Can the mechanical CT Scanner perform heart scanning as well as EBCT?

 

   

Acceptance of coronary calcification as a marker for coronary disease by the medical community has now led various researchers and manufacturers to begin exploring the use of mechanical CT scanner for the quantification of coronary calcium. Some American facilities and manufacturers have begun to use mechanical scanners (Spiral/helical/multi detector) to detect coronary calcium and to ‘promote / claim’ this to the medical community as the same ‘heart scan’ they have heard about.  Scoring accuracy, reproducibility, and clinical interpretability are some of the factors affecting Coronary Artery Calcium Scans.

The objective of clinicians measuring coronary calcium is to:

1.    Determine the presence and extent of atherosclerosis

2.    Determine the absence of atherosclerosis

3.    Help determine the risk of a subsequent coronary event

4.    Recommend and monitor patient treatment

5.    Follow the progression or regression of coronary artery disease

In order to achieve this objectives one needs:

1. A reliable and reproducible calcium score

2. An established method of interpreting the score

3. A convenient, cost-effective, and non invasive method of collecting the data

Calcium Scoring Accuracy

The accuracy of artery scanning is dependent upon immobility. Coronary arteries even at it’s most quiescent cardiac cycle moves 10-20mm/sec. hence motion artifacts are important consideration. This motion artifacts contribute to the variability and accuracy of calcium score on coronary artery scanning. EBCT has an exposure time of 100 msec. and is capable of ‘freezing’ the image to produce an accurate calcium score. Mechanical CT that have image acquisition range from 300 to 1000 msec (exposure time), have more motion artifacts thereby contributing to a wider variability of calcium scores, resulting in poor reproducibility

Reliability, Reproducibility and Acceptability

To date, there are a few comparative studies and abstracts published on EBCT and mechanical CT. Becker, et al, (Eur Radiol 1999) , Baskin et al (Circ. 1995;92:651), Budoff, et al (below) have shown that calcific deposits are blurred due to cardiac motions and small calcifications may be missed, hence, poor correlation at lower score and significant inter-scan variability. Becker demonstrated an overall 42% inter-modality variation in symptomatic patients and 91% inter-modality variability in patients with non-obstructive disease. Budoff’s result have shown inter-modality variability of  79.3 %.

A comparative study between EBCT and Spiral CT by Budoff, et al, has shown that spiral CT has a sensitivity of 67%, specificity of 70% and an overall diagnostic accuracy of 74%. There were 5 patients with positive EBCT but negative spiral CT and  vice versa. The positive and negative predictive value were 84% and 58% respectively.

These studies did show some correlation at high calcium scores but the variations of over 100 points on average are far too great to:

a)      track atherosclerosis for progression/regression  

b)      accurately risk assess patients

c)      correctly predict those with or without atherosclerosis (negative predictive value has been very well established in countless studies with EBCT scanner) 

	Mechanical CT scanner	EBCT scanner
Scanning speed	300-800 msec	50-100 msec
Radiation dose	240 mrem or more	60 mrem
Sensitivity	Low for moderate & low calcium score	High
Specificity	Low for moderate & low calcium score	High
Reproducibility	Moderate	High
Negative predictive value	Moderate	Very High
Use in patient management	Poor	Yes
Clinical/scientifically proven	No	Yes
Widely used & accepted	No	Yes
FDA approval	No	Yes

 Clinical Interpretation of Calcium Score Guidelines

Hundreds of EBCT papers have already published in peer reviewed journals over the past 15 years. American Heart Association 1996 statement paper states ‘only EBCT can quantitate the amount or volume of calcium and a negative EBCT coronary calcium imply a very low likehood of significant luminal obstruction’. Arad, et al. (Circulation 1996; 93; 1951-1953) demonstated that EBCT scores above 4 had prognostic significance.

In 1999, Rumberger and other experienced EBCT researchers published a consensus paper (Mayo Clinic Proceeding 1999; 74:243-252) on EBCT’s Coronary Artery Calcium Scanning guidelines.  Budoff, et al. (Clin Cardiol 1999 22:554-558) review paper proposed a treatment algorithm for different calcium scores. These and other published papers formed the basis of clinical interpretation and guidelines for the utilisation of calcium score in assessing and management of patients.

The poor correlation between EBCT and mechanical CT’s calcium score (esp.at lower score) simply does not allow EBCT guidelines and data to be used as the later’s clinical interpretation

If  used similarly, the dire consequences would be:

a)      false reassurance would be greatly magnified in patients with negative results

b)      false interpretation of calcium scores influencing patient’s treatment and management

c)      no applicable prognostic value which is dependent on the calcium score and age-adjusted  percentile