Magnification in radiography occurs because the x-rays are divergent or spread out from the x-ray source. Therefore, the object will appear larger on the detector than the true object size. Magnification in radiography is defined as (Image Size/Object Size) and is equal to the (SID/SOD) which is the source to image distance divided by the source to object distance.
For more details and examples please visit our page on magnification: howradiologyworks.com/magnifi...
Chapters
00:00 Intro
00:30 Magnification Principle
02:21 Magnification Definition
03:52 SID/SOD
05:05 Image Size/ Object Size
05:54 Magnification Off-center
06:49 Outtakes
We will start with an analogy that almost everyone will be familiar with from their childhood or parenthood. If you have a flashlight pointed at the wall you can make a shadow puppet with your hand such as a bird or a bunny.
Remember that as you move closer to the flashlight, the projection of the bunny is going to be larger on the wall. Then as your fingers move further away from the flashlight, the projection of the bunny is going to end up smaller. This process is called magnification as objects closer to the source are going to be larger (if the image position is kept fixed). Since visible light and x-rays travel in straight lines the magnification phenomena is the same.
The light comes out of a light bulb or flashlight in all directions is called a divergent. In the same way the x-rays coming out of the x-ray tube are divergent as well (i.e. going in all directions within the collimated region).
Instead of a shadow puppet we are concerned with imaging something that is residing inside of the patient. We can think about an object that is in a plane within the patient parallel to the imaging receptor (e.g. the detector). In reality we are concerned with making images of the heart, lungs or bones but lets start by thinking about a simple object with is just a straight line as this is simplest to think about.
The definition of the magnification is the relationship between the object plane and the image plane. The magnification is defines as the (Image Size)/(Object Size). Since the x-rays are spreading out (i.e. diverging) the magnification will always be a number that is greater than 1.
How is magnification in Radiography dependent on the object position?
Since we often have control of the magnification in radiography exams it is important to understand the distances that control the magnification. In the figure below we define the Source to Object Distance (SOD) and the Source to Image Distance (SID) (note sometimes you may see others call this the Source to Detector Distance (SDD), this is referring to the same distance).
In the figure below you can see that we can make one triangle that has the object size and the SOD, and another triangle with the image size and the SID. As shown these triangles are similar triangles. The SID is proportional to this image size, in the same way that this SOD is proportional to this object size. Therefore, the ratio of the sides are equal, i.e Image Size/Object Size = SID/SOD. You can see that this is our definition of magnification, so the magnification=SID/SOD.
We can also solve this relationship for other variables. Most typically we would want to solve for the image size or the object size.
For instance, if you are taking a chest radiograph and the true object is 10mm long what will be its length in the image plane if the SOD is 170 cm and the SID is 180 cm?
The image size=(SID/SOD)*Object Size, which will be (180/170)*10 = 10.6mm. From this is example you can see that typically chest radiography will have a relatively small magnification and the size of the object in the image plane will only be slightly larger than the true size.
Another example would be an interventional angiography scenario where the SOD is 50cm and the SOD is 100cm, with the same object length of 10mm.
For this case the image size=(SID/SOD)*Object Size, which will be (100/50)*10 = 20mm. From this is example you can see that typically chest radiography will have a relatively small magnification and the size of the object in the image plane will be significantly larger than the true object size.
At a high level these clinical radiography scenarios have a low magnification as it is typically desirable to have the detector fairly close to the patient:
Chest Radiography
Mammography
Abdominal Radiography
On the other hand these clinical scenarios typically have a high magnification:
Interventional Radiography
Angiography / Cardiac Cath labs
Magnification Mammography