Wednesday, July 15, 2015

Computerized Tomography

Computerized Tomography or CT scan is a marriage between x-ray and a computer. This has several advantages over x-ray. First the images have much more detail than plain film.

Second, the data can be manipulated to depict various planes through the body – frontal, sagittal, coronal, etc. - much like MRI. They can even be viewed in 3D. Finally, the level of x-ray exposure (ionizing radiation) is much reduced.

CT scan, just like plain x-ray, differentiates tissue based on density. The more dense the tissue, the more x-ray is absorbed, the less strikes the computer receptors, the lighter the shade. So bone (very dense) shows up as white and air shows up as black. All the other tissues are in between, showing up as varying shades of gray.

Again, just like x-ray, CT scan sees bone extremely well and the absence of bone (think bone tumor or fracture) is readily seen in contrast. By comparison, MRI sees soft tissue very well, but does not image bone readily. I’ll review MRI in detail next week.

Motion is a real issue for all imaging studies. X-ray solves this issue by taking single exposures in a tenth of a second. CT scan and MRI have issues in this regard as they collect data over a long period of time. So tissues that move, like the heart, lungs, and the digestive tract cannot easily be captured. Blood will appear black as it courses through the arteries and veins of the body.

High speed CT scans eliminates motion allowing clear images of moving tissue. This study can visualize calcium deposits in the artery walls of the heart. The radiologist rates the amount of calcification as the “calcium score”. High calcium scores have led some physicians and many patients to fear calcium intake as a cause of heart disease. However, that fear is misplaced.

Calcium is laid down in the pre-existing plaque in the artery walls as it becomes loose or “vulnerable”. This is a defense mechanism by the body to strengthen the binding capacity of the plaque so it is less likely to break loose, creating an embolism. Emboli that travel to the heart cause heart attacks, those that end up in the brain create strokes. If the emboli travels to the lung, it is labeled as a pulmonary embolism. The calcium actually decreases the chances of these events. However, the presence of calcium in plague does indicate that it has become vulnerable and that the risk of CVA (cardiovascular accident) is high.

Cardiologists and insurance companies have been very slow to accept the findings of high speed CT scanning. I can only assume that is because it is a very inexpensive test (under $200) that can often replace a full cardiac workup which costs several thousand dollars. However, if you have a high speed CT scan of your heart and present it to your cardiologist, they cannot ignore the results.

The short fall of CT scanning is missing soft tissue findings as noted above. If you physician believes you have some physical change in an organ, gland or other soft tissue, they will probably order an MRI. However, if a bone problem is suspected, then CT scan is the test of choice. CT scan is also much more cost effective than MRI.

The Bottom Line:
When your physician recommends imaging studies, please ask why they prefer one study over another. If you have a history or family history of heart disease, please consider having a high speed CT scan of the heart to generate your calcium score. The test is inexpensive and might provide valuable insight to your cardiac health.

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