Biomedical subjects

D B Twieg

Publications and source records attributed to D B Twieg.

31 records · Page 2Linked to original sources

PubMed1990

Spin echo 31P spectroscopic imaging in the human brain.

Spectroscopic imaging of phosphorus metabolites in the human brain has been carried out with two data acquisition methods: by observation of the free induction decay (FID) signal and by a short spin echo sequence. The resultant spectral images and spatially resolved spectra are compared. Spin echo observation is found to provide spectra of superior quality, and by suitably selecting the sequence timing, no significant increase in T2 losses, as compared with the FID method, is encountered. 31P images with approximately 3.5 cm spatial resolution are obtained within times of 37 min at 2.0 T field strength.

Brain↗

PubMed1990

Image-guided 31P magnetic resonance spectroscopy of normal and transplanted human kidneys.

Image-guided 31-phosphorus magnetic resonance spectroscopy (MRS) was used to obtain spatially localized 31P spectra of good quality from healthy normal human kidneys and from well-functioning renal allografts. A surface coil of 14 cm diameter was used for acquiring phosphorus signals solely from a volume-of-interest located within the kidney. To determine the effects of kidney transplantation on renal metabolism, patients with well functioning allografts were studied. Little or no phosphocreatine in all spectra verifies the absence of muscle contamination, and is consistent with proper volume localization. The intensity ratio of phosphomonoesters (PME) to adenosine triphosphate (ATP) resonances in transplanted kidneys (PME/ATP = 1.1 +/- 0.4) was slightly elevated (P = 0.2) compared to that of healthy normal kidneys (PME/ATP = 0.8 +/- 0.3). The inorganic phosphate (Pi) to ATP ratio was similar in the two groups (Pi/ATP = 1.1 +/- 0.1 in transplanted kidneys vs. 1.2 +/- 0.6 in normal kidneys). Acid/base status, as evidenced from the chemical shift of Pi, was the same in both normal controls and transplanted kidneys. Despite the practical problems produced by organ depth, respiratory movement, and tissue heterogeneity, these results demonstrate that image-guided 31P MR spectra can reliably be obtained from human kidneys.

Adult↗

PubMed1989

Multiple-output chemical shift imaging (MOCSI): a practical technique for rapid spectroscopic imaging.

A new technique for rapid multiple-frame chemical shift imaging and localized spectroscopy is reported. Unlike phase-encoding techniques and multiple-delay chemical shift imaging techniques, this technique permits spectroscopic imaging at high spatial resolution and moderately high spectral resolution with acquisition times as short as 10 min or less. Unlike other rapid spectroscopic imaging techniques which have been reported, it uses cyclic trapezoidal gradient waveforms achievable in standard clinical magnetic resonance imaging systems. Multiple-output chemical shift imaging images and spectra from a human study are shown.

Body Water↗

PubMed1989

Phosphorus-31 magnetic resonance spectroscopy in humans by spectroscopic imaging: localized spectroscopy and metabolite imaging.

In in vivo phosphorus magnetic resonance spectroscopy (MRS), spectroscopic imaging (SI) can be used as a flexible localization technique, producing spectra from multiple volumes in a single examination. Presented here are phosphorus SI studies of human organs in which a selective-volume SI reconstruction was used rather than the usual array-format SI reconstruction. A linear predictor technique was used to estimate the initial points of the free induction decay missing because of the delay needed for phase-encoding gradients, significantly reducing the baseline artifacts which commonly complicate interpretation of SI spectra. In studies of heart, brain, liver, and kidney, the performance of SI was found to compare favorably with that of ISIS. SI phosphorus metabolite intensity images from a brain tumor patient were obtained at 2 X 2-cm in-plane resolution (with "slice" thickness of roughly 16 cm, determined by coil sensitivity) in 34 min, demonstrating the feasibility of obtaining clinically useful metabolite images in clinically reasonable examination times.

Astrocytoma↗

PubMed1989

Clinical magnetic resonance spectroscopy of brain, heart, liver, kidney, and cancer. A quantitative approach.

