CRI Advanced Imaging Techniques 3 — Questions and Answers
Question 1: In computed tomography, which artifact appears as streaks or bands between two highly attenuating objects such as metal fasteners?
- Ring artifact
- Beam hardening artifact (Correct answer)
- Partial volume artifact
- Scatter artifact
Correct answer: Beam hardening artifact
Beam hardening causes streaks between dense objects because lower-energy photons are preferentially absorbed, shifting the beam spectrum and causing cupping or streaking.
Question 2: When selecting a scintillator for a flat panel detector used in high-energy industrial radiography (>1 MeV), the preferred scintillator material is typically:
- Cesium iodide (CsI:Tl)
- Gadolinium oxysulfide (Gd2O2S)
- Lutetium oxyorthosilicate (LSO) (Correct answer)
- Calcium tungstate (CaWO4)
Correct answer: Lutetium oxyorthosilicate (LSO)
Lutetium oxyorthosilicate and similar dense scintillators are preferred at MeV energies because their high atomic number and density provide adequate stopping power for high-energy photons.
Question 3: In laminography (also called computed laminography), the imaging geometry differs from standard CT in that:
- The object rotates 360° around a vertical axis
- The X-ray source and detector tilt around an oblique axis allowing flat extended objects to be imaged (Correct answer)
- The detector array is curved rather than flat
- Multiple focal spots are used simultaneously
Correct answer: The X-ray source and detector tilt around an oblique axis allowing flat extended objects to be imaged
Laminography uses a tilted rotation axis so the object can extend beyond the field of view, making it ideal for large flat components like circuit boards or fuselage panels.
Question 4: The dynamic range of a digital detector is most directly measured by:
- The smallest detectable defect size
- The ratio of the maximum to minimum measurable signal (often expressed in bits) (Correct answer)
- The number of gray levels visible to the human eye
- The detector's spatial resolution in line pairs per millimeter
Correct answer: The ratio of the maximum to minimum measurable signal (often expressed in bits)
Dynamic range is the ratio of the largest to smallest signal the detector can reliably measure, typically expressed as the number of bits in the analog-to-digital converter.
Question 5: In phase-contrast radiography, image contrast is generated primarily by:
- Differences in mass attenuation coefficients between materials
- Refraction and diffraction of X-rays at material interfaces (Correct answer)
- Compton scatter from low-density materials
- Fluorescent emission from trace elements
Correct answer: Refraction and diffraction of X-rays at material interfaces
Phase-contrast imaging exploits the refraction of X-rays at interfaces between materials with different refractive indices, producing edge-enhancement effects even for low-attenuation materials.
Question 6: A digital radiography system has a signal-to-noise ratio (SNR) that improves by a factor of 2 when the exposure is:
- Doubled
- Quadrupled (Correct answer)
- Halved
- Increased by 10%
Correct answer: Quadrupled
Because quantum noise follows Poisson statistics (σ ∝ √N), doubling the SNR requires quadrupling the number of detected photons.
Question 7: In industrial CT, the term 'voxel size' is analogous to 'pixel size' in 2D radiography and is primarily determined by:
- The X-ray tube focal spot size alone
- The detector pixel pitch and geometric magnification (Correct answer)
- The reconstruction kernel selected
- The rotation step angle used during scanning
Correct answer: The detector pixel pitch and geometric magnification
Voxel size in CT is fundamentally governed by the detector pixel pitch mapped back through the geometric magnification (source-to-object / source-to-detector ratio).
CRI Advanced Imaging Techniques 3
In computed tomography, which artifact appears as streaks or bands between two highly attenuating objects such as metal fasteners?