5 keys for Digital Radiography

dose-creep

By: CE4RT

Computed Radiography (CR) and Direct Radiography (DR), are now the standard at most hospitals and imaging centers. There are some key concepts that every X-Ray tech should know about these digital radiography systems.

 

1. Digital Vs. Film, Know the Difference

Although film radiography had made advances from the early days such as high speed film, rare earth screens, etc. digital systems are finally the norm. CR uses digital cassettes that are portable and hold a latent image, DR captures the image directly onto a plate and immediately transfers it. The use of digital systems for x-ray offers several radiation protection advantages over the use of film. For one thing, modern digital receptors are more sensitive than film. This offers the opportunity to reduce mAs on technique charts. Additionally, digital systems can reduce repeat exams in several ways.

  • The images produced by digital systems are typically more clear than those from film.
  • Digital images can easily be edited and enhanced with viewing tools such as invert, zoom, etc. for multiple ways to look at the subject on a single exposure. Such enhancements do not alter the original diagnostic copy of the image.
  • When properly archived, digital images are almost never lost and don’t go away when the patient needs them checked out of the hospital.
  • Processing and delivery of digital images is much faster than film. An earlier diagnosis can increase the chance of effective treatment.
  • With film, processors are occasionally problematic. Processor rollers break and films get stuck, chemical temperatures and concentrations can vary if the processors are not operating correctly. Although occasionally there are technical problems with digital delivery, these are rare. The main cause of problems in digital radiography is human error.
  • Some digital systems have automatic exposure controls which reduce the chance of incorrect techniques.

As the technology has advanced, there are very few advantages remaining for film. The primary advantage remaining is that once the film is taken and developed, it can be viewed without any electricity whatsoever. This may seem trivial until it is needed in a remote location or during a blackout. And this can be overcome if the situation is anticipated by printing from digital to film.
Also, although in the long run for large scale imaging centers, digital offers a cost advantage, the cost of implementing and maintaining a film system for low volume imaging is still lower than a digital system considering the staff and equipment to maintain the computer ifrastructure required to practically implement digital systems. It used to be a barrier to entry into digital that there was a learning curve for the computer software needed by the physicians and ancillary staff, however nowadays everyone has basic computer skills.

Many experts claim that the techs who take the best digital radiographs are the techs who learned on analog. This is because they learned to rely on good technique rather than depending on the equipment. The younger generation of techs will probably never know the smell of developer.

2. Follow the CR / DR Checklist

CR and DR exams are now the cornerstone of general medical radiography, which is a major source of manmade background radiation. Radiologic technologists play a key role in protecting the public by following best practices when performing digital radiography exams. The following is a checklist of best practices when performing a CR or DR exam adapted from the Image Wisely campaign. If using film, modify the checklist to include practices specific to film including film selection, developing, processor maintenance, etc.

As you greet the patient

  • Has the patient been properly identified?
  • Is the order appropriate and match the patient’s complaints?
  • Has the exam been explained to the patient and/or their parent or guardian?
  • If female, has the patient’s reproductive status been verified?

Before you begin

  • Is the exam information acquired from a modality work list?
  • Have all unnecessary persons been cleared from the room prior to exposure?
  • Is there alignment of the beam and body part and image receptor?
  • Is the signal to image distance (SID) correct?
  • Is a grid needed and appropriately placed?
  • Is the beam properly angled and collimated?
  • Have markers been placed?
  • Is shielding necessary and placed correctly?
  • Have the correct technical factors been selected?
  • Have positioning and breathing instructions been given and understood?

During the exam

  • Are images processed correctly in the reader?
  • Does the correct exam information appear on the images?
  • Is the exposure index correct for the exam?
  • Is the image masked correctly?
  • Are any digital annotations needed?
  • Is the image processed correctly?
  • Have any necessary notes regarding the exam such as medications been charted?
  • Are all images visible and correct in the diagnostic viewing system?

3. Watch Exposure Indicators

When using CR or DR, an exposure indicator provides feedback by displaying the relative exposure at the image receptor, which is dependent on the efficiency and sensitivity of the digital receptor. Without an exposure indicator, verification of the appropriate exposure factor selection is impossible. There is, however, a learning curve for technologists to understand the distinctions of digital image acquisition, processing, and display. One problem is that the manufacturers of digital radiography equipment each use different ways of determining and reporting the exposure indicator. Variations in terminology, units, mathematical formulas, and calibration conditions of these exposure indicators create some confusion for technologists, radiologists, and physicists. This confusion grows when the technologist must work with systems from more than one vendor, or when proper training is not provided for the available unit.

