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Lawrence L Wald

Publications and source records attributed to Lawrence L Wald.

At least 19 recordsLinked to original sources
PubMed2006

Frontal connections and cognitive changes in normal aging rhesus monkeys: a DTI study.

Recent anatomical studies have found that cortical neurons are mainly preserved during the aging process while myelin damage and even axonal loss is prominent throughout the forebrain. We used diffusion tensor imaging (DT-MRI) to evaluate the hypothesis that during the process of normal aging, white matter changes preferentially affect the integrity of long corticocortical association fiber tracts, specifically the superior longitudinal fasciculus II and the cingulum bundle. This would disrupt communication between the frontal lobes and other forebrain regions leading to cognitive impairments. We analyzed DT-MRI datasets from seven young and seven elderly behaviorally characterized rhesus monkeys, creating fractional anisotropy (FA) maps of the brain. Significant age-related reductions in mean FA values were found for the superior longitudinal fasciculus II and the cingulum bundle, as well as the anterior corpus callosum. Comparison of these FA reductions with behavioral measures demonstrated a statistically significant linear relationship between regional FA and performance on a test of executive function. These findings support the hypothesis that alterations to the integrity of these long association pathways connecting the frontal lobe with other forebrain regions contribute to cognitive impairments in normal aging. To our knowledge this is the first investigation reporting such alterations in the aging monkey.

Aging↗
PubMed2006

Effect of spatial smoothing on physiological noise in high-resolution fMRI.

Physiological noise dominates the SNR of the fMRI time-course at commonly used spatial resolutions at field strengths of 3 T and above. Operating in this physiological noise dominated regime limits some benefits of high field acquisition since increases in image SNR produce only modest increases in time-course SNR. Although previous studies have shown that the physiological noise dominance can be mitigated by using higher spatial resolutions, not all functional studies require voxel sizes smaller than the thickness of the human cortex. In this study, we examine the effect of acquiring high spatial resolution, thermal noise dominated time-courses and spatially smoothing the images to lower resolutions, which would otherwise be physiological noise dominated. At high field strengths, where physiological noise is most problematic, this strategy lowered the overall time-course variance compared to direct acquisition at commonly used spatial resolution. At 7 T for example, 5 x 5 x 3 mm3 resolution images derived from smoothing 1.5 x 1.5 x 3 mm3 data improved time-course SNR by a factor of 1.89 compared to a time-series acquired at 5 x 5 x 3 mm3. Presumably, this effect was derived from the reduced physiological-to-thermal noise ratio in the high spatial resolution data followed by a smoothing operation that improves SNR without adding physiological noise. Our findings demonstrate that in contrast to conventional SNR penalties associated with spatially smoothing Fourier data, the time-course SNR of smoothed high-resolution data can be improved compared to direct acquisition at the desired resolution.

Artifacts↗
PubMed2006

Bright and black blood imaging of the carotid bifurcation at 3.0T.

PURPOSE: The aim of this study was to evaluate our preliminary experience at 3.0 T with imaging of the carotid bifurcation in healthy and atherosclerotic subjects. Application at 3.0 T is motivated by the signal-to-noise gain for improving spatial resolution and reducing signal averaging requirements. MATERIALS AND METHODS: We utilized a dual phased array coil and applied 2D, 3D time of flight (TOF) and turbo spin echo (TSE) sequences with comparison of two lumen signal suppression methods for black blood (BB) TSE imaging including double inversion preparation (DIR) and spatial presaturation pulses. The signal-to-noise ratios (SNR) of healthy carotid vessel walls were compared in 2D and 3D BB TSE acquisitions. The bright and black blood multi-contrast exam was demonstrated for a complex carotid plaque. RESULTS: Contrast-to-noise (CNR) greater than 150 was achieved between the lumen and suppressed background for 3D TOF. For BB, both methods provided sufficient lumen signal suppression but slight residual flow artifacts remained at the bifurcation level. As expected 3D TSE images had higher SNR compared to 2D, but increased motion sensitivity is a significant issue for 3D at high field. For multi-contrast imaging of atherosclerotic plaque, fibrous, calcified and lipid components were resolved. The CNR ratio of fibrous (bright on PDW, T2W) and calcified (dark in T1W, T2W, PDW) plaque components was maximal in the T2W images. The 3D TOF angiogram indicating a 40% stenosis was complemented by 3D multi-planar reformat of BB images that displayed plaque extent. Detection of intimal thickening, the earliest change associated with atherosclerotic progression was observed in BB PDW images at 3.0 T. CONCLUSIONS: High SNR and CNR images have been demonstrated for the healthy and diseased carotid. Improvements in RF coils along with pulse sequence optimization, and evaluation of endogenous and exogenous contrast mechanisms will further enhance carotid imaging at 3.0T.

Adult↗
PubMed2006

Accelerated volumetric MRI with a SENSE/GRAPPA combination.

PURPOSE: To combine the specific advantages of the generalized autocalibrating partially parallel acquisitions (GRAPPA) technique and sensitivity encoding (SENSE) with two-dimensional (2D) undersampling. MATERIALS AND METHODS: By splitting the 2D reconstruction process into multiple one-dimensional (1D) reconstructions, the normal 1D GRAPPA method can be used for image reconstruction. Due to this data-handling process, a GRAPPA reconstruction is performed along the phase-encoding (PE) direction and effectively a SENSE reconstruction is performed along the partition-encoding (PAE) direction. RESULTS: In vivo experiments demonstrate the successful implementation of the SENSE/GRAPPA combination. Experimental results with up to 9.6-fold acceleration using a prototype 32-channel receiver head coil array are presented. CONCLUSION: The proposed SENSE/GRAPPA combination for 3D imaging allows the GRAPPA method to be applied in combination with 2D undersampling. Because the SENSE/GRAPPA combination is not based on knowledge of spatial coil sensitivities, it should be the method of choice whenever it is difficult to extract the sensitivity information.

Algorithms↗
PubMed2006

Direct parallel image reconstructions for spiral trajectories using GRAPPA.

The use of spiral trajectories is an efficient way to cover a desired k-space partition in magnetic resonance imaging (MRI). Compared to conventional Cartesian k-space sampling, it allows faster acquisitions and results in a slight reduction of the high gradient demand in fast dynamic scans, such as in functional MRI (fMRI). However, spiral images are more susceptible to off-resonance effects that cause blurring artifacts and distortions of the point-spread function (PSF), and thereby degrade the image quality. Since off-resonance effects scale with the readout duration, the respective artifacts can be reduced by shortening the readout trajectory. Multishot experiments represent one approach to reduce these artifacts in spiral imaging, but result in longer scan times and potentially increased flow and motion artifacts. Parallel imaging methods are another promising approach to improve image quality through an increase in the acquisition speed. However, non-Cartesian parallel image reconstructions are known to be computationally time-consuming, which is prohibitive for clinical applications. In this study a new and fast approach for parallel image reconstructions for spiral imaging based on the generalized autocalibrating partially parallel acquisitions (GRAPPA) methodology is presented. With this approach the computational burden is reduced such that it becomes comparable to that needed in accelerated Cartesian procedures. The respective spiral images with two- to eightfold acceleration clearly benefit from the advantages of parallel imaging, such as enabling parallel MRI single-shot spiral imaging with the off-resonance behavior of multishot acquisitions.

Algorithms↗
PubMed2006

Dynamic magnetic resonance inverse imaging of human brain function.

MRI is widely used for noninvasive hemodynamic-based functional brain imaging. In traditional spatial encoding, however, gradient switching limits the temporal resolution, which makes it difficult to unambiguously identify possible fast nonhemodynamic changes. In this paper we propose a novel reconstruction approach, called dynamic inverse imaging (InI), that is capable of providing millisecond temporal resolution when highly parallel detection is used. To achieve an order-of-magnitude speedup in generating time-resolved contrast estimates and dynamic statistical parametric maps (dSPMs), the spatial information is derived from an array of detectors rather than by time-consuming gradient-encoding methods. The InI approach was inspired by electroencephalography (EEG) and magnetoencephalography (MEG) source localization techniques. Dynamic MR InI was evaluated by means of numerical simulations. InI was also applied to measure BOLD hemodynamic time curves at 20-ms temporal resolution in a visual stimulation experiment using a 90-channel head array. InI is expected to improve the time resolution of MRI and provide increased flexibility in the trade-off between spatial and temporal resolution for studies of dynamic activation patterns in the human brain.

Brain Mapping↗
PubMed2006

Parallel RF transmission with eight channels at 3 Tesla.

Spatially selective RF waveforms were designed and demonstrated for parallel excitation with a dedicated eight-coil transmit array on a modified 3T human MRI scanner. Measured excitation profiles of individual coils in the array were used in a low-flip-angle pulse design to achieve desired spatial target profiles with two- (2D) and three-dimensional (3D) k-space excitation with simultaneous transmission of RF on eight channels. The 2D pulse excited a high-resolution spatial pattern in-plane, while the 3D trajectory produced high-quality slice selection with a uniform in-plane excitation despite the highly nonuniform individual spatial profiles of the coil array. The multichannel parallel RF excitation was used to accelerate the 2D excitation by factors of 2-8, and experimental results were in excellent agreement with simulations based on the measured coil maps. Parallel RF transmission may become critical for robust and routine human studies at very high field strengths where B(1) inhomogeneity is commonly severe.

Equipment Design↗
PubMed2006

Hippocampal volume, PTSD, and alcoholism in combat veterans.

Studies imposing rigorous control over lifetime alcohol intake have usually not found smaller hippocampal volumes in persons with posttraumatic stress disorder. Because the majority of negative studies have used adolescent samples, it has been suggested that chronicity is a necessary condition for such findings. To test the hypothesis that a smaller hippocampus in PTSD is unrelated to comorbid alcoholism or to chronicity, this study estimated hippocampal volume in a relatively large group (N=99) of combat veterans in which PTSD, lifetime alcohol abuse/dependence, and Vietnam versus Gulf War service were crossed. In subjects with histories of alcoholism, unadjusted hippocampal volume was 9% smaller in persons with PTSD than in those without PTSD. In nonalcoholic subjects, the PTSD-related difference in hippocampal volume was 3%. The failure to observe a strong association between PTSD and hippocampal volume in nonalcoholic subjects was not ascribable to younger age, reduced PTSD chronicity, or lower PTSD symptom severity. The possibility that smaller hippocampal volume is limited to groups in which PTSD is compounded by comorbid alcoholism is not necessarily incompatible with results suggesting a smaller hippocampus is predispositional to PTSD. Further examination of the role of alcoholism and other comorbid conditions in studies of brain structure and function in PTSD appears warranted.

Adult↗
PubMed2005

Detection of entorhinal layer II using 7Tesla [corrected] magnetic resonance imaging.

The entorhinal cortex lies in the mediotemporal lobe and has major functional, structural, and clinical significance. The entorhinal cortex has a unique cytoarchitecture with large stellate neurons in layer II that form clusters. The entorhinal cortex receives vast sensory association input, and its major output arises from the layer II and III neurons that form the perforant pathway. Clinically, the neurons in layer II are affected with neurofibrillary tangles, one of the two pathological hallmarks of Alzheimer's disease. We describe detection of the entorhinal layer II islands using magnetic resonance imaging. We scanned human autopsied temporal lobe blocks in a 7T human scanner using a solenoid coil. In 70 and 100 microm isotropic data, the entorhinal islands were clearly visible throughout the anterior-posterior extent of entorhinal cortex. Layer II islands were prominent in both the magnetic resonance imaging and corresponding histological sections, showing similar size and shape in two types of data. Area borders and island location based on cytoarchitectural features in the mediotemporal lobe were robustly detected using the magnetic resonance images. Our ex vivo results could break ground for high-resolution in vivo scanning that could ultimately benefit early diagnosis and treatment of neurodegenerative disease.

Entorhinal Cortex↗
PubMed2005

Functional MRI using regularized parallel imaging acquisition.

Parallel MRI techniques reconstruct full-FOV images from undersampled k-space data by using the uncorrelated information from RF array coil elements. One disadvantage of parallel MRI is that the image signal-to-noise ratio (SNR) is degraded because of the reduced data samples and the spatially correlated nature of multiple RF receivers. Regularization has been proposed to mitigate the SNR loss originating due to the latter reason. Since it is necessary to utilize static prior to regularization, the dynamic contrast-to-noise ratio (CNR) in parallel MRI will be affected. In this paper we investigate the CNR of regularized sensitivity encoding (SENSE) acquisitions. We propose to implement regularized parallel MRI acquisitions in functional MRI (fMRI) experiments by incorporating the prior from combined segmented echo-planar imaging (EPI) acquisition into SENSE reconstructions. We investigated the impact of regularization on the CNR by performing parametric simulations at various BOLD contrasts, acceleration rates, and sizes of the active brain areas. As quantified by receiver operating characteristic (ROC) analysis, the simulations suggest that the detection power of SENSE fMRI can be improved by regularized reconstructions, compared to unregularized reconstructions. Human motor and visual fMRI data acquired at different field strengths and array coils also demonstrate that regularized SENSE improves the detection of functionally active brain regions.

