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Biomedical subjects

D M Kramer

Publications and source records attributed to D M Kramer.

At least 37 records · Page 2Linked to original sources
PubMed1994

Lumenal side histidine mutations in the D1 protein of Photosystem II affect donor side electron transfer in Chlamydomonas reinhardtii.

Site-directed mutants of the D1 protein generated in Chlamydomonas reinhardtii have been characterized to determine whether specific lumenal side histidine residues participate in or directly influence electron transfer. Histidine 195 (H195), a conserved residue located near the amino-terminal end of the D1 transmembrane alpha-helix containing the putative P680 chlorophyll ligand H198, was changed to asparagine (H195N), aspartic acid (H195D), and tyrosine (H195Y). These H195 mutants displayed essentially wild-type rates of electron transfer from the water-oxidizing complex to 2,6-dichlorophenolindophenol. Flash-induced chlorophyll a (Chl a) fluorescence yield rise and decay measurements for Mn-depleted membranes of the H195Y and H195D mutants, however, revealed modified YZ to P680+ electron transfer kinetics. The rate of the variable Chl a fluorescence rise was reduced approximately 10-fold in H195Y and H195D relative to the wild type. In addition, the rate of Chl a fluorescence decay in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea was approximately 50-fold more rapid in H195D than in the wild type. These results can be accommodated by a change in the midpoint potential of YZ+/YZ which is apparent only upon the removal of the Mn cluster. In addition, we have generated a histidine to phenylalanine substitution at histidine 190 (H190), a conserved residue located near the lumenal thylakoid surface of D1 in close proximity to the secondary donor YZ. The H190F mutant is characterized by an inability to oxidize water associated with the loss of the Mn cluster and severely altered donor side kinetics. These and other results suggest that H190 may participate in redox reactions leading to the assembly of the Mn cluster.

Animals↗
PubMed1993

A technique for assessment of bone marrow composition using magnetic resonance phase interference at low field.

In disorders which involve the bone marrow, the fat/water ratio of the marrow is often an indicator of the progress of disease. A noninvasive method of monitoring this ratio in bone marrow could be clinically useful. We have investigated a method of bone marrow assessment, using magnetic resonance imaging at low field (0.064 Tesla). The method has been tested using calibrated phantoms; we also present results from normal human subjects.

Adult↗
PubMed1992

Analysis of hybrid imaging techniques.

Hybrid imaging techniques have been proposed as a means of decreasing imaging time without the cost and technical constraints of echo planar imaging. With this technique phase encoding measurements acquired at different echo times in more than one multiecho experiment are used to form a single image. This study analyzes the trade-offs which occur in the selection of this technique over more conventional imaging in terms of signal/noise, contrast, resolution, imaging time, and efficiency. Hybrid imaging is shown to be advantageous when raw speed is essential such as in abdominal or pediatric imaging. When coverage rather than time becomes the important factor, hybrid imaging does not offer a significant advantage over conventional methods. T2 decay will also serve as a roll-off filter which will reduce the spatial resolution, but not the noise, for short TE hybrid imaging.

Brain Neoplasms↗
PubMed1991

Preparation and characterization of the water-soluble heme-binding domain of cytochrome c1 from the Rhodobacter sphaeroides bc1 complex.

The ubiquinol:cytochrome c2 oxidoreductase (bc1 complex) of Rhodobacter sphaeroides consists of four subunits. One of these subunits, cytochrome c1, is the site of interaction with cytochrome c2, a periplasmic protein. In addition, the sequences of the fbcC gene and of the cytochrome c1 subunit that it encodes suggest that the protein should be located on the periplasmic side of the cytoplasmic membrane and that it is anchored to the membrane by a single membrane-spanning alpha-helix located at the carboxyl-terminal end of the polypeptide. Site-directed mutagenesis of the fbcC gene was used to alter the codon for Gln228 to a stop codon. This results in the production of a truncated version of the cytochrome c1 subunit that lacks the membrane anchor at the carboxyl terminus. The bc1 complex fails to assemble properly as a result of this mutation, but the Rb. sphaeroides cells expressing the altered gene contain a water-soluble form of cytochrome c1 in the periplasm. The water-soluble cytochrome c1 was purified and characterized. The amino-terminal sequence is identical with that of the membrane-bound subunit, indicating the signal sequence is properly processed. High pressure liquid chromatography gel filtration chromatography indicates it is monomeric (28 kDa). The heme content and electrochemical properties are similar to those of the intact subunit within the complex. Flash-induced electron transfer kinetics measured using whole cells demonstrated that the water-soluble cytochrome c1 is competent as a reductant for cytochrome c2 within the periplasmic space. These data show that the isolated water-soluble cytochrome c1 retains many of the properties of the membrane-bound subunit of the bc1 complex and, therefore, will be useful for further structural and functional characterization.

Amino Acid Sequence↗
PubMed1991

Magnetic resonance imaging characteristics of multiple sclerosis plaques imaged with two-dimensional and three-dimensional Fourier transform techniques at low and mid field strengths.

A total of 58 multiple sclerosis lesions from 16 patients were used to characterize the performance of a low field system for the detection of edematous (e.g., water-elevating) brain lesions. Contrast, signal-to-noise, and their product (signal difference to noise) were measured for two-dimensional and three-dimensional Fourier transform techniques at low field strength (640 G) and compared to two-dimensional sequences at mid field strength (3,500 G). The results showed numerically that low-field strength magnetic resonance imaging can reliably detect multiple sclerosis lesions, and, by extension, other water-elevating lesions, although with lower confidence levels.

Adult↗
PubMed1991

Impact of section doubling on lesion contrast.

Misregistration between section position and a feature of interest, such as a lesion, can result in loss of contrast on magnetic resonance images. Because the section direction typically has the poorest resolution, reduction of section misregistration can improve lesion contrast. Lesions occur at random sites; hence, to reduce misregistration, it is necessary to repeat imaging with a small amount of offset. This is impractical. Therefore, a reconstruction algorithm that yields offset sections was implemented, in which sections were produced with a zero- and a half-section shift. This algorithm was tested in 14 patients with multiple sclerosis. As expected, because of randomness associated with section placement relative to lesions, the half-shifted sections provided greater contrast as often as the zero-shifted sections. The combined set improved contrast significantly in 39% of lesions; improvement was greatest in lesions with lesser contrast. Signal-to-noise ratios were not affected by this process, which appears to be useful in magnetic resonance image reconstruction.

Adult↗
PubMed1990

Rapid radiofrequency calibration in MRI.

A collection of modified procedures for setting rf transmitter levels using a three-pulse sequence is described. Based on a geometrically weighted ratio of four signals, an estimate of the flip angle is calculated and used for an updating prescription. These techniques are designed to perform rapidly yet eschew systematic errors due to relaxation during the pulse sequence.

Algorithms↗
PubMed1990

MR imaging of the knee with a 0.064-T permanent magnet.

Magnetic resonance (MR) imaging has proved beneficial in the evaluation of internal derangements of the knee. A limitation to general acceptance of MR imaging of the knee has been availability and cost. The recent introduction of low-field-strength MR imaging has shown promise in decreasing the cost and increasing the availability of this modality. High-resolution (pixel size, 0.7 mm2), three-dimensional Fourier transform (3DFT), thin-section (3.5 mm) imaging performed on a 0.064-T permanent magnet was used to evaluate 117 knees in 114 consecutive patients. The appearance of normal anatomy and internal derangements of the knee at low-field-strength imaging is described. Arthroscopic correlation was available for 28 knees. Findings from low-field-strength MR imaging and arthroscopy agreed in 79% of cases in the determination of meniscal tears. Partial-flip-angle techniques with 3DFT produced thin-section images of the knee of diagnostic quality. The authors conclude that in patients with internal derangements of the knee, low-field-strength (0.064-T) MR imaging may provide useful information.

Adolescent↗
PubMed1989

Measuring signal-to-noise ratios in MR imaging.

The signal-to-noise ratio (S/N) in magnetic resonance imagining is one of the variables that must be measured when comparing the relative performance of different techniques. Although various investigators and official groups have proposed different methods for measuring S/N, these are generally not practical for use by a physician working in a clinical situation. The authors present a simple method that should serve for estimating S/N in most cases.

Magnetic Resonance Imaging↗
PubMed1989

Physics of thin-section MR imaging at low field strength.

Thin-section magnetic resonance imaging at low field strengths requires analysis of the relative merits of data collection techniques for direct three-dimensional versus multisection two-dimensional imaging. This analysis was done with specific emphasis on shortened relaxation times and the use of reduced magnetic field gradients. Three-dimensional Fourier transform techniques can provide thin-section images of good diagnostic quality when combined with partial flip-angle gradient-reversal techniques.

Humans↗
PubMed1988

Signal-to-noise ratio and section thickness in two-dimensional versus three-dimensional Fourier transform MR imaging.

The efficiency and quality of three-dimensional Fourier transform (3DFT) magnetic resonance imaging were evaluated and compared with those of 2DFT imaging. For a fixed imaging time and number of sections, 3DFT imaging with conventional spin echoes always had a worse signal-to-noise ratio (S/N) than did 2DFT imaging, when both were optimized with respect to choice of repetition time (TR). With partial-flip magnetic resonance (MR) imaging, S/N was nearly equal for 2DFT and 3DFT imaging when both were optimized with respect to TR and flip angle. 3DFT imaging can have cross-section artifacts that exceed those of 2DFT imaging. For very thin sections these artifacts may be lessened, and 3DFT imaging can achieve this with smaller gradient pulses. Over-all, 3DFT imaging was found to be advantageous only for the very-thin-section imaging and in combination with partial-flip MR imaging.

Fourier Analysis↗
PubMed1985

Fundamentals of magnetic resonance imaging.

Magnetic resonance (MR) is a new method for obtaining high quality tomographic images of potentially greater pathophysiologic specificity than conventional imaging methods. The basic concepts necessary for understanding MR instrumentation and image interpretation are outlined. A fundamental knowledge of MR is needed to appreciate future clinical applications.

Humans↗
PubMed1984

Volume and planar gated cardiac magnetic resonance imaging: a correlative study of normal anatomy with thallium-201 SPECT and cadaver sections.

Magnetic resonance (MR) gated cardiac imaging was performed in ten subjects using a prototype 0.15-T resistive magnet imaging system. Volume and planar imaging techniques utilizing saturation recovery, proton Tl-weighted relaxation time pulse sequences produced images of the heart and great vessels with exquisite anatomic detail that showed excellent correlation with cadaver sections of the heart. The left ventricular myocardial segments also showed excellent correlation with the thallium-201 cardiac single photon emission computed tomography images. Volume acquisition allowed postprocessing selection of tomographic sections in various orientations to optimize visualization of a particular structure of interest. The excellent spatial and contrast resolution afforded by MR volume imaging, which does not involve the use of ionizing radiation and iodinated contrast material, should assure it a significant role in the diagnostic assessment of the cardiovascular system.

Aorta↗
PubMed1984

Basic principles of magnetic resonance imaging.

This article presents a description of how magnetic resonance signals arise, how they are observed, and how they are used to form images. Also discussed are what these signals can reveal about various properties of tissue and what the roles of key instrument settings are in maximizing the information.

Brain↗