Biomedical subjects

Jeff H Duyn

Publications and source records attributed to Jeff H Duyn.

18 recordsLinked to original sources

PubMed2006

An adaptive filter for suppression of cardiac and respiratory noise in MRI time series data.

The quality of MRI time series data, which allows the study of dynamic processes, is often affected by confounding sources of signal fluctuation, including the cardiac and respiratory cycle. An adaptive filter is described, reducing these signal fluctuations as long as they are repetitive and their timing is known. The filter, applied in image domain, does not require temporal oversampling of the artifact-related fluctuations. Performance is demonstrated for suppression of cardiac and respiratory artifacts in 10-minute brain scans on 6 normal volunteers. Experimental parameters resemble a typical fMRI experiment (17 slices; 1700 ms TR). A second dataset was acquired at a rate well above the Nyquist frequency for both cardiac and respiratory cycle (single slice; 100 ms TR), allowing identification of artifacts specific to the cardiac and respiratory cycles, aiding assessment of filtering performance. Results show significant reduction in temporal standard deviation (SD(t)) in all subjects. For all 6 datasets with 1700 ms TR combined, the filtering method resulted in an average reduction in SD(t) of 9.2% in 2046 voxels substantially affected by respiratory artifacts, and 12.5% for the 864 voxels containing substantial cardiac artifacts. The maximal SD(t) reduction achieved was 52.7% for respiratory and 55.3% for cardiac filtering. Performance was found to be at least equivalent to the previously published RETROICOR method. Furthermore, the interaction between the filter and fMRI activity detection was investigated using Monte Carlo simulations, demonstrating that filtering algorithms introduce a systematic error in the detected BOLD-related signal change if applied sequentially. It is demonstrated that this can be overcome by combining physiological artifact filtering and detection of BOLD-related signal changes simultaneously. Visual fMRI data from 6 volunteers were analyzed with and without the filter proposed here. Inclusion of the cardio-respiratory regressors in the design matrix yielded a 4.6% t-score increase and 4.0% increase in the number of significantly activated voxels.

Adult↗

PubMed2006

Large-amplitude, spatially correlated fluctuations in BOLD fMRI signals during extended rest and early sleep stages.

A number of recent studies of human brain activity using blood-oxygen-level-dependent (BOLD) fMRI and EEG have reported the presence of spatiotemporal patterns of correlated activity in the absence of external stimuli. Although these patterns have been hypothesized to contain important information about brain architecture, little is known about their origin or about their relationship to active cognitive processes such as conscious awareness and monitoring of the environment. In this study, we have investigated the amplitude and spatiotemporal characteristics of resting-state activity patterns and their dependence on the subjects' alertness. For this purpose, BOLD fMRI was performed at 3.0 T on 12 normal subjects using a visual stimulation protocol, followed by a 27 min rest period, during which subjects were allowed to fall asleep. In subjects who were asleep at the end of the scan, we found (a) a higher amplitude of BOLD signal fluctuation during rest compared with subjects who were awake at the end of the scan; (b) spatially independent patterns of correlated activity that involve all of gray matter, including deep brain nuclei; (c) many patterns that were consistent across subjects; (d) that average percentage levels of fluctuation in visual cortex (VC) and whole brain were higher in subjects who were asleep (up to 1.71% and 1.16%, respectively) than in those who were awake (up to 1.15% and 0.96%) at the end of the scan and were comparable with those levels evoked by intense visual stimulation (up to 1.85% and 0.76% for two subject groups); (e) no confirmation of correlation, positive or negative, between thalamus and VC found in earlier studies. These findings suggest that resting-state activity continues during sleep and does not require active cognitive processes or conscious awareness.

Adult↗

PubMed2006

Analysing radio-frequency coil arrays in high-field magnetic resonance imaging by the combined field integral equation method.