Brain Mapping↗
PubMed2004

Parallel imaging reconstruction using automatic regularization.

Increased spatiotemporal resolution in MRI can be achieved by the use of parallel acquisition strategies, which simultaneously sample reduced k-space data using the information from multiple receivers to reconstruct full-FOV images. The price for the increased spatiotemporal resolution in parallel MRI is the degradation of the signal-to-noise ratio (SNR) in the final reconstructed images. Part of the SNR reduction results when the spatially correlated nature of the information from the multiple receivers destabilizes the matrix inversion used in the reconstruction of the full-FOV image. In this work, a reconstruction algorithm based on Tikhonov regularization is presented that reduces the SNR loss due to geometric correlations in the spatial information from the array coil elements. Reference scans are utilized as a priori information about the final reconstructed image to provide regularized estimates for the reconstruction using the L-curve technique. This automatic regularization method reduces the average g-factors in phantom images from a two-channel array from 1.47 to 0.80 in twofold sensitivity encoding (SENSE) acceleration. In vivo anatomical images from an eight-channel system show an averaged g-factor reduction of 1.22 to 0.84 in 2.67-fold acceleration.

Algorithms↗
PubMed2003

Stereopsis activates V3A and caudal intraparietal areas in macaques and humans.

Stereopsis, the perception of depth from small differences between the images in the two eyes, provides a rich model for investigating the cortical construction of surfaces and space. Although disparity-tuned cells have been found in a large number of areas in macaque visual cortex, stereoscopic processing in these areas has never been systematically compared using the same stimuli and analysis methods. In order to examine the global architecture of stereoscopic processing in primate visual cortex, we studied fMRI activity in alert, fixating human and macaque subjects. In macaques, we found strongest activation to near/far compared to zero disparity in areas V3, V3A, and CIPS. In humans, we found strongest activation to the same stimuli in areas V3A, V7, the V4d topolog (V4d-topo), and a caudal parietal disparity region (CPDR). Thus, in both primate species a small cluster of areas at the parieto-occipital junction appears to be specialized for stereopsis.

Animals↗
PubMed2003

A wavelet-based approximation of surface coil sensitivity profiles for correction of image intensity inhomogeneity and parallel imaging reconstruction.

We evaluate a wavelet-based algorithm to estimate the coil sensitivity modulation from surface coils. This information is used to improve the image homogeneity of magnetic resonance imaging when a surface coil is used for reception, and to increase image encoding speed by reconstructing images from under-sampled (aliased) acquisitions using parallel magnetic resonance imaging (MRI) methods for higher spatiotemporal image resolutions. The proposed algorithm estimates the spatial sensitivity profile of surface coils from the original anatomical images directly without using the body coil for additional reference scans or using coil position markers for electromagnetic model-based calculations. No prior knowledge about the anatomy is required for the application of the algorithm. The estimation of the coil sensitivity profile based on the wavelet transform of the original image data was found to provide a robust method for removing the slowly varying spatial sensitivity pattern of the surface coil image and recovering full FOV images from two-fold acceleration in 8-channel parallel MRI. The results, using bi-orthogonal Daubechies 97 wavelets and other members in this family, are evaluated for T1-weighted and T2-weighted brain imaging.

Algorithms↗
PubMed2003

Degenerate mode birdcage volume coil for sensitivity-encoded imaging.

A volume birdcage coil for accelerated image encoding with parallel acquisition methods such as SENSE is demonstrated. The coil is degenerately tuned with both the standard homogeneous mode and the first gradient mode of the birdcage coil resonant at the Larmor frequency. Conventional and antisymmetric coupling structures allow imaging from each of these modes simultaneously. The coil for SENSE-type reconstruction with acceleration factors of up to 2-fold is demonstrated. The spatial distribution of the added noise from the SENSE reconstruction (g-factor map) due to geometrical arrangement of the two-channel system is estimated. The spatially averaged g-factors were found to be 1.21, 1.36, and 1.55 for 1.3, 1.6, and 2-fold accelerations, respectively. The system was demonstrated in vivo using accelerated and nonaccelerated anatomical brain images at 1.5 T. The maximal 2-fold acceleration in this dual-mode degenerate birdcage coil offers the potential to extend SENSE-type image reconstruction methods to applications demanding uniform whole brain coverage.

Brain↗
PubMed2003

In vivo tracing of major rat brain pathways using manganese-enhanced magnetic resonance imaging and three-dimensional digital atlasing.

