Sunday, July 22, 2007

Calcium Score: Part 1

Everyone has heard of atherosclerosis; it is the term for the cardiovascular disease that is the outcome of atherogenesis (the accumulation of plaque in the arteries). An atheromatous plaque forms due to atheroma or the accumulation of lipoproteins (cholesterol) and connective tissue inside the arterial wall. Shear stress and other factors that thin the arterial wall can sometimes cause the wall containing the ‘vulnerable’ plaques (or soft plaques) to rupture. If the rupture clogs the coronary arteries, this can result in ischemia or even a heart attack (infarction of cardiac tissue).

With age, if the soft plaques do not rupture, the layer between them and the vessel wall near the lumen can undergo microcalcification, and in general, create calcium deposits in the arterial wall. As a result, the amount of calcium in the coronary arteries is an established number for identifying the severity of coronary artery disease. The number is commonly referred to as ones calcium score and anyone who gets a CTA (CT angiography) done can obtain their calcium score from their physician. However, one should be cautious since the distribution of the calcium in the coronary arteries (whether the calcium is diffused or focused) also plays an important role. Furthermore, the number does account for the number of soft plaques or vulnerable plaques, which in fact pose a greater risk since they have the potential to rupture. Evidently, there is no index out there that quantifies the vulnerability of plaques since they are difficult to identify using most imaging modalities or software. In fact, the software that I’ve been using only quantifies calcium in the coronary arteries.

In particular, I’ve been using software developed by GE called Smart Score (figure) to assign patients a calcium score. That is, I’ve been highlighting the calcium in the arteries using CTA and differentiating the calcium for the left anterior descending artery (LAD), left circumflex (LCX), and the right coronary arteries (RCA). Doing this is pretty straight-forward. I just click on a button labeled LAD if I want to highlight the calcium deposits in the LAD, RCA for calcium deposits in the right coronary artery, and LCX for deposits in the left circumflex. The software automatically segments the calcium deposits within the region I highlight using a simple thresholding algorithm. When I do this for all of the transverse slices of the heart, I record 8 numbers. One number corresponds to the overall volumetric score or the total volume of calcium in the coronary arteries, and three other numbers for the volume in the LAD, LCX and RCA. Similarly, an older and more commonly used number for calcium scoring, known as the agatson number, is recorded. The agatson number, unlike the volumetric score, scales the area of segmented calcium in each slice by the average Hounsfield value/ brightness of the segmented region. I believe this older method was developed to account for partial volume effect in CT images. In fact, even though the volumetric score is more accurate, the agatson score is still used due to legacy. I guess it doesn’t really matter. I have looked at both numbers for 200+ patients, and I can say that they don’t differ by a concerning amount.

I guess some of you might be wondering why I’m collecting this data in the first place. I’ll discuss the details of that later this week.

No comments: