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Biomedical subjects

G M Pohost

Publications and source records attributed to G M Pohost.

At least 37 records · Page 2Linked to original sources
PubMed1998

Changes in left ventricular mass and volumes in patients receiving angiotensin-converting enzyme inhibitor therapy for left ventricular dysfunction after Q-wave myocardial infarction.

OBJECTIVES: We evaluated global and segmental left ventricular (LV) mass and LV mass/volume ratio in patients with LV dysfunction receiving angiotensin-converting enzyme (ACE) inhibitor therapy after acute myocardial infarction (MI). BACKGROUND: ACE inhibitors attenuate LV dilatation and compensatory hypertrophy after acute MI in animal models. However, LV remodeling in patients after acute MI has been largely defined on the basis of changes in chamber volume alone. METHODS AND RESULTS: Twenty-nine patients with LV ejection fraction <40% received the ACE inhibitor ramipril (range 2.5 to 20 mg/day) within 5 days of their first Q-wave MI. Magnetic resonance imaging was performed at baseline and at 3 months, providing global and regional LV volumes and mass from summated serial short-axis slices. Mean arterial blood pressure was unchanged from baseline to 3-month follow-up (89 +/- 10 to 92 +/- 17 mm Hg). LV mass decreased (90 +/- 25 to 77 +/- 21 gm/m2, p < 0.0005) as LV end-diastolic volumes increased (65 +/- 13 to 73 +/- 22 ml/m2, p < 0.01). Global LV mass to volume ratio decreased from 1.40 +/- 0.28 to 1.08 +/- 0.18 gm/ml (p < 0.0001), as did circumferential wall thickness to volume ratio of noninfarcted myocardium at the base of the LV (0.06 +/- 0.02 to 0.05 +/- 0.02 mm/ml, p < 0.001). LV ejection fraction increased from 35 +/- 6 to 40 +/- 9% (p < 0.001) in the presence of an increase in calculated end-systolic wall stress (185 +/- 57 to 227 +/- 54 gm/cm2, p < 0.01). CONCLUSIONS: ACE inhibitor therapy was associated with improved LV function in the face of a decrease in mass to volume ratio of the LV as well as a decrease in wall thickness to volume ratio of noninfarcted myocardium. Whether ACE inhibitor therapy had direct or indirect effects on these changes in LV mass and function are open questions that require further investigation.

Adult↗
PubMed1998

Magnetic resonance angiography.

Since the emergence of magnetic resonance (MR) imaging, its clinical applications have seen a logarithmic growth. The advantage of MR imaging is that it offers a vast amount of important clinical information with minimal risk to the patient, and promises to reduce the need for angiographic studies with their attendant morbidity and mortality. We review the applications and recent advances of MR imaging to include coronary imaging of native, bypassed and stented vessels, carotid arteries, peripheral arteries, and the aorta. In addition, we discuss issues of patient preferences and the future directions of MR imaging. We predict that the clinical utility of MR angiography will grow with refinements that will improve speed, resolution, and even the use of spectroscopy to characterize plaque.

Aorta↗
PubMed1997

Correlation of thallium uptake with left ventricular wall thickness by cine magnetic resonance imaging in patients with acute and healed myocardial infarcts.

Myocardial infarction (MI) is characterized by cellular necrosis which undergoes fibrotic transformation over time. Cine magnetic resonance imaging (MRI) offers high-resolution 3-dimensional images of the left ventricular myocardium, allowing sampling of the myocardial wall thickness over the entire left ventricle. Tomographic (single-photon emission computed tomography [SPECT]) thallium images also provide 3-dimensional information on the location and level of thallium uptake, which has been shown to correlate with myocardial viability. The purposes of this study were: (1) to examine the relation between both end-diastolic and end-systolic wall thickness and normalized thallium-201 uptake over the left ventricle in a group of patients with MI, (2) to examine the relation between regional wall thickening and normalized thallium uptake, and (3) to examine the relation between thallium uptake and wall thickness both early and late after infarction. Twenty-four patients with MI underwent stress, redistribution, and reinjection thallium SPECT imaging and cine MRI within several days. Seventeen patients underwent imaging late after infarction and 7 underwent imaging early after infarction. Normalized thallium activity was correlated with MRI wall thicknesses at both end-diastole and end-systole for 18 segments for each ventricle. In addition, end-diastolic and end-systolic wall thicknesses were grouped by their corresponding thallium activity levels into percentiles. End-systolic wall thickness correlated significantly with normalized thallium uptake in 14 of 18 segments, end-diastolic wall thickness in only 4 of 18 segments, and wall thickening in only 3 of 18 segments. Mean values for end-diastolic and end-systolic wall thicknesses corresponding to severely reduced (<50%) normalized thallium activity were 9.9 +/- 1.1 and 8.5 +/- 0.6, respectively. Using receiver-operating curve analysis, end-systolic wall performed as a better diagnostic parameter than end-diastolic wall for identifying severely reduced thallium activity levels. For all levels of thallium activity, end-diastolic wall thicknesses were all thinner late versus early after MI, whereas end-systolic wall thickness was thinner only in the segments corresponding to severely reduced thallium activity. Based on these results, end-systolic wall thickness is the best noninvasive anatomic parameter of myocardial scar.

