Soft-tissue detectability in cone-beam CT: Evaluation by 2AFC tests in relation to physical performance metrics

D. J. Tward, J. H. Siewerdsen, M. J. Daly, S. Richard, D. J. Moseley, D. A. Jaffray, N. S. Paul

Research output: Contribution to journalArticlepeer-review

25 Scopus citations


Soft-tissue detectability in cone-beam computed tomography (CBCT) was evaluated via two-alternative forced-choice (2AFC) tests. Investigations included the dependence of detectability on radiation dose, the influence of the asymmetric three-dimensional (3D) noise-power spectrum (NPS) in axial and sagittal or coronal planes, and the effect of prior knowledge on detectability. Custom-built phantoms (∼15 cm diameter cylinders) containing soft-tissue-simulating spheres of variable contrast and diameter were imaged on an experimental CBCT bench. The proportion of correct responses (Pcorr) in 2AFC tests was analyzed as a figure of merit, ideally equal to the area under the receiver operating characteristic curve. Pcorr was evaluated as a function of the sphere diameter (1.6-12.7 mm), contrast (20-165 HU), dose (1-7 mGy), plane of visualization (axial/sagittal), apodization filter (Hanning and Ram-Lak), and prior knowledge provided to the observer [ranging from stimulus known exactly (SKE) to stimulus unknown (SUK)]. Detectability limits were characterized in terms of the dose required to achieve a given level of Pcorr (e.g., 70%). For example, a 20 HU stimulus of diameter down to ∼6 mm was detected with Pcorr 70% at dose ≥2 mGy. Detectability tended to be greater in axial than in sagittal planes, an effect amplified by sharper apodization filters in a manner consistent with 3D NPS asymmetry. Prior knowledge had a marked influence on detectability-e.g., Pcorr for a ∼6 mm(20 HU) sphere was ∼55%-65% under SUK conditions, compared to ∼70%-85% for SKE conditions. Human observer tests suggest practical implications for implementation of CBCT: (i) Detectability limits help to define minimum-dose imaging techniques for specific imaging tasks; (ii) detectability of a given structure can vary between axial and sagittal/coronal planes, owing to the spatial-frequency content of the 3D NPS in relation to the imaging task; and (iii) performance under SKE conditions (e.g., image guidance tasks in which lesion characteristics are known) is maintained at a lower dose than in SUK conditions (e.g., diagnostic tasks in which lesion characteristics are unknown).

Original languageEnglish (US)
Pages (from-to)4459-4471
Number of pages13
JournalMedical physics
Issue number11
StatePublished - 2007
Externally publishedYes


  • 2AFC
  • Alternative forced-choice test
  • Cone-beam CT
  • Detectability
  • Flat-panel detector
  • Image-guided interventions
  • Image-guided radiation therapy
  • Imaging performance
  • Noise-power spectrum

ASJC Scopus subject areas

  • Biophysics
  • Radiology Nuclear Medicine and imaging


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