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Biomedical subjects

Xavier Golay

Publications and source records attributed to Xavier Golay.

36 records · Page 2Linked to original sources
PubMed2004

Flow territory mapping of the cerebral arteries with regional perfusion MRI.

BACKGROUND AND PURPOSE: Conventional contrast-enhanced angiography is the gold standard for visualization of the vascular tree supplied by the major cerebral arteries and assessment of collateral flow. Thus far, however, no methods are available to assess the actual flow territories of the individual cerebral arteries. In the present study, we evaluate a noninvasive arterial spin labeling MRI method for selective mapping of the flow territories of the left and right internal carotid arteries and posterior circulation (basilar artery and vertebral arteries). METHODS: A spatially selective labeling approach, regional perfusion imaging, was developed on the basis of selective slab inversion of the arterial water with a pulsed arterial spin labeling sequence. The selectivity of this method was demonstrated. RESULTS: Regional perfusion imaging enables assessment of the perfusion territories of the major cerebral arteries. With selective labeling of an internal carotid artery, signal is present in both the ipsilateral anterior cerebral artery and ipsilateral middle cerebral artery flow territory. With labeling of the basilar artery, perfusion-weighted signal is symmetrically present in both posterior cerebral artery flow territories. Cerebral blood flow values measured with regional perfusion imaging in the complete hemisphere (40.1 mL x min(-1) x 100 g(-1) tissue), white matter (22.1 mL x min(-1) x 100 g(-1) tissue), and gray matter (65.8 mL x min(-1) x 100 g(-1) tissue) are in agreement with data in the literature. CONCLUSIONS: We present the first imaging method capable of evaluating both quantitatively and qualitatively the flow territories of the individual brain-feeding arteries in vivo.

Adult↗
PubMed2004

Multiple acquisitions with global inversion cycling (MAGIC): a multislice technique for vascular-space-occupancy dependent fMRI.

Recently, a new fMRI technique, termed vascular-space-occupancy (VASO), was introduced that uses T1-based blood nulling to detect cerebral blood volume (CBV) changes during brain activity. However, similar to other T1-preparation methods, this technique is hampered by the fact that there is only one zero-crossing on the relaxation curve, presently limiting its application to single-slice studies. A multislice VASO-fMRI method is presented that employs a series of nonselective 180 degrees pulses to periodically invert the magnetization and maintain it around zero, while acquiring slices in between. The effects of magnetization transfer and signal contamination by stimulated echoes are discussed. Solutions to reduce the effect of T1-signal decay as a function of slice number are provided. Phantom data show excellent agreement between experiments and numerical simulations. Multislice VASO-fMRI images of visual stimulation show effective blood nulling in all slices and appropriate functional activations in all volunteers (n=4).

Blood Volume↗
PubMed2004

Determining the longitudinal relaxation time (T1) of blood at 3.0 Tesla.

It is important to determine the longitudinal relaxation time of blood for black blood imaging, as well as for quantifying blood flow by arterial spin labeling (ASL). In this study a circulation system was used to measure blood T1 under physiological conditions at the new clinical field strength of 3.0T. It was found that 1/T1 in s(-1) was linearly dependent (P < 0.05) on hematocrit (Hct) within a normal range of 0.38-0.46. The relationships were 1/T1 = (0.52 +/- 0.15). Hct + (0.38 +/- 0.06) and 1/T1 = (0.83 +/- 0.07). Hct + (0.28 +/- 0.03) for arterial (oxygenation = 92% +/- 7%) and venous blood (69% +/- 8%), respectively, which led to estimated T1 values of 1664 +/- 14 ms (arterial) and 1584 +/- 5 ms (venous) at a typical human Hct of 0.42. The temperature dependencies of blood T1 were 22.3 +/- 0.6 ms/ degrees C and 19.8 +/- 0.8 ms/ degrees C for Hct values of 0.42 and 0.38, respectively. When a head coil transmit/receive setup was used, radiation damping caused a slight reduction (19 ms) of the measured T1 values.

Animals↗
PubMed2004

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.

Artifacts↗
PubMed2004

Perfusion imaging using arterial spin labeling.

Arterial spin labeling is a magnetic resonance method for the measurement of cerebral blood flow. In its simplest form, the perfusion contrast in the images gathered by this technique comes from the subtraction of two successively acquired images: one with, and one without, proximal labeling of arterial water spins after a small delay time. Over the last decade, the method has moved from the experimental laboratory to the clinical environment. Furthermore, numerous improvements, ranging from new pulse sequence implementations to extensive theoretical studies, have broadened its reach and extended its potential applications. In this review, the multiple facets of this powerful yet difficult technique are discussed. Different implementations are compared, the theoretical background is summarized, and potential applications of various implementations in research as well as in the daily clinical routine are proposed. Finally, a summary of the new developments and emerging techniques in this field is provided.

Animals↗
PubMed2004

Parallel imaging techniques in functional MRI.

Originally developed for increased scanning velocity in cardiac imaging, parallel imaging (PI) techniques have recently also been applied for the reduction of artifacts in single-shot techniques. In functional brain imaging (fMRI) techniques, PI has been used for several purposes. It has been applied to reduce the distortions caused by the length of the echo-planar imaging readout, diminution of the gradient-related acoustic noise, as a means to increase acquisition speed or to increase the achievable brain coverage per unit time. In this article, the different applications of PI techniques in fMRI are reviewed, together with the basic theoretical background and the recently developed hardware necessary to achieve rapid, high signal-to-noise ratio PI-fMRI.

Artifacts↗
PubMed2004

Sustained poststimulus elevation in cerebral oxygen utilization after vascular recovery.

The brain's response to functional activation is characterized by focal increases in cerebral blood flow. It is generally assumed that this hyperemia is a direct response to the energy demands of activation, the so-called flow-metabolism coupling. Here we report experimental evidence that increases in oxygen metabolism can occur after activation without increases in flow. When using multimodality functional MRI (fMRI) to study visual activation in human brain, we observed a postactivation period of about 30 seconds during which oxygen consumption remained elevated, while blood flow and volume had already returned to baseline levels. The finding of such a prolonged and complete dissociation of vascular response and energy metabolism during the poststimulus period indicates that increased metabolic demand needs not per se cause a concomitant increase in blood flow. The results also show that the postactivation undershoot after the positive blood-oxygen-level-dependent hemodynamic response in fMRI should be reinterpreted as a continued elevation of oxygen metabolism, rather than a delayed blood volume compliance.

Adult↗
PubMed2004

High-resolution diffusion tensor imaging of the brain stem at 3 T.

Diffusion tensor imaging with 1.8-mm isotropic resolution was performed to delineate structures of the brain stem. High-resolution single-shot imaging was achieved by the combination of a high-field magnet (3T) and the SENSitivity Encoding (or SENSE) parallel imaging technique. Various structures in the brain stem, such as the inferior olivary nuclei, deep cerebellar nuclei, some cranial nerves, and white matter tracts were identified, which have been difficult to appreciate by conventional MR techniques.

Adult↗
PubMed2003

Sickle cell disease: continuous arterial spin-labeling perfusion MR imaging in children.

Cerebral blood flow (CBF) was measured with continuous arterial spin-labeling perfusion magnetic resonance (MR) imaging in 14 children with sickle cell disease and seven control subjects. Mean CBF values were higher in patients (P <.005) than in control subjects in all cerebral artery territories. Three patients had decreased CBF in right anterior and middle cerebral artery territories compared with CBF on the left, and one patient had a profound decrease in CBF in all three territories in the right hemisphere. Baseline CBF was significantly decreased in territories seen as unaffected on conventional MR images and MR angiograms in four children with sickle cell disease.

Anemia, Sickle Cell↗
PubMed2003

Functional magnetic resonance imaging based on changes in vascular space occupancy.

During brain activation, local control of oxygen delivery is facilitated through microvascular dilatation and constriction. A new functional MRI (fMRI) methodology is reported that is sensitive to these microvascular adjustments. This contrast is accomplished by eliminating the blood signal in a manner that is independent of blood oxygenation and flow. As a consequence, changes in cerebral blood volume (CBV) can be assessed through changes in the remaining extravascular water signal (i.e., that of parenchymal tissue) without need for exogenous contrast agents or any other invasive procedures. The feasibility of this vascular space occupancy (VASO)-dependent functional MRI (fMRI) approach is demonstrated for visual stimulation, breath-hold (hypercapnia), and hyperventilation (hypocapnia). During visual stimulation and breath-hold, the VASO signal shows an inverse correlation with the stimulus paradigm, consistent with local vasodilatation. This effect is reversed during hyperventilation. Comparison of the hemodynamic responses of VASO-fMRI, cerebral blood flow (CBF)-based fMRI, and blood oxygenation level-dependent (BOLD) fMRI indicates both arteriolar and venular temporal characteristics in VASO. The effect of changes in water exchange rate and partial volume contamination with CSF were calculated to be negligible. At the commonly-used fMRI resolution of 3.75 x 3.75 x 5 mm(3), the contrast-to-noise-ratio (CNR) of VASO-fMRI was comparable to that of CBF-based fMRI, but a factor of 3 lower than for BOLD-fMRI. Arguments supporting a better gray matter localization for the VASO-fMRI approach compared to BOLD are provided.

Adult↗
PubMed2003

Measurements of cerebral perfusion and arterial hemodynamics during visual stimulation using TURBO-TILT.

Estimation of cerebral blood flow (CBF) in functional perfusion imaging could benefit from a method capable of separating effects of arterial arrival time and trailing edge. To accomplish this, the transfer insensitive labeling technique (TILT) was combined with a train of 13 consecutive acquisitions, called TURBO-TILT. Visual activation maps obtained at 13 postlabeling delay times (TI) showed a spatial shift from regions surrounding the arterial vasculature at short TI to brain parenchyma at longer delay times. High baseline CBF and short arrival times were found for the voxels with maximum activation at short TI (<1200 ms), while CBF values (43 ml / 100 g tissue/min) and its increase upon activation (55%) at longer TI were in agreement with literature data on regional cerebral perfusion.

Adult↗
PubMed2003

Defining thresholds for changes in size of simulated T2-hyperintense brain lesions on the basis of qualitative comparisons.

OBJECTIVE: Our purpose was to define thresholds below which trained reviewers cannot detect changes in the size of T2-hyperintense brain lesions. MATERIALS AND METHODS: We generated T2-weighted brain MR images (TR/TE, 4000/80) with simulated hyperintense lesions derived from a real multiple sclerosis plaque. The size of the original multiple sclerosis lesion was varied by scaling up or down the lesion using a bicubic interpolation method. Three hundred seventy-eight composite images, in which two T2-weighted images containing lesions were paired, were presented to three equally trained neuroradiologists to define thresholds below which changes in original lesion size could not be detected. Stepwise logistic regression was used to evaluate the dependency of size thresholds on the original size of the lesion. RESULTS: Thresholds ranged from a 5% to 15% increase in the original lesion diameter. For increases greater than 15%, all three reviewers detected the change in lesion size irrespective of the diameter of the original lesion. There was a dependency of the threshold on the diameter of the original lesion (p = 0.02). CONCLUSION: Using an MR simulator, we can define thresholds below which changes in original lesion size cannot be reliably detected. These results may guide the design of clinical trials that rely on trained reviewers to assess change in lesion burden.

Brain↗
PubMed2002

Scan time reduction in proton magnetic resonance spectroscopic imaging of the human brain.

A simple technique is described for scan time reductions in proton magnetic resonance spectroscopic imaging (MRSI) of the human brain. Scan time is reduced by approximately 35% while preserving spatial resolution by reducing the field of view (FOV) and number of phase-encoding steps in the transverse direction of the brain. A multislice MRSI of the brain is demonstrated which takes approximately 20 min with a square FOV, and 13 min with a reduced FOV. The signal-to-noise ratio (SNR) in the reduced FOV scan was measured to be 15% lower than that of the full FOV scan, which is close to the expected theoretical value of 19% based on the square root of the scan time. The method can be applied with any sequence, and requires minimal software and no hardware modifications. Scan time in MRSI is minimized in this method by using FOVs no larger than the dimensions of the object to be imaged. The method may also be combined with other fast MRSI techniques to provide further scan time reductions.

