Plain X-ray imaging uses a single kilovoltage beam generated by a normal x-ray generator. The beam passes through the target and is attenuated by the tissues therein. It then strikes radiographic film where it causes a chemical reaction to occur, leading to image production.
Attenuation of kilovoltage photons
Due to the kilovoltage energies used, plain x-rays are attenuated predominately by the photoelectric effect. Attenuation is therefore related to the cube of the atomic number (Z3). In human tissues, this leads to a marked difference in attenuation between soft tissues and fat (Z ~ 8) and bone (Z ~ 13). This is seen on the image, where bone causes significant attenuation, soft tissues cause some attenuation and air/lung cause minimal attenuation.
The radiographic film is covered by a filter which reduces the amount of scattered x-rays which can cause a chemical reaction (coherent scattering is more common at very low kilovoltage energies and can reduce image contrast). The film itself is a polyester sheet covered with a silver bromide (Ag+ Br- emulsion. When ionising radiation interacts with the emulsion, free electrons may be captured by the silver ion, forming a stable silver atom. This atom comes out of the solution and attaches to the polyester film. When treated, areas exposed to ionising radiation will appear darker on the film due to the silver atoms.
Use of plain X-ray imaging
Plain x-ray was the principle planning method until the advent of computerised tomography. By aligning the x-ray beam with the desired megavoltage field, a 'beams eye view' was created. This would allow visulations of bony landmarks which were then used to plan radiotherapy treatment. Differentiation between soft tissues was poor.
- Inexpensive ($30-$50 per scan)
- Low amount of ionising radiation to obtain image
- Good contrast between bone, soft tissue, lung and air (in two dimensions)
- Contrast may be used to outline hollow viscera (gastrointestinal tract, urinary tract)
- Fluroscopy allows visualisation of organ/tumour movement in two dimensions, if the tumour is located in the lung.
- Poor differentiation between soft tissues
- Two dimensional image only
- Attenuation information for kilovoltage photons only
- Further measurements are needed (eg. patient surface contour) to plan radiotherapy treatment