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There are other fundamentals which I want to share here.

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This is a scan diagram of a CT scanner,

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a side view. I just want to define a couple of things because this

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impacts a lot of the image quality aspect.

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If you look at the CT scanner, the gantry of a CT

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scanner is typically between approximately 70 centimeter in

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diameter.

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In radiation oncology, we have even wider bore, that is suffice to

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say. Generally, all these CT scanners are 70

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centimeter in diameter.

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However, the image reconstruction is defined to

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less than the actual gantry opening.

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We call that as a maximum sampling region.

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The maximum sampling region is approximately 50

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centimeter in diameter.

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That's the reason why you see sometime on obese

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patient, the shoulder or hip chopped up because the

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image reconstruction goes up to this point.

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Beyond which it's not, it's chopped off, and therefore, any

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object inside this max sampling area is what result

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in the image reconstructed.

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In addition, we also have what is called as user-defined

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scan field of view.

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This is usually less than or equal to the maximum scan

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sampling.

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The other factors which you need to remember is the CT table these

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days will have a very high patient load

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capacity. They can take up to 500 pounds of

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patient. The reason why it's challenging because when the

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scanner table moves inside, it doesn't collapse, it doesn't

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fluctuate like it can't deliver. It remains stable,

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and that depends on the technology, how they're able to hold it.

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So

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in this diagram, the XY is defined as the axial plane,

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and the Z is called the in and out of the gantry or the Z direction.

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So here is an example. In radiation therapy,

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we have what is called as the wide bore,

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which is slightly larger than this

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particular size,

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and we also have what is called as extended field

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of view.

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Because with the current field of view, the object will be chopped

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off here, but since CT simulator is used for cancer

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treatment, they want the visibility of the entire anatomy,

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so the extended view usually goes beyond the normal

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limit. The accuracy of the CT number is not that good

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compared to the other one area, but still it helps too, and this

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is called the extended field of view.