Correlation of radionuclide and optical tracer assessments of fluid flow in artificial ventricles

Characterization of flow properties of an artificial ventricle may aid development of designs to minimize thrombosis. Techniques for determining two such flow properties, viz. ventricular clearance rate and ejection fraction, are compared and validated here for polyurethane and silicone rubber ventricles operated in a mock circulatory loop at various stroke volumes and pulse rates. Ventricular clearance rats were measured both by clinical radionuclide tracer techniques and by an optical tracer method. Ejection fractions were measured by radionuclide imaging and validated by direct measurements of flow rate and ventricular volume. Results from the two methods for ventricular clearance are in close agreement. The optical tracer method is superior in spatial resolution, convenience and economy, but the radionuclide tracer method for ejection fraction gives excellent agreement with the absolute measurements.

Chronic mesenteric ischemia: use of in vivo MR imaging measurements of blood oxygen saturation in the superior mesenteric vein for diagnosis

PURPOSE

To determine if dogs and humans with chronic mesenteric ischemia demonstrate a decrease in the percentage of oxygenated hemoglobin (%HbO2) in the superior mesenteric vein (SMV) after a meal.

MATERIALS AND METHODS

In 10 dogs, ameroid rings were surgically implanted around the superior mesenteric arteries to create gradual stenosis. Pre- and postoperative angiograms and pre- and postprandial magnetic resonance (MR) oximetry measurements of the SMV %HbO2, with flow-independent T2 measurements of venous blood, were obtained at different times. In 10 patients with atherosclerotic disease and six patients with symptomatic chronic mesenteric ischemia, the same measurements were obtained after at least 6 hours of fasting and at 15, 35, and 45 minutes after ingestion of a liquid nutritional supplement.

RESULTS

In seven dogs, the postprandial SMV %HbO2 increased an average of 2.5% +/- 0.8 before surgery and decreased an average of 6.3% +/- 2.1 when hemodynamically significant (>70%) stenosis of the superior mesenteric artery developed 7-14 days after surgery. In the 10 patients without ischemia, the SMV %HbO2 increased by 4.6% +/- 0.6, whereas in the symptomatic patients a postprandial decrease of 8.8% +/- 0.7 occurred (P < .0001).

CONCLUSION

Measurement of the SMV %HbO2 with MR oximetry is a promising test for diagnosis of chronic mesenteric ischemia.

Flow-independent angiography for peripheral vascular disease: initial in-vivo results

Flow-independent angiography (FIA), an approach that isolates arterial blood using MR relaxation characteristics rather than flow effects, was evaluated for application in peripheral vascular disease (PVD). First, pilot studies were conducted in which FIA coronal projection images were obtained from controls and symptomatic patients with PVD to assess clinical utility. All control images corresponded to the expected leg arterial anatomy with little interference from deep veins (one of five) and muscle (zero of five). Superficial venous signal was less well suppressed in comparison to deep veins (four of five). Images of symptomatic patients were less consistent with difficulty suppressing muscle and deep venous signal in some cases and edema when present. We then compared T2 values for muscle (T2m, tibialis anterior), arterial blood (femoral and popliteal arteries), and venous blood (femoral, popliteal, and saphenous veins) in controls (n = 8) and symptomatic patients with intermittent claudication (n = 5) or ischemic rest pain (n = 7). Changes in T2 measurements of various tissues accounted for poorer contrast in symptomatic patients. Patients with ischemic rest pain had significantly higher T2m compared with controls (T2m = 39.3 +/- 2.1 (1 standard error of the mean [SEM]) versus 30.9 +/- .4, P < .01). For all measurements, other than saphenous vein, variances were greater in symptomatic patients. To realize the inherent advantages of FIA for this clinical application, additional work on suppression of signals from muscle, veins, and edema is required. One promising approach involves shifting from projection images to three-dimensional acquisitions for improved tissue suppression.

Repeated dose toxicity study (28 days) in rats and mice with N-methylpyrrolidone (NMP)

Twenty-eight day feeding studies were conducted to evaluate the repeated dose toxicity of NMP, a widely used industrial solvent, in Crl:CD BR rats and B6C3F1 mice. Groups of 5 male and 5 female rats each were fed either 0, 2,000, 6,000, 18,000, or 30,000 ppm NMP; similar groups of mice were fed either 0, 500, 2,500, 7,500, or 10,000 ppm. In vivo parameters, hematology and clinical chemistry parameters, and complete pathology evaluations were conducted after approximately 28 days. Decrements in mean body weight gains, reflecting decreases in food consumption and efficiency, were seen in male rats fed 18,000 ppm and in both sexes fed 30,000 ppm. In rats, clinical chemical changes, indicating possible compound-related alterations in lipid, protein, and carbohydrate metabolism, occurred at 18,000 ppm in males and 30,000 ppm in both sexes. No histopathological changes in rats were judged to be directly related to NMP exposure. Hematological (mild to moderate leukopenia) and histopathological alterations (hypocellular bone marrow, testicular degeneration and atrophy, and thymic atrophy) were judged to be secondary to nutritional and body weight effects in male and/or female rats at 30,000 ppm. In mice, cloudy swelling of the epithelia of the distal parts of the renal tubuli was observed in 4 males and 3 females at 10,000 ppm and in 2 male mice at 7,500 ppm. For both rats and mice, abnormal urine coloration was observed (in mice at 2,500 ppm and above, and in rats at 18,000 ppm and above). The discoloration was interpreted as a sign of systemic availability of the test substance, but not as an adverse effect. The NOAEL was 6,000 ppm for male rats and 18,000 ppm for female rats. In mice, the NOAEL was 2,500 ppm based on the kidney histopathology.

An analytical model of restricted diffusion in bovine optic nerve

An analytical model of restricted diffusion in bovine optic nerve is presented. The nerve tissue model is composed of two different objects: prolate ellipsoids (axons) and spheres (glial cells) surrounded by partially permeable membranes. The free diffusion coefficients of intracellular and extracellular water may differ. Analytical formulas for signal loss due to diffusion in the pulsed gradient spin echo (PGSE) experiment for this tissue model are derived. The model is fitted to experimental data for bovine optic nerve. The obtained model parameters are shown to be reasonable. The model describes all of the characteristics of the PGSE data: anisotropy, upward curvature of decay curves, and diffusion time dependence. The validity and sensitivity of the model are also discussed.

In vivo magnetic resonance evaluation of blood oxygen saturation in the superior mesenteric vein as a measure of the degree of acute flow reduction in the superior mesenteric artery: findings in a canine model

RATIONALE AND OBJECTIVES

The authors tested the hypothesis that changes in oxygen saturation (%HbO2) in the superior mesenteric vein (SMV), as measured with in vivo magnetic resonance (MR) oximetry, correlate with the degree of acute superior mesenteric artery (SMA) flow reduction.

METHODS

Ten mongrel dogs were studied. A catheter was inserted into the SMV, and a perivascular ultrasonic flow probe and an adjustable mechanical occluder were placed around the SMA. MR oximetry was carried out at the resting state and after the SMA was constricted to predetermined levels (0%-75% of initial flow). In seven dogs, SMV blood samples were obtained immediately before and after each MR measurement; %HbO2 was measured simultaneously by using an oximeter. With linear regression analysis, the SMV %HbO2 measurements obtained at MR imaging were compared with those obtained at oximetry. With a logistic model, MR imaging changes in SMV %HbO2 were compared with the degree of SMA flow reduction.

RESULTS

SMV %HbO2 measurements obtained with MR imaging correlated well with those obtained with oximetry (r = .97). Changes in SMV %HbO2 measured at MR imaging also correlated well with the degree of SMA flow reduction, as determined with a logistic model (P = .01).

CONCLUSION

Noninvasive in vivo MR measurements of SMV %HbO2 can be used to determine the degree of acute SMA flow reduction with a high degree of accuracy in a canine model.

Isopropanol: summary of TSCA test rule studies and relevance to hazard identification

The toxicity of isopropanol (IPA) has been extensively studied as a result of a Test Rule under Section 4 of the Toxic Substances Control Act. In general, the data showed that IPA has a low order of acute and chronic toxicity; does not produce adverse effects on reproduction; is neither a teratogen, a selective developmental toxicant, nor a developmental neurotoxicant; and is not genotoxic or an animal carcinogen. IPA is, however, a potential hazard for transient central nervous system depression at high exposure levels. In addition, IPA produced effects to several rodent toxicity endpoints at high dose levels (i.e., motor activity, male mating index, and exacerbated renal disease) which are of unclear relevance to human health. The data generated by these studies confirmed that IPA acts as a typical short-chain alcohol in mammalian biological systems. It produces a significant narcotic effect upon exposure at high levels for extended periods of time, with no irreversible effects even after repeated exposure, which is consistent with other short-chain alcohols. The metabolism of IPA appears equivalent across species with rapid conversion to acetone and carbon dioxide. Overall, these studies demonstrate IPA exposure is a low potential hazard to human health. This information will allow for an improved assessment of the human health risks from IPA exposure.

Performance comparison of two Wolter type II telescopes in the far ultraviolet

Experimental results for image quality and scatter in far-UV light are used to choose between the conventionally polished Solar Extreme-Ultraviolet Rocket Telescope and Spectrograph (SERTS-A) and computer-controlled polished SERTS-C Wolter type II solar rocket telescopes for an extreme-UV flight instrument. In 124-nm light the SERTS-C telescope image had a 0.67-arcsec full width at half-maximum (FWHM), compared with a 1.25-arcsec FWHM for the SERTS-A telescope. In addition SERTS-C had twice the peak irradiance of SERTS-A and an order of magnitude lower near-angle scatter. The inflight performance of the telescopes is consistent with laboratory findings.

Coronary angiography with magnetization-prepared T2 contrast

A magnetization-prepared, T2-weighted sequence (T2 Prep) is used to suppress muscle and venous structures. When combined with lipid suppression, this technique improves the visualization of the coronary arteries. T2 Prep was designed to be rebust in the presence of flow as well as B0 and B1 inhomogeneities and may be combined with virtually any imaging technique. Here, it is implemented with both a single-slice spiral acquisition and a multi-slice spiral method that acquires up to 15 slices in a single breath-holding interval.

Oxygen saturation of blood in the superior mesenteric vein: in vivo verification of MR imaging measurements in a canine model. Work in progress

PURPOSE

To evaluate the accuracy of magnetic resonance (MR) imaging in estimating oxygen saturation of blood (%HbO2) in the superior mesenteric vein (SMV) of a canine model in vivo.

MATERIALS AND METHODS

MR imaging was used to measure the T2 of blood in samples obtained via a catheter placed in the SMV in seven mongrel dogs. %HbO2 was measured with a reflectance oximeter. These measurements were obtained at the resting state, during superior mesenteric artery occlusion, and after reperfusion. MR imaging and oximeter measurements were then compared by using linear regression analysis.

RESULTS

Refocusing intervals (tau 180) of 12 and 24 msec were used for 17 and 18 %HbO2 measurements with MR imaging, respectively. With tau 180 of 12 msec, there was an excellent correlation between MR imaging measurements and oximeter measurements (r = .969). The intercept was 5.3% and the slope was 0.959. With tau 180 of 24 msec, r = .953, the intercept was 15.4%, and the slope was 0.817.

CONCLUSION

Estimates of %HbO2 in the SMV with MR imaging are accurate in the range of most clinical interest.

Incorporating lactate/lipid discrimination into a spectroscopic imaging sequence

A spectroscopic imaging sequence incorporating a two-shot lactate editing method was used in two human brain studies to image lactate and NAA. The subtractive editing method allows separate images of lactate, NAA, and lipids to be collected during a single study with no SNR penalty. The sequence uses a spectral-spatial excitation for slice selection and water suppression, and employs inversion recovery and an echo time of 136 ms for additional lipid suppression.

Pulsed saturation transfer contrast

In vivo 1H conventional NMR image contrast generation usually relies on the macroscopic T1 and T2 relaxation parameters of the tissues of interest. Recently cross-relaxation related image contrast has been reported by Wolff and Balaban in animal models. Due primarily to the broad lineshape of the intended saturation spin pool and the use of off-resonance irradiation, high specific absorption rate and an auxiliary RF amplifier have been necessary to produce these images. The relatively long spin-lattice relaxation property of this spin pool, however, suggests the use of pulse methods to achieve saturation. In this paper, we show that short-T2 spin pools can be selectively saturated with short intense RF pulses. Cross-relaxation time constants can be measured using the technique of saturation recovery. In vivo magnetization-transfer-weighted images can be produced using pulses on commercial whole-body imagers without additional hardware.

1991 I.I. Rabi Award. Estimating oxygen saturation of blood in vivo with MR imaging at 1.5 T

The use of magnetic resonance (MR) imaging is investigated for noninvasively estimating the oxygen saturation of human blood (%HbO2) in vivo by means of relaxation characteristics identified in earlier MR spectrometry studies. To this end, a sequence is presented for determining the T2 of vascular blood in regions in which motions of the body and of the blood itself present a major challenge. With use of this sequence on a commercial 1.5-T whole-body imager, the relationship between the T2 and %HbO2 of blood is calibrated in vitro for the conditions expected in vivo. T2 varies predictably from about 30 to 250 msec as %HbO2 varies from 30% to 96%. T2 values measured in situ for vascular blood in the mediastinum of several healthy subjects qualitatively reflected the behavior observed in vitro. Estimates of %HbO2 for these vessels obtained with the in vitro calibration appear reasonable, particularly for venous blood, although difficulties arise in selecting the appropriate calibration factors. These encouraging initial results support a more systematic study of potential sources of error and an examination of the accuracy of in vivo measurements by comparison with direct measurements of %HbO2 in vessels.

Flow-independent magnetic resonance projection angiography

The performance of current, flow-based sequences for imaging vasculature using MR is severely restricted in regions with inherently slow flow. We address this problem with a flow-independent imaging method. Specifically, we generate projection images of blood in the limbs while suppressing signal from all other tissues (primarily skeletal muscle, bone marrow, and subcutaneous fat) using a flow-compensated, water-selective, short TI inversion recovery sequence with a long echo time. We experimentally evaluate the effectiveness of this sequence and present in vivo results clearly demonstrating the method's potential.

Computing material-selective projection images in MR

We detail a robust, general method for computing projection images of individual materials in a volume as linear combinations of MR projection images with different material-dependent weightings. Signal per unit volume for each material in each raw image is acquired directly for accurate cancellation of undesired, overlapping materials. The weighted sum of the input images is determined to maximize the signal-to-noise ratio (SNR) and minimize inhomogeneity effects in the material-selective images. We tested the implementation experimentally in both phantom and human studies, producing selective images with reasonable SNRs and material isolation. With further development of sequences to rapidly acquire input images having greater material differentiability, we envision the application of the selective projection imaging format to screening studies searching over large volumes for diseased tissues.

Tetany and myocardial arrhythmia due to hypomagnesaemia. A case report

An 11-year-old boy with acute intestinal obstruction presented with tetany, and developed myocardial arrhythmia during anaesthesia. The procedure was abandoned and the patient further investigated. He was found to have hypomagnesaemia and normocalcaemia. Intravenous magnesium sulphate corrected the tetany, and no further arrhythmia occurred during subsequent surgery.

Noise in stenosis measurement using digital subtraction angiography

This paper examines statistical errors in the measurement of arterial stenoses by digital videodensitometry. Images of vessel phantoms were acquired using digital subtraction angiographic techniques with low concentrations of an iodine contrast medium and low levels of x-ray exposure. Effects of the spatial and temporal averaging of image information on signal-to-noise ratios in the stenosis measurement were of primary interest. The influences of iodine concentration, x-ray scatter, veiling glare, x-ray energy spectrum, x-ray exposure, and detective quantum efficiency of the system were also included in the theoretical analysis. The agreement between theoretical calculation and experimental measurement of a simulated vessel was verified using measured values of the imaging system parameters. With a 14.2 mg/ml iodine concentration, using 20 mR per image at the entrance to a 13-cm water phantom, and averaging over a 6-mm length of a vessel 6.2 mm in diameter, the standard deviation in a measurement of a vessel's relative cross-sectional area was about 0.05. The extension of these results to practical applications in vivo is discussed.

The Corrosion of Carbon Steel and Stainless Steel in Simulated Geothermal Media

The corrosion rates of low-carbon steel, and 304, 316 and 410/420 stainless steels in simulated geothermal media containing hydrogen sulfide have been measured by means of the polarization resistance technique. Good agreement was found between weight-loss and polarization resistance measurements of the corrosion rate for all the metals tested. Carbon steel formed a non-adherent film of mackinawite (Fe1 + xS). The lack of protection afforded to the steel by the film resulted in an approximately constant corrosion rate. The stainless steels also exhibited corrosion rates that were independent of time. However, the 410 and 420 alloys formed an adherent film consisting mainly of troilite ( FeS ) which provided only limited passivity. In contrast, the 304 and 316 alloys appeared to be essentially protected by a passive film which did not seem to involve an iron sulfide phase. However, all the stainless steels, particularly the 410 and 420 alloys, showed pitting, which indicated that some breakdown of the passive films occurred.

Electrochemical Phase Diagrams for the Fe/S/H2O System under Geothermal Conditions

Previously published electrochemical phase (potential-pH) diagrams for the Fe/S/H2O system at elevated temperatures are assessed. It is concluded that the potential-pH equations derived by Biernat and Robins1 can be used to provide reliable diagrams over the temperature range 298-573 K. These equations were used to derive a set of diagrams relevant to geothermal fluids in general terms, so that the corrosion products of iron and ferrous alloys immersed in these fluids may be predicted. In addition, diagrams are presented for geothermal fluids in the Broadlands and Wairakei fields in New Zealand. The significant changes in the E-pH diagrams as the temperature increases over the range 298-573 K are that the region of stability for Fe(OH)3- widens and the regions of stability for FeS2 and Fe3O4 and, at higher temperatures, FeS, shrink in size.