About 8% of men express insufficient PSMA for the scan to work and it seems Chris was one of them.
The PSMA scan is very similar to having an MRI scan in my experience. although you wait for your individually produced nuclear injection which comes in a specially protected syringe, (which I think in turn is inside a box to contain radiation). After the scan I was told to keep away from people for a couple of hours and then for another four hours to not get close to children or pregnant women. It seems different centres give slightly different directions regarding timing. There was nothing else of significance that I can remember.
Prior to the PSMA scan, I had a Choline scan, which led to UCLH concluding I had cancer in an Iliac Lymph node due to an amount of Choline uptake. However, I wanted other opinions on this so contacted 2 other hospitals in the UK and the hospital where I had my RT in Heidelberg back in 2008. All believed it was unlikely that the node was affected by cancer. Heidelberg compared the copy of the Choline scan with the MRI's they had previously done and said the size and shape of the nodes had not changed. They suggested I have a 68 Gallium PSMA scan (which incidentally was formulated in Heidelberg in conjunction with the DKFZ there). Unfortunately, UCLH would not give me this scan on the NHS so I paid for one privately at the Paul Strickland Scanner Centre. There was no sign of any uptake in the aforementioned node, although a small tumour in the Prostate. This was very important because if the node had been affected I would have to rely on systemic treatment. UCLH would not offer more focal treatment if the node had been affected. So the PSMA scan, which I copied to UCLH could well have changed treatment options. I am now awaiting the result of a biopsy to check whether another HIFU treatment would be appropriate.
It is the case that sometimes cancer cells can be too few and or dispersed to be seen on a PSMA scan but the chance of finding tumours increases with higher PSA in patients expressing sufficient PSMA.