A radiologist wants a projection radiograph to reveal a small liver lesion of…

PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Radiation physics and diagnostic imaging · Radiation physics and diagnostic imaging

Life at Thermal Energy, Ch Images from attenuation and from decay

Problem

A radiologist wants a projection radiograph to reveal a small liver lesion of diameter

4.0\,\mathrm{mm}

that has a subject contrast

C_{\rm s} = 0.020

against the surrounding liver tissue, using a beam of

60\,\mathrm{keV}

photons. The beam crosses

18\,\mathrm{cm}

of soft tissue before the lesion's shadow reaches the detector. Take the soft-tissue attenuation coefficient as

\mu = 0.217\,\mathrm{cm^{-1}}

60\,\mathrm{keV}

and its mass energy absorption coefficient as

\mu_{\rm en}/\rho = 3.3\times10^{-3}\,\mathrm{m^2\,kg^{-1}}

. Use the Rose criterion with

k_{\rm R} = 5

, treat the lesion as a uniform disc of the stated diameter seen face-on, and assume an ideal photon-counting detector with no scatter. ((a)) Find the minimum number of photons

N

that must be collected over the lesion's projected area for it to be seen. ((b)) Find the lesion's projected area and hence the photon fluence that must reach the detector under the lesion. ((c)) Find the fluence that must enter the patient's skin, given the transmission through

18\,\mathrm{cm}

of soft tissue. ((d)) Find the least entrance dose this requires.

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