Friday, February 24, 2006

Heart Tests: Coronary angiogram - Non Invasive

Here is part 21 of the heart of the matter. Part 20 is here and the dislcaimer is here.

This is a picture of a 64 slice multi-slice CT coronary angiogram

Computerize tomography is a form of X-ray imaging of the heart. It involves taking serial X-rays of the heart, thinly slicing the heart using an X-ray knife, then re-constructing them (3D reconstruction) with the use of sophisticated computer software, to form a virtual heart. The pictures indeed look seductive.

Heart arteries can be targeted by localizing the arteries with contrast dye. We can then also re-construct the arteries. This sounds simple enough, except that it is not quite so simple. Is what you see, the true picture? Or is it not?

One of the biggest problem with sequential X-rays of the heart is motion. The heart moves, from about 70 beats per minute, to anywhere near 100beats per minute when the subject is excited, anticipating the CT scans. The increased heart rate will cause motion artifacts (the X-ray camera is filming slower then the target object, that is in motion).

The other technical problem, is localizing the heart arteries with the dye. Without the dye, the X-ray camera cannot differentiate the arteries. Since the dye is injected into an arm vein, it takes time to circulate to the heart artery, and in the process, also becomes diluted, so that the localizing of the heart arteries, will not be as precise as the selective coronary (invasive) angiogram.

The other obvious difficulty in heart artery imaging is the tortuous, twisting and winding course of the heart artery. No two persons have the same size artery, running at exactly the same course. More so when we know that Asian arteries are smaller then Caucasian arteries, and maybe more calcified.

Therefore, when the CT scanner takes pictures of the heart arteries at standard projections (guided by software meant for Caucasians), it may X-ray slice arteries at uneven places, giving false information. The arteries may contain speckles of calcium which cast interfering shadows called "calcium artifacts", or the heart arteries may be small, and cutting it unevenly, may cause impression of false blockages, or miss blockages that are there.

The patient is under the X-rays scanner for about 20-30secs. That's how long it takes for an average scan, and the rest of the data is acquired post-procedure, (off-line heart artery reconstruction). The reconstructions accuracy is heavily dependent on the adequacy of the initial on-line scan acquisition (poor initial scan acquisition means poor data input, and so false picture reconstruction. Garbage In Garbage Out is something they still teach in computer schools).

Suffice to say that there is a " professional learning curve " required, to try and learn the skills of proper scan acquisition for our Asian arteries, which are somewhat different, from the Dutch, German, or American arteries. We all want to see our arteries to make sure that they are okay, but be aware that you may see a picture of your heart arteries, but what you see may not exactly be the true picture of your arteries, given all the artifacts and limitations and assumptions made by the scanner software.

Basically, the multi-slice CT angiogram, is an evolving technique, and even now, refinements are being made. The very first scanners were single slice CT, mainly to assess calcium score. Then came the 4 slice CT, then the 8 slice CT, then the 8slice, then the 16 slice. The current scanners in Malaysia are mainly the 64 slice. The 128 slice scanners are already on display waiting to be marketed. I hear that a 256 slice prototype scanner by Toshiba will be exhibited soon.

The progress in technology is amazing. The high radiation exposures are being looked at critically. Suffice to say that the early machines exposes the subjects to (for a CT coronary angiogram) the equivalent of 500 chest X-rays. A total body CT scan, exposes the subject to almost 2,000 chest X-rays. Certainly not small amount of radiation.

To slow the heart rate (to avoid motion artifacts), the subjects are given beta-blockers. Some subjects can't tolerate the beta-blockers and fatalities have been recorded.

Calcium artifacts are difficult to overcome.

Poor on-line technique cause some scans to be inadequate for interpretation, or some sections of the heart artery to be inadequate for interpretation. In fact, some medical studies show that as much as 5-10% of heart artery segments may not be assessable using this scan technique.

Perhaps one of the biggest issue with this CT angiogram, is the lack of medical data, what with these new machines coming out so fast that it's almost tempting to belief that such upgrades are driven by sales and marketing, not so much by medical need. "Evidence based medicine" type doctors find it difficult to recommend the use of this machine for general population screening because of a lack of medical evidence of it's usefulness and it's dangers We believe that it does have a role to play in the assessment of CAD but surely not population screening of healthy adults.

Although invasive coronary angiogram is invasive, but with advances in technology, it too has become minimally invasive but remains definitive and is still the "gold standard" for the diagnosis of CAD.

To summarize

  1. The invasive technique is invasive as it's name implies. MSCT is non invasive.
  2. The invasive technique has a 100% accuracy for both positive and negative prediction purposes. The best MSCT shops are scoring around 92%-95% (postive) and 95% (negative) in terms of predictive value.
  3. The radiation risk of invasive angiograms are less and the exposure risk for MSCT can only increase.
  4. Invasive techniques avoid motion and calcium artifacts in the image. MSCT carries these artifacts. Worse yet, it may not be obvious since the image is post-procedure processed by software not customised for your ethnic group.
  5. The cost of an invasive angiogram is more at approx RM 4000 (USD 1000) with the cost of MSCT is at around RM 2700 (USD USD 700).
  6. The invasive equipment you use is probably mature and any changes would be minor. The MSCT equipment keeps on changing at an alarming rate.

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