Queensland Diagnostic Imaging

General X-Ray - Computed Radiography

What is an X-Ray Examination?

The familiar 'X-ray' was the earliest of the currently utilized diagnostic imaging techniques to be applied to clinical practice. X-rays are still widely used in general medicine particularly for imaging bones and the chest. A radiograph is a visible photographic record produced on a special type of film by X-rays passing through a body part. A specially trained radiographer positions the patient on the 'X-ray' table and aligns the 'X-ray' beam. X-rays have enough energy to pass through body tissue, however some X-rays are absorbed or scattered. Dense tissue such as bone absorbs more radiation than soft tissue. The remaining X-rays pass into a film cassette where they are absorbed by a chemical layer in the cassette known as a phosphor screen. The phosphor emits light proportional to the intensity of the 'X-ray' beam striking it. In turn, this light exposes the 'X-ray' film contained in the cassette. The exposed film is then processed in an automatic developer.

What are X-rays ?

Radio waves, light and X-rays are all forms of electromagnetic radiation. X-rays have a higher energy and a shorter wavelength than visible light. It is the high energy of X-rays that enables penetration through solid material. X-rays were first discovered in 1895 by a German physicist, Wilhelm Conrad Roentgen. X-rays are produced by a vacuum tube which accelerates electrons across a high voltage in the order of 100,000 volts. The electrons are produced by heating a small filament similar to that in a light bulb. The accelerated electrons strike a target made of tungsten, chosen for its high melting point and high density. The electrons striking the target release most of their energy as heat but a small amount is transformed into X-rays Modern tubes have rotating targets to assist with heat dissipation.

Tomography

Many general 'X-ray' rooms are fitted with a tomographic attachment to facilitate the production of special radiographs known as tomograms. During the 'X-ray' exposure, the tube is moved across in a smooth arc over the patient. The result is a picture where a single layer in the patient is in clear focus and where the layers above and below the focal plane are blurred. This technique is useful in more clearly defining the kidneys during a procedure known as an IVP. Tomograms are also sometimes required to better display abnormalities in bones.

Computed radiography

Computed radiography (CR) is a relatively new technique where the information obtained from an x-ray exposure is recorded on a special electrostatic plate rather than on x-ray film. The plate is subsequently read by a laser film reader which employs an analogue to digital converter. The digital information acquired is then made available to an image storage and processing network. The radiologist views the images on a high resolution high contrast monitor. In a hospital environment, referring clinicians can view the images on their own monitors. Hard copy images can be produced by sending data to a laser film printer. If required, images can be archived for future reference.

Direct digital radiography is similar to the above but the digital image is acquired instantaneously via a special detector plate that is exposed directly by the x-ray beam once it has passed through the patient. The information is immediately available to the network thereby cutting out the additional step required in 'reading' the exposed plate when using CR.

Advantages of CR:

• One advantage of these techniques is that the resultant image has greater latitude which means that the actual x-ray exposure used is not as critical to the result. This means less reject exposures as occurs when a conventional film is too light or too dark. For the patient this means less radiation exposure.

• Overall, the exposure required to produce a good quality image is less with CR than with conventional radiography.

•The images have inherent edge enhancement which improves perception of subtle abnormalities.

•The images can be manipulated on the monitor by the radiologist to assist diagnosis. By changing the brightness or contrast and zooming in on areas of interest lesions can be detected more confidently. Measurements of any detected abnormalities can be performed easily by the interpreting radiologist to improve the precision of the report.

•The images are rapidly available to an entire network are are readily stored using picture archiving systems

•The images can be sent via the internet to radiologists on-call or to referring clinicians.

For more information please see Imaging Updates Volume 7 Number 4

Edge Enhancement in CR

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Edge enhancement

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Edge enhancement

Special X-ray procedures

X-rays may be obtained following the administration of some form of contrast material. Contrast agents contain elements like Iodine or Barium with a high atomic number. Contrast agents absorb X-rays more than soft tissues and appear white on standard 'X-ray' pictures. Procedures performed in the general 'X-ray' room include:

• IVP an IntraVenous Pyelogram where contrast injected into an arm vein is followed through the kidneys into the bladder.

• Sialogram where contrast is injected via a small catheter into a salivary gland.

• Sinogram where contrast is injected into a discharging sinus on the skin surface to outline an associated abscess cavity.

You can learn more about these procedures by visiting the Procedures Web page.

The General X-Ray - Computed Radiography Room

An X-ray room includes an X-ray generator and an X-ray tube assembled on a tube stand, connected to the generator by high tension cables. A special table with a moving table top is used for positioning the patient. Under the table is a film cassette holder called a bucky tray into which the cassette is inserted in readiness for the exposure. Another bucky is mounted on a wall for erect films.

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Link to frequently asked questions in	• General X-Ray