Image Artefacts in Digital Radiography: What Technicians Miss

Learn the causes of image artefacts in digital radiography, including detector faults, tube issues, operator errors, QA gaps, and correction techniques.

DIGITAL RADIOGRAPHY AND DR PANELS

8/24/20264 min read

Image Artefacts in Digital Radiography: Causes and Correction Techniques

The transition from film-based systems to Digital Radiography (DR) and Computed Radiography (CR) has revolutionized medical and industrial imaging by enhancing diagnostic precision and dose efficiency. However, digital systems are susceptible to image artefacts—unintended optical features that can obscure minute anatomical or structural details and lead to clinical misdiagnosis. In India, the Central Drugs Standard Control Organization (CDSCO) reclassified X-ray machines as Class C (Moderate to High Risk) medical devices on January 1, 2021, precisely to address the spurt in misbranded and inferior products that cause unacceptably inaccurate results.

This article explores the technical causes, operator-led errors, and mandatory correction techniques required to maintain image quality and adhere to the ALARA (As Low As Reasonably Achievable) principle.

1. Technical Causes: Generator and Tube Performance

The stability and quality of the X-ray beam are the primary factors influencing the presence of technical artefacts. If the functional performance of the generator drifts from its baseline values, the resulting images may lose contrast or resolution.

A. kVp and Timer Inaccuracy

Operating Potential (kVp): The peak potential determines the penetrating power of the beam; if the delivered kVp is not close to the operator's setting, the image may suffer from poor contrast or excessive noise. The regulatory tolerance for kVp accuracy is ± 5 kV.
Exposure Timer: A malfunctioning timer leads to radiographs that are under-exposed or over-exposed. This often necessitates image retakes, which effectively double the radiation dose to the patient. The acceptable percentage error for an exposure timer is ± 10%.

B. Effective Focal Spot Blooming

The ability to resolve small anatomical details depends entirely on the effective focal spot size. Over time, thermal stress can cause the focal spot to enlarge or "bloom".

Impact: A blooming focal spot results in blurred images and a loss of spatial resolution.
Benchmarks: For a focal spot smaller than 0.8 mm, the mandatory tolerance is +0.5 f.

2. Detector-Based Artefacts and Failure Modes

The Imaging Device—defined as the detector unit or array that receives X-rays and produces electrical or light signals—is the most sensitive component in the digital chain.

A. Dead Pixels and Spatial Resolution Loss

Spatial resolution refers to the system's ability to visualize small objects in the image.

Pixels and DPI: Resolution is influenced by detector characteristics and scanning settings; for instance, images scanned at 300 dots per inch (DPI) may approach the theoretical threshold for resolution, while settings of 600 DPI may be needed to resolve finer details.
Dead Pixels: Clusters of non-responsive pixels create "dead spots" or streaks that obscure clinical data. Systems must be able to resolve at least a mesh pattern of 30 lines/inch or a bar pattern of 1.5 lp/mm to be considered authoritative.

B. Image Lag and Ghosting

While not explicitly named "ghosting" in the training modules, the AERB Safety Code mandates that digital detectors be calibrated periodically to prevent residual signals from previous exposures from contaminating new images.

CR Plates: For Computed Radiography, plates must be periodically evaluated for artifacts and cleaned according to manufacturer procedures to ensure they perform satisfactorily in service.

3. Operator-Led and Technique Errors

Even with high-spec equipment, artefacts often arise from improper handling or positioning during the procedure.

A. Positioning and Angulation Errors

Packet Placement: In dental and general radiography, incorrect receptor placement is the most frequent error, often leading to missing the area of clinical interest.
Horizontal and Vertical Angulation: Inaccurate beam angles cause image distortion, where the object appears elongated or foreshortened.
Cone Cut: A "cone cut" artefact occurs when the primary beam is not centered with the receptor, leaving a portion of the image unexposed.

B. Beam and Field Misalignment

Congruence: The X-ray radiation field must be perfectly aligned with the optical light field so the operator can accurately position the patient.
Tolerance: Discrepancies between the optical and radiation fields must not exceed 2% of the focal spot-to-image receptor distance (S).
Central Beam Alignment: If the beam is not perpendicular to the image receptor (within a < 1.5° tolerance), the resulting image distortion results in a loss of minute anatomical details.

4. Correction and Prevention Techniques

Maintaining Expertise, Authoritativeness, and Trustworthiness (E-E-A-T) in radiography requires a systematic approach to Quality Assurance (QA) and maintenance.

A. Mandatory Biennial Quality Assurance (QA)

The "end user" (Licensee) is legally responsible for ensuring that periodic QA of X-ray equipment is carried out by authorized agencies once every two years or after major repairs.

Linearity Tests: QA must verify the Coefficient of Linearity (CoL) of radiation output, ensuring it remains less than 0.1 as current (mA) or time (s) stations are changed.
Output Consistency: The Coefficient of Variation (CoV) for radiation output must be less than 0.05 across various available kV stations to ensure predictable imaging results.

B. Detector Calibration and Cleaning

Digital Calibration: Digital detectors must be calibrated periodically as per the manufacturer’s recommendations to maintain functional performance.
Plate Hygiene: CR imaging plates require a specific cleaning frequency to remove dust and debris that cause physical artefacts on the digital record.

C. Specialized Software and PACS Integrity

AI and CAD: Modern systems often utilize Artificial Intelligence (AI) and Computer-Aided Detection (CAD) to provide automated, standardized interpretation of images, particularly for systematic TB screening.
PACS Quality: Facilities using Picture Archiving and Communication Systems (PACS) must ensure that patient images are stored for over three years without being lost or unintentionally altered.

5. Operational Discipline and Training

The ultimate responsibility for ensuring radiation safety and image quality rests with the Employer and the Licensee.

Staff Qualifications: X-ray installations must have qualified operators—such as radiologists or technologists with adequate knowledge of radiation protection—to handle equipment.
Standard Protocols: Facilities should set and follow standard exposure protocols for both adult and pediatric patients to optimize image quality while minimizing dose. For children, operators should use the shortest possible exposure time and use anti-scatter grids only when absolutely necessary.
Safety Accessories: To protect operators while they troubleshoot or position patients, the use of 0.25 mm lead equivalent aprons and 1.5 mm lead equivalent mobile barriers is mandatory.

Conclusion: Achieving Diagnostic Excellence

A "good" diagnostic X-ray procedure provides the optimum quality information at the lowest possible radiation risk. By understanding the mechanical causes of artefacts—such as kVp drift and focal spot blooming—and maintaining religious adherence to biennial QA testing, facilities can ensure their digital imaging systems remain authoritative and reliable.

Failure to comply with these national safety regulations is a punishable offense under the Atomic Energy Act, 1962, emphasizing that in the world of modern radiology, technical precision is the bedrock of public trust and legal compliance.

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