7.3 Technique Compensation: Age, Pathology, Cast and Orthopaedic Devices, Screen Speed & Beam Restriction
Key Takeaways
- Paediatric technique reduces mAs substantially below the adult value for the same part while keeping exposure times as short as possible, and geriatric technique reduces mAs for osteoporotic bone but often needs longer immobilisation support rather than higher factors.
- A dry plaster cast typically requires doubling the mAs or adding about 8 to 10 kVp, a wet plaster cast requires roughly tripling the mAs, and a fibreglass cast requires only about a 25 to 30 percent mAs increase or no change at all.
- Additive pathology such as pleural effusion, ascites, pneumonia, Paget disease and osteoblastic metastasis requires an increase in technique, while destructive pathology such as emphysema, osteoporosis, osteolytic metastasis and bowel obstruction requires a decrease.
- Changing intensifying screen speed follows the relative speed formula, in which the new mAs equals the old mAs times the old relative speed divided by the new relative speed, so moving from 200 to 400 speed halves the required mAs.
- Adding or tightening a beam restrictor removes scatter and therefore reduces receptor exposure, requiring a compensating increase in mAs; the general rule for changing from a large to a small field is roughly a 30 to 50 percent mAs increase.
7.3 Technique Compensation: Age, Pathology, Cast and Orthopaedic Devices, Screen Speed & Beam Restriction
Section 7.2 covered the inverse square law, the 15% rule and grid conversion factors. The Enhanced TOS competency 2.3 of Radiographic Contrast lists eight conversion situations and awards 5 items to solving them. The remaining four — technique change when using a beam restrictor, correction factor for varying intensifying screen speeds, age correction factor, and correction factor for orthopaedic radiography — are the subject of this section, together with the pathology compensation that examiners pair with them.
1. The Master Rule
Every compensation question is the same question: something has changed how many photons reach the image receptor, so how much must mAs change to restore receptor exposure?
- If the change removes photons from the receptor (thicker part, tighter collimation, adding a grid, slower screen, wet cast), increase mAs.
- If the change delivers more photons (faster screen, removing a grid, thinner or less dense part), decrease mAs.
Adjust mAs, not kVp, when your goal is to restore density or receptor exposure while holding contrast constant. Adjust kVp when you need penetration.
2. Age Correction
Age changes tissue composition, not just size, so a paediatric or geriatric patient is not simply "a smaller adult".
Paediatric
| Age band | Typical mAs relative to the adult value for the same part | Rationale |
|---|---|---|
| Neonate / infant | About 25-30% | Minimal muscle mass, high water content, poorly mineralised bone |
| 1-5 years | About 30-50% | Progressive mineralisation |
| 6-12 years | About 50-75% | Approaching adult tissue density |
| Adolescent | Approaching 100% | Adult technique with size adjustment |
Beyond arithmetic, paediatric technique obeys three rules:
- Shortest possible exposure time, achieved with the highest mA the tube allows, because motion is the dominant cause of repeats in children.
- Higher kVp with lower mAs where contrast permits, to cut dose.
- Immobilisation before restraint. Use the department's approved immobilisation devices; a parent or guardian who assists must be given a lead apron and gloves and must never be someone who is or may be pregnant.
Geriatric
Osteoporotic bone attenuates less, so reduce mAs by roughly 25-30% for the spine, pelvis and hip in a patient with known significant osteoporosis. What geriatric patients usually need is not a technique change but time, support and clear instruction: kyphosis alters the relationship between the part and the receptor, hearing impairment affects breathing instructions, and reduced mobility limits positioning. Compensating with technique for a poorly supported position produces motion, not diagnosis.
3. Casts and Orthopaedic Devices
This is the "correction factor for orthopaedic radiography" the TOS names explicitly.
| Device | Compensation |
|---|---|
| Fibreglass cast | No change, or increase mAs by about 25-30% for a thick cast |
| Dry plaster cast (small to medium) | Double the mAs, or add about 8-10 kVp |
| Dry plaster cast (large / hip spica) | Increase mAs by about 2 to 3 times, or add about 10 kVp |
| Wet plaster cast | Increase mAs by about 3 times, or add about 10-12 kVp — water in the plaster is a major additional attenuator |
| Plaster cast with a metal reinforcing bar | Do not compensate for the bar; change the projection if it obscures anatomy |
| Air splint | Decrease mAs slightly — the splint attenuates less than the soft tissue it replaces |
| Fibreglass or plastic splint | Usually no change |
| Orthopaedic hardware (plates, rods, prostheses) | Do not increase overall technique to "see through" metal. Metal is essentially opaque; instead collimate, use an appropriate projection, and consider metal-artifact strategies on cross-sectional imaging |
A recurring exam trap: traction apparatus and external fixators do not require a technique increase — they require a projection that works around them, plus scrupulous attention to not moving the limb.
4. Pathology Compensation
The Patient Care subject links this to Effect of Pathologic Conditions on the Relative Attenuation of X-rays; the Image Production subject tests the arithmetic.
| Direction | Mechanism | Examples | Typical adjustment |
|---|---|---|---|
| Additive (increase technique) | Fluid, tissue or mineral replaces air, or bone becomes denser | Pleural effusion, empyema, haemothorax, ascites, pneumonia, atelectasis, pulmonary oedema, congestive heart failure, Paget disease, osteoblastic metastasis, osteopetrosis, acromegaly, hydrocephalus | Increase mAs by about 35-50%, or add about 5-8 kVp (more for large effusions) |
| Destructive (decrease technique) | Air or lucent tissue replaces denser tissue, or bone demineralises | Emphysema, pneumothorax, bowel obstruction, free intraperitoneal air, osteoporosis, osteomalacia, osteolytic metastasis, multiple myeloma, degenerative arthritis in the elderly, severe emaciation | Decrease mAs by about 25-50%, or reduce kVp by about 5-8 |
The judgement rule: compensate only when the pathology is known and diffuse. A focal lesion should never drive a global technique change, and compensating for a suspected but unconfirmed condition risks masking the very finding being sought.
5. Intensifying Screen Speed Conversion
new mAs = old mAs x (old relative speed / new relative speed)
| Screen system | Relative speed |
|---|---|
| Detail / extremity (slow) | 100 |
| Par speed | 200 |
| Regular / medium | 400 |
| High speed | 800 |
| Very high speed | 1200 |
Worked example 1. An acceptable knee radiograph used 20 mAs with a 100-speed detail screen. What mAs is needed on a 400-speed system?
20 x (100 / 400) = 5 mAs. Four times faster means one quarter the mAs.
Worked example 2. A technologist must move from a 400-speed system to a 100-speed detail cassette for a finger. Original technique was 3 mAs.
3 x (400 / 100) = 12 mAs.
The trade-off you must be able to state. Faster screens use larger phosphor crystals and a thicker phosphor layer, which increases light output and therefore lowers patient dose, but increases quantum mottle and light diffusion, which degrades spatial resolution (recorded detail). Slow detail screens are used for extremities precisely because resolution matters more than dose in a thin, low-dose part.
Screen-film relative speed also depends on kVp: rare-earth screen speed rises with kVp, so a technique chart calibrated at 70 kVp may over-expose at 100 kVp on the same system.
6. Beam-Restrictor (Collimation) Change
Tightening collimation removes scattered photons that were contributing to receptor exposure. The image gets cleaner but lighter, so mAs must rise.
| Field change | Typical mAs compensation |
|---|---|
| Large field to a moderately smaller field | Increase mAs by about 30% |
| Large field to a tightly collimated small field on a thick part | Increase mAs by about 50%, or more on the abdomen and lumbar spine |
| Removing a cone or restrictor (opening the field) | Decrease mAs correspondingly |
The magnitude scales with the amount of scatter that was present: on a thin extremity, collimating changes almost nothing; on a large abdomen at 80 kVp, it can change receptor exposure substantially. Never let this compensation become an argument against collimating — collimation is the primary scatter-control and dose-reduction tool, and modern automatic exposure control largely handles the compensation for you provided the correct chamber is selected.
7. Consolidated Conversion Reference
| Situation | Formula or factor |
|---|---|
| Distance change (maintain density) | new mAs = old mAs x (new SID squared / old SID squared) |
| 15% rule (maintain density) | Increase kVp by 15% and halve mAs; decrease kVp by 15% and double mAs |
| Grid change | new mAs = old mAs x (new grid factor / old grid factor); typical factors: no grid 1, 5:1 grid 2, 6:1 grid 3, 8:1 grid 4, 12:1 grid 5, 16:1 grid 6 |
| Screen speed change | new mAs = old mAs x (old speed / new speed) |
| Dry plaster cast | mAs x 2, or kVp + 8 to 10 |
| Wet plaster cast | mAs x 3, or kVp + 10 to 12 |
| Fibreglass cast | mAs x 1.25 to 1.3, or no change |
| Additive pathology | mAs x 1.35 to 1.5, or kVp + 5 to 8 |
| Destructive pathology | mAs x 0.5 to 0.75, or kVp - 5 to 8 |
| Paediatric (infant) | About 25-30% of adult mAs |
| Tighter collimation | mAs x 1.3 to 1.5 |
Worked composite example. An adult knee is imaged at 65 kVp, 8 mAs, 100 cm SID, 400-speed screens, no grid. The next patient is an adult in a dry plaster long-leg cast, and the department has only 100-speed detail cassettes left.
- Cast:
8 x 2 = 16 mAs. - Screen speed:
16 x (400 / 100) = 64 mAs.
Final technique: 65 kVp, 64 mAs at 100 cm SID. If 64 mAs forces an exposure time long enough to risk motion, apply the 15% rule instead — raise to about 75 kVp and halve the mAs to 32 — accepting the slight loss of contrast in exchange for a motion-free image.
An acceptable forearm radiograph was produced at 60 kVp and 6 mAs. The patient returns with a wet plaster cast on the same forearm. Which technique is most appropriate?
A knee radiograph made with a 400 relative speed screen-film system required 5 mAs. The department must use a 100 relative speed detail cassette for the repeat. What mAs is required, and what is the trade-off?
A chest radiograph is requested on a patient with a large unilateral pleural effusion, using a technique chart calibrated for a normal chest. What adjustment is appropriate and why?
A radiographer tightens collimation on an abdominal radiograph from a 35 by 43 cm field to a 24 by 30 cm field. What happens to receptor exposure and what compensation is appropriate?