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Fast Computed Tomography for Quantitative Cardiac Analysis—State of the Art and Future Perspectives

1990; Elsevier BV; Volume: 65; Issue: 10 Linguagem: Inglês

10.1016/s0025-6196(12)62145-4

1942-5546

Erik L. Ritman,

Advanced X-ray and CT Imaging

Two fast computed tomographic scanners, designed primarily for imaging cardiac structures and function, have been in use since the early 1980s. The technical aspects of both systems have been described previously in detail, and a considerable body of scientific literature now documents the biomedical capabilities of these scanners. This review examines these biomedical capabilities as applied to quantitative analysis of the heart and pulmonary circulations. On the basis of this overview, some speculations about the current strengths and possible further developments of the fast computed tomographic approach in these applications are made. Two fast computed tomographic scanners, designed primarily for imaging cardiac structures and function, have been in use since the early 1980s. The technical aspects of both systems have been described previously in detail, and a considerable body of scientific literature now documents the biomedical capabilities of these scanners. This review examines these biomedical capabilities as applied to quantitative analysis of the heart and pulmonary circulations. On the basis of this overview, some speculations about the current strengths and possible further developments of the fast computed tomographic approach in these applications are made. In the decade after the initial clinical introduction of x-ray computed tomography (CT) by EMI, Inc. in 1972 for studies of the body and, in particular, the brain, several fast CT scanners were proposed by numerous investigators,1Sturm RE, Ritman EL, Wood EH: Quantitative three-dimensional dynamic imaging of structure and function of the cardiopulmonary and circulatory systems in all regions of the body. In Cardiovascular Imaging and Image Processing: Theory and Practice—1975. Edited by DC Harrison, H Sandler, HA Miller. Palos Verdes Estates, California, Society of Photo-Optical Instrumentation Engineers, 1975, pp 103–122Google Scholar, 2Iinuma TA Tateno Y Umegaki Y Watanabe E Proposed system for ultrafast computed tomography.J Comput Assist Tomogr. 1977; 1: 494-499Crossref PubMed Scopus (27) Google Scholar, 3Haimson J X-ray source without moving parts for ultra-high speed tomography.IEEE Trans Nucl Sci. 1979; 26: 2857-2861Crossref Scopus (10) Google Scholar, 4Maydan D Shepp LA Cho ZH A new design for high speed computerized tomography.IEEE Trans Nucl Sci. 1979; 26: 2870-2871Crossref Scopus (8) Google Scholar, 5Boyd DP Gould RG Quinn JR Sparks R Stanley JH Herrmannsfeldt WB A proposed dynamic cardiac 3-D densitometer for early detection and evaluation of heart disease.IEEE Trans Nucl Sci. 1979; 26: 2724-2727Crossref Scopus (104) Google Scholar primarily for studies of the beating heart. Currently, two types of fast, multislice scanning CT systems are in use in biomedical institutions: the commercially available Picker Fastrac (formerly, Imatron C-100) scanner and the unique dynamic spatial reconstructor (DSR) (Mayo Foundation). Both systems have previously been described in considerable technical detail.6Ritman EL Robb RA Harris LD Imaging Physiological Functions: Experience With the Dynamic Spatial Reconstructor. Praeger Publishers, New York1985Google Scholar, 7Boyd DP Lipton MJ Cardiac computed tomography.Proc IEEE. 1983; 71: 298-307Crossref Scopus (220) Google Scholar The key features that distinguish these scanners from other existing CT scanners are that they complete a scan within a fraction of a second, they scan multiple adjacent slices (thereby scanning a volume), and they can repeat these scans at frequent intervals for a specified period. The important difference between these two types of scanners is that the DSR has greater functional flexibility in that spatial, temporal, and contrast resolution can be adjusted to favor one aspect of resolution over the other.8Behrenbeck T Kinsey JH Harris LD Robb RA Ritman EL Three-dimensional spatial, density, and temporal resolution of the dynamic spatial reconstructor.J Comput Assist Tomogr. 1982; 6: 1138-1147Crossref PubMed Scopus (32) Google Scholar This flexibility facilitates basic research applications. The Picker Fastrac scanner has less functional flexibility in that scanned slice thickness is fixed at 8 mm (in the multislice mode of operation) and scan duration is fixed at 1/20 second. In combination with its superior contrast resolution,9Boyd DP Computerized transmission tomography of the heart using scanning electron beams.in: Higgins CB CT of the Heart and Great Vessels: Experimental Evaluation and Clinical Application. Futura Publishing Company, Mount Kisco, New York1983: 45-60Google Scholar however, these characteristics allow greater clinical utility. Because both scanners have been in operation for several years, some objective conclusions can be drawn about the essential features, current roles, and possible future directions of fast CT in cardiovascular imaging. In general terms, the major advantage of x-ray over other energy forms for rapidly generating tomographic image data is that the flux of x-ray photons necessary for a good signal-to-noise ratio in images can be delivered in a brief period, at a high repetition rate, and in a highly controlled geometric arrangement. X-ray energy, however, has two disadvantages relative to the other forms of radiant energy used to generate tomographic images. The first disadvantage is that x-rays ionize; hence, a biologically determined upper limit exists for the number of photons that can be delivered. This constraint on radiation absorbed dose in turn limits the image detail (that is, the spatial, temporal, and contrast resolution) that can be achieved.10Motz JW Danos M Image information content and patient exposure.Med Phys. 1978; 5: 8-22Crossref PubMed Scopus (154) Google Scholar The limit, however, is somewhat flexible because the risk associated with the irradiation and the risk of not diagnosing the disease must be carefully considered. The other limitation is the nature of interaction between the x-rays and matter. Under the usual diagnostic conditions, the x-ray image is related to electron density, which in turn is related to tissue density.11Victoreen JA Probable x-ray mass absorption coefficients for wave-lengths shorter than the K critical absorption wave-length.J Appl Phys. 1943; 14: 95-102Crossref Scopus (25) Google Scholar Although this characteristic limits the image resolution in terms of soft tissue discrimination, the introduction of high-density atoms in millimolar concentrations can partially overcome this limitation. In general, these iodinated contrast agents are used to enhance x-ray contrast of an anatomic or vascular space selectively through transient x-ray opacification. Appropriate use of contrast agents expands the utility of the images despite the limited contrast resolution. Their use, however, reinforces the need for the ability to scan rapidly so that image artifacts, due to motion of the contrast agent, are minimized. The ability to produce accurate images of the transient concentration distribution of intravascularly administered contrast agents provides the two most powerful attributes of fast CT in cardiology—the accurate imaging of the geometry of rapidly moving cardiovascular structures and the application of a wide range of indicator- dilution methods used for microcirculation studies inaccessible to conventional x-ray CT scanners. As a consequence, fast CT image data are well suited to the accurate quantitation of intravascular mass transport, parenchymal perfusion, vascular anatomy, and cardiac function. Future developments in fast CT for cardiac applications, then, should be viewed in this perspective. Fast CT, especially when used to produce volume (that is, three-dimensional) image data or to produce time sequence images (or both), generates a considerable demand for rapid, user-friendly, image display and analysis techniques. Indeed, until these needs were met, fast CT was primarily of academic interest only. Hence, the practical utility of fast CT is inexorably inter-twined with the ability to display and analyze effectively and rapidly the image data generated by the scanners. In this review, we explore several specific examples of use of fast CT and methods for analysis and display of the data. In all these applications, three basic display methods are used, as illustrated in Figure 1. These display methods are vitally important and essential for identifying regions of biomedical interest and for making the measurements in the correct anatomic or temporal locations (or both), even though the important endpoints of the analysis are often numbers and graphs rather than the image displays themselves. In the subsequent discussion, examples of the role of fast CT in quantitating the structure and function of the heart and great vessels are presented. Fast CT scanning of the heart involves either injection of contrast medium into the venous circulation by means of an infusion over sufficient time to opacify both the right and the left side of the heart simultaneously12Reiter SJ Rumberger JA Feiring AJ Stanford W Marcus ML Precision of measurements of right and left ventricular volume by cine computed tomography.Circulation. 1986; 74: 890-900Crossref PubMed Scopus (164) Google Scholar or a software “addition” of the dextrophase and levophase after brief bolus injections on the right side of the heart.13Sinak LJ Hoffman EA Ritman EL Subtraction gated computed tomography with the dynamic spatial reconstructor: simultaneous evaluation of left and right heart from single right-sided bolus contrast medium injection.J Comput Assist Tomogr. 1984; 8: 1-9Crossref PubMed Scopus (13) Google Scholar The latter method entails injection of contrast medium during the performance of frequent scans during the dextrophase and levophase of the resulting angiogram. Each scanned cardiac cycle can then be represented by sequential volume images with either the right or the left chambers opacified. By matching equal time intervals from the R wave of the electrocardiogram, the volume (three-dimensional) images of the levophase can be digitally added (voxel for voxel) to the images of the dextrophase in a nonlinear fashion so that the contrast brightness in the right ventricular chamber is scaled to match the brightness of the opacified left ventricle. The resulting images contain equally enhanced left and right ventricular chambers and can be used for retrospective analysis, including display of static and dynamic oblique planar images. Once the chambers have been clearly and accurately delineated in three dimensions, absolute chamber volumes, stroke volumes, and myocardial mass can also be accurately estimated from these image data. Hoffman and Ritman14Hoffman EA Ritman EL Shape and dimensions of cardiac chambers: importance of CT section thickness and orientation.Radiology. 1985; 155: 739-744PubMed Google Scholar showed (Fig. 2) that the absolute volume and circumference (as an index of shape) of all cardiac chambers can be estimated with 95% accuracy without the need to use a simplified model of the shape of the chambers. The actual three-dimensional shape of the chambers is difficult to express quantitatively, but three-dimensional surface displays help to show that the fast CT image data are qualitatively accurate in detail. These studies showed that although the detailed chamber shape is sensitive to slice thickness, spacing, and orientation, chamber volumes may be accurately conveyed by the fast CT scans. Iwasaki and associates15Iwasaki T Sinak LJ Hoffman EA Robb RA Harris LD Bahn RC Ritman EL Mass of left ventricular myocardium estimated with dynamic spatial reconstructor.Am J Physiol. 1984; 246: H138-H142PubMed Google Scholar used the DSR scanner to estimate left ventricular myocardial volume and chamber volume in dogs. Values of the left ventricular muscle estimated with the DSR (y) showed a good correlation with the postmortem values (x) (r = 0.99; y = 0.94x + 4.1). Feiring and colleagues,16Feiring AJ Rumberger JA Reiter SJ Skorton DJ Collins SM Lipton MJ Higgins CB Ell S Marcus ML Determination of left ventricular mass in dogs with rapid-acquisition cardiac computed tomographic scanning.Circulation. 1985; 72: 1355-1364Crossref PubMed Scopus (128) Google Scholar who conducted a similar study with use of the Picker Fastrac scanner, found that myocardial mass estimated with this scanner (y) was related to the postmortem mass (x) (r = 0.99; y = 0.97x + 1.98), and repeat scans analyzed in any one dog showed high reproducibility. In addition to absolute volumes of myocardium and the cardiac chambers, the change in these volumes throughout the cardiac cycle is also accurately measured with use of fast CT. Thus, the ability to measure wall thickness and chamber volume at 0.05-second intervals throughout a cardiac cycle was demonstrated by Sinak and co-workers.17Sinak LJ Iwasaki T Hoffman EA Ritman EL Regional myocardial function: evaluation with DSR.in: Sigwart U Heintzen PH Ventricular Wall Motion. Thieme-Stratton, Stuttgart, Germany1984: 57-62Google Scholar Similarly, Reiter and associates,18Reiter SJ Rumberger JA Stanford W Marcus ML Quantitative determination of aortic regurgitant volumes in dogs by ultrafast computed tomography.Circulation. 1987; 76: 728-735Crossref PubMed Scopus (37) Google Scholar using the Picker Fastrac scanner, showed that stroke volume can be measured with sufficient accuracy to detect valvular incompetence by the difference between right and left stroke volumes. The accuracy is illustrated in Figure 3, in which left ventricular stroke volume is plotted against simultaneously recorded flowmeter estimates of stroke volume. These estimates now allow accurate quantitation of valvular incompetence, as illustrated in the right panel. Using the volume-scanning capability of the DSR, Hoffman and Ritman19Hoffman EA Ritman EL Invariant total heart volume in the intact thorax.Am J Physiol. 1985; 249: H883-H890PubMed Google Scholar showed that the total volume of the heart (that is, the volume contained within the pericardial sac) is essentially invariant during the cardiac cycle. Although this observation was reported almost 60 years ago by Hamilton and Rompf,20Hamilton WF Rompf JH Movements of the base of the ventricle and the relative constancy of the cardiac volume.Am J Physiol. 1932; 102: 559-565Google Scholar only with the availability of fast CT was it first quantitatively confirmed in vivo. The mechanism of cardiac contraction responsible for this observation is that the epicardial ventricular apex and the dorsal surface of the atria remain essentially fixed, whereas the atrioventricular valve plane moves toward the apex or base—similar to a piston in a reciprocating engine. Finally, with use of fast CT images recorded during several scans, regional myocardial function can be related to pathologic conditions in a quantitative and readily comprehensible way by use of computer-generated displays. Thus, a fundamental problem is the display of a three-dimensional array of time-varying distributions of brightness values on a two-dimensional television screen. One approach that compresses the information from a temporal sequence of many three-dimensional images into one two-dimensional image is to generate images of the spatial distribution of the mean transit time (that is, the center of gravity of the time-dependence curves in each region) and to present these data as a gray scale proportional to the length of transit time. Although this approach has been applied primarily to head scans,21Holden JE Ip WR Continuous time-dependence in computed tomography.Med Phys. 1978; 5: 485-490Crossref PubMed Scopus (9) Google Scholar it is equally applicable to display of perfusion patterns in other organs such as the myocardium, in which the location and the severity of regional perfusion deficits would be expected to be particularly helpful in the diagnosis of coronary artery disease. Another solution is to borrow a concept from cartographers, who have developed many three-dimensional to two-dimensional transformations. Of particular use have been topograms in which the qualitative essence of the three-dimensional geometry is not lost despite the distorting mathematical transformation of the data. Schwartz and associates22Schwartz RS Bove AA Bahn RC Ritman EL A method for quantitative display of three-dimensional regional myocardial function.IEEE Trans Med Imaging. 1985; MI-4: 208-214Crossref Google Scholar have shown that topograms relating rate of systolic wall thickening, coronary anatomy, and histologic evidence of scarring can be conveniently displayed. These examples suggest that the general capability of convenient concurrent display of data from various sources (such as coronary arteriography and ventriculography) in a single display will enable the observer to deduce cause and effect, as well as consequences or interrelationships of physiologic or disease processes. The major branches of the pulmonary arterial tree are generally confined to a volume that can be imaged by a single volume scan of the DSR and of the Picker Fastrac scanner. When the volume image reconstruction process has been completed, the data can be displayed and analyzed in several ways. The original set of transverse images can be viewed sequentially through space or through time (or both), or the volume image data can be reformatted to produce images of slices in sagittal, coronal, or oblique planes. Reformatting can be achieved with use of an operator-interactive computer program that provides the user with various choices of volume image presentation. Display tools of particular use in this application are a varifocal mirror23Harris LD Camp JJ Display and analysis of tomographic volumetric images utilizing a vari-focal mirror.Proc SPIE. 1984; 507: 38-45Crossref Scopus (14) Google Scholar and pseudo three-dimensional displays such as a projection dissolution rotation display24Harris LD Robb RA Yuen TS Ritman EL Display and visualization of three-dimensional reconstructed anatomic morphology: experience with the thorax, heart, and coronary vasculature of dogs.J Comput Assist Tomogr. 1979; 3: 439-446Crossref PubMed Scopus (63) Google Scholar or a shaded surface display.25Robb RA Barrillot C Interactive 3-D image display and analysis.Proc SPIE. 1988; 939: 173-202Crossref Scopus (37) Google Scholar In the projection type of display, in which the voxels are summed along the lines of sight of a theoretical observer, the resultant picture resembles a conventional radiograph. Unlike conventional angiography, however, numerical projection can be used to create a display from any conceivable angle of view by using only one set (that is, one angiogram) of volume image data. Moreover, the original data set can be mathematically manipulated to “dissolve” or “dissect” away unwanted superposing tissues (for example, the mediastinum or chest wall). The shaded surface type of display shows a structure, in a selected x-ray density range, as if it were a three-dimensional solid object. Light shading is used to convey a sense of depth perspective. Color can also be added to convey additional information to help identify anatomic structures or to convey functional qualities of the anatomic structure being displayed.26Hoffman EA Acharya RS Wollins JA Computer-aided analysis of regional lung air content using three-dimensional computed tomographic images and multinomial models.Int J Math Model. 1986; 7: 1099-1116Crossref Scopus (16) Google Scholar For example, a patient with complex pulmonary artery anatomy due to a congenital malformation and surgical corrections was scanned in the DSR,27Liu Y-H Mair DD Hagler DJ Seward JB Julsrud PR Ritman EL Angiography for delineation of systemic-to-pulmonary shunts in congenital pulmonary atresia: evaluation with the dynamic spatial reconstructor.Mayo Clin Proc. 1986; 61: 932-941Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar and the resulting three-dimensional image data were used to create a “surface” display. The numerous vascular shunts can be enhanced by the addition of color to the three-dimensional surface display. When precise measurements of vessel diameters are needed, accuracy is maximized if the measurements are made from the appropriate oblique-plane images. The three-dimensional image data can be used to localize an area of interest and to orient the desired two-dimensional section before analysis.28Harris LD Identification of the optimal orientation of oblique sections through multiple parallel CT images.J Comput Assist Tomogr. 1981; 5: 881-887Crossref PubMed Scopus (24) Google Scholar The accuracy with which these three-dimensional image data of pulmonary arterial trees can be used to measure vascular geometry has been evaluated in a glass model,29Block M Liu Y-H Harris LD Robb RA Ritman EL Quantitative analysis of a vascular tree model with the dynamic spatial reconstructor.J Comput Assist Tomogr. 1984; 8: 390-400Crossref PubMed Scopus (32) Google Scholar in an anesthetized dog,30Liu Y-H Hoffman EA Ritman EL Measurement of three-dimensional anatomy and function of pulmonary arteries with high-speed x-ray computed tomography.Invest Radiol. 