# Michael Joy (1940–2020)

This week I belatedly learned that Mike Joy died. This was sad news indeed. Joy was a Canadian electrical engineer who measured current density in the body using magnetic resonance imaging. In Chapter 8 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I include a homework problem in which the magnetic field throughout the body is known and the student’s task is to determine the current density.

Section 8.6

Problem 24. The differential form of Ampere’s law, Eq. 8.24, provides a relationship between the current density j and the magnetic field B that allows you to measure biological current with magnetic resonance imaging (see, for example, Scott et al. (1991)). Suppose you use MRI and find the distribution of magnetic field to be

B_x = C(yz^2 − yx^2)

B_y = C(xz^2 − xy^2)

B_z = C4xyz

where C is a constant with the units of T m^−3. Determine the current density. Assume the current varies slowly enough that the displacement current can be neglected.

To solve this homework problem, calculate the curl of the magnetic field to get, within a proportionality constant, the current density. By the way, the problem doesn’t ask you to do this, but you might want to verify that the divergence of B is zero as it must be according to Maxwell’s equations, and that the divergence of j is zero (conservation of current).

The article we cite in IPMB is a beautiful paper by Greig Scott, Robin Armstrong, Mark Henkelman, and Joy. At that time Scott was Joy’s graduate student at the University of Toronto.

Scott GC, Joy MLG, Armstrong RL, Henkelman RM (1991) Measurement of nonuniform current density by magnetic resonance. IEEE Transactions on Medical Imaging Volume 10, Pages 362–374.

Using MRI to measure current density was one of those ideas I wish I’d thought of, but I didn’t. When Peter Basser and I wrote a paper analyzing an alternative (and less successful) method to detect action currents using MRI, we cited four of Joy’s articles in our very first sentence! I first met Joy when we co-chaired a session at the 2009 IEEE Engineering in Medicine and Biology Society Conference in Minneapolis. I had the honor of being the external examiner for one of Joy’s graduate students, Nahla Elsaid, at her 2016 dissertation defense. Joy was a delightful guy, and a joy to work with. I’ll miss him.

Below is Joy’s obituary.

MICHAEL LAWRENCE GRAHAME JOY (July 31, 1940-July 5, 2020) was born in Toronto and died at Drynoch Farm in Caledon, on his own terms, in his own time. He was predeceased by his wife Jane (née Andras) and will be dearly missed by his wife Carol Fanning, his son Rob, his daughters Gwen and Ellen, their partners, his grandchildren (Asha, Nel, Tallulah, Freya, Kelvin, and Skyler) and generations of nieces, nephews, cousins, former students, friends and colleagues.

Mike was professor emeritus at the University of Toronto; Institute of Biomaterials & Biomedical Engineering; Department of Electrical & Computer Engineering. He was a pioneer in the development of Magnetic Resonance and Electric Current Density Imaging and earned numerous significant grants, awards and citations.

Mike, (Muncle Ike, Zeepa) was truly a unique individual. He was a man of many interests who always had time for the numerous children who would follow him like shadows as he puttered on his latest amazing project. He could turn the most mundane chore into both an adventure and a learning experience. He imparted his love of nature, enquiry and adventure on his young assistants, whether tinkering on his jet boat Feeble, constructing a zip line, building model rockets, fishing, or going on long walks where “getting lost” was all part of the fun.

Mike enjoyed being surrounded by those he loved. His birthday parties at the Bay were the highlight of the summer while the Christmas tree parties at the Farm kicked off the festive season. Whether at summer picnics, Church, dinners, gatherings, bridge games, visiting family at Nares Inlet or summer afternoons on the side porch, he was always at the center of things with his distinctive laugh and quick sense of humour.

Mike left his imprint on so many. His was a life well lived and well loved. In lieu of flowers, please consider a donation to the Georgian Bay Land Trust, one of the many conservation projects Mike supported.

Originally published at http://hobbieroth.blogspot.com.

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Professor of Physics at Oakland University and coauthor of the textbook Intermediate Physics for Medicine and Biology.

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