The respiratory motion-induced errors in tumor quantification and delineation are highly dependent upon the motion amplitude, tumor location, tumor size, and choice of the attenuation map for PET image attenuation correction.
Purpose: Dual-energy CT (DECT) is arguably the most accurate energy mapping technique in CTbased attenuation correction (CTAC) implemented on hybrid PET/CT systems. However, this approach is not attractive for clinical use owing to increased patient dose. The authors propose a novel energy mapping approach referred to as virtual DECT (VDECT) taking advantage of the DECT formulation but using CT data acquired at a single energy (kV P ). For this purpose, the CT image acquired at one energy is used to generate the CT image at a second energy using calculated kV P conversion curves derived from phantom studies. Methods: The attenuation map (l-map) at 511 keV was generated for the XCAT phantom and clinical studies using the bilinear, DECT, and VDECT techniques. The generated l-maps at 511 keV are compared to the reference derived from the XCAT phantom serving as ground truth. PET data generated from a predefined activity map for the XCAT phantom were then corrected for attenuation using l-maps generated using the different energy mapping approaches. In addition, the generated l-maps using the above described methods for a cylindrical polyethylene phantom containing different concentrations of K 2 HPO 4 in water were compared to actual attenuation coefficients. Likewise, CT images of five clinical whole-body studies were used to generate l-maps using the various energy-mapping approaches were compared with l-maps acquired at 511 keV using 68 Ge/ 68 Ga rod sources for the clinical studies. Results: The results of phantom studies demonstrate that the proposed method is more accurate than the bilinear technique. All three l-maps yielded almost similar results for soft and lung tissues whereas for bone tissues, the DECT and the VDECT methods produced a much smaller mean relative difference (3.0% and 2.8%, respectively) than the bilinear approach (11.8%). Likewise, the comparison of PET images corrected for attenuation using the various methods showed that the proposed method provides better accuracy (6.5%) than the bilinear method (13.4%). Clinical studies further demonstrated that, compared to the bilinear method, the VDECT approach has better agreement for bony structures with the DECT technique (1.5% versus 8.9%) and transmission scanning (8.8% