The principles and evolution of magnetic resonance imaging

Magnetic Resonance Imaging (MRI) offers the best textbook example for the exploitation of both Heike Kamerlingh Onnes's major discoveries, namely the liquefaction of liquid helium and superconductivity. This paper will briefly describe how MRI images are acquired, give a historical account about the early days, and then focus on the magnet. It provides a window into the world of physics and engineering of magnet technology and it will chart the magnet design evolution since inception. For example, over the past 20 years, magnet designers have taken 1.5T MRI magnets from a weight of 13tonnes and a length of 2.40m in 1989 to a weight of 3.2tonnes and a length of 1.37m in 2009. The 3T products have also undergone similar developments. Coils experience mechanical forces of order 380tonnes. The cryogenics for MRI magnets have also undergone major improvements. Today's magnets do not use liquid nitrogen and most importantly have zero helium loss, thus needing no refill. Combined with the weight, this makes the body scanner accessible to developing countries and easy to site in any room. Although the future may eliminate the need for liquid helium completely, MRI will always continue to depend on superconductivity.