Adult↗
PubMed1997

Rapid cardiac imaging with turbo BRISK.

A variety of variable and constant rate, sparse sampling strategies have previously been proposed to rapidly image dynamically changing objects. The majority of these strategies compile a k-space data set for any given time point by substituting k-space data from the most recently sampled time positions (extracted from the sparsely sampled set). The BRISK technique, is a variable rate, sparse sampling technique which additionally incorporates an interpolation scheme to more accurately represent k-space data at positions which were not directly sampled. Here, strategies are introduced that allow tubo concepts to be incorporated with BRISK. Simulations are conducted to compare the efficacy of the turbo BRISK acquisition and processing strategy against a constant rate, sparse sampling strategy with direct substitution of the most recently acquired k-space lines. It is shown that turbo BRISK generates images of similar quality in approximately half the time as the uniform sampling rate, sparse sampling strategy. Data from turbo BRISK acquisitions of multicardiac phase image sets, obtained on a normal volunteer and cardiac patients are presented.

Heart↗
PubMed1997

Automatic cardiac MR image segmentation using edge detection by tissue classification in pixel neighborhoods.

A highly sensitive edge detector has been developed that uses tissue classification of pixels based on analysis of data in their local neighborhoods. In conjunction with recursive region growing, it has been used successfully to define regions of interest (ROI) when applied specifically to gradient echo MR images of the heart. The detector adapts to nonuniformity by carrying out an independent analysis at each location. If two tissues are present in a neighborhood and the pixel at that location cannot be classified with the seed pixel, a region edge has been crossed and recursion is stopped. No geometric assumptions relating to object shape such as definition of a region center and radial search are required. The detector was applied to multi-slice, multi-phase images of the heart from 26 subjects. A segmentation strategy specified slice processing order, graded ROIs, and used successfully detected ROIs to guide subsequent detection. Segmentation of all images resulted in a 90.3% median edge pixel detection efficiency.

Heart↗
PubMed1997

Common k-space acquisition: a method to improve myocardial grid-tag contrast.

A current limitation of many myocardial tag acquisitions employing SPAMM encoding is the relatively rapid loss of tag contrast over the cardiac cycle. Acquisition schemes that apply line tags produce prolonged tag contrast compared with directly excited grid tags. However, a grid-tag series can be generated by combining two orthogonal line-tag series. To be time efficient, each line-tag series can be acquired with the read gradient oriented perpendicular to the line-tag direction. There are several disadvantages associated with this approach, including the requirement to avoid signal fold-over and that fat shift artifacts appear in different directions in each line-tag series. Presented here is a method of applying separate line tags that does not require interchanging the read and phase encoding gradients and does not extend the scan time compared with a conventional grid-tag acquisition. Additionally, the means of generating grid tags results in a doubling of the tag contrast to noise ratio compared a line-tag set. Computer simulations are presented along with phantom and volunteer scans.

Heart↗
PubMed1997

A method to measure arbitrary k-space trajectories for rapid MR imaging.

A method to measure arbitrary k-space trajectories was developed to compensate for nonideal gradient performance during rapid magnetic resonance (MR) imaging with actively or nonactively shielded gradients at a magnetic field strength of 4.1 T. Accurate MR image reconstruction requires knowledge of the k-trajectory produced by the gradient waveforms during k-space sampling. Even with shielded gradients, residual eddy currents and imperfections in gradient amplifier performance can cause the true k-space trajectory to deviate from the ideal trajectory. The k-space determination was used for spiral gradient-echo imaging fo the human brain. While individual calibrations are needed for new pulse sequences, the method of k-space determination can be used for any sequence of preparation pulses and readout gradient waveforms and should prove useful for other trajectories, including the rastered lines of echo-planar imaging.

