In the transition from film-screen to digital radiography, one thing remains constant: artifacts degrade image quality and can lead to misdiagnosis. However, digital artifacts behave differently from film artifacts. A scratch on a CR plate looks nothing like a scratch on film, and a DR detector with dead pixels presents a challenge that didn't exist in the analog era. Every radiologic technologist must be able to recognize artifacts, understand their root cause, and know how to prevent them — not just for image quality, but for the ARRT exam, which consistently tests artifact identification and remediation.
Artifacts in digital radiography fall into two broad categories: hardware-related artifacts (originating from the detector, grid, or x-ray tube) and software-related artifacts (arising from image processing, histogram analysis, or post-processing algorithms). This guide covers both, organized by source so you can systematically troubleshoot any artifact you encounter in clinical practice.
The cardinal rule of digital artifact troubleshooting: If you see an artifact on one image, check if it reproduces on subsequent images. A fixed, reproducible artifact points to a hardware issue (dead pixel, plate scratch, grid damage). A random or variable artifact suggests technique error (motion, positioning, exposure) or a processing anomaly. This simple distinction saves hours of troubleshooting time.
Computed radiography (CR) uses photostimulable phosphor (PSP) plates housed in cassettes. Because the plates are handled, transported, and processed mechanically, they are susceptible to a unique set of artifacts that have largely disappeared with direct digital radiography — yet CR remains widely used in portable radiography, smaller facilities, and many imaging departments worldwide. Understanding CR artifacts is essential for the ARRT exam and for any technologist working with CR systems.
Physical scratches on the PSP plate surface appear as fine, linear radiolucent lines on the image. Unlike grid lines (which are evenly spaced and parallel), scratches are irregular in width, length, and orientation. They occur when dust or grit particles are trapped between the plate and the readout mechanism, or when the plate is improperly handled during cassette loading and unloading.
Prevention: Handle cassettes carefully. Clean CR plates regularly according to the manufacturer's specifications. Always transport cassettes in protective carriers. Replace plates that show extensive scratching — the image quality degradation is permanent.
Static discharge artifacts appear as branching, jagged black or white lines (depending on the image polarity) that resemble lightning bolts. They are caused by a buildup of static electricity on the PSP plate, typically in low-humidity environments (below 40% relative humidity). When the charged plate comes into contact with the readout mechanism, a sudden discharge occurs, exposing the phosphor layer and creating the characteristic branching pattern.
Prevention: Maintain ambient humidity above 40% in the CR processing area. Use antistatic sprays and mats. Ground the CR reader properly. Allow cassettes to acclimate to room temperature before processing if they've been stored in a cold area.
A ghost image — a faint remnant of a prior exposure — appears when the PSP plate is not fully erased before reuse. CR plates store residual energy in the phosphor layer after readout; the erasure process (exposure to bright light) must completely release this remaining energy. If the erasure lamp is failing, the exposure time is insufficient, or the plate has been exposed to a very high dose, residual signal remains.
Ghost images appear as faint, superimposed anatomy that doesn't match the current patient. They are most visible in the collimated (unexposed) borders of the image, where any residual signal stands out against the black background.
Prevention: Perform daily CR reader QC per manufacturer guidelines. Monitor erasure lamp output during preventive maintenance. If ghosting is persistent, increase erasure time in the reader settings or flag the reader for service.
The ARRT frequently tests artifact appearance and cause. Memorize this pattern: Branching lines = static. Straight, sharp lines with regular spacing = grid lines. Irregular, thin lines of varying direction = scratches. Faint residual anatomy = incomplete erasure. Bright white spots = dust on the PSP plate.
Direct digital radiography (DR) uses flat-panel detectors that convert X-rays into an electronic signal. These detectors are more robust than CR plates but introduce their own artifact patterns. DR artifacts are often hardware-related and can signal detector degradation.
A dead pixel appears as a small, round white or black dot in a consistent location on every image. Dead pixels occur when individual detector elements (del) fail and no longer respond to X-ray exposure. Modern DR detectors maintain a "bad pixel map" that automatically interpolates values for known dead pixels, replacing them with an average of neighboring pixel values.
When dead pixels cluster (too many in one region) or a new dead pixel appears suddenly, it may indicate progressive detector failure. A cluster of dead pixels appears as a visible patch of incorrect density that is reproducible from image to image.
Action: Run the detector's dead pixel calibration routine. If the artifact persists, the detector needs service. The ARRT exam expects you to know that a single dead pixel is clinically acceptable with interpolation, but large clusters (>10 pixels in a 1 cm area) degrade diagnostic quality.
DR detectors require a flat-field (gain) calibration to compensate for differences in pixel sensitivity across the detector surface. If the calibration is outdated or performed incorrectly, the resulting image shows subtle, large-area density variations — one region may appear slightly brighter or darker than another, even when imaging a uniform phantom. This is known as a "gain artifact" or "shading artifact."
Gain artifacts become more common as the detector ages or when the detector temperature changes significantly (e.g., moving from a cold storage area to a warm examination room). Temperature-related gain artifacts appear as gradual density gradients across the image.
