Radiology

Introduction

Radiology describes the use of x-rays to examine the skeletal system by producing a two dimensional picture of the bony structures in situ. X-rays are electromagnetic energy waves that are produced when electric and magnetic fields are altered and are measured in terms of wavelength, frequency (Hz) and energy (photons). Compared to other sources of energy waves such as infra red, for example, x-rays are high frequency, short wavelength and high energy waves. The high energy permits them to pass through soft tissue structures - with dense bone, they are absorbed or scattered. The 2 dimensional image that is produced diagnostically occurs when x-rays that penetrate through an object collide with the film and the radiation blackens the film. Where dense bony tissue has absorbed or scattered the x-rays, there is less damage to the film and an opaque image is left on the film once developed.

X-ray equipment

X-ray machines have an area to create X-rays known as the head and a control panel to set the parameters of the current which is measured in voltage (measured in kilovolts or kV) and amperage (measured in milliamperes (mA)), and the time (measured in seconds). X-ray machines can be portable, mobile or fixed. Each machine head contains a glass tube with an anode and a cathode; current applied to the cathode from the mains via a transformer within the head releases electrons which are attracted to the anode at the end of the tube. The tube contains a vacuum and focusing cup to ensure the electrodes can only travel in the direction of the anode. An electrical field within the tube controls the speed at which the electrons travel towards the anode.

Within the anode are the atoms which will produce X-rays once the electrons collide with them at tremendous speeds. Only 1% of the energy of the electrons colliding with the atoms is transformed into X-rays - the rest generates heat. The elements of both the cathode and the anode are constructed primarily of tungsten as this metal can withstand extreme heat. This heat must be dispersed, and X-ray machines differ in how this is achieved; stationary anodes use a radiator external to the tube, whereas rotating anodes provide an ongoing circulating disc of cooler metal for electron collision - these anodes have a two stage timing process on the control panel that allows the anode to start rotating prior to the second timer which controls exposure.

The X-rays produced in the head exit from the tube via an opening in the casing - this forms the primary beam which is directed at the area of interest in the patient. X-rays which are oriented away from this opening are absorbed by the lead casing which surrounds the tube.

Also see Contrast Radiography