Applications of digital angiography: percutaneous transluminal coronary angioplasty (1987)

CKER Applications of Digital Angiography: Percutaneous Transluminal Coronary Angioplasty tricu- ':An David A. Sato, Jonathan Tobis, Orhan Nalcioglu, Warren D. Johnston, and Walter L. Henry ttion. sure- :rmi- >ition ttion N RECENT YEARS computers have had a profound impact on all facets of medicine. In cardiovascular radiology, the potential applica- tions of angiography have broadened because of computer applications. Digital angiography is more versatile than standard film-based angi- ography because the data is readily accessible for manipulation and analysis by computer algorithms. 1..,; Computer programs have been developed to quantitate the degree of coronary stenoses by either edge detection or by videoden- sitometry. The physiologic significance of coro- nary stenoses can be assessed by measuring coro- nary flow before and after hyperemic stimulus or by evaluating left ventricular wall motion during atrial pacing. With the introduction of percutaneous trans- luminal coronary angioplasty (PTCA), cardiac angiographers have become more aggressive in the intervention of coronary artery disease. 1· 10 Because of its versatility, digital angiography can be of benefit to the interventional angiographer. Using a digital system, the angiographer can generate a digital road map to aid balloon and guidewire postitioning. 11 With this method, a previously obtained digital coronary image is interlaced with the live fluoroscopic image pro- viding a reference for balloon and guidewire placement. Digital images are available for more immediate review in the catheterization labora- tory, therefore, the result of a procedure can be quickly assessed using quantitative and physio- logic digital methods. 12·15 I COMPUTER PROCESSING In conventional film-based angiography, ra- diographic film is exposed and subsequently developed to yield the angiographic image. In digital angiography, images are transformed by an image-processing computer into a matrix con- sisting of 512 x 512 or 262, 144 segments or pixels. In the digitization process, the electric signal in each pixel is assigned an integer number corresponding to the degree of x-ray absorption and hence contrast density in that segment. The range of contrast density depends upon the num- ber of bits available for storage. Typically, 8 bits are available, which corresponds to 28 or 256 shades of gray. The digital information in each frame is then stored in computer memory. In the original stud- ies, computer memory was limited. Images were obtained at 30 frames/sec, post-processed in real time and stored on videotape. The digital-to- analog data conversion resulted in image degra- dation. More recently, larger computer storage capabilities have become available, allowing 1,800 frames of data to be acquired per study. Data can be subsequently manipulated to opti- mize contrast visualization, to quantitate coro- nary artery disease or to provide information about coronary flow. Because the angiogram is stored as a series of numbers, the images can be manipulated by a variety of mathematical formulas, including addition, subtraction, logarithmic amplification, convolution algorithms, and Fourier transforma- tion.16-18 Digital angiography was originally studied as a method for enhancing low concen- trations of contrast following intravenous (IV) injection. Mask mode subtraction was one of those methods studied. 19 In this process, a digital radiograph obtained prior to contrast injection is stored as a mask. The digital data in this mask is then subtracted pixel by pixel from subsequent frames following contrast injection. In the result- ing images, background structures including lungs, skeletal structures, and soft tissues will be subtracted and iodine contrast will be enhanced. This method is useful for visualizing the left or right ventricle with IV contrast injection or with direct intraventricular contrast injection using lower amounts of iodine contrast than are neces- sary with film-based systems. If the patient moves or breathes after acquisi- tion of the mask, pixels with a given x-y coordi- From the Division of Cardiology and Department of Radiological Sciences, University of California, Irvine, Orange. Address reprint requests to Jonathan Tobis, MD, Division of Cardiology, University of California at Irvine, Medical Center, Orange, CA 92668. © 1987 by Grune & Stratton, Inc. 0887-797I/87/0101--0005$05.00/0 American Journal of Cardiac Imaging, Vol 1, No 1 (January). 1987: pp 51-57