Digital radiography: Practical advantages of Digital Radiography. Practical Advantages in image quality

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1 Digital radiography: Digital radiography is set to become the most common form of processing radiographic images in the next 10 years. This is due to a number of practical and image quality issues. Practical advantages of Digital Radiography No need for a darkroom- with associated problems of safe light, light tightness and ventilation problems No chemicals required and no disposal problems for used chemicals No cleaning of automatic processor/tanks or trays No boxes of film required with risk of fogging etc Digital radiography is much faster Less prone to human error No storage problems for radiographs and no degradation of film with time Ease of retrieval and storage of radiographs using digital archiving software/ CDs. No need for light boxes, hot lights to read radiographs Ease of transfer of images Practical Advantages in image quality Increased dynamic range: This allows the assessment of soft tissues and bone in the same exposure by altering the image in the monitor. Post processing of images i.e. the ability manipulate the image to change contrast, brightness, edge enhancement etc. Magnification of areas of interest Wide exposure latitude: the range of exposures used to obtain a diagnostic image is very wide. Therefore digital radiography systems can compensate to some degree for over or underexposure. This means fewer repeat films due to incorrect exposure settings. There are a few disadvantages

2 Practical disadvantages of Digital Radiography Initial cost of setting up digital radiography system- however, once the system is set up, the cost of consumables e.g. film,cassettes, chemicals etc is non existent/negligible Technical failure of the computer/ reader / digital display results in no radiographs. Definitely need fast, efficient support team if have digital radiography system. Versatility - can no longer take some views e.g. intra- oral views if have DDR Degradation of image with time if using CR Disadvantages in image quality of Digital Radiography? The spatial resolution in conventional film/screen radiography is higher - but the difference is not discernible to the the naked eye.? Is this really a disadvantage Types of Digital Radiography: There are two broad categories of digital radiography available Computed radiography, CR Direct digital radiography, DDR (This is commonly called DR - direct radiography which is a little confusing) Computed radiography (CR): Image production takes place in two stages 1. A detector or image plate for acquiring the image: This is a flexible image plate containing photosensitive phosphors and looks similar to a traditional

3 intensifying screen. This plate is placed within a rigid cassette and is positioned in the traditional manner. The X-ray beam excites the electrons in the crystalline phosphor to a higher energy level where they are trapped. X-rays Photodiode converts x-rays to electrical energy TFT detects and reads the electric charge Readout circuit 2. A device to read the plate: The cassette is partially inserted into the reader and the image plate extracted where is is fed through a series of rollers. The laser beam then scans the image plate. The laser beam causes the energy from the trapped electrons to return to their original energy level. Due to the specially selected photostimulable luminescence of the phosphor the released energy is in the form of visible light. This light is collected and a photomultiplier converts the light in to an electrical signal and into a digital image. This image is then transmitted to a computer for digital processing. Direct Digital Radiography (DDR):

4 In contrast to both conventional radiography and CR, in DDR the detector that captures the incident X-rays produces the digital signal that is sent to a computer. There are different ways in which this is achieved. Thus DDR is sub divided into: Scintillator Converts x-rays to light 1. Direct conversion digital radiography (or true DDR): 2. Indirect conversion:. There are two types of indirect conversion used Flat panel scintillator Charge coupled device (not a true indirect conversion system) Direct conversion digital radiography: Direct conversion digital radiography transforms the energy of the X-rays directly in to an electric impulse. This is also a flat panel system. The components of these systems are: A semiconductor material (amorphous selenium) coated on a plate that converts x-ray energy directly to electrical energy A thin film transistor (TFT) is directly coupled to the semiconductor detector, which detects and reads the electric charge

5 X-RAYS!!!! Lens focus the light to smaller Indirect conversion digital radiography: These systems first transform the X-ray energy to light and then transform the light in to an electric impulse. Flat panel scintillator The components of this system include: CCD converts light to electrical energy which it reads Readout circuit Scintillation material (GdOxS or CsI) coated on a plate that converts x-rays to flash of light A photodiode converts flash of light to electric charge A thin film transistor (TFT) is directly coupled to the semiconductor detector, which detects and reads the electric charge

6 !!!! Photodiode converts x-rays to electrical energy TFT detects and reads the electric charge!!!! Readout circuit! Charged couple detectors: These systems are strictly speaking not a type of indirect conversion digital radiography but often included as they send the image directly to computer. This system is not considered as efficient as the previously described systems.

7 The components of this system include: Scintillation material (GdOxS or CsI) coated on a plate which converts x-rays to light A reducing lens to minify the light beam to the dimensions of the charge coupled device (CCD) -chip The chip converts light energy into electrical charges and then to a digital format for readout. Scintillator Converts x-rays to light

8 CR vs. DR Both systems have more than adequate image resolution for accurate radiological interpretation. CR is significantly cheaper. CR is slower (readout time, 45 seconds to 2 minutes) and DDR acquisition and image display is immediate. Upgrading from conventional radiography to CR requires no adaptation of the X-ray equipment. DDR systems need to be coupled with the generator and X-ray tube to synchronize exposure and reading. CR laser reader is sensitive to dust and moving parts require repair and maintenance DR plates are sensitive to temperature changes. CR plates can be reused many times but their lifetime is limited. S/F CR DDR Linking to X-ray machine No No Yes/No Cassette used in radiography Yes Yes No Darkroom required Yes No No Chemicals / film Yes No No Speed of radiograph production mins secs 0-3 sec Set up cost

9 S/F CR DDR On going costs of consumables Yes No No Pre-processing /processing image manipulation No Yes Yes Post processing image manipulation No Yes Yes DQE* Poor Fair/ Good Good Life span 7-10 years unknown unknown * DQE =Detection Quantum efficiency Manipulation of images There are three stages when the image can be manipulated- preprocessing, processing and post processing. However the criteria for preprocessing and processing are similar so they will be discussed as a single entity. Preprocessing / processing manipulation A major advantage of digital detectors is that they have a very wide dynamic range, which means that they will produce a diagnostic image over a wide range of exposure settings. The image produced is formed on a rectangular pixel matrix with discreet pixel values representing the measured exposure generated from the x-ray. This can be represented in histogram format In most systems, exposure recognition is automatic and the lowest and highest pixel values are accordingly assigned -these histograms are called look up tables ( LUT). The LUT is then manipulated depending on what area is to be viewed. These are often preset in your system and combined with other factors to set up algorithms for various parts of the body. It is important to make sure that the algorithms set up in the system give an image that you are happy to interpret. It should also be remembered that if you are able to alter LUT and send that information to your computer that you may lose some of the raw data -so you need to be sure you are happy with the images you have sent. Post processing In practically all systems the image can be altered to improve visualization of various structures but these changes are transient do not actually alter the stored

10 image. the image can be altered in various ways. when viewing an image, the contrast and brightness can be manipulated by windowing the image width and/ or level. The window width refers to the range of pixel values that are displayed out of the available pixel values. Viewing with a wide window generally leads to lower contrast while viewing with a narrow window leads to higher contrast. The window level refers to the midpoint of the pixel values that are displayed. Increasing the level generally increases the brightness of the image. Post processing also allows you to magnify an area of interest and invert an image if required. Some systems can further manipulate an image and allow image sharpening, edge enhancement, and smoothing Artifacts in digital radiography: There are a number of artefacts that are common to all types of radiography eg. movement blur, grid cut off etc which we will not elaborate on here.however, some artefacts are unique to CR and/ or DDR and can occur during image acquisition, digital processing or image display. Artifacts common to CR and DDR Exposure artifacts: The appearance of under or overexposure is completely different to conventional radiography. In film/screen radiography, film blackening is related to exposure. In digital radiography, the image can be adjusted by changing image brightness and contrast in the display making detection of exposure artifacts difficult. Underexposure: When the image is underexposed, the detector does not have enough information to produce a good quality image. This is seen as increased graininess in the final radiograph and is also called noise or quantum mottle.

11 This is similar to the grainy appearance of digital photos taken in poor lighting conditions. Overexposure: Overexposure is almost impossible to detect unless it is extreme in which case you get black burned out areas. Generally speaking, in digital radiography, image quality increases with exposure up to a certain level. Every digital system gives a numerical reading when an image is acquired that correlates with the amount of radiation reaching the detector. This is generally called the exposure index (EI). It is very important to find out what the EI range for your system so that you can maximize image quality while minimizing radiation dose to personnel and stress to your radiographic equipment. Halo artifact Uberschwinger: This appears as a radiolucent (black) halo surrounding highly radiopaque objects.e.g. orthopedic implants such as screws. This can mimic bone lysis or resorption similar to what is caused by implant related osteomyelitis or bone necrosis. This artefact is particularly obvious when edge enhancement filters are applied to the image which is often the case when reviewing bone radiographs. Selection of the wrong algorithm If you use the incorrect algorithm then the image will have chosen the incorrect LUT and filters for the area and result in a suboptimal image CR artifacts Fogging: Fogging is only seen in CR. The phosphor plates are very sensitive to background and scattered radiation. If they are left in the X-ray room, they will pick up scatter

12 and this will result in decreased image quality. To avoid it, the phosphor plates should be kept away from radiation and erased regularly as part of the daily maintenance of the system. Double exposure and ghost images: Is caused when the phosphor plate is not read or fully erased between two exposures and results in two different images superimposed. Scratches and dirt: This occurs in CR, when the phosphor is dirty or cracked. This will result in areas were no signal is detected because there is phosphor material missing (cracks in the plate) or dirt that blocks the laser and or light emitted from the film during read out- the result is white lines (dust on laser reader) or white dots (dirt on screens). Black lines occur due to PMT malfunction/dirt Moire pattern This has a wavy lined appearance due to the sampling pattern running parallel to the grid lines. To rectify cane direction of the grid DR artifact Dead pixels: This is DR specific. When one of the small detectors in the TFT is faulty, no information will be available for that part of the image. This would normally appear as tiny black dots in the image, similar to a dead pixel in a screen. However, most

13 DR systems detect the dead pixel and fill the gap using interpolation of the data from adjacent pixels.