Let's talk about PSMA PETscans.
A patient is given an infusion of Gallium-68 and left for an hour for it to absorb. The cubicle is lead lined with lead lined swing doors. The isotope dispenser is lead covered and in a lead lined box. The person who administers the Gallium-68 has a badge underneath his gloves and another in his pocket that is checked regularly for the amount of radiation he has absorbed.
The half life of Gallium-68 is 68 minutes and so in about eleven hours it is it one one-thousandth of its strength. Drinking water after a scan helps to flush it out and meanwhile, stay away from children and pregnant women for a day and if one uses a public restroom then flush twice.
To form the image, which is produced in three dimensions, the radioactive isotope Gallium-68 is administered by injection. The isotope attaches to a molecule (ligand), such as PSMA-11 (also known as gozetotide). This tracer is injected into the patient and binds to prostate-specific membrane antigen (PSMA) proteins, which are overexpressed on the surface of most prostate cancer cells. The isotope then emits positrons, which are detected by the PET scanner to create detailed imagea.
It may occur to you to wonder how Gallium-68 is delivered, because with such a short half-life, how is it stored before it is delivered?
Let’s say a patient is going to be given gallium 68 which has a half-life of 68 minutes at a certain time on a certain day. How do the technicians get the gallium-68 to administer because it itself will already have started to decay?
The answer is that the hospital does not store Ga-68 itself. They store a longer-lived parent isotope called germanium-68, which has a half-life 271 days. And a generator continually produces gallium-68.
Ge-68 decays into Ga-68 at a steady rate. Because Ge-68 lasts months, the generator provides a small but predictable supply of fresh Ga-68 throughout the day.
When the patient needs an injection, the technician “elutes” the generator. That means they flush a saline solution through the generator and it washes out the newly-formed Ga-68, which is then used immediately to prepare the radiotracer.
Because Ga-68 decays quickly, timing is tight. So once eluted, the Ga-68 is prepared and administered quickly so the activity is correct for the scan.
The generator can be eluted multiple times per day. But each elution yields less if done too frequently. Typically, it’s done once a day or according to a schedule that maximizes the available activity.