Artifacts↗
PubMed1997

1H NMR measurement of triacylglycerol accumulation in the post-ischemic canine heart after transient increase of plasma lipids.

This study tests the hypothesis that increased levels of plasma lipids can accelerate accumulation of myocardial triacylglycerols in post-ischemic but viable myocardium. Two groups of dogs underwent 90 min of left anterior descending coronary artery (LAD) occlusion followed by 240 min of reperfusion. The first group of saline-treated dogs (n = 7) had physiological levels of plasma lipids during reperfusion: a second group treated with Liposyn and heparin (n = 5) experienced increased plasma lipids during reperfusion. The transmural content of triacylglycerols was determined during ischemia and reperfusion using 1H NMR one-dimensional chemical shift imaging (1D CSI), and at the end of reperfusion using Oil Red-O staining and chemical assay. TTC staining was used to identify the extent of irreversibly injured myocardium. Subepicardial and plasma triacylglycerol content, measured both by 1D CSI and chemically, did not change during reperfusion in saline-treated dogs. Infusing dogs with Liposyn and heparin for 90 min during reperfusion transiently elevated their plasma triacylglycerols, which returned to normal levels following Liposyn wash-out. During Liposyn wash-out, myocardial triacylglycerols measured by 1D CSI preferentially increased in the subepicardium of area-at-risk myocardium (P < 0.05). Triacylglycerol content, measured chemically, also increased in area-at-risk compared to non-ischemic subepicardium (P < 0.001). Significant endocardial damage occurred in both groups, but elevated levels of plasma lipids did not increase the size of the area-at-risk. Therefore, elevated plasma lipids caused a preferential accumulation of triacylglycerols in area-at-risk myocardium during reperfusion without exacerbating irreversible ischemic injury. These results are consistent with either inhibited fatty acid oxidation or mis-matched fatty acid extraction and oxidation in area-at-risk myocardium.

Animals↗
PubMed1997

Effect of physiological factors on proximal flow convergence upstream of an incompetent valve: an in-vitro study.

The flow (Q) through regurgitant valves may be quantified by multiplying the area of an isovelocity contour (isovel) by its velocity. This was tested computationally and experimentally (using MRI). Q = 14 to 141 ml/s, using flat and conical orifice plates. Plotting Q versus isovelocity radius, a plateau was found which, for low flow, corresponded to the true Q. At higher flow or large confinement, Q was overestimated. For conical plates, angle correction worked at low Q but not at higher values due to the formation of separation regions. These converted the cone plate into a flat plate. MRI produced similar results at 57 ml/s in that Q was correct with no angle correction. At low flow, MRI was too noisy to produce a clear plateau consistently.

Aortic Valve Insufficiency↗
PubMed1996

Accuracy of biplane long-axis left ventricular volume determined by cine magnetic resonance imaging in patients with regional and global dysfunction.

Left ventricular (LV) volumes and ejection fraction can be obtained by applying Simpson's rule to multiple short-axis tomographic planes. A simpler method for determining LV volumes using the area-length equation is widely accepted and requires less time to acquire and analyze. Its accuracy, however, is questionable in deformed or asymmetrically contracting ventricles. This study compares biplane long-axis to serial short-axis computed LV volumes obtained by cine gradient-echo magnetic resonance imaging (MRI) in 2 distinct patient populations: (1) patients with global LV dysfunction, and (2) patients with regional LV dysfunction. A total of 114 patients were studied using both methods. Among 37 patients with global LV dysfunction, there was no statistically significant difference between methods (long axis vs short axis) for determining LV end-diastolic volume (203 +/- 91 vs 201 +/- 90 ml), end-systolic volume (142 +/- 81 vs 141 +/- 82 ml), and ejection fraction (33 +/- 12 vs 33 +/- 13%). However, in the 77 patients with regional dysfunction, LV end-diastolic volume was statistically slightly higher when obtained using the long-axis approach (157 +/- 53 vs 152 +/- 51 ml; p=0.004). Otherwise, end-systolic volume (97 +/- 49 vs 95 +/- 49 ml) and ejection fraction (40 +/- 13 vs 40 +/- 13%) were similar (p=NS). The correlation between LV volumes and ejection fractions for both groups was excellent (r >0.91). Thus, in this study group, biplane long-axis and serial short-axis computed LV volumes and ejection fractions were similar in patients with global or regional LV dysfunction. In critically ill patients unable to complete a comprehensive MRI examination, the biplane long-axis-derived volumes provide adequate data.

