Chest: Coronary Artery Evaluation
One of the areas of greatest interest with multidetector CT is in the evaluation of the coronary arteries. The interest is in part a follow through of developments with the electron beam CT scanner (EBCT) where there was significant research as to the ability to detect and quantify coronary artery calcification. Coronary artery calcification represents the presence of artherosclerosis and is a potential indicator of significant coronary risk in the future. The presence of any calcification is indicative of coronary artery disease. Whereas stress testing is usually positive with a 50% stenosis, CT detection of coronary disease may be able to detect disease at an earlier stage. Currently the goal would be to obtain a baseline in a patient and then do follow-ups to determine progression of disease and determine if medical intervention (i.e. diet, medication, etc.) is successful in reducing the risk of a cardiac event. Although the timing of follow-up studies is still under evaluation many centers recommend a follow-up at 3 years.
Multidetector CT is now being used for the evaluation of the coronary arteries by using prospective cardiac gating. Although there has been some controversy as to the merits of MDCT versus electron beam scanning the consensus is that they provide similar information. An advantage of MDCT is its inherent higher spatial resolution which may result in increased detection of even minimal calcification. An ECG-tracing of the patient is done and a delay time of about 80% of the RR interval is selected to try to define the diastolic phase of the heart. With multislice CT with a 500 ms rotation time we obtain 250 ms temporal resolution. A typical scan of the coronary arteries can be obtained in about 15 seconds with 4 adjacent slices obtained per perspective ECG-trigger.
Calcium scoring is based on the Agatston scoring algorithm, which was developed for EBCT scoring. A CT threshold score of 130 HU is selected and a coronary score for each of four main coronary branches (left main coronary artery, left anterior descending coronary artery, circumflex coronary artery and right coronary artery) is obtained. The score is computer generated by measuring the volume of coronary calcification (mm2) and multiplying it by a factor (between 1 and 4) based on the peak attenuation value of the lesion. The scores are stratified to a scale with 4 categories; no calcification, minimal calcification, moderate calcification and extensive calcification. A lack of calcification correlates with a lack of coronary artery disease (95% negative predictive value), while a higher score can give a prognosis concerning risk from a myocardial event. The typical scoring system can also be divided into score and their clinical significance. One of the scoring systems with clinical significance is listed in table 1.
One of the article that clearly shows that helical CT and EBCT are essentially of equal value was by Carr et al. found that "A general purpose, current generation helical CT scanner equipped for retrospective cardiac gating can accurately quantify coronary calcium, and the results are highly correlated to scores obtained with electron beam CT."
We are currently using a Siemens Somatom Plus VolumeZoom scanner (Siemens Medical Systems, Iselin, NJ) for our coronary artery study. Our protocol consists of:
4 x 2.5 mm collimation
2.5 mm slice width
500 msec rotation w/ 250 msec spatial resolution
140 kVp
20-40 mAs (usually 40 mAs)
1.5 second cycle time (heart rate dependent)
Z-coverage of 120mm in 18 seconds
We use prospective gating for all of our studies. Because of some concern for Agatston score reproducibility on EBCT scanners we are routinely obtaining two acquisitions for each patient. Our results on our first hundred patients have shown little interscan variability with no changes in risk stratification.
Although it is uncommon to find significant stenosis of the coronary arteries in the absence of vessel calcification, it still may occur in around 8-9% of patients. There is now interest with multidetector CT for the detection of soft plaque or non-calcified plaque. These plaques are more likely to rupture causing acute myocardial infarction. Currently the gold standard for detection of these lipid rich lesions is intravascular ultrasound. In order to do this with MDCT we must do a CT angiogram with iodinated contrast material and the use of 1 mm collimation and retrospective cardiac gating. Preliminary results have been promising but additional correlative studies will be needed.
Coronary artery calcification scoring is an easy exam to do technically and the scoring process is equally simple. Some limitations on the significance and reliability of the study include:
true correlation of significance of presence of calcification and presence/degree of stenosis.
true correlation of presence of calcification and risk of plaque rupture.
motion artifacts due to breathing.
cardiac motion related artifacts.
partial volume effects.
poor EKG gating.
image noise.
study reproducibility. The role of coronary artery calcification scoring is still controversial. The American College of Cardiology and the American Heart Association recently published an expert consensus panel on the use of CT (EBCT) for the diagnosis and prognosis of coronary artery disease. The consensus statement felt that the true cost-effectiveness of the study for diagnosis of coronary artery disease and for its ability to predict and modify the outcome of early disease was still not defined in a rigorous scientific manner and that further research had to be done. But they did reach several conclusions:
a negative EBCT makes the presence of atherosclerotic plaque, including unstable plaque, very unlikely.
a negative test is highly unlikely in the presence of significant luminal obstructive disease.
negative tests occur in the majority of patients who have angiographically normal coronary arteries.
a negative test may be consistent with a low risk of a cardiovascular event in the next 2 to 5 years.
a positive EBCT confirms the presence of a coronary atherosclerotic plaque.
the greater the amount of calcium, the greater the likelihood of occlusive CAD, but there is not a 1- to -1 relationship, and findings may not be site specific.
the total amount of calcium correlates best with the total amount of atherosclerotic plaque, although the true "plaque burden" is underestimated.
a high calcium score may be consistent with moderate to high risk of a cardiovascular event within the next 2 to 5 years.
Despite these conclusions the consensus panel still reached what might be called a split decision in their recommendation; "Although preliminary data are intriguing with respect to risk prediction in the asymptomatic patient, available data are insufficient to support recommending EBCT to asymptomatic members of the general public or for routine clinical use. Further studies are enthusiastically recommended for determining the additive predictive effect of the calcium score in patients with intermediate risk, particularly in the elderly. The use of EBCT in selected asymptomatic patients can be justified when performed in the context of a medical assessment only after the more standard cardiac risk assessment is considered insufficient by the physician to direct further therapy plans."
Table 1
Calcium Score
Evaluation
Clinical Significance
Recommendation
no calcification
CHD
eliminated (95%)
preventive health
1-10
minimal calcification
stenosis unlikely
preventive health
11-100
slight calcification
CHD possible
evaluate risk factors
101-400
intermediate calcification
CHD with stenosis
treatment of risk possible factors
cardiac exam over 400
extensive calcification
high probability stress ECG, for significant stenosis
? cardiac cath
Table 2
Evaluation of the Coronary Arteries: Calcium Scoring
Agatston scoring algorithm threshold of 130 HU used score for each lesion is given by calculating area of each area of a lesion (mm2) with a co-factor (between 1 and 4) that depends on the HU peak value scores are defined for left main (LM), left anterior descending (LAD), circumflex (CX) and right coronary artery (RCA).
Some recent articles on the subject include:
"A general purpose, current generation helical CT scanner equipped for retrospective cardiac gating can accurately quantify coronary calcium, and the results are highly correlated to scores obtained with electron beam CT." Evaluation of Subsecond Gated Helical CT for Quantification of Coronary Artery Calcium and Comparison with Electron Beam CT
Carr JJ et al.
AJR 2000;174:915-921
Interscan Variation in Coronary Artery Calcium Quantification in a Large Asymptomatic Patient Population
Yoon HC et al.
AJR 2000;174:803-809
"Interscan variation remains an important limitation of electron beam CT in the examination of asymptomatic patients." Interscan Variation in Coronary Artery Calcium Quantification in a Large Asymptomatic Patient Population
Yoon HC et al.
AJR 2000;174:803-809
"Although preliminary data are intriguing with respect to risk prediction in the asymptomatic patient, available data are insufficient to support recommending EBCT to asymptomatic members of the general public or for routine clinical use. Further studies are enthusiastically recommended for determining the additive predictive effect of the calcium score in patients with intermediate risk, particularly in the elderly. The use of EBCT in selected asymptomatic patients can be justified when performed in the context of a medical assessment only after the more standard cardiac risk assessment is considered insufficient by the physician to direct further therapy plans." American College of Cardiology/ American Heart Association Expert Consensus Document on Electron Beam Computed Tomography for the Diagnosis and Prognosis of Coronary Artery Disease
O'Rourke RA, Brundage BH, Froelicher VF et al.
Circulation 2000; 102:126-140
a negative EBCT makes the presence of atherosclerotic plaque, including unstable plaque, very unlikely.
a negative test is highly unlikely in the presence of significant luminal obstructive disease.
negative tests occur in the majority of patients who have angiographically normal coronary arteries.
a negative test may be consistent with a low risk of a cardiovascular event in the next 2 to 5 years.
a positive EBCT confirms the presence of a coronary atherosclerotic plaque.
the greater the amount of calcium, the greater the likelihood of occlusive CAD, but there is not a 1- to -1 relationship, and findings may not be site specific.
the total amount of calcium correlates best with the total amount of atherosclerotic plaque, although the true "plaque burden" is underestimated.
a high calcium score may be consistent with moderate to high risk of a cardiovascular event within the next 2 to 5 years.
American College of Cardiology/American Heart Association Expert Consensus Document on Electron Beam Computed Tomography for the Diagnosis and Prognosis of Coronary Artery Disease
O'Rourke RA, Brundage BH, Froelicher VF et al.
Circulation 2000; 102:126-140