Alignment checks of the light and X-ray fields

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I was doing a first year imaging lab (in radiography course). We were supposed to check the alignment of the collimation light, and how accurate it represents the x-ray "beam". One of the question in the task question is to determine "the x-ray fields (express this answer in cm and a percentage of the SID)"

The bold part got me puzzled, x-ray field: isn't it supposed to be area of the x-ray? then how do we measure x-ray in cm?. Well, I think I gotta be wrong on that assumption but my head doesn't give me any other suggestion. I've looked up 4 different book about medical imaging but none mentioned the matter.

I've attached a copy of the prac sheet to make it clearer.

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There are two things this test checks for: x-ray and light field congruence (how far off is the center of the radiation field to the center of the light field) and x-ray/light field size (are they the same size).

The light field is supposed to represent the area that is supposed to be irradiated by the x-ray beam. This means that the center of the light field and x-ray field should coincide. Because of how the light field is projected, this can be affected by the position of the light bulb, the type of light bulb and the position/angle of the mirror. In the US, the x-ray/light field alignment needs to be within 2% of the SID (~2 cm for a 40"/101.8 cm).

The other part of the test checks whether the size of the light field is the same as the size of the radiation field produced. If you set the collimators to produce a 24x30 cm light field, the radiation field produced should also be 24x30 cm. In the US, the difference between the length/width of the light field and the radiation field needs to be less than 2% SID, and the sum of the length and width differences needs to be less than 4% SID.

In the questions part of your worksheet, the second and third line are both part of the same question, not two different questions.

Tip: When you do the test, use the small focal spot. It will give you better definition of the collimator edges.

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