Figure 1: Cardiac catheterization lab setup
In this picture, we see the table that the patient lies supine on during the procedure. The image intensifier (which detects the X-rays to form an image) is anterior to the patient, and the X-ray source is posterior to the patient. The concept of backlighting is very important in developing an intuitive understanding of the images.
Because the image intensifier is what collects X-rays to create an image, we will represent it with a camera. Similarly, the X-ray source will be represented by a light bulb.
Figure 2: Schematic illustration of C-arm
Because of this backlighting, we are really looking at “shadow puppets” of the coronary arteries.
<aside> ⭐
The further an object is from the image intensifier, the closer it must be to the x-ray source. Counterintuitively, this means images that are further from the image intensifier actually appear larger!
</aside>
This key concept is illustrated with the graphics below:
Figure 3: Object far from light source appears relatively small
First, the blue star is relatively far from the light source, resulting in a relatively small shadow.
Figure 4: Object near to light source appears relatively large
Next, we move the blue star relatively closer to the light source, resulting in a larger shadow, even though it is further from the source
To see this concept in action, consider the following cineangiogram:
Video 1: Left anterior oblique caudal angiography
Video 1: Left anterior oblique caudal angiography
Here is a still image with the LAD and LCx labeled.
For some, this intuitively appears as if the proximal LAD and LCx are close to the viewer and move further away (i.e. “dive into” the screen) as the vessels course toward the apex. However, this image is actually obtained by viewing the heart from the apex. Therefore, the left main, proximal LAD, and proximal LCx are actually the furthest from the viewer, and the distal vessels are closest (i.e. they are “emerging from” the screen).