Such a scan isn't possible with present technology. What is possible are scans and other methods of recording information that could conceivably be relevant to an attempt to reconstruct your mind at a future time. If the argument is simply that brain scans "couldn't hurt"... well, 'duh'. But making a diary of your frequent thoughts or videotaping yourself as you go about your day couldn't hurt either. Knowing the regional blood-flow patterns in your brain in response to narrow and limited stimuli is not in a significantly different category.

The question is whether the cost and time involved in these endeavors is is better than plausible counter-factual spending. My point isn't just that SI shouldn't purchase a machine; it's that giving to existential risk research, or ensuring the financial stability of your cryonics organization probably has a better return for your own long-term survival than getting frequent fMRI scans does. The point that spending money on brain scans has a better return than an expenditure that probably lowers your long-term survival rate (due to car accidents) is not a strong argument.