Biomedical subjects

J McCubbin

Publications and source records attributed to J McCubbin.

10 recordsLinked to original sources

PubMed2007

Searching for the best model: ambiguity of inverse solutions and application to fetal magnetoencephalography.

Fetal brain signals produce weak magnetic fields at the maternal abdominal surface. In the presence of much stronger interference these weak fetal fields are often nearly indistinguishable from noise. Our initial objective was to validate these weak fetal brain fields by demonstrating that they agree with the electromagnetic model of the fetal brain. The fetal brain model is often not known and we have attempted to fit the data to not only the brain source position, orientation and magnitude, but also to the brain model position. Simulation tests of this extended model search on fetal MEG recordings using dipole fit and beamformers revealed a region of ambiguity. The region of ambiguity consists of a family of models which are not distinguishable in the presence of noise, and which exhibit large and comparable SNR when beamformers are used. Unlike the uncertainty of a dipole fit with known model plus noise, this extended ambiguity region yields nearly identical forward solutions, and is only weakly dependent on noise. The ambiguity region is located in a plane defined by the source position, orientation, and the true model centre, and will have a diameter approximately 0.67 of the modelled fetal head diameter. Existence of the ambiguity region allows us to only state that the fetal brain fields do not contradict the electromagnetic model; we can associate them with a family of models belonging to the ambiguity region, but not with any specific model. In addition to providing a level of confidence in the fetal brain signals, the ambiguity region knowledge in combination with beamformers allows detection of undistorted temporal waveforms with improved signal-to-noise ratio, even though the source position cannot be uniquely determined.

Biophysical Phenomena↗

PubMed2006

Optimal reduction of MCG in fetal MEG recordings.

Recording fetal magnetoencephalographic (fMEG) signals in-utero is a demanding task due to biological interference, especially maternal and fetal magnetocardiographic (MCG) signals. A method based on orthogonal projection of MCG signal space vectors (OP) was evaluated and compared with independent component analysis (ICA). The evaluation was based on MCG amplitude reduction and signal-to-noise ratio of fetal brain signals using exemplary datasets recorded during ongoing studies related to auditory evoked fields. The results indicate that the OP method is the preferable approach for attenuation of MCG and for preserving the fetal brain signals in fMEG recordings.

Algorithms↗

PubMed2004

How many channels are needed for MEG?

Channel count in modern MEG systems has been steadily increasing, but are more channels necessary? Assuming that the spatial sampling considerations are satisfied, this question can be answered by examining the MEG system's ability to localize and resolve brain sources. For the simple situation where only uncorrelated sensor noise is present, dipole localization accuracy monotonically increases with increasing number of channels, while for spatially correlated brain noise the accuracy increases only until the number of channels reaches 100 to 200. Beyond this limit the inter-channel separation is comparable to the brain noise correlation distance and increasing the channel count does not help. Contrary to the above dipole result, we show by simulations with up to several thousand channels, that if the data is analyzed by beamformers even in the presence of correlated brain noise, the two-source resolvability and single-source localization accuracy monotonically improve with increasing number of channels. We demonstrate such behavior for a 275 channels system, where we have inserted an artificial dipole into real measured brain noise and resampled the number of channels to 138. Beamformer analysis of the data shows markedly improved localization accuracy when the number of channels is increased from 138 to 275. This finding also signifies that the beamformer performance is not limited by system imperfections when the number of channels is as large as 275. To clarify these results, we illustrate analytically the mechanism of beamformer resolution dependence on the number of channels, using an example of a simple system containing two dipole sources, and uncorrelated sensor noise.

Brain↗

PubMed2004

Forward solution ambiguity for fMEG model.

Accurate knowledge of the electromagnetic model of the fetal brain is required for quantitative fetal MEG (fMEG) signal analysis. The model is defined by the fetal head position, assuming that volume currents are confined to it. Head position can be located by 3D ultrasound. Since the ultrasound is presently not integrated with fMEG, there is a potential for fetal head motion when the mother moves between the two instruments. To interpret the fMEG data without accurate information about the fetal head position, a model search must be undertaken to localize not only the fMEG source within the fetal head, but also the fetal head within the maternal abdomen. Such model search results in multiple solutions with different source amplitudes, but with roughly the same fit errors [Vrba, 2004]. We show that this behaviour is caused by the forward solution ambiguity. For a given measurement, there is a large region within the maternal abdomen within which all fetal head centers produce reasonably good explanation of the observed signal. The source positions within these different fetal heads are different, but the resulting signals are indistinguishable in the presence of even a small noise. Similar ambiguity also exists in adult MEG, where it usually does not pose a problem because the head position is well known.

Computer Simulation↗

PubMed2004

Advanced electronics for the CTF MEG system.

Development of the CTF MEG system has been advanced with the introduction of a computer processing cluster between the data acquisition electronics and the host computer. The advent of fast processors, memory, and network interfaces has made this innovation feasible for large data streams at high sampling rates. We have implemented tasks including anti-alias filter, sample rate decimation, higher gradient balancing, crosstalk correction, and optional filters with a cluster consisting of 4 dual Intel Xeon processors operating on up to 275 channel MEG systems at 12 kHz sample rate. The architecture is expandable with additional processors to implement advanced processing tasks which may include e.g., continuous head localization/motion correction, optional display filters, coherence calculations, or real time synthetic channels (via beamformer). We also describe an electronics configuration upgrade to provide operator console access to the peripheral interface features such as analog signal and trigger I/O. This allows remote location of the acoustically noisy electronics cabinet and fitting of the cabinet with doors for improved EMI shielding. Finally, we present the latest performance results available for the CTF 275 channel MEG system including an unshielded SEF (median nerve electrical stimulation) measurement enhanced by application of an adaptive beamformer technique (SAM) which allows recognition of the nominal 20-ms response in the unaveraged signal.

