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Technical Innovation |
1 Russell H. Morgan Department of Radiology and Radiological Science, Johns
Hopkins Medical Institutions, 601 N. Caroline Street, Rm. 3254, Baltimore, MD
21287-0801.
2 Hip Graphics, Baltimore, MD.
Received February 6, 2004;
accepted after revision July 8, 2004.
Address correspondence to L. P. Lawler.
Introduction
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The initial relatively high-speed acquisition of electron beam CT (EBCT) [3] provided some of the earliest quantitative CT information on ventricular size and shape and on systolic function. However, this complex technology was never widely available, was limited to prospective gating, and is fast being replaced by the the more versatile mechanical CT where its acquisition parameters in terms of temporal resolution are rapidly approaching those of EBCT. Earlier work with single-detector helical scanning was able to produce animated 2D images of the heart with ventricular values that closely correlated with conventional ventriculography [4].
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Axial planar data for 3D and 4D postprocessing are reconstructed from the raw data using retrospective gating, which defines a portion of the cardiac cycle as a percentage of the R-R interval. For 4D imaging, a contiguous sequence of nine or more separate "whole-heart" volumes, of equal duration intervals, is generated. These volumes represent the distinct phases (i.e., "time windows") of cardiac motion (e.g., 10%, 20%, 30%, and so forth up to 90% of the R-R interval). This is a semiautomatic standard process of all cardiac software and is performed at the scanner. The user simply defines the percentage reconstruction desired, and the result is a series of separate stacks of axial images with each stack representing a particular period of the cardiac cycle. Within each time window, all voxels of the heart are represented. The total number of images may be more than 2,000 images, depending on the slice reconstruction used (0.75–1 mm). The data are sent to a 3D workstation that incorporates commercially available volume-rendering software and supports a work-in-progress version of 4D software.
4D Reconstruction, Visualization, and Measurement
Four-dimensional CT of the heart is not yet universally defined. We define
it as a moving 3D representation of the cardiac cycle. The 4D data display is
based on an existing 3D platform (InSpace, Siemens Medical Solutions) that
permits the user to view the data with a variety of techniques—maximum
intensity projection, multiplanar reconstruction, and volume rendering. Unlike
3D imaging, which requires only a single time window of helical data, 4D
imaging requires the entire set of sequential time windows (representing the
full R-R cycle) to be loaded into the 4D program; this loading takes
approximately 60–90 sec. The nine volumes of data are then sorted on the
basis of their sequential temporal interrelationship within the cardiac cycle,
which is deduced from the ECG map of cardiac motion over time. The computer
performs this task automatically. Thus, a coherent cine loop is generated, and
the user defines its cycle rate.
Existing interactive, real-time 3D tools are preserved for the user, and the beating heart may be also displayed in 2D axial and multiplanar formats. The user maintains control over variables such as opacity, brightness, and window width and level. Regions of interest are applied to remove unwanted data from the image. Infinite, real-time interactive planes and projections using clip-editing plane slab segmentation of data allow display of standard ventricle short and long axes and four-chamber views. Planes orthogonal to specified wall segments of interest show the presence or absence of cyclical thickness changes to best effect. Patient- or abnormality-specific planes may also be drawn; for example, planes may be drawn through the valve to depict the moving leaflets (Figs. 1A, 1B, 1C, 1D, and 1E).
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The data obtained may be quantified. By viewing the 4D images, the user may first observe the most diastolic and most systolic phases of cardiac motion as defined by chamber size, wall thickness, and valve motion. Existing cardiac software platforms allow one to define the endocardial and epicardial surfaces by region of interest drawing or density threshold techniques. From this systolic and diastolic information on cardiac volume and regional wall thickness, changes may be assessed and ejection fraction and stroke volume may be measured. As in echocardiography and conventional ventriculography, experience allows one to learn the patterns of abnormal wall motion including akinesis and dyskinesis by observing normal and abnormal 4D sequences, which to date remains largely subjective [5].
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MDCT generates isotropic voxels that allow a perspective of interpretation independent of the plane of acquisition. This independent perspective permits us to view animated 3D images that simulate the cardiac motion. In the future, 4D CT will be indexed to the particular patient's heart rate and rhythm to reflect the individual cardiac cycle. Improved temporal resolution of MDCT will enhance the systolic image quality. The visualized motion will be reconciled to existing wall volume and thickness mapping derived from planar data. It is hoped that a method will be developed to quantifiably assess myocardial displacement, which is contained in the data. High contrast and spatial resolution information reflecting chamber performance is already contained within routine retrospectively gated data sets. Four-dimensional cardiac CT will make a major contribution to harnessing the full morphologic and functional potential of high-quality MDCT data acquired from a single cardiac CT study.
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TopIntroductionMaterials and MethodsDiscussionReferences |
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