Portable Chest at 100 Centimeters
You are called to the ICU for a portable AP chest on a 70-kg, intubated patient with a right internal jugular central line and bilateral chest tubes. The bed is wedged against the wall and the maximum achievable source-to-image-receptor distance is 100 cm instead of the 180 cm you would normally use. You shoot with your standard 180-cm technique. The DR system reports a deviation index (DI) of +2.4; your department's acceptable DI range is ±1. The radiologist's read on the first image: "loss of contrast; line tip near the carina cannot be confidently localized."
1. Explain numerically what a DI of +2.4 means and why the first image came out that way.
Read the worked answer
The formula: DI = 10 × log10(EI actual ÷ EI target).
So DI = +2.4 means the receptor received 10^0.24 ≈ 1.74 times the target exposure - roughly three-quarters of a stop over.
Why: if the mAs is unchanged and the tube moves from 180 cm to 100 cm, receptor exposure rises by the inverse square:
(180 ÷ 100)² = 3.24×
Uncompensated, that would have produced a DI near +5. The measured +2.4 tells you that only part of the inverse-square gain was absorbed - either the technique you actually used was already lower than your 180-cm reference, or the patient/field differs from your reference case. Either way, the receptor is still about 74% over target.
Exact compensation from the measured DI: multiply mAs by 1 ÷ 1.74 ≈ 0.57. Compensation from geometry alone: multiply mAs by 0.31. Do not apply both - pick the one that matches the baseline you are correcting from.
2. What will you change for the next exposure, and what will you refuse to change?
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Change:
- Set mAs to about 57% of the value that produced DI +2.4 (or 31% of your true 180-cm technique if that is what you started from). Verify with the DI on the next image rather than assuming.
- Push the SID as far as the room physically allows before accepting 100 cm - angle the tube housing, move the bed 20 cm if the wall permits. Even 120 cm cuts magnification meaningfully.
Refuse to change:
- Do not drop the grid. At 100 cm with an adult chest, the air gap is unchanged (that is OID, not SID) and scatter reaching the receptor is significant. Losing the grid trades an exposure problem for a contrast problem.
- Do check the grid's focal range. A grid labeled for 180 cm used at 100 cm produces peripheral cut-off - density falling off toward the edges that looks like a positioning error and is not correctable by mAs. If the grid is out of its focal range, switch to a grid rated for the working distance rather than removing it entirely.
Separate the two complaints. Loss of contrast on a DR image is usually a scatter/geometry problem, not a dose problem. The DI and the contrast complaint may have different causes; do not fix one by making the other worse.
3. Do you repeat the image? Defend your answer.
Read the worked answer
First, interrogate the image you already have. Zoom and re-window the existing file and look specifically at the central line tip relative to the carina. A receptor exposure 1.74× target does not destroy line visibility on a DR system - overexposure generally gives you more signal, and the reported contrast loss is more likely scatter and windowing. If the tip can be localized within a centimeter or two, there is nothing to gain from another exposure.
Repeat only if the clinical question is unanswerable. Justify it explicitly:
- The question is line position. If it cannot be answered, the exam has failed.
- The patient is intubated and unstable; a repeat means another manual hold, another set of hands, and coordination with respiratory therapy. That risk is real but acceptable if the alternative is a malpositioned line.
- A repeat is justified at a corrected technique plus tighter collimation, not at the same technique.
If you repeat without changing anything, you have added dose and learned nothing - that is the definition of a preventable repeat.