus 115 (67) minutes, p < 0.03). There was little variation in the diameter of the proximal stomach with either LOX or placebo. In the distal stomach marked contractile activity was observed during LOX (ACF 2.9 (0.2) versus 1.5 (2.9) during placebo, p < 0.01). AMP also increased during LOX compared with placebo (56 (22)% versus 27 (16)%, p < 0.001). CONCLUSION: The increases in antral motility are likely to contribute to the acceleration of GE and suggest that CCK may regulate GE by acting on the distal stomach although an effect on the proximal stomach cannot be excluded.

Evaluation of delayed gastric emptying in diabetic patients with autonomic neuropathy by a new magnetic resonance imaging technique and radio-opaque markers.

OBJECTIVE: Our objective was to validate a new noninvasive magnetic resonance imaging (MRI) technique for diagnosis of delayed gastric emptying by using radio-opaque markers (ROMs) in diabetic patients with and without cardiovascular autonomic (CAN) and peripheral sensomotoric neuropathy (PSN). RESEARCH DESIGN AND METHODS: Fifteen diabetic outpatients were recruited, eight with CAN and PSN (group A, age 28-61 years, mean diabetes duration 27 years) and seven without CAN (group B, age 28-60 years, mean diabetes duration 16 years). Gastric emptying and motility were assessed with ROMs and MRI in random order. After an overnight fast either a test meal (451 kcal) containing a capsule with 10 ROMs is eaten and a supine plain abdominal X ray is taken after 6 h or 500 ml intralipid 10% (550 kcal) is swallowed for the MRI study, using a 1.5 Tesla Gyroscan ACS II (Philips, Eindohoven, The Netherlands). Computer-assisted segmentation of images was used to measure gastric emptying (T1/2, min) over 125 min, contraction frequency (F, min-1), mean contraction amplitude (CA, % basal), and velocity (V, cm/s). Blood glucose was kept constant at 5.0-8.0 mmol/l. RESULTS: In group A, 6.1 +/- 1.36 ROMs (mean +/- SE) were retained in the stomach after 6 h and 0 ROM in group B, indicating a significant delay of gastric emptying in patients with CAN. The MRI study revealed a significantly longer gastric emptying (P < 0.005) in group A (T1/2 = 124 +/- 10 min) as compared with group B (T1/2 = 85 +/- 18 min). There was no difference in F, CA, and V between the two groups: F 2.9 +/- 0.07 and 2.7 +/- 0.1 (min-1), CA 26.8 +/- 1.2 and 29.6 +/- 1.6 (% basal), V 0.43 +/- 0.02 and 0.40 +/- 0.02 (cm/s), respectively. CONCLUSIONS: MRI offers the possibility of visualizing and examining exactly the mechanisms responsible for gastric emptying and is characterized by a high specificity but a lower sensitivity as compared with ROMs, which proved to be an ideal screening test for diagnosis of gastroparesis in clinical practice.

Measurement of proximal and distal gastric motility with magnetic resonance imaging.

The precise motor mechanisms associated with gastric emptying of nutrient liquids are unclear, in part because of difficulties in measuring the motility from the proximal and distal stomach simultaneously. We have now examined proximal and distal gastric motility, using a novel magnetic resonance imaging (MRI) technique. In seven healthy volunteers (4 males, 3 females; 27-37 yr), gastric emptying and motility were determined on two occasions after ingestion of 500 ml 10% and 25% dextrose labeled with 1 mM gadolinium tetraazacyclododecane tetraacetic acid, using a 1.5-tesla Philips Gyroscan ACS II scanner. Gastric emptying was determined every 15 min with a series of transaxial scans. After each series of transaxial scans, 120 coronal scans, 1.2 s apart, were performed through the antrum and proximal stomach. For each coronal slice the diameters of the proximal stomach and the antrum were measured to determine the number of contractions per minute and depth (%basal diameter). Gastric emptying (half-emptying time) was faster after ingestion of 10% compared with 25% dextrose (49 +/- 15 vs. 118 +/- 37 min; P < 0.01). After both meals, the diameter of the proximal stomach remained relatively constant, whereas there were marked fluctuations in the diameter of the antrum. Mean (+/- SD) frequency (2.8 +/- 0.6 vs. 2.0 +/- 0.8/min; P < 0.001) and depth (40 +/- 17% vs. 34 +/- 16%; P < 0.04) of antral contractions were higher after 10% dextrose compared with 25% dextrose. Rapid MRI techniques allow simultaneous measurement of both gastric emptying and motor function of different gastric regions. The increase in the frequency and depth of distal gastric contractions during ingestion of 10% compared with 25% dextrose supports the concept that the antrum contributes to the regulation of gastric emptying of nutrient liquids.

Real-time image reconstruction system for interventional magnetic resonance surgery.

Interventional surgery techniques under the control of fast acquired magnetic resonance (MR) images may become important in interventional radiology in the near future. One of the components needed for an interventional MR scanner is the real-time reconstruction of the acquired MR images. However up to now no real-time reconstruction systems are readily available for MR images. Therefore a reconstruction device was developed, which allows reconstruction and display of MR images with a delay of less than 50 ms. Additional to the high performance, the main characteristics of the presented device are its full compatibility with different MR acquisition techniques and its moderate cost. The device can be operated with most types of commercial scanners. It is especially suited for interventional MR systems but has also applications with conventional MR scanners.

Measurement of gastric emptying and gastric motility by magnetic resonance imaging (MRI).

Hitherto it has been impossible to measure noninvasively gastric emptying and motility from multiple regions of the stomach in humans. We describe the development of a novel methodology to achieve this using magnetic resonance imaging (MRI). Initial validation studies performed in five healthy volunteers demonstrated similar gastric emptying curves and secretion rates after ingestion of 10% dextrose labeled with [Gd]DOTA, assessed by MRI ([Gd]DOTA as meal marker) and a simultaneous double-indicator technique. Comparison between MRI and scintigraphy in five patients also gave similar results. Application of newer MRI technology allowing a series of coronal scans 1.2 sec apart permitted quantification of wall motion in the proximal and distal stomach in seven healthy volunteers. These results indicate that MRI provides new insights into the mechanisms responsible for both normal and disordered emptying.