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Bit Depth and Dynamic Range (rad techs]

The signal range over which the detector can faithfully represent the measured signal generated by x-rays is the dynamic range. The bit depth is the number of separate computer bits used in saving the value for each DEL. For an illustrated guide to digital sampling see our post: https://howradiologyworks.com/digital... . Chapters: 00:00. Bit depth taught with Legos 05:50. Analog to digital system 06:55 Dynamic range of digital systems 09:00 Trade-offs between sampling and dynamic range The majority of new x-ray systems in the United States use digital x-ray detectors (indirect or direct conversion flat panel detectors), also so called computed radiography equipment uses a digital readout as well. Since you are a Radiologic Technologists or a student and you use or will be soon using these systems many times a day, you are likely interested in the important concepts around how the digital image is formed. The x-rays interact with the detector creating an analog signal, this signal is then converted to a digital signal (a number for each detector element) in the detector. Bit Depth We will start with an example of the most common x-ray detector in clinical use. In an indirect x-ray detector when X-rays hit the detector they are converted to visible light photons. These light photons are measured by a photo diode which convert them to electrons. In this case the number of electrons is the analog signal and it is digitized as the electrons go through circuits, and a single number is assigned to each detector element. The digital circuit converts the energy deposited in the detector to a sampled (i.e. digitized) number. In the figure you can see the effect of the number of bins on the digitization. On the left is the true energy. On the right are the digitized versions of the signal with different bit depths. When the signal is digitized every bit will be set to either 0 or 1. It is set to 1 if the true signal is above the level and set to 0 if the true signal is below the level. The number of levels in the digitization is directly related to how many bits the detector has in the analog to digital conversion circuit: number of Levels = 2N , where N is the bit depth. So, if we use a 4 bit conversion of the energy to digital signal, the accuracy will be much less than in case of 8 or 16 bit conversion. In general, the more levels you have (i.e. the higher the bit depth) the more accurate the image will be. In all digital x-ray detectors an analog signal is converted into a digital signal and the conversion is more accurate when higher bit depths are used. Dynamic Range Dynamic range is also part of the digitization process and is related to the bit depth. The dynamic range is the range over which the signal will be properly digitized. For instance for signals that are higher than then upper end of the dynamic range the signal that is read out will be saturated as it can not handle the high signal levels. As discussed in the section above there is a desire to make the size of each digitization bin smaller. This can be accomplished by adding more bins as discussed above. The length of each bin can also be reduced by reducing the supported range. This range of supported signal levels is referred to as the dynamic range of the system. The height of each digitization bin is simply: Digital bin height = Dynamic Range / (Number of Bins -1). In this figure you can see the issues that can occur and why the dynamic range and the bit depth must be chosen carefully. In the figure the dynamic range is changed while leaving the bit depth constant. If the dynamic range of the system is too small then signals with a very high signal level will be saturated and the true value will not be recorded, rather just the highest value that the system can record with be used. On the other hand, if the dynamic range is too large then there will be wasted bits in the conversion that are never used, and each bit will cover a greater signal range. Since there is a desire to have each bit cover a smaller signal range, an overly large dynamic range is also not optimal. In the optimal case the dynamic range of the system will cover most all signal levels that are expected on the system so that saturation does not occur, but it will not be so large that there are significant digitization errors. When the dynamic range is chosen appropriately this is the ‘well sampled’ region in the figure. In clinical images if the detector does not have a large enough dynamic range the values in areas of very high signal, such as the lungs, will be saturated and structural differences in the lung tissue will be lost. Rad Take-home Point: The range of all values that are properly digitized is known as the dynamic range of the detector and the dynamic range must be chosen appropriately to reduce the size of each sampling bin, but without having saturation.