Whatever they use, ultra sound etc etc, it has to be something that gives you a live view of the needle + tumor when you do the injection. Whatever technology that is, it will never be as good as a MRI. They will have MRI images - where you can see tumors, small and large. The smallest ones you can detect on a MRI image is likely too small to detect on the live imaging when doing injection. Small tumors may also be very tricky to safely put a needle into without it moving around (you need to be sure your are inside the tumor with the needle). If live imaging is limited then I can see that they don't want to take a chance on the smallest ones.
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