We present the combined field integral equation (CFIE) method for analysing radio-frequency coil arrays in high-field magnetic resonance imaging (MRI). Three-dimensional models of coils and the human body were used to take into account the electromagnetic coupling. In the method of moments formulation, we applied triangular patches and the Rao-Wilton-Glisson basis functions to model arbitrarily shaped geometries. We first examined a rectangular loop coil to verify the CFIE method and also demonstrate its efficiency and accuracy. We then studied several eight-channel receive-only head coil arrays for 7.0 T SENSE functional MRI. Numerical results show that the signal dropout and the average SNR are two major concerns in SENSE coil array design. A good design should be a balance of these two factors.

Algorithms↗

PubMed2006

Accelerated parallel imaging for functional imaging of the human brain.

Accelerated parallel imaging (PI) techniques have recently been applied to functional imaging experiments of the human brain in order to improve the performance of commonly used single-shot techniques like echo-planar imaging (EPI). Potential benefits of PI-fMRI include the reduction of geometrical distortions due to off-resonance signals, the reduction of signal-loss in areas with substantial signal inhomogeneity, increases of the spatial and temporal resolution of the fMRI experiment and reduction of gradient acoustic noise. Although PI generally leads to a substantial decrease in image signal-to-noise ratio (SNR), its effect on the temporal stability of the signal, which ultimately determines fMRI performance, is only partially determined by image SNR. Therefore, the penalty for using PI is generally not as severe as the SNR reduction. The majority of problems related to single-shot techniques become more severe at an increased magnetic field strength, making PI an important tool in achieving the full potential of fMRI at high field.

Algorithms↗

PubMed2005

Technological advances in MRI measurement of brain perfusion.

Measurement of brain perfusion using arterial spin labeling (ASL) or dynamic susceptibility contrast (DSC) based MRI has many potential important clinical applications. However, the clinical application of perfusion MRI has been limited by a number of factors, including a relatively poor spatial resolution, limited volume coverage, and low signal-to-noise ratio (SNR). It is difficult to improve any of these aspects because both ASL and DSC methods require rapid image acquisition. In this report, recent methodological developments are discussed that alleviate some of these limitations and make perfusion MRI more suitable for clinical application. In particular, the availability of high magnetic field strength systems, increased gradient performance, the use of RF coil arrays and parallel imaging, and increasing pulse sequence efficiency allow for increased image acquisition speed and improved SNR. The use of parallel imaging facilitates the trade-off of SNR for increases in spatial resolution. As a demonstration, we obtained DSC and ASL perfusion images at 3.0 T and 7.0 T with multichannel RF coils and parallel imaging, which allowed us to obtain high-quality images with in-plane voxel sizes of 1.5 x 1.5 mm(2).

Cerebrovascular Circulation↗

PubMed2005

Optimizing brain tissue contrast with EPI: a simulated annealing approach.

A new magnetization preparation and image acquisition scheme was developed to obtain high-resolution brain images with optimal tissue contrast. The pulse sequence was derived from an optimization process using simulated annealing, without prior assumptions with regard to the number of radiofrequency (RF) pulses and flip angles. The resulting scheme combined two inversion pulses with the acquisition of three images with varying contrast. The combination of the three images allowed separation of gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF) based on T1, contrast. It also enabled the correction of small errors in the initial T1 estimates in postprocessing. The use of three-dimensional (3D) sensitivity-encoded (SENSE) echo-planar imaging (EPI) for image acquisition made it possible to achieve a 1.15(3) mm3 isotropic resolution within a scan time of 10 min 21 s. The cortical GM signal-to-noise ratio (SNR) in the calculated GM-only image varied between 30 and 100. The novel technique was evaluated in combination with blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) on human subjects, and provided for excellent coregistration of anatomical and functional data.

Brain Mapping↗

PubMed2005

Resolution and reproducibility of BOLD and perfusion functional MRI at 3.0 Tesla.

Visual and somatosensory activation studies were performed on normal subjects to compare the spatial discrimination and reproducibility between functional MRI (fMRI) methods based on blood oxygen level-dependent (BOLD) and perfusion contrast. To allow simultaneous measurement of BOLD and perfusion contrast, a dedicated MRI acquisition technique was developed. Repeated experiments of sensory stimulation of single digits of the right hand showed an average variability of activation amplitude of 25% for BOLD data, and a significantly lower variability of 21% for perfusion data. No significant difference in the variability of the locus of activity was observed between the BOLD and perfusion data. In somatotopy experiments, digits II and V were subjected to passive sensory stimulation. Both the BOLD and perfusion data showed substantial overlap in the activation patterns from the two digits. In a retinotopy study, two stimuli were alternated to excite different patches of V1. Again there was substantial overlap between the activation patterns from both stimuli, although the perfusion performed somewhat better than the BOLD method. Particularly for the visual studies, the overlap in activation patterns was more than expected based on the fine-scale retinotopic mapping of cortical activity, suggesting that both BOLD and perfusion contrast mechanisms contribute substantially to the point-spread function (PSF).

Adult↗

PubMed2004

Temporal dynamics of the BOLD fMRI impulse response.

Using computer simulations and high-resolution fMRI experiments in humans (n=6) and rats (n=8), we investigated to what extent BOLD fMRI temporal resolution is limited by dispersion in the venous vasculature. For this purpose, time-to-peak (TTP) and full-width at half-maximum (FWHM) of the BOLD impulse response (IR) function were determined. In fMRI experiments, a binary m-sequence probe method was used to obtain high-sensitivity model-free single-pixel estimates of IR. Simulations of postcapillary flow suggested that flow-related dispersion leads to a TTP and FWHM increase, which can amount to several seconds in larger pial veins. fMRI experiments showed substantial spatial variation in IR timing within human visual cortex, together with a correlation between TTP and FWHM. Averaged across the activated regions and across subjects, TTP and FWHM were 4.51+/-0.52 and 4.04+/-0.42 s, respectively. In regions of interest (ROI) weighted toward the larger venous structures, TTP and FWHM increased to 5.07+/-0.64 and 4.32+/-0.48 s, respectively. In rat somatosensory cortex, TTP and FWHM were substantially shorter than in humans (2.73+/-0.60 and 2.28+/-0.63 s, respectively). These results are consistent with a substantial macrovascular dispersive contribution to BOLD IR in humans, and furthermore suggest that neurovascular coupling is a relatively rapid process, with a resolution below 2.3 s FWHM.

Adult↗

PubMed2004

Unraveling multisensory integration: patchy organization within human STS multisensory cortex.

Although early sensory cortex is organized along dimensions encoded by receptor organs, little is known about the organization of higher areas in which different modalities are integrated. We investigated multisensory integration in human superior temporal sulcus using recent advances in parallel imaging to perform functional magnetic resonance imaging (fMRI) at very high resolution. These studies suggest a functional architecture in which information from different modalities is brought into close proximity via a patchy distribution of inputs, followed by integration in the intervening cortex.

Acoustic Stimulation↗

PubMed2004

EPI-BOLD fMRI of human motor cortex at 1.5 T and 3.0 T: sensitivity dependence on echo time and acquisition bandwidth.

PURPOSE: To investigate the sensitivity dependence of BOLD functional imaging on MRI acquisition parameters in motor stimulation experiments using a finger tapping paradigm. MATERIALS AND METHODS: Gradient-echo echo-planar fMRI experiments were performed at 1.5 T and 3.0 T with varying acquisition echo time and bandwidth, and with a 4 mm isotropic voxel size. To analyze the BOLD sensitivity, the relative contributions of BOLD signal amplitude and thermal and physiologic noise sources were evaluated, and statistical t-scores were compared in the motor area. RESULTS: At 1.5 T, the number of activated pixels and the average t-score showed a relatively broad optimum over a TE range of 60-160 msec. At 3.0 T, an optimum range was observed between TEs of 30-130 msec. Averaged over nine subjects, maxima in the number of pixels and t-score values were 59% and 18% higher at 3.0 T than at 1.5 T, respectively, an improvement that was lower than the observed 100% to 110% increase in signal-to-noise ratio at 3.0 T. CONCLUSION: The somewhat disappointing increase in t-scores at 3.0 T was attributed to the increased contribution of physiologic noise at the higher field strength under the given experimental conditions. At both field strengths, reducing the effective image acquisition bandwidth from 35 to 17 Hz per pixel did not affect or only marginally affect the BOLD sensitivity.

Adult↗

PubMed2004

Signal-to-noise ratio and parallel imaging performance of a 16-channel receive-only brain coil array at 3.0 Tesla.

The performance of a 16-channel receive-only RF coil for brain imaging at 3.0 Tesla was investigated using a custom-built 16-channel receiver. Both the image signal-to-noise ratio (SNR) and the noise amplification (g-factor) in sensitivity-encoding (SENSE) parallel imaging applications were quantitatively evaluated. Furthermore, the performance was compared with that of hypothetical coils with one, two, four, and eight elements (n) by combining channels in software during image reconstruction. As expected, both the g-factor and SNR improved substantially with n. Compared to an equivalent (simulated) single-element coil, the 16-channel coil showed a 1.87-fold average increase in brain SNR. This was mainly due to an increase in SNR in the peripheral brain (an up to threefold SNR increase), whereas the SNR increase in the center of the brain was 4%. The incremental SNR gains became relatively small at large n, with a 9% gain observed when n was increased from 8 to 16. Compared to the (larger) product birdcage head coil, SNR increased by close to a factor of 2 in the center, and by up to a factor of 6 in the periphery of the brain. For low SENSE acceleration (rate-2), g-factors leveled off for n>4, and improved only slightly (1.4% averaged over brain) going from n=8 to n=16. Improvements in g for n>8 were larger for higher acceleration rates, with the improvement for rate-3 averaging 12.0%.

Brain Mapping↗

PubMed2004

Scalable multichannel MRI data acquisition system.

A scalable multichannel digital MRI receiver system was designed to achieve high bandwidth echo-planar imaging (EPI) acquisitions for applications such as BOLD-fMRI. The modular system design allows for easy extension to an arbitrary number of channels. A 16-channel receiver was developed and integrated with a General Electric (GE) Signa 3T VH/3 clinical scanner. Receiver performance was evaluated on phantoms and human volunteers using a custom-built 16-element receive-only brain surface coil array. At an output bandwidth of 1 MHz, a 100% acquisition duty cycle was achieved. Overall system noise figure and dynamic range were better than 0.85 dB and 84 dB, respectively. During repetitive EPI scanning on phantoms, the relative temporal standard deviation of the image intensity time-course was below 0.2%. As compared to the product birdcage head coil, 16-channel reception with the custom array yielded a nearly 6-fold SNR gain in the cerebral cortex and a 1.8-fold SNR gain in the center of the brain. The excellent system stability combined with the increased sensitivity and SENSE capabilities of 16-channel coils are expected to significantly benefit and enhance fMRI applications.

Brain↗

PubMed2004

Hunting for neuronal currents: absence of rapid MRI signal changes during visual-evoked response.

While recent reports have advocated the use of magnetic resonance imaging (MRI) to detect the effects of neuronal currents associated with human brain activity, only preliminary experimental data have been presented so far to demonstrate the feasibility of the method. Furthermore, it has not been adequately demonstrated that (1) MRI can separate neuronal current (NC) effects from other effects such as blood oxygen level-dependent (BOLD) contrast; (2) MRI has adequate sensitivity to detect NCs in vivo. In this work, we introduce a method that can separate slow (e.g., BOLD) processes from potential rapid (e.g., NC) processes and apply this method to investigate whether MRI allows detection of an NC response to a visual stimulus. MRI studies (n = 8) at 3.0 T using a sensitive multichannel detector showed insignificant effects related to NCs (averaged t < 0.05), in the presence of a highly significant BOLD signal (t = 6.15 +/- 0.90). In contrast, magnetoencephalography (MEG) experiments performed under similar conditions on the same subjects showed highly significant electrical activity (t = 7.90 +/- 2.28). It is concluded that, under the conditions used in this study, the sensitivity of MRI to detect evoked responses through NCs is at least an order of magnitude below that of BOLD-based functional MRI (fMRI) or MEG and too low to be practically useful.

Data Interpretation, Statistical↗

PubMed2002

Design of a SENSE-optimized high-sensitivity MRI receive coil for brain imaging.

An 8-channel receive-only detector array was developed for SENSE MRI of human brain. The coil geometry was based on a gapped element design and used ultra-high impedance preamplifiers for mutual decoupling of the elements. Computer simulations of the electric and magnetic fields showed that excellent signal-to-noise ratio (SNR) and SENSE performance could be achieved by placing the coil elements close to the head and maintaining a substantial gap between the elements. Measurements with a 1.5 T prototype coil showed a 2.7-fold improvement of the SNR averaged over the brain compared to a conventional quadrature birdcage receive coil and an average geometrical noise amplification factor (g-value) of 1.06 and 1.38 for rate-2 and rate-3 SENSE, respectively.

Brain↗

PubMed2002

Application of sensitivity-encoded echo-planar imaging for blood oxygen level-dependent functional brain imaging.

The benefits of sensitivity-encoded (SENSE) echo-planar imaging (EPI) for functional MRI (fMRI) based on blood oxygen level-dependent (BOLD) contrast were quantitatively investigated at 1.5 T. For experiments with 3.4 x 3.4 x 4.0 mm(3) resolution, SENSE allowed the single-shot EPI image acquisition duration to be shortened from 24.1 to 12.4 ms, resulting in a reduced sensitivity to geometric distortions and T(*)(2) blurring. Finger-tapping fMRI experiments, performed on eight normal volunteers, showed an overall 18% loss in t-score in the activated area, which was substantially smaller than expected based on the image signal-to-noise ratio (SNR) and g-factor, but similar to the loss predicted by a model that takes physiologic noise into account.

Brain↗

PubMed2002

Reduction of gradient acoustic noise in MRI using SENSE-EPI.

A new approach to reduce gradient acoustic noise levels in EPI experiments is presented. Using multichannel RF receive coils, combined with SENSE data acquisition and reconstruction, gradient slew-rates in single-shot EPI were reduced fourfold for rate-2 and ninefold for rate-3 SENSE. Multislice EPI experiments were performed on three different scanner platforms. With 3.4 mm in-plane resolution, measuring 6 slices per second (12 slices with 2000 ms TR), this resulted in average sound pressure level reductions of 11.3 dB(A) and 16.5 dB(A) for rate-2 and rate-3 SENSE, respectively. BOLD fMRI experiments, using visually paced finger-tapping paradigms, showed no detrimental effect of the acoustic noise reduction strategy on temporal noise levels and t scores.

Adult↗

PubMed2002

Cortical systems associated with covert music rehearsal.

Musical representation and overt music production are necessarily complex cognitive phenomena. While overt musical performance may be observed and studied, the act of performance itself necessarily skews results toward the importance of primary sensorimotor and auditory cortices. However, imagined musical performance (IMP) represents a complex behavioral task involving components suited to exploring the physiological underpinnings of musical cognition in music performance without the sensorimotor and auditory confounds of overt performance. We mapped the blood oxygenation level-dependent fMRI activation response associated with IMP in experienced musicians independent of the piece imagined. IMP consistently activated supplementary motor and premotor areas, right superior parietal lobule, right inferior frontal gyrus, bilateral mid-frontal gyri, and bilateral lateral cerebellum in contrast with rest, in a manner distinct from fingertapping versus rest and passive listening to the same piece versus rest. These data implicate an associative network independent of primary sensorimotor and auditory activity, likely representing the cortical elements most intimately linked to music production.

Adult↗

PubMed2002

Pulsed arterial spin labeling: comparison of multisection baseline and functional MR imaging perfusion signal at 1.5 and 3.0 T: initial results in six subjects.

Flow-alternating inversion-recovery magnetic resonance imaging was performed at 3.0 T to measure cerebral perfusion during rest and motor activation in six healthy adult volunteers. Results were compared with those at 1.5 T. The mean signal-to-noise ratio for both gray matter and white matter perfusion measured with and without vascular suppression at 3.0 T was significantly (P <.01) higher (n = 6) than that at 1.5 T. Brain perfusion activation maps collected during a motor task showed a substantially larger number of activated pixels (>80%) at 3.0 T, with activation in the supplementary motor area in the 3.0-T data that was not present on 1.5-T perfusion maps.

Adult↗