Angiography at reduced frame rates: How low can you go?

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    Angiography at Reduced FrameRates: How Low Can You Go?John Warner, MD, and Thomas M. Bashore, MDDuke Medical CenterDurham, North Carolina

    In this issue of Catheterization and CardiovascularInterventions, Dr. Guo and Dr. Sheehan describe ventricu-lographic results obtained at 30 frames/sec. comparedwith 15 frames/sec. They do this by creating 3 directories.The original data were acquired at 30 frames/sec. Thenthe even numbered and the odd numbered images wereseparated into two subsets to simulate acquisition at 15frames/sec. Each set of images were then reviewed, andthe end-systolic and end-diastolic frames were traced.Left ventricular volume, the ejection fraction, and a wallmotion score, (using the centerline method that this grouphas popularized), were then assessed. They compared thesubset with odd numbered frames to the full data set,matching its corresponding frame to the same frame infull data set. Similarly, they compared the even numberedframes to the corresponding even numbered frame in thefull data set. Since the same frame was being assessed inboth the subset and the full data set, a measure ofintraobserver variability was obtainable. The end-diastolic or end-systolic image was then selected from thefull set of images and compared to the end-diastolic orend-systolic image from the even or odd frame subsets.Any variability observed in this latter comparison wasthen felt attributable to the reduced frame rate.

    The authors found that there was little difference ineither the intraobserver variability or in the errors due tothe reduced framing rate. Given the relatively broaderrors inherent in the derivation of ventricular volumes,the inability to find a significant difference may not comeas too much of a surprise to many cardiologists.

    This study emphasizes the importance of samplingrates. Effectively, the authors are sampling the end-diastolic and the end-systolic portions of the ventricularvolume curve. The basic question is how frequently mustthe angiographic ventricular volume curve be sampled toprovide the true end-diastolic and end-systolic volumetricinformation. Failure to sample frequently enough intro-duces an artifact called aliasing.

    To help understand the issue, consider the following.Whenever any dynamic event (in this case the ventricular

    volume curve) occurs, the waveform can be analyzed byFourier analysis. Fourier analysis breaks down any wave-form into a series of harmonics. The harmonic content inany cyclic event is determined by noting how manyharmonics must be added together to reproduce the event.The ventricular volume curve can be reproduced byadding back a total of about 10 harmonics (or sinewaves). The ventricular volume curve waveform, there-fore, has about 10 harmonics (or 10 Herz) in it. Samplingtheorem dictates that one must sample at 2 times themaximum Herzian content in any waveform to be sure thewaveform can be reproduced. In theory, therefore, theventricular volume curve should be sampled at least 20times per R-R interval or cycle. Sampling at less than thatmay result in missing the true end-systolic or trueend-diastolic frame.

    When the heart rate increases, the sampling rate shouldalso increase to insure that the end-systolic and end-diastolic frames are adequately captured. This becomes aparticular issue in the pediatric age group, but is certainlyan issue in angiography whenever images are acquired athigh heart rates. The paper by Guo and Sheehan does notaddress this issue. The inability of the authors to demon-strate a difference between 15 frames/sec and 30 frames/sec may also be due to the fortunate fact that diastolictime changes more than systolic time when the heart rateis varied. For instance, as the heart rate increases, the timefrom end-diastole to end-systole (ejection time) willshorten much less than the time from end-systole toend-diastole (diastolic time). This fact may lessen theoreti-cal concern regarding variability in the heart rates.

    Thus, comforting to realize that despite these concerns,using a less than theoretically acceptable frame rate doesnot appear to make that much difference in the finalanalysis. This is an issue that has also come to theforefront because of the welcomed advent of digitalimaging. With the use of computer imaging and non-interlaced computer image display, slower frame ratesmay now be readily displayed without flicker. This hasallowed for a progressive reduction in the framing ratesneeded during angiography, while preserving acceptableimage display and viewing.

    There are many advantages in using a reduced frame rate.As digital technology advances, the requirements for digi-tal storage and for digital image transfer and display willincrease. As cardiac catheterization laboratories gradually

    Catheterization and Cardiovascular Interventions 48:2223 (1999)

    r 1999 Wiley-Liss, Inc.

  • move from 512 3 512 3 8 bit digital image acquisitionand display systems to 1024 3 1024 3 10 bit systems, theamount of information will grow exponentially. The needto network systems and transfer this large amount of datarapidly over these networks creates a need for researchinto better methods of image compression as well as aclose examination of framing rates necessary to capturethe information required. The ability to use a lowerframing rate to reduce the acquisition time and to reduce

    the need for so much digital data storage is obviouslyrelevant.

    The paper by Dr. Guo and Dr. Sheehan takes a first steptoward addressing the clinical relevance of such aliasingissues as we continue to redefine how we perform cardiaccatheterization. How low can you go?apparently in theadult patient undergoing ventricular angiography (presum-ably at normal heart rates) we can acquire acceptableinformation at 15 frames/sec.

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