How would a film radiograph with excessive density be described when viewed on a display monitor?

Digital Radiography, Radiology

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Knowing how number is used is key to controlling exposure.

Imaging in a radiology film environment is much like playing Goldilocks and the Three Bears. You take your image, hold it up to the viewbox and say: “This image is too light”; “This image is too dark”; or, “This image is just right!” If you underexpose your image, it will be too light, and if you overexpose the image, it will be too dark (See figure 1). The density and contrast of the image on film is controlled by the kV, mAs and other exposure factors.

However, with digital imaging devices, brightness and contrast are no longer linked to exposure factors. Digital systems produce images with consistent density and contrast regardless of the exposure factors (See figure 2). So how does a radiographer know if a digital image is over- or under-exposed?

The potential for gross overexposure is one issue we encounter when a radiology department or clinic changes to a digital image receptor. The reason for this increased risk is that we’ve lost the visual connection between the exposure and an image’s appearance. That’s why it’s so important for the radiographer to understand how to read and utilize the exposure indicators.

Read the related blog on Diagnostic Reference Levels.

On digital imaging systems, an exposure indicator provides useful feedback to the radiographer about exposures delivered to the image receptor (ASRT, 2010). An over- or under-exposed image will deliver an incorrect exposure indicator; whereas a correct exposure will provide a corresponding exposure indicator. The indicator is a vendor-specific value that provides the radiographer with an indication of the accuracy of their exposure settings for a specific image (ASRT, 2010). The exposure indicator has as many different names as there are vendors in the market. The names include S-number, REG, IgM, ExI and Exposure Index.

Carestream’s computed radiography (CR) and digital radiography (DR) systems both reference their exposure indicator as the exposure index or EI. After an exposure is made, the resulting image appears on the monitor and displays a number in the Exposure Index field. The number is a representation of the average pixel value for the image in a predefined Region of Interest (ROI).

The exposure index allows the radiographer to match the exposure to the desired speed class of operation. The speed class is set in a given department by consulting with an interpreting radiologist. The radiologist’s feedback on sample images helps determine the level of image noise he or she can accept. It’s important to note that, as speed class increases, so does the amount of image noise. Once an acceptable noise level is established, a radiographer can identify the speed class of operation for the imaging system and the corresponding technique charts. It’s the responsibility of the radiographer to select a technique that provides enough exposure to reduce the amount of noise while also adhering to ALARA standards.

The exposure index is indirectly proportional to the speed class of operation. If you’re using the Carestream Exposure Index values, for every 300-exposure-index increase, the speed class is reduced by half. In other words, if the exposure index increases from 1400 to 1700, the speed class is reduced from a 400-speed class to a 200-speed class. The Carestream EI is not necessarily unique to the receptor type. However, CR systems typically operate at a lower speed class than DR systems.

IEC Exposure Index is international standard

Remember that each radiology imaging manufacturer has its own method of providing exposure indicators. This can be confusing to radiographers who have multiple vendors within their facility. Fortunately, there is a standard for exposure index for digital X-ray imaging systems. Developed concurrently by the International Electrotechnical Commission (IEC) and the American Association of Physicists in Medicine (AAPM), in cooperation with digital radiography system manufacturers, the index has been implemented as an international standard. It’s known as the IEC exposure index. Carestream systems are configurable for the user to display the Carestream EI, the IEC EI, or both.

The IEC exposure index is unique to the receptor type being used and to the exam performed. Three default Target Exposure Index (TEI) values are preloaded into the system. The three values represent the default Target EI for bucky, non-bucky and pediatric exams.

Once the operating speed class is determined, the key operator can adjust the Target EIs to correspond to the recommendations made by the facility’s physicist. After an exposure, the IEC EI will display, followed by the deviation index (DI) in parentheses. The deviation index quantifies the difference between the actual EI and the Target EI, and this feedback allows the radiographer to track and adjust his or her exposures. When the actual EI is equal to the Target EI, the DI will equal 0. A positive or negative DI indicates the amount of exposure greater or lesser than the target EI. It does not necessarily mean that an image needs to be

repeated. If the deviation is greater than +3, the exposure index displays in red to indicate a high/low exposure that might need further review.

The DI chart below outlines how to use the deviation index. In the example above, the DI was calculated as 1.06. In the chart you’ll see that a DI of 1 means the resulting exposure was ~26% higher than the Target EI. The initial DI was 1.06, so we can estimate that we are slightly higher, perhaps closer to 30%. Although it might be a good image, it is merely an indicator to the radiographer that he/she might be able to reduce the exposure factors the next time a particular exam is performed- reducing the dose to the patient while still acquiring an acceptable image.

Martin Pesce, RT, is Clinical Development Manager at Carestream

The wide exposure latitude of digital radiography devices can result in a wide range of patient doses, from extremely low to extremely high. An "appropriate" patient dose is that required to provide a resultant image of "acceptable" image quality necessary to confidently make an accurate differential diagnosis. If the detector is underexposed due to inadequate radiographic technique factors, even though the image can be amplified and rescaled to present a good grayscale rendition, the quantum mottle in the image is likewise amplified, resulting in a noisy and grainy image. This causes the low contrast resolution sensitivity to be compromised, and often necessitates a retake. At least in this situation the underexposure is easy to recognize based upon the appearance of the image.

A more problematic situation occurs with detector overexposure caused by inappropriately high radiographic technique factors, resulting in needless patient dose. Except for extreme overexposures, images that are produced are usually of excellent radiographic quality with high contrast resolution sensitivity and low quantum mottle, due to the ability of the digital detector system to rescale the high signals to a grayscale range optimized for viewing on a soft copy monitor or hard copy film. Unfortunately, the patient in this situation has received needless radiation exposure, often without the knowledge of anyone involved in the acquisition or reading of the case. In some cases, a three to five times overexposure or more can happen, without any complaints from anyone. A phenomenon known as "dose creep" can occur based on the visible negative impact that underexposure can have on image appearance, and lack of perceived negative impact when the patient is overexposed but with beautiful electronic images. In the analog screen-film detector paradigm, the fixed speed of the detector requires that the exposure be correct, otherwise the response of the film optical density in the processed image is either too light (underexposure) or too dark (overexposure). Since there is no direct correlation with image appearance and grayscale rendition (brightness/contrast) in the digital image acquisition, the immediate feedback is lost. Fortunately, 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. Unfortunately, each manufacturer has a unique way of indicating this exposure indicator feedback signal. Until a formal exposure index standard is adopted by all manufacturers, it is imperative that technologists and radiologists become familiar with the specific way a given digital detector device indicates and reports the relative exposure intensity at which the image was acquired. This allows identification of under and overexposed examinations (and patients) and assists the technologist in performing adjustments in radiographic techniques to achieve consistency in radiation exposure and to optimize image quality simultaneously with safety to the patient.

Figure 1 shows the comparison of the classic characteristic curve response of a variety of screen-film detector "speeds" as a function of incident exposure, and comparison to a generic digital radiography detector response. Clearly, the latitude of the digital detector spans a large range of "equivalent speed class" screen-film detectors. Of note is the extremely large range of very high exposures (red ellipsoid) that fall on the linear response curve of the digital detector, which is a cause for concern when digital feedback signals (exposure indices) are not tracked.

Figure 1. Characteristic curve response of screen film detectors of various radiographic speeds and digital radiography detectors.

The outcomes of wide latitude response of digital radiography devices are illustrated in Figure 2, demonstrating a set of images of a chest phantom at various exposure levels (an exposure level of 1 X is comparable to a 200 speed screen-film detector response). Screen-film image response in terms of optical density is strongly affected by the variation in incident exposure levels. Digital radiography images are scaled uniformly, despite the incident exposure variation; however, as the contrast resolution phantom depicts in the lower row, larger statistical variations in the underexposed images have a larger impact on the ability to resolve small, low contrast signals, whereas at very high exposures (compare 2.5X to 5X images) the image contrast resolution / sensitivity responses do not benefit significantly from increasing the dose to the patient. In fact, at even higher exposures, a loss of contrast resolution occurs from inclusion of other non-stochastic noise sources (e.g., detector imperfections) and saturation of the signals.

Figure 2. Digital radiography phantom images acquired with screen-film (top row), computed radiography (middle row), and an extracted and magnified insert from the digital images (bottom row). The variation in incident exposure in each column corresponds to a range from one-half up to five times the exposure of a typical "200 speed" screen-film detector.

A clinical example of underexposure is illustrated in Figure 3, demonstrating the lack of detail in the image and preponderance of a grainy, mottled appearance. This underexposure is likely due to improper radiographic technique (mAs too low) or Automatic Exposure Control phototimer malfunction. A repeat exposure of the same patient is shown in Figure 4, clearly demonstrating improved image quality and diagnostic information not shown in the underexposed image.

Figure 3. Underexposed computed radiography image of the abdomen (click on image for full sized version).

Same patient, proper exposure is shown in Figure 4.

Figure 4.Properly exposed computed radiography image of the abdomen. (click on image for full sized version).

In some cases, particularly in areas of the image with little or no attenuation, overexposure of the patient and the digital detector can result in saturation and a loss of image information beyond the linear operating range of the detector, as shown in Figure 5 for lung areas and un-collimated areas adjacent to the patient anatomy.

Figure 5. Overexposure and saturation of areas of the digital image, in which digital data is lost and unrecoverable (click on image for full sized version).