Introduction to X-rays

In X-ray radiography images are produced by casting an X-ray shadow onto a photographic film or digital detector:

Like Gamma rays, X-rays can travel through soft tissues in the body with little attenuation, and are only “stopped” by high density tissues such as bone. - IOP

Attenuation is the reduction of the intensity of an X-ray beam as it passes though matter. 

Contrast distinguishes, based on density, differences between organs, tissues, bones, or blood vessels during  imaging.

kVp and mAs

kVp     - kilovolts

Is the voltage difference between the anode and the cathode.

"p" stands for peak - meaning the peak or highest voltage reached in one electrical cycle.

mAs - milliampere seconds

Is the current flowing through the X-ray tube multiplied by the the time it flows.

These are the two primary controls that we have with an X-ray tube. 

They control the quality and the amount of the  X-ray beam.

kVp

• As we increase the kVp or the voltage difference, we increase the speed at which the electrons traverse the chamber.

• They impact the anode at higher and higher energies, and create higher energy X-rays.

• The X-rays become more penetrating.

• If we decrease the voltage, then we will see a decrease in the energy of the X-rays.

• The X-rays become less penetrating.

mAs

• is directly proportional to the number of electrons that come off the filament, and that are accelerated across the gap

• is directly proportional to the radiation dose

• If we double the mAs, then we’re going to double the radiation dose to the patient.  

Summary of steps

1. Filament current applied through tungsten filament at cathode.

2. Heats up filament to produce enough energy to overcome binding energy of electrons (thermionic emission).

3. Electrons released from filament.

4. Tube voltage is applied across the x-ray tube.

5. Electrons, therefore, are accelerated towards positively charged anode, which gives them a certain energy.

6. The electrons strike the anode and the energy released via interaction with the anode atoms produces x-ray photons.

7. These x-ray photons leave the x-ray tube through the window in an x-ray beam towards the patient.

8. They pass through the patient to the detector to produce the x-ray image.

X-ray Intensity & Attenuation

Usually, when talking about intensity, we are talking about the amount of energy of particles, or radiation, per unit area.

Attenuation is the reduction of the intensity, or, energy, of an x-ray beam as it travels through matter.

The attenuation coefficient is a measure of how much the incident energy beam (e.g. ultrasound or x-rays) is weakened by the material it is passing through.

Now that we've had some fun with exponential equations, and, with 'e' and 'ln', it is ok I think to introduce you to the equation for x-ray attenuation through matter: