Clinical Imaging

There are 3 main goals of Diagnostic Radiography:

• Production of images of dianostic quality to determine the existence of a pathology determining the correct treatment and care for the patient
• Dose is to be minimised as much as possible (As Low As Reasonably Achievable) to prevent the ionising radiation causing stochastic and nonstochastic effects on a patient.
• Patient/staff safety in regards to positioning and infection control 

The X-ray source needs to produce a uniform beam in terms of their kV (energy) set by the radiographer. kV causes the amount of contrast on the image; due to the penetration through the object (patient).. mAs causes the Blackening of the X-rays due to image density. The X-ray beam attenuates after being distributed, it is either absorbed or scattered, once interacting with matter the properties of the beam alter and the object becomes magnified or distorted.

When electrons are produced from a cathode (the source of electrons - made of a filament and a focusing cup) they are then accelerated through thermionic emission towards the anode target usually made of tungsten (melting point of 3410 degrees), as it approaches the target it is suddenly decelerated by braking radiation and produces an x-ray photon. X-rays should come from a point source, should be controllable and safe.

Typically we want X-rays to come out at a point source to reduce penumbra (fuzziness of xrays) this is created by a small anode angle (17degrees) but this then produces the Anode heel effect. This effect states that as the x-ray photons come closer to the anode angle (C) there is less intensity as it is parallel to the anode and the inverse square law. This says that the further away the object is from the anode, the more diverse the x-rays become therefore more penumbra is produced.

Fine focus produces less penumbra and a more detailed X-ray image, due to the source being smaller this is used when geometric factors limit image quality and this reduces tube loading. Broad focus is used when less detail is needed (Abdominal X-ray) but dose produce more penumbra (Figure H) This image is limited by patient attenuation,there is higher tube loading and more heat dissipation.

Fine Focus

Broad Focus

The use of filters can reduce X-ray dose by removing the low kV X-rays, filters typically are made of aluminium and low penetrating X-rays are 'absorbed' by the aluminium.