Biomedical subjects

T R Nelson

Publications and source records attributed to T R Nelson.

84 records · Page 5Linked to original sources

PubMed1987

Severe polyhydramnios: incidence of anomalies.

The sonograms of 195 singleton pregnancies complicated by polyhydramnios were reviewed, and follow-up information was obtained on 191 patients. A grading system was developed that differentiated mild from severe polyhydramnios using real-time or static sonographic equipment. Mild polyhydramnios was present in 138 (71%), and severe polyhydramnios was present in 57 (29%). Previously it has been reported that 60% of cases of polyhydramnios are idiopathic and the pregnancies have a normal outcome. Twenty percent are associated with maternal abnormalities and 20% are associated with fetal anomalies. In this study, pregnancies with severe polyhydramnios had a much greater prevalence of fetal anomalies (75%) than pregnancies with mild polyhydramnios (29%). The 57 singleton pregnancies with severe polyhydramnios were analyzed in depth. Fourteen (25%) of the fetuses were normal; 43 (75%) had significant congenital abnormalities that predominantly involved the CNS, gastrointestinal tract, heart, and genitourinary tract. In all fetuses with primary CNS abnormalities, polyhydramnios was diagnosed at or before 30 weeks of gestation, while in most of the fetuses (83%) with gastrointestinal abnormalities it was diagnosed after 30 weeks. Sonographic findings correlated closely with the findings noted at birth or autopsy. In patients with severe polyhydramnios, normal sonograms were sensitive in excluding major congenital anomalies and, thus, were helpful in providing the parents with favorable prognoses. Sonograms should be performed in patients with polyhydramnios to identify congenital anomalies and to provide information regarding prognosis for fetal outcome.

Adult↗

PubMed1985

Fluorine nuclear magnetic resonance: calibration and system optimization.

Fluorine-19 magnetic resonance imaging (MRI) offers advantages for imaging organs and tissues. 19F is readily synthesized into a variety of compounds and offers the potential for in-vivo imaging as a complement to hydrogen MRI. The purpose of this work was to determine the minimum detection sensitivity for a fluorinated compound (CF3-CO2H) as a function of pulse sequence, interpulse times (TE, TI, and TR), gradient values and the number of data averages. CF3-CO2H was chosen because it has a single spectral line and exhibits a minimal frequency shift under the experimental conditions used for this experiment. A resistance MR scanner operating at a resonance frequency of 6.255 MHz was used for imaging both fluorine (.156 T) and hydrogen (.147 T). Critical factors determining the minimum detection sensitivity included system signal-to-noise ratio (S/N), acquisition time, relaxation times (T1, T2), and sample volume. Samples were measured over the range of 0.05 M to 20.0 M and showed a linear relationship between signal strength and concentration. The minimum detection sensitivity was 0.1 M. Use of higher static fields and optimized coils as well as improved system signal-to-noise ratios will improve detection sensitivity. We conclude that imaging of fluorine on low-field system is feasible, although it is necessary to optimize many parameters to maximize detection sensitivity.

Fluorine↗

PubMed1984

Optimizing tissue contrast in magnetic resonance imaging.

Magnetic resonance imaging demands that tissue contrast and signal-to-noise advantages be sought in each component of the imaging system. One component of magnetic resonance imaging in which contrast and signal-to-noise ratios are easily manipulated is in the choice of pulse sequences and interpulse delay times. This article provides a general method for determining the best choices of interpulse delay times in pulse sequences and applies that method to saturation recovery, inversion recovery, and spin-echo sequences. Saturation recovery and inversion recovery sequences with rephasing pulses, and tissues with unequal hydrogen densities are considered. Optimization of pulse sequences is carried out for the two distinct cases of (a) a fixed number of sequence repetitions and (b) a fixed total imaging time. Analytic expressions are derived or approximate expressions are provided for the interpulse delay times that optimize contrast-to-noise ratios in each pulse sequence. The acceptable range of interpulse delay times to obtain reasonable contrast using each pulse sequence is discussed.

Animals↗

PubMed1984

Phase detection and contrast loss in magnetic resonance imaging.

