Scaled activity and a minimum diagnostic floor, preparation that decides the scan before it starts, immobilisation before sedation, and the equipment a children's list needs that an adult list never asked for.
A department that has run adult lists for two years and takes its first paediatric session discovers the difficulty within the first hour, and it is not the dose calculation. It is that a four-year-old will not lie still for twenty minutes, and almost every other adjustment follows from that one fact.
Children are not small adults for dosimetry either, but the dosimetry is the part with a published method. The rest is organisation, equipment and patience, and it is where departments either build a paediatric service or quietly stop accepting the referrals.
Paediatric activity is scaled from the adult value by body weight or body surface area using a published paediatric dosage reference. The department should pick one reference, have the physician and the medical physicist agree it, and post it in the hot lab where the person dispensing can see it. Working it out from memory, or halving an adult dose because the child looks about half-sized, is how a department ends up with a scan that has to be repeated.
Two consequences matter more than the arithmetic itself.
The first is that there is a minimum activity below which the study is not diagnostic. Below that floor you have given the child a radiation dose and obtained nothing, which is a worse outcome than not scanning. Dosage references carry minimum values for this reason. A department that is uncomfortable with paediatric dose should reduce the number of studies it requests, not the activity in the syringe.
The second is that a repeat scan is a second dose. That single line justifies spending more time and money on preparation and immobilisation than seems reasonable, because the cheapest dose reduction available to any department is the scan it did not have to do twice.
The hot lab has its own problem here. Small activities mean small volumes, and the residual left in the syringe and needle is a much larger proportion of a paediatric dose than of an adult one. Assay before and after administration, record the residual, and use the recorded net activity in the report and in any quantitative calculation. Weigh the child on the day, on the department's own scale. The weight written on the referral card was taken at some point in the past and children change.
Preparation is the highest-yield part of paediatric nuclear medicine and it costs nothing but organisation.
Almost everything a department can do without a drug should be tried before a drug is considered. Feed and wrap for infants. A moulded or vacuum support that holds position without force. Soft straps and sandbags. A parent seated in the room wearing an apron, with a hand where the child can feel it and out of the field of view. Distraction, which in practice means a phone or tablet held by a parent, and which should be treated as legitimate departmental equipment rather than tolerated. Timing the acquisition to coincide with a natural nap.
Sedation is a service, not a drug order. It requires a written policy, a person qualified to administer and monitor it who is not simultaneously running the acquisition, monitoring equipment, oxygen and suction, a recovery area, and a discharge criterion. If the hospital cannot provide all of those for the session, the department should not be sedating, and should say so at the point the list is booked rather than at the point the child is on the table. Sedated cases also occupy a much longer slot than the acquisition time suggests, and the booking system has to reflect that or the rest of the list collapses.
Here is the bind. A child has smaller organs and receives less activity, so there are fewer counts available per unit time, and the classic answer of imaging for longer is exactly the thing the patient will not permit. Every paediatric protocol is a negotiation between those two.
Things that buy counts:
Things that buy tolerance:
Much paediatric nuclear medicine is quantitative and repeated: relative renal function in a child with hydronephrosis is followed over years and used to decide whether to operate. The number is only useful if it was produced the same way each time. Fix the software, the background subtraction method and the region-drawing convention, record them with the report, and note the acquisition parameters so the study done in three years is comparable rather than merely similar. A change in relative function that turns out to be a change in method is a decision made on an artefact.