The magnetic resonance egg timer

BMJ 2007; 335 doi: (Published 19 July 2007) Cite this as: BMJ 2007;335:151
  1. Brian Witcombe, consultant in radiology, Gloucester Royal NHS Trust
  1. brian.witcombe{at}

    Unless something is happening, an x ray department can seem as uninteresting as an empty garage or aircraft hangar, but it is unethical to let visitors watch patients being examined. Fortunately, when the distinguished members of the Cirencester Science and Technology Society visited us to learn about radiological scanning, one of our secretaries agreed to act as a model, so demonstrating ultrasonography was not a problem. The visitors could see her heart and aorta pulsating, learn how a Doppler signal can be used to assess vascular flow, and witness how abdominal anatomy can be obscured by calcium in the ribs or air in the bowel.

    A selection of foods and other items hidden in a cardboard box proved a popular way to demonstrate computed tomography. The visitors enjoyed being quizzed about the contents of the box, and having to distinguish cherries from grapes, a banana from a courgette, and a bruised apple from a sound one. They were also asked to distinguish a length of skirting board from a piece of “tongue-and-groove” plank, and the grain of the timber was shown exquisitely. Scans were completed in seconds, sections in all three orthogonal planes were quickly constructed, and post-processing techniques such as surface rendering were demonstrated.

    We showed off magnetic resonance imaging (MRI) using a different phantom, a chicken carcase filled with eggs. The MR images not only revealed the detail of the chicken's anatomy but also distinguished each egg's embryo, yolk, and albumen. The differences between a fresh egg, a bad egg, and a chocolate cream egg were discernable, and one egg was made invisible by wrapping it in aluminium foil. The foil acts like a Faraday cage: it is a barrier to the passage of the radio waves which cause the resonance of protons on which MR signal is dependent.

    An unforeseen effect, however, was how cooking an egg changes its MR signal. The albumen of a fresh egg seems white on T2 images, but the signal is lost on cooking so the white turns to black. The change from white to black extends in from the outside as the egg cooks. When an egg is ready to eat and the albumen has solidified, the white of an egg is completely black on T2 images. This occurs, as may be guessed from breakfast experiences, after boiling for a little over three minutes.

    Our finding suggested commercial potential, but there are practical constraints. The actual process of scanning takes a minute or two to complete, and the egg has to be cooked at a distance from the scanner to prevent the egg pan and heat source becoming stuck to the magnet. Moving a boiling egg in and out of the scanner is hardly practicable and, even if these matters could be solved, an MR egg timer is unlikely to become a cost effective alternative to the standard sandglass.

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