Life as an Engineer

As a postdoc at Stanford, I have quite a bit of latitude to seek out interesting projects. Here are a few things I've looked at in my first year here.

RF Safety Prescreening

Currently the #1 safety issue in MRI is the existence of implanted wires in patients. These are most commonly found in pacemakers, but also in neurostimulators, abandoned catheter leads, etc. A wire in a patient tends to be similar to in length to a wavelength of RF radiation, and can therefore absorb a lot of energy, potentially burning the patient or causing the associated device to malfunction.
The red curve shows the S-Parameter spectrum of an MRI coil loaded with a (simulated) human head. If we add a conductor at a near-resonant length, we see a splitting of the spectrum (blue), which is characteristic of coupled resonators. We propose to use this as a technique to screen patients with potentially dangerous implants.

ISMRM 2013 Conference Abstract (p4411, PDF): A Low-Power, Offline Prescreen to Detect and Suppress Dangerous Currents
ISMRM 2013 Conference E-Poster (p4411, Power Point, ~ 27MB): A Low-Power, Offline Prescreen to Detect and Suppress Dangerous Currents

Reverse-Polarization Imaging

This is a natural continuation of the RF safety prescreen. Our group has invented a very cool technique for imaging dangerous implanted wires.

ISMRM 2013 Conference Abstract (p2840, PDF): Contrast in Visualized Currents Using Reverse Polarization and Pre-Spoiling Twister Gradients
ISMRM 2013 Conference Poster (p2840, PDF): Contrast in Visualized Currents Using Reverse Polarization and Pre-Spoiling Twister Gradients

Wireless Devices

In kind of a Skunkworks moment, I decided to build some wireless sensors for motion detection in the MRI scanner. My idea was to use an array of accelerometers to measure the tilt at various points along the patient's chest. Using the tilt instead of double-integration has some SNR advantages, and may be useful for types of non-rigid motion. The first version made use of an Arduino and XBee radio, and is described in the abstract below. The abstract, by the way, was rejected for (I assume) being too innovative.

A newer version of the device is pictured in the margin on the right, and uses no wires at all, except for a short run to connect to the battery (which might also disappear in the next iteration). The newer version also includes an RF power measurement IC which I use to do wireless current measurments in wires. We've also used a pair of these to study propagation of 2.4GHz in the magnet bore. Here is an abstract describing the first version:

A Wireless Accelerometer Array for Respiratory Motion Tracking

Life as a Physicist

For a summary of the work I did during my PhD, check out my Thesis Page

Research on ultracold matter enjoys a natural advantage in Toronto
This page contains some of the work produced during my master's and PhD, including a brief stint as a biologist. My MSc project was completed in the so-called "photon lab" of Aephraim Steinberg, and I moved to the "atom lab" for my PhD where we work with a Bose-Einstein Condensate.

For incrementally more detailed explanations of my PhD research, have a look at my thesis page:

Bose-Einstein Condensate Wavefunction Reconstruction Through Collisions with Optical Potentials

Getting 100,000 or so Rubidium atoms down to absolute zero is an intensely technical task, so look through for some fun details.

Refereed Publications

Oral and poster presentations


Check out some pictures of our condensate condensing. These three, sequential pictures show our rubidium cloud as it is cooled across the Bose-Einstein transition temperature. The rightmost image is around 200 nano-Kelvin above absolute zero, and about half the atoms are in the same quantum state (the condensate).

Very few people have been lucky enough to witness this very special state of matter - and I got to make it on a (nearly) daily basis!

Master's Project

I've recently unearthed a copy of my master's thesis. It's not "archival quality," which I think excuses how obviously it was prepared during a couple of all-nighters (during which I was living at the lab anyway!). It was a fun time! I've redone the typesetting since I used an odd piece of software to do that the first time.

Two Photon Process Tomography and the Superoperator

Academic Geneaology

I found a similar page prepared by my supervisor, Aephraim Steinberg, and have reproduced it here with my name grafted to the bottom:


Albert Michelson was largely self-taught, although he worked with both Simon Newcomb and Hermann von Helmholtz.

Henry Gale was a student of Albert Michelson at the University of Chicago.

William Smythe was a student of Henry Gale at the University of Chicago.

Charles Townes was a student of William Smythe at the California Institute of Technology.

Ray Chiao was a student of Charles Townes at the Massachusetts Institute of Technology.

Aephraim Steinberg was a student of Ray Chiao at the University of California, Berkeley.

Christopher Ellenor was a student of Aephraim Steinberg at the University of Toronto, Canada.

Aephraim adds the following comment:

Although it is always dangerous to extrapolate, one notes that the Nobel Prize appears to skip two generations in this family tree. By this reckoning, I expect one of my students to eventually earn a trip to Stockholm. I therefore encourage all future potential Nobel Prize winners to immediately start thinking about applying to do graduate work at the University of Toronto!