What's all the hype behind 64-slice scanners?
TYP LIBRARY
64-slice CT scanners featured on Oprah! Just what does this mean? Do these new devices provide any advantages in your heart disease prevention program?
The October 19th, 2005 episode of the popular Oprah Winfrey daytime talk show featured an hour-long conversation about the new 64-slice CT scanners. Oprah's guest, thoracic surgeon Dr. Mehmet Oz, gushed, "If you think about it, the CAT scan takes a picture. If it's your gallbladder, it just sits there and waits for the photo shoot. But to really capture what is happening with a heart, you need a scanner that can take pictures so fast it can catch the heart in a beat. These new scanners can take almost 200 pictures a second so they can get your heart at so many different angles that you capture it. What makes this technology so cool, is you can actually go beyond calcium which is just a symptom."
Is this true? What's the advantage of such rapid imaging? Is calcium just a "symptom" but not the disease itself? Should we all run out and get a 64-slice scan? Dr. Oz was unclear on this issue when asked who should consider such a scan. He stated that if you are willing to make changes in your life, then you might consider one.
What's the real story?
We are thrilled that the concept of CT heart scanning is gaining a broad audience. If it makes it to Oprah, it's not so much groundbreaking as mainstream. Oprah, after all, is hardly a forum for scientific discourse. But it is a powerful mainstream media outlet.
So what's the real story with the 64-slice scanners?
Our view: the 64-slice scanners are the latest and greatest version of the multi-slice scanners first introduced around a decade ago. The earliest versions of these devices were called spiral or helical scanners because the x-ray detector needed to rotate around the body part being scanned to generate a 360-degree image. The problem was that it required time to revolve around the body, usually a second or two. This simply was not fast enough to image a moving object like the heart. The heart moves several times through several phases of motion in that one or two seconds of time.
Design engineers therefore provided a solution by building scanners with more than one x-ray detector in these devices: initially 2, 4, 8, 16, and now 64-detectors. This allows faster imaging with less radiation (though still not negligible; see below). There's no argument that these devices represent an improvement over its predecessors like the 16-slice devices. Preliminary studies suggest that angiograms using x-ray dye do indeed provide detailed images of the coronary arteries.
Does this detailed imaging potential mean we should throw away plain old heart scans and all get 64-slice coronary angiograms? After all, Dr. Oz says if you're interested in changing your lifestyle, you should consider having one.
Well, consider:
* First and foremost, angiography, regardless of how it is performed, merely determines if a blockage is present. Unfortunately, 90% of all heart attacks are the result of plaque ruptures at sites that have minor blockage. Calcium scoring detects these silent killers.
* The coronary calcium scan is quantitative. In other words, you get a specific and precise score that tells just how much plaque your have. Recall that, although calcium is being measured, calcium is simply a means to measure total plaque since it consistently occupies 20% of plaque volume. A coronary angiogram, regardless of which device is used to generate it, is non-quantitative. The best you could do is say that there's a 40% blockage at one site along an artery. This is not a full longitudinal quantification of plaque.
* Most people do not need screening angiography that requires x-ray dye and a substantial quantity of radiation, even with the 64-slice devices. Estimates vary, but you're exposed to more radiation than a conventional heart catheterization by going through a 64-slice angiogram (the same as about 40 or more chest x-rays!). Yes, it's a wonderful technique, but angiography is not suited to be a screening test. Heart scanning to obtain a heart scan score also subjects us to radiation, but a relatively modest quantity suitable for a screening test. <
* Many centers acquiring 64-slice devices are hospitals. Hospitals as a general rule are not interested in prevention. They are interested in generating more heart procedures like bypass surgery. Shockingly, even though the 64-slice scanners are able to obtain heart scan scores, many of these centers don't really care about coronary calcium scoring. They only want the angiograms, since these often lead to costly procedures. Don't count on helpful counseling after your scan if it comes from a hospital.
64-slice scanners can provide calcium scores just like any other scanner. But that capability seems to have been forgotten in the excitement over improved angiographic imaging.
In all practicality, the 64-slice scanners provide reliable calcium scoring just like any other scanner, both multi-slice and EBT. Angiographic capability is indeed enhanced, but this should not prompt you to get a coronary angiogram. It's simply not necessary in most cases.
Many scan centers have been able to perform confident non-invasive coronary angiograms in patients for several years on the EBT and 16-slice multi-slice devices. When are such studies truly indicated? Most cardiologists familiar with the technique would agree that they can be useful when:
o A stress test (e.g., stress thallium or equivalent) is equivocal—a common situation. In other words, the stress test shows a questionable, though potentially important, abnormality.
o Symptoms are unusual (e.g., shoulder or abdominal pain)
• Follow-up of persons with prior procedures, e.g., bypass surgery, and perhaps stents (though stent imaging is still a difficult hurdle due to the steel)
o Congenital anomalies are suspected—CT angiography, in fact, is great for this question
The "virtual" images provided by CT coronary angiography, are a tremendous innovation and offer a non-invasive method of detecting blockages. Unfortunately, the majority of heart attacks occur at sites with little or no blockage and calcium scoring remains the best method of detecting this risk. CT or "virtual" angiograms also suffer from many of the same limitations as virtual colonoscopies. If a polyp is imaged that needs to be removed, a real colonoscopy needs to be performed. Likewise, if a severe blockage is revealed in the coronaries, a real angiogram and perhaps angioplasty/stent, etc. may need to be performed.
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