Radiography Exam Digital Radiography and PACS 1 — Questions and Answers

Question 1: In computed radiography (CR), the image is captured by a photostimulable phosphor (PSP) plate. To read the image, the PSP plate is exposed to:

A laser beam that stimulates the stored energy to emit visible light (Correct answer)
A high-voltage electric field
Ultraviolet light
A second x-ray exposure

Correct answer: A laser beam that stimulates the stored energy to emit visible light

In CR, the PSP plate stores x-ray energy as trapped electrons; a laser beam stimulates these electrons, causing photostimulated luminescence (PSL) that is detected and converted to a digital image.

CR imaging plates contain barium fluorohalide phosphors (e.g., BaFBr:Eu²⁺). X-ray exposure creates electron-hole pairs with electrons trapped in 'F-centers' (color centers). In the reader, a red laser beam scans the plate, stimulating electrons to release their stored energy as blue/green light (photostimulated luminescence, PSL). A photomultiplier tube detects this light and converts it to a digital signal. After reading, the plate is flooded with white light to erase residual latent image.

Question 2: Direct digital radiography (DR) flat panel detectors using amorphous selenium (a-Se) are classified as:

Direct conversion detectors (x-rays directly produce electrical charge) (Correct answer)
Indirect conversion detectors (x-rays produce light, then electrical charge)
Photostimulable phosphor detectors
Computed tomography detectors

Correct answer: Direct conversion detectors (x-rays directly produce electrical charge)

Amorphous selenium flat panel detectors are direct conversion detectors — x-rays directly ionize the selenium layer, producing electron-hole pairs that are collected as an electrical signal without intermediate light production.

Direct conversion DR detectors use a-Se (amorphous selenium) photoconductor. X-rays directly ionize the selenium, producing electron-hole pairs. An applied electric field sweeps these charges to thin-film transistor (TFT) arrays, which read out the image. Direct conversion preserves spatial resolution because there is no light spreading (as occurs in indirect conversion). Indirect conversion detectors (e.g., CsI:Tl or GOS + a-Si TFT) convert x-rays to light first, then to electrical charge.

Question 3: In digital radiography, the exposure index (EI) number primarily reflects:

The amount of radiation exposure received by the image detector (Correct answer)
The mAs used for the exposure
The kVp selected for the technique
The SID used during the exposure

Correct answer: The amount of radiation exposure received by the image detector

The exposure index (EI) is a standardized number that indicates the exposure reaching the image receptor — it reflects the radiation dose to the detector, not the technique factors.

The AAPM/IEC standardized exposure index (EI) was developed to create a uniform indicator across manufacturers. EI ∝ detector radiation exposure. A target EI is established for each examination type (e.g., EI=300 for chest PA). The deviation index (DI = 10 log(EI/EI_target)) indicates over- or underexposure: DI 0 = correct, DI +1 = 25% overexposure, DI -1 = 20% underexposure. EI monitoring helps identify exposure creep (gradual overexposure) and technique errors.

Question 4: PACS (Picture Archiving and Communication System) uses which standard protocol for storing, transmitting, and displaying medical images?

DICOM (Digital Imaging and Communications in Medicine) (Correct answer)
HL7 (Health Level 7)
JPEG 2000
PDF

Correct answer: DICOM (Digital Imaging and Communications in Medicine)

DICOM is the universal standard for medical imaging data, defining how images are stored, formatted, transmitted, and displayed — it is the foundation of PACS interoperability.

DICOM (NEMA standard PS 3.x) defines the file format and network communication protocols for medical images. Every DICOM file contains the pixel data plus a header with patient demographics, study information, modality, and acquisition parameters. DICOM services include Store (C-STORE), Query/Retrieve (C-FIND/C-MOVE), Modality Worklist, and Structured Reporting. PACS, modalities, workstations, and RIS must be DICOM-compliant to communicate. HL7 is used for administrative/clinical data exchange (not imaging).

Question 5: The primary advantage of using indirect conversion flat panel detectors with cesium iodide (CsI:Tl) over gadolinium oxysulfide (GOS) screens is:

CsI:Tl has a needle-like crystal structure that channels light to the detector, reducing light spreading and improving spatial resolution (Correct answer)
CsI:Tl requires less radiation dose
CsI:Tl is more resistant to humidity
CsI:Tl produces more light per x-ray photon

Correct answer: CsI:Tl has a needle-like crystal structure that channels light to the detector, reducing light spreading and improving spatial resolution

CsI:Tl's columnar needle-like crystal structure acts as a light guide, channeling emitted light perpendicular to the detector surface, which minimizes lateral light spreading and preserves spatial resolution.

CsI:Tl scintillator crystals grow in needle-like columns (similar to fiber optics), guiding emitted light toward the photodetectors with minimal lateral spreading. This maintains spatial resolution even with thick CsI layers (which improve x-ray absorption/detection efficiency). GOS (gadolinium oxysulfide) screens have random granular crystals that scatter light in all directions, reducing sharpness at the same layer thickness. CsI detectors therefore achieve better DQE (detective quantum efficiency) and higher MTF than GOS-based detectors.

Question 6: Detective quantum efficiency (DQE) in digital radiography describes:

How efficiently the detector converts x-ray exposure into a useful signal relative to ideal detector performance (Correct answer)
The number of pixels per square millimeter in the detector
The maximum dose the detector can receive before saturation
The speed at which the detector reads out the image

Correct answer: How efficiently the detector converts x-ray exposure into a useful signal relative to ideal detector performance

DQE is the ratio of output SNR² to input SNR² across spatial frequencies, measuring the efficiency with which the detector uses the incident x-ray photons to produce a noise-limited image.

DQE(f) = [SNR_out(f)]² / [SNR_in(f)]² represents a detector's overall imaging performance at each spatial frequency. A DQE of 1.0 means all input photons are used perfectly (ideal). Typical DR detectors achieve DQE of 0.4–0.8 at low frequencies. High DQE means lower patient dose is required for equivalent image quality. DQE is affected by x-ray absorption efficiency, noise sources (electronic, structural), and spatial resolution (MTF). It is the key metric for comparing digital detector performance.

Radiography Exam Digital Radiography and PACS 1

In computed radiography (CR), the image is captured by a photostimulable phosphor (PSP) plate.
To read the image, the PSP plate is exposed to: