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Physics
Ch 11
| Question | Answer |
|---|---|
| What is scatter radiation? | X‑ray photons that have changed direction after interacting with matter. |
| What causes scatter radiation in radiography? | Interactions (mainly Compton scattering) between x‑ray photons and patient tissues. |
| Why is scatter radiation undesirable in radiographic imaging? | It degrades image contrast and adds unwanted dose to patient and operator. |
| What is the primary method to reduce scatter reaching the image receptor? | Use of a grid. |
| What is a grid in radiography? | A device placed between patient and image receptor that absorbs scatter before it reaches the receptor. |
| What does grid “ratio” refer to? | The height of lead strips divided by the distance between them (height/spacing). |
| How does increasing the grid ratio affect scatter cleanup? | Higher grid ratio absorbs more scatter photons. |
| What is a disadvantage of a high‑ratio grid? | Requires increased exposure (mAs), increasing patient dose. |
| What is grid “frequency”? | Number of lead strips per centimeter or inch. |
| How does grid frequency affect image quality? | Higher frequency reduces visible grid lines but may increase absorption of primary beam. |
| What is grid “cutoff”? | Loss of primary beam and image density due to misalignment or misuse of the grid. |
| Name three common causes of grid cutoff. | Wrong SID, improper grid‑detector alignment, grid angled incorrectly. |
| How does field size affect scatter production? | Larger field size → more scatter. |
| How does collimation affect scatter? | Reducing field size via collimation decreases scatter. |
| What is beam restriction? | Limiting the x‑ray beam to the area of interest using collimators or apertures. |
| Why is beam restriction important? | Reduces scatter, lowers patient dose, improves image quality/contrast. |
| How does patient thickness affect scatter production? | Thicker patients → more scatter due to increased interaction volume. |
| What is compression in radiography? | Applying pressure or immobilization to reduce patient thickness. |
| How does compression affect scatter and image quality? | Reduces scatter, lowers dose, improves contrast. |
| What is a moving grid (reciprocal grid)? | Grid that moves during exposure to blur its lines and reduce artifacts. |
| What is a stationary grid? | A fixed grid that does not move during exposure. |
| What type of grid is typically used in general radiography? | Moving (reciprocal) grid. |
| What is the recommended grid ratio for general radiography? | Often 8:1 or 10:1 (varies by exam and technique). |
| What is the effect of using no grid on image quality and dose? | Lower image contrast (more scatter) but lower patient dose and simpler positioning. |
| What interaction primarily produces scatter in diagnostic imaging? | Compton scattering. |
| How does increasing kVp affect scatter production? | Higher kVp increases scatter proportion relative to absorption. |
| What is the scatter‑to‑primary ratio (SPR)? | The ratio of scattered photons to primary (useful) photons reaching the receptor. |
| How does SPR affect image contrast? | Higher SPR → lower contrast (more fog). |
| What is backscatter? | Scatter radiation that is reflected back toward the source or operator from patient or surfaces. |
| How does backscatter affect occupational exposure? | Increases operator dose if shielding or distance not adequate. |
| What is a radiographic apron used for? | To shield personnel from scatter and backscatter radiation. |
| What is lead equivalent of a protective barrier/apron? | The thickness of lead that provides equivalent attenuation (often 0.5 mm Pb for aprons). |
| What is the effect of using beam‑limiting devices on patient dose? | Reduces unnecessary exposure by limiting irradiated volume. |
| What is the primary disadvantage of excessive collimation? | Potential for cutoff of anatomy of interest if misaligned. |
| What is “grid ratio vs scatter cleanup” tradeoff? | Higher grid ratio → better scatter removal but increased dose and need for precise technique. |
| What is the purpose of a moving grid blur? | To eliminate visible grid lines on the radiograph. |
| What should be considered when selecting a grid for a specific exam? | Patient size, exam type, kVp, field size, expected scatter amount. |
| What is grid cutoff caused by improper SID? | If SID is outside the recommended range, primary beam may be absorbed by grid strips → underexposure. |
| What is the benefit of using a high‑frequency grid? | Less visible grid lines on the image. |
| What is cross‑hatch grid? | Grid with lead strips in two perpendicular directions (rarely used, requires precise alignment). |
| Why are cross‑hatch grids not commonly used in general radiography? | Difficult to align central ray and receptor — easy to cause severe cutoff. |
| What is a moving grid “oscillating” along during exposure? | It moves the lead strips laterally to blur out grid lines. |
| How does improper centering affect grid performance? | Increases risk of grid cutoff and uneven receptor exposure. |
| What is quantum noise from scatter? | Variation in receptor exposure caused by random scatter photons reducing image contrast. |
| What is receptor exposure “fog”? | Unwanted receptor exposure primarily from scatter radiation, lowering contrast. |
| What is subject contrast? | Contrast resulting from differences in absorption by patient tissues. |
| How does scatter affect subject contrast? | Scatter adds uniform exposure, reducing contrast between tissues. |
| What is the effect of increasing mAs to compensate for grid absorption? | Increases patient dose, may increase scatter production. |
| What is “air gap” technique? | Increasing OID (object-to-image distance) to reduce scatter reaching the receptor. |
| How does increasing OID reduce scatter? | Scatter photons diverge and miss the receptor due to distance. |
| What is a common disadvantage of the air gap technique? | Increased magnification and potential for unsharpness. |
| What is a bucky tray? | The part of the x‑ray table or wall unit that holds and moves the grid. |
| Why must the grid‑tray align properly with central ray? | To avoid grid cutoff and uneven exposure. |
| What is backscatter guard? | Lead shield behind patient or table to reduce scatter toward operator. |
| What is the relationship between patient dose and scatter control? | Better scatter control reduces required radiation, lowering dose while improving image quality. |
| What is the ideal balance for scatter control in radiography? | Sufficient primary x‑rays for image formation + minimal scatter — achieved via optimal kVp, mAs, collimation, grid use. |
| What is “scatter fraction”? | Portion of total radiation reaching the receptor that is scatter (vs primary). |
| How does grid use affect scatter fraction? | Reduces scatter fraction by absorbing scatter before it reaches receptor. |
| What is the effect of poor grid maintenance (damaged grid) on images? | Uneven exposure, grid lines, cutoff — poor image quality. |
| What is “grid alignment test”? | QC procedure to ensure grid is properly aligned to tube and receptor for correct use. |
| What is a “moving‑grid blur pattern test”? | QC check to verify moving grid is functioning and grid lines are properly blurred. |
| What is the definitions of grid selectivity vs contrast improvement ratio? | Selectivity: ability to transmit primary vs scatter; Contrast improvement: degree of contrast increase when using grid. |
| Why might a technologist omit a grid for pediatric or portable extremity exams? | Because small field size and low scatter make grid unnecessary — reduces dose and simplifies setup. |
| What is the benefit of using a grid for chest radiography? | Removes high scatter due to large anatomy — improves contrast and diagnostic quality. |
| What does “beam‑restricting device” refer to? | Collimator, cones, or aperture diaphragm used to shape the beam and reduce scatter. |
| What is “gonadal shielding” in context of scatter control? | Lead shields placed over radiosensitive organs to reduce unnecessary scatter or direct radiation exposure. |
| How does correct patient positioning contribute to scatter control? | Minimizes tissue overlap, limits irradiated volume, reduces scatter. |
| What is the effect of beam energy (kVp) on scatter-to-primary ratio? | Higher kVp → increased scatter relative to absorption → higher SPR. |
| What is a drawback of using too low kVp to reduce scatter? | Increased patient dose, increased absorption, lower penetration for thick anatomy. |
| What combination of techniques yields best image quality with minimal dose? | Proper kVp/mAs, collimation, grid selection, collimation, minimal OID, compression when appropriate. |
| What is the radiographer’s role in scatter radiation control? | Select appropriate technique, use collimation, choose/add grids when needed, maintain QC of grids and beam‑limitation. |
| What is the relationship between field size and patient dose? | Larger field size → more tissue irradiated → higher patient dose. |
| What is the main purpose of QC for grid and beam‑restriction devices? | Ensure consistent scatter control, prevent artifacts, and maintain image quality while managing dose. |
| What is “contrast improvement factor (K)”? | Ratio of contrast with grid to contrast without grid — measures how much contrast is improved by grid use. |
| What is a “grid conversion factor (GCF)”? | The factor by which mAs must be increased when switching from non‑grid to grid technique to maintain receptor exposure. |
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