Manbir Singh

An electronically collimated gamma camera for single photon emission computed tomography. Part I: Theoretical considerations and design criteria

The detection and imaging characteristics of a new type of g a m m a c a m e r a for s i n g l e p h o t o n e m i s s i o n c o m p u t e d t o m o g r a p h y have been investigated. Unlike conventional gamma cameras which use mechanical collimation, the new gamma camera utilizes e l e c t r o n i c c o l l i m a t i o n which is obtained from a sequential interaction of gamma radiation with a dual position‐and‐energy sensitive detectionsystem. Coincident counting between the two detectors provides localization of activity upon a multitude of conical surfaces throughout the object, wherefrom the three‐dimensional activity distribution can be reconstructed. Not only does electronic collimation provide simultaneous multiple views of the object, but a large gain in sensitivity is also indicated over a conventionally collimated gamma camera under conditions of similar spatial resolution.Detector optimization studies have been performed to design a prototype system comprising a 33×33 array of high‐purity germanium detectors coupled to an uncollimated conventional scintillation camera. The cumulative signal‐to‐noise ratio in projection images obtained with this system is expected to be about a factor of 4 higher (sensitivity about a factor of 15 higher) than that obtained in a corresponding projection image with a conventional gamma camera for imaging a uniformly distributed Tc‐99m source in a 20‐cm‐diam ×20‐cm‐tall cylinder. A similar gain is expected in the tomographic images.

An Evaluation of Methods for Neuromagnetic Image Reconstruction

In this paper, we discuss several aspects of a potential new medical imaging modality for producing a quantitative three-dimensional map of neuron current densities associated with brain function. The neuromagnetic image is produced by reconstructing a current dipole field from external magnetic field measurements made with an array of superconducting quantum interference device (SQUID) detectors. This field is produced by numerical inversion of the Biot-Savart equation. The purpose of the work is to investigate fundamental limits on the feasibility of the proposed system under ideal conditions. The following problems are addressed: 1) What are the factors limiting resolution of the system? 2) What is a suitable model for neural activity in the brain? 3) What classes of algorithms are suitable for estimating the model parameters? The major conclusion of this work is that the inversion problem is severely ill-posed and the choice of model and estimation algorithm are crucial in obtaining reasonable solutions. A class of solutions, termed minimum dipole, is proposed as a means of obtaining more acceptable results.

An electronically collimated gamma camera for single photon emission computed tomography. Part II: Image reconstruction and preliminary experimental measurements

Iterative algorithms have been investigated for reconstructing images from data acquired with a new type of gamma camera based upon an electronic method of collimating gamma radiation. The camera is composed of two detection systems which record a sequential interaction of the emitted gamma radiation. Coincident counting in accordance with Compton scattering kinematics leads to a localization of activity upon a multitude of conical surfaces throughout the object. A two-stage reconstruction procedure in which conical line projection images as seen by each position sensing element of the first detector are reconstructed in the first stage, and tomographic images are reconstructed in the second stage, has been developed. Computer simulation studies of both stages and first-stage reconstruction studies with preliminary experimental data are reported. Experimental data were obtained with one detection element of a prototype germanium detector. A microcomputer based circuit was developed to record coincident counts between the germanium detector and an uncollimated conventional scintillation camera. Point sources of Tc-99m and Cs-137 were used to perform preliminary measurements of sensitivity and point spread function characteristics of electronic collimation.

Visual speech perception without primary auditory cortex activation

Speech perception is conventionally thought to be an auditory function, but humans often use their eyes to perceive speech. We investigated whether visual speech perception depends on processing by the primary auditory cortex in hearing adults. In a functional magnetic resonance imaging experiment, a pulse-tone was presented contrasted with gradient noise. During the same session, a silent video of a talker saying isolated words was presented contrasted with a still face. Visual speech activated the superior temporal gyrus anterior, posterior, and lateral to the primary auditory cortex, but not the region of the primary auditory cortex. These results suggest that visual speech perception is not critically dependent on the region of primary auditory cortex.

Vibrotactile activation of the auditory cortices in deaf versus hearing adults.

Neuroplastic changes in auditory cortex as a result of lifelong perceptual experience were investigated. Adults with early-onset deafness and long-term hearing aid experience were hypothesized to have undergone auditory cortex plasticity due to somatosensory stimulation. Vibrations were presented on the hand of deaf and normal-hearing participants during functional MRI. Vibration stimuli were derived from speech or were a fixed frequency. Higher, more widespread activity was observed within auditory cortical regions of the deaf participants for both stimulus types. Life-long somatosensory stimulation due to hearing aid use could explain the greater activity observed with deaf participants.

