The Mobile Unit That Sometimes Looks Brighter
A battery-powered mobile DR unit serves the ICU of a tertiary hospital. During the monthly QC check at 70 kVp and 5 mAs, three consecutive exposures measured at 100 cm gave 12.1, 11.4 and 13.0 mR. At 10 mAs the same setup gave 24.9, 25.6 and 24.2 mR. The last annual half-value layer measurement was 2.1 mm Al at 80 kVp. Technologists report that the unit 'sometimes looks brighter' from one day to the next, that the collimator light is dim, and that the light field looks off-centre. It is the only mobile unit available for a 20-bed ICU, and its service contract has expired.
1. Assess exposure reproducibility and mAs linearity, showing your reasoning, and state whether the unit should stay in service.
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Reproducibility at 5 mAs: mean = (12.1 + 11.4 + 13.0)/3 = 12.17 mR. Standard deviation is about 0.66 mR, so the coefficient of variation is roughly 0.66/12.17 = 5.4%, and the spread from lowest to highest reading is 1.6/12.17 = about 13%. Successive exposures at identical settings should agree within about 5%. This fails.
Linearity: mean at 10 mAs = (24.9 + 25.6 + 24.2)/3 = 24.9 mR. 24.9/12.17 = 2.05 against an expected 2.00, a deviation of about 2.5%, which is inside the usual plus-or-minus 10% linearity tolerance. This passes.
An output that varies by 13% at fixed settings, with the anecdotal day-to-day brightness changes, points to an unstable generator, timer or tube output - not to technique. The unit should be withdrawn from clinical use until serviced. Being the only unit is a reason to escalate fast, not a reason to keep exposing patients with unpredictable output.
2. What does the half-value layer value mean for patient dose and image quality, and what would you check?
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At 80 kVp the minimum half-value layer should be 2.3 mm Al; 2.1 mm Al is below that. Low HVL means a relatively greater proportion of low-energy photons that are absorbed in the patient's skin and contribute nothing to the image - so skin dose rises for the same detector exposure, and image contrast is not improved.
Check added filtration and whether it has been removed or damaged, check kVp accuracy (a generator that indicates 80 kVp but delivers more will change the HVL that should be achieved), and check the tube for target pitting and filtration damage. Re-measure HVL after any service. Note that the dim collimator lamp and off-centre light field are separate faults with the same root cause category - the unit has not been maintained - and both increase dose through unnecessary field size.
3. Set out the corrective action, the administrative steps and the re-test plan for a unit with an expired service contract.
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Corrective action: tag the unit out of service, remove it from the schedule, record the failing values in the QC log, and notify the radiation safety officer and hospital administration the same day. Do not lend, borrow or 'use it just for tonight'.
Administrative: raise a service request even though the contract has lapsed - most vendors accept a one-off repair order - and put the contract renewal and the QC programme in writing to administration with the failed results attached. Arrange interim cover: another department's unit, patient transfer to the department for essential exams, or rescheduling elective portable work. Record the temporary change in service.
Re-test after repair and before returning to use: exposure reproducibility, mAs linearity, kVp accuracy, HVL, collimator/light-field alignment and beam centring. Retain the records, and review the images taken on the unit in the preceding weeks for evidence of the instability.