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Biomedical subjects

G M Pohost

Publications and source records attributed to G M Pohost.

At least 91 records · Page 5Linked to original sources
PubMed1992

Slice thickness reduction by partial overlapping presaturation.

We present a new approach to slice thickness reduction that does not require strengthening the selection gradient or increasing the RF pulse duration. The basis of the approach is to employ a thick slice and suppress a portion of its width, thereby forming a thin slice. Two distinct implementations of the technique are introduced and demonstrated by imaging slice profiles in a phantom and generating images of a volunteer's knee.

Humans↗
PubMed1992

Relationship between septal perfusion, viability, and motion before and after coronary artery bypass surgery.

The etiology of abnormal interventricular septal motion occurring after open-heart surgery using cardiopulmonary bypass has not been clarified. Intraoperative ischemic septal injury has been proposed as one explanation for this finding. To examine this possibility, resting septal perfusion and viability were studied using rest and redistribution thallium-201 scintigraphy in 16 patients before and after coronary artery bypass surgery. The results were compared with septal motion on preoperative and postoperative resting gated blood pool scans. Preoperatively, septal thallium uptake was normal in 10 of 16 patients, and septal motion was normal in 14 of 16. Postoperatively, septal thallium uptake was normal in 11 of 16 patients, while septal motion was abnormal in all. Thus abnormal postoperative septal motion is usually associated with normal septal perfusion and viability on thallium scans and therefore is not the result of septal ischemic injury in a majority of patients.

Adult↗
PubMed1992

Amiloride in ouabain-induced acidification, inotropy and arrhythmia: 23Na & 31P NMR in perfused hearts.

The increase in intracellular sodium (Nai), resulting from inhibition of the Na/K ATPase by cardiac glycosides, is known to increase calcium influx via Na(+)-Ca2+ exchange, and thereby increase contractility. This increase in intracellular Ca2+ has been related to the development of intracellular acidification and enhanced activity of the Na(+)-H+ exchanger as a measure by the cell to prevent further acidification. Thus, the efflux of the H+ ions results in an additional increase in Nai. This may subsequently lead to an increased rate of Ca2+ influx and therefore to the potentiation of the effects of cardiac glycosides. To assess the role of Na(+)-H+ exchange in the mechanism of ouabain action in the beating heart we used amiloride, a known inhibitor of Na(+)-H+ exchange. Isolated rat hearts were perfused with either ouabain (50 microM) alone (n = 8, Group I), amiloride (1.0 mM) + ouabain (50 microM) (n = 8, Group II), or amiloride (1.0 mM) alone as a control group (n = 4, Group III). 23Na and 31P NMR spectroscopy were used to assess the changes in Nai and intracellular pH (pHi), respectively, while simultaneous and continuous monitoring of left ventricular pressure was carried out. Perfusion with both ouabain alone (Group I) or ouabain + amiloride (Group II), resulted in a time dependent increase in Nai levels, reaching (within 25 mins) a maximum of 200 +/- 7% of control in Group I, and 170 +/- 10% of control in Group II. Concurrently, a mild but significant decrease in pHi was observed in both groups. This decrease, however, was significantly higher in Group II compared to Group I (0.34 pH units vs. 0.19 pH units, respectively; P less than 0.05), suggesting that inhibition of Na(+)-H+ exchange by amiloride limits the recovery from ouabain-induced intracellular acidification. While developed pressure gradually increased in Group I to a maximum of 268 +/- 52% of control, the addition of amiloride in Group II substantially reduced the positive inotropic effect. Ventricular fibrillation (VF) developed in three of the eight hearts in Group I within 10-13 mins after the addition of ouabain. Interestingly, the rate of Nai increase in hearts that sustained VF was significantly higher compared to those without VF (mean slope 10.1 +/- 2.11 vs. 3.9 +/- 1.0, respectively; P less than 0.0001). Ventricular fibrillation did not develop in Group II or III.(ABSTRACT TRUNCATED AT 400 WORDS)

Amiloride↗
PubMed1992

Outflow refreshment angiography: a bright blood, bright static tissue technique.

