Computer Measurements of Coronary Artery Disease

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Computer Measurements of Coronary Artery Disease IMAGING processing of the coronary arteriogram by computer has been extended by the commercial availability of software for the measurement of stenoses, coronary flow, flow reserve, and myocardial perfusion. Whilst it is accepted that further development is needed, do the present techniques work, and are they Coronary stenoses may be assessed by two different approaches. In the geometric approach, the X-ray image of the selected segment of coronary artery, including the narrowing and the supposedly normal vessel proximal and distal to it, is transformed into a digital signal which is processed by a computer. Then the machine, with a variable degree of assistance from the operator, draws an outline of the stricture, and of the assumed vessel outline before the stricture had developed, and calculates the degree of narrowing, the pressure drop on the basis of assumed blood flow volumes, and consequent interference with flow. Digital processing of the X-ray image on cine film can be achieved directlyl or orthogonal projections can be traced and the tracings computerised.1 Both methods depend on a model of the geometry and physical properties of the vessel and of the stricture. In the second approach X-ray transmission is measured along the contrast-filled artery. Transmission is inversely related to the logarithm of the iodine content of the vessel, and so, if the concentration of iodine is constant, the depth of vessel through which the X-rays have passed at any point, including the stricture, can be calculated. No assumptions about the shape of vessel or stricture are required. Both techniques have been applied more recently by direct

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digital radiography, which is processed by temporal mask mode subtraction or multiple differential filtration. Coronary flow measurements by computer sound impressive but existing methods depend purely on the rate of movement of the contrast front along the vessel-ie, its velocity multiplied by the crosssectional area of the vessel. The computer merely uses complex calculations to define the contrast front and simple methods to measure its progress. Other methods, which make use of the rate of build-up and fall-off of video density or contrast along a vessel, have been applied only in non-coronary vascular disease.3 Assessment of myocardial perfusion with computer techniques merely inscribes the radiological equivalent of a dye-dilution curve or dye-washout curve to a segment of myocardium, again by measuring X-ray transmission against time and plotting the resultant curve. These techniques place a 1. Reiber 2.

JHC, Serruys PW, Slager CJ. Quantitative coronary and left ventricular cineangiography. Lancaster: Martinus Nijhof, 1986. Brown BG, Bolson E, Frimer M, Dodge HT Quantitative coronary arteriography.

Circulation 1977; 55: 329-37. 3. Lewis BD, Enzman DR, Guthaner DF, Brody WR. High frame-rate DSA imaging technique for evaluation of aorta femoral run off. Radiology 1984, 151: 789-90.

linearity of imaging system contrast response at low levels, a particular problem in relation

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cine films. When studying the coronary evaluation of coronary artery

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arteriogram for disease,

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valuable-the cross-sectional area of the stenosis4 (its shape, length, and multiplicity may also be relevant) and the percentage reduction of cross-sectional area produced by the stricture, as compared with the "normal" vessel above its Thus the widely used percentage reduction of diameter, often estimated visually but variably6 to within a few percent on the coronary arteriogram, is based on the assumption that a 60% diameter narrowing, approximating to an 80% cross-sectional area reduction, is a "significant" stricture.7 Significance in this sense implies a stricture of sufficient severity to impede the increase in blood flow that the myocardium requires during exercise-ie, to reduce the coronary flow reserve.8 The edge detection method undoubtedly measures diameters of experimentally strictured tubes, performing better when the stricture is concentrid rather than eccentric,9 and calculates cross-sectional area, pressure drop, and flow resistance when applied to coronary arteriograms on an assumed mode1.1 In real life the so-called normal artery is unlikely to be normal, since atheroma is widespread along the coronary tree. Moreover, the stricture itself is unlikely to have a circular cross-section; it may be elliptical or even crescentic. Many factors, both biological and technical,11,12 influence the computer measurement. The X-ray transmission methods 13 " are not limited by geometrical considerations. However, although X-ray transmission can be measured, the accuracy of measured change, and the corrections required for technical imperfections in the imaging chain, make the results of uncertain value unless very strict quality assurance and control procedures are always applied.

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4. Harrison DG, White CW, Hiratzka LF, et al The value of lesion cross-sectional area determined by quantitative coronary angiography in assessing the physiologic significance of proximal left antenor descending coronary arterial stenosis. Circulation 1984, 69: 1111-19. 5 Wijns W, Serruys PW, Reiber JHC, et al Quantitative angiography of the left anterior descending coronary artery; correlations with pressure gradient and results of exercise thallium scintigraphy. Circulation 1985; 71: 273-79. 6 DeRouen TA, Murray JA, Owen W. Variability in the analysis of coronary arteriograms Circulation 1977; 55: 324-28. Gould KL, Lipscombe K, Hamilton GW. Physiologic basis for assessing critical coronary stenosis. Am J Cardiol. 1974; 33: 87-94 8. Hoffman JIE. Maximal coronary flow and the concept of coronary vascular reserve. Circulation 1984; 70: 153-59. 9. Key H, Jackson PC, Thomas EA, et al The accuracy of digital subtraction angiography for the quantification of atherosclerosis. Br J Radiol 1987, 60:

1083-88. 10. Viodaver

Z, Frech R, Van Tassel RA, Edwards JE. Correlation of the antemortem coronary arteriogram and the post mortem specimen. Circulation 1973, 47: 162. 11. Spears JR, Sander T Quantitation of coronary artery stenosis seventy: limitations of and computerised information extraction In. Reiber JHC, Serruys PW, eds. State of the art in quantitative coronary arteriography Lancaster: Martinus Nijhoff, 1986: 103-24 Selzer RH, Shircone A, Lee PL, et al. A second look at quantitative coronary angiography Some unexpected problems. In Reiber JHC, Serruys PW, eds State of the art in quantitative coronary artenography Lancaster Martinus Nijhoff,

angiography

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1986: 125-43. 13 Kishon Y, Yerushalami

S, Deutsch V, Neufeld HN Measurement of coronary arterial lumen by densitometric analysis of angiograms. Angiology 1979; 39: 304-11. 14. Sander T, Als AV, Paulin S Cine-densitometric measurement of coronary arterial stenoses. Cathet Cardiovasc Diagn 1979; 5: 299-345.

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stricture studied by edge detection and transmission X-ray techniques may give different under certain results circumstances,l5 the densitometric method being more accurate.9 same

flow measurements depend on accurate of the velocity of the contrast front, which must be measured in a segment of vessel not subject to foreshortening so that the distance the front has travelled is known.16 Accurate assessment of vessel cross-section requires accurate determination of the vessel diameter (any error is squared). These techniques, even if they determine coronary flow at rest in human subjects (and the complex anatomy of the beating heart makes such measurements very difficult), do not answer the most important question-how flow will increase with exercise. The myocardial wash-in and wash-out techniques have the potential to compare myocardial flow in myocardium that is apparently normally perfused with that in tissue subserved by a strictured vessel. Rapid, gated exposures, arrested respiration, highly accurate X-ray generation and recording, and a convenient arrangement of vessels and myocardium for making observations are all useful. Nevertheless, the technique correlates variably with experimental measurements of blood flow.17 It has been combined with coronary flow velocity measurements to give an index of myocardial perfusion that correlates fairly well with measured changes in coronary flow, as determined experimentally. 18 A more immediately fruitful area for the application of X-ray image measurements may be comparisons of myocardial flow in the same subject before and after induced hyperaemia; by this means the importance of a lesion and the effect of intervention may be assessed, with the patient acting as his own control.l9 In dogs, reactive hyperaemia can be estimated by inscribing X-ray transmission dye-dilution curves over the coronary arteries after contrast injection and comparing areas under the curves before and after

