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.
