Digital Imaging Systems, PACS, and Image Post-Processing

Digital Imaging Systems in Radiologic Technology

Modern radiologic practice relies almost entirely on digital image acquisition. Unlike conventional film-screen radiography, digital systems capture x-ray energy electronically, allowing images to be displayed, adjusted, stored, and transmitted without chemical processing. Two broad categories of digital imaging are used in clinical practice: computed radiography (CR) and digital radiography (DR).

Computed Radiography (CR)

CR uses a flexible photostimulable phosphor (PSP) plate housed in a cassette that resembles a traditional film cassette. When exposed to x-rays, the phosphor crystals store a latent image as trapped electrons in excited energy states. The cassette is then fed into a CR reader, where a laser beam scans the plate and causes the stored electrons to release their energy as visible light — a process called photostimulated luminescence (PSL). A photomultiplier tube detects this light and converts it to a digital signal.

After readout, the plate is exposed to bright white light to erase any residual image so it can be reused. Failure to erase a plate fully can leave a ghost image (erasure artifact) on the next exposure. CR plates must be protected from background radiation and used within a reasonable time after exposure, because the latent image fades.

CR is considered an indirect capture system and requires more steps than DR, but its cassette-based design makes it compatible with existing x-ray rooms and portable equipment.

Digital Radiography (DR)

DR systems capture the image directly in the detector panel, eliminating the cassette-to-reader step. Most modern DR units use a flat-panel detector (FPD) containing a large array of thin-film transistors (TFTs). Two main designs exist:

• Indirect conversion FPD: X-rays strike a scintillator layer (commonly cesium iodide or gadolinium oxysulfide) that converts x-ray energy to light, which is then detected by a photodiode array.
• Direct conversion FPD: A photoconductor material (commonly amorphous selenium) converts x-ray energy directly to an electrical charge without a light step, reducing blur.

DR images appear on the monitor within seconds of exposure, improving workflow efficiency. Because there is no cassette exchange, positioning errors are identified faster, reducing patient exposure from repeat examinations.

Exposure Indicators

All digital systems use an exposure indicator (EI) — a numeric value calculated from the detector signal — to report whether the receptor received an appropriate amount of radiation. The International Electrotechnical Commission (IEC) established a standardized EI system with a target EI and a deviation index (DI) to compare the actual exposure to the target. A DI near zero indicates proper exposure. A significantly positive DI suggests overexposure; a significantly negative DI suggests underexposure.

Because the digital system can brighten an underexposed image through post-processing, the technologist must not rely on image appearance alone to judge exposure adequacy. Always check the exposure indicator against the department's reference range. Repeated overexposure increases patient dose unnecessarily — a direct patient safety concern — while underexposure introduces quantum noise and may compromise diagnostic quality.

Image Post-Processing

Once acquired, digital images can be adjusted before or after transmission. Key post-processing tools include:

• Windowing and leveling (W/L): Window width controls image contrast; window level (center) controls image brightness. Adjusting W/L allows the technologist or radiologist to optimize visualization of different tissue types without repeating the exposure.
• Annotations: Patient data labels, side markers, and technical notes are added electronically. Electronic markers do not replace proper physical side marking at the time of exposure.
• Image orientation and cropping: Images may be flipped or rotated to match anatomical display conventions.

PACS and RIS Workflow

A Picture Archiving and Communication System (PACS) stores, retrieves, and transmits digital images across a network. It works alongside a Radiology Information System (RIS), which manages patient scheduling, orders, and reports. Together they replace film libraries and paper orders.

A typical technologist workflow includes: (1) verify patient identity using at least two identifiers; (2) confirm the exam order in the RIS; (3) perform the examination; (4) flag the acquired images in PACS for quality review; (5) apply appropriate W/L and annotations; (6) route the completed study to the radiologist's reading queue. Each step must follow department protocol to maintain image integrity and patient privacy under applicable health information regulations.

Digital Image Artifacts

Common digital artifacts include:

• Grid lines: Anti-scatter grid misalignment or motion during exposure.
• Erasure (ghost) artifact: Incomplete erasure of a CR plate from a previous exposure.
• Quantum noise (mottle): Insufficient x-ray photons reaching the detector; seen as a grainy image with a low exposure indicator.
• Detector damage artifacts: Dead pixels or lines from physical damage to the flat-panel detector.

Identifying artifacts accurately is essential. A misidentified artifact may lead to a repeated exposure, increasing patient dose, or to a missed pathology if an artifact mimics anatomy.

Safety Considerations

Digital imaging does not eliminate radiation risk. The technologist remains responsible for applying ALARA (As Low As Reasonably Achievable) principles, accurate collimation, appropriate technique selection, and shielding where indicated. Overexposure is easier to commit in digital imaging because a poor technique may still produce a visually acceptable image — this phenomenon is sometimes called exposure creep. Regular audit of exposure indicators and adherence to technique charts are the primary safeguards.