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Biomedical subjects

J N Lee

Publications and source records attributed to J N Lee.

At least 19 recordsLinked to original sources
PubMed1992

Ovarian and circulating levels of oxytocin and arginine vasopressin during the estrous cycle in the rat.

Ovarian extracts of Long-Evans rats separated using high performance liquid chromatography (HPLC) were measured by radioimmunoassays (RIAs) for the presence of oxytocin and arginine vasopressin (AVP). The results showed that the ovary contains both, and that they are indistinguishable from the respective standard synthetic peptides. During the estrous cycle, the ovarian content of oxytocin was 10-fold higher (p less than 0.01) in estrus than in the other phases, while AVP was 16- and 25-fold higher (p less than 0.01) in metestrus than in the other phases. In contrast, the plasma levels of oxytocin showed no significant difference among the various phases of the estrous cycle. However, the plasma level of AVP level was significantly higher (p less than 0.01) in diestrus than in other phases. The present study thus strongly supports the hypothesis that both oxytocin and AVP can be produced by the ovary itself in the rat. The possible roles of oxytocin and AVP in the reproductive cycle are discussed.

Animals↗
PubMed1992

[The mechanism of the pineal gland in inhibiting sexual receptivity of female rats: (II) Does it concern progesterone?].

The present study was designed to find the role of progesterone in modulating the effect of pineal gland upon reproductive behavior in the female rat. Monosodium L-glutamate (MSG) was used as a neurotoxin to induce hypogonadal function. In the first part of the experiment, rats were divided into four groups: (1) normal, (2) pinealectomized (Px), (3) MSG-treated (MSG), and (4) pinealectomized MSG-treated (Px-MSG), as immature rats became mature, the IEC (Incidence of Estrous Cycle) in each group was determined by vaginal smear for three weeks. Serum samples for estimating the levels of progesterone were collected at estrous. Then, ovariectomy was performed. Two weeks later, estradiol was primed for three days following progesterone administration and sexual receptivity as well as solicitation were observed. The results showed that, the serum progesterone levels and IEC of Px-MSG rats were significantly higher than that of the MSG group. In addition, Px and Px-MSG rats showed a higher sexual receptivity than normal and MSG rats, respectively. In the second part of the experiment, rats were divided into two groups: (1) normal and (2) Px. All rats were ovariectomized at 2.5 months of age and implanted with a 2-mm 20% estradiol benzoate-filled silastic capsule. Then, sexual receptivity to various doses of progesterone (0, 0.02, 0.1, 0.5 mg/rat) were observed. The results showed that receptivity of Px rats was significantly higher than that of the normal ones in a dose-dependent manner. It is concluded that pineal gland plays an important role in the regulation of ovary function as well as sexual receptivity in female rats. Also, progesterone may be one of the factors modulating the effect of pineal gland on sexual receptivity.

Animals↗
PubMed1990

CNR enhancement in the presence of multiple interfering processes using linear filters.

Given several images of the same slice, a linear filter can produce an image in which the contrast-to-noise ratio (CNR) between pathological and normal tissues is greater than in any of the initial images. To distinguish the pathology from more than one tissue, the filter should optimize the set of CNRs between the pathology and each of the interfering tissues. We define the optimal filter as the one which provides the largest value for the minimum CNR in the set and show how it is selected from a field of only four possibilities. The filter is demonstrated with both experimental phantom studies and clinical cases. Filter performance is compared with that of other techniques for distinguishing a desired feature from more than one interfering process.

Image Enhancement↗
PubMed1990

Rapid MR imaging of blood flow with a phase-sensitive, limited-flip-angle, gradient recalled pulse sequence: preliminary experience.

To assess blood flow rapidly, a limited-flip-angle, gradient recalled pulse sequence was modified to acquire two views at the same phase-encoding step in successive repetitions. One view is obtained with first-moment flow compensation, while the second view is obtained with selectable flow encoding (non-zero first moment) along one direction. Blood flowing along the encoded direction acquires a phase difference between the two views, resulting in signal dependent on both direction and speed of flow. Stationary tissues undergo no phase change. Therefore, the phase shift between the two views produces an image that spatially renders flow direction and velocity. With a 24-msec repetition time, a 256 X 128 matrix, and two excitations, data acquisition is completed in 13 seconds per location (both a magnitude image and a flow image are produced at each location). Images generated with flow phantoms confirmed the accuracy of this method. Preliminary clinical evidence in 23 human subjects suggests that this method is useful in evaluating portal hypertension, distinguishing arterial from venous flow, distinguishing between slow flow and clot, and confirming the presence of clot. This method appears to be a fast, easy way to assess blood flow in large vessels.

Adolescent↗
PubMed1989

Flow-compensated limited flip angle MR angiography.

A method of subtraction angiography that has an acquisition time of 8 s per slice is described. Flow-compensated and uncompensated measurements are acquired in an interleaved fashion using limited flip angles and gradient refocusing. Magnitude images are reconstructed and subtracted to generate the angiogram. Results were generated in vivo in the imaging of the carotid bifurcation of several human volunteers. Susceptibility and inhomogeneity induced artifacts are prominent in thick slices, but can be greatly reduced by imaging several thin slices and adding them together. Thin slices do not require dephasing gradients to reduce the dynamic range, and there is no signal cancellation in overlapping vessels. The method is ideal for acquiring scout angiograms, and with averaging may produce images of diagnostic quality.

Blood Flow Velocity↗
PubMed1989

MR fluoroscopy: initial clinical studies.

Magnetic resonance (MR) fluoroscopy is a method for high-speed MR image acquisition with the goals of short acquisition time per image (500 msec or less), high image rate (10 images or more per second), and high-speed image reconstruction (150 msec or less from data acquisition to image display). The authors present their results with the first two goals in volunteers. MR fluoroscopic image data were acquired with a limited flip angle pulse sequence with reduced repetition times (TRs) and fewer phase encodings used per image. The sequence was applied continuously, and images were formed by updating one set of data with data from the most recently taken measurements. Sample head images were generated with TR/echo times as small as 11/5.5 msec and 48 phase encodings for a total acquisition time of about 500 msec. Images were acquired while the volunteer flexed his head. Artifacts from the motion became less evident on images as progressively shorter acquisition times were used.

Brain↗
PubMed1988

MR fluoroscopy: technical feasibility.

A method of magnetic resonance image acquisition and reconstruction is described in which high imaging rates and fast reconstruction times are allowed. The acquisition is a modification of the basic FLASH sequence but with a restricted number N of phase encodings. The encodings are applied sequentially, periodically, and continuously. Images are formed by sliding a window of width N encodings along the acquired data and reconstructing an image for each position of the window. In general the acquisition time per image exceeds the time between successive images, and the method thus has a temporal lag. Experimental studies were performed with a dynamic phantom using 48 phase encodings and a TR of 20 ms, for an image acquisition time of about 1 s. The image display rate in the reconstructed sequence was 12.5 images/s, and the image sequence portrayed the motion of the phantom. Additional studies were done with 24 encodings. It is shown how the sliding window technique lends itself to high-speed reconstruction, with each newly acquired echo used to quickly update the image on display. The combination of the acquisition technique described and a hardware implementation of the reconstruction algorithm can result in realtime MR image acquisition and reconstruction.

Fluoroscopy↗
PubMed1988

Fast limited flip angle MR subtraction angiography.

A fast MR angiography method is introduced that is capable of generating difference images of blood vessels in scan times of 10-20 s. This is an order of magnitude faster than many previous methods. The fundamental concept of this approach is to use cardiac gating and acquire several phase encodings at least twice during each cardiac cycle using limited flip angles (LFAs) and repetition times in the 20 to 50 ms range. The encodings acquired during diastole are subtracted from those acquired during systole to generate the difference image. The contrast in the difference image is due both to the influx of unsaturated spins and to the loss of phase coherence of systolic blood moving at high velocity along a magnetic gradient. The systolic peak of the cardiac cycle is determined during reconstruction by shifting the systolic and diastolic "windows" until the difference signal is maximized. Ghost artifacts due to pulsatile flow are eliminated by a phase reordering technique similar in concept to those developed for suppression of breathing artifacts. Arteries in thick slices are successfully imaged and initial in vivo results are presented.

Angiography↗
PubMed1987

The contrast-to-noise in relaxation time, synthetic, and weighted-sum MR images.

The contrast-to-noise ratio (CNR) in three types of computed MR images is compared in a computer simulation. The original data consist of two spin-echo or two saturation-recovery images. Each pair of images is used to generate a relaxation time image, a synthetic image at arbitrary echo or repetition time, and an image which is a weighted sum of the original images. The CNR produced by these three methods is compared for signals spanning a wide range of relaxation times. In every comparison an optimally weighted sum produces the highest CNR that is statistically attainable. The CNR in the optimum synthetic image equals this bound only if contrast reversal does not occur in the original images. The CNR in relaxation time images is always less than the statistical bound and can be less than the CNR in the original images.

Brain Diseases↗
PubMed1987

Optimizing the precision in T1 relaxation estimation using limited flip angles.

In this article we describe the precision in the estimation of the spin-lattice relaxation time T1 from MRI signals acquired for various flip angles with the repetition time TR held constant. We review the estimation procedure itself and present a model for the propagation of noise in the signal into the calculated T1. This model is verified by both Monte Carlo simulations and experimental data taken on image phantoms. Based on this model, we find that for a given TR/T1 there exist two optimal flip angles that will minimize the uncertainty in the estimated T1. We also show how two optimal angles can be selected for a given range of TR/T1 values. In addition, T1 estimation using the two optimal angles can be comparable to or better than using multiple evenly spaced angles. Finally, in an initial comparison with the two-point saturation recovery method of calculating T1, results for equal total scanning time TRTot suggest that for T1 greater than 0.5 TRTot, the limited flip angle approach gives better T1 precision whereas for T1 less than 0.5 TRTot the saturation recovery approach is better.

Humans↗
PubMed1986

Instrumentation for rapid MR image synthesis.

MR image synthesis has previously been developed as a means of retrospectively optimizing contrast of arbitrary materials in MR images. The first step of this process is to form computed N(H), T1, and T2 images from source images acquired at a variety of echo delay and repetition times. The second step is to take these computed images, along with operator-selected timing parameters, and mathematically generate a synthesized image. Computation is carried out pixel by pixel according to the equation describing the chosen pulse sequence. This paper presents a study of design considerations for a digital image processor capable of rapidly performing the second step, the actual synthesis. In this work the computations inherent to image synthesis are identified, and the feasibility of performing them in high-speed hardware examined. An analysis of the imprecision due to bit-limited calculations shows that an error bound of 0.4% is possible with a 16-bit processor design. A method is described which uses a commercially available image processor by which images can be synthesized according to any of the standard pulse sequences in less than 600 ms.

Computers↗
PubMed1986

A modified saturation-recovery approximation for multiple spin-echo pulse sequences.

In equations describing multiple spin-echo pulse sequences, the T1 dependence is often approximated by a saturation-recovery expression, which is valid only when the repetition time TR is much greater than the time required for the echoes. A slight modification of this approximation greatly increases its accuracy for small TR values.

Humans↗
PubMed1986

Optimum acquisition times of two spin echoes for MR image synthesis.

Spin-echo images can be synthesized at arbitrary values of echo time TEs if two images are acquired at the same repetition time and two different echo times TE1 and TE2. Depending on the value of TEs, the noise in the synthetic images can either be greater or less than the acquisition noise. This note shows that if the time between the acquired echoes TE2 - TE1 is equal to T2, the noise level in the synthetic images is no larger than the acquisition noise for TEs greater than or equal to TE1. This is the lowest possible noise bound for two-echo acquisition. Also, the noise bound for images synthesized with O less than or equal to TEs less than or equal to TE1 is minimized by making TE1 as short as possible.

Biometry↗
PubMed1986

Pulse sequence extrapolation with MR image synthesis.

Previous reports have presented validation studies of magnetic resonance (MR) image synthesis in which multiple spin-echo (MSE) source data were used to generate spin-echo images for various echo times and repetition times (TRs). A new method-"pulse sequence extrapolation" -synthesizes images for pulse sequences different from that of the acquisition. MSE data acquired in a time equivalent to a TR of 2,000 msec can be used to generate inversion-recovery (IR) images for arbitrarily chosen TI inversion times. Other combinations of pulse sequences were also studied, and synthetic images were compared visually and quantitatively to directly acquired images with corresponding parameters. Synthetic IR signals of the brain parenchyma consistently matched directly acquired signals to within 6%, with respect to the full magnetization signal. The noise level of synthetic signals was generally no more than twice that of direct acquisition signals, as predicted. This method can achieve selective fat suppression and enhancement in IR imaging.

Biophysical Phenomena↗
PubMed1986

Magnetic resonance image synthesis. Clinical implementation.

Magnetic resonance (MR) image synthesis is a technique enabling the retrospective optimization of scanning parameters. This paper describes the methods used to enable the clinical implementation of this technique. It is shown that effective synthetic images can be generated from only three acquired images per slice. Data for 16 slices and TR times of 500 and 2000 ms can be efficiently acquired in a multi-slice multi-echo dual-TR pulse sequence. Computation of T1, T2, and density images and the subsequent synthesis of images for arbitrary TR and TE times can be performed at high speed with dedicated hardware. The method is seen as one of standardizing acquisition protocols and thereby improving patient throughput. Specific clinical applications are discussed.

Humans↗
PubMed1986

The influence of estrogen on the hyperglycemic action of alloxan in female rats.

The purposes of this study were to investigate (1) the relationship between estrogen and glucose levels in the circulation during an estrous cycle of the rat and (2) the effect of estrogen on the hyperglycemic action of alloxan. Three-month old Long Evans strain rats were used. The present study established for the first time the correlation between estrogen and glucose levels in the circulation during an estrous cycle. A significant (p less than 0.01) negative correlation was observed with the lowest glucose level at the proestrus stage where estrogen is at peak. The severity of hyperglycemia induced by alloxan was positively correlated with the circulating level of endogenous estrogen, but was negatively correlated with the fasting glucose level. A similar pattern was observed in ovariectomized female rats that were followed by estradiol implantation to achieve a constant level of circulating estrogen. It may be concluded from the present study that estrogen, under physiological conditions, participates in the regulation of glucose metabolism during various stages of the estrous cycle in the rats, and that glucose is likely able to protect the pancreatic beta cells of the animals against the hyperglycemic action of alloxan.

Alloxan↗