The future of computerized cardiac angiography.

The Future of Computerized Cardiac Angiography WALTER L. HENRY, MD, and JONATHAN TOBIS, During the past 2 decades, digital computers have had an enormous impact in many areas, including medicine. Initially, the benefits to be derived from computers for the medical care system were obscured by high equip- ment costs and difficulty in interacting with the devices. During the past 5 years, however, the cost of computer hardware has decreased dramatically and improve- ments in software have allowed more effective human- computer interaction. These recent developments have produced equipment in which the advantages of com- puter technology are obvious to the physician-user. For example, most, if not all, present-day medical imaging devices are basically specialized computers programmed to process images coming from a variety of sources, such as x-ray machines, gamma cameras or ultrasound transducers. Computer processing of x-ray images is a clinical application that has occurred only recently. This is particularly true in the field of cardiac angiography, in which many frames of angiographic images must be processed each second. In addition, each cardiac angi- ographic frame must be converted into a computer format in such a way that very small detail in the image is preserved. This requirement for fine detail and high frame rates made computer processing of cardiac an- giograms possible only during the past few years, as large and relatively inexpensive computers and so- phisticated digital storage devices were developed. Advances in computer technology are still occurring very rapidly, and computer processing of ventricular and coronary angiograms is now feasible in a routine clinical environment. Computer processing of cardiac angiograms has many advantages. One of the major advantages is that images can be manipulated using various algorithms to enhance the visibility of iodinated contrast material. For ex- ample, the mask subtraction algorithm allows studies to be done with a smaller amount of contrast medium, which, in turn, may result in less risk and less discomfort to the patient. This algorithm has been used by several groups, who have shown that digital subtraction left ventriculograms can be obtained with direct intracar- diac injection of one-fourth the standard amount of contrast material. The lower amount of contrast pro- duces fewer ectopic beats and fewer hemodynamic changes during and after the injection. A second major advantage of computer processing of ventricular and MD coronary angiograms is the ability to quantify disease in a way that has been cumbersome, if not impossible, using film-based tine methods. Although methods for performing quantitative analysis of coronary angio- grams are being developed, preliminary studies indicate that it should be possible in a routine clinical environ- ment to quickly and accurately quantify the reduction of a coronary artery lumen produced by atherosclerotic lesions. The ability to estimate the minimal luminal diameter of coronary lesions should provide signifi- cantly improved information upon which clinical deci- sion making can be based. This information may prove especially useful before and during coronary angioplasty procedures. In addition, these methods will facilitate an analysis of the impact of various interventions, in- cluding angioplasty, on coronary artery disease. Computer processing of cardiac angiograms is not without problems and cost. Financially, present digital angiographic systems are expensive-$200,000 to $300,000. This cost is partially offset by savings in film and film processing equipment. These film savings do not nearly offset the cost of image processing computers. Some indirect costs, however, may be significantly re- duced by computer processing of cardiac images, such as the technician and physician time required to analyze quantitatively ventricular and coronary angiographic studies. When the cost of physician and technician time is factored in, present devices are somewhat more cost competitive. Another problem with computer image processing relates to the use of mask subtraction for enhancing ventricular and coronary images. With this method, a mask image of the heart must be obtained before the injection of contrast material into the ventricle or cor- onary arteries. Therefore, the patient must not move between the time the mask is obtained and the subse- quent images are processed. Otherwise, misregistration artifacts appear in the images. This problem can be minimized by having the patient hold his breath just before and during the injection of contrast material. Also, various algorithms built into the image processing computer can be used to move the mask relative to the contrast-filled images in order to reduce misregistration artifact. Thus, even when patients move during the study, the digitized and subtracted ventricular and coronary images can be processed and misregistration artifact minimized. Alternatively, other image pro- cessing methods that do not require the use of a mask, such as recursive filtering, can be used to enhance im- ages. Coronary stenosis can also be quantified using the unsubtracted images. In our experience, the unsub- tracted coronary images that are stored digitally, edge enhanced and magnified are as good for analysis as standard film-based images. Although the spatial res- From the Department of Medicine, University of California, Irvine, California College of Medicine, Orange, California. Manuscript received February 13, 1984, accepted February 25, 1984. Address for reprints: Walter L. Henry, MD, University of California, Irvine, California College of Medicine, 101 City Drive South, Orange, California 92668.