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intervention,17 or by measuring changes in contrast appearance, time, or peak density or even by combining these features in a colour-mapping display. In animal experiments, the acquisition of reasonable correlations between these methods and measured flow by means of electromagnetic flow meters (which themselves give some anomalous results during

injection) requires scrupulous experimental technique and attention to detail of the biological variables in the dog, and also electrocardiographic gating. contrast

Serruys PW, Reiber JHC, Wijns W, et al Assessment of percutaneous transluminal coronary angioplasty by quantitative coronary angiography diameter versus densitometric area measurements Am J Cardiol 1984; 54: 482-88. 16. Spiller P, Schmiel FK, Politz B, et al. Measurement of systolic and diastolic flow rates in the coronary artery system by X-ray densitometry. Circulation 1983; 68: 337-47 17. Nissen SE, Elion JL, Booth DC, et al. Value and limitations of computer analysis of digital subtraction angiography in the assessment of coronary flow reserve. 15.

Circulation 1986; 73: 562-71. 18

Hodgson JM, LeGrand V, Bates ER, et al Validation in dogs of a rapid digital angiographic technique to measure relative coronary blood flow during routine cardiac catheterisation. Am J Cardiol 1985, 55: 188-93. 19. Zulstra F, van Ommeren J, Reiber JHC, Serruys PW. Does the quantitative assessment of coronary artery dimensions predict the physiologic significance of a coronary stenosis? Circulation 1987, 75: 1154-61.

technique of digital angiography enables images to be directly recorded in digital form. Commercial programs are now supplied to calculate stricture severity by both diameter measurement and X-ray absorption techniques, and to study myocardial perfusion. There is clearly a need for rigorous X-ray system quality assurance if the units are to be used for anatomical and physiological measuring instruments rather than for imaging only. Design improvements to control X-ray output quality will be required. Extensive development of the software and attention to practical angiographic techniques will be necessary if the reliability of these measurements is to rise to the point where they will rival those already familiar in nuclear medicine. The results may be pretty-but will they be useful? The

CHASING RAINBOWS NO MORE: DAMAGE LIMITATION AT THE MRC PRESENTATION of the 1986/87 Medical Research Council annual report! provided an opportunity for Dr Dai Rees, the new Secretary of the MRC, to talk about the difficult times facing the Council. For several years now the MRC has staggered from crisis to crisis. "We can’t continue like this" said Dr Rees; crisis management will henceforth be abandoned. Its place will be taken by planned allocation of funding to achieve a realistic balance within the financial constraints-in other words the MRC will emerge severely

pruned, but without the enchanced potential for growth that characteristically follows in the garden. The new version of the Council’s corporate plan reflecting these principles will be published next year and is unlikely to make cheerful reading. The MRC receives grant-in-aid of some C140 million; each year this amount is reduced by 1-2 million in real terms as a result of pay awards. Small injections of government money help the acute sickness but do nothing for the chronic malaise. To overcome the immediate crisis the Council needs about 10 million; to restore medical research to full health would require an extra 40 million a year. The MRC has to decide what to do when, not if, they do not receive the money. The Council is determined to stick by its abiding principles-to support outstanding investigators, to maintain an interest in all its current spheres of activity, and to sponsor clinically oriented research towards better health in the community-and it is equally determined that it will not resort to heavy-handed management and effectively stifle research initiative. It can only hope to achieve these aims in the future by adopting a policy of damage limitation. Already many worthwhile applications are turned down -eg, up to 50% of project grants. Moreover, for historical reasons the longer term programme grants tend to come up for renewal in blocks: 1988 will see a 50% increase in such applications. The MRC AIDS programme has received special government funding towards drug design and vaccine development. The MRC now perceives a need for detailed clinical studies, especially in patients with AIDSrelated dementia, for epidemiological studies to establish modes of transmission in heterosexuals in the UK, and for research into HIV 2, for which the Council’s unit in The 1 MRC Annual

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Report 1986· 87. Medical Research Council, 20 Park Crescent, London Pp 96. £6.