Digital Radiography, Tips & Tricks
21st century radiography has gone digital. This technology enables X-ray technologies and testing experts to view the X-ray image at a whole new level, in a combination of benefits that was not possible in the analog world of X-ray film. There is a whole new potential of image analysis out there!
What is the Difference Between Film Density and Saturation in DR ?
Film density is a measure of the degree of film darkening. Technically, it should be called "transmitted density" when associated with transparent-based film since it is a measure of the light transmitted through the film. Density (D) is a logarithmic unit that describes a ratio of two measurements: Specifically, it is the log of the intensity of light incident on the film (I0) to the intensity of light transmitted through the film (It). In Digital Radiography (DR) the imagers are so sensitive that even with minimal amount of energy an image can be produced that is equivalent to the film density between 0-7. In order to cover all the film densities, it is recommended to use at least 80% saturation from 14 Bit (16,384 grey scale) or 16 Bit (65,536 grey scale) imager. A Digital Detector Array or DDA (also known as Detector or Imager) with a high Grey Value will enable operators to best leverage their DR results. The higher Grey Value leads to Higher Signal To Noise Ratio. One of the key concepts is the usage of signal/noise (SNR) measurements as equivalent to the optical density of film and film system class.
What is the Meaning of Dynamic Range in Digital Radiography ?
In Digital Radiography the image is represented as a scale of grey values. The amount of grey levels, which a DDA can produce, determines its dynamic range. The dynamic range is represented in bits. A 14 bit or 16 bit imager can produce images with 16,384 or 65,536 different grey level values. The dynamic range is parallel to what would be referred to as "Latitude" in film X-ray and it represents the range of exposures in which an image and contrast will be formed. While in film, there is often a trade-off between "Latitude" and "Contrast" (because of the way images are formed using X-ray film), in digital radiography there is no such limitation. The sensitive imagers respond to X-ray exposure and are able to produce a high contrast image on a wide range of X-ray exposure values (represented by the grey scale). For digital detectors, dynamic range is the range of x-ray exposure over which a meaningful image can be obtained. Digital detectors have a wider and a linear dynamic range, virtually eliminates the risk of a failed exposure.
How is Basic Spatial Resolution (BSR) Defined in Digital Radiography?
SRB is the Spatial detector resolution of the image in X-and Y direction and it is the Smallest detail perpendicular to the X-ray beam resolved in the image. The SRB is also known as the effective pixel size that corresponds to 1/2 of the detector un-sharpness. The SRB is measured with a Duplex wire (EN462-5 or ASTM E2002) and calculated out of the visibility of Duplex Wires using line profile. It is defined by an indication of 20% dip in the grey level values of the digital image of the area between the two lines of a pair. The various standards differ between them as to the exact location of this 20% dip with regards to BSR. Some standards define this as the first line pair not to meet the 20% dip and others define it as the last line pair to meet the 20% values dip.
What is a DDA or Imager?
A Digital Detector Array (DDA) is also sometimes called a detector or imager. The imager replaces the X-ray film, it detects the X-rays that have penetrated through the inspected object and produces the digital radiography image by converting the detected X-rays into digital information.
The X-rays, which have penetrated through the object will first hit a scintillator or a fluoroscopic screen. This screen absorbs the ionizing radiation and converts it into visible light. The light is then converted into a digital X and Y axis readout by an element that is sensitive to light (such as Photo Diode). The readout is then converted into a digital signal, which is recorded as an image composed of many pixels.
What are Pixels in Digital Radiography?
In every digital image, no matter how it was generated, the image is displayed with pixels. Pixels are the smallest element in an image. Digital radiography is a method in which the X-rays are detected using sensors or imagers that produce a digital image - an image which is displayed and represented in pixels. The pixel is the smallest unit which can be measured in digital radiography. The pixel is a digital sample of the image and its color (or grey level) represents the amount of X-ray that penetrated the inspected object to reach the detector. The higher the amount of pixels in an image the higher the resolution. On the other hand, more pixels usually mean that each pixel is smaller in size. Small pixels cause lower penetration per dose as less light can be captured by each pixel. The pixel size, along with the signal to Noise Ratio and scintillator thickness will all determine the effective spatial resolution (BSR) and if a small discontinuity will be detected.
How is Signal to Noise Ratio (SNR) Defined?
Every digital device produces both a signal and noise. The signal is the data, which is desired, whereas the noise is the effect of physical electronics processing in the device and it obscures the signal. The Signal to Noise Ratio is defined as the ratio of signal power to the noise power. The less noise, more clear and precise will be our results (image). If a DDA is properly constructed, and a digital radiography system properly designed, the effects of the electronic noise inherent to this equipment can be reduced. A good imager offers a higher Signal to Noise Ratio. High SNR will enable operators to see even the smallest of discontinuities.
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