A brain MRI scan lasts and runs at the whole-body radio-frequency limit of throughout

PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Radiation physics and diagnostic imaging · Nuclear medicine, dosimetry and radiotherapy

Life at Thermal Energy, Ch Echoes, precession and the therapeutic beam

Problem

A brain MRI scan lasts

20\,\mathrm{min}

and runs at the whole-body radio-frequency limit of

2\,\mathrm{W\,kg^{-1}}

throughout. A separate patient receives a radiotherapy course that delivers a total prescribed dose of

60\,\mathrm{Gy}

to the tumour, given as

30

fractions of

2\,\mathrm{Gy}

. Take the specific heat capacity of tissue as

c_p = 3600\,\mathrm{J\,kg^{-1}\,K^{-1}}

. ((a)) Calculate the energy per kilogram the MRI scan leaves in tissue, and the temperature rise this would cause with no blood flow to carry the heat away. ((b)) State the energy per kilogram the radiotherapy course leaves in the tumour, in the same units, using

1\,\mathrm{Gy} = 1\,\mathrm{J\,kg^{-1}}

. ((c)) By what factor does the MRI scan's energy per kilogram exceed the radiotherapy course's? Explain, in terms of the form each energy takes, why the course still kills the tumour while the scan is harmless.

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