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O Nalcioglu

Publications and source records attributed to O Nalcioglu.

18 recordsLinked to original sources
PubMed1992

Correction of chemical-shift artifacts in 19F imaging of PFOB: a robust signed magnitude method.

This paper describes a method for correcting the chemical-shift artifacts in 19F NMR imaging of perfluoroctylbromide emulsion (PFOB) by utilizing the two spectral peaks of PFOB which have a long T2 value in conjunction with the Dixon method. Corrected images are obtained from the magnitude of the measured images using the sign determined from the phase images. The method was tested in the presence of several phase deformation factors, such as static magnetic field inhomogeneity and inaccurate time shift of the pi refocusing pulse, which affect the phase errors of each pixel in the reconstructed image. The advantage of the signed magnitude method is demonstrated experimentally by comparing it with the currently used complex and magnitude summation/subtraction methods.

Artifacts↗
PubMed1992

Correction of chemical-shift artifacts in multislice F-19 imaging with perfluorooctyl bromide.

Chemical-shift artifact correction methods for multislice F-19 imaging with perfluorooctyl bromide are described and results obtained with physical phantoms presented. Utilization of the two long-T2 spectral peaks of PFOB in multislice imaging enhances the imaging efficiency considerably. Simultaneous imaging of the adjacent slices is achieved by appropriately adjusting the slice selection gradient amplitude, the bandwidth, and the slice position offset frequency of the selective RF pulses in relation to the frequency separation between the two peaks.

Algorithms↗
PubMed1991

A modified pulse sequence for in vivo diffusion imaging with reduced motion artifacts.

A modified spin-echo diffusion imaging pulse sequence incorporating bipolar gradients that is less sensitive to macroscopic motion-induced artifacts is presented. Expressions for apparent diffusion coefficient in the presence of microcirculation and macroscopic motions and contrast-to-noise ratio in the diffusion map are derived. Diffusion coefficients of liquid phantoms have been measured using a 1.5-T GE Signa system. Experiments performed to demonstrate the immunity to motion-induced errors are reported. Brain images from human volunteer have also been included to show the potential clinical application of the proposed pulse sequence.

Brain↗
PubMed1991

Correction for chemical-shift artifacts in 19F imaging of PFOB: simultaneous multislice imaging.

One of the difficulties encountered in 19F NMR imaging of fluorinated blood substitutes is that these compounds often exhibit complex multipeak spectra. These peaks result in chemical-shift artifacts along the readout direction and blurred images. In addition, each peak excites a different slice (mis-selection) when a slice selection gradient is applied during the selective rf pulse. A simultaneous multislice imaging method has been developed to solve the inherent problem of mis-selection. The essence of this method is to use the two strongest peaks of the spectrum to excite controlled different multiple slices simultaneously, with or without a slice gap. The images corresponding to the two spectral lines are then separated from in- and out-of-phase images (Dixon method). This method corrects the problem of mis-selection and either improves the SNR or increases the number of slices over spectrally selective methods which image only one peak.

Artifacts↗
PubMed1991

NMR angiography with enhanced quasi-half-echo scanning.

Flow dephasing effects in NMR images can be significantly reduced by the use of gradient quasi-half-echo signals. They can also be reduced by moment-nulling techniques. In this paper, an efficient imaging pulse sequence, the flow-insensitive enhanced quasi-half-echo method is developed in which these two techniques are combined. This pulse sequence is used to reduce dephasing effects in images acquired to enhance blood vessels in gradient echo subtraction angiography. Both phase corrected and uncorrected quasi-half-echo reconstruction techniques are used to determine the effect on image resolution and vessel enhancement.

Blood Vessels↗
PubMed1991

Spatially resolved flow velocity measurements and projection angiography by adiabatic passage.

This paper describes the basic principles of gradient modulated adiabatic passage using a CW radiofrequency excitation. The possible applications of this technique include a direct assessment of in-plane and oblique directional flow velocities, and visualization of flow velocity profiles. Flow angiography based on the time-of-flight technique is also discussed with experimental results.

Angiography↗
PubMed1991

Absolute cross-sectional area measurements in quantitative coronary arteriography by dual-energy DSA.

