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Mobile Computed Radiography Imaging

George Tsoukatos, BPS, RT(R)

   *Digital Product Specialist, NY Imaging Service, Inc.
   Address correspondence to: George Tsoukatos, BPS, RT(R), Digital Product Specialist, NY Imaging Service, Inc, 85 Dickson Street, Suite #103, Newburgh, NY 12550. E-mail: georget@nyimagingservice.com.

Disclosures: The author reports having no significant financial or advisory relationships with corporate organizations related to this activity.

ABSTRACT

Mobile radiography is a fundamental aspect of the medical imaging field. These examinations are difficult due to a variety of potential obstacles and variables. In addition, the basic radiographic technology has evolved over the years and has been considerably impacted by the use of computed radiography (CR). Digital technology has improved the transition of workflow and reduced the need to repeat a portable study due to exposure variables. This article provides a review of current mobile radiographic technology coupled with the use of CR, either integrated within the unit or using imaging plates with the system.

Introduction
Mobile radiographic examinations are difficult due to a variety of potential obstacles and variables. In many instances, a portable examination is requested because a patient's condition precludes transportation to the main department, where the imaging environment is more amenable to producing a high-quality radiograph. Portable chest radiographs are the most common form of portable imaging, and comprise 10% to 15% of the images acquired in a typical hospital practice.1 This article will highlight important considerations for the successful delivery of mobile computed radiography (CR) services.

Important Considerations for Portable Radiography of the Chest
The mobile radiographic examination is often the ultimate test of the radiographer's competence and skill. Accomplishing an optimal quality image without stationary equipment on the most difficult patient population is a prime achievement. An understanding of the special considerations involved in mobile examinations is important for the radiographer. Some of the most important considerations are listed below:

  • Patient position
  • Positioning of the patient's shoulders due to the clinical limitations for chest radiography
  • Placement of the CR plate within the variables and potential artifacts presented
  • Potential for an unintentional lordotic radiograph due to positioning or angle distortion
  • Limitations in obtaining the proper source to image distance (SID) and magnification factors

In a surgical setting or the emergency/trauma unit, the stress of performing in a tense environment may compound existing challenges with positioning the patient and equipment. In addition, limitations due to sterile conditions and the presence of additional critical equipment (eg, ventilators, multiple intravenous solutions, cardiac lines, or operating room equipment) can compound the challenges of performing mobile CR.

Good communication with both the patient and the specialty healthcare personnel treating the patient is key to performing a quality examination. The patient's permission must be obtained before proceeding with the examination, an explanation of the procedure should be given, and some rearrangement of equipment and room furnishings are usually required before bringing the mobile unit into the room. The unconscious or incognizant patient requires the same explanation that a cognizant patient would receive. In many instances, conscious but incognizant patients will be more cooperative if they hear a kind voice of explanation prior to being touched. In surgery, the attending physician or operating room nurse must be consulted before entering, and this procedure should be followed, even in an emergency suite.

Features of Portable X-Ray Units
TableX-ray units are available in a wide array of sizes and capabilities, many of which can be classified as mobile. There are 2 major classifications of mobile equipment: portable light duty units and full-power institutional units. Light duty mobile units and some full-power units obtain power from wall outlets. The more advanced full-power units use batteries for a power supply and are capable of greater voltages. Specialized generators have been developed specifically for mobile equipment. Capacitor discharge units and battery-powered units are the 2 most common generators. Capacitor discharge units produce a constant potential output, whereas battery operated units essentially produce 3-phase output.2 Adding battery power to machines not only makes them more portable, but also allows them to be used in locations where wall plugs are not readily available. Newer, longer-life batteries mean machines can be ready to go whenever they are needed. Special features of portable units are summarized in the Table.

Proper Patient Positioning and Pathology
Patients requiring examinations with mobile equipment are often unable to assume standardized positions. Taking radiographs outside of a standard room in the main department presents special challenges that must be addressed with portable systems. The primary challenge is maneuverability. In response to this need, manufacturers are now beginning to offer machines with articulated arms. However, clinical experience and a thorough knowledge of acceptable positioning variants are critical to overcome maneuverability issues. For example, the inability of the patient to sit on the side of a bed requires an anteroposterior (AP) projection of the chest instead of the preferred PA projection. A patient in orthopedic traction may not be able to straighten the knee. Thus, a distal knee joint, mid-shaft and proximal hip exposure will be required to avoid gross distortion and to satisfactorily demonstrate the entire femur from a single projection. Other techniques may include cross-stretcher lateral projections, as well as the utilization of unique angles and concepts to obtain the necessary radiographic views without further endangering the patient's condition. These techniques go hand in hand with setting up the proper SID, and utilization of accessories such as grids, cassette holders and other devices.

Conventional Artifacts and "Obstacles" in Mobile Imaging
Because the mobile examination is performed at the patient's bedside, there is an increased possibility of artifacts, such as personal items (eg, dropped hair pins or jewelry), layers of coverings (eg, insulated blankets or several sheets), and medical equipment (eg, nasogastric tubes, intravenous lines, catheters, electrocardiographic leads, or clamps). A careful and tactful examination of the area of interest should be performed prior to exposure to locate and remove as many of these artifacts as possible. It is usually possible to carry out this examination during positioning, which also presents an opportunity to check for a necklace, bra, brace, and so on. Clamps holding various tubes and lines should be moved as far from the area of interest as possible. For example, intensive care unit chest radiography often requires movement of intravenous, respiratory, and cardiac lines from over the lung fields. A nursing staff member or critical care hospitalist should be consulted before moving any lines, especially venous and arterial lines. For optimal imaging, it is desirable to limit patient coverings to a single smoothed layer of gown or sheet.

During a mobile examination, the radiographer is bringing a radiation hazard into an area not designed for radiation protection. Professional responsibility for ensuring radiation protection becomes a fundamental operating procedure during all mobile radiography. This is a prime opportunity for radiographers to educate the public, health professionals, physicians, and other patients concerning proper radiation protection practices. A duty exists to protect the patient, health workers, the public, and oneself. The mobile unit should not be used as a portable shield. When standing behind a mobile unit, a lead apron is required to ensure sufficient protection.

Radiographers also should move to a maximum distance from the mobile unit by extending the exposure control cord prior to making the exposure. Other recommendations are as follows:

  • Mobile radiography is one of the areas in which the radiographer may receive a high exposure. Utilization of radiation recording devices is essential, as are proper documentation and safety protocols.
  • The radiographer should stand at a right angle to the tube and scattering object, with a minimum 6-foot cord for mobile radiography.
  • Always carry 2 aprons with the machine (for the radiographer and the patient) and/or other gonadal shielding devices.
  • Lead aprons should be worn with the proper thickness and protection factor, as well as other protective devices as deemed necessary (ie, glasses or thyroid shield)
  • Always properly label the cassettes and use the proper marker system for examination and patient position.
  • A dose-monitoring software package should be acquired and used when possible.
  • The radiographer should never place a hand or any other body part in the primary beam.
  • Lead protective apparel must be provided to any parties holding a patient or cassette, as well as proper documentation with a dosimeter of any dose received.
  • Lead shielding must be used for all patients unless it will interfere with the examination or anatomical region of interest.

Source to Image Distance
A primary cause of the need for repeated mobile exposures is failure to measure distance. Every mobile procedure must have SID measured. Most systems have a tape measure connected to the collimator or some other type of measuring system to determine this length. Many departments use technique charts for mobile imaging with different SIDs taken into consideration. In the main radiographic department, posteroanterior/AP upright chest radiography is performed at 72 inches SID to reduce magnification of the heart silhouette. The portable AP chest radiograph is routinely taken at 50 inches SID.3 Mobile variations in technique should take into account the inverse square law when calculating changes as well as other factors, such as utilization of grids.4

Conventional Grids
Proper alignment to a grid is difficult when the patient is supine but the grid is on a soft bedding surface, which easily permits the grid to tilt, unless the patient's weight is almost exactly distributed around the center of the grid. Proper alignment is even more difficult when the film or CR plate is at an angle other than parallel or perpendicular. The radiographer must attempt to align the central ray and the cassette perpendicular to one another.

Grid applications and concepts are recommended on anatomic body parts that are greater than 12 cm in thickness and for chest radiographs of more than 26 cm.5 A focused grid means that the lead strips are placed perpendicularly in the center, and then angled diagonally to correspond to the divergence of the X-ray beam. A crosshatch or cross grid involves 2 sets of lead strips arranged perpendicularly to one another. Positioning is critical with this type of grid to avoid "cutoff." A stationary grid is any grid not associated with a moving device applied to it. A moving grid is a stationary grid that has a motor attached to it to allow movement during exposure, and thereby reduces the visibility of grid lines on the finished radiograph.

Grid specifications and choices are based on the below parameters:

  • Grid Ratio: A ratio of the height of the lead strip to the distance between them by the interspace material. A grid ratio of 8:1 is recommended when filming below 90 kilovoltage peak (kVp), and a grid ratio of 10 or 12:1 is recommended for examinations requiring kVp ranges greater than 90 kVp.6
  • Grid Frequency: The number of lead strips or line pairs per inch or line pairs per millimeter (lp/m).
  • Contrast Improvement Factor: Ratio of the contrast of a finished radiograph made with the grid when compared to the contrast of a radiograph made without the grid.
  • Grid Cutoff: Uneven density or loss of density on the resultant image due to undesirable absorption of the primary X-ray beam by the grid.

Scatter radiation affects the contrast of the final image quality. Most stationary grids run parallel to the length of the cassette. Grid lines should run parallel to the cassette angle. Proper collimation allows the region of interest to be centered so that the image processing algorithm can determine the density of the region. Collimation also reduces the amount of scatter radiation, which results in improved contrast. Ideally, the radiographer should center the body part to the middle of the cassette, with 4 margins of collimation.

Most technologists can draw a simple angle, such as a 45° angle, quite accurately, but accuracy becomes more difficult when the baseline is at an irregular angle. Angling off-center to a focused grid by as little as 5° can result in sufficient grid cutoff that visibly reduces image density.5

Grids that permit wide exposure and centering latitude assist in these problems. Low-ratio gridsFigure 1 (5:1, 6:1, and 8:1) are often preferred for mobile radiography.6 Use of a parallel grid instead of a focused grid also increases latitude.

Some equipment manufacturers provide an "add on attachment" (Figure 1) to mobile radiographic systems that allow for safe placement of the grid. With this attachment, the technologist always has the grid ready for use in a safe and secure location.7

CR Concepts and Workflow
Workflow is an important consideration when considering the integration of a mobile radiographic system with an existing CR service. The 4 major components associated with CR are photostimulable phosphor detector, which is commonly referred to as an imaging plate; the laser imaging plate reader, or CR reader; a computer for processing images (central processing unit and viewing software); and a quality assurance (QA) monitor or tech review station. The mobile system either has an integrated reader or the technologist transfers the plate to the closest reader or back to the main department.

In most instances, when performing mobile digital imaging, a technologist takes the imaging phosphor CR plate with the mobile radiographic system, performs the study, and returns to the closest reader to process the plate and review the image. Some hospitals with large intensive care units (ICUs) have a CR reader stationed in the ICU and a network connection so that the image can be directed to the facility's picture and archiving communication system (PACS), and sent to the radiologist or clinician for review.

Figure 2A few examples that integrate a CR reader within the console of the mobile radiographic system are shown in Figure 2.8,9

Computed radiography 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 CR imaging requires the radiology department or outpatient center to replace conventional film-screen cassettes with digital imaging cassettes, and also requires the replacement of the conventional processor with a laser reader. The digital cassettes can be referred to as imaging plates or image receptors. In terms of spatial resolution, conventional film-screen technology can resolve between 6 to 10 lp/m, whereas CR is limited to approximately 2.5 lp/m.10 Even though the spatial resolution is lower with digital imaging, it is the improvement in contrast resolution that makes the final image quality preferable.

