Helical CT

Obtaining slices one by one, each time moving the patient slightly, is a relatively slow process which becomes an issue in hospital efficiency requiring more dedicated time to monitor a single CT scan. Alternatively, a method of CT scanning since the late 1980s involves moving the table continuously while the X-ray apparatus revolves; effectively tracing a helical beam path on the patient as seen in the figure to the right. This method is called spiral/helical computed tomography. In helical CT scans, the pitch a dimensionless quantity that is defined as the ratio of the table displacement (d) per revolution of the X-ray apparatus over the slice thickness (S) or

The pitch is useful in determining the radiation doses in a given field-of-view. For example, a p > 1 indicates fewer projections of the patient’s tissues within the FOV and therefore a lower image quality (decreased Signal-Noise Ratio) but benefits from exposing the patient to a lower radiation dosage. The opposite is true for p > 1 where the output image possesses a higher SNR with a trade off that a volume of tissue is irradiated in more than one revolution. Selecting a suitable pitch can depend on many factors including the examination type (eg. chest imaging), and the patient’s age, size and medical history [1].

Table feed per gantry revolution, d

Width of the active detector array, S

A trace of the path the X-ray tube travels as it scans along the patient's abdomen. The width of the active detector array is equivalent to the slice thickness. (Source: Healthcare Technology Management)

References:

[1] Smith N. B., Webb A. (2010), "Introduction to Medical Imaging", Cambridge University Press Textbooks.