A new "bright blood" strategy, outflow refreshment imaging, is introduced in which a number of overlapping slices are excited in rapid succession. Flowing spins that refresh each overlapped slice portion contribute a bright signal. Additionally, static tissue in each non-overlapped slice portion also yields a bright signal. However, the flow/static contrast is comparable to that produced in inflow refreshment images, and angiograms can be generated by conventional maximum intensity projection processing. The dual ability to visualize angiograms and static tissue images is a major benefit of the strategy. Computer simulations of flow sensitivities and in vivo results are presented which compare the outflow and inflow refreshment imaging strategies.

Blood Flow Velocity↗
PubMed1992

Time-varying wall stress: an index of ventricular vascular coupling.

Previous work in the isolated heart and intact circulation has suggested that the relationship between wall stress and time during left ventricular (LV) ejection is linear and that the slope, which will be referred to as time-varying wall stress, increases in response to augmentation in afterload. However, the etiology of the increase in slope has not been determined in an intact animal. Magnetic resonance imaging coupled with high-fidelity LV pressure measurement using a nonferrous catheter-tip manometer generates a detailed assessment of wall stress in an animal model where the thorax and pericardium have never been disturbed. Accordingly, six anesthetized dogs were studied during autonomic blockade with atropine and propranolol during angiotensin infusion, producing three widely disparate left ventricular systolic pressures (87 +/- 7 vs. 124 +/- 13 vs. 152 +/- 10 mmHg, P less than 0.001). Time-varying wall stress did not change from low to medium load (-42.4 +/- 9.5 to -27.3 +/- 22.3 g.cm-2.ms-1) but increased significantly at high load (-21.7 +/- 14.9 g.cm-2.ms-1, P less than 0.05). Analysis of the relative contribution of pressure, chamber radius, wall thickness, and long-axis dimension to the changes in time-varying wall stress demonstrated only the pressure component to change its relative contribution at medium (P less than 0.001) and high load (P less than 0.001). Therefore, we conclude that the increase in time-varying wall stress results from augmentation of pressure in the latter one-half of systole that is incompletely offset by shortening and wall thickening.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Assessment of postreperfusion myocardial hemorrhage using proton NMR imaging at 1.5 T.

BACKGROUND: Intramyocardial hemorrhage occurs frequently after reperfusion of acute myocardial infarction. However, its significance has not yet been established, mainly because of the lack of methods for detecting such hemorrhage. The following ex vivo study was carried out to assess the potential of nuclear magnetic resonance (NMR) imaging to detect and quantitate postreperfusion intramyocardial hemorrhage. METHODS AND RESULTS: Sixteen adult mongrel dogs underwent 3 hours of coronary occlusion followed by 1 hour of reperfusion, and three dogs underwent 4 hours of occlusion without reperfusion. Radiolabeled microspheres and 51Cr-labeled red blood cells were used to assess flow and evaluate the extent of hemorrhage. These results were later compared with both NMR and histology. Spin-echo NMR imaging was performed on the excised hearts using a 1.5-T system. Macroscopic assessment of the sliced myocardium revealed the existence of hemorrhage in 14 of the 16 dogs that underwent reperfusion but in none of those with occlusion only. In all 16 dogs with reperfusion, zones of increased signal intensity (SI) ratio (1.68 +/- 0.41 compared with control, p less than 0.05) were seen in regions relating to the distribution of the occluded coronary artery, whereas in 13 of the 16 dogs, areas of decreased SI within the zone of increased SI ratio (0.81 +/- 0.16 compared with control, p less than 0.05) were also seen, corresponding to regions with macroscopic hemorrhage. In contrast, in the three dogs without reperfusion, no macroscopic hemorrhage was observed, and likewise, no NMR zones of reduced SI were detected. Hemorrhage size by NMR (decreased SI zones), correlated well with hemorrhage size calculated from tissue slices (r = 0.96, SEE = 0.92%, p less than 0.01), or by 51Cr labeling (r = 0.78, SEE = 1.5, p = 0.1). In the reperfusion group, T2 relaxation times in the infarcted hemorrhagic zone (58 +/- 9 msec) were significantly lower than the infarcted zones without hemorrhage (98 +/- 13 msec, p less than 0.001). In contrast, when compared with control (964 +/- 72 msec), T1 relaxation times were significantly increased in both infarct zones, either with (1,284 +/- 176 msec) or without (1,266 +/- 103 msec) hemorrhage. The selective shortening of T2 relaxation times in the hemorrhagic regions is consistent with the paramagnetic effects of deoxyhemoglobin. CONCLUSIONS: NMR imaging may provide a noninvasive approach for the detection and quantitation of intramyocardial hemorrhage. This observation may provide a means to further characterize pathological processes associated with acute myocardial infarction and assess the role of myocardial hemorrhage after reperfusion therapy.

Animals↗
PubMed1992

In vivo 31P nuclear magnetic resonance spectroscopy of human temporal lobe epilepsy.

We performed localized 31P nuclear magnetic resonance (NMR) 1H-image-guided in vivo spectroscopy to study regional high-energy phosphate levels in the brains of normal controls and in patients with intractable unilateral temporal lobe epilepsy. We did not observe differences in intracellular pH between controls and patients. The phosphocreatine/inorganic phosphate ratio was reduced by 50% in the epileptogenic temporal lobe compared with controls (p less than 0.005) and by 35% when compared with the unaffected contralateral temporal lobe (p less than 0.05). We did not observe differences in the ratio of phosphomonoesters to phosphodiesters between controls and patients. These findings suggest that in vivo 31P NMR spectroscopy yields a distinctive interictal metabolic profile in patients with intractable unilateral temporal lobe epilepsy and may permit noninvasive lateralizing evidence of the seizure focus.

Adolescent↗
PubMed1991

Interstudy reproducibility of biplane cine nuclear magnetic resonance measurements of left ventricular function.

Cine nuclear magnetic resonance (NMR) imaging, as a noninvasive and high-resolution imaging modality, has been shown to be reliable for determining absolute left ventricular (LV) volumes and ejection fraction. A relatively new gradient echo cine NMR approach using 2 orthogonal long-axis planes (2- and 4-chamber) aligned with the true axes of the left ventricle has been previously developed and validated against radiographic biplane LV cineangiography. The aim of the present investigation was to determine the reproducibility of this more rapid cine NMR approach for the measurement of LV volumes and ejection fraction. Eighteen normal subjects underwent 2 cine NMR studies, on different days, using a 1.5-tesla clinical imaging system. Studies were analyzed on-line and blindly by 2 independent observers. Intraobserver error was also determined in a blinded manner. Mean values of measurements determined by this method in this group of normal subjects were end-diastolic volume (120 +/- 20 ml), end-systolic volume (39 +/- 9 ml) and ejection fraction (67 +/- 4%). Paired analysis of data revealed no significant bias between interstudy, interobserver or intraobserver measurements, except for interobserver end-diastolic volume, where the first observer measurements were slightly elevated (5.6 +/- 7.8 ml) compared with the second. This resulted in a small difference in ejection fraction (1.7 +/- 2.3%) between observers. The absolute variation between measurements (square root of variance components) was low for all interstudy, interobserver and intraobserver comparisons: end-diastolic volume was less than +/- 6.7 ml, end-systolic volume less than +/- 3.5 ml and ejection fraction less than +/- 2.4%.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

31P NMR spectroscopy in chronic adriamycin cardiotoxicity.

