Radiography Exam Digital Radiography and PACS 2 — Questions and Answers
Question 1: In digital radiography, 'exposure creep' refers to:
- A gradual increase in patient exposure over time because digital processing masks overexposure by adjusting image brightness automatically (Correct answer)
- Progressive decrease in image resolution due to detector aging
- Slowly increasing quantum noise in digital images
- Gradual misalignment of the x-ray beam with the detector
Correct answer: A gradual increase in patient exposure over time because digital processing masks overexposure by adjusting image brightness automatically
Exposure creep occurs when radiographers gradually increase mAs over time because the digital processing corrects for overexposure, masking the fact that patients are receiving excessive dose.
Unlike film-screen systems where overexposure produces an obviously dark image, digital systems automatically apply lookup table (LUT) corrections to normalize brightness. This means moderate overexposure looks identical to a properly exposed image on the display. Without dose monitoring via EI/DI tracking, radiographers may gradually increase technique unnecessarily. Monitoring EI values and deviation indices, combined with regular QC audits, is essential to detect and prevent exposure creep.
Question 2: Which type of artifact in CR imaging appears as a faint previous image superimposed on the current image?
- Ghost artifact (residual image artifact) (Correct answer)
- Aliasing artifact
- Quantum noise artifact
- Grid cutoff artifact
Correct answer: Ghost artifact (residual image artifact)
Ghost artifacts occur in CR when the PSP plate was not completely erased after the previous exposure — residual electrons release during the next scan, creating a faint superimposed image from the previous study.
CR ghost artifacts (residual image artifacts) occur when the erasing process (white light flooding) is incomplete or the plate was stored in a dark environment where spontaneous release of trapped electrons occurs slowly. Ghost artifacts can mimic pathology or obscure anatomy. Prevention includes proper erasing cycles, regular plate replacement, and storing plates away from radiation exposure. Ghost artifacts are unique to CR and do not occur with DR flat panel detectors.
Question 3: The image matrix size in digital radiography refers to:
- The number of pixels arranged in rows and columns that make up the digital image (Correct answer)
- The physical size of the detector panel
- The number of gray levels in the image
- The spatial frequency at which the image is acquired
Correct answer: The number of pixels arranged in rows and columns that make up the digital image
The image matrix defines how many pixels are arranged in rows × columns to form the digital image — a larger matrix (more pixels) means higher spatial resolution.
Common radiographic matrix sizes: 2048×2048 or 2560×3072 for chest DR. Pixel size = FOV / matrix size. Smaller pixels = better spatial resolution. A 35×43 cm chest detector with a 3000×3000 matrix has ~117 μm pixels. Bit depth (e.g., 12-bit = 4096 gray levels, 14-bit = 16,384 gray levels) determines the number of distinct intensities. Modern DR detectors use 14-bit depth to capture the wide dynamic range of diagnostic imaging.
Question 4: In a PACS environment, the RIS (Radiology Information System) primarily manages:
- Patient scheduling, order management, and radiology report generation (Correct answer)
- Image storage and retrieval
- Image acquisition and transmission
- QC monitoring of imaging equipment
Correct answer: Patient scheduling, order management, and radiology report generation
The RIS manages the administrative and workflow functions of the radiology department: scheduling, patient demographics, orders, billing, and report management — it interfaces with PACS for complete workflow.
The RIS (Radiology Information System) handles patient scheduling, order entry, demographics, report dictation/transcription, billing, and result communication. It interfaces with the hospital HIS (Hospital Information System) via HL7 and with PACS via DICOM Modality Worklist (so patient information auto-populates on modalities). Together, RIS+PACS form the core of modern filmless radiology departments. Electronic health record (EHR) integration now often incorporates RIS functionality.
Question 5: What is the primary difference between lossy and lossless image compression in medical imaging?
- Lossy compression permanently discards some image data (irreversible), while lossless compression preserves all original data (reversible) (Correct answer)
- Lossy compression is faster but lossless is better for large files
- Lossless compression creates larger files than lossy compression
- There is no practical difference for diagnostic images
Correct answer: Lossy compression permanently discards some image data (irreversible), while lossless compression preserves all original data (reversible)
Lossy compression (e.g., JPEG) permanently discards image data to achieve smaller file sizes, while lossless compression (e.g., JPEG 2000 lossless, RLE) preserves all original data perfectly.
Lossless compression (e.g., run-length encoding, JPEG-LS lossless) perfectly reconstructs the original image with compression ratios of approximately 2:1 to 3:1. It is required for primary diagnostic use under ACR guidelines. Lossy compression (e.g., JPEG, JPEG 2000 lossy) achieves much higher ratios (10:1 to 50:1) by permanently discarding data imperceptible to the human eye. Lossy compression is controversial for primary diagnosis but may be acceptable for archiving or non-diagnostic uses. DICOM supports both.
Question 6: When evaluating monitor quality for diagnostic radiology, the minimum luminance ratio (maximum to minimum luminance) and the maximum luminance for primary diagnostic displays should be:
- Luminance ratio ≥ 250:1 and maximum luminance ≥ 350 cd/m² (Correct answer)
- Luminance ratio ≥ 100:1 and maximum luminance ≥ 100 cd/m²
- Luminance ratio ≥ 50:1 and maximum luminance ≥ 200 cd/m²
- Luminance ratio ≥ 500:1 and maximum luminance ≥ 500 cd/m²
Correct answer: Luminance ratio ≥ 250:1 and maximum luminance ≥ 350 cd/m²
AAPM TG-18 recommends primary diagnostic displays have a maximum luminance ≥ 350 cd/m² and a luminance ratio (Lmax/Lmin) ≥ 250:1 to adequately display the full diagnostic gray scale.
AAPM Task Group 18 (TG-18) establishes performance criteria for medical displays. Primary diagnostic workstation monitors must meet: maximum luminance (Lmax) ≥ 350 cd/m², luminance ratio (Lmax/Lmin) ≥ 250:1, DICOM GSDF (Gray Scale Display Function) calibration within ±10%, uniformity within 30% across display area. Clinical workstations (for referring clinicians, not primary diagnosis) have lower requirements. Regular monitor QC (luminance checks, GSDF calibration) is required — typically daily for primary displays.
Radiography Exam Digital Radiography and PACS 2
In digital radiography, 'exposure creep' refers to: