David N. Levin

The spatial location of EEG electrodes: locating the best-fitting sphere relative to cortical anatomy

The location of the international 10-20 system electrode positions and 14 fiducial landmarks are described in cartesian coordinates (+/- 1.4 mm average accuracy). Six replications were obtained on 3 separate days from 4 normal subjects, who were compared to each other with a best-fit sphere algorithm. Test-retest reliability depended on the electrode position: the parasagittal electrodes were associated with greater measurement errors (maximum 7 mm) than midline locations. Location variability due to head shape was greatest in the temporal region, averaging 5 mm from the mean. For each subjects electrode locations a best-fitting sphere was determined (79-87 mm radius, 6% average error). A surface-fitting algorithm was used to transfer the electrode locations and best-fitting sphere to MR images of the brain and scalp. The center of the best-fitting sphere coincided with the floor of the third ventricle 5 mm anterior to the posterior commissure. The melding of EEG electrode location information with brain anatomy provides an empirical basis for associating hypothetical equivalent dipole locations with their anatomical substrates.

Interactive 3-D Patient-Image Registration

A method has been developed which allows accurate registration of 3D image data sets of the head, such as CT or MRI, with with the anatomy of the actual patient. Once registration is accomplished, the patient and image spaces may be interactively explored, and any point or volume of interest in either space instantly transformed to the other. This paper demonstrates the use of this technology in accurately transferring radiation therapy treatment plans from the 3D image space in which they are simulated, to the physical patient. This provides a heretofore missing objective link between 3D image-based simulations and actual treatment delivery.

Theory and simulation of NMR spectroscopic imaging and field plotting by projection reconstruction involving an intrinsic frequency dimension

Intrinsic frequency differences in an NMR signal may be regarded as describing a projection of a pseudo-object on a nonspatial axis. The frequency differences produced by imposition of a linear field gradient on an object with a single sharp resonance line describe a projection of the object on axes in ordinary space. When differences of both kinds exist in a signal, it corresponds to a projection of a pseudoobject along some direction in a space defined by an intrinsic frequency axis as well as by the usual spatial axes. The projection directions relative to the intrinsic frequency axis in this space depend upon the gradient amplitudes. This general formulation permits such intrinsic frequency differences as those resulting from spectra and magnetic field inhomogeneities to be recovered by projection reconstruction methods, and clearly resolved images of spatial distributions of the nuclear spin magnetization to be obtained in the presence of such differences. In the full-volume case, because rapidly switched gradients are not required to do volume imaging with this technique, it may be used with any NMR spectrometer (even with cw spectra); this is not the case with techniques employing selective excitation and multidimensional Fourier transforms (I). We present here the general theoretical formulation of this new technique and computer simulations in two

The mechanical state of intracranial tissues in elderly subjects studied by imaging CSF and brain pulsations

The biomechanical properties of intracranial tissues influence the mechanical coupling of brain and CSF oscillations to the driving vascular pulsations. Dynamic phase contrast MRI was used to measure the transfer functions that characterize these couplings in normal elderly subjects and patients with Alzheimers disease. The transfer functions of both groups were significantly different from the previously reported transfer functions of normal young subjects. The data show that vascular pulsations tend to cause greater spinal cord movements and smaller CSF oscillations in the older subjects than in the younger ones. These results are likely to be due to age-related changes in the mechanical state of intracranial tissues.

Retrospective fusion of radiographic and MR data for localization of subdural electrodes

Prior to epilepsy surgery, subdural electrodes are often implanted and monitored for a few days to identify the focus of abnormal electrical activity. During the implantation and subsequent brain resection, there may be uncertainty about the exact location of the electrodes with respect to features of brain anatomy such as specific gyral convolutions or lesions. In experiments with a phantom and patients, implanted electrodes were imaged with multiplanar skull radiographs (or CT scans). After retrospective registration with pre-implantation MR data, the electrodes were mapped from these studies onto an MR-derived three-dimensional brain model. The resulting multimodality displays showed the relationship of the electrodes to brain anatomy. In one patient the position of each electrode with respect to a metabolic lesion was also displayed by mapping preimplantation PET data onto the same brain model. This new display of electrode positions may strengthen the interpretation of subdural electrical recordings and thereby reduce uncertainty in planning the resection of epileptic tissue.

An fMRI Study of the Effect of Amphetamine on Brain Activity

Functional magnetic resonance imaging was used to evaluate the effects of oral d-amphetamine on brain activation elicited by auditory and simple motor tasks in ten normal right-handed subjects. We measured the percent signal change and number of voxels activated by a tone discrimination task and a right hand finger-tapping task after 20 mg of d-amphetamine and after placebo. Compared to placebo, amphetamine significantly increased the number of activated voxels in the left and right primary auditory cortices during the tone discrimination task and increased the number of activated voxels in the ipsilateral primary sensorimotor cortex and right middle frontal area during the motor task. Although highly specific vascular effects of drug cannot be ruled out as an explanation, these results could also mean that amphetamine increases the neuronal activity associated with each of these two tasks.

RAGOBOT: a new platform for wireless mobile sensor networks

We present Ragobot, a fully-featured validated platform for use in mobile sensor networks. Ragobot is a robot of small dimensions with features that surpass those provided by many other robots in this category. Ragobot hardware and software are implemented with modularity as one of the main considerations; therefore, these are easy to upgrade and customize according to the needs of each specific application. Moreover, we present Ragoworld, a controlled physical space for the development and evaluation of mobile sensor network algorithms.

Using an image database to constrain the acquisition and reconstruction of MR images of the human head

A training set of MR images of normal and abnormal heads was used to derive a complete set of orthonormal basis functions which converged to head-like images more rapidly than Fourier basis functions. The new image representation was used to reconstruct MR images of other heads from a relatively small number of phase-encoded signal measurements. The training images also determined exactly which phase-encoded signals should be measured to minimize image reconstruction error. These signals were nonuniformly scattered throughout k-space. Experiments showed that head images reconstructed with the new method had less serious truncation artifacts than conventional Fourier images reconstructed from the same number of signals. The resulting images were characterized by spatially variable spatial resolution and were particularly well-resolved in regions where the training images had structural detail.

Complete duplication of the urethra with vaginal stenosis.

Complete urethral duplication, clitoral hypertrophy and vaginal urethra in a girl with normal adrenal function are described. These anomalies caused severe micturitional disturbance by several mechanisms. Reported cases of complete duplication form a continuous spectrum of abnormalities and were classified according to anatomical pattern.

Assessment of the biomechanical state of intracranial tissues by dynamic MRI of cerebrospinal fluid pulsations: a phantom study

We used a cranial phantom to investigate how intracranial mechanical factors [brain compliance and the resistance to the flow of cerebrospinal fluid (CSF)] affect the way in which CSF pulsations are driven by pulsatile transcranial blood flow. Dynamic phase-contrast magnetic resonance imaging (MRI) was used to measure the transfer function between vascular pulsations and pulsatile response of the CSF below the foramen magnum of the phantom. We found that the coupling between the high frequency components of cervical CSF flow and transcranial blood flow was decreased when the phantom was modified to simulate increased brain compliance and increased resistance to CSF flow.