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Appendix A — Exposure & Acquisition Reference Tables

Purpose

This appendix provides quick access to essential technical data frequently referenced in radiographic positioning and exposure selection.
It is designed for fast clinical lookup and supports efficient decision-making during image acquisition.

Exposure Factor Reference by Proportional Anatomy

Proportional Anatomy Group (Examples) Typical SID kVp Range mAs Range Grid Use Technique Notes (Technical Insights)
Distal Extremities
Hands, wrists, feet, ankles		 40 in (102 cm) 55 – 65 2 – 6 No Use short exposure times to minimize motion blur; employ small focal spot for sharp cortical detail.
Shoulder–Knee–Cervical Group
Shoulder, humerus, knee, cervical spine		 40 in 65 – 75 6 – 12 Optional For parts > 10 cm, add +15 % kVp or switch to grid; consider compensating filter for shoulder or knee thickness variation.
Hip–Femur–Pelvis–Abdomen Group
Hip, femur, pelvis, lumbar spine, abdomen		 40 in 75 – 90 20 – 60 Yes Apply anode-heel effect (anode toward head) for uniform exposure; use AEC (center + outer cells) for large field coverage.
Thoracic–Cervical Lateral Group
Thoracic and lateral cervical spine		 40 – 48 in 75 – 95 20 – 60 Yes Use air-gap technique (especially lateral C-spine) to reduce scatter and enhance contrast; compensate with slight mAs increase.
Chest (PA & Lateral) 72 in (183 cm) 110 – 125 2 – 6 Yes Use very short exposure (< 10 ms) to freeze motion; high kVp for long-scale contrast; grid ratio ≥ 8:1.
Skull–Facial–Sinus Group 40 in 75 – 85 10 – 30 Yes Employ tight collimation and small focal spot; reduce mAs ≈ ½ for air-filled sinuses.
Contrast Studies (Barium or Iodine) 40 in +30 % kVp above non-contrast ¼ mAs of non-contrast Yes Use high kVp for penetration of dense contrast; avoid AEC over-response by using manual exposure control.

Technique Adjustment Rules

Situation Adjustment
15 % increase in kVp ≈ 2× mAs Maintains density, changes contrast
30° oblique projection 1.5× mAs
45° oblique projection 2× mAs or +15 % kVp
Lateral projection 4× mAs and +15 % kVp
Barium studies +30 % kVp / ¼ mAs
Grid conversion (non-grid → grid) 2× mAs or +15 % kVp

Clinical Notes

  • Select exposure factors by proportional anatomy rather than by exam name.
    This approach groups regions that share similar tissue thickness and attenuation patterns, allowing consistent technique selection even when imaging different body parts.
  • Examples of proportional relationships:
    • Shoulder, knee, and cervical spine: comparable tissue thickness and density; similar midrange kVp (65–75) and moderate mAs.
    • Hip, femur, pelvis, and abdomen: higher tissue volume and bone density; typically 75–90 kVp range with grid use.
    • Extremities (hands, feet, forearms, ankles): thin, low-attenuation structures imaged without a grid (55–65 kVp).
  • This grouping method allows radiographers to:
    • Anticipate technical adjustments based on size and density rather than memorizing exposure numbers.
    • Apply the 15% kVp rule and proportional mAs scaling confidently.
    • Reduce repeat exposures and improve consistency between manual and AEC-based techniques.
  • Maintain consistent contrast using fixed-kVp charts.
    In a fixed-kVp system, each body part or proportional group uses a set kVp chosen for optimal subject contrast, and only the mAs is adjusted for patient size or pathology.
    Example: All abdomen exams may use 80 kVp, increasing or decreasing mAs for thin or large patients.
  • Adjust exposure proportionally using variable-kVp charts (Base 60 Method).
    In a variable-kVp chart, exposure is calculated from measured part thickness:
    60 kVp + 2 kVp per cm of tissue.
    This method increases kVp with thicker anatomy, improving beam penetration while keeping mAs constant—useful where consistent size measurement is possible.
  • Key comparison:
    • Fixed-kVp: faster, more consistent contrast across technologists and machines.
    • Variable-kVp: more individualized; useful in teaching settings to show how attenuation affects exposure.
  • Always record adjustments that “work” for your department to create reproducible technique charts.