1987; 22: 28-36Crossref PubMed Scopus (25) Google Scholar and, to a limited extent, in children with surgically introduced vascular conduits of known size.31Liu Y-H Hoffman EA Hagler DJ Seward JB Julsrud PR Mair DD Ritman EL Accuracy of pulmonary vascular dimensions estimated with the dynamic spatial reconstructor.Am J Physiol Imaging. 1986; 1: 201-207PubMed Google Scholar In the glass model of a “vascular tree,” the measurements of branch cross-sectional area, branch segment length, and branching angles were all within 3% of the direct measurement. In two patients with right ventricular outflow tract-to-pulmonary artery conduits, the DSR values and directly measured conduit cross-sectional area were 45 mm2Iinuma TA Tateno Y Umegaki Y Watanabe E Proposed system for ultrafast computed tomography.J Comput Assist Tomogr. 1977; 1: 494-499Crossref PubMed Scopus (27) Google Scholar versus 50 mm2Iinuma TA Tateno Y Umegaki Y Watanabe E Proposed system for ultrafast computed tomography.J Comput Assist Tomogr. 1977; 1: 494-499Crossref PubMed Scopus (27) Google Scholar, respectively, and 120 mm2Iinuma TA Tateno Y Umegaki Y Watanabe E Proposed system for ultrafast computed tomography.J Comput Assist Tomogr. 1977; 1: 494-499Crossref PubMed Scopus (27) Google Scholar versus 115 mm2Iinuma TA Tateno Y Umegaki Y Watanabe E Proposed system for ultrafast computed tomography.J Comput Assist Tomogr. 1977; 1: 494-499Crossref PubMed Scopus (27) Google Scholar, respectively. The possible utility of such vascular measurements for evaluating functional impairment is illustrated in Figure 4. The data in Figure 4 are the result of quantitative analysis of the dynamic geometry of pulmonary arteries of pigs with aorta-to-pulmonary artery shunts.32Wu X Latson LA Driscoll DJ Ensing GJ Ritman EL Dynamic three-dimensional anatomy of pulmonary arteries in pigs with aorto-to-pulmonary artery shunts.Am J Physiol Imaging. 1987; 2: 169-175PubMed Google Scholar The DSR was used by Block and associates33Block M Bove AA Ritman EL Coronary angiographic examination with the dynamic spatial reconstructor.Circulation. 1984; 70: 209-216Crossref PubMed Scopus (27) Google Scholar to scan the coronary arteries of dogs. After one injection of contrast medium into the aortic root of each dog, all major epicardial coronary arteries and the septal artery could be imaged. Projection dissolution displays were generated to show the arteries from all possible views—even the strictly cranial view—so that superposition of arteries could be avoided (Fig. 5). The geometric accuracy of the images was evaluated by comparing coronary arterial segment length measured from the DSR images with postmortem measurements (r = 0.99; standard error of the estimate [SEE] = 3.12 mm) and by measuring the stenosis produced by nine hollow plastic cylinders lodged in the coronary arterial lumina by percutaneous catheterization. The DSR overestimated the length of 3- to 6-mm-long stenoses by 0.4 ± 0.5 mm. The area reduction caused by the hollow cylinders varied from 53 to 92% and was underestimated by the DSR a mean of 7%. The volume of plastic in each plug (mean, 13.2 mm3Haimson J X-ray source without moving parts for ultra-high speed tomography.IEEE Trans Nucl Sci. 1979; 26: 2857-2861Crossref Scopus (10) Google Scholar), calculated from the length, cross-sectional area, and degree of the stenosis, showed a correlation of r = 0.90 (SEE = 2 mm3Haimson J X-ray source without moving parts for ultra-high speed tomography.IEEE Trans Nucl Sci. 1979; 26: 2857-2861Crossref Scopus (10) Google Scholar) with the actual volume of plastic and showed no significant difference from the line of identity (P>0.05). In a similar study by Spyra and colleagues,34Spyra WJT Bell MR Bahn RC Zinsmeister AR Ritman EL Bove AA Detection of mild coronary stenoses using the dynamic spatial reconstructor.Invest Radiol. 1990; 25: 472-479Crossref PubMed Scopus (9) Google Scholar hollow plastic cylinders were embolized into the left coronary arteries of dogs to produce 25 to 56% reductions in 2- to 3-mm arterial lumen diameters. For each dog, one three-dimensional image of the heart was reconstructed from a DSR scan sequence recorded during a 30-ml injection of contrast medium into the aortic root. Four independent observers participated in blinded visual analysis of multiview projection images computed from the single three-dimensional image with use of the workstation-based analysis software described by Robb and co-workers.35Robb RA Heffernan PB Camp JJ Hanson DP A workstation for multi-dimensional display and analysis of biomedical images.Comput Methods Programs Biomed. 1987; 25: 169-184Abstract Full Text PDF PubMed Scopus (13) Google Scholar Postmortem coronary angiograms of the isolated heart were used for accurate determination of the anatomic location of the stenoses. With use of the fast CT image data, the overall sensitivity of the detection procedure was 89%, the specificity was 81%, and the sensitivity of detecting stenoses of 50% or more was 98%. Bove and associates36Bove AA Block M Smith HC Ritman EL Evaluation of coronary anatomy using high-speed volumetric computed tomographic scanning.Am J Cardiol. 1985; 55: 582-584Abstract Full Text PDF PubMed Scopus (13) Google Scholar showed that a DSR scan obtained during clinical coronary arteriography provides information comparable to that from quantitative analysis of a conventional multiview coronary arteriogram, as shown in Figure 6. That a fast CT-based method could possibly be used for performing intravenous coronary tomography was suggested by Ritman and Bove.37Ritman EL Bove AA A tomographic approach to intravenous coronary arteriography.in: Reiber JHC Serruys PW State of the Art in Quantitative Coronary Arteriography. Martinus Nijhoff Publishers, Dordrecht, Netherlands1986: 67-78Crossref Google Scholar Although some mild stenoses can clearly be detected with a fast CT scanner such as the DSR, the hemodynamic significance of those stenoses is unlikely to be deduced from the apparent severity of the stenoses. Koiwa and colleagues,38Koiwa Y Bahn RC Ritman EL Regional myocardial volume perfused by the coronary artery branch: estimation in vivo.Circulation. 1986; 74: 157-163Crossref PubMed Scopus (45) Google Scholar however, demonstrated that the cross-sectional area of a coronary artery in dogs is related to the volume of myocardium perfused by that artery. The cross-sectional area (CSA) of the coronary artery supplying a volume of myocardium (Vdsr) was related to that volume as follows: Vdsr = 6.34CSA − 2 (r = 0.88; P<0.001). This equation suggests that the coronary artery luminal cross-sectional area is linearly related to the volume of muscle it perfuses. This relationship may be useful in expressing the physiologic significance of coronary arterial narrowing. If this linear relationship is also found in humans and is quantitatively similar, the physiologic significance of the cross-sectional area of a coronary artery, whether it be diffusely narrowed or locally stenosed, could perhaps be expressed as the percent of cross-sectional area predicted from the volume of myocardium perfused. Indicator-dilution curves are readily obtained by measuring the image brightness in a selected region of interest in sequential cardiac cycles during the passage of the bolus of the contrast medium (Fig. 7). By using myocardial and aortic root dye-dilution curves measured with fast CT, considerable information can be derived for the intramyocardial microscopic vessels and microcirculation, which are not resolvable by existing x-ray CT scanners. A portion of the myocardium consists of intravascular blood. Depending on what fraction of the myocardium is blood and how easily this blood moves within the intravascular space in response to changes in regional compression, this factor may have a considerable influence on myocardial mechanical properties and may be of importance for reflecting the vascular supply, especially in ventricular hypertrophy. Iwasaki and Ritman39Iwasaki T Ritman EL Intramyocardial blood volume dynamics in the cardiac cycle (abstract).Fed Proc. 1984; 43: 422Google Scholar performed aortic root injections of x-ray contrast agent in dogs that were scanned in the DSR. The fraction of blood in the myocardium was computed as the ratio of increased x-ray opacity of the heart muscle to the increase in opacity of the aorta. From these studies, they found that, at end-diastole, blood volume was 13.4 ± 0.3% of muscle volume under control conditions. In a similar study,40Wu X Chung N Stray-Gundersen J Ritman EL Intramyocardial blood volume in dogs with exercise hyp