Biomedical subjects

Peter Kellman

Publications and source records attributed to Peter Kellman.

30 records · Page 2Linked to original sources

PubMed2004

Artifact suppression in imaging of myocardial infarction using B1-weighted phased-array combined phase-sensitive inversion recovery.

Regions of the body with long T1, such as cerebrospinal fluid (CSF), may create ghost artifacts on gadolinium-hyperenhanced images of myocardial infarction when inversion recovery (IR) sequences are used with a segmented acquisition. Oscillations in the transient approach to steady state for regions with long T1 may cause ghosts, with the number of ghosts being equal to the number of segments. B1-weighted phased-array combining provides an inherent degree of ghost artifact suppression because the ghost artifact is weighted less than the desired signal intensity by the coil sensitivity profiles. Example images are shown that illustrate the suppression of CSF ghost artifacts by the use of B1-weighted phased-array combining of multiple receiver coils.

Artifacts↗

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↗

PubMed2003

Automatic in-plane rotation for doubly-oblique cardiac imaging.

PURPOSE: To develop and test a method for automatically calculating the in-plane rotation for doubly-oblique slice geometry in order to minimize wrap artifacts for a given FOV. MATERIALS AND METHODS: The equations for in-plane rotation were formulated for doubly-oblique imaging of a cylindrical body with elliptical cross-section. Based on this formulation, automatic in-plane rotation was implemented and tested on a commercial scanner using nominal values for ellipticity of the body. RESULTS: Short axis, doubly oblique, cardiac imaging were acquired with and without in-plane rotation. The desired in-plane rotation proved to be relatively insensitive to the ellipticity of the body, permitting an automatic solution based on a nominal value. CONCLUSION: In-plane rotation is desirable for doubly oblique imaging (e.g., cardiac applications), particularly for reduced-FOV accelerated imaging such as SENSE. The proposed method, which provides an approximate solution for automatic, in-plane rotation for doubly-oblique imaging, was demonstrated.

Artifacts↗

PubMed2003

Real-time accelerated interactive MRI with adaptive TSENSE and UNFOLD.

Reduced field-of-view (FOV) acceleration using time-adaptive sensitivity encoding (TSENSE) or unaliasing by Fourier encoding the overlaps using the temporal dimension (UNFOLD) can improve the depiction of motion in real-time MRI. However, increased computational resources are required to maintain a high frame rate and low latency in image reconstruction and display. A high-performance software system has been implemented to perform TSENSE and UNFOLD reconstructions for real-time MRI with interactive, on-line display. Images were displayed in the scanner room to investigate image-guided procedures. Examples are shown for normal volunteers and cardiac interventional experiments in animals using a steady-state free precession (SSFP) sequence. In order to maintain adequate image quality for interventional procedures, the imaging rate was limited to seven frames per second after an acceleration factor of 2 with a voxel size of 1.8 x 3.5 x 8 mm. Initial experiences suggest that TSENSE and UNFOLD can each improve the compromise between spatial and temporal resolution in real-time imaging, and can function well in interactive imaging. UNFOLD places no additional constraints on receiver coils, and is therefore more flexible than SENSE methods; however, the temporal image filtering can blur motion and reduce the effective acceleration. Methods are proposed to overcome the challenges presented by the use of TSENSE in interactive imaging. TSENSE may be temporarily disabled after changing the imaging plane to avoid transient artifacts as the sensitivity coefficients adapt. For imaging with a combination of surface and interventional coils, a hybrid reconstruction approach is proposed whereby UNFOLD is used for the interventional coils, and TSENSE with or without UNFOLD is used for the surface coils.

Animals↗

PubMed2003

Assessment of regional systolic and diastolic dysfunction in familial hypertrophic cardiomyopathy using MR tagging.

Diastolic and systolic left ventricular (LV) dysfunction often significantly contribute to disabling symptoms in familial hypertrophic cardiomyopathy (FHC). This study compares regional LV function (midwall circumferential strain) during systole and diastole in eight FHC patients and six normal volunteers (NVs) using MR tagging. A prospectively-gated fast gradient-echo sequence with an echo-train readout was modified to support complementary spatial modulation of magnetization (CSPAMM) tagging and full cardiac cycle data acquisition using the cardiac phase to order reconstruction (CAPTOR), thus providing tag persistence and data acquisition during the entire cardiac cycle. Total systolic strains in FHC patients were significantly reduced in septal and inferior regions (both P < 0.01). Early-diastolic strain rates were reduced in all regions of the FHC group (all P < 0.03). The combination of CSPAMM and CAPTOR allows regional indices of myocardial function to be quantified throughout the cardiac cycle. This technique reveals regional differences in systolic and diastolic impairment in FHC patients.

Cardiomyopathy, Hypertrophic↗

PubMed2003

Method for functional MRI mapping of nonlinear response.

Nonlinear systems analysis combining blood oxygen level dependent (BOLD), functional magnetic resonance imaging (fMRI) and m-sequence stimulation paradigms are proposed as a new method for exploring neuronal responses and interactions. Previous studies of electrical activity in the human visual cortex have observed significant nonlinearities of task-induced activity with temporal dynamics on a timescale of 10-20 ms. Despite the confounding effect of the seconds-long hemodynamic response, it is demonstrated that BOLD fMRI can be used to probe neuronal interactions on a time scale of tens of ms. Visual activation experiments were performed with various stimuli, and amplitude maps of first and second order kernel coefficients were generated using correlation analysis. Second order nonlinearities in BOLD fMRI were observed and attributed to temporal contrast caused by transitions in the stimulus sequence. In addition, the kernel maps showed significant differences between second order nonlinearities of foveal and peripheral vision. By including a reference experiment with a slightly modified stimulus presentation, a distinction could be made between (fast) neuronal nonlinearities and hemodynamic effects on the time scale of the seconds. The results indicate that BOLD fMRI can probe fast neuronal nonlinearities.

Brain Mapping↗

PubMed2002

Phase-sensitive inversion recovery for detecting myocardial infarction using gadolinium-delayed hyperenhancement.

After administration of gadolinium, infarcted myocardium exhibits delayed hyperenhancement and can be imaged using an inversion recovery (IR) sequence. The performance of such a method when using magnitude-reconstructed images is highly sensitive to the inversion recovery time (TI) selected. Using phase-sensitive reconstruction, it is possible to use a nominal value of TI, eliminate several breath-holds otherwise needed to find the precise null time for normal myocardium, and achieve a consistent contrast. Phase-sensitive detection is used to remove the background phase while preserving the sign of the desired magnetization during IR. Experimental results are presented which demonstrate the benefits of both phase-sensitive IR image reconstruction and surface coil intensity normalization for detecting myocardial infarction (MI). The phase-sensitive reconstruction method reduces the variation in apparent infarct size that is observed in the magnitude images as TI is changed. Phase-sensitive detection also has the advantage of decreasing the sensitivity to changes in tissue T(1) with increasing delay from contrast agent injection.

Artifacts↗

PubMed2002

Multishot EPI-SSFP in the heart.

Refocused steady-state free precession (SSFP), or fast imaging with steady precession (FISP or TrueFISP), has recently proven valuable for cardiac imaging because of its high signal-to-noise ratio (SNR) and excellent blood-myocardium contrast. In this study, various implementations of multiecho SSFP or EPI-SSFP for imaging in the heart are presented. EPI-SSFP has higher scan-time efficiency than single-echo SSFP, as two or more phase-encode lines are acquired per repetition time (TR) at the cost of a modest increase in TR. To minimize TR, a noninterleaved phase-encode order in conjunction with a phased-array ghost elimination (PAGE) technique was employed, removing the need for echo time shifting (ETS). The multishot implementation of EPI-SSFP was used to decrease the breath-hold duration for cine acquisitions or to increase the temporal or spatial resolution for a fixed breath-hold duration. The greatest gain in efficiency was obtained with the use of a three-echo acquisition. Image quality for cardiac cine applications using multishot EPI-SSFP was comparable to that of single-echo SSFP in terms of blood-myocardium contrast and contrast-to-noise ratio (CNR). The PAGE method considerably reduced flow artifacts due to both the inherent ghost suppression and the concomitant reduction in phase-encode blip size. The increased TR of multishot EPI-SSFP led to a reduced specific absorption rate (SAR) for a fixed RF flip angle, and allowed the use of a larger flip angle without increasing the SAR above the FDA-approved limits.

Artifacts↗

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

Phased array ghost elimination (PAGE) for segmented SSFP imaging with interrupted steady-state.

Steady-state free precession (SSFP) has recently proven to be valuable for cardiac imaging due to its high signal-to-noise ratio and blood-myocardium contrast. Data acquired using ECG-triggered, segmented sequences during the approach to steady-state, or return to steady-state after interruption, may have ghost artifacts due to periodic k-space distortion. Schemes involving several preparatory RF pulses have been proposed to restore steady-state, but these consume imaging time during early systole. Alternatively, the phased-array ghost elimination (PAGE) method may be used to remove ghost artifacts from the first several frames. PAGE was demonstrated for cardiac cine SSFP imaging with interrupted steady-state using a simple alpha/2 magnetization preparation and storage scheme and a spatial tagging preparation.

Artifacts↗

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↗