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EDITORIAL Table of Contents   
Year : 2003  |  Volume : 13  |  Issue : 1  |  Page : 11-12
CT coronary angiograms


Joint Editor & Secretary, Indian Journal of Radiology and Imaging, India

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How to cite this article:
Kohli A. CT coronary angiograms. Indian J Radiol Imaging 2003;13:11-2

How to cite this URL:
Kohli A. CT coronary angiograms. Indian J Radiol Imaging [serial online] 2003 [cited 2019 Oct 21];13:11-2. Available from: http://www.ijri.org/text.asp?2003/13/1/11/28614
Coronary artery disease is the largest epidemic sweeping across our country. It strikes 10 years earlier, is more extensive and severe than in westerners. Often when it strikes early it is fatal, without a warning sign, without adequate time for reparative measures. If a myocardial infarction is the presentation, and fortunately if the individual survives the episode there is some amount of compromised cardiac function. Risk factors for coronary artery disease are Diabetes, Hypertension, Hyperlipidemia, Family history, Smoking and Stress. It is well documented that only mental stress is a very significant player in the development of Coronary artery disease. Several studies have demonstrated similarities between the effects of physical and mental stress. In fact what has been demonstrated is mental stress causes silent ischemia with no signs and symptoms as compared to physical stress which often presents with angina.[1]

Who can survive in this world without mental stress? It is important to screen for coronary artery disease in all individuals. Investigations used to screen for Coronary artery Disease at present, in addition to laboratory tests for diabetes and hyperlipidemia are ECG, Stress test, Stress Thalium, Electron Beam CT and Catheter Coronary Angiography.

ECG is the most commonly used and available test to evaluate for Coronary artery disease following chest pain. Unfortunately an ECG is the most insensitive means to answer this question as a normal ECG, neither can exclude ischemia nor infarction. Stress test is the next most commonly used test today. It has a sensitivity of 68% and a specificity of 77%.[2] Its disadvantages are that nearly 3 out of 10 indivuals with coronary artery disease will have a normal stress test and thereby be under the false impression that all is well. Stress thalium tests which combine a stress test and a thalium to study cardiac perfusion are more sensitive taking the sensitivity upto 85%.[3] These two tests however have one major disadvantage, they require the heart to be stressed to levels which would rarely be required. As a result these tests can precipitate acute myocardial infarction, arrhythmias, cardiac arrest and death. Such complications are accepted today in view of the lack of another less risky screening test.

The gold standard for evaluating Coronary artery disease is a catheter coronary artery angiogram. This entails a femoral arterial puncture and has its own share of morbidity and mortality. Alas a too invasive study to use for screening. The ideal screening test for Coronary artery disease would be a noninvasive test which does not stress the heart to produce potential complications. Electron Beam CT initially showed a lot of promise especially with its ability to quantify the amount of calcium in the coronary arteries. The presence of significant coronary calcium is a strong indicator to the presence of coronary artery disease. However it is more commonly the presence of lipid rich plaques which precipitate an acute coronary event. The unstable lipid plaque ruptures and embolises downstream to occlude the coronary artery with consequent myocardial infarction. Therefore it is more important to detect soft plaques than calcific plaques. EBCT further lost out to Multi detector CT as the spatial resolution of MDCT was higher at 10lp/cm as compared to 7 lp/cm for EBCT.[4] Of course MDCT had an obvious advantage as it was not restricted to the heart like EBCT and can evaluate noncardiac causes of chest pain.

Since the advent of Spiral CT and especially now multidetector CT, CT angio is being accepted as the most reliable noninvasive modality to study vasculature. The reason for this is simple, it is based on the same principles of catheter based angiographies, direct visualization of contrast in the vessel. It does not suffer from issues of operator dependence and flow, issues which plague other noninvasive modalities. But when it comes to imaging of the cardiac vasculature CT has significant limitations. The coronary arteries are small vessels ranging from 1 to 4mm in diameter. To image these small vessels high spatial resolution will be required. Thin sections ranging from 0.5 to 0.75 mm with overlapping spirals, usually a pitch of 0.3 will result in good quality images of the coronary arteries. To cover the entire heart a spiral of 12 cm is required. Considering these parameters on a 4 slice Multidetector CT, a scan time of 42-48 seconds will be required. This is not advisable as it will be difficult for the patient to hold their breath for so long. Further the heart rate will vary during the scan, a major cause for poor coronary artery studies. The ideal and possibly the only Multidetector CT to be used for coronary artery imaging are 16 detector scanners. These require 12-14 seconds to acquire a scan. This breath hold is very comfortable for the patient and also there is a reduced incidence of variation of the heart rate. Another limitation of the coronary arteries is that they are in close contact with the myocardium which contracts vigorously especially during systole. The coronary arteries undergo significant motion during systole resulting in considerable image blur. During diastole there is minimal motion. To eliminate the motion blur it is ideal to obtain images for reconstruction during diastole. To achieve this ECG and image acquisition are done simultaneously. Only the data available during diastole is used for reconstruction. Data during systole is discarded. The disadvantage of this technique is that with a high heart rate, diastole time is shorter. With a heart rate of approximately 60 a temporal window of approximately 250 msec is available for data acquisition. This drops to 150 msec as the heart rate rises to 100. The ideal solution to this problem is to administer beta blockers either orally or intravenously. This reduces and also makes the heart rate steady. It is possible to obtain good quality images also with patients with higher heart rates by utilizing data from 2 or more consecutive cardiac cycles. As the data obtained during systole is not used, most CT scanners reduce the tube output during systole to reduce the radiation exposure. With the basic issues of spatial and temporal resolution sorted out it is very important to obtain images with a good contrast to noise ratio ie excellent contrast enhancement. This will help to provide high quality images of the coronaries detect wall irregularities and resolve small density differences such as atherosclerotic plaques with different composition/attenuation properties. To obtain high iodine density in the coronary arteries it is ideal to use the highest concentration of nonionic iodine contrast. The highest nonionic Concentration available today is iomeron which has an iodine concentration of 400mg%. Unfortunately this contrast is not yet available in this country. The ideal would also be to image the coronary arteries when there is maximum contrast in the coronaries. To determine the time when the contrast will reach the coronaries, it is ideal to use a test bolus of contrast and time the transit time of contrast from a peripheral vein to the coronaries. Another technique is an automated technique which starts the scan when a predetermined threshold is reached. Also it is important to maintain the maximum contrast pacification in the coronaries during the entire length of the study. This is best done by biphasic injection rates. An initial bolus of contrast at 5-6 ml/sec followed by a maintainance at 3 ml/sec is injected one major limitation of CT coronary angio is calcification in the coronary arteries. In severely calcified vessels it is difficult to assess the patency and diameter of the lumen, infact it may cause artefactual narrowing of the lumen.

The pathological process of developing coronary artery disease is triggered by an injury to the endothelial cells resulting in an inflammatory reaction and deposits of fat accumulated on the surface of the intima. These grow causing narrowing of the vessel leading to ischemia and finally infarction. Lipid plaques are nonstenotic but tend to rupture with distal remobilization leading to myocardial infarction. CT can with help of attenuation values differentiate the different types of plaques. Plaques consisting of tissue less than 50 HU is lipid, between 50-130 HU are fibrous and more than 130 HU calcification. The only other modality which can determine plaque composition is intravascular ultrasound. An extremely invasive option. If lipid rich plaques are detected statins can be used to stabilize the plaque and limit the possibility of plaque rupture, subsequent remobilization and consequent myocardial infarction. Thus CT coronary angiograms provide not only an anatomical map of the coronary arteries but also provide morphological information of the heart and most important details of plaque morphology.

Imaging of the coronary arteries is undoubtedly the biggest commercial opportunity for radiologists. To reach this pot of gold they will need to address these very specific issues of Spatial, Temporal and contrast resolution and of course protect this turf from invasive cardiologists.

 
   References Top

1.Guibbini R etal; Effect of mental stress on myocardial perfusion in patients with ischemic heart disease. Circulation 1991; 83 suppl:100-107.  Back to cited text no. 1    
2.Gianrossi R, et al Exercise induced ST depression in the diagnosis of coronary artery disease - A meta-analysis. Circulation 1989;80:87-98.  Back to cited text no. 2    
3.Berman DA, et al. Risk Assessment in patient with stable CAD : Incremental value of nuclear imaging. J Nucl Card 1996;3:S41-9.  Back to cited text no. 3    
4.Ihnesorge B M, Becker C R, Flohr T G, Reiser M F : Multislice CT in cardiac imaging : Springer 2002 pg 65.  Back to cited text no. 4    

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Correspondence Address:
Anirudh Kohli
Joint Editor & Secretary, Indian Journal of Radiology and Imaging
India
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Source of Support: None, Conflict of Interest: None


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