Fuji of Japan uses a sensitivity number (S) which mimics the concept of “speed class” familiar to technologists who have used film-screen radiography. Carestream of New York labels its exposure indicator “exposure index,” which represents the average pixel value of the clinical region of interest. Agfa of Belgium has a CR exposure indicator, lgM, which represents the logarithm of the median exposure value within a region of interest. Most DR manufacturers were slow in developing exposure indices at the image receptor.

Fuji
corporation S Number 
 Agfa corporation IgM  Kodak/Carestream – Exposure Index   Detector Exposure – Estimate (mR) Action
> 1000 < 1.45 < 1250 < 0.20 Underexposed: repeat
601-1000 1.45-1.74 1250-1549 0.2-0.3 Underexposed: QC
301-600 1.75-2.04 1550-1849 0.3-0.7 Underexposed: Review
 150-300  2.05-2.35 1850-2150 0.7-1.3  Acceptable range
 75-149  2.36-2.65  2151-2450  1.3-2.7  Overexposed: Review
 50-74 2.66-2.95  2451-2750 2.7-4.0 Overexposed: QC
 < 50  > 2.95  >2750  > 4.0  Overexposed: repeat if
necessary

Source Image Gently: Using Exposure Indicators To Improve Pediatric Digital Radiography – Abbreviation: QC = quality control.

4. Avoid Dose Creep

Dose creep is the unintentional increase of exposure that can occur in digital capture systems because of the digital systems’ flexibility in processing a wide range of exposures into good images. With analog screen-films, the fixed speed of the film along with standard processor controls requires that the exposure is correct, otherwise the image is either too light (underexposure) or too dark (overexposure). In digital image acquisition, this immediate feedback is lost. DR and CR systems work very well in compensating for poor technique. A problematic situation called ‘dose creep’ can occur with digital radiography systems, especially CR type systems that do not use automatic exposure controls. This is because except for extreme overexposures, images that are produced are usually of excellent radiographic quality due to the ability of the digital detector system to optimize images for viewing on a soft copy monitor or hard copy film. Some estimates say that Digital image processing can compensate by 100% for underexposure and more than 500% for overexposure. Unfortunately, the patient in this situation can receive unnecessary radiation exposure, often without the knowledge of anyone involved in the acquisition or reading of the case. It’s not inconceivable for a patient to receive three to five times overexposure without any complaints from anyone.

Dose creep can occur because underexposures can look bad, but overexposures look fine. This creates an incentive for the technologist to overexpose. The goal as always is to get an image with the highest diagnostic quality using the lowest possible exposure. To help technologists accomplish this, most digital detector systems have an “exposure indicator” that provides some feedback as to the relative exposure that was incident on the detector based upon the analysis of the raw image data intensity and subsequent scaling necessary to produce an image with appropriate brightness and contrast settings.

5. Collimate

This should go without saying, but it is essential that radiographers carefully use collimation to the appropriate anatomy of interest when performing examinations to minimize patient exposure and prevent errors in processing of the digital image data. By limiting the anatomy that receives radiation, a smaller area of the patient’s tissue is exposed, thereby reducing patient dose and minimizing scatter radiation to the patient. Collimation is also important in radiography because the resulting image might demonstrate reduced image contrast due to excess scatter radiation striking the receptor. Digital radiography systems have software that provides electronic masking (collimation) based on recognition of the borders of the exposed area of the image receptor; radiographers may need to adjust the electronic masking to accurately align it to the exposure field. But masking, shuttering or cropping should not be used as replacements for beam restriction achieved through physical collimation of the x-ray field size. Masking never should be used to cover anatomy that is contained within the exposure field at the time of image acquisition because of legal and radiation safety concerns. Also in digital systems, mathematical algorhythms are used to adjust brightness and contrast and in some systems an image with white space can throw those calculations off.
A best practice in radiography is to collimate the x-ray beam to the anatomic area appropriate for the procedure. Electronic masking to improve image viewing conditions should be applied in a manner that demonstrates the actual exposure field edge to document appropriate collimation. Masking must not be applied over anatomy that was contained in the exposure field at the time of image acquisition.

 

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