Recent studies have emphasized the limitations of conventional coronary angiography. These limitations include the lack of correlation between the severity of coronary stenosis as estimated from coronary angiograms and the actual severity of stenotic lesions measured in postmortem hearts. As a result, attempts have been made to quantitate luminal dimension more precisely. The application of quantitative digital subtraction angiography (DSA) in the assessment of coronary artery lesion dimension has been limited by cardiac and respiratory motion artifacts. We have reported previously on a motion-immune dual-energy (DE) cardiac mode in which kVp and filtration are switched at 30 Hz. To assess the potential advantages of a videodensitometric technique for quantification of absolute vessel cross-sectional area (CSA), three different quantitative coronary arteriography (QCA) algorithms were compared. The three algorithms under comparison were a videodensitometric (V) algorithm, which does not require any geometric assumption for absolute vessel CSA measurement, and videodensitometric (VC) and edge detection (ED) algorithms, which do require the assumption of circular cross-section for CSA measurements. A cylindrical vessel phantom (0.5-4.75 mm in diameter) and a crescentic vessel phantom, producing 25% to 90% area stenosis, were imaged over the chest of a humanoid phantom. The low- and high-energy images were corrected for scatter and veiling glare before energy subtraction. For CSA measurements in crescentic vessel phantoms, the V algorithm produced significantly improved results (slope = 0.87, intercept = 0.51 mm2, r = .95) when compared to the VC (slope = 1.05, intercept = 4.19 mm2, r = .75) and the ED (slope = 1.57, intercept = 5.21 mm2, r = .60) algorithms.

Algorithms↗
PubMed1991

Quantification of magnetic resonance scans for hippocampal and parahippocampal atrophy in Alzheimer's disease.

The brains of patients with Alzheimer's disease (AD) invariably exhibit neuropathology in the hippocampus and entorhinal cortex when examined postmortem. Magnetic resonance imaging (MRI) offers a noninvasive, high-resolution method for quantifying volumetric changes in the AD brain antemortem. Eight patients diagnosed with probable AD and 7 age-matched controls had MRI scans and were tested on a battery of cognitive and olfactory tests. The hippocampus and entorhinal cortex (parahippocampal gyrus) showed significant atrophy, with over 40% reduction in size. Areas of the brain that are not highly involved in the degenerative state of AD, such as the striatum, did not show significant volumetric changes. Hippocampal and parahippocampal gyrus volumes had the highest correlation with scores on the Mini-Mental State Examination (r = 0.89), with lower correlations for a smell identification test (r = 0.65), odor match-to-sample test (r = 0.72), and a visual match-to-sample test (r = 0.26).

Aged↗
PubMed1991

Measurement of three-dimensional radiation dose distributions using MRI.

Recent investigations have shown that nuclear magnetic resonance (NMR) can be used in conjunction with a suitable chemical dosimeter to estimate the dose from ionizing radiation (Gore et al., Phys Med. Biol. 29, 1189-1197, 1984). Based on this fact it was proposed that spatial dose distributions can be measured in gels infused with the chemical dosimeter using NMR imaging. There have been few such attempts and they provided only qualitative results. In this paper, we report results demonstrating the feasibility of obtaining quantitative dose distribution measurements by this technique. It is shown that quantitative dose distribution measurements necessitate the calculation of relaxation rate maps. We have determined that the spin-spin relaxation rate is a more sensitive parameter than the spin-lattice relaxation rate. It is also demonstrated that the addition of chemical sensitizers could improve the dose sensitivity of the measured NMR parameters. The two features characterizing a photon beam, depth-dose relationship, and beam profile as measured by this technique are in good agreement with the measurements using conventional methods, ionization chambers, and film dosimetry.

Ferrous Compounds↗
PubMed1990

Projection images of the position-velocity joint spin density distribution.

A new concept of phase encoding called position-velocity combined oblique Fourier phase encoding is introduced. It encodes both spatial and velocity information in a single oblique direction in the position-velocity space. Using this method, two-dimensional projection images of the three-dimensional position-velocity joint spin density distribution along different directions can be obtained. These projection images can provide detailed information on the flow system under study. The imaging of the two-dimensional projection is less time consuming compared to three-dimensional Fourier flow imaging and can be easily implemented on a conventional magnetic resonance imaging scanner.

Fourier Analysis↗
PubMed1990

Investigation of blood flow dynamics by NMR angiography.

Gradient-echo sequences with different amount of flow velocity compensation have been studied using cardiac gating in cine mode MR angiography. The initial results on the leg of a healthy volunteer indicate that the circulatory dynamics may be studied qualitatively by means of these flow-sensitive angiograms.

Blood Flow Velocity↗
PubMed1989

Differential flow imaging by NMR.

A new method for spatially resolved NMR flow measurements, named differential flow imaging (DFI), is introduced and experimentally verified. The DFI technique is based on the fact that flow velocity in any direction may cause a pixel position shift in the phase-encoding direction of a 2DFT NMR image. In this method two flow-influenced magnitude images are obtained by properly encoding and/or compensating the flow velocity. A spatial map of the desired component of the flow velocity can consequently be calculated from these two images. Since the DFI technique uses only the magnitude information of the complex images, it is not sensitive to systematic phase errors in contrast to other methods which are based on the phase measurements. On the other hand, the DFI technique can be combined with the phase measurement methods to perform multidimensional flow measurements in a shorter data acquisition time when the phase errors are small or corrected.

Magnetic Resonance Imaging↗
PubMed1987

Videodensitometric determination of minimum coronary artery luminal diameter before and after angioplasty.