Cluster Analysis↗

PubMed2004

Human fetal brain imaging by magnetoencephalography: verification of fetal brain signals by comparison with fetal brain models.

Fetal magnetoencephalogram (fMEG) is measured in the presence of a large interference from maternal and fetal magnetocardiograms (mMCG and fMCG). This cardiac interference can be successfully removed by orthogonal projection of the corresponding spatial vectors. However, orthogonal projection redistributes the fMEG signal among channels. Such redistribution can be readily accounted for in the forward solution, and the signal topography can also be corrected. To assure that the correction has been done properly, and also to verify that the measured signal originates from within the fetal head, we have modeled the observed fMEG by two extreme models where the fetal head is assumed to be either electrically transparent or isolated from the abdominal tissue. Based on the measured spontaneous, sharp wave, and flash-evoked fMEG signals, we have concluded that the model of the electrically isolated fetal head is more appropriate for fMEG analysis. We show with the help of this model that the redistribution due to projection was properly corrected, and also, that the measured fMEG is consistent with the known position of the fetal head. The modeling provides additional confidence that the measured signals indeed originate from within the fetal head.

Algorithms↗

PubMed1999

Development of an instrument to measure community acceptance of nurse practitioners and physician assistants.

The purpose of this study was to identify the significant dimensions of the concept of community acceptance of nurse practitioners/physician's assistants and to construct a reliable and valid instrument which would reflect these dimensions. The methodological approach included: conceptualization of categories, development of items for each category, development of the tool, administration of the tool, and psychometric analysis of results. Community input through focus-group interviews and post-administration questions provided qualitative data. The survey tool, consisting of items in four conceptualized categories (knowledge, access, competence, and trust), was administered in five rural communities. The responses of 967 residents were analyzed through factor analysis. The criterion, eigenvalue > or = 1.0, resulted in seven factors. Oblique rotation was applied to the seven factors and marker variables (loadings > .70) facilitated the identification of the underlying dimensions of each factor. Overall, 98% of the items assigned to the original categories were maintained after factor analysis. The identification of these dimensions helped to simplify the description and understanding of community acceptance of nurse practitioners and physicians' assistants. Community acceptance of these advanced health care providers is a necessary precursor to use of services.

Adult↗

PubMed1998

Acceptance of nurse practitioners and physician assistants in meeting the perceived needs of rural communities.

Nurse practitioners and physician assistants have provided a partial solution to the shortage of primary care services in medically underserved rural areas. This paper describes the results of a study exploring community acceptance of nurse practitioners and physician assistants in rural medically underserved areas. Community acceptance in the context of this study implies not only satisfaction with care received, but also willingness of the community to support NP/PA practice through its infrastructure and encourage members to initially seek and continue to receive care from an NP or PA. Five focus groups were conducted in each of five rural medically underserved communities. The two most pervasive findings were the lack of previous exposure to NPs and PAs and the general belief that NPs and PAs would be accepted in these communities if certain conditions could be met. The theme of conditional acceptance included both personal and system factors. Personal factors included friendliness, competence, willingness to enter into the life of the community, and the ability to keep information confidential. System factors considered critical for acceptance included service type, integration with the existing health care system, cost, geographic proximity, and availability. The results of this study offer insight into community attitudes and suggest marketing strategies for those who plan to introduce NP or PA services into rural communities.

Focus Groups↗

PubMed1991

Biomagnetometers for unshielded and well shielded environments.

The paper describes magnetometers for operation in well shielded and unshielded environments. For unshielded environments, the noise cancellation is accomplished by spatial filtering using higher-order gradiometers which are formed in software. The theory of the software formation of high-order gradiometers has been successfully tested experimentally to the second-order gradient level, using a seven-channel prototype system. An unshielded, open environment system, designed to operate with third-order noise cancellation is presently under construction. It is a whole head system, consisting of a minimum of 64 sensing channels, and is expected to be operational by late 1991. In the opposite extreme, a magnetometer is being developed for operation in a well shielded environment of a whole body superconducting shield. The high temperature superconducting shield will operate in liquid nitrogen, while the magnetometer detection system will operate in liquid helium. The high level of shielding will allow detection of biomagnetic signals using magnetometers. The whole body high temperature superconducting shielded system is presently under development and a half scale working shield already exists. The complete whole body shielded system is expected to be operational in 1994/95.

Equipment Design↗

PubMed1988

Personal health training and the severely handicapped: a curriculum based research investigation.

Project TRANSITION studied, as part of a three year field initiated research project, the effect of training selected personal health skills and their relationship to adaptive behavior. The subjects were moderately, severely, and profoundly retarded individuals residing in a state-operated developmental center. Eighty-eight subjects were selected and then randomly assigned to one of four experimental groups and participated in a 14-week training program. To test the effects of the training, valid and reliable curriculum-embedded tests were developed and used. A curriculum-embedded test uses the material to be learned as the basis for assessing the degree to which the material has been learned. These tests were designed to measure and monitor subjects' independent ability to perform each task analyzed skill. The Adaptive Behavior Scale was used to measure changes in adaptive behavior. Results of the personal health skill testing yielded significant group differences on the independent indices developed for the Project. No meaningful significant differences were found in adaptive behavior. The data indicated that the developed curriculum and scoring system was effectively used to train and monitor selected independent personal health skills of the severely mentally retarded adults in this study.

Adaptation, Psychological↗