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 such as aluminum in the back. The insides of conventional radiology cassettes, compared to those of a CR system, exhibit greater differences. The CR cassette holds within it an imaging plate, or photostimulable phosphor detector. The CR cassette also can 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. Positioning plays a much more important role with a CR system than it does with conventional film because of inherent problems in reading the imaging plate. This greater need for specificity in positioning and requirement to be in the center of the imaging field is due to the way the laser reads and converts the image. More minor differences between traditional cassettes and CR cassettes are the fact that CR cassettes have a barcode identification window and a green orientation strip along one edge. If this orientation strip is not placed at the top or on the patient's right side (AP), the image will appear upside down when processed.

The imaging plates can be reused unless they become scratched or otherwise damaged, or until they reach their life expectancy (based upon exposures or age by manufacturer and QA testing results). Unlike film, imaging plates are not exposed by normal room light and may be removed from the cassette for cleaning or inspection.

The imaging plate (Figure 3), similar to that of conventional film, has many layers.11 In both conventional film and the CR imaging plate there are protective layers, a reflective layer, and a support layer. In CR imaging, the reflective layer is used to project light away from the imaging plate when it is being processed by the laser reader; in conventional film, the reflective layer is used to keep light near the film.12

Figure 3 

The largest difference between a conventional film-screen 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 CR is termed photostimulable phosphor. This photostimulable phosphor is made of barium fluorohalide doped with europium (BaFx: Eu2; Figure 3), which is used as an activator to create a luminescence center. The 2 most common types of phosphor configurations in the imaging plate for CR systems are turbid or structured.13 A turbid phosphor configuration is a phosphor layer with a random distribution of phosphor crystals within the active layer. With structured (needle) phosphor, a phosphor layer with columnar phosphor crystals is within the active layer, which resembles needles lined up on end and packed together.14

Components of an imaging phosphor plate include the following:

  • Protective Layer: A thin layer of transparent film that protects the phosphor.
  • Rare Earth Phosphor Layer: A closely dispersed layer of fine-grained crystals of yttrium, gadolinium, or lanthanum, which immediately convert X-rays into visible light, with a minimum of afterglow.
  • Barium Fluorohalide Phosphor Layer: A closely dispersed of fine-grained, photostimulable, europium-activated, barium fluorohalide crystals, that store the latent image until released when restimulated during processing.
  • Light Reflective Layer: This layer increases the intensity of light being emitted from the crystals by reflecting it back toward the reader, instead of it being absorbed.
  • Conductive Layer: This is a light absorbing layer, made up of conductive needle-like crystals that absorb any unreflected light as well as any electrostatic charges.
  • Polyester Support Layer: Made from a polyester material, this layer gives structural rigidity and a base for the coating of all of the other layers. Polyester is used because of its excellent stability as well as its durability and flexibility.
  • Light Shielding Layer: This is a carbon particle layer that prevents the light from leaking from the rear of the imaging plate.
  • Backing Layer: This is a protective layer made from a soft polymer that prevents scratching when the plates are stacked during the manufacturing process.

Once an exposure has been taken using an imaging plate, the imaging plate must be processed to release the manifest (invisible) image. The manifest image, or stored energy, is released from the phosphor layer by placing the imaging plate into a laser reader. The CR laser reader uses a red laser light to scan the information from the imaging plate. When the red laser hits the imaging plate, the plate releases a blue, visible light. This light is then captured to create the latent image. When using a mobile radiographic system with an integrated CR reader, the information technology department usually establishes a secure wireless communication or hardwire network connection for communication to the radiology information system, hospital information system, or PACS  using Digital Imaging and Communications in Medicine (DICOM) protocols.

All CR systems offer a degree of exposure latitude much greater than that with film-screen receptor systems. Consider that the dynamic range of exposure for photostimulable phosphors is linear over a range of greater than 10 000 to 1,15 whereas the dynamic range of analog radiographic images produced by screens is roughly 40 to 1. With this substantial increase in dynamic range, overexposure or underexposure of radiographic images seen in conventional film-screen imaging is virtually eliminated by photostimulable phosphor technology imaging. This does not mean that images acquired at the extreme low and high values can be optimized into a high-quality image, but it does mean that all values of an exposure can be represented and discriminated on the final image. The database of the CR system can correct for extreme overexposure and underexposure. Although this ability to optimize image quality can be quite valuable, the radiographer may unknowingly overexpose the body part being imaged and deliver an unnecessary dose to the patient. With a better understanding of how digital radiography systems operate, radiologic technologists should be able to work with their physicist and equipment services agent to adjust the techniques used with the aim of minimizing patients' radiation doses and avoiding the phenomenon known as dose creep (the inadvertent use of more radiation exposure to ensure a high-quality image).

Other key terms to image processing and CR system functionality are as follows15:

  • Detective Quantum Efficiency is a measure of a receptor's ability to create an output signal that accurately represents the input signal (X-ray beam).
  • Photostimulable Phosphor Conversion Efficiency is the ability of the storage phosphor to convert the signal exiting the patient into trapped electrons.
  • Figure 4Image Smoothing: Used to reduce random noise by averaging neighboring pixel intensity values. Averaging causes the odd pixel to blend with the surrounding pixels, thereby reducing its sharpness to the eye; however, some detail is lost.
  • Image Sharpening: This filtering process enhances the edge detail to increase the detail of small, high contrast structures. This technique is often used to evaluate C-spines or other small boney areas, important with portable radiography for trauma imaging. High contrast resolution provides the ability to distinguish between similar tissue densities by capturing thousands of shades of gray, many more than the human eye can pick up.
  • Modulation Transfer Function (MTF) defines border characteristics, or how clearly and sharply edges can be distinguished. It is one thing to be able to discern individual line pairs, but MTF is an indication of how sharp the line pairs appear. Simply stated, MTF measures spatial resolution.
  • Window Level controls the brightness of the image. Increasing the window level provides a darker image and decreasing the window level provides a lighter image (Figure 4).16

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 then put into a graph, called a histogram. The histogram is being adjusted when the technologist 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. An example of a chest-specific histogram is shown in Figure 5.15,17

Figure 5

The process of enhancing the raw image data is called image segmentation.18 The image must then be adjusted before the data are sent to workstations, or printed. The raw data are subjected to various algorithms and lookup tables (LUT) that define areas of interest and collimated areas. The average density and LUT control the overall density and contrast of an image. The final image is first available on the remote operator panel (ROP) also known as the "tech QA or review station." What is important for the technologist to understand is that specific software algorithms must be applied to the image prior to presenting it as a finished radiograph. These modifications of the image occur in the reader programs and at the workstation using LUT as reference. The reader must then be told what cassette contains the image, which is accomplished by barcoding the cassette with the appropriate algorithm selected at the reader or ROP.

Once the study is selected and the cassette is barcoded, the technologist may proceed using the cassette as one would use a screen-film cassette. In digital imaging, algorithms are selected rather than cassette types. In screen-film imaging, the technologist may use a different screen-film type for a kidney-ureter-bladder image than they would for a forearm image. In digital imaging, the same cassette is used, but the computer's software selects the appropriate processing algorithm to process the photostimulable plate. This is a very important difference between screen-film imaging and CR.

In summary, during X-ray exposure the incoming X-ray energy is absorbed and stored in the barium fluorohalide phosphor layer of the imaging plate at a given absorption quantum efficiency. During image processing, laser light excitation of the phosphor crystals in this layer releases the converted X-ray energy stored in the imaging plate as blue-violet light.19 The information stored on the imaging plate is in analog form and is not converted to digital information until the image is processed. Regardless of the type or manufacturer of the CR imaging system, the technologist must view the image on the reader station monitor and either accept or reject the image based on the exposure index. An accepted image is then sent to PACS for image review on network workstations, or the image can be printed for conventional reading and filing. The plate is read, and then it is erased by exposure to a white light and then replaced back into the cassette, ready for the next study. It should be noted that most CR imaging plates have a relative sensitivity equivalent to 200 to 300 speed film-screen combinations and may be used for approximately 10 000 images before the image quality begins to degrade (see the manufacturer of origin operators manual and guidelines for specifics).

Radiographic Techniques with CR
Manual techniques are extremely important in digital imaging because a variable kVp or variable milliampere seconds (mAs) chart will help the technologist achieve uniform exposure indexes for portable images. One cannot use identical exposure techniques (kVp or mAs) as those used for film. Unfortunately, there is no golden rule to follow, because techniques vary for each device manufacturer. Even for a particular manufacturer, technique depends on the type of screen or detector that is used. Correct techniques and positioning are very important when using LUT, or pathology may be lost. A technical factor that may not be well understood in CR is that kVp does not influence contrast. This presupposes that the kVp used to obtain an image is adequate to penetrate the anatomical part. In digital imaging, the technologist is able to use higher kVp values then conventional, resulting in lower patient dose. Overexposure lacks any value whatsoever in digital imaging. For example, in screen-film conventional imaging, overpenetration of the chest may be needed, to accentuate the placement of a line. This is not necessary with CR imaging because this detail can be achieved through the image processing.

The role of kVp and mAs are different in CR and screen-film modalities. CR uses image processing algorithms to control density and contrast. In film screen, kVp primarily controls contrast and mAs controls density. In CR, kVp still affects contrast, but image processing LUTs control the majority of the contrast seen on the image. The new role for mAs with the help of kVp is to control the amount of graininess or noise seen in the image. When adjusting technique for CR, the technologist should use the same kVp values to properly penetrate the body part in question and utilize mAs for the amount of tolerable noise.

Exposure Indexes
Because CR systems are not integrated into the X-ray equipment, they generally provide an indication, by number, that tells whether an exposure is within range. Manufacturers use different values and different terminology to indicate overexposure and underexposure, but they essentially convey the same information. The confusing part is that with some devices, the number should be low for less exposure, and other devices use lower values for higher exposure. Importantly, technologists should check directly with the manufacturer to verify how their system's exposure index works and relates to both dose and image quality improvement.

Other factors contributing to exposure index numbers include:

  • Scatter (more scatter = higher exposure index numbers)
  • Distance: SID (dose and scatter)
  • Collimation (correct collimation reduces scatter)
  • Examination selected at the imaging display terminal (due to histogram analysis) x Delay in processing from time of exposure

Examples of various manufacturer terms for exposure index include logarithm of the median exposure number, sensitivity number, exposure index value, or reached exposure value.3

The Importance of Collimation and Scatter Radiation Control
The impact of scatter radiation is even more pronounced on CR. If one does not properly collimate the area to be exposed, the result is unacceptable image quality. This is especially important with imaging extremities. Thorough technologist training is therefore necessary. An important note is that the black mask, border, or shuttering should not be used in place of effective beam restriction. It is critical to apply the black mask or shuttering to an image outside the actual collimated border of the exposure field. This is important to proving that a technologist exercised prudent judgment in using beam restriction. An additional device that also may provide scatter reduction is a customized filter. Depending on the body part to be radiographed and viewed, a custom filter can be placed into the collimator rail and used for density compensation on body parts that have wide variations.

Computed radiography is more sensitive to scatter radiation. Noise manifests itself in 2 ways in CR imaging. Black pepper noise, also known as stale plate noise, is due to residual exposure on the plate. This can be caused by the cassette laying dormant and picking up residual radiation, or sitting in the radiographic suite. That is why it is prudent QA practice to erase unused imaging plate cassettes every 24 hours. White noise represents under penetration or quantum noise. Exposure index levels can be used to characterize the level of under penetration. Under-penetrated images in CR appear to be noisy or have a grainy, mottled look to them.

CR Grids
When using CR and grids in a mobile environment, very specific line spacing (grid frequency) is required to avoid the Moiré pattern, which is a summation artifact caused by the scanning laser beam overlapping with the grid line structure.3 Moiré artifacts can occur if the laser scan of the CR runs parallel to the grid lines. Most grid frequencies are between 80 and 178 lines per inch; the higher grid frequency provides better image quality.20 Furthermore, grid suppression software will help reduce the visibility of this artifact. Grid suppression algorithms also can help eliminate the visibility of grid lines when utilizing a stationary grid. The Moiré pattern is really an interference pattern generated by the coincidence of the grid lines with the scanning laser beam.

Due to the high cost of both conventional and custom CR grids, it is highly recommended that a protective encasement be purchased with the grid. Encasements come in a variety of options including a grid cap ("drop on"), encasement with channels ("slip on"), or a flat encasement. The materials that the encasements are made of are usually aluminum or lightweight polypropylene.

Cassette Imaging Plate Care and Cleaning
Cassette imaging plate care and cleaning are especially important for CR. The CR cassettes easily collect dust. This dust can be seen on the images, and can even damage the plates by creating scratches if it gets inside the CR readers. The solution to avoid dust is quite simple: once a month, or more often if the environment is dusty, each plate should be taken out of its cassette and cleaned with the vendor-recommended solution. CR cassettes should be wiped down after every patient and their hinges checked every day for proper unloading into a laser reader. Infection control protocols should take into consideration the use of the cassette in specialty areas and the potential for cross contamination, especially for mobile or trauma applications. Many facilities include cassette covers in the routine cleaning protocol. In addition to checking the hinges, the barcode on the imaging plate should be inspected to insure that it can be properly read and recognized by the computer. The QA Coordinator and/or Biomed/Service agent should assign this task to a dedicated staff member.

Reject Analysis
The image undergoing QA could be deemed to be unsatisfactory, either due to patient movement, the fact that the required anatomy is not shown, improper technique, or a variety of other reasons.

In the film world, the number of rejects was easy to track-one could just review the contents of the "reject" bin and do an analysis. The equivalent of the reject bin is the recycle bin on the workstation; however, someone needs to review these rejects, match them with the technologist performing the examination, analyze the results, and, on a monthly basis, schedule retraining and/or make changes in workflow to reduce these occurrences in the future. Many of the CR systems have the software to calculate repeat reject analysis by repeat rate either by anatomical region or examination type. Analysis of this data should be conducted by the QA coordinator.

Benefits and Advantages of CR in a Mobile Imaging Environment
Vendors are introducing new technologies, such as double-sided CR plates, and new compounds for their screens that result in increased sensitivity. In general, CR plates appear to require a greater dose to get to the same level of image quality; however, as stated, it depends heavily on the plate manufacturer. A new concept from some manufacturers is to offer specialty visualization software options; for example a companion image is produced from the original exposure. This software automatically enhances the regions of interest without window and leveling. By eliminating the need to adjust the image, the radiologist's reading efficiency can be improved.

One of the niceties of CR is that image data are already in digital form so it can easily be linked onto the PACS network. Because CR adheres to DICOM standards, these units adhere to the various subclass standards for compatibility. From the reader a link can be established directly to a wet or dry laser printer using DICOM Print Management Service Class, and to PACS storage servers using DICOM Query/Retrieval Service Class. The images also can be displayed to any workstation in the PACS network, which significantly decreases emergency department/trauma wait time.

The following is a summary of the special advantages of digital CR that cannot be achieved by analog screen-film imaging:

  1. X-ray exposure and display of the image are uncoupled; therefore characteristics of image presentation, mainly optical density and contrast, become less significant in the raw data.
  2. There are a limitless number of "original images" available for viewing which can be outputted to multiple stations simultaneously without intermediate copying of the images, as with screen-film radiographs.
  3. Digital images can be transferred over a local area network or wide area network without any deterioration for all degrees of image spatial frequency. This includes CD-ROM, Internet, and teleradiology.
  4. A film cost savings is definitely possible if viewing over a workstation is the primary means of display and multiple images are printed on a single sheet, when measurement is not a consideration.
  5. The digital image can be adapted to any viewer's requirements by image processing algorithms and post-processing functions of software.