Adult↗
PubMed2002

Origin and minimization of residual motion-related artifacts in navigator-corrected segmented diffusion-weighted EPI of the human brain.

Motion sensitivity in diffusion-weighted imaging (DWI) can be effectively suppressed using single-shot echo-planar imaging (EPI). However, segmented (multishot) EPI is often used to increase resolution and reduce spatial distortions, which in turn increases susceptibility to brain motion. The sources of these residual motion artifacts in navigator-echo-corrected segmented EPI images of the brain were investigated. The results indicate that the dominant source of these artifacts is cardiac pulsation with occasional involuntary movement of the subject. The relationship between the cardiac cycle and motion artifacts shows that optimum timing for the data acquisition is possible. In addition it is shown that the effects of involuntary motion can be removed by swapping k-space data between redundant datasets.

Artifacts↗
PubMed2002

High-resolution isotropic 3D diffusion tensor imaging of the human brain.

High-resolution cardiac-gated 3D diffusion tensor imaging (3D-DTI) is demonstrated in vivo for several areas of the human brain. Anatomical mapping of subcortical white matter (WM), as well as definition and identification of major WM bundles from the brainstem were performed in humans for the first time using this technique. Improved intrinsic signal-to-noise ratio (SNR) and relatively reduced sensitivity to physiological motion (e.g., brain pulsations) with respect to cardiac-gated multislice acquisition are demonstrated. The advantages and weaknesses of this approach are discussed.

Brain↗
PubMed2002

Reproducibility of primary motor cortex somatotopy under controlled conditions.

BACKGROUND AND PURPOSE: The somatotopic organization of the contralateral primary motor cortex (M1) and its intra- and intersubject reproducibility has been the subject of many investigations and controversies. A potential explanation for a least some of the conflicting results could be the lack of movement control in the studies performed. The purpose of this study was to investigate these issues under controlled experimental conditions. METHODS: Functional MR imaging was performed in 12 healthy volunteers performing hand, finger, wrist, elbow, foot, and tongue movements. Two experimental sessions were separated by 2 weeks. Controlled conditions were achieved by means of a custom-designed arm and hand manipulandum providing standardization of the movements within and across subjects. RESULTS: The experiments revealed a clear large-scale somatotopy of the contralateral M1 with distinct subregions controlling the foot, arm, and tongue. Despite considerable overlap of the volumes, geometric centers of gravity (COGs) showed statistically significant differences in coordinates between the elbow, wrist, fingers, and hand. COGs showed a high degree of intra- and interindividual reproducibility, particularly for the upper limb movements, in contrast to the activation volumes that proved to be unreliable parameters, despite the controlled conditions. CONCLUSION: These findings support the existence of a gross-scale somatotopic organization yet also demonstrate a clear, fine-scale somatotopy of the within-arm representations. Furthermore, they reveal high reproducibility of the COGs when standardized conditions are applied. This observation highlights the need for movement control to allow for intra- and intersubject comparison.

Adult↗
PubMed2002

Intervoxel heterogeneity of event-related functional magnetic resonance imaging responses as a function of T(1) weighting.

Inflow effects on activation-related BOLD signal changes in event-related fMRI experiments were assessed by varying the repetition time (TR) and flip angle (FA) values for gradient-echo echo-planar imaging (GE-EPI). Surprisingly, both increases and decreases were detected in these signal changes with increased T(1) weighting (reduced TR, increased FA). The well-known "positive" effect is attributed to inflow of fresh spins in the slice, leading to an apparent reduction in T(1). The "negative" effect is attributed to voxels containing pure parenchyma, where large-vessel inflow effects are very small and the BOLD effect is dominated by microvascular blood volume and oxygenation changes. Because blood T(1) is greater than tissue T(1) at 1.5 T, the fractional BOLD effect decreases with increased T(1) weighting. To aid in the interpretation of these experimental results, numerical simulations were performed based on a physiological multicompartment model, including pure tissue, large vessels (arteries, veins), microvessels (arterioles, capillaries, venules), and cerebrospinal fluid.

Adult↗