Quantitative measurements of coronary stenoses were made from digital coronary angiograms in 19 patients before and after percutaneous transluminal coronary angioplasty (PTCA). Two methods of measurement were compared. Mean stenosis before PTCA was 67 +/- 10% by the edge detection method and 67 +/- 12% by videodensitometry (difference not significant). After PTCA, the mean stenosis was 32 +/- 14% by edge detection and 30 +/- 13% by videodensitometry (difference not significant). In addition, a new method was developed to rapidly calculate the absolute minimum luminal area and diameter by videodensitometry. The minimum luminal diameter before PTCA was 1.0 +/- 0.5 mm and after PTCA increased to 2.4 +/- 0.5 mm (p less than 0.001). The validity of the videodensitometric method was analyzed in a series of Lucite phantom studies, which suggested that when there is an irregular angiographic appearance, the densitometric method may be more accurate than standard edge detection methods. Digital acquisition of coronary angiograms provides a means for rapid application of quantitative analysis during coronary interventional procedures.

Angioplasty, Balloon↗
PubMed1985

Digital coronary roadmapping as an aid for performing coronary angioplasty.

In an attempt to improve visualization of the position of the guidewire and dilatation balloon during coronary angioplasty, a method was developed called digital coronary roadmapping. With this method a digitally acquired coronary angiogram is interlaced with the live fluoroscopic image of the guidewire and balloon catheter. The digital coronary angiogram is superimposed at the same magnification and radiologic projection as the live fluoroscopic image onto the video monitor above the catheterization table. The digital roadmap image thus provides immediate feedback to the angiographer to assist in directing the guidewire into the appropriate coronary artery branch and to help in placement of the balloon so that it straddles the site of stenosis.

Adult↗
PubMed1979

Post-reconstruction method for beam hardening in computerised tomography.

A method for correcting the beam hardening artefacts in computerised tomography is introduced. After an initial reconstruction of the object the uncorrected image is used to estimate the amount of bone and tissue along each ray. The bone and tissue lengths obtained from the initial reconstruction are used to add a correction term to each original projection. A second reconstruction using the corrected projection data yields the final beam hardening corrected image. The results are presented showing the application of this formalism to a mathematical phantom. The instability of the correction method with respect to various possible sources of error is examined.

Tomography, X-Ray Computed↗
PubMedDate not supplied

Reblurred deconvolution method for chemical shift removal in F-19 (PFOB) MR imaging.

Perfluorocarbons such as perfluoroctylbromide (PFOB) can be used as contrast agents in the vascular system for fluorine-19 magnetic resonance imaging or as synthetic oxygen carriers. F-19 imaging has been proposed for studying the vascular system, capillary flow, tissue perfusion, and tumor oxygenation. A major difficulty is that F-19 compounds often have complex multipeak spectra. These peaks result in chemical shift artifacts, lower signal-to-noise ratios, and blurred images. Each peak also excites a different section when a section-select gradient is applied. Direct inverse filtering is the simplest deconvolution method for correcting such artifacts; however, two major difficulties present themselves: functional singularity and noise amplification at high frequencies. The use of a new reblurred deconvolution (RED) method appears to overcome these problems. Although this method is based on iterative deconvolution in the spatial domain, the computational overhead is negligible. Since the point spread function and object data are already available in the time domain as FID data, RED appears to be useful for eliminating chemical shift artifacts and suppressing noise amplification while restoring the original image without loss of resolution.

Contrast Media↗
PubMedDate not supplied

Reduced-bandwidth method for F-19 imaging of perflubron.

A reduced-bandwidth imaging method has been developed to eliminate the chemical shift artifacts in magnetic resonance (MR) imaging of the blood substitute perflubron (PFB) and simultaneously enhance the signal-to-noise ratio (SNR). The two strongest spectral peaks, which have relatively long T2 values (247 and 471 msec), were used. When the receiver bandwidth is reduced substantially by increasing the data acquisition time Ts, the bandwidth across the object becomes less than the chemical shift frequency. The reduced bandwidth eliminates misregistration by displaying the images corresponding to multiple spectral peaks on the same image plane simultaneously. An additional gain due to the reduced bandwidth is the reduced thermal Gaussian noise. Unfortunately, the increased Ts results in an increased TE, which causes the signal to be attenuated by T2 relaxation. The optimum measured Ts (and TE) values for successful image separation and maximum SNR were 120 and 144 msec for the two spectral peaks, respectively. The long TE also suppresses the rest of the downfield spectral peak cluster of PFB. The degree of magnetic field inhomogeneity and tissue susceptibility across the object may cause some limitations in the application of this technique; however, a composite radio-frequency pulse that will allow use of additional spectral lines and/or localized volume imaging techniques may be incorporated to overcome these limitations.

Artifacts↗