Abstract for presentation at 36th Annual Scientific Meeting of the Australian and New Zealand Society of Nuclear Medicine

Converting CT Numbers to Linear Attenuation Coefficients for Various Single Photon-Emitting Radionuclides

  • A/Prof Dale Bailey, Royal North Shore Hospital, Australia
  • Saxby Brown, University of Sydney, Australia
  • Dr Clive Baldock, The University of Sydney, Australia
  • Aims:
    To define the relationship between the linear attenuation coefficient (μ) for different radionuclide photon energies and CT numbers (Hounsfield Units (HU)) for biologically relevant materials, such as soft tissue or bone, to convert the CT images for SPECT attenuation correction (AC) maps. This work examines the relationship between HU and μ for the single photon emitting radionuclides 99mTc, 67Ga, 123I, 131I, 201Tl and 111In.
    Two lead containers were modified to produce narrow-beam and broad-beam photon beams from the radionuclide placed inside. Each material (water, oil, sawdust, wood, perspex, bone) was positioned between the modified container and the gamma-camera. The linear attenuation coefficients were obtained by increasing the thickness of the material and observing the attenuation curve. The narrow-beam linear attenuation coefficients were compared to the documented photon cross-sections ain the XCOM library, provided by the National Institute of Standards and Technology (NIST). A series of CT scans were then acquired (120kVp, 200mAs) to estimate a mean HU value for each material.
    The narrow-beam attenuation coefficients were plotted against the HU value for each material and at each photon energy. A bilinear trend was fitted with a change in slope at 0HU. For each radionuclide, the equations of the two lines are given in the general form μ = y0+a.HU with y0 and μ shown in the table below.
    The coefficients in the table provide a conversion from HU to linear μ values, to produce μ maps from CT scans. There is an increasing change in the slope of the two lines for any particular radionuclide at HU=0 as photon energy increases.

    Energy (keV y0 (cm-1) a(x10-3);HU<0 a(x10-3);HU>=0
    75 0.16197 0.14823 0.16623
    140 0.14873 0.15175 0.11406
    159 0.13566 0.13788 0.12558
    167 0.13372 0.13587 0.12538
    185 0.12962 0.13167 0.11677
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