The magnetic resonance imaging (MRI)-detectable T1 contrast agent manganese (Mn2+) has recently been introduced as a neural tracer in rodents, birds, and monkeys. We have tested to what extent this in vivo method is useful for three-dimensional (3-D) survey of connectivity patterns in the rat somatosensory system. A commonly available 3 T human clinical MRI scanner was used to trace neural pathways following focal injection of manganese chloride (MnCl2) in the somatosensory cortex. Six to 10 h after MnCl2 injection, we found significant signal enhancement in major projection systems, including corticocortical, corticostriatal, corticothalamic, corticotectal, corticopontine, and corticospinal pathways. To facilitate the assignment of anatomic localization to the observed Mn2+ signal enhancement, we registered the MRI data with a 3-D digital reconstruction of a stereotaxic rat brain atlas. Across-animal comparison using the digital model allowed demonstration of a corticothalamic 3-D topographic organization in agreement with previously published two-dimensional topographic schemes based on classical neural tracing data. We conclude that anterograde MnCl2/MRI tracing allows rapid analysis of topographic organization across multiple brain regions. The method allows a higher data throughput for 3-D studies of large-scale brain connectivity than conventional methods based on tissue sectioning.

Animals↗
PubMed2003

Comparison of cardiac MRI on 1.5 and 3.0 Tesla clinical whole body systems.

RATIONALE AND OBJECTIVES: A cardiac imaging pilot study was performed on 1.5 and 3.0 Tesla (T) whole body magnetic resonance units equipped with identical gradient sets and geometrically equivalent body coils. The goals were to compare the signal-to-noise (SNR) and contrast-to-noise (CNR) ratios on matched studies conducted at both field strengths and demonstrate the potential for functional and morphologic cardiac evaluation at 3.0 T. METHODS: Short axis cine true fast imaging with steady precession (True FISP) was compared at 1.5 and 3.0 T using the body coil in transmit-receive mode and transmit-only with single loop and phased array receiver coils. SNR of the myocardium and CNR of the ventricular blood and myocardium were calculated from a quantitative region of interest analysis of these data. Additionally at 3.0 T, long axis and 4-chamber cine as well as "dark blood" imaging are demonstrated with sequence and parameter settings comparable to current state of the art for cardiac evaluation at 1.5 T. RESULTS: The 3.0 T data consistently demonstrates increases in SNR when all imaging conditions are closely matched but the increase has a large variability ranging from 20 to 85% depending on the radiofrequency coil configuration. Ventricular blood-myocardium CNR greater than 30 is obtained at 3.0 T, which is comparable to an optimized 1.5 T acquisition despite the specific absorption rate limitation of flip angle to nearly one half the value. The increased SNR at 3.0 T improves detection of fine anatomic detail, such as the chordae tendineae and mitral valve structure. CONCLUSIONS: Increased specific absorption rate can be a limiting fact; however, we have demonstrated that 3.0 T cardiac imaging shows gains in SNR while maintaining the CNR. The SNR gain is advantageous, and phased array coil technology is key for improving cardiac magnetic resonance imaging at 3.0 T.

Aged↗
PubMed2002

Modulation of brain and serum glutamatergic concentrations following a switch from conventional neuroleptics to olanzapine.

BACKGROUND: Recent evidence suggests that effects upon glutamatergic transmission may contribute to the therapeutic action of certain atypical antipsychotic agents. METHODS: Glutamate concentrations were measured in serum and were estimated (Glx/Cr) in cingulate cortex by proton magnetic resonance spectroscopy (MRS) in schizophrenia patients while they were being treated with conventional neuroleptics and then 8 weeks after switching to olanzapine. Serum glutamate concentrations were obtained from 11 subjects, and MRS estimates of Glx/Cr were available from 10 subjects at both time points. RESULTS: Serum glutamate concentrations significantly increased after the switch from conventional agents to olanzapine; brain glutamate (Glx/Cr) did not change significantly; however, brain glutamate (Glx/Cr) concentrations increased significantly in patients who exhibited an improvement in negative symptoms with olanzapine compared with patients with no change or worsening of negative symptoms. CONCLUSIONS: Comparisons performed following the switch from conventional agents to olanzapine are consistent with previous studies of clozapine and provide additional preliminary evidence supporting the hypothesis that effects on excitatory amino acid activity may contribute to olanzapine's efficacy for treating negative symptoms.

Adult↗