CR & DR Systems: Digital Imaging Workflow & Artifacts
Overview of Digital Imaging Systems
Digital imaging has replaced film-screen systems in virtually all modern radiology departments. The ARRT Radiography exam tests your understanding of computed radiography (CR) and digital radiography (DR) workflows, image processing, exposure indicators, artifacts, and quality control protocols.
Definition: Computed Radiography (CR) uses a photostimulable phosphor (PSP) plate inside a cassette to capture the latent image, which is later read by a laser scanner. Digital Radiography (DR) uses a flat-panel detector (FPD) permanently installed in the table or wall bucky, delivering an image directly to the workstation.
Key areas to master for the exam:
- Step-by-step CR and DR workflows
- Image processing and PACS/DICOM/RIS integration
- Exposure indicators and deviation index (DI)
- Common digital artifacts and their causes
- Quality control responsibilities
CR & DR Imaging Workflows
Understanding the CR workflow in sequence is a high-yield exam topic. Each step has a testable detail.
- Positioning & Exposure: The patient is positioned using standard technique. The CR cassette containing the photostimulable phosphor (PSP) plate is placed in the X-ray beam path.
- Cassette Transport: After exposure, the cassette is carried to the CR reader unit. The latent image is stored on the PSP plate as trapped electrons in higher energy states.
- Laser Scanning: Inside the reader, a laser beam scans the plate, releasing the stored energy as visible light — a process called photostimulated luminescence (PSL).
- Signal Detection: The emitted light is captured by a photomultiplier tube (PMT), which converts it into a digital signal for image processing.
- Plate Erasure: The plate is exposed to bright white light to erase any residual signal, then returned to service.
Positioning Trap: If a CR plate is not fully erased before reuse, a ghost image (remnant of the previous exposure) will appear on the next image. This is one of the most commonly tested CR artifacts.
DR eliminates cassette handling entirely, making it faster and more efficient than CR.
- Positioning & Exposure: The patient is positioned and the exposure is made directly to the flat-panel detector (FPD), which is permanently installed in the table or wall bucky.
- Immediate Image Display: The image appears on the PACS workstation typically within 5–15 seconds — no cassette transport or laser reading required.
- No Plate Erasure: Because there is no PSP plate, there is no erasure step. However, DR detectors can exhibit lag artifact (residual signal from a previous exposure) if not properly managed.
| Feature | CR | DR |
|---|---|---|
| Detector type | PSP plate in cassette | Flat-panel detector (FPD) |
| Image availability | Minutes (reader required) | 5–15 seconds |
| Cassette handling | Required | Not required |
| Erasure step | Required (bright light) | Not required |
| Ghost image risk | Yes (incomplete erasure) | No (lag artifact instead) |
| Efficiency | Lower | Higher |
Exam Tip: DR is faster and more efficient than CR. If an exam question asks which system provides the most immediate image availability, the answer is DR.
Image Processing & Information Systems
After acquisition, digital images undergo electronic processing before display. The technologist interacts with multiple information systems daily.
Image Processing Steps: Raw image data is processed through histogram analysis, look-up table (LUT) application, edge enhancement, and noise reduction before display on the PACS workstation.
| System | Function |
|---|---|
| PACS (Picture Archiving and Communication System) | Stores, retrieves, and distributes digital radiographic images |
| DICOM (Digital Imaging and Communications in Medicine) | Universal standard for medical image format and communication |
| RIS (Radiology Information System) | Manages patient scheduling, reporting, and workflow |
Exam Tip: DICOM is the universal standard that allows images from different vendors to be shared across systems. PACS is the storage and distribution platform. RIS handles the administrative/scheduling side. Know the distinction — the ARRT exam tests all three.
Exposure Indicators & Quality Control
Digital systems do not show overexposure or underexposure the same way film did. Exposure indicators are the primary tool for evaluating technique accuracy in CR and DR.
Definition — Exposure Index (EI): A numeric value generated by the imaging system that reflects the amount of radiation reaching the detector. Each vendor uses a different scale.
| Vendor System | Indicator Name | Direction |
|---|---|---|
| Fuji | S-number | Inverse — higher number = lower exposure |
| Kodak | EI | Direct — higher number = higher exposure |
| AGFA | lgM | Direct (logarithmic) |
| Canon | REI | Direct |
| IEC Standard | Deviation Index (DI) | Vendor-neutral; target DI = 0 |
Critical Threshold — Deviation Index (DI): The IEC Deviation Index (DI) is a vendor-neutral measure of technique accuracy. A DI of 0 means the exposure matched the target exactly. Acceptable ranges and target EI values are set by the medical physicist. A significantly positive DI indicates overexposure; a significantly negative DI indicates underexposure. The ARRT exam may ask you to interpret DI values or identify which vendor scale is inverse.
Scenario: A technologist performs a chest PA and the Fuji S-number reads 800 instead of the expected 200. This indicates underexposure (higher S-number = lower dose on Fuji's inverse scale), and the image may show excessive quantum noise.
Digital artifacts are frequently tested on the ARRT exam. You must be able to identify the artifact, its cause, and how to prevent it.
| Artifact | Cause | System |
|---|---|---|
| Ghost image | Incompletely erased CR plate | CR only |
| Lag artifact | Residual signal from previous exposure | DR only |
| Grid cutoff | Grid misalignment (off-level, off-center, upside-down) | CR & DR |
| Collimator shadow | Collimator blades visible as white border on image | CR & DR |
| Quantum noise (mottle) | Insufficient mAs — too few photons reaching detector | CR & DR |
| Detector pixel dropout | Dead pixels appearing as dark or bright dots | DR |
| Moiré pattern | Stationary grid frequency matches detector sampling frequency | CR & DR |
Common Errors to Know:
- Ghost image = always a CR erasure failure — never occurs in DR.
- Quantum noise = caused by low mAs, not low kVp. Increasing mAs reduces noise.
- Moiré pattern = use a moving (Bucky) grid or change grid frequency to eliminate.
- Grid cutoff = lines across the image from grid misalignment; most commonly tested with focused grids used at the wrong SID or tilted.
Quality control (QC) for CR and DR systems is a shared responsibility between the technologist and the medical physicist. The ARRT exam tests which tests are performed and at what frequency.
| QC Test | Purpose | Frequency |
|---|---|---|
| Flat field correction (uniformity test) | Ensures uniform detector response across the entire field | Daily / as needed |
| Spatial resolution test (line pair phantom) | Evaluates sharpness and resolving capability of the detector | Monthly / annually |
| Noise measurement | Assesses signal-to-noise ratio of the detector | Monthly / annually |
| Artifact evaluation | Identifies dead pixels, ghost images, or processing errors | Daily |
Exam Tip: Target EI values and acceptable DI ranges are established by the medical physicist, not the technologist. The technologist's role is to apply those standards and report deviations. Daily QC tasks (flat field, artifact check) are typically performed by the technologist; annual testing is performed by medical physics.
Summary & Exam Takeaways
KEY CONCEPTS — Test-Ready Recall
- CR workflow: PSP plate → cassette → reader → laser → PMT → digital signal → erase plate with bright light.
- DR workflow: FPD permanently installed → image available in 5–15 seconds → no cassette or erasure step.
- Ghost image = CR only (incomplete erasure). Lag artifact = DR only (residual signal).
- Fuji S-number is inverse — higher S = lower exposure. All other major systems (Kodak EI, AGFA lgM, Canon REI) are direct.
- Deviation Index (DI): Target = 0. Positive DI = overexposure; negative DI = underexposure. Set by the medical physicist.
- Quantum noise is caused by low mAs — increase mAs to reduce noise.
- Moiré pattern = stationary grid frequency interferes with detector sampling — use a moving Bucky grid.
- PACS = image storage/distribution. DICOM = universal image standard. RIS = scheduling/reporting.
- QC: Flat field and artifact evaluation are daily tasks; spatial resolution and noise testing are monthly/annual tasks performed with medical physics oversight.
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