Biomedical subjects

S K Nagle

Publications and source records attributed to S K Nagle.

5 recordsLinked to original sources

PubMed1999

Locally focused contrast-enhanced carotid MRA.

With conventional Fourier transform (FT) magnetic resonance imaging (MRI), it is difficult to perform contrast-enhanced three-dimensional (3D) MR angiography (MRA) with the temporal and spatial resolution necessary to depict the carotid arteries. However, locally focused (LF) MRI is a more efficient method that utilizes prior knowledge of the image content to reconstruct images from sparse k-space samples. In this paper, we show how LF MRI can be used to perform high-resolution gadolinium (Gd)-enhanced 3D carotid MRA in less than 10 seconds. First, the accuracy of the technique was demonstrated by comparing LF and conventional (FT) images of a vascular phantom. Then the method was used to perform Gd-enhanced 3D MRA of a patient's carotid arteries. Instead of using bolus timing, the arterial phase was retrospectively identified in a consecutive series of images, just as in X-ray angiography.

Adult↗

PubMed1999

Multiple region MRI.

Traditional Fourier MR imaging (FT MRI) utilizes the Whittaker-Kotel'nikov-Shannon (WKS) sampling theorem. This theorem specifies the spatial frequency components which need to be measured to reconstruct an image with a known field of view (FOV). In this paper, we generalize this result in order to find the optimal k-space sampling for images that vanish except in multiple, possibly non-adjacent regions within the FOV. This provides the basis for "multiple region MRI" (mrMRI), a method of producing such images from a traction of the k-space samples required by the WKS theorem. Image reconstruction does not suffer from noise amplification and can be performed rapidly with fast Fourier transforms, just as in conventional FT MRI. The mrMRI method can also be used to reconstruct images that have low spatial-frequency components throughout the entire FOV and high spatial frequencies (i.e. edges) confined to multiple small regions. The greater efficiency of mrMRI sampling can be parlayed into increased temporal or spatial resolution whenever the imaged objects have signal or "edge" intensity confined to multiple small portions of the FOV. Possible areas of application include MR angiography (MRA), interventional MRI, functional MRI, and spectroscopic MRI. The technique is demonstrated by using it to acquire Gd-enhanced first-pass 3D MRA images of the carotid arteries without the use of bolus-timing techniques.

Carotid Arteries↗

PubMed1996

Single residue substitutions that change the gating properties of a mechanosensitive channel in Escherichia coli.

MscL is a channel that opens a large pore in the Escherichia coli cytoplasmic membrane in response to mechanical stress. Previously, we highly enriched the MscL protein by using patch clamp as a functional assay and cloned the corresponding gene. The predicted protein contains a largely hydrophobic core spanning two-thirds of the molecule and a more hydrophilic carboxyl terminal tail. Because MscL had no homology to characterized proteins, it was impossible to predict functional regions of the protein by simple inspection. Here, by mutagenesis, we have searched for functionally important regions of this molecule. We show that a short deletion from the amino terminus (3 amino acids), and a larger deletion of 27 amino acids from the carboxyl terminus of this protein, had little if any effect in channel properties. We have thus narrowed the search of the core mechanosensitive mechanism to 106 residues of this 136-amino acid protein. In contrast, single residue substitutions of a lysine in the putative first transmembrane domain or a glutamine in the periplasmic loop caused pronounced shifts in the mechano-sensitivity curves and/or large changes in the kinetics of channel gating, suggesting that the conformational structure in these regions is critical for normal mechanosensitive channel gating.

Amino Acid Sequence↗

PubMed1996

Towards an understanding of the structural and functional properties of MscL, a mechanosensitive channel in bacteria.

Whether it be to sense a touch, arterial pressure, or an osmotic gradient across a cell membrane, essentially all living organisms require the capability of detecting mechanical force. Electrophysiological evidence has suggested that mechanosensitive ion channels play a major role in many systems where mechanical force is detected. But, despite their biological importance, determination of the most basic structural and functional features of mechanosensitive channels has only recently become possible. A gene called mscL, which was isolated from Escherichia coli, was the first gene shown to encode a mechanosensitive channel activity. This channel directly responds to tension in the membrane; no other proteins are required. MscL appears to be a homohexamer of a 136 amino acid polypeptide that is highly alpha helical, contains two transmembrane domains, and has both the amino and carboxyl termini in the cytoplasm. The study of the MscL protein remains, to date, one of the most viable options for understanding the structural and functional characteristics of a mechanosensitive channel.

Bacterial Proteins↗

PubMedDate not supplied

Locally focused MRI of interventions.

Certain interventional MR procedures would benefit from T2-weighted imaging because of the sensitivity of T2-weighted images to tissue damage and target lesion contrast. To acquire such images with reasonable temporal resolution, a single-shot acquisition should be used because of the inherently long TR needed for T2 weighting. Unfortunately, most scanners require long readout times (eg, greater than 150 msec) and high bandwidths (eg, greater than 120 kHz) to perform conventional single-shot imaging with high spatial resolution. The resulting images are thus degraded by unacceptable artifacts and noise levels. This study illustrates how to create locally focused MR images that have high spatial resolution in a region of interest and lower spatial resolution elsewhere in the image. Because these images can be created from sparse k-space data, a scanner with modest gradients (eg, 10 mT/m maximal amplitude, 500 microsec minimal rise time) can acquire them after a single excitation with relatively short readout time and low bandwidth. This technique may make it practical to monitor interventions with T2-weighted imaging. The method was illustrated by reconstructing dynamic changes, which were simulated experimentally by moving objects in the vicinity of a normal human head.

Brain↗