Steven R. Fleagle

Measurement of left ventricular mass in vivo using gated nuclear magnetic resonance imaging

Alterations of left ventricular mass occur in a variety of congenital and acquired heart diseases. In vivo determination of left ventricular mass, using several different techniques, has been previously reported. Problems inherent in some previous methods include the use of ionizing radiation, complicated geometric assumptions and invasive techniques. We tested the ability of gated nuclear magnetic resonance imaging to determine in vivo left ventricular mass in animals. By studying both dogs (n = 9) and cats (n = 2) of various sizes, a broad range of left ventricular mass (7 to 133 g) was examined. With a 0.5 tesla superconducting nuclear magnetic resonance imaging system the left ventricle was imaged in the transaxial plane and multiple adjacent 10 mm thick slices were obtained. Endocardial and epicardial edges were manually traced in each computer-displayed image. The wall area of each image was determined by computer and the areas were summed and multiplied by the slice thickness and the specific gravity of muscle, providing calculated left ventricular mass. Calculated left ventricular mass was compared with actual postmortem left ventricular mass using linear regression analysis. An excellent relation between calculated and actual mass was found (r = 0.95; SEE = 13.1 g; regression equation: magnetic resonance mass = 0.95 X actual mass + 14.8 g). Intraobserver and interobserver reproducibility were also excellent (r = 0.99). Thus, gated nuclear magnetic resonance imaging can accurately determine in vivo left ventricular mass in anesthetized animals.

Feasibility of identifying amyloid and hypertrophic cardiomyopathy with the use of computerized quantitative texture analysis of clinical echocardiographic data

Ultrasound tissue characterization, the evaluation of certain physical properties of a tissue based on its acoustic properties, is an evolving application in echocardiography. The ability to identify acutely and chronically injured tissue has been demonstrated in a number of animal studies, but data in humans are limited. The present study tested the hypothesis that quantitative echocardiographic texture analysis, a method of evaluating the spatial pattern of echoes in echocardiographic images, would differentiate amyloid and hypertrophic cardiomyopathy from normal myocardium. Routine clinical echocardiographic data were obtained on 34 subjects at the Mayo Clinic (10 normal subjects, 10 patients with amyloid heart disease, 8 patients with hypertrophic cardiomyopathy and 6 patients with left ventricular hypertrophy due to hypertension). Standard videotape recordings of these echocardiograms were analyzed at the University of Iowa. Echocardiographic data were digitized with use of a calibrated, 256 gray level digitization system. Quantitative texture analysis was performed on data from the ventricular septum and posterior left ventricular wall in end-diastolic and end-systolic, short-axis and long-axis echocardiographic images. The gray level run length texture variables were able to discriminate hypertrophic cardiomyopathy and amyloid heart disease from normal myocardium and from each other (p less than 0.0083 for comparisons of the quantitative texture features of amyloid versus hypertrophic cardiomyopathy versus normal by multivariate analysis of variance). The texture of the myocardium in hypertensive left ventricular hypertrophy not associated with amyloid or hypertrophic cardiomyopathy was in general not significantly different from that of normal myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Automated analysis of coronary arterial morphology in cineangiograms: geometric and physiologic validation in humans

A method of coronary border identification is discussed that is based on graph searching principles and is applicable to the broad spectrum of angiographic image quality observed clinically. Cine frames from clinical coronary angiograms were optically magnified, digitized, and graded for image quality. Minimal lumen diameters, referenced to catheter size, were derived from automatically identified coronary borders and compared to those defined using quantitative coronary arteriography and to observer-traced borders. computer-derived minimal lumen diameters were also compared to intracoronary measurements of coronary vasodilator reserve, a measure of the functional significance of a coronary obstruction. To test the robustness of the present border detection method, computer-derived coronary borders were compared to independent standards separately for good and poor angiographic images. The accuracy of computer-identified borders was similar in the two cases.

AUTOMATED IDENTIFICATION OF LEFT VENTRICULAR BORDERS FROM SPIN-ECHO MAGNETIC RESONANCE IMAGES : EXPERIMENTAL AND CLINICAL FEASIBILITY STUDIES

Gated cardiac magnetic resonance imaging (MRI) permits detailed evaluation of cardiac anatomy, including the calculation of left ventricular volume and mass. Current methods of deriving this information, however, require manual tracing of boundaries in several images; such manual methods are tedious, time consuming, and subjective. The purpose of this study is to apply a new computerized method to automatically identify endocardial and epicardial borders in MRIs. The authors obtained serial, short-axis, spin-echo MRIs of 13 excised animal hearts. Also obtained were selected short-axis, spin-echo ventricular images of 11 normal human volunteers. A method of automated edge detection based on graph-searching principles was applied to the ex vivo and in vivo images. Endocardial and epicardial areas were used to compute left ventricular mass and were compared with the anatomic left ventricular mass for the images of excised hearts. The endocardial and epicardial areas calculated from computer-derived borders were compared with areas from observer tracing. There was very close correspondence between computer-derived and observer tracings for excised hearts (r = 0.97 for endocardium, r = 0.99 for epicardium) and in vivo scans (r = 0.92 for endocardium, r = 0.90 for epicardium). There also was a close correspondence between computer-generated and actual left ventricular mass in the excised hearts (r = 0.99). These data suggest the feasibility of automated edge detection in MRIs. Although further validation is needed, this method may prove useful in clinical MRI.

Nuclear magnetic resonance relaxometry of the normal heart: relationship between collagen content and relaxation times of the four chambers.

The use of nuclear magnetic resonance (NMR) relaxation time measurements for characterization of abnormal cardiac tissue depends upon knowledge of variations of relaxation times of normal myocardium and determinants of these variations. We calculated in vitro NMR T1 and T2 relaxation times of canine myocardium from the four cardiac chambers, and determined hydroxyproline concentration (as a measure of collagen) and percent water content of the samples. We found both water content and T1 relaxation time of the right ventricle to be significantly greater than the left atrium (p less than 0.05). T2 relaxation time of the left ventricle was found to be shorter than each of the other three chambers (p less than 0.05). There were significant correlations between the spin-lattice relaxation time and both percent water content (r = 0.58) and hydroxyproline concentration (r = 0.45). A significant correlation was also found between T2 relaxation time and hydroxyproline concentration (r = 0.49). When T1 and T2 were adjusted for water and hydroxyproline content, there was no longer any evidence for significant interchamber differences for either T1 or T2. These data suggest that differences in NMR relaxation times exist among the four chambers of the normal canine heart. Furthermore, a major determinant of myocardial spin-lattice relaxation time is tissue water content while both collagen content and percent water content significantly contribute to variability in cardiac chamber T2 relaxation times.

Effect of tissue fat and water content on nuclear magnetic resonance relaxation times of cardiac and skeletal muscle

Understanding tissue determinants that affect the nuclear magnetic resonance (NMR) properties of myocardium would improve noninvasive characterization of myocardial tissue. To determine if NMR relaxation times would reflect changes in tissue fat content, two experimental models were investigated. First, an idealized model using mixtures of beef skeletal muscle and beef fat was studied to investigate the effects of a wide range of tissue fat content. Second, myocardium with varying fat content from hogs raised to have varying degrees of ponderosity was analyzed. Tissue fat and water contents and spin-lattice (T1) and spin-spin (T2) relaxation times at 20 MHz were measured. The skeletal muscle/fat mixtures ranged in fat content from 35% to 95% with water content variations from 50% to 75%. Water content decreased as fat content increased. A significant inverse linear relationship was found between T1 and sample fat content (r = -0.997). Spin-spin relaxation times showed a significant positive curvilinear relationship with fat content (r2 = 0.96). In the animal experiments, 18 hogs were studied with samples obtained from both right and left ventricular (LV) free walls, with care taken to avoid epicardial fat. Myocardial fat content ranged from 3% to 25%. A significant correlation was found between LV fat content and corrected LV mass (r = 0.62), which suggested that the increase in LV mass could be explained, at least in part, by changes in myocardial fat content. Similar to the muscle/fat mixture model, a significant positive curvilinear relationship was found between myocardial T2 and tissue fat content (r2 = 0.67) for all the myocardial samples.(ABSTRACT TRUNCATED AT 250 WORDS)

A three-dimensional graph searching technique for cardiac border detection in sequential images and its application to magnetic resonance image data

Developed is a new method of automated border detection using graph searching principles. The method takes into account the added information available in temporal or spatial sequences of images that are commonly available in biomedical applications. This method is applied to detect endocardial and epicardial borders in short-axis magnetic resonance images of the heart. Preliminary results show that it may be more robust in areas of noise when compared to a method based only on data from a single image. They suggest that this method could potentially improve performance of automated edge detection methods when spatial or temporal sequences of frames are available.<<ETX>>

Videodensitometric analysis of coronary stenoses in vivo geometric and physiologic validation in humans

Assessment of the severity of coronary stenoses on arteriograms conventionally is based on subjective estimates of percent luminal diameter narrowing. However, in studies in patients with multivessel coronary artery disease, we have found a poor correlation between percent stenosis and the physiologic significance of an individual coronary obstruction. The purpose of this study was to determine whether computerized videodensitometry would allow estimation of coronary luminal area and therefore prediction of the physiologic significance of individual coronary stenoses in humans. Videodensitometry was used to define the minimal luminal area of 15 left anterior descending, 15 circumflex, and 15 right coronary artery segments in 43 patients. Computer-assisted quantitative coronary arteriography (method of Brown et al) was used to determine the minimal luminal cross-sectional area of these same segments. In each arterial segment, coronary vasodilator reserve was assessed using intraoperative (n = 18 segments) or intracoronary (n = 27 segments) Doppler measurements of coronary vasodilator reserve. Videodensitometric estimates of coronary luminal area correlated well with minimal luminal area defined using the independent geometric technique of quantitative coronary arteriography (r = 0.82, y = 0.97 X + 0.71, SEE = 1.83 mm2, n = 45) and with lesion physiologic significance as defined by studies of the peak-to-resting velocity ratio (r = 0.71, 0.92, and 0.74 for the left anterior descending, circumflex, and right coronary arteries, respectively). Thus, videodensitometry is a promising method that may supplement geometric approaches to quantitative analysis of coronary arteriograms in humans.

Feasibility of quantitative texture analysis of cardiac magnetic resonance imagery: preliminary results

Texture analysis has proven useful in the evaluation of many forms of digital imagery. Our group has shown that texture analysis of echocardiograms may be useful for characterization of myocardial ischemia, contusion, and cardiomyopathies. Recent work in tissue characterization of the brain has used texture analysis of magnetic resonance images to discriminate normal brian tissue from edema and tumor. Magnetic resonance images of the heart have been noted to exhibit visually apparent abnormalities in region image intensity in disease. Based upon these qualitative visual observations, our previous work in cardiac ultrasound texture analysis and the recent application of texture analysis to magnetic resonance brain images, we hypothesized that texture analysis would be useful in evaluating myocardial tissue characteristics from cardiac magnetic resonance images. As a first step towards the goal of cardiac tissue classification, we evaluated the ability of texture analysis to discriminate the left ventricular myocardium from other tissues using magnetic resonance images.

Effects of Instrument Adjustments on Quantitative Echocardiographic Gray Level Texture Measures

Quantitative echocardiographic gray level texture measures have proved useful in characterizing a variety of cardiac abnormalities, including contusion, ischemia, and cardiomyopathies. However, alterations of echocardiographic signal processing controls could potentially affect the resulting data. The purpose of this study was to assess the effect of common echocardiographic instrument adjustments on texture data. We imaged a graphite-in-gel phantom on five separate occasions, each time using 12 combinations of adjustments of transmit power, compression, and postprocessing curves over a clinically applicable range. Image texture was assessed using gray level run length and gray level difference texture measures; 42 individual measures were calculated. Significance of texture measure variations was assessed by analysis of variance. Changes in all three controls were associated with significant alterations in texture measures. Adjustments of transmit power and compression produced larger changes in texture measures than did adjustments of postprocessing curves. There were significant transmission-compression-postprocessing interaction effects (p less than 0.05) for all but eight of the texture measures. For each texture measure, there were significant transmission-compression interaction effects (p less than 0.05). In summary, instrument adjustments had significant effects on quantitative texture measures, and these effects should be considered when evaluating quantitative echocardiographic gray level texture measures.