The following tips and techniques for acquiring and processing a quality CR image should be considered when converting from conventional screen-film technology:

  1. If a cassette has not been used in 48 hours, it should be erased using the Secondary Erasure. Use Primary Erase for direct X-ray exposures (exposure errors) on the imaging plate. Do not leave CR plates propped up in a room during examinations. Do not place CR plates near any radiation emitters.
  2. Use the smallest imaging plate as possible and appropriate for each examination. When doing extremity work, 2 or more views can be accomplished with 1 cassette. Keep the views close together and use lead strips (blockers) to mask the views. Take caution not to overlap exposure areas. Collimate to the proper field size: avoid having extra anatomy in the image.
  3. When part thickness is greater than 3 inches, a grid is recommended (ie, shoulder and knee). Ensure that grid ratio, focal film distance, and lp/mm match correctly with the CR system's specifications. Use PACS pre-fetching and access to radiologists of relevant pre-day prior mobile studies for comparison.
  4. Use the correct algorithm. The algorithms that process digital images are set so that the automatic rescaling and LUT will give the appropriate density and contrast for each body part. If the image does not appear correct due to technique, location, or another reason, do not attempt another algorithm. Chronically wrong algorithms should be adjusted.
  5. Use the correct technique. Strive to use a technique that puts the image exposure indicator in the center of the range for the system. Doing so will keep the image from being under- or overexposed and limit patient exposure. Consider using higher kVp and lower mAs based on the 15% rule. Post an accurate and proper technique chart to match the radiographic system generator output and the CR capture device.
  6. Collimate instead of crop whenever possible. Limiting field size by collimation both reduces patient dose and limits scatter radiation.
  7. Appropriate kVp for portable chest (non-grid) should be 70 to 85 kVp. Do not use above 90 kVp without a grid. In portable chests with a grid, the kVp should never be higher than 110 kVp.
  8. Use markers. Place markers before exposure to eliminate any questionable results from image manipulation. This practice is better from a legal perspective and less confusing for others who view the images. Consider using the latest marker technology designed for digital imaging.
  9. Perform quality control activities. Digital systems must be checked periodically just like screen-film systems. Images must be checked for positioning errors and for exposure values. This includes a cleaning and inspection protocol for imaging phosphor plates and cassettes. Repeats and accepted images should be checked to ensure dose creep is kept at bay, and review monitors should be evaluated for any flaws.
  10. Centering and positioning are very important! Keep the patient well centered on the cassette. Poor positioning is one of the leading causes of repeating a study.
  11. Limit image manipulation. Digital images should be manipulated as little as possible before being sent to a PACS. The more an image is manipulated, the less information, or data, that is sent to PACS, which means the radiologist has less information with which to work.
  12. Seek educational opportunities through books, online courses, or seminars. Digital imaging is still relatively new. Books and online courses allow for convenient study, while seminars put technologists face-to-face with others who are finding their way through the digital age of radiography.

Conclusions
In the fast-paced emergency department environment or ICU, a portable system utilizing CR technology can have a positive impact by speeding up the acquisition of images, enabling instant image viewing and wireless transmission, and providing an image that can be manipulated for soft tissue or bone window viewing. This all plays a role in the overall diagnosis and patient care decisions made by the clinician.

The factors that make a good technologist are identical in both the screen-film world or a digital world. Although digital systems are more forgiving in variances in technique, the fact is that a poorly positioned examination will be repeated, regardless of systems. The skills do not change, but some new skills are required, including understanding computer systems as well as aspects of image processing to ensure optimal image quality.

References
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Comments/Questions

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Mobile Computed Radiography Imaging

 
» Comment From: blessed2007 » Posted on: 10/28/2010 20:42 PM
This article was like a refresher article of what we learned in radiologic program
 
» Comment From: oca » Posted on: 11/06/2010 9:55 AM
very informative
 
» Comment From: dfisher390 » Posted on: 11/10/2010 18:54 PM
this did not post to my account and I cannot print my certificate with the CE code on it
 
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