Correlation between BOLD-fMRI and EEG signal changes in response to visual stimulus frequency in humans

The correlation between signals acquired using electroencephalography (EEG) and fMRI was investigated in humans during visual stimulation. Evoked potential EEG and BOLD fMRI data were acquired independently under similar conditions from eight subjects during stimulation by a checkerboard flashed at frequencies ranging from 2–12 Hz. The results indicate highly correlated changes in the strength of the EEG signal averaged over two occipital electrodes and the BOLD signal within the occipital lobe as a function of flash frequency for 7/8 subjects (average linear correlation coefficient of 0.76). Both signals peaked at approximately 8 Hz. For one subject the correlation coefficient was 0.20; the EEG signal peaked at 6 Hz and the BOLD signal peaked at 10 Hz. Overall, the EEG and BOLD signals, each averaged over 40‐sec stimulation periods, appear to be coupled linearly during visual stimulation by a flashing checkerboard. Magn Reson Med 49:108–114, 2003.

Three-dimensional maximum-likelihood reconstruction for an electronically collimated single-photon-emission imaging system

A three-dimensional maximum-likelihood reconstruction method is presented for a prototype electronically collimated single-photon-emission system. The electronically collimated system uses a gamma camera fronted by an array of germanium detectors to detect gamma-ray emissions from a distributed radioisotope source. In this paper we demonstrate that optimal iterative three-dimensional reconstruction approaches can be feasibly applied to emission imaging systems that have highly complex spatial sampling patterns and that generate extremely large numbers of data values. A probabilistic factorization of the system matrix that reduces the computation by several orders of magnitude is derived. We demonstrate a dramatic increase in the convergence speed of the expectation maximization algorithm by sequentially iterating over particular subsets of the data. This result is also applicable to other emission imaging systems.

Novel diffusion tensor imaging methodology to detect and quantify injured regions and affected brain pathways in traumatic brain injury

PURPOSE To develop and apply diffusion tensor imaging (DTI)-based normalization methodology for the detection and quantification of sites of traumatic brain injury (TBI) and the impact of injury along specific brain pathways in (a) individual TBI subjects and (b) a TBI group. MATERIALS AND METHODS Normalized DTI tractography was conducted in the native space of 12 TBI and 10 age-matched control subjects using the same number of seeds in each subject, distributed at anatomically equivalent locations. Whole-brain tracts from the control group were mapped onto the head of each TBI subject. Differences in the fractional anisotropy (FA) maps between each TBI subject and the control group were computed in a common space using a t test, transformed back to the individual TBI subjects head space, and thresholded to form regions of interest (ROIs) that were used to sort tracts from the control group and the individual TBI subject. Tract counts for a given ROI in each TBI subject were compared to group mean for the same ROI to quantify the impact of injury along affected pathways. The same procedure was used to compare the TBI group to the control group in a common space. RESULTS Sites of injury within individual TBI subjects and affected pathways included hippocampal/fornix, inferior fronto-occipital, inferior longitudinal fasciculus, corpus callosum (genu and splenium), cortico-spinal tracts and the uncinate fasciculus. Most of these regions were also detected in the group study. CONCLUSIONS The DTI normalization methodology presented here enables automatic delineation of ROIs within the heads of individual subjects (or in a group). These ROIs not only localize and quantify the extent of injury, but also quantify the impact of injury on affected pathways in an individual or in a group of TBI subjects.

Fast MLE for SPECT using an intermediate polar representation and a stopping criterion

The maximum-likelihood estimation (MLE) approach to SPECT (single-photon-emission computed tomography) has received considerable interest since the application of the expectation maximum (EM) algorithm to emission tomography. In addition to the appealing asymptotic properties of MLE solutions, the EM algorithm guarantees convergence to a positive solution. In practice, several drawbacks to an MLE approach are encountered: the computational demands are quite large, especially in comparison to noniterative techniques; reconstructions tend to take on an increasingly, nonsmooth quality as the MLE solution is approached, so the algorithm is usually terminated long before convergence. The authors address these problems in two ways: they perform the actual source estimation on a rectangular grid but use a polar pixel representation of the source during forward/back projection operations to reduce the computational requirements; they use a chi-squared test to statistically determine a suitable cutoff point for the iterative process. >

Reconstruction of Images from Neuromagnetic Fields

In response to specific stimuli, the human brain emits a measurable magnetic field from regions actively involved in processing the stimulus. We have implemented an iterative algorithm to reconstruct images from the neuromagnetic field. Computer simulation studies performed to develop the algorithm are reported. Experimental measurements of a human visually-evoked field and images reconstructed therefrom are also reported. The results demonstrate, for the first time, the feasibility of imaging multiple sources within the brain that produce a magnetic field.