0:00

Finally, let's talk briefly about X-ray geometry.

0:05

There are a couple of standard definitions.

0:08

SID means source to image distance.

0:10

As I mentioned, for tabletop radiography, the SID is 40

0:14

inches or 100 centimeter approximately.

0:17

Source to object distance. Source to object distance, what is the

0:21

source to where the patient is?

0:24

And object to image distance.

0:27

Why is this important? This geometry becomes important in

0:31

terms of how much the beam get diverged, the patient image get

0:35

magnified, or if there is any unsharpness caused

0:39

due to the positioning, and so forth.

0:42

In this geometry,

0:44

the object here,

0:46

and there's a gap before the image receptor.

0:50

In this case, the focal spot is very small.

0:53

Because of that, there is very less penumbra,

0:58

which means this is very sharp edge image is created.

1:03

On the other hand, if there is focal spot

1:07

slightly larger than the point source,

1:10

the X-rays hitting here has from different

1:14

area that will cause a fuzziness around the edge.

1:18

There is a geometric unsharpness is caused.

1:22

If it is a high magnification, you have the X-ray tube

1:26

the same location, but the patient is moved up closer.

1:29

At this time, the image looks like this.

1:32

There is a central core, and then there is a shadow

1:35

region. And that will determine the sharpness,

1:39

which depends on the focal spot.

1:42

The intensity of the X-rays also varies where the object is,

1:47

and that goes by the inverse square law.

1:49

The X-ray intensity is inversely related to the square of the

1:53

distance from the source.