X-ray Detector Theory
There are hundreds of types of X-ray detectors. The technology for X-ray detection has been focused on speed, clarity or resolution of the image, robust operation with radiation present, and physical size and weight. Film was widely used for many years to create a radiographic image. There were several film speeds that were selected based on the application requirements of the radiographer. Phosphorus coated screens were used to convert X-ray photons to light photons by early radiographers. The radiographer would use red goggles to filter the green light emitting from the phosphorous screens in different shades or levels of brightness depending on how much of the radiation was absorbed by the patient or object of interest.
It is a key point to take away that the efficient conversion of X-ray to light play an important part in improving the diagnostic capability of the X-ray image. Not only were the phosphor screens dangerous because the radiographer was in the direct path of the radiation but they also provided a low light level output or low quantum efficiency.
Phosphor and rare earth screens were also used in film cassettes to convert the X-rays to light so that the film would expose to a usable density quicker and a patient would get less radiation. Also, the opportunity for scatter radiation to expose the film and blur the image was reduced. If one thought in digital terms, the signal to noise increased. The photonic science of the spectrum of light created also played an important part in the proper conversion of the screen. A key feature of the X-ray film screen cassette was that it pressed the screen tightly against the film. When the screen became worn or warped, the film would have blurry or out of focus areas on the image because the transfer of light would scatter similar to having a focal spot that is to large. Thus the measurement of the homogeneous accuracy of a conversion layer on a cassette is achieved by setting a screen mesh pattern directly on the cassette and exposing it so that you can be assured the image sharpness is the same throughout the entire image. This is an important factor with flat panel digital detector that use phosphor screens to keep the resolution at the highest possible level throughout the whole image plane.
The linear array detector and the flat panel detectors are made up of several photo diodes in a strait line or linear array. The first CT machines built used a photomultiplier tubes as the measurement device of the light. There are many configurations for photomultiplier circuits. When a bias voltage is applied to a photo multiplier tube and it is dark, there is a slight current flow which is called dark current. The amplifier section of the circuit has a measured and calibrated gain to null out the dark current because each photomultiplier is slightly different. When light hits the photomultiplier, electrons are able to flow to a level that is consistent with the amount if light seen. The amplifier then converts the current flow into a voltage. The voltage is applied to an analog to digital (A/D) converter such that as the light increases, the signal increases and the digital value increases. Thus, the cell will provide us with a digital value corresponding to the light that is applied to it. In an 8 bit system the value of 1111 1111 binary or 255 decimal would be maximum light and 0000 0000 binary or 0 decimal would be minimum light. The same holds true for 12 bits or 16 bits etc... Today the photo multiplier tube has been replaced by the photo diode; they are much smaller and do not require as much circuitry to achieve a functional signal. The photodiode or phototransistor will conduct harder as light increases. One photodiode is contained in each of the pixels. Each pixel measures the light that is present. The variables involved with the conversion of X-rays to usable light are vast and require precise attention to each stage of the conversion.
Detector Quantum Efficiency (DQE)
Detective quantum efficiency (DQE) is a measurement of image quality in radiography and refers to the efficiency of a detector in converting incident x-ray energy into an image signal; it is the ratio of the squared output signal-to-noise ratio (SNRo)2 to the squared input signal-to-noise ratio (SNRi)2 of the imaging detector. This measurement takes into account the modulation transfer function (MTF), spatial frequency, detector material and radiation. In other words, given a specific X-ray beam through an object with a specific absorption factor one can quantify the contrast, resolution and gain of a detector. A high DQE is desirable and means that for a given input radiation the results are better. In medical applications this means that the patient receives less dose during the exam. For the NDT RT and CT radiographer this means that a greater level of image quality can be achieve quicker and more efficiently. We will discuss this as it applies to different detectors.