Abnormal cardiac energy metabolism has been postulated as a mechanism for adriamycin induced cardiotoxicity. This study was designed to determine high energy phosphate stores at rest and with hemodynamic stress in perfused rat hearts after animals had been chronically exposed to adriamycin (2 mg/kg weekly for 14 weeks). Morphologic and hemodynamic changes were mild in this model. Phosphorus-31 NMR determined intracellular pH and levels of inorganic phosphate (Pi) and ATP were comparable in treated and control hearts. Phosphocreatine (PCr) levels were markedly decreased in treated hearts (0.89 +/- 0.07 units/g versus 1.7 +/- 0.13 units/g, p less than 0.001). The PCr/Pi ratio decreased in both groups during hemodynamic stress. It recovered earlier in controls and there was a marked over-shoot after cessation of rapid pacing in this group which was not present in adriamycin treated hearts. These results suggest that metabolic regulation in response to hemodynamic stress is impaired after chronic adriamycin exposure. PCr depletion and delayed metabolic recovery after hemodynamic stress appear to be potentially useful markers for the effect of adriamycin on the heart.

Adenosine Triphosphate↗
PubMed1991

Development of a micromanometer-tip catheter to record high-fidelity pressures during cine-gated NMR without significant image distortion.

We report a new micromanometer-tip catheter to record high-fidelity pressures during cine-gated gradient echo NMR imaging in an intact animal model. Of the various metals and alloys tested for use in its construction, brass produced the smallest NMR artifact with minimal magnification. With the use of this new catheter design, there was little signal loss and no distortion of endocardial borders during imaging.

Alloys↗
PubMed1991

Visualization of altered myocardial lipids by 1H NMR chemical-shift imaging following ischemic insult.

Acute myocardial infarction is associated with an accumulation of lipids. Spectroscopic and chemical-shift imaging strategies which can depict the spatial distribution of these chemical species are evolving. The present study was undertaken to test whether the Dixon method could detect spatially lipids known to accumulate in myocardium after an ischemic insult. Seven dogs underwent a 24-h coronary artery occlusion (LAD = 4, Cx = 3). Post mortem, hearts were removed and imaged ex vivo. Myocardial samples were also evaluated by high-resolution 1H NMR spectroscopy. Lipid images revealed regions of increased signal intensity, in the regions corresponding to the myocardial infarction, particularly in the periphery of the infarction. An increase in mobile lipids was observed by 1H NMR spectroscopy of myocardial samples with moderately reduced blood flow and corresponding to regions with increased signal intensity on the lipid image. This study shows that chemical-shift imaging may be useful for detecting alterations in myocardial lipid levels following an ischemic insult.

Animals↗
PubMed1991

A new acquisition mode for 2D inflow refreshment angiography.

Time-of-flight angiography methods rely on blood/tissue contrast generated when fresh blood spins enter a partially saturated region. A series of projective angiographic views is generated by application of a maximum intensity projection algorithm to data acquired from a 3D volume. When the data are acquired as a set of 2D slices, the angiograms exhibit sensitivity to body motion, resulting in the generation of artifacts. We present a new 2D time-of-flight angiographic technique, STREAM, Suppressed Tissue with Refreshment Angiography Method. It exhibits low sensitivity to internal body motion, thus allowing acquisition of good quality, single average angiograms. To suppress the static tissue signal a unique "active" suppression strategy is employed. Additionally, to further suppress the lipid signal, the chemical-shift phenomenon is exploited in the STREAM sequence.

Algorithms↗
PubMed1991

Direct projective time-of-flight NMR angiography within 4s.

To enhance contrast between flow and static tissue, NMR projective angiographic methods usually require data subtraction, or alternatively the application of special data processing routines such as the "maximum intensity projection" algorithm. We have successfully developed a direct projection method which yields sufficient flow/static contrast in a single acquisition, producing angiograms in under 4 s. Direct projection angiograms of the cervical and abdominal vessels were obtained. The cervical vessels were uniformly depicted in a number of different projection views. The abdominal vessels exhibit intensity variations due to the pulsatile flow pattern. A series of abdominal angiograms were acquired, each triggered at a different starting cardiac phase. When displayed as a movie sequence the pulsatile flow pattern was seen to advance along the arteries. To more uniformly represent the abdominal vessels a 24-s average was adequate. This new technique should allow NMR angiograms to be obtained in a "fluoroscopic" mode.

Abdomen↗
PubMed1991

Sequential 2D inflow venography: initial clinical observations.

Sequential 2D inflow NMR (SDI) venography was employed to define venous anatomy and pathology in five healthy subjects and five patients, mean age 48 +/- 7 years, with documented deep vein thrombosis. SDI images were graded independently by two observers on a scale of 1,2 nondiagnostic and 3,4 diagnostic categories. All iliac and femoropopliteal SDI venograms were diagnostic. In contrast, only one out of nine calf SDI venograms was diagnostic. SDI is a promising new method to assess the proximal peripheral venous system. Improved imaging strategy is needed to produce flow images of the calf veins.

Femoral Vein↗
PubMed1991

Improvement of spectral resolution in shift-reagent-aided 23Na NMR spectroscopy in the isolated perfused rat heart system.

The level of intracellular sodium (Nai) is maintained at approximately 14 mM in healthy myocytes. When myocytes are damaged, Nai increases and therefore the level of Nai may be a means of evaluating myocardial cell integrity. A particularly useful method to monitor Nai levels is 23Na NMR spectroscopy. However, because of the isochronous nature of the extracellular sodium (Nao) and Nai NMR signals, paramagnetic lanthanide shift reagents (LSR), such as dysprosium triphosphate, Dy(PPP)7-(2), have been used to shift the Nao signal. This reveals the unshifted Nai signal and allows the NMR monitoring of Nai in isolated perfused hearts and other systems. A major shortcoming of this method (the "shift-only" method) is in the need to minimize the Nao signal by not submerging the perfused hearts in Na(+)-containing buffer. An equally undesirable alternative is the utilization of relatively high concentrations of LSR to shift a large Nao signal sufficiently to enable reasonable resolution and quantitation of Nai. We present here a method, the "shift-relaxation" method, which is a combination of using a mixture of Dy(PPP)7-(2), a shift reagent, and gadolinium triphosphate, Gd(PPP)7-(2), a relaxation agent, with data acquisition using an inversion-recovery (IR) pulse sequence. This combination allows differentiation between Nao and Nai by the difference in their respective T1 values in addition to the shift between them. With this technique we can selectively minimize the extracellular signal and therefore minimize the need for a large Dy-induced shift, as well as allow data acquisition on a heart submerged in Na(+)-containing perfusate. The resulting improved discrimination between Nai and Nao at relatively low levels of LSR should be helpful for ultimate in vivo applications and potential clinical applications, where a lower dose of LSR also means a decreased possibility of physiologically deleterious effects. Also included in this paper is a method for the quick determination of an accurate 180 degrees pulse which is required for the optimization of the IR method.

Animals↗
PubMed1991

SLIP, a lipid suppression technique to improve image contrast in inflow angiography.

In rapidly acquired gradient echo images, lipid regions usually appear as areas of high signal intensity. In 2D inflow angiograms processed by the maximum intensity projection algorithm, this high lipid signal may prevent angiographic visualization of some blood vessels. We present a new approach to suppress the lipid signal, SLIP, spatially separated lipid presaturation. The SLIP technique suppresses lipid signal over a narrow region by exploiting the chemical-shift effect. SLIP requires an arrangement of a thin slice and a wide presaturation slab already available in 2D inflow refreshment angiography. Thus SLIP was incorporated to an inflow angiography sequence without increasing the acquisition time. The use of SLIP resulted in considerable improvement in angiographic contrast and increased the number of vessels detected in projection angiograms. The SLIP technique does not require high homogeneity of the main magnet field.

Algorithms↗