CT report that includes all data boosts benefits

October 1, 2007
Heon Lee, MD, PhD

,
Pal Suranyi, MD, PhD

,
U. Joseph Schoepf, MD

Accurate and reproducible assessment of left ventricular function is crucial for differential diagnosis, risk stratification, treatment planning, and predicting prognosis in patients with heart disease. Evaluation of global and regional LV wall motion, valvular function, myocardial mass, and, in some cases, right ventricular function produces valuable ancillary pieces of clinical information that can be obtained from a contrast-enhanced multislice CT scan.

Accurate and reproducible assessment of left ventricular function is crucial for differential diagnosis, risk stratification, treatment planning, and predicting prognosis in patients with heart disease. Evaluation of global and regional LV wall motion, valvular function, myocardial mass, and, in some cases, right ventricular function produces valuable ancillary pieces of clinical information that can be obtained from a contrast-enhanced multislice CT scan.

No physician interpreting cardiac CT would neglect to report an incidental finding outside the pericardium during the evaluation of a cardiac CT. In the same vein, we take this opportunity to plead with our colleagues-radiologists and cardiologists alike-to draw on the rich pool of potentially useful clinical information that can be obtained with a single contrast-enhanced, retrospectively gated CT scan.

The most common method used for cardiac CT acquisition these days is retrospective ECG gating. While this approach provides great flexibility to identify the most suitable phase of the cardiac cycle with the least cardiac motion for image reconstruction, the main disadvantage of this technique is the relatively high radiation dose that results from continuous irradiation of the patient paired with a very slow, oversampled pitch.

If only a few selected phases are reconstructed for morphological analysis, most of the applied radiation will not be converted into useful image information and thus will be wasted. Sophisticated technical features, such as ECG pulsing with reduced tube current during phases that ordinarily do not yield motion-free images, can significantly reduce radiation exposure, although not to the level that would be applied with the historic method of prospective ECG triggering that applies radiation only during a short, predefined phase in the cardiac cycle, leaving the tube turned off during the remainder of the RR interval.

However, as the goal of retrospective ECG gating is to acquire images of every portion of cardiac anatomy at any given time-point throughout the cardiac cycle (i.e., essentially a 4D data set), this approach also forms the foundation of CT-based functional analysis of the heart. With newer MSCT scanners and dedicated analysis software, both qualitative and quantitative assessment of global and regional ventricular function have become possible.

According to several recent studies, there is no doubt that functional analysis based on CT is accurate compared with clinical reference standards such as echocardiography, ECG-gated SPECT, and MRI.

For functional analysis, additional overlapping image series can be reconstructed retrospectively at 5% or 10% increments across the cardiac cycle based on the same raw MSCT data set that's acquired for morphologic analysis at no extra cost in terms of contrast medium or radiation exposure. Furthermore, reconstructions for functional analysis need not be as detailed (a slice thickness of 1.5 mm at 1.5-mm increments should suffice) as those of the dedicated coronary arteriograms (slice thickness of 0.75 mm is recommended at 0.3 to 0.4-mm increments), which saves time and data storage space. Thus, a few additional mouse clicks yield 4D data sets of the cardiac anatomy in all phases of the cardiac cycle, enabling analysis of global and regional functional parameters.

Global cardiac function is defined by ejection fraction, stroke volume, and cardiac output, which are derived from the measurement of end-diastolic and end-systolic left ventricular volumes. Regional myocardial wall thickness or regional function parameters such as systolic wall thickening or wall motion provide useful information on the functional state of ischemic and nonischemic myocardial segments.

Although regional wall thinning or abnormal resting function is not a fully reliable indicator of nonviability (for that, one would need to apply pharmacological stress), it stands to reason that if a region is found dysfunctional, further evaluation is warranted. Thus, decreased function may be an early harbinger of underlying disease and, therefore, functional analysis should be an integral part of the clinical report of patients assessed for coronary artery disease.

A standardized template for reporting CT coronary angiography findings in an organized manner may be a very useful tool to streamline the reporting process and to ensure that all bases are covered. Our reports describe all potential abnormalities, variations, and changes that may be visible when reviewing various reconstructions.

In the procedures section of the report, we provide a description of items pertinent to appropriate billing in our local healthcare environment and the most basic imaging parameters, such as section thickness; use of retrospective ECG gating; contrast material type, volume, and injection speed; medications (if used); and image postprocessing methods, such as MPR and 3D reconstruction. In the findings section, the gross cardiac and great vessel anatomy are described. We comment on the myocardium (thickness, areas of hypoattenuation or suspected infarction, calcified scars, etc.), cardiac chambers, valves, pericardium, pulmonary veins, pulmonary arteries, and aorta. A section is dedicated to incidental findings of the lungs, chest wall, and mediastinum.

In the dedicated cardiac section, we report the presence and location of various cardiac devices and catheters. When coronary artery bypass grafts are present, we describe the type, origin, course, and site of the anastomoses; the presence, location, and degree of graft stenoses; and the quality of runoff within the grafted vessel distal to the anastomosis. Each of the main coronary arteries is described separately, noting in detail the presence and type of atherosclerotic plaque burden.

Finally, we add the results of the functional analyses, such as LV end-diastolic volume, end-systolic volume, ejection fraction, stroke volume, and LV mass. Using dedicated segmentation algorithms, these parameters are quickly and easily calculated with little or no observer input. Color-coded maps of ventricular function, either as cine 4D display or as the traditional polar map intuitively inform the reader about regional functional deficits. Abnormal findings can easily be correlated with multiplanar reformatted cine-loop displays in long-axis, short-axis, or four-chamber views to diagnose hyperkinesis, akinesis, hypokinesis, or dyskinesis.

The functional data are not mere by-products of coronary CT angiography but valuable additional information, gained without further cost, that should be considered an integral component of every cardiac CT exam. Although it may increase the routine reading time by a couple of minutes, considering the study costs, the radiation exposure, and, most important, the useful diagnostic information that can be obtained, it appears well worth the effort to include functional results in our reports.

Why allow so much radiation to go to waste when, with so little input, so much can be gained? We recommend complete utilization of the entire 4D data set to provide the referring physician with the most comprehensive report.

Dr. Lee and Dr. Suranyi are radiologists at the Medical University of South Carolina in Charleston. Dr. Schoepf is an associate professor of radiology and director of CT research and development at the same institution.