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It is vital for any radiologic technologist working with digital imaging systems to be educated on the equipment they are using. This lesson covers the topic of digital imaging. The advantages and disadvantages of the different systems available are mentioned. Basic digital imaging terminology, the process of capturing and viewing a digital image, and basic quality control to be done in a digital imaging department are discussed. Also contained in the lesson is a breakdown of a PACS. The lesson will also discuss possible ways to decrease patient dose with digital imaging systems.
Author: Jennifer M. Schmidt BS, R.T. (R)(M), written on Friday April 10th 2009 - 6:20 PM Credits: 0
The A.R.R.T. Continuing Education Requirements for Renewal of Registration states that Internet activities reported in a biennium may not be repeated for credit in the same or subsequent biennium. If you have already completed this article, DO NOT take this CEU article again.
Upon completion of this course the participant should be able to do the following:
Describe how a digital image is formed.
Discuss the various materials used in manufacturing of digital imaging plates and detectors.
Define basic digital imaging terminology.
Understand why new exposure techniques can be used to decrease a patient's dose.
Explain the basics of a PACS.
Be able to compare and contrast a CR and DR system.
Computed Radiography (CR)
Digital Radiography (DR)
Flat Panel Detector
Comparison of CR vs. DR
Advantages & Disadvantages of DR
Advantages & Disadvantages of CR
Look Up Table
Window & Level
Detective Quantum Efficiency
Sensitivity to Scatter Radiation
Forming the Digital Image
Quality Control in Digital Radiography
Service Engineer & Physicist
The world of x-ray is always is changing. The name of the field, radiologic technology, alone gives the indication that the latest technology is used in the profession. In recent years the jump to using the latest technology has never been truer. More and more radiology departments are converting from conventional radiography departments to digital imaging departments with complicated computers. In the middle of all the conversion we have radiologic technologist trying to keep up.
To insure that technologist have all the latest information to provide the best quality patient care, this publication will break down how a digital image is formed, basic terminology used with digital imaging, possible ways to decrease the patient's dose, and a brief discussion on quality control is provided. At the completion of this article it is the intent of the author for a radiologic technologist to be able to offer the best quality care with their new knowledge of how digital imaging works.
Digital imaging can be broken down into two categories: computed radiography and digital radiography. Digital imaging is the proper term to use when collectively speaking about any type of image that is taken with digital media, whether a cassette or no cassette is involved.
Computed radiography (CR) is a way to capture x-rays in a digital format using a cassette and laser reader system. It is commonly referred to as CR imaging. The conversion to a CR system has many benefits, the biggest being the use of the same x-ray table and generator, in most cases. The conversion to CR imaging requires the radiology department to replace the conventional film-screen cassettes with digital imaging cassettes and replacing of a processor with a laser reader. The digital cassettes can be referred to as imaging plates or image receptors.
The cassette used for CR systems looks very similar to that of a conventional radiography cassette. The actual cassette is made of a radiolucent material such as plastic in the front and a radiopaque material like aluminum in the back. Inside the cassette of a CR system is where the big difference begins. The “film” inside the CR cassette is called an imaging plate. Unlike conventional film, the imaging plate can be used over and over. It can also be positioned the same as a conventional radiography cassette. This ability to position the imaging plate similar to a conventional film-screen cassette can help in the transition to digital imaging.
Conventional film-screen imaging requires the use of wet processing with harsh chemicals to form the latent (visible) image. A laser reader is used to produce the latent image in CR imaging systems. The harsh chemicals have been replaced by a red laser beam.
Once an exposure has been taken using an imaging plate, the imaging plate must be "processed" to release the manifest (invisible) image. Releasing of the manifest image is accomplished by placing the imaging plate into a laser reader. The laser reader uses a red laser light to scan the information from the imaging plate. When the red laser hits the imaging plate it (the imaging plate) releases a blue light which is visible. This blue visible light is then captured to create the latent image.
CR offers smooth transitions for departments by requiring fewer equipment changes. Those being the ability to use the same x-ray table and x-ray tube. There are still many other hurdles that can come up when implementing any new system including digital imaging equipment.
Digital radiography (DR) another category under the umbrella of digital imaging does not use cassettes. Digital radiography can be referred to as DR or a cassette-less system. A radiology department that decides DR is the best method will need to replace all the equipment when converting from conventional screen-film. Depending on the manufacturer of the DR system there are a few different set-ups available. One of the set-ups is similar to that of a fluoroscopy table where the table does not move up or down, but does move from horizontal to vertical. There is U-shaped digital radiography equipment where the x-ray tube and film capturing device are connected in a sort of "U" fashion. Which set-up is right for a radiology department depends upon the department's needs.
The DR, cassette-less system captures the x-ray image with a flat panel detector, containing a photoconductor. The flat panel detector takes the place of a traditional Bucky. Manufacturers of DR equipment have made two versions of the flat panel detector. One version is direct conversion the other is indirect conversion.
The difference between direct and indirect digital conversion is very slight. The name of each basically gives it away. With the direct conversion the x-rays are directly converted to a digital signal. With indirect conversion x-rays are first converted to visible light, and then to a digital signal. Theoretically indirect conversion would lose more information compared to direct conversion, if a person was following the rule of thumb that every time a signal is converted a part of it is lost. However, most DR systems are manufactured with an indirect conversion flat panel detector, so the loss of information, if any, is minimal.
There are advantages and disadvantages to each DR system. Each radiology department will need to reflect on their specific needs to decide which is best for their situation. The largest advantage with a DR room is the increase in speed. DR rooms eliminate all steps pertaining to the placement and processing of a cassette, allowing for the manifest image to be seen very quickly. The biggest disadvantage of the DR system would be the need to replace all the x-ray equipment which can be more expensive then a budget has room for.
The largest advantage of a CR system would be the ability to keep most x-ray tables and x-ray tubes in a department that wants to convert. This would allow even a small budget to convert to digital imaging. The largest disadvantage of a CR system would be it is not the fastest system available.
In the end, it is each department’s individual decision which system is right. The important item to remember is with either system over time they will be more cost-effective and environmentally safer than conventional film-screen. This happens through eliminating the purchase of film and harsh chemicals to process the film.
Look Up Table (LUT)
Taking the actual x-ray of the patient with any digital imaging system is just where the fun begins. Now comes all the computer processing. The first step to processing an image digitally by the computer is selecting the anatomical part being imaged on the look-up table, which is referred to as the LUT. The LUT is programmed for various body parts this programming gives the computer a point of reference how to process the manifest image into the latent image.
Each anatomical button is specifically programmed, even down to the position. It is vital to choose the proper part along with its position. When an anatomical part and position are selected this activates the pre-programmed algorithms to interpret the x-ray. An algorithm is a set of mathematical equations the computer uses, in this case, to translate the x-ray into a latent image on the computer monitor. Every button in the LUT is programmed with its own algorithm. If the incorrect part and position are selected it would be similar to doing a multiplication math problem when you were asked to do subtraction. In both instances you will end up with a number or in our case a latent image, but the answer will be incorrect and the image will be of poor quality. Other than for this reason, it is also important to select the proper part and position as this is how the exam is labeled in the archiving system.
A function of the algorithms is to activate a tool called automatic rescaling. Automatic rescaling is a compensatory tool. It can compensate for slight variances in part thickness and moderate technique errors. Automatic rescaling should not be relied upon, but is a nice feature.
Along with the compensatory tool automatic rescaling, digital imaging equipment has wide latitude. Latitude can be associated with a margin of error. Compared to conventional film-screen, digital imaging is much more forgiving of technique errors and variances in patient density. What would have been considered under or over exposed with film-screen systems can often times be salvaged with digital imaging systems. However, take caution with this information. This does not mean you can use whatever technique you want and your image will be diagnostic. A proper technique should always be employed to insure an optimal diagnostic image and as low as reasonably achievable (ALARA) radiation doses.
What is it about a digital imaging system that allows for this wide latitude? Part of the reason is the image manipulation tools. There are several tools available to manipulate the final image. There are simple tools like image orientation that allow users to place the image in proper reading placement. Then there are more complicated tools like image stitching that allows a user to piece several x-rays together to form one. Image stitching is commonly used with scoliosis or leg length films.
In addition to these tools there are also tools available that can alter the contrast and density of the final image. These tools help provide the wide latitude for digital imaging systems. Window and level functions are easily manipulated with the movement of a mouse. Moving the mouse adjusts the gray tones on the image. Instead of being stuck with whatever contrast you have from you exposure, you can now manipulate the contrast in an effort to salvage a once repeatable exposure into an exposure of diagnostic quality. It is this piece that makes the latitude of digital imaging wider than conventional film-screen.
Shuttering is another handy tool provided by a digital imaging system. Similar to the idea of masking in mammography, to allow for better quality diagnosis digital imaging systems allow the background to be black instead of white. So if you can visualize a view box with a hand film hanging you can visualize how difficult it can be to spot hair line fractures because of the brightness around the image. This is where another x-ray or piece of paper is held up around the hand to better visualize it. That is known as shuttering with a digital imaging system. No more finding a sheet of paper, the computer does it for you.
Detective Quantum Efficiency (DQE)
There is a slight variance between all digital imaging systems and how well they can convert x-rays into a useful image. The measurement of the x-rays being converted into a useful signal is termed detective quantum efficiency (DQE). The DQE of a unit is dependent upon what it is made of and how well it is converting the absorbed x-rays into the useful image. The direct signal DR systems have had the highest DQE over both CR and in-direct DR systems. Now there is not much difference with the newer materials being used. Although, digital imaging has not been around for very long advancements and betterments are being found continually.
DQE is more than just the amount of useful x-rays. DQE affects the resolution of the final product. The DQE of any digital imaging system is much greater than that of conventional film-screen systems. Digital imaging systems are much more efficient in absorbing x-rays and converting them to the useful information on an image. This increase in DQE is another aspect of why digital imaging has wide latitude compared to conventional film-screen imaging.
Now that you know digital imaging systems offer wide latitude it is important that you realize there are some things a digital imaging system cannot fix and there are some ways in which a digital imaging system is pickier than conventional film-screen. This is most certainly true when dealing with scatter radiation.
It cannot be stressed enough how important it is with any system to use tight collimation especially to decrease the patient dose. With a digital imaging system the need for tight collimation is even greater.
A part of why digital imaging systems have wide latitude is due to the fact that the imaging plate is sensitive to more energy levels of x-ray, including lower energy scatter. Digital imaging plates should never be left anywhere in a room where they could come in any remote contact with scatter radiation. Imaging plates should be erased even if they are left in the back of an x-ray room outside of the control panel's secondary barrier. Being exposed to secondary radiation before or after a patient x-ray will decrease the diagnostic quality of the image.
The sensitivity to more energy levels of x-ray is the reason tight collimation must be used for every procedure from fingers all the way to chest x-rays. It is important for every technologist to go back to positioning class and remember that the central ray for a PA chest radiograph is placed at T7. Too often technologists place the top of the light field at the top of the shoulders, thus including far too much of the abdomen structures. It is through this act that the diagnostic quality of digital images is compromised. Not only is the image compromised because of the increase in unnecessary scatter radiation, but also because the algorithm programmed to digitize the image is not set up to read half a lung field and half an abdomen field!
Along with proper use of collimation, lead masking should be employed as well. Lead masking should with no doubt be used if the digital imaging system you are working allows for two exposures on one film. It depends upon how the algorithms of each system are set-up whether it allows for two-on-one exposures. Not only is it vital to use lead masking with two-on-one exposures, but lead masks should be used for lateral thoracic spines, lateral sternums, and any other anatomy being imaged that can produce significant amounts of scatter on a final image.
Although digital imaging brings wide latitude in regards to the technique used, it surely does not bring wide latitude for positioning errors. If anything the room for error gets smaller with digital imaging systems.
Of course proper positioning must be employed for the diagnostic quality of an image, but in addition to that because of the way the algorithms in the LUT are programmed a radiologic technologist needs to work diligently to have the part being imaged be in the center of the imaging field.
The need for the part to be in the center of the imaging field is due to the way the laser reads and converts the image. The laser reads from the middle of the imaging plate outward. The format in which digital images are processed makes it very vital for the radiologic technologist to be very proficient in making sure the part being imaged has a central location on the cassette to aid in the proper development of the image. The laser reading from the center out is in addition to the programmed algorithms.
Take for example a chest x-ray, the central ray is to be located on T7, if an image was taken with the central ray located at T12 the computer does not know that and will process the image as if the central ray is at T7 leaving for the distal portion of the lung field to be processed with the contrast and density of the mid-portion of the thorax. This becomes a problem with heavier patients because the computer does not know to compensate for the miss-position of the central ray, leaving the area around the costophrenic angles of poor diagnostic quality.
There are some exceptions to this idea, and that is when the digital imaging system is programmed to read two images on one imaging plate. Not all systems are programmed to allow two or more images on one cassette, for example a finger x-ray, whether the digital imaging system is programmed for two or more images on one cassette would need to be asked of the installers of the system.
The amount of spatial resolution available on a digital image can vary just like that of conventional film-screen images. Similar to film-screen images the properties of the emulsion; thickness of the active layer, number of crystal-halides, and the size of the crystal halides the properties of the active layer of a digital imaging plate can affect the spatial resolution.
The big difference between the spatial resolution available with conventional film screen combinations and that of digital imaging is what you can do after the image is taken. All radiologic technologists who have worked with conventional film know that once the image is processed there is nothing more to be done. Digital imaging on the other hand does have some tools that can be used to help improve the spatial resolution of an image.
Edge enhancement and smoothing are two of the tools available to control spatial resolution. To help process images faster and require less storage space pixel frequencies are averaged across an entire image. This can lead to some areas being grainy. Edge enhancement makes the field of pixels being averaged smaller making the image have higher resolution. Smoothing is very similar to edge enhancement it averages pixel frequencies, but on a smaller scale. Making smoothing the appropriate tool to increase the spatial resolution of small structures, such as a finger, on a digital image.
The negative side to these tools is once they are used and the image is saved you cannot retrieve the original image. One would think no big deal, but it is a big deal. The computer monitors in the x-ray suite have significantly less resolution available on them compared to that of the Radiologist reading station. Therefore what looks like a better picture in regards to spatial resolution in the x-ray suite may be significantly different on a reading station. The best practice to increase spatial resolution without manipulating the image is to use close collimation, decrease the object-to-image distance, and use a small focal spot size whenever possible.
The imaging plate, similar to that of conventional film has many layers. In both conventional film and the CR imaging plate there are protective layers, a reflective layers, and a support layer. In the case of the reflective layer, it is used to project light away from the imaging plate when it is being processed by the laser reader, where in conventional film the reflective layer is used to keep light near the film.
The largest difference between conventional film and a CR imaging plate is the active layer. In conventional film the active layer is the emulsion. Comprised within the emulsion are silver halide crystals in a gelatin suspension. The active layer of the CR imaging plate used for computed radiography is termed the photostimulable phosphor. This photostimulable phosphor is made of barium fluorohalide.
How the x-rays pass through the patient has not changed with digital imaging. What happens after they go through the patient is where the big change occurs. X-rays upon exiting the patient interact with the imaging plate’s active layer made of barium fluorohalide. The x-rays excite the electrons in the barium fluorohalide to the point where they become trapped within the conductive layer of the imaging plate. The conductive layer of an imaging plate is meant to reduce static electricity and absorb the excited electrons.
The absorbed x-rays in the conductive layer are stuck here until they are released by the laser reader. The x-rays will remain stuck here forever until they interact with a red laser, but the interesting part is that immediately after the x-ray exposure the conductive layer actually starts to lose some of the recorded information. An imaging plate should be processed as soon as possible. Probably more interesting than loosing information starting immediately is the fact that a portion of the image will always remain on the imaging plate even after being struck by the red laser and erased. The amount of the image that will remain is very minimal but over time an imaging plate will need to be replaced because of the residual build up of x-rays in the conductive layer. Even with this need for replacement of the imaging plate eventually, CR cassettes are still more economical when you compare them to conventional radiography cassettes with film and how they need to be processed.
After the manifest image is captured on the imaging plate in the conductive layer it must be placed in a laser reader to form the latent image. In the laser reader the imaging plate is slid out of the protective cassette at which time a red laser beam is directed at the imaging plate. The red laser causes the information stuck in the conductive layer of the imaging plate to be emitted as visible blue light. This blue light is than captured by a photodetector, which in turn allows the manifest image to become the latent image.
Once the information has been captured by the photodetector the digital imaging system must convert it to a digital signal so it can be viewed. To do this a number must be assigned to every blue light photon. The number assigned will denote how many gray tones should be assigned to the photon to establish the latent image. These numbers are associated with the histogram that has been preselected on the LUT. The numbers also determine how many gray tones for each light photon will be available in the final image.
Histograms are programmed for each body part to aid in the processing of an image. As the laser translates the information into a digital signal it will read from the center of the film to the collimated edge. The information that is within the collimated field is put into a graph. This graph is called a histogram. The histogram is being adjusted when a person adjusts the window and level settings of an image. Some digital imaging systems allow a viewing of the graph or histogram while an image is being post-processed.
Digital radiography systems use a flat-panel detector to capture the x-rays once they have passed through the patient. Direct converters absorb x-rays in an amorphous selenium material called a scintillator then convert the x-rays into a digital signal. The scintillator used for in-direct conversion captures the x-rays and then converts the x-rays to a visible signal. The visible signal then interacts with a photodetector which converts the visible signal into a digital signal.
A big part of how the final image looks is to do with the matrix it is viewed on. A matrix is a collection of many boxes that forms a digital image. An entire TV is considered a matrix as is the computer monitor considered a matrix. What makes one matrix different from the other is the number of boxes, called pixels contained within the matrix. A pixel is one box, the smallest visible unit of an image. As the number of pixels increase in a matrix so does the resolution of the image.
This is a matrix made of 80 pixels.
Take for example the new digital television sets available, they are sold in 720p or 1080p. The “p” denotes how many pixels are contained within the television set or matrix. To enhance the amount of detail or resolution you see on your television you would want to purchase the highest number of pixels available. The catch here is that television stations only broadcast in 720p, the 1080p televisions are mainly for individuals that watch a lot of movies or play video games.
The resolution of the monitor in the x-ray suite is usually around 1000p. The resolution of the Radiologist reading station is anywhere from 2000p upwards to a 5000p. The resolution needs to be this high to allow for optimal diagnosis of the x-ray images.
There is one other piece of the puzzle a person should look into before automatically picking the highest pixels available and that is pixel pitch. Pixel pitch is how much space is in between each pixel in a matrix. The information that takes up the space between each pixel is an average of all the other pixels around it affecting the final resolution of an image. One can imagine what the image resolution would look like if the pixel pitch was an inch or more big. The resolution would be poor, so a bigger matrix is not always better unless the pixel pitch is minimal.
Forming the Digital Image
No longer can anyone look at an x-ray image and decipher whether it is over or under exposed with digital radiography systems. The only way this can be determined is by evaluating the exposure indicator. Each brand of digital imaging system has their own way of reporting exposure indicators but they all have the same principle behind them. Carestream (Kodak) and Agfa digital imaging systems use a system where as the exposure indicator number increases so has the radiation absorbed by the imaging plate or flat-panel detector increased. They are directly proportional. Fuji’s digital imaging system is indirectly proportional. As the radiation absorbed by the imaging plate or flat-panel detector increases the exposure indicator number decreases. Exposure indicators are not exactly transferable to the amount of radiation received by the patient, but they can give a ball park estimate of over or under exposure.
It is through these exposure indicators that the over or under exposure of a film must be evaluated. The ranges of exposure indicators are primarily set by the manufacturers of the digital radiography system. However, they can be set-up too wide, meaning images can fall within range, but it is not diagnostic. In this situation a radiologic technologist should work with the lead Radiologist and physicist to narrow the acceptable range or exposure indicators.
Because digital imaging systems have wide latitude for exposure techniques there are new ways to decrease patient dose. Any radiologic technologist that has worked with conventional film-screen systems can attest to the need for proper techniques to be utilized. Especially in regards to kilovoltage peaks (kVp) used because this had such an impact on the contrast of an image. Now instead of kVp being the main effecter on contrast it is the algorithms programmed for each body part and position.
It is still important to use reliable exposure factors, but there is room for improvement because the contrast of an image is not mainly dependent upon kVp. What does this all mean? Well let’s review what kVp does to a patient’s radiation dose. As kVp increases the energy of the x-ray beam increases allowing for the beam to pass through the body more easily and leaving less harmful radiation behind in the body. That is why there is less radiation dose to a patient on a chest x-ray compared to that of an abdomen x-ray. Also remember that with an increase in kVp one could decrease the milliamperage (mA) used. This decrease in mA is the primary reason the patient radiation dose went down with higher kVp.
Now let’s put all of the known factors together. We know that with film screen kVp is the main controlling factor of contrast on an image. To produce an image with mainly black and white necessary for a hand or foot a low kVp between 50-60 would be recommended. When radiographing the lung field to get a full perspective of the differencing densities with a long gray scale a high kVp would be employed, 90-120 would be recommended.
In digital imaging the main controlling factor over contrast is the algorithms and histograms programmed in the computer. kVp still plays a role in determining the contrast it should not be completely disregarded. So, if contrast is no longer primarily controlled by kVp it can be fluctuated to decrease the patient’s radiation dose while still producing proper contrast on a film because it is the algorithms and histograms, not the kVp that is the main controlling factor.
The decrease in the patient’s radiation dose comes from a combination of increasing the energy of x-ray beam (increase in kVp) so it can leave less harmful radiation behind and being able to decrease the mA all together because of the increase in kVp. If the kVp was increased with no decrease in mA the patient would actually receive a higher radiation dose. The decrease in mA is vital to lowering the patient’s radiation dose.
The catch to this philosophy of raising the kVp as high as possible and lowering the mA as low as possible is that digital imaging systems all require some minimum mA used. Of course that minimum mA amount is not known so it potentially becomes a trial and error procedure to find the most optimal radiation exposure that produces a diagnostic image.
For example with film screen the technique for finger is on average 55-60kVp with about 2-5mA/seconds (mAs) used. If the kVp was increased to 80 the mAs would need to be 0.5-2.5 to keep the density the same on the film. Some x-ray units do not go to that low of a mAs and more importantly digital imaging systems cannot produce an acceptable latent image with that little amount of mA being produced by the tube.
The other obstacle to keep in mind with trying to increase the kVp and decreasing the mA to decrease the patient’s radiation dose is how sensitive digital imaging systems are to scatter radiation. With an increase in kVp you have an increase in the production of scatter radiation. The morale of the story is to use As Low As Reasonably Achievable (ALARA)! You want to use the lowest radiation dose possible to produce a diagnostic image. If a patient is radiated with any amount of radiation but no diagnostic image is produced then the patient has been radiated for no diagnostic purpose.
It is all about the exposure indicator the digital imaging system reads out. The goal of any radiologic technologist should be ALARA with the production of exposure indicators the objectives to meeting this goal are more easily achieved if the radiologic technologist is diligent in achieving it. X-rays images are given a range of acceptable exposure indicators it is the job of the radiologic technologist to produce a diagnostic image with the least amount of radiation as possible. Whatever the range given for a particular radiograph it should be the goal of the radiologic technologist to be as close as possible to being underexposed but still being within the acceptable exposure indicator range. For example with a Fuji system that is indirect, the exposure indicator would want to be as high as possible without being outside the range.
So why are we seeing dose “creep up” in the radiology departments that have converted to digital imaging systems? The main reason is because it is a known fact that if you slightly overexpose the image with a digital imaging system you can window and levels the image back into a diagnostic range, similar to using a hot light on over exposed conventional film. In contrast to an image being underexposed the digital image can not be salvaged. With a better understanding of how digital radiography systems operate radiologic technologists should be able to work with their physicist to adjust the techniques used and decrease patients’ radiation doses.
Quality Control in Digital Radiography
Along with an understanding of technology advances in the field of radiography just as was done with film, quality management must be administered with digital radiography systems. Quality management procedures need to be in place throughout the entire radiology department to ensure the best quality of care for patient. The new digital radiography systems are not exempted from these best practices.
Quality control for digital imaging systems has similar items compared to conventional radiography and some items that are completely new. The number of quality control responsibilities for a radiologic technologist has decreased slightly with the elimination of the wet processor, but there is still plenty to do. To provide the best quality control one person should be designated to perform and oversee all the quality control procedures in a radiology department.
Repeat/reject analysis must still take place, but it has few changes compared to CR. Repeat/reject analysis can take place on a daily basis, but it is advisable to perform a repeat/reject analysis on a monthly basis. Repeat/reject analysis can be much more simplified with digital imaging systems compared to CR systems because technologists label the reason they are rejecting/repeating an image. The only problem with this is consistency. What one technologist may label a positioning error another may label as motion. To insure the quality of a repeat analysis program each technologist will need to be educated on how best to label their repeats/rejects.
Repeat/reject analysis in a digital radiography room can be broken down by each technologist, but in a CR room each technologist would need to type in their initial manually. It is not necessary to record repeat rates by individual technologists in a general radiography department, but it can lead to pin pointing areas of difficulty. It is recommended that if a radiology department records repeat/reject rates by individual technologists those numbers not be posted for all members of a department to view. This could negatively affect a radiologic technologist to not repeat/reject an image that is non-diagnostic so they have the lowest repeat/reject rate.
Along with doing a repeat/reject analysis on a monthly basis the imaging plates should be removed from computed radiography cassettes. Once the imaging plate is removed it should be looked over for any damage or particles that could cause an artifact on the final image. While the imaging plate is being evaluated it should be cleaned in accordance with the manufacturer’s guidelines. If an imaging plate is damaged and needs to be replaced, the damaged plate must be disposed of properly according to the state and U.S. Environmental Protection Agency regulations because it contains barium.
On a weekly basis the cassettes and imaging receptors should be cleaned if not more frequently especially in a busy department. A deep cleaning should be done on the cassettes and image receptors in addition to a cleaning after each patient to insure the stoppage of infectious diseases from transferring from one patient to another.
Equipment should also be looked over for any damage on a weekly basis. Cords and wires should be evaluated for any cracks. The computer monitors should be cleaned weekly along with the keyboards and mouse. This helps insure adequate viewing conditions and to help decrease the spread of infectious diseases. Laser readers have an air intake vent that allows it to keep all the working mechanisms cool. This vent should be cleaned on a weekly basis to ward off any malfunctions with the laser reader due to poor air quality or flow.
In addition to these simple weekly tasks the manufactures’ of each digital imaging system will have recommendations to follow to insure a properly functioning unit. The quality control technologist will want to diligently follow those recommendations to insure the best quality care and to extend the life of the digital imaging system. Some of the manufacturers’ recommendations may consist of checking the integrity of the system with a designated cassette and testing for proper image acquisition with a phantom.
On a daily basis every CR cassette should go through an erasing cycle to eliminate any amount of scatter radiation. Each imaging plate does go through an erasing cycle in the laser reader after it has been read, however in addition to this every cassette in a department should be erased on a daily basis. This helps eliminate any unnecessary scatter radiation that the cassette may have absorbed since they are so much more sensitive compared to conventional film.
CR cassettes should be wiped down after every patient and their hinges checkedevery day for proper unloading into a laser reader. In addition to checking the hinges the barcode on the imaging plate should be inspected to insure it can be properly read and recognized by the computers information.
The major point for quality control with digital imaging systems is to follow the manufacturers’ recommendations. Most state health departments do not have laws in place yet because this technology is so new they are not sure how to regulate it. To find out your states regulations on quality control contact your states Department of Health. They are there to help you not hinder you.
In addition to the radiologic technologist monthly, weekly, and daily quality control responsibilities a service engineer and physicist will do addition quality control tests on digital imaging systems similar to those performed on conventional radiography systems. The service engineer usually does testing every 6 months to insure quality images are being taken and low exposure rates are being employed. The physicist, depending upon the requirements of the facility, will do their checks for quality on a 6 month or yearly basis.
Picture Archiving and Communication System (PACS)
Maybe one of the greatest advantages for a health care system that comes with a digital imaging system is the lack of storage space needed. No longer will hospitals and clinics need to rent out warehouse buildings to store every x-ray for every patient for at least 7 years. So, where are all the x-rays going? Well, into a PACS of course! PACS is a Picture Archiving and Communication System. This system uses a computer server, which takes up extremely less space compared to housing film. The computer server stores all x-rays taken for a specified amount of time and can even house digitized previous x-rays for comparison. It is not recommended for any health care system to digitize every previous x-ray, only those needed for a specific patient.
The other piece of PACS is the communication part. It is with the computer server that allows physicians the ability to view an image from an authorized location this is considered a form of communication. Where ever a PACS monitor is available a physician can log-in and view the image, this can even be done from home or cell phone in some cases.
In order for images to be saved into a PACS the imaging equipment must contain a DICOM compatible with the PACS. DICOM is Digital Imaging and Communications in Medicine. Since 1985 it has been standard for imaging equipment to contain a DICOM, but the fine print must be read to insure compatibility.
On a very simple basis a PACS is broken down to image acquisition, a computer monitor and a server. Of course it can get much more difficult than just image acquisition, one monitor and a server, but these basics relatively remain the same.
Image acquisition is the taking and digital formatting of the x-ray. The final product after the digital formatting is termed a soft copy. A soft copy is what is seen on the computer monitor. If one were to print a copy of the x-ray this would be a hard copy. Soft copy image acquisition has been performed in special modalities for a long time so the transition to viewing the images only as a soft copy was more like a natural step.
Now what needed to happen was for multiple computer monitors to be able to view the soft copy from various locations. That is where display stations come into the picture. Display stations are computer monitors set up to be able to view images on a PACS. The images available to view on a display station have been approved by the radiologic technologist, so no repeats will be available to be seen. Display stations can be setup throughout a hospital or clinic. They are however very expensive because of the need for high resolution (more pixels) to get a proper view of the image. In some cases, if a physician is ok with poor resolution they may be able to view a soft copy on their Personal Digital Assistant (PDA). It would not be recommended to do a diagnostic reading from a PDA, but it could be used for quick glances. It is all through a PACS that makes any of this possible.
The PACS has really helped the speed of patient care in a radiology department without infringing on quality of care. Before PACS hard copy films would need to be hung on a view box and go through several steps before a report was ready for the ordering physician to read. Then if the ordering physician needed to see the images they would either need to go to the radiology department or wait for the images to be delivered to them. Now with PACS diagnostic imaging has become almost instantaneous.
Additional to being able to almost instantaneously view diagnostic images a person can somewhat manipulate the image on a display workstation. There are applications available that one can use to evaluate the image. The image can be magnified, the contrast can be adjusted, and there are even measuring tools available. In most cases these applications can not be saved to the image, the image will convert back to its original state.
There are also several different types or grades of display workstations. Due to the cost of high resolution monitors usually only those of the Radiologist reading station will be the highest resolution. An ordering or referring physicians display workstation will more likely be of a lesser resolution compared to that of the Radiologist. The radiologic technologist display workstations and QC stations will be of similar resolution of the ordering physicians, usually about 1000 pixels, but will have different applications available to be used; tools that can delete repeated images and tools to add text to the image.
The radiologic technologists display workstation comes with a minimal amount of memory available to archive images. The length an image remains available on these display workstations depends upon how busy of a department they are in. Some of the display workstations will have images from a month ago, while others will only have the days images available.
In order to view the images on the display workstation they (the images) must be saved in an archive. The archive for PACS is computer servers. These computer servers replace the customary file room. The volume of images needing to be saved will determine how big or how many servers are needed for a facility.
There are a few different ways a PACS can communicate with the display workstations and each comes with its own set of pros and cons. There is a client/server-based system which immediately sends the images to an archival server upon acceptance. Distributed systems are also available where images are sent to specific reading stations. For example if MRIs were only read from one location they could always only be sent to that specific location. There is also a web-based system available. Teleradiology uses a web-based system.
A small hurdle in the beginning of a transition from conventional film to soft copy is what to do with old images for a patient that may be needed for comparison or hard copy images a patient may bring from another facility. The simplest solution is to digitize the hard copy films and add a soft copy to the patient’s PACS file. It is not advisable to digitize all old films only those necessary for comparison. The digitizing of any unneeded hard copy would take up valuable archive, computer server space.
The same idea goes for if a patient needs to take their soft copy images from one facility to another. Instead of printing a hard copy most facilities are burning the data onto a CD for the patient to take with them to the other facility. In some cases the burning may not be necessary. Some facilities may be able to access each other's PACS. This is however rare due to the security that would need to be in place to insure there were no HIPPA violations.
It is absolutely vital for anyone working with digital imaging to learn everything they can. The more one understands about the equipment they are using the safer they can use it. This is so important to keep in mind in a radiology department. Radiation is a known carcinogen, it causes cancer no doubt about it, the experts just don't exactly know how much, kind of like cigarettes. It is the responsibility of the radiologic technologist to stay current with the new technologies to ensure best practices are being employed, which includes ALARA!
Carter C. Digital Radiography and PACS. St. Louis, MO: Mosby/Elsevier; 2008.