Effect of parameters on spectrum

It is important to know how parameters affect the X-ray spectrum. And how this translates into practice and your recording.

Increasing the mAs number
When the mAs number is increased, more X-rays will come out of the tube. Since you will often not be able to set the milliampere rate of the device, you can only influence the mAs value by setting the time.

Of all photon energies in the spectrum (Figure 1), the intensity increases proportionally with the increase in mAs number. So with twice the mAs, the intensity of all photon energies will have doubled. Nothing changes in the properties of the beam. The average photon energy remains the same. The contrast in an image will also remain the same.

Impact in pictures
With films, you would see a darker picture; with digital techniques, overexposure can easily be overlooked. Too low a mAs number gives an image with a lot of noise.

Increasing the kV
The effect of increasing the tube voltage is shown in Figure 2. The maximum photon energy in keV is equal to the set kV of the device. You can see the spectrum expanding to the right at higher kV. What is also noticeable is that you get more radiation, the area under the curve increases. The efficiency of an X-ray tube increases with higher kV. Therefore, the mAs number must be adjusted when the kV is increased. A rule of thumb is, if the kV is increased by 10 kV, the mAs (time) can be halved. With an exposure automatic, this is done automatically.

Impact in pictures

A higher kV gives less contrast in the image, the difference between soft tissue and bone becomes smaller.

Additional filtering
When additional filters are applied, mainly the low-energy X-ray photons will be filtered out. What remains is a spectrum with an average higher energy. Some of the higher photon energies will also be filtered out. In manual exposure, a higher mAs number must be chosen for equal exposure. Most devices have types of shots programmed where kV, mAs, filtering depend on your choice of element or type of client.

Impact in pictures
More filtering gives less contrast in image, the difference between soft tissue and bone becomes smaller.

Sources:

Physics for imaging and radiotherapy; J. Scheurleer, 2017

Introduction to Radiation Hygiene; A.J.J. Bos et al; 2009