CT scan series part 11 – Inside the “Donut” - How many scans ...?

[ThreeWs]

When a CT scan is ordered for you, think of it as a “Session” because unless you ask you won't know how many actual scans you are getting. A typical CT session may be one, two or three scans. In certain machines you will be as a loaf of bread being “sliced” by bursts of radiation where each slice is typically 5mm thick. There are also machines that use a spiral use of radiation to generate the required images. These may be referred to as Helical CTs and are newer and faster but do not necessarily use less radiation than other non helical machine designs. There are also single & dual source configured machines referring to the number of places in the scanner the radiation is projected. The key to lowest dose in radiation exposure is not in the hardware but in the software used for reconstructing the data into the final image. Dual source machines and helical designed CTs are newer and can be assumed to operate with low dose software but some single source CTs can operate with low dose software also. Older machines may or may not have the capability to be upgraded for use with low dose software. You need to question this.

Ionizing radiation exposure for you will be determined by your body size and weight and what tissue is actually scanned. A scan of just your head/neck will not require as much radiation as other softer tissues.

From the 2008 AAPM report 96 - 3.8 Effective Dose (E)

“A 100-mGy dose to an extremity would not have the same potential biological effect (detriment) as a 100-mGy dose to the pelvis35.” In section 4.4 Size-or Weight-based Technique Charts:

“CT numbers represent a fixed amount of attenuation relative to water) ensures that the image always appears properly exposed. As a consequence, CT users are not technically compelled to decrease the tube-current-time product (mAs) [milli Amperes] for small patients, which may result in excess radiation dose for these patients. It is, however, a fundamental responsibility of the CT operator to take patient size into account when selecting the parameters that affect radiation dose, the most basic of which is the mAs12,14”

Each Clinical Trial CT-scanning session may typically be 3 CT-scans with two overlapping areas of tissue comprising the Aortal Arch and Upper Lung tissue which lies between the head/neck scan and the chest scan. Scanning overlap of kidney/liver organs lie between the Chest scan and the Pelvic/Abdominal scan. Both overlapping tissue areas will receive, on the older machines with higher radiation dosing, between 600 and 900 chest X-rays of radiation. OK..., so how many scan sessions can the Clinical Trial patient expect to have in the course of a Trial? Because some of the sessions may only be one scan with no tissue overlap or two scans with one tissue overlap, that number is hard to answer precisely but I will provide a real life example with some hypothetical variation dependent on model of CT scanner and software used in image reconstruction. More later.

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