There’s nothing better than seeing the insides of the body part you are interested in in fine detail. When you use CT a lot, you get to know about how physics affects your image quality. Once everyone has a 64 slice helical machine, we won’t have to worry about tweaking the protocols to get the most out of a study, but in the meantime, a little knowledge about helical CT physics will help you get the best scans out of your machine.
Helical CT is quite a revolution from the axial scanners that preceded it. The x-ray tube spins continuously as the table moves past it, rather than the spin/stop/table advance of earlier generations. Helical CT lets you scan much faster because of continuous table motion. The standard rotation time for a single slice helical scanner is 1 second, but newer multislice machines are down to half a second. The way to describe the table increment/slice thickness is pitch. For instance, if the table moves 10 mm during that singe rotation of the gantry, and the collimation is 5 mm, the pitch is 2. Increasing pitch allows you to scan even faster, as the gantry doesn’t have to complete a 360 degree rotation around the patient to create an image. This is a big advantage in scanning large areas, or those prone to motion such as the thorax.
So why not scan at a pitch of 5 and be done in 10 seconds? There is a limit to how far the CT can “interpolate” the spiral 3D data it collects, or average the data into a perpendicular slice or image. The gantry needs to rotate at least halfway around the patient to get data from all sides. So the upper limit of a single slice unit is a pitch of 2, but even so you will get some artifacts. A pitch of 1.5-1.7 will not result in loss of image quality, and you can use it to your advantage.
The two things you can gain with increasing pitch are
1. Increased scan speed – same volume covered in less time (less chance of respiratory motion artifact)
2. Thinner slices in the same time – If you increase the pitch, you can decrease the slice thickness or collimation, and scan the same volume in the same time.
Since thinner slices and faster scans are almost always good, use pitch to your advantage.