Below is the information I received regarding a spleen scan.
The typical platelet study entails the following:
1. We take a small volume (approx 50 ml) of the patient's blood on DAY 1
2. We isolate the patient's platelets aseptically, and label them with a small amount (4 MBq, or 108 micro-Curie) In-111
3. We inject this into the patient's antecubital fossa (arm vein).
4. We take images at 1hr, 4hr (on DAY 1), 24hr (on DAY 2) and 48hr (on DAY 3) post-injection, and also take a small blood sample at each time point
5. The nuclear medicine consultant (sometimes in conjunction with haematologists) then compares the pictures and blood 'clearance' (pharmacokinetics) of the patient's platelets, and the study findings (report) is sent to your referring doctor within 3 days of the study.
The risk of such a study is small. The "effective dose" for a platelet study is 1 mSv (staff working in radiology departments are not allowed to receive more than 20 mSv body dose per year, for example). This 1 mSv effective dose is equivalent to 50 chest x-rays, or about a third of the natural background radiation received in a year, or an increased risk of 1 in 77,000. A full-body CT scan typically has an effective dose of 15 mSv, thus 15 times more radiation than the platelet study. I guess another risk (much more difficult to quantify) is the prick of the needle, but I am sure the patient has 'suffered' that many times before.