Image Production and Quality Flashcards

A digital radiograph of a knee appears excessively noisy and grainy. The exposure indicator (EI) is significantly below the target range. Which of the following is the most likely cause for this appearance?

Answer: Insufficient mAs

A grainy or noisy appearance, known as quantum mottle, is caused by an insufficient number of x-ray photons reaching the detector. The mAs setting directly controls the quantity of photons produced. A low exposure indicator value confirms that the detector received inadequate exposure, which is most commonly due to insufficient mAs.

Which of the following technical factor adjustments will increase the penetrability of the x-ray beam, resulting in lower image contrast?

Answer: Increasing the kVp

Kilovoltage peak (kVp) controls the kinetic energy of the electrons striking the anode, which in turn determines the maximum energy and overall quality of the x-ray photons. Increasing the kVp produces a higher-energy, more penetrating x-ray beam. This increased penetration leads to less differential absorption between tissues, resulting in a longer scale of contrast (more shades of gray).

An MRT is performing a CT scan of the head. The resulting axial images show dark, streak-like artifacts and a 'cupping' artifact where the central portion of the brain parenchyma appears artificially dark. This is most characteristic of:

Answer: Beam hardening artifact

Beam hardening artifacts occur because a polychromatic x-ray beam becomes 'harder' (higher average energy) as it passes through dense objects like the skull, as lower-energy photons are preferentially absorbed. This differential absorption is not fully corrected by standard reconstruction algorithms, leading to characteristic dark streaks and 'cupping' where the center of a uniform object appears less dense than its periphery.

In digital radiography, what is the primary purpose of applying a look-up table (LUT) during image processing?

Answer: To map raw pixel data to specific brightness and contrast levels for display

A look-up table (LUT) is a post-processing tool that translates the raw numerical pixel values from the detector into specific grayscale values for display on a monitor. Each LUT is designed to provide the optimal brightness and contrast for a specific anatomical part, ensuring consistent image presentation regardless of minor exposure variations.

Which combination of geometric factors will result in the highest degree of image unsharpness (penumbra)?

Answer: Large focal spot, short SID, long OID

Geometric unsharpness, or penumbra, is the blurring at the edges of a structure. It is maximized by using a large focal spot, a short source-to-image distance (SID), and a long object-to-image distance (OID). Each of these factors increases the divergence of the x-ray beam as it passes the edge of the object, creating a larger, more poorly defined shadow on the detector.

An MRT performs a portable chest x-ray using a focused grid. The resulting image demonstrates severe grid cut-off on both lateral edges, while the central portion of the image is properly exposed. What is the most likely cause of this artifact?

Answer: The grid was placed upside-down

A focused grid has its lead strips angled to match the divergence of the x-ray beam at a specific SID range. If the grid is placed upside-down, the strips are angled in opposition to the beam's divergence. This causes the primary beam to be absorbed at the periphery, resulting in severe bilateral cut-off, while only the central, most perpendicular rays can pass through to the detector.