Radiography Exam Quality Control & Assurance 2 — Questions and Answers

Question 1: Which QC test is performed to evaluate the accuracy of the radiographic timer?

Sensitometry
Spinning top test (Correct answer)
Step wedge test
Half-value layer measurement

Correct answer: Spinning top test

The spinning top test evaluates timer accuracy in single-phase or three-phase generators by counting the number of dots or arc produced during a timed exposure. Digital systems require a synchronous spinning top or electronic timer test.

For single-phase generators, the spinning top creates discrete dots per exposure. At 1/60 second, one dot (one pulse) should appear. For three-phase, 12-pulse equipment, an arc is created. Digital (constant-potential) generators require a synchronous spinning top (motor-driven at known speed) or electronic timer accuracy tests since the output is nearly constant. Timer accuracy should be within ±5% of the set time. This test is part of routine QC for radiographic rooms.

Question 2: The purpose of performing a sensitometric strip in film-based radiography QC is to:

Measure focal spot size
Evaluate processor performance by measuring film base-plus-fog, speed, and contrast indices (Correct answer)
Assess collimator alignment
Test kVp accuracy

Correct answer: Evaluate processor performance by measuring film base-plus-fog, speed, and contrast indices

Sensitometry monitors film processor performance by measuring base-plus-fog (Dmin), speed index (SI), and contrast index (CI) from a standardized sensitometric strip. Deviations indicate processor drift requiring maintenance.

A sensitometer exposes a film strip to a standardized, reproducible light source with 21 density steps. The strip is processed and read with a densitometer. Key values include: base-plus-fog (should be ≤0.20), speed index (mid-density point), and contrast index (difference between two specific steps). Daily sensitometric monitoring catches processor temperature, chemistry, and replenishment problems before they affect clinical images. Control charts are used to track trends.

Question 3: In computed radiography (CR), the erasure cycle of the imaging plate is performed to:

Increase the sensitivity of the plate
Remove residual latent image from a previous exposure (Correct answer)
Improve spatial resolution
Calibrate the plate reader

Correct answer: Remove residual latent image from a previous exposure

The erasure cycle (flooding with intense white light) removes residual latent image trapped in the phosphor crystals of the imaging plate, preventing ghost images from appearing on subsequent exposures.

CR imaging plates use photostimulable phosphors (PSP) that trap electrons when exposed to X-rays. After readout, residual electrons may remain. The erasure cycle exposes the plate to intense fluorescent light for a standardized time, releasing all remaining electrons and returning the plate to baseline sensitivity. Inadequate erasure leaves a faint ghost image from the previous exam. Erasure should be verified during QC, especially after high-dose exposures. Plates not erased within a set time period are automatically re-erased before use.

Question 4: A lead apron QC program should include which of the following tests performed at least annually?

Tensile strength testing
Fluoroscopic or radiographic inspection for cracks or defects (Correct answer)
Weight measurement and calibration
pH testing of the lead lining

Correct answer: Fluoroscopic or radiographic inspection for cracks or defects

Lead aprons must be inspected fluoroscopically or radiographically at least annually (and after suspected damage) to detect cracks, voids, or separation in the lead shielding that would reduce protective effectiveness.

Visual inspection of lead aprons is insufficient because lead can crack internally without visible surface damage — especially at fold points. Regulatory and accreditation standards (NCRP, state law, ACR) require periodic fluoroscopic or radiographic inspection to detect: cracks in the lead, thin spots, holes, and delamination. Any apron with defects covering a critical area (gonads, chest) should be removed from service. Inspection logs must be maintained. Folding aprons rather than hanging them accelerates cracking.

Question 5: Which phantom is most commonly used for QC testing of fluoroscopic image intensifier systems?

AAPM CT phantom
Leeds TO.16 or RMI 156 fluoroscopic phantom (Correct answer)
Alderson Rando phantom
Acrylic step wedge

Correct answer: Leeds TO.16 or RMI 156 fluoroscopic phantom

Fluoroscopic QC phantoms such as the Leeds TO.16 or RMI 156 contain test patterns (resolution, contrast, line pairs) used to assess image intensifier resolution, contrast, and distortion.

Fluoroscopic QC tests assess: spatial resolution (line pair patterns), low-contrast resolution, image intensifier conversion factor, automatic brightness control (ABC), and geometric distortion. Phantoms like the Leeds TO.16 or Nuclear Associates fluoroscopic test tool contain wire mesh, resolution bar patterns, and contrast targets. Results are compared to baseline measurements and manufacturer specifications. QC must be performed according to NCRP Report 99 or state regulatory requirements, typically quarterly or semi-annually.

Question 6: Automatic exposure control (AEC) QC testing evaluates:

Focal spot size only
Consistency of mAs output across different anatomical part thicknesses (Correct answer)
Darkroom safelight integrity
Generator ripple percentage

Correct answer: Consistency of mAs output across different anatomical part thicknesses

AEC QC testing verifies that the system delivers consistent, appropriate exposure across different thicknesses and at different density settings. The AEC should maintain consistent optical density/detector dose regardless of patient thickness.

AEC systems use ionization chambers or solid-state detectors to terminate exposure when the IR reaches a preset dose. QC testing includes: (1) reproducibility — repeat exposures at the same settings should produce the same output; (2) density consistency across different attenuator thicknesses; (3) kVp compensation — changing kVp should not require manual mAs adjustment; and (4) backup time settings. Tolerance is typically ±10% variation in receptor dose. AEC failures can cause systematic over- or under-exposure.

Radiography Exam Quality Control & Assurance 2

Which QC test is performed to evaluate the accuracy of the radiographic timer?