Electron beam computed tomography (EBCT) (C150XP
Imatron) is used to determine coronary calcium. The scanner
operates in the high resolution volume mode (630 Ma, 130kV,
scanning 100 ms, 40 slices @ 3mm, 512 matrix, 26-cm field of
view, sharp kernel . Prospective electrocardiographic triggering
in mid-diastole (80% of the RR interval) is applied during a
single inspiratory breath-hold. The calcium score is calculated
with Imatron's implemented software, according to the method
of Agatston et al. A calcified plaque was defined as a lesion
of at least two adjacent pixels (>0.51mm2) with a signal density
>130 Houndsfield units. EBCT can accurately detect coronary
calcium, which indicates the presence of coronary atherosclerosis.
Coronary calcium, as seen in the early stage of coronary artery
(CAD), is closely correlated with fatty "soft" plaque, which
may rupture and cause myocardial infarction. But the correlation
between arterial mineralization and the probability of plaque
rupture is unknown. In symptomatic patients, EBCT calcium screening
has been used to estimate the severity of stenosis. No close
correlation, however, could be found between the amount of calcium
and arterial lumen narrowing.
A recent study of 1,764 symptomatic patients with suspected
CAD showed that calcium screening with EBCT is a highly sensitive
and moderately specific test to predict stenotic disease. Exclusion
of coronary calcium defines a substantial subgroup of patients,
albeit symptomatic, with a very low probability of significant
stenoses.
The calcium scores for men and women shown
in
table 1 and table
2 indicate that:
1)
the mean score in men was significantly higher than that in
women at each age class;
2)
for both genders, there was a significant increase in calcification
with age;
3)
the variability of scores within each age class was high; this
was caused by a minority of patients with excessive calcification;
and
4)
the difference between patients with and without significant
CAD was highly significant for the total group of patients and
within each age group.
Exclusion of calcium in this study: No calcium
was found in 128 (23.7%) of men and in 116 (40.8%) of 284 women
without significant CAD, as compared with 5 (0.7%) of 685 men
and 0 of 255 women with coronary stenoses approximately 50%
(table 3). Thus, exclusion
of calcium was associated with an extremely low probability
of significant stenosis in men and women.
In contrast, there was a considerable number
fo patients without stenoses who, although symptomatic, did
not reveal any calcification: 55% of symptomatic men and women<
40 years of age had no calcification. In this group, the risk
of stensis was 0%.
Clinical useful score cutpoints
for management decisions:
In figure
158a (for men) and figure
158b (for women), the scores are divided into three levels
for each age group: the lower range defines 95% of patients
without stenoses. Thus, up to the lower score threshold, significant
CAD in a symptomatic person is very unlikely. The upper range
includes 90%of patients with significant stenoses. Calcium levels
beyond the upper score threshold are associated with obstructed
disease in the majority of patients. In the white zone, the
diagnosis is uncertain. The white zone consisted of 98 men (8%)
and 40 women (7.40 %). A narrow white area indicates an excellent
test variable, with only a few patients in the indefinite zone
(approaching a "yes" or a "no" decision). According to this,
EBCT calcium screening was especially helpful in men<50 years
old and in women<60 years old.
Mechanism of calcification:
Coronary calcium predominantly consists of
calcium phosphate, which is not only absorbed or deposited passively,
but also forms in situ by organized, regulated process.
It is strongly associated with the total plaque
burden, but it is not a direct marker of the vulnerable plaque;
but the greater the calcium score, the greater the potential
for increased numbers of potentially lipid-rich plaques, which
are widely thought to be the culprit lesions of acute coronary
syndromes.
Lesser obstructive plaques are associated
with a higher incidence of coronary occlusion, as compared with
severely obstructed plaques. This occurs not because of the
higher risk of such plaques in and of themselves, but because
of their much greater number.
Clinical implications:
In patients with typical angina (provoked
by physical or emotional stress) and/or a definite diagnosis
of myocardial ischemia on conventional stress tests, there is
no indication for EBCT calcium screening because of the extremely
high pretest likelihood of significant stenosis.
Coronary angiography is required for the
definite diagnosis and treatment in these patients.
There is an indication for calcium screening
with EBCT in patients with atypical chest pain, especially if
noninvasive tests are not feasible or inconclusive. Calcium
screening has potential as a filter in symptomatic patients
to reduce the number of invasive procedures which do not lead
to intervention.
A recent study shows that EBCT is an efficient
and safe test to use in patients presenting to the Emergency
Department with acute chest pain, allowing rapid discharge with
a negative scan.It also confirms that the absence of calcium
implies a very low (0.6% annual) incidence of coronary events,whereas
the presence of coronary calcium is an independent predictor
of future cardiac events,supporting prior reports showing that
coronary calcium to be a strong predictor of future cardiovascular
events.
Reference: Haber, R. et als., Correlation
of Coronary Calcification and Angiographically Documented Stenoses
in Patients With Coronary Artery Disease:Results of 1,764 Patients,JACC2001;37:451-7.
Reference:Georgiou,D. and Others,Screening
Patients With Chest Pain in the Emergency Department using Electron
Beam Tomography:AFollowupStudy,JAMCC,Vol.38,No.1,2001,pp.104-109.