Clinical studies using 31P and 1H MRS with a whole body 2.0 T MRI/MRS system are described. In most cases, techniques to quantitate absolute molar concentrations of metabolites in various organs were used. In the brain, AIDS, chronic stroke, and white matter lesions were associated with alterations of brain 31P metabolites. Epilepsy was associated with increased pH in the seizure focus. In the heart, dilated cardiomyopathy was associated with increased PDE/ATP while PCr/ATP was unchanged. In the liver, alcoholic hepatitis and cirrhosis were associated with diminished hepatic ATP while alcoholic hepatitis had increased pH and cirrhosis had decreased pH. This allowed differentiation of normal liver, alcoholic hepatitis, and alcoholic cirrhosis without biopsy. In the prostate, malignancy was associated with increased PME/ATP and decreased PCr/ATP. The PME/PCr was greatly increased in malignant prostate with no overlap in normals. Other cancers outside the brain had increased PME and effective treatment was often associated with diminished PME. 1H MRS of the brain was performed using ISIS and outer volume suppression pulses for volume localization. Excellent high resolution 1H water-suppressed spectra were obtained at echo times as short as 30 ms, showing well resolved peaks for lactate, N-acetylaspartate, glutamate, choline, creatinine, and inositol. 1H MRS demonstrated that the uptake of ethanol by the brain was slower than the rise of ethanol in blood. 31P spectroscopic imaging of the brain with resolution of 2.25 x 2.25 x 2.5 cm produced metabolic images and high resolution spectra from desired regions of interest.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain↗

PubMed1989

Clinical MRS studies of the brain.

Image-guided 31P and 1H magnetic resonance localized spectroscopy was performed on patients with brain tumors, temporal lobe epilepsy, chronic brain stroke, and deep white matter lesions. Absolute molar concentrations of metabolites, peak area ratios, and pH were obtained. The important findings were that 31P metabolite concentrations were significantly reduced in tumors, infarcts, and deep white matter lesions. Similarly, 1H metabolite intensities were reduced in chronic stroke. In the seizure foci of epilepsy patients, in tumors, and in chronic stroke, the pH was more alkaline than the normal pH. Peak area ratios were altered in tumors (reduction of phosphocreatine/inorganic phosphate (PCr/Pi) and in chronic stroke (large increases in Cr/NAA and Cho/NAA). Finally, the spectroscopic imaging technique offers a versatile alternative to the "single point" techniques, producing spectra or images of the spatial distribution of individual 31P metabolites.

Brain↗

PubMed1988

Detection of motion using B1 gradients.

An NMR method which makes use of a radiofrequency (RF) field gradient to detect diffusion, perfusion, or flow is demonstrated. The technique is analogous to the detection of motion using pulsed B0 gradients. The simplest form of the experiment is as follows: A spatially imhomogeneous RF field generated by a surface coil produces dispersal of magnetization in the Y Z plane. After a delay, the RF field is applied within reversed phase to restore polarization along +Z, which is then sampled using an observe pulse or pulse sequence. If molecular motion occurs during the delay period the amplitude of Z magnetization will be reduced. The lengths of the RF pulses and the delay period are varied so that the effects of relaxation, flow, and diffusion or perfusion can be distinguished. The present study demonstrates the use of this method to detect slow fluid flow. Advantages of this method include the availability of large RF gradients, and the avoidance of eddy currents. It is suggested that the method may have application to the study of perfusion and flow in vivo.

Diffusion↗

PubMed1988

Application of image-guided surface coil P-31 MR spectroscopy to human liver, heart, and kidney.

Localized phosphorus-31 magnetic resonance (MR) spectroscopy in humans has previously been accomplished with surface coils by means of depth-resolved surface coil spectroscopy or rotating frame experiments, in which the extent of tissue sampled critically depends on surface coil placement. The authors' goal was to modify the surface coil image-selected in vivo spectroscopy (ISIS) experiment to accomplish three-dimensional volume selection through application of selective pulses in the presence of B0 gradients. Advantages of ISIS include the ability to use proton images to define the volume of interest (VOI) and reduced dependence on exact positioning of the surface coil. However, rapid replication of the surface coil ISIS experiment can cause spectral contamination from signals originating outside the VOI. A modified version of the ISIS experiment was developed to alleviate contamination under conditions of rapid replication. Applications of localized P-31 MR spectroscopy for observation of high-energy phosphorus metabolites are presented in human liver, heart, and transplanted and normal kidney.

Computer Simulation↗

PubMed1987

A general treatment of NMR imaging with chemical shifts and motion.

A general treatment of nuclear magnetic resonance imaging (MRI) and spectroscopic imaging (MRSI), which takes into account the effects of chemical shift, motion, field inhomogeneity, and relaxation times, is presented. A graphical representation based on the k trajectory formalism which includes these effects is then developed for MRI and MRSI acquisition processes. These considerations should be useful in the study and design of flow-sensitive MRI and MRSI methods and the accurate prediction of motion artifacts in conventional MRI and MRSI techniques. We conclude by presenting examples illustrating applications of the general theory to specific MRSI and flow imaging methods.

Fourier Analysis↗

PubMed1985

Acquisition and accuracy in rapid NMR imaging methods.

The echo-planar imaging method (EPI) and its variants can acquire data sufficient to form an image in a much shorter time than the Fourier or projection-reconstruction methods. Fundamental requirements on gradient waveforms and signal sampling are discussed for EPI and SEPI (a variant using sinusoidal gradient waveforms), based on the k-trajectory formulation of acquisition in NMR imaging. Using a new simulation method, image defects in EPI and SEPI are demonstrated which result from failure to meet these requirements. Faithful rendition of theoretically ideal gradient waveforms places severe demands on gradient system response in rapid imaging. It is shown, however, that nonideal gradient system response need not degrade images if appropriate reconstruction computations are used. Transfer functions and noise power spectra are derived for EPI and SEPI, and the ratio of these two functions is used to derive ideal observer signal-to-noise ratios for simple visual interpretive tasks. It is shown that ideal observer performance can differ significantly between EPI and SEPI for some visual tasks, using the same NMR imaging system.

Magnetic Resonance Spectroscopy↗

PubMed1977

Effects of noise on the determination of ejection fraction from left ventricular time-activity curves.

The effects of Poisson noise on three estimates of ejection fraction made from left-ventricular time-activity curves have been investigated. All three methods are based on a sinusoidal model of left-ventricular volume changes. The first, developed by Schelbert et al., overestimates the ejection fraction for low-activity levels and low ejection fractions. The second estimate, which is merely a first-order correction for the contribution of Poisson noise to the first estimate, appears to be more accurate when both estimators are applied to simulated time-activity curves, and the resulting ejection fractions are compared. A third, "maximum likelihood" estimator, when applied to the same data, is apparently more accurate than the first two.

Heart Function Tests↗

PubMed1976

Influence of dobutamine on hemodynamics and coronary blood flow in patients with and without coronary artery disease.

The influence of dobutamine on hemodynamics and coronary blood flow was studied in patients after routine cardiac catheterization. The data demonstrated that dobutamine is a powerful inotropic agent at a dose that has a relatively small influence on heart rate. In patients without coronary artery disease dobutamine greatly increased coronary arterial perfusion. In patients with severe coronary artery diseases dobutamine resulted in a much smaller increase in coronary perfusion, and the pattern of perfusion became more inhomogeneous. The results suggest that dobutamine has a potential inotropic value but raise concern about its influence on regional myocardial perfusion in patients with serious coronary artery disease.

Blood Pressure↗

PubMedDate not supplied

The k-trajectory formulation of the NMR imaging process with applications in analysis and synthesis of imaging methods.

The fundamental operations of nuclear magnetic resonance (NMR) imaging can be formulated, for a large number of methods, as sampling the object distribution in the Fourier spatial-frequency domain, followed by processing the digitized data (often simply by Fourier transformation) to produce a digital image. In these methods, which include reconstruction from projections, Fourier imaging, spin-warp imaging, and echo-planar imaging, controllable gradient fields determine the points in the spatial-frequency domain which are sampled at any given time during the acquisition of data (the free induction decay, or FID). The detailed time dependence of the resulting trajectory of sample points (the k trajectory) determines the relative weight and accuracy with which image information at each spatial frequency is measured, establishing theoretical limitations on image quality achievable with a given imaging method. We demonstrate here that these considerations may be used to compare the theoretical capabilities of NMR imaging methods, and to derive new imaging methods with optimal theoretical imaging properties.

Image Enhancement↗