A variety of methods are used to describe and control the radiation output of a CT scanner. Different approaches may serve to emphasize one facet of a particular scan over another, favor the lexicon and style of a specific manufacturer or simply reflect history. The multiplicity of terms often adds confusion to what should be a simple task: to characterize the output of a CT machine in terms of the radiation dose to a cylinder. We will begin by examining radiation dose to cylinderswith an emphasis on isolating superfluous parameters.
We start, as in TG111, with an infinite helical scan of an infinitely long cylindrical phantom. During such a scanthe dose at any point in the cylinder will asymptotically build up to its ultimate value Deq. The spatial average of this value, , can serve as an indicator of scanner output and along with the total number of rotations N determine Etot, the total energy absorbed by the cylinder. If a finite cylinder is long enough ( ≥Leq), a scan through it from one end to the other will yield these values in its central plane.
The approach to Deq can conveniently be described by h(L), a robust function of scan length L which depends only weakly on z axis collimation and tube potential. This function facilitates the simple calculation of the lower dose to the central plane resulting from a finite scan. If the length of the cylinder is finite, as it is for the standard CTDI phantom, the dose is reduced further in a predictable fashion resulting in good correlation between Deq,bar and CTDIvol which in turn has been shown to be a useful tool in equating the radiation output between scanners differing in model and vendor.
Though CTDIvol (and Deq,bar) are useful for characterizing CT machine output, we need to include the effect of patient size in assessing patient dose. TG204 has used measurements on phantoms of varying size to determine a size specific dose estimate (SSDE) which serves to accomplish this task. The SSDE has also proven to be very helpful in designing protocols for patients of varying size, especially pediatric patients.
Objectives:
1)To describe the radiation dose to a cylinder in uncomplicated terms, starting with the particularly simple case of an infinite scan to an infinite cylinder.
2)To extend the description to finite scans to infinite cylinders in terms of the h(L) function and then to the standard CTDI phantoms.
3)To describe some of the subtleties in making reliable CTDI measurements.
4)The demonstrated fact that dose indices (e.g., CTDIvol) provide a reliable indicator of machine output will be used to show, for example, how patient scans from machines of varying model and manufacturer can be compared.
5)To describe how characterizing the dose to cylinders of varying size can be used both to provide a reasonable index of patient dose and as a useful tool in the design of pediatric protocols.