The Cannula: — Bruising, Pain and Difficult Veins

The sensation is similar to a blood test — a brief sharp scratch at insertion. For most people it is manageable and lasts only a few seconds. What can make it feel worse is anxiety, cold skin, or needing more than one attempt because the first vein was not accessible. If you have found cannulation very painful before, mention it when you book so the team can allow extra time and explain each step as they go.

The team will try the most accessible site first and, if that does not work, try a different vein. If your veins are severely damaged, a more experienced nurse may take over, or ultrasound guidance may be used to find a deeper vein. In rare cases no vein can be accessed and the scan may need to be rescheduled. This is uncommon but does happen in patients who have had many cycles of chemotherapy. Telling the team in advance gives them the best chance of success at the first attempt.

Chemotherapy can affect vein walls and, in some people, platelet counts — both of which make bruising more likely. Some drugs also thin the skin over time. A bruise at the cannula site is more common in cancer patients and does not mean anything went wrong. Press gently on the site for a few minutes when the cannula comes out. Contact the scanning centre if the bruise spreads, the area becomes hard or warm, or if you feel pain or tingling in the arm.

Yes, and you can ask for it when you book or on arrival. It needs time to take effect, so tell the team as early as possible rather than asking once you are already in the chair. It reduces the sharp sting at insertion but does not eliminate all sensation. If anxiety about the cannula is significant, mention that too — teams are experienced with this and can walk you through each step as they go.

A small bruise that appears after the cannula is removed is common and settles on its own. Press gently on the site for a few minutes immediately after removal to help. Contact the scanning centre the same day if the bruise is spreading, the area becomes hard or warm, or if you develop pain, numbness or tingling in the arm. These signs are uncommon but worth reporting promptly rather than waiting to see what happens.

Tell the team which side your mastectomy was on when you book. A cannula is generally not placed in the arm on the same side as a mastectomy if lymph nodes were removed from that side, because the reduced lymphatic drainage raises the risk of swelling. The team will use the other arm. If you are unsure whether your lymph nodes were removed, check with your surgeon's team before the scan so the right decision can be made.

The radiotracer used in a PET scan has to be injected directly into a vein so that it circulates throughout your body within a controlled time. There is no tablet or inhaled version for standard PET scans. The cannula stays in during the waiting period after injection and during the scan itself, in case a second dose is needed. It is removed before you go home. The procedure does not require an overnight stay.

If the tracer does not enter the vein fully — a situation called extravasation — it can affect the quality of the scan images and the scan may need to be rescheduled. The team watch the site during injection for signs of swelling or discomfort. If it happens they note it and the reading radiologist takes it into account. Tell the team immediately if you feel pain, burning or swelling at the site during the injection — do not wait until after the scan.