Chapter 10 Lecture Notes

Filtration

Filtration - Adding filtration to the useful x-ray beam reduces patient dose.

X-Ray Quality

Penetrability - refers to the ability of X-rays to traverse an object. It is strongly affected by the X-ray energy, determined by factors such as peak voltage, anode target material, and filtration used. The penetrating power of a certain X ray spectrum is given by the half value layer (HVL).

Attenuation - process by which radiation loses power as it travels through matter and interacts with it. Beam attenuation is the basis of the contrast observed in all X-ray based imaging methods.

Half-Value Layer - the amount of material that is required to reduce the intensity of an X-ray beam to half. For X-ray beams, this is normally expressed in aluminium or copper thickness, but can also be expressed in other materials or media, such as water.

This is a diagram of the path of x-rays from the tube to the radiation detector.

This is a graph of the exposure rate versus the thickness of copper to find the Half-Value Layer (HVL). In this case the HVL is about 1.86 mm of copper.

Here is another graph for the HVL calculuation. In this case it is x-ray quantity versus aluminium thickness. The HVL is about 2.4 mm of aluminium.

Steps to Determine the HVL

1. Find the intensity of the x-ray beam with no absorbing material in the beam and then find the intensity when different known thickness of an absorber, usually aluminum.

2. Plot the ordered pairs (thickness of absorber, x-ray intensity).

3. Find the x-ray intensity equal to half the original intensity.

4. Draw a horizontal line to the curve from the half intensity mark.

5. Draw a vertical line from the curve where the half intensity was marked to the horizontal axis.

6. Estimate the thickness on the x-axis for the HVL value.

Example:

The following data were obtained with a fluoroscopic x-ray tube operated at 80 kVp. The exposure levels were measured 50 cm above the table top with aluminum absorbers positioned on the surface. Estimate the HVL by graphing the data.

 Added mm Al mR 0 65 1 48 3 30 5 21 7 16 9 13

First I graph the data on a graph.

Then I draw a line horizontal from half of the maximum (65 mR), so at 32.5 mR. I next draw a vertical line down from where the horizontal line intersected the graph. Then I estimate the millimeters of aluminum that is needed.

This looks like about 2.8 mm of Aluminium for the HVL.

HVL is the best method for specifying x-ray quality.

Factors Affecting X-Ray Quality

Kilovolt Peak (kVp) - Increasing the kVp increases the quality of an x-ray beam.

 Approximate Relationship Between kVp and HVL kVp HVL (mm of Al) 50 1.9 75 2.8 100 3.7 125 4.6 150 5.4

Filtration - removal of parts of the X ray spectrum using absorbing materials in the X-ray beam. The X-ray spectrum reaching the patient is filtered by attenuating material in its path. Filtering of the beam is used in order to modify the spectral or spatial distribution of X-rays, or both.

The diagram below shows what happens when filtration is added to the beam. Notice that the quantity goes down, but the quality goes up. The peak is moving to the right.

Types of Filtration

Inherent Filtration - the filtration of an X-ray beam by any parts of the X ray tube or tube shield through which the beam must pass. The parts include the glass envelope of the X-ray tube, the oil cooling the tube and the exit window in the tube housing. The inherent filtration corresponds to approximately 0.51 mm of aluminium.

Added Filtration - This filtration is for normal X-ray purposes commonly made of aluminium or copper. The purpose of inserting such extra filtration into the X-ray beam serves the following purposes:
To remove the low-energy photons that never would have been able to reach the film and produce an image. These photons would, if present, only increase the radiation dose given to the patient.

The End