Adolescent↗
PubMed1996

1H NMR spectroscopic imaging of myocardial triglycerides in excised dog hearts subjected to 24 hours of coronary occlusion.

BACKGROUND: Myocardial ischemic insult causes depression of fatty-acid beta-oxidation and increased fatty-acid esterification with triglyceride (TG) accumulation. This accumulation has been demonstrated to occur in the territory with diminished blood flow surrounding an infarct, ie, the region at risk. To evaluate whether the extent of TG accumulation in the canine heart after 24 hours of ischemia could be detected, we applied myocardial 1H nuclear magnetic resonance (NMR) spectroscopic imaging (SI). METHODS AND RESULTS: Seven adult mongrel dogs underwent 24 hours of left anterior descending coronary artery occlusion. Postmortem, the hearts were excised and the size and location of the infarct were determined. With a Philips 1.5-T clinical NMR imaging/spectroscopic system, two-dimensional (2D) 1H NMR SI was performed. TG 1H NMR chemical shift images were reconstructed from the frequency domain spectra by numerical integration. A statistically significant (P < .05) increase in TG signal intensity was demonstrated in the region at risk compared with the nonischemic control region. There was an intermediate quantity of TG in the infarct region. Biochemical determination of tissue TG content (milligrams per gram wet weight) in the control, at-risk, and infarct regions confirmed the 1H NMR measurements. Histological evaluation with oil red O staining also demonstrated graded TG accumulation in myocytes. The highest TG levels were found in the at-risk region and the lowest levels in the control region. CONCLUSIONS: By use of 2D 1H NMR SI, the present study confirms and extends previous work that demonstrates preferential accumulation of TG in the reversibly injured myocardium after 24 hours of coronary occlusion. This study provides an important step toward the clinical application of TG imaging. When TG imaging is ultimately possible, resultant data would have diagnostic, prognostic, and therapeutic implications.

Animals↗
PubMed1996

Simultaneous multicompartment intracellular Ca2+ measurements in the perfused heart using 19F NMR spectroscopy.

Although Ca2+ transport regulation at subcellular organelles is of great interest, only limited methodology has been available for measuring organellar [Ca2+] levels. The present study employs the 19F NMR resonance frequency of 4F-BAPTA to measure free [Ca2+]. In 4F-BAPTA loaded perfused rabbit hearts, two 19F NMR resonances were clearly observed. The frequency of one was consistent with cytosolic [Ca2+] levels. Responses to agents that after sarcoplasmic reticulum function identified the other resonance as originating from that organelle. The experiments demonstrate the utility of NMR shift indicator methodology in obtaining simultaneous multi-compartment intracellular [Ca2+] measurements and in enabling organellar [Ca2+] measurements to be made from within intact living tissue.

Animals↗
PubMed1996

Spectroscopic imaging of human brain glutamate by water-suppressed J-refocused coherence transfer at 4.1 T.

The authors reported the development and implementation of a water-suppressed J-refocused coherence transfer sequence to observe glutamate in human brain at 4.1 T. The sequence is modeled for I2S2 and I2S2M spin systems analytically and plotted for a range of echo times. In this sequence, water suppression and refocusing of J-coupled resonances are achieved through a brief multiple quantum step without significant loss of signal. Phantom data are shown. Human brain spectroscopic imaging of glutamate, acquired with a total echo of 36 ms, demonstrates the application of the sequence to observe gray and white matter differences in glutamate content.

Aspartic Acid↗
PubMed1996

Quantitative 1H spectroscopic imaging of human brain at 4.1 T using image segmentation.

Metabolic differences in the content of N-acetylaspartate (NAA), creatinine (CR), and choline (CH) in cerebral gray and white matter can complicate the interpretation of 1H spectroscopic images. To account for these variations, the gray- and white-matter content of each voxel must be known. To provide these data, a T1-based image segmentation scheme was implemented at 4.1 T. The tissue composition of each voxel was determined using the point-spread function of the spectroscopic imaging acquisition and the segmented anatomical image. Pure gray- and white-matter values for CR/NAA and CH/NAA, and the content of CR, CH, and NAA, were determined using a linear-regression analysis of 984 voxels acquired from 10 subjects using white-matter CR as an internal standard. This information was used to establish means and confidence intervals for CR/NAA and CH/NAA from a voxel of arbitrary tissue composition. Using a single-tailed t test, the extent and locations of the metabolic abnormalities (P < 0.05) in a patient with multiple sclerosis were identified.

Aspartic Acid↗
PubMed1996

Evaluation of multiple sclerosis by 1H spectroscopic imaging at 4.1 T.

The authors report on high-field (4.1 T) magnetic resonance 1H spectroscopic imaging studies on eight patients with relapsing remitting multiple sclerosis (mean expanded disability status scale (EDSS) 1.0) and eight normal controls. Using T1-weighted imaging to determine lesion position, the authors found the ratios of choline/N-acetyl (NA) compounds and creatine/NA were increased significantly in the multiple sclerosis (MS) patients relative to controls in lesioned tissue, adjacent to lesion, far removed from lesions as well as in periventricular tissue. The gray matter creatine/NA was mildly increased (P < 0.01) in the MS patients, whereas the elevated gray-matter ratio of choline/NA was of borderline significance (P = 0.13). A more detailed comparison of white-matter and mean gray-matter metabolite values indicates that creatine is increased greatest in areas far from lesions. This is in contrast to choline, which was greatest in lesions, and NA, which was smallest in lesions. It is postulated that the creatine increase may reflect an astrocytic (gliotic) or oligodendrocytic remyelinating process. The increased choline most likely reflects varying levels of inflammation and membrane turnover, whereas the NA decrease is representative of axonal dysfunction or loss.

Adult↗
PubMed1995

In vivo MRI visualization of acute myocardial ischemia and reperfusion in ferrets by the persistent action of the contrast agent Gd (BME-DTTA).

BACKGROUND: Contrast agent-enhanced magnetic resonance imaging (MRI) has the potential to visualize myocardial ischemia. To date, however, no agent has been found that has a sustained effect that allows MRI detection for the entire duration of ischemia and reperfusion and thus is useful in conjunction with stress test MRI. In this article, we introduce the gadolinium complex of N3,N6-bis(2'-myrisotyloxyethyl)-1,8-dioxo-triethylene- tetraamine-N,N,N1,N1-tetraacetic acid [Gd(BME-DTTA)], an agent potentially useful for such a purpose. METHODS AND RESULTS: Four protocols were carried out. ECG-triggered, partially T1-weighted, spin-echo MRI was used in protocols A through C. In protocol A, in nonischemic ferrets, 50 mumol/kg Gd(BME-DTTA) induced a 70 +/- 5% intensity enhancement lasting 3 hours. In protocol B, the left anterior descending coronary artery was occluded, and a 99mTc-sestamibi-induced autoradiographic contrast verified (r = .87, P < .01) a Gd(BME-DTTA)-induced (n = 5) or Gd(DTPA)-induced (n = 4) MRI contrast. In the Gd(BME-DTTA) group a sustained contrast and in the Gd(DTPA) group a short-lived contrast were observed. In protocol C (n = 11), during ischemia, a 31 +/- 3.3% (P < .02) contrast was evident between the ischemic and nonischemic myocardial regions. Upon reperfusion, a contrast of 19 +/- 3% (P < .05) and 13 +/- 4.5% (P < .05) persisted for 5 and 15 minutes, respectively. Beyond 15 minutes, the contrast continued to diminish gradually. Nonradioactive microspheres verified (r = .87, P < .05) ischemia and reperfusion in this model. In protocol D (n = 4), blood delta R1 data showed that the blood pool retained Gd(BME-DTTA) for the entire time frame of the experiment at high enough concentration to provide an appropriate wash-in effect during the initial contrast enhancement and during reperfusion. CONCLUSIONS: This study demonstrates that Gd(BME-DTTA) induces a sustained MRI contrast between regions of normal versus ischemic myocardium, showing the potential of this agent for the diagnosis of ischemic heart disease in conjunction with stress tests.

Animals↗