Grid Use

Grids are designed to improve image contrast by absorbing scatter radiation before it reaches the image receptor. Scatter reduces subject contrast and can obscure low-contrast detail, especially in thick or dense body regions. Correct use of either physical or virtual grids helps maintain diagnostic image quality while optimizing radiation dose.

Physical Grid Use

  • Use a physical grid whenever part thickness exceeds 10 cm (4 in) or when scatter visibly reduces image contrast.
  • Grid ratios:
    • 8:1 — routine use for average adult body parts.
    • 10:1 to 12:1 — for larger patients, mobile chest, or abdomen imaging where scatter is increased.
  • Centering: Align the x-ray beam precisely to the grid’s center to prevent grid cut-off and ensure uniform exposure.
  • SID tolerance: Adhere to the grid’s specified focal range to maintain image uniformity.
  • Removal for thin parts: Remove the grid for pediatric, extremity, or thin anatomy imaging (< 10 cm) to reduce patient dose and unnecessary exposure.
  • Technique compensation: When adding a grid, increase mAs or kVp per departmental grid conversion factors (e.g., +15 % kVp or 2× mAs).

Virtual (Digital) Grids & Hybrid Use

Modern digital radiography systems increasingly employ virtual grids (also called software scatter correction or grid-less scatter reduction). These post-processing algorithms estimate and suppress scatter radiation electronically, improving image contrast without requiring a physical grid.

  • Mechanism: Virtual grids analyze raw image data and model scatter distribution, then subtract or correct it before image display.
  • Performance: Studies show that, for many body parts (e.g., chest, abdomen, pelvis), virtual grids can produce image contrast comparable to a physical grid—often with reduced patient dose when the physical grid is eliminated and exposure factors are appropriately lowered to compensate.
  • Grid ratio emulation: Some systems allow users to select a virtual grid ratio (e.g., 6:1, 10:1, 20:1) that adjusts the algorithm’s scatter-suppression strength.
  • Workflow advantage: Eliminates alignment errors and simplifies mobile or trauma imaging where physical grid centering is difficult.
  • Noise consideration: Unlike physical grids, virtual grids may slightly increase image noise when heavy scatter correction is applied. Use department-approved presets to balance contrast and noise.

Hybrid Use (Physical + Virtual)

In cases of very large or high-scatter patients, both a physical and virtual grid may be used together:

  • The physical grid intercepts a large portion of scatter before it reaches the detector.
  • The virtual grid algorithm further refines image contrast by suppressing residual scatter.
  • This hybrid approach maximizes image quality for bariatric patients or high-scatter projections (e.g., lateral abdomen, cross-table spine).
  • Ensure exposure technique adjustments account for grid use (typically 2× mAs for the physical grid component).

Grid Selection by Part Thickness and Scatter Level

To select the level of grid use, measure or estimate part thickness and consider the level of scatter that will be produced. If the part is thin and the scatter risk is low, no grid is needed. If the part is thin/medium and the scatter risk is moderate a virtual (digital) grid may be used. If the part is medium/thick and there is a high risk of scatter a physical grid should be used. If the patient is morbidly obese, a hybrid approach, using both a physical grid and a virtual grid may be employed.

Technique Notes

  • Virtual grid: Removing the physical grid and enabling virtual correction often allows mAs reduction while maintaining contrast—verify with vendor guidance and track EI/DI.
  • Hybrid (very large patients): Keep the physical grid and enable virtual correction to preserve contrast; expect higher mAs (watch heat/motion).
  • Air-gap option: On some laterals (e.g., C-spine), a modest air-gap + longer SID can reduce scatter like a low-ratio grid; raise mAs to compensate.
  • Tight collimation: Always reduces scatter at the source.

Clinical Notes

  • For grid-less exams, remember that proper collimation and tight field size remain the most effective scatter control method.
  • Always verify the virtual grid option is correctly selected on the workstation before exposure.
  • Physical grids require more precise centering; virtual grids reduce positioning error but must be properly configured in software calibration.
  • Document any hybrid grid use in technique protocols for reproducibility and training.

Clinical Application Notes

These reference values represent standard departmental averages.
Always verify local protocol, equipment calibration, and patient-specific considerations such as body habitus, age, or pathology.
Use these tables as a guide to maintain image quality, minimize dose, and ensure reproducible results.

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Clinical Preceptor Reference Guide for Student Competencies Copyright © 2025 by Carla M. Allen is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, except where otherwise noted.

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