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9.1 Introduction Helical CT (also called spiral CT) was introduced commercially in the late 1980s and early 1990s. Helical CT has expanded the traditional CT capability by enabling the scan of an entire organ in a single breath-hold. It is safe to state that helical CT is one of the key steps that moved CT from a slice-oriented imaging modality to an organ-oriented modality. The difference in the naming convention between helical and spiral CT is due mainly to different CT manufacturers. For all practical and technical purposes, there is no difference between the two. To avoid confusion, we will use the term “helical” throughout this chapter. 9.1.1 Clinical needs All previous chapters have focused on a single scanning protocol: the step-and-shoot mode. This scanning protocol contains both a data acquisition period and non-data-acquisition period. During the data acquisition period, the patient remains stationary while the x-ray tube and detector rotates about the patient at a constant speed. Once a complete projection dataset is acquired for the slice, the non-data-acquisition period starts. The x-ray tube is turned off and the patient is indexed to the next scanning location. For typical CT scanners, the minimum non-data-acquisition period is on the order of seconds as a result of both mechanical and patient constraints. The mechanical constraint is due to the fact that a typical patient weighs over 45 kg, and the patient table requires a certain amount of time to move a large mass from one location to another. The cause of the patient constraint may not be as obvious. From the law of physics we know that to move a resting object over a short distance, first we must accelerate the object up to a certain speed and decelerate the object when it is near the target location. Since the distance between adjacent scanning locations is typically a few millimeters, the amount of acceleration and deceleration is fairly large. A human body is not rigid (the internal organs can move and deform), so the acceleration and deceleration will likely induce motion in the patient. As a result, a certain amount of time must elapse to minimize motion artifacts. In the late 1980s, the CT scan speed approached one second per revolution.
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