from Robin, Leif and Susan's older son

You get two emails from me, to give you a sense of this guy.

I work in an ICU. I was on night-shift, and I sent him an email which he replied to during the day, and I read the next night at work.

I wrote…

I'm sitting down in MRI monitoring my patient.  The tech tells me it's a superconducting magnet cooled with helium and that's what the whooshing sound is.  To make the images they use RF coils arrayed in each of 3 dimensions to disrupt the magnetic field?  Cool.

He wrote…

Oh cool! I've been in an MRI machine at about that time - well a few hours earlier. The tech was happy.
 
Let's see - ok I'm making this up from debris floating around in there.
 
The signal comes from a change in the power absorbed from the applied rf as the rf frequency is swept (varied in very small steps) over the range where the protons in the matter will "flip".their spin directions - sucking energy from the passing rf. The magnetic field sets the frequency and the strength of the signal. The magnetic field has to be very uniform over the area being measured.
 
aside:  (I tried to use NMR (nuclear magnetic resonance) but called MRI in public because nuclear is evil) to measure the AGS main magnet field once. There the field is by design changing with location. (It has a gradient as well as an offset value - the magnet works both as a dipole (the offset) and a quadrupole (the gradient). The gradient forced the MRI area sensitive to the applied rf to be very tiny. My efforts to "flatten" the field were not adequate to give me a signal I could measure and so I failed to measure the field. (The result would have been an absolute measurement - holy grail - so long as you can measure the frequency - usually relatively easy and absolute so long as we agree as to the length of a second.)

Ok, so in my experience use MRI to measure the field. But you know the field and use the strength of the response - the power sucked from the rf circuit - as the frequency is changed to learn the density of the material. Maybe not flipping proton spin but other stuff? Ok, now you are beyond my pay grade — being retired. I don't know how the correlation between power loss and material being sampled is accomplished. Oh yes, and then there is the need to unfold the three dimensional scan into a three dimensional density picture. Oh, that's just programming — right.
 
So I think they vary the rf frequency, measure the power loss, and the magnetic field doesn't know a thing.
 
cold!
 
da

For this other email — I started doing travel nursing, which is like contract work around the county, about three years ago. And it's a bit of a leap to take — going back to being a gypsy at thirty — I had a lot of apprehension, but wanted to do it.

So he sent me an email with a picture he took of this giant cloud of swallows down at the cliff over the Long Island Sound in Wading River. And he wrote…

The thousands of swallows milling above the stairs are about to travel. They seem full of anticipation and maybe the joy of flying. No second thoughts. Oh, to be a swallow. Ok, they are just doing what swallows do. So maybe not.
 
Enjoy your adventure. It is time. We do get some sustenance or energy from your being. Please keep in touch.
 
da