Formation of the radiographic image

Question 1

Describe the process of X-ray formation. 
Mention the roles of:
* Cathode filament
* Transformers
* Vacuum chamber
* Anode
* Lead shielding
* Aluminum filter

Answer 1

The step down transformer sends current to the cathode filament which produces electrons.
The filament metal has a high melting point, and thus can be heated to a high degree. Electrons accumulate on it.
The filament is heated, and electrons are “boiled off” aka thermionic emission.

The cathode and anode are connected to transformers. The purposes of this connection is that it creates a high voltage field within the X ray tube, which accelerates electrons. This electric field accelerates electrons toward anode.

Meanwhile, the vacuum chamber removes obstructions to electrons. Air molecules block the path of electrons toward the target/anode.

The step up transformer takes 240V and increases it to the KV rating of the machine. The electrons, travelling at high speed, collide into the anode/target. This causes massive deceleration of electron, which results in an X ray photon and heat being released.

The anode rotates to distribute the heat caused by the collision.

Photons are produced at a range of energies. Low energy photons get absorbed into soft tissue, but has no useful contribution. This must be filtered to minimize exposure to radiation. Aluminum disks absorbs these low energy photons

2% x-rays are created, 98% heat is emitted

Question 2

What factors affect the geometric appearance of the radiograph?

Answer 2

Geometric characteristics are affected by:
• Small focal spot, small tooth to film distance, and large source to film distance.
• Sharpness can be affected by patient moving
• Resolution can be affected by horizontal positioning
• Elongation and shortening is affected by vertical positioning

Question 3

How do you achieve a perfect radiograph?

Answer 3

1. Focal spot (beam where x-rays come out of) as small as possible for sharpness
2. Tooth as far as possible from x ray source to minimise chances of magnification and achieve anatomical accuracy
3. Tooth to film distance as close as possible
4. Film parallel to the tooth (?)
5. X-ray beam, perpendicular to tooth and film