Determination of Coronary Calcium

Agatston Score

The Agatston score, the original means by which coronary calcification was quantified, was derived from the use of electron beam CT (EBCT) scanning. Calcium scoring by EBCT and cardiac CT, though, are not wholly equivalent.

Most of the literature on coronary calcification was derived using Agatston scoring on now-obsolete EBCT equipment. The determination of coronary calcification by other scores has less presence in the literature.

The Agatston score was derived from two-dimensional axial scans with 3-mm slice thickness, with calcium weighted according to its density as determined by HU (higher weight for greater HUs).

The first problem encountered with calcium scoring by the Agatston EBCT method was reproducibility: its accuracy was ±14% to ±51% (±40% average). Reproducibility even by now-obsolete four-slice CCT appeared superior: ±20%, and by 16-slice CCT reproducibility was even greater: ±13%. The better reproducibility of Agatston score by CCT may be the result of better spatial resolution.

The Agatston score is vulnerable to the partial volume effect. Phantom studies suggest that body size and level of scanning influence attenuation and calcium score. As the intra- and interobserver variability of coronary artery calcium scores by EBCT technology are low, ECG gating should be used to reduce the variability.

Agatston calcium scoring is influenced by the different equipment used for the scan.

In general, the literature suggests that coronary calcium scores lower than 400 by CCT and EBCT are similar but that older-generation CCT may not yield the same correlation as do more contemporary CCT scanners.

Problems with the Agatston score include:

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  Susceptibility to partial volume effects

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  Submitral annular calcium sampling

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  Effects of slice thickness

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  Effects of cardiac motion

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  Very long breath-holds needed in EBCT systems

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  Variability depending on the type and make of equipment used

Use of average density improves reproducibility.

Calcium Volume

The calcium volume score is the product of the number of voxels exceeding threshold multiplied by the volume of a voxel. It is theoretically independent of slice thickness—an improvement over the Agatston score. Calcium volume appears more reproducible than Agatston score: ±14%. However, calcium volume does not have as much literature to back it up.

Calcium (“Hydroxyapatite” or “Mineral”) Mass

Use of calcium or hydroxyapatite (HA) mass as a measure of the total amount of calcification offers improvements over the Agatston score but does not have as much literature to support it. It has become a preferred means of describing coronary calcium because it:

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  Automatically adjusts/corrects for partial volume effects

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  Is less affected by scan parameters

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  Has the best reproducibility: ±9%

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  Correlates well with Agatston score: r ² = 0.97, y − 0.1922 x ( Table 14-1 )

  TABLE 14-1

  Variation of Agatston Score and Mass Score by EBCT and CCT

  	EBCT	CCT PROSPECTIVE	CCT RETROSPECTIVE	CCT RETROSPECTIVE	CCT RETROSPECTIVE
  Width	3 mm	4 x 2.5 mm	4 x 2.5 mm	4 x 2.5 mm	4 x 1.0 mm
  Agatston Score
  Mean	35.5	20.8	21.6	24.1	28.8
  σ	3.3	5.8	6.1	4.8	1.6
  CV(%)	9.3	27.9	28.3	19.8	5.9
  HA Mass
  Mean	6.3	3.9	4.2	4.8	4.9
  σ	0.3	0.9	1.0	0.8	0.2
  CV(%)	5.3	22.2	23.9	16.1	4.1

  CCT, cardiac CT; CV(%), mean coefficient of variance = σ/mean ¹⁰ ; EBCT, electron beam CT; HA, hydroxyapatite.

Mass score is 0.2 times the Agatston score.