A mammography detector pixel is square

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

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

Problem

A mammography detector pixel is

0.10\,\mathrm{mm}

square, so its area is

A = 1.0\times10^{-8}\,\mathrm{m^2}

. The beam is monoenergetic at

20\,\mathrm{keV}

and passes through

4.0\,\mathrm{cm}

of breast tissue with linear attenuation coefficient

\mu = 0.80\,\mathrm{cm^{-1}}

before reaching the detector. The mass energy absorption coefficient of breast tissue at

20\,\mathrm{keV}

\mu_{\rm en}/\rho = 0.0417\,\mathrm{cm^2\,g^{-1}}

. The detector is ideal: it counts every photon that reaches it, and no photon is scattered into or out of the pixel. Take the Rose criterion

k_{\rm R} = 5

. ((a)) State, citing Chapter 2's rule that the variances of independent, rare events add, why this pixel's photon count

N

satisfies

\mathrm{Var}(N) \approx \bar N

; you do not need to repeat the derivation. ((b)) A lesion casts a subject contrast of

C_{\rm s} = 3.0\,\%

in this pixel. Using the Rose criterion, find the number of photons

N

that must be collected at the detector over this pixel to see this contrast, and hence the entrance fluence and entrance dose (in mGy) needed just outside the breast, using the attenuation and fluence-to-dose tools of Chapter 6.

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