Image Production · Exposure and contrast3 min read5-question check
Pathology and Cast Technique Adjustments
Fluid, bone growth and dense tissue are additive, so raise technique; air, bone loss and wasting are destructive, so lower it.
Additive conditions
Some diseases make tissue harder to penetrate. These additive conditions need more technique than the chart's standard setting. In the chest they include pleural effusion, pneumonia, atelectasis, pulmonary edema, congestive heart failure, and cardiomegaly. In bone they include Paget disease, osteoblastic metastases, osteopetrosis, and acromegaly. In the abdomen, ascites and a large aortic aneurysm are additive, as are hydrocephalus and edema. A useful memory aid: added fluid, added bone, or added tissue means added technique. Many charts handle this with about a 15 percent kVp increase, or roughly 50 to 100 percent more mAs depending on severity, but follow your department chart.
Destructive conditions
Destructive conditions make tissue easier to penetrate, so technique goes down. In the chest these are emphysema and pneumothorax, where extra air replaces tissue. In bone they include osteoporosis, osteolytic metastases, multiple myeloma, osteomalacia, and hyperparathyroidism. Bowel obstruction with large gas-filled loops and severe weight loss, called emaciation, are also destructive. Using a standard technique on these patients overexposes the area and can bury fine detail, such as a thin pneumothorax line.
Casts
Casts add material the beam must cross. A small to medium dry plaster cast needs roughly 50 to 60 percent more mAs, or about 5 to 7 kVp. A large dry plaster cast needs about double the mAs, or about 8 to 10 kVp. A wet plaster cast needs about triple the mAs, or about 10 to 12 kVp, because water adds density. Fiberglass casts add very little, about 25 to 30 percent more mAs or 3 to 4 kVp. Splints follow the same logic by material and thickness.
Other patient factors
Age and body habitus matter too. Children and older adults with thin, osteoporotic bones need less technique. Larger patients need more, and contrast in the bowel or vessels, such as barium, needs higher kVp to penetrate. With AEC, the system adjusts mAs for most of these changes on its own, as long as the right chamber sits under the anatomy. Manual technique and technique charts need the adjustments made deliberately. When the history mentions a disease, check whether it adds or removes tissue before you set the technique.
Clinical Example
A patient with known emphysema comes for a PA chest. Using the standard chest technique, the last study showed burned-out lung periphery. This time the radiographer lowers the technique slightly per the chart, and the image shows lung markings out to the chest wall with a thin pneumothorax line easy to see.
Common Mistakes
- Raising technique for emphysema because the patient seems sick; air is destructive.
- Treating a fiberglass cast like plaster and overexposing the image.
- Forgetting that a wet plaster cast needs more than a dry one.
- Placing multiple myeloma with the additive bone conditions; it is lytic.
Why It Matters
The 2027 Image Production outline lists technique adjustments for casts, pathology, age, BMI, and contrast media. Questions give a diagnosis and ask whether to raise or lower technique, so knowing which list a disease belongs on is the whole skill.
Key Relationships
- Fluid, consolidation, sclerotic bone → additive → increase
- Air, lytic bone, wasting → destructive → decrease
- Small dry plaster → about +50% mAs; large dry plaster → double; wet plaster → triple
- Fiberglass → small increase
- AEC compensates if the chamber is under the anatomy