Introduction to Direct  Digital Radiography (DR)

Introduction to Direct Digital Radiography (DR)

Kat Evans

Veterinary practices are frequently described as having digital radiography, however, this can be a broad and often inaccurate description. To prevent confusion, it is recommended to use the terminology Direct Digital Radiography (DR) or Computed Radiography (CR) when describing ‘digital radiography’. In this article, we will discuss the theory behind DR, and answer some frequently asked questions.

What is DR?

Put simply, DR systems convert x-rays directly into a digital signal after they have passed through the patient. This information is then relayed to a computer to enable you to see the radiograph on a monitor. This differs from CR which requires a cassette to be read or processed to enable visualisation of the image.

All digital systems will also apply algorithms, which are also known as LUTs (Look Up Tables). These take the raw image, which is normally very flat, and enhance it to give us a usable image. Algorithms, and therefore image appearance, can vary enormously depending upon the quality of the equipment from different manufacturers. Different DR systems will use different receptor technologies to capture the raw image, but most are based around a layer of Thin Film Transistors, known as flat panel detectors. These are further sub-divided into direct conversion and indirect conversion detectors.

Direct conversion DR detector

In direct conversion detectors (Figure 1) x-ray photons hit the receptor and interact with the atoms of the photoconductor, converting the energy from the x-ray photons into an electrical charge. The electrical charge is then used by the thin film transistor array to produce a digital image. The information that is used to create the image is the location and strength with which the photons hit the TFT layer. This interaction works most effectively if the incoming x-ray photons have a high energy level, i.e. are produced using a high kV.

Physical manufacturing limitations restrict the resolution achievable by direct conversion. An array of tiny detectors is used because it is physically impossible to manufacture one single large detector and inevitably this results in small ‘gaps’ between the individual detectors. The photoconductor is designed to maximise resolution by channelling incoming ions towards the detectors. There are a range of photoconductors used (the most common is amorphous selenium) which vary in their sensitivity to radiation, resolution and their ability to cope with environmental stresses.

Figure 1. Direct conversion DR detector

Indirect conversion DR receptor

Indirect DR receptors (Figure 2) have more layers than direct DR receptors. There is a layer of scintillation material which covers the front of the photodiode, this effectively works in the same way as the intensifying screen in a traditional X-ray cassette. The scintillation material emits light when stimulated by radiation. The indirect stage of the conversion is then the transmission of this light by the photodiode into an electrical signal. In comparison to direct conversion DR receptors, the ‘extra layer’ allows a lower exposure to be used when acquiring an image. The increase in number of layers (or times the information is ‘processed’) before an image is generated also results in a slight loss of resolution. Therefore DR receptors utilising indirect conversion will inherently result in images with lower resolution than direct conversion DR receptors.

Figure 2. Indirect conversion DR receptor

DR FREQUENTLY ASKED QUESTIONS

Which should I choose?

Historically, indirect conversion receptors have been preferred due to the lower radiation dose required.  Direct conversion is now becoming more popular as the sensitivity of TFT layers improves, however, there have been more issues related to the stability of the direct conversion systems; some detector panels fail if they are taken outside certain temperature ranges!

Currently indirect conversion is still more common in human medicine, with very few direct panels in use.

Why do some panels cost more than others?

Very often quality is the answer. In the same way a television or computer monitor is made, there will always be a number of ‘duff’ pixels on a DR panel. When quality assurance is applied, they are then categorised by the failure rate, i.e. the less failed pixels, the lower the failure rate and the higher quality the panel. When calibrating a detector panel, the DR system builds up a map of defective pixels and then hides them, so that you don’t see them on the image, but this doesn’t mean they are not there!

Fundamentally, higher resolution panels are harder to make, so this will impact cost.

Why does temperature matter?

All DDR panels are sensitive to temperature changes and this can affect the required X-ray exposure. To compensate for this, many systems will recalibrate if they detect a temperature change, however, all will perform less well in extreme cold because they were originally designed to be used at room temperature. Therefore, we recommend never leaving them in unheated vehicles overnight. It is also worth remembering that wireless systems rely on batteries within the plate. Modern, rechargeable batteries have improved a great deal, but still don’t cope well with the cold which can cause rapid loss of charge.

Why do they break when dropped?

The scintillation layer is normally coated onto a sheet of glass. It needs to be affixed to a thin, strong clear surface and glass functions well. The panel can cope with normal use and much work has gone into making them drop resistant, but they are very complex inside. Direct blows from a horse’s hoof or repeatedly being dropped onto a hard surface will take its toll. Most are designed to hold the weight of a human patient with the weight spread evenly over the plate. The higher risk of damage comes when pressure is applied to a point. A kick is likely to be most damaging, but this risk can be reduced by using a plate protector.

Why does image quality change over time?

All DDR technologies will age over time as radiation can be harmful to electronics, eventually affecting the sensitivity of a system. This impact can be reduced by servicing, performing gain calibrations and offset recalibrations. This will also help with any pixel drop out that has occurred over time.

Why does it take so long to repairs plates?

Unlike repairs to most systems, which may be possible via computer remote, or in-house if electrical, or maybe a simple part replacement, the repair of a DR receptor panel requires a very specific environment (sterile, static-free, dust-free and airlocked) and technical skill level. This level of repair is typically only available direct from the panel production facility.

IMV imaging supplies a variety of fixed and portable DR systems into the South African market, all with specialist vet software and pricing options to fit your practice best. Contact us via our website http://www.imv-imaging.co.za/ for more information, or call Tim on 082 616 4685.

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