Action: Perform a fresh flat-field calibration. Allow the detector to reach thermal equilibrium before use. If shading persists, there may be detector element degradation.
Image lag occurs when a DR detector retains signal from a previous exposure into the next image. This appears as a faint ghost image — similar to CR erasure artifacts but caused by trapped charges in the detector's thin-film transistor (TFT) array. Image lag is most noticeable when a high-exposure image is followed by a low-exposure image (e.g., a dense pelvis followed by a hand).
Some image lag is normal — the ARRT recognizes that up to 5% residual signal from the prior image is acceptable. If lag exceeds 5%, the detector may need recalibration or replacement.
Action: Allow adequate reset time between exposures. Perform dark-field and offset calibrations. If lag is excessive, contact the manufacturer.
Grids are essential for scatter reduction, but they produce distinctive artifacts in digital imaging that differ from film-screen artifacts. Understanding these artifacts is critical because the wrong corrective action can make things worse.
Grid cutoff appears as a band of decreased density — typically at the lateral edges of the image or as a broad, uniform underexposure. In digital radiography, grid cutoff is often masked by automatic rescaling, making it harder to detect than on film. The rescaling algorithm may increase brightness in the affected area, creating a paradoxically noisier image with poor contrast that the technologist might mistake for an underexposure problem.
Common causes of grid cutoff include off-center CR, angled CR relative to the grid plane, improper SID for a focused grid, and upside-down focused grid placement. On the ARRT exam, upside-down grid is a frequently tested cause of severe bilateral cutoff.
A Moiré pattern is a wavy, rippling artifact — often described as a "watered silk" or "zebra stripe" appearance — that results from interference between the grid lines and the detector's sampling frequency. This is a uniquely digital artifact with no film-screen equivalent.
Moiré patterns occur when the grid frequency is close to the Nyquist frequency (half the sampling frequency) of the digital detector. For example, a detector with a pixel pitch of 143 μm has a Nyquist frequency of approximately 3.5 line pairs per millimeter (lp/mm). Using an 8:1 grid with 80 lines/cm (8 lp/mm) with this detector creates a beat frequency that produces visible Moiré patterns.
Solution: Use a grid with a frequency higher than the Nyquist frequency of the detector. Many modern DR detectors use high-line-rate grids (100-120 lines/cm) specifically to avoid Moiré interference. Alternatively, some detectors apply digital filtering to suppress Moiré patterns in post-processing.
If the detector is stationary relative to the grid during exposure, the grid lines themselves may be visible as thin, parallel, evenly spaced lines across the image. Most DR detectors use software algorithms to "de-grid" (remove grid lines) during processing, but these algorithms can fail if the grid frequency is unusual, if the grid is damaged (bent or buckled), or if the grid lines are not perfectly parallel to the detector rows.
Solution: Ensure the grid is not damaged. Verify that the detector's grid-line removal algorithm is configured for the correct grid frequency. A drifting grid line removal algorithm is a common cause of subtle grid artifacts that come and go.
While motion and positioning artifacts are not unique to digital imaging, their appearance in digital detectors has distinct characteristics that technologists must recognize.
Patient motion produces blurring or "ghosting" of anatomical structures — edges appear duplicated or smeared. In digital radiography, motion artifacts are often mistakenly attributed to detector or processing errors because the motion may be subtle. The telltale sign: motion blur affects only the anatomy, not the background or collimated borders.
Voluntary motion (breathing, movement during exposure) produces uniform blurring. Involuntary motion (peristalsis, cardiac motion) produces localized blurring. The classic example is the "double-lung" appearance from a patient breathing during a chest X-ray — the lung borders appear duplicated.
Solution: Use the shortest possible exposure time. Increase mA rather than exposure time to achieve the same mAs. Use proper immobilization. For pediatric patients, timing the exposure at the end of expiration for chest X-rays reduces motion.
Mobile DR detectors can shift during exposure, particularly in portable radiography. This produces sharp, straight-edge borders at the detector edges that don't align with anatomy, accompanied by subtle blurring. Unlike patient motion, detector motion affects the entire image uniformly, including the collimated borders.
Solution: Ensure the detector is securely positioned under the patient table. Use the detector locking mechanism. In portable radiography, stabilize the detector behind the patient before positioning the tube.
One of the biggest challenges in digital radiography is that image processing algorithms can create, hide, or mimic artifacts. Understanding these processing artifacts is essential for the modern rad tech.
Digital detectors use histogram analysis to automatically rescale images for optimal display. The detector analyzes the distribution of pixel values, identifies the anatomy boundaries, and applies a look-up table (LUT) to map the raw data to the display range. If the histogram analysis misidentifies the anatomy — for example, if collimation edges are included in the analysis, or if a high-density object (pacemaker, surgical hardware) occupies too much of the image — the rescaling algorithm produces an image that is too light, too dark, or has incorrect contrast.
The classic example: a chest X-ray with the collimation too close to the lung apices. The histogram sees a large area of black (the collimated border) and rescales to make the grays brighter — resulting in a washed-out, overexposed appearance.
