Dedicated breast CT: radiation dose for circle-plus-line trajectory

Abstract

Purpose: Dedicated breast CT prototypes used in clinical investigations utilize single circular source trajectory and cone-beam geometry with flat-panel detectors that do not satisfy datasufficiency conditions and could lead to cone beam artifacts. Hence, this work investigated the glandular dose characteristics of a circle-plus-line trajectory that fulfills data-sufficiency conditions for image reconstruction in dedicated breast CT. Methods: Monte Carlo-based computer simulations were performed using the GEANT4 toolkit and was validated with previously reported normalized glandular dose coefficients for one prototype breast CT system. Upon validation, Monte Carlo simulations were performed to determine the normalized glandular dose coefficients as a function of x-ray source position along the line scan. The source-to-axis of rotation distance and the source-to-detector distance were maintained constant at 65 and 100 cm, respectively, in all simulations. The ratio of the normalized glandular dose coefficient at each source position along the line scan to that for the circular scan, defined as relative normalized glandular dose coefficient RD g N À Á , was studied by varying the diameter of the breast at the chest wall, chest-wall to nipple distance, skin thickness, x-ray beam energy, and glandular fraction of the breast. Results: The RD g N metric when stated as a function of source position along the line scan, relative to the maximum length of line scan needed for data sufficiency, was found to be minimally dependent on breast diameter, chest-wall to nipple distance, skin thickness, glandular fraction, and x-ray photon energy. This observation facilitates easy estimation of the average glandular dose of the line scan. Polynomial fit equations for computing the RD g N and hence the average glandular dose are provided. Conclusions: For a breast CT system that acquires 300-500 projections over 2p for the circular scan, the addition of a line trajectory with equal source spacing and constant x-ray beam quality (kVp and HVL) and mAs matched to the circular scan, will result in less than 0.18% increase in average glandular dose to the breast per projection along the line scan. V C 2012 American Association of Physicists in Medicine. [http://dx

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“…Sechopoulos et al (2010) simulated a DBCT commercial system (Koning Corp., West Henrietta, NY) using Monte Carlo simulations and a hemi-ellipsoidal representation of the breast of varying sizes and compositions and provided the DgN CT coefficients for the x-ray spectrum used by this specific DBCT system. For possible DBCT acquisitions that involve a circle-plus-line trajectory, Vedantham et al (2012c) introduced the relative normalized glandular dose coefficient (RDgN(y)) which provides the MGD resulting from the projection acquired when the x-ray source is at position y below the line of the circular scan.…”

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“…The solid line shows the linear fit with zero-intercept. Current estimates [35][36][37][38] for normalized glandular dose coefficients (D g N CT ) assume a linear relationship between CNL and D eff . The data indicate that the choice of CNL = 0.7014D eff is suitable, when a linear fit with zero-intercept is assumed.…”

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“…For the computation of MGD to the breast, Monte Carlo simulations were performed by modifying a previously described setup (Vedantham et al, 2012a). Specifically, the imaging geometry was modified to reflect a clinical DM/DBT system with a source to image-receptor distance (SID) of 70 cm as shown in Fig.…”