Several recent articles have assessed the relative efficiency of nuclear magnetic resonance (NMR) pulse sequences. One consideration that has received little attention is the effect on image contrast of displaying images without information on the sign of the reconstructed signals. The radiofrequency receivers currently used on most NMR imaging systems are quadrature detectors that preserve both the magnitude and sign of the NMR signal. Usually, however, sign or phase information is not used in the final image presentation. We point out that in imaging sequences that may have negative signals, such as inversion recovery, this loss of sign information produces a reduction in contrast between some tissues in an NMR image. We discuss the tissue parameters and interpulse delay times that result in contrast loss in inversion recovery and indicate the extent of contrast loss. We point out that for some tissues with unequal hydrogen spin densities, the region of contrast loss coincides with the region where maximum contrast would occur if sign information were preserved.

Brain↗

PubMed1984

Selection of pulse sequences producing maximum tissue contrast in magnetic resonance imaging.

The importance of spin density [N(H)] and spin-lattice (T1) and spin-spin (T2) relaxation in the characterization of tissue by nuclear magnetic resonance (NMR) is clearly recognized. This work considers which optimized pulse sequences provide the best tissue discrimination between a given pair of tissues. The effects of tissue spin density and machine-imposed minimum rephasing echo times (TEMIN) for achieving maximum signal tissue contrast are discussed. A long TEMIN sacrifices T1-dependent contrast in saturation recovery (SR) and inversion recovery (IR) pulse sequences so that spin-echo (SE) becomes the optimum sequence to provide tissue contrast, due to T2 relaxation. Pulse sequences providing superior performance may be selected based on spin density and T1 and T2 ratios for a given pair of tissues. Selection of the preferred pulse sequence and interpulse delay times to produce maximum tissue contrast is strongly dependent on knowledge of tissue spin densities as well as T1 and T2 characteristics. As the spin density ratio increases, IR replaces SR as the preferred sequence and SE replaces IR and SR as the pulse sequence providing superior contrast. To select the optimal pulse sequence and interpulse delay times, an accurate knowledge of tissue spin density, T1 and T2 must be known for each tissue.

Animals↗

PubMed1984

Fetal age estimation by ultrasound: the impact of measurement errors.

Ultrasound estimation of fetal age is a routine, safe, and valuable diagnostic procedure. The overall accuracy of ultrasound measurement of femur length and biparietal diameter was investigated using a dowel phantom, chicken bones, and a bottle. Objects were imaged in a water bath using commercially available static, linear, and sector scanners. Transducer type and frequency, depth, object dimension, and observer variation were evaluated. Our results show that there was no appreciable difference between transducer types; the overall accuracy of the measurements improved with increasing transducer frequency; the lateral measurements consistently overestimated the object length; the absolute error in lateral measurements was between 0-12 mm; the absolute error in axial measurements was between 0-4 mm; the interobserver variability was +/- 1.3 mm, and intraobserver variability was +/- 1.0 mm. We conclude that transducer type does not affect measurement accuracy, measurement errors are greater in the lateral dimension as opposed to the axial dimension, proper quality assurance and performance evaluation procedures are necessary to ensure optimal results, and femur length measurements will continue to be useful in evaluation of gestational age.

Age Determination by Skeleton↗

PubMedDate not supplied

Automatic analysis of left ventricular ejection fraction using stroke volume images.

The purpose of this study was to analyze, validate, and report on an automatic computer algorithm for analyzing left ventricular ejection fraction and to indicate future applications of the technique to other chambers and more advanced measurements. Thirty-eight patients were studied in the cardiac catheterization laboratory by equilibrium radionuclide ventriculography and concurrent contrast ventriculography. The temporal and spatial behavior of each picture element in a filtered stroke volume image series was monitored throughout the cardiac cycle. Pixels that met specific phase, amplitude, and derivative criteria were assigned to the appropriate chamber. Volume curves were generated from regions of interest for each chamber to enable calculation of the left ventricular ejection fraction. Left ventricular ejection fractions showed a good correlation (r = 0.89) between the two techniques. Ejection fractions ranged between 0.12 and 0.88, showing a wide range of application. It is concluded that automatic analysis of left ventricular ejection fraction is possible using the present algorithm and will be useful in improving the reproducibility and providing more accurate information during exercise protocols, pharmaceutical interventions, and routine clinical studies.

Cardiac Output↗

PubMedDate not supplied

Effect of patient imaging angle on apparent cardiac volumes and the potential impact on measurement of valvular regurgitant fractions.

The accurate measurement of cardiac chamber volume is of major importance in assessing cardiac performance. Accurate equilibrium radionuclide volume estimations are difficult to obtain, due to the geometry of the chambers, and the physical characteristics of the imaging system. The purpose of this study was to examine the effects of imaging projections on relative cardiac chamber volumes, indexes, and stroke volume ratios. Twenty-two male patients, free of clinical evidence of disease, were studied. A series of four 2-minute acquisitions were made with the patient successively imaged in the anterior, 30 degrees left anterior oblique (LAO), 45 degrees LAO, and 60 degrees LAO projections with 15 degrees of caudal inclination. Filtered stroke volume and original images were used by the operator to assign right ventricular (RV), left ventricular (LV), and a combined right and left ventricular (TOT) regions-of-interest. From the data we determined end-diastolic counts (EDC), end-systolic counts (ESC), stroke counts (SC), ejection fractions (EF), and R/L stroke count ratios. The following changes were observed as the projection was moved from the anterior to 60 degrees LAO: 1) all RV parameters decreased in value, including, RVEDC (P less than .001), RVESC (P less than .01), RVESC (P less than .01) and RVEF (P less than .001); 2) LVEDC and LVESC (both P less than .01) increased while LVEF decreased (P less than .004); and 3) the R/L stroke count ratio decreased (P less than .001). Variability could be explained by 1) chamber overlap and geometry; 2) patient variability; and 3) intrachamber, interchamber and chest wall photon attenuation and scatter. We suggest that close attention to detail, with computer assistance, to optimally position the patient may reduce the effect of inherent limitations in radionuclide volumetric measurements, thus improving the reliability and usefulness of existing studies.

Aged↗

PubMedDate not supplied

Menstrual variation of normal breast NMR relaxation parameters.

Nuclear magnetic resonance (NMR) offers great promise in noninvasively distinguishing normal, benign, and malignant tissues in the breast. However, well-documented changes that occur in breast tissues during the normal menstrual cycle suggest that lesion detectability will depend on knowledge of the variations in NMR parameters that occur as a consequence of these changes. Correct selection of optimal NMR pulse sequences and interpulse times, based on spin density and T1 and T2 values, to maximize tissue contrast and lesion detectability, will likewise depend on our understanding of variations in NMR parameters of normal breast tissues. The purpose of this study is to determine the range and variability of NMR parameters (spin density, T1 and T2) associated with changes in normal breast tissues during the menstrual cycle, as measured by NMR imaging. Initial findings suggest that there are measurable differences between breast tissues as a function of time in the menstrual cycle.

Breast↗

PubMedDate not supplied

Magnetic resonance imaging: the basic physical and clinical concepts. Part I.

Noninvasive evaluation of the human body has become an essential tool in the diagnosis of disease processes. Recent developments in nuclear magnetic resonance imaging (MRI) have extended a powerful chemical analytic technique to the medical imaging field. This article presents the fundamental principles of MRI in terms of the concept of transmission and reception of energy. The physical principles of spin resonance are discussed in Part I. Spin relaxation and pulse sequences will be examined in Part II and III. A selected bibliography directs the reader to a more detailed discussion of the pertinent physics.

Humans↗

PubMedDate not supplied

Magnetic resonance imaging: the basic physical and clinical concepts. Part II.

Noninvasive evaluation of the human body has become an essential tool in the diagnosis of disease processes. Recent developments in nuclear magnetic resonance imaging (MRI) have extended a powerful chemical analytic technique to the medical imaging field. This article presents the fundamental principles of MRI in terms of the concept of transmission and reception of energy. The physical principles of spin resonance were discussed in Part I. Spin relaxation and pulse sequences will be examined in Part II and III. A selected bibliography directs the reader to a more detailed discussion of the pertinent physics.

Biophysical Phenomena↗

PubMedDate not supplied

Magnetic resonance imaging: the basic physical and clinical concepts. Part III.

Noninvasive evaluation of the human body has become an essential tool in the diagnosis of disease processes. Recent developments in nuclear magnetic resonance imaging (MRI) have extended a powerful chemical analytic technique to the medical imaging field. This article presents the fundamental principles of MRI in terms of the concept of transmission and reception of energy. A selected bibliography directs the reader to a more detailed discussion of the pertinent physics.

Humans↗