Solution: Proper collimation is the first line of defense. Most DR systems allow the technologist to define the anatomical region of interest (the "anatomy ROI" or "exposure field recognition") so the histogram analysis ignores collimated borders. Ensure the correct anatomical program is selected before exposing.
Digital radiography's automatic rescaling has an unintended consequence: exposure creep. Because the system compensates for overexposure by rescaling the image to a normal brightness, a technologist who consistently uses excessive technique will never see a "dark" image. The images look good, but the patient receives unnecessary dose. The only signs are a low deviation index (DI) or high exposure index (EI) in systems that report these values. Always monitor your exposure indicators and resist the temptation to increase technique "just to be safe."
Most DR systems apply edge enhancement algorithms to improve the visibility of anatomical borders. When applied too aggressively, these algorithms produce a "halo" or "ring" artifact around high-contrast edges — bones appear to have a bright white line on one side and a dark line on the other. This is known as the "overshoot" or "ringing" artifact.
Excessive edge enhancement also amplifies image noise, giving the image a grainy or "digital" appearance. This is most noticeable in low-dose images where the noise floor is higher.
Solution: Use manufacturer-recommended processing parameter sets for each anatomical region. Adjust edge enhancement strength (kernel size and gain) during protocol configuration. If a specific image is too sharp, request reprocessing with softer parameters.
Aliasing artifacts appear as jagged or stair-step edges on structures that should be smooth — most commonly seen on the borders of the diaphragm, heart, or metallic objects. Aliasing occurs when the spatial frequency of the anatomical edge exceeds the Nyquist frequency of the detector, causing undersampling.
Aliasing is more prominent with larger pixel pitches (lower resolution detectors). It can be minimized with anti-aliasing filters applied during image processing, though these filters may reduce overall sharpness.
Solution: Use the highest-resolution detector matrix available for the anatomical region (e.g., use a small focal spot and a detector binning of 1×1 rather than 2×2 for extremity work). Most DR systems apply anti-aliasing in the acquisition software.
Understanding the differences between CR and DR artifacts helps technologists identify the source of the problem and take appropriate corrective action.
| Artifact Type | CR Appearance | DR Appearance | Root Cause |
|---|---|---|---|
| Scratches | Irregular radiolucent lines | Rare (no moving parts) | Physical damage to PSP plate |
| Static electricity | Branching "lightning" lines | Rare (detector is sealed) | Low humidity + PSP handling |
| Ghost image | Faint prior anatomy | Faint prior anatomy (lag) | CR: incomplete erasure; DR: TFT charge retention |
| Dead pixels | N/A (CR reads whole plate) | Small white/black dots, fixed location | Failing detector elements |
| Grid lines visible | Rare (CR motor moves plate) | Parallel lines across image | Grid frequency / detector sampling mismatch; failed de-grid algorithm |
| Moiré pattern | N/A | Wavy "watered silk" pattern | Grid frequency near Nyquist frequency |
| Shading/gradient | Uneven illumination (laser scanner) | Gradual density change across image | CR: laser scanner degradation; DR: gain calibration outdated |
| Dust/dirt artifacts | White spots on image | N/A (sealed detector) | Dust on PSP plate during readout |
| Backscatter | Increased density behind cassette | Increased density behind detector | Improper positioning; no lead backing |
When you encounter an unexpected artifact, follow this step-by-step approach:
To quickly check a DR detector for dead pixels or calibration issues, perform a "flash" test: expose the detector at approximately 50% of the mAs used for a typical abdomen technique, with no grid and no anatomy in the beam. The resulting image should be uniformly gray. Any bright or dark spots, bands, or gradients indicate detector problems that need addressing. Document the results in your QC log.
Most digital artifacts are preventable with proper technique and routine maintenance. Here are the key prevention strategies every rad tech should follow:
Perform daily QC on every DR detector before patient imaging. A simple flat-field image identifies dead pixels, gain shifts, and grid issues before they affect clinical images. For CR, clean PSP plates weekly and verify erasure lamp output monthly. The ARRT exam includes questions about recommended QC frequencies — memorize that daily QC is the minimum standard for DR detectors and weekly plate cleaning is recommended for CR.
DR detectors are expensive, sensitive devices. Never drop a detector. Never place heavy objects on top of a detector. Keep detectors away from liquids. Store detectors in a temperature-controlled environment (15-30°C). Sudden temperature changes cause condensation that can damage the TFT array and create permanent artifacts. For portable DR, always use a protective cover and transport case.
Inspect grids regularly for physical damage (dents, buckled interspaces). Store grids vertically in a grid cabinet — never lay them flat under heavy objects. Clean grid surfaces with manufacturer-recommended cleaning solutions. Replace grids every 5-7 years of clinical use, as the lead strips can degrade over time.
Use the exposure technique chart for each anatomical region — do not "eyeball" technique factors. Monitor exposure indicators (EI, DI, or similar) on every image. If your EI is consistently higher than the target range, reduce your technique. If the image is noisy, increase technique cautiously. Remember: in digital radiography, a good-looking image does not guarantee appropriate patient dose.
Try these ARRT-style multiple choice questions based on this article. Click an option to check your answer — correct answers turn green, wrong ones turn red.