Joseph J. McInerney

The Measurement of Multidimensional Myocardial Dynamics Using Scattered Radiation Fields

A new radiographic device, based upon the analysis of scattered radiation fields, has been developed to measure myocardial dynamics. The device consists of an array of detectors arranged to monitor photons scattering from the epicardial surface. Data synthesis permits real-time dynamic displays of the epicardial surface in two and three dimensions. The system has been tested on close-chest canines. Epicardial surface displacements within the closed chest cavity can be measured to 0.1 mm (S.D.). Right or left ventricular surfaces may be monitored on a given scan. Surfaces behind or in front of outer myocardial boundaries within the direct field of view of the detectors are located with equal accuracy. Except for the use of low levels of fluoroscopic x-rays, the procedure is completely noninvasive. Radiation dose levels and component costs for the prototype system are modest. The detector system attaches to a standard fluoroscopic x-ray generator.

Determination of regional myocardial perfusion by x-ray fluorescence.

Validation studies were performed to demonstrate the effectiveness of an x-ray induced fluorescence system in quantitating regional myocardial perfusion in vivo. In a series of 13 open-chested canines, x-ray induced fluorescence was used to simultaneously monitor iodine concentration transients which arose in the left ventricular lumen and in the myocardium after the intravenous injection of an iodinated flow tracer. Deconvolution of the recorded transients produced a transfer function from which the mean transit time for the tracer to travel between the left ventricular lumen and the myocardium was calculated. Measurements of regional myocardial perfusion (Q) made by radioactive microspheres were compared with the reciprocals of the mean transit times (MTT-1) and gave a linear correlation (n = 38): MTT-1 = 0.033 + 0.069 Q, r = 0.71. Comparison of the percent change in perfusion (dQ) relative to a control study for each dog with the percent change in the respective reciprocals of the mean transit times (dMTT-1) produced a linear correlation coefficient of r = 0.88 for the regression line dMTT-1 = 0.46 dQ - 10.7. The x-ray induced fluorescence system may provide a minimally invasive means for monitoring iodine concentration transients and determining relative, if not absolute, measures of regional myocardial perfusion.

High precision Compton backscatter maps of myocardial wall dynamics. Theory and applications

Compton backscatter imaging (CBI) is a technique that uses x-rays scattered from the closed-chest surface of the heart to obtain high frequency (5 msec) and high precision (+/- 0.1 mm SD) measurements of regional surface displacements and velocities. These measurements are acquired in a three-dimensional format that allows the reconstruction of the epicardial surface and the creation of color coded displacement and velocity maps at many time points during the cardiac cycle. Applications of the technique are shown to characterize detailed regional normal wall displacement and velocity patterns, and the significant alteration of those patterns after coronary embolization. The technique is also applied to the characterization of early diastolic wall dynamics. CBI measurements show that a brief and somewhat paradoxical inward displacement of the anterior ventricular wall occurs during early diastole in normal canines. The wall dynamics associated with this inward displacement suggest a brief collapse of the ventricle subsequent to aortic valve closure. Diastolic collapse velocities and displacements are significantly altered subsequent to coronary occlusion with mean and maximum collapse velocities decreasing by 50% and concomitant inward displacements decreasing by 40%. Data acquisition with CBI is non-invasive, does not require contrast agents or radioisotopes, and uses low irradiation levels (125 kVp, 3-5 ma). The average radiation dose to the heart for a typical study is 250 mrem, significantly lower than that of other radiation based imaging techniques.

Convective-dispersive characteristics of tracer transport calculated from transfer function analysis of biological indicator-dilution curves

A solution to the convection-dispersion model of tracer transport in biological systems is presented. This solution provides for the characterization of tracer transport through a network of blood vessels based on the tracer transients recorded at the inlet and outlet points of the circulation under investigation. Fourier transformations of the transients are used to produce a transfer function from which the mean transit time as well as other transport characteristics can be calculated. The practical aspects of applying transfer function analysis to retrieve transport characteristics under experimental conditions are also presented. The application demonstrates the efficacy of the transfer function analysis method independent of tracer recirculation effects and a priori knowledge of actual tracer concentrations.

The Use of Left Ventricular Epicardial Surface Velocity Patterns as a Marker of Pacing Site

Ectopic ventricular foci were simulated at selected endocardial sites in 15 closed‐chest canines using ventricular pacing. During this pacing, a noninvasive x‐ray backscatter imaging technique was used to measure epicardial LV displacements at 5‐ms intervals during the cardiac cycle. These displacement measurements were used to calculate epicardial surface velocities in each study and were presented as a time sequence of color coded velocity maps. Characteristic patterns in the timing and spatial propagation of LV surface velocities were noted for each pacing site, particularly during the expansion of the LV during isovolumic contraction and the inward motion of the LV during ejection. Average surface velocity maps for the 15 canines were computed for each pacing site. These average maps were used as standards for comparison with individual pacing studies to determine the probable site of pacing. Comparisons were made using a computer algorithm, based upon auto‐ and cross‐correlation techniques in the time domain. This algorithm correctly identified pacing sites with sensitivities of HA 74%, LV 76%, RV 79%, and RVOT 77% and specificities of RA 98%, LV 96%, RV 90%, and RVOT 93%. The results show that this noninvasive mapping procedure has potential for identifying the location of an ectopic ventricular focus.

A tomographic imaging system utilizing X-ray Compton backscatter

A preliminary version of a thoracic imaging system developed to help facilitate the noninvasive locating of a coronary bypass graft for subsequent flow patency evaluation by iodine X-ray fluorescence is discussed. The imaging technique utilizes the selective measurement of Compton backscattered X-rays to inexpensively construct an effective tomographic image of the heart-lung field where grafts are located. Background theory, system design, and preliminary results are presented.<<ETX>>

The Effects of Pacing Site on Left Ventricular Epicardial Surface Velocity Patterns During Systole

Changes in epicardial LV velocity patterns during isovolumic contraction and ejection as induced by ventricular pacing were studied in 15 canines. A noninvasive imaging technique that provided high temporal resolution was used to study the timing of an outward expansion of the LV during isovolumic contraction and the propagation pattern of an inward LV velocity wavefront during ejection. With this technique, surface displacements were measured (± 0.1 mm SD) at 50–70 locations on the LV free wall at 5‐msec intervals. Velocities were calculated by differentiating the surface displacement waveforms and an interpolation procedure was used to provide detailed color coded velocity maps of the LV surface. LV surface velocities were determined from data obtained during closed‐chest endocardial pacing from each of four sites: right atrium, right ventricular apex, left ventricular apex, and right ventricular outflow tract. These surface velocities showed a distinct spatial and temporal pattern for each pacing site. The results show that this noninvasive mapping procedure has potential for determining the location of an ectopic ventricular focus.

A line-focus collimator with a field of view of uniform thickness

A new line-focus x-ray collimator features a field of view (FOV) with uniform thickness in the near field between the collimator and focal line. General design equations were developed and then constrained to define such a uniform FOV. A prototype collimator was experimentally evaluated using a Compton backscatter imaging technique. The full-width-tenth-max (FWTM) thickness, measured at 420 locations in the near field, showed good uniformity (1.51 +/- 0.06 cm) and closely approximated the nominal design thickness (1.8 cm).

A model to aid in the calculation of regional myocardial perfusion

The authors present a model of tracer transport which could be useful for determining perfusion with indicator-dilution methods. The model is based on convection-dispersion transport of the tracer through the coronary circulation and requires the simultaneous recording of tracer concentrations in the left ventricular lumen and in the region of interest in the myocardium. Application of the model to predict myocardial flows in animal studies is also presented.<<ETX>>

Dynamic Radiography: A New Technique For Epicardial Imaging*

A new radiographic system has been developed for use in mapping three-dimensional epicardial motion. The new radiograph consists of an array of detectors arranged such that they monitor 1 cm strip of scattering photons along a contour across the cardiac surface. Synthesis of the detector data permits immediate display of the beating cardiac contour on a CRT screen for observation and analysis. The entire radiographic system is under microprocessor control. Location of the cardiac surface and initial cardiac contour display is possible within ten seconds of a scans initiation. The system has been carefully validated on mechanical models. Studies with anesthetized canines demonstrate the new radiograph in a physiological environment. Eventual clinical use is expected to provide the physician with accurate data on ventricular function at patient costs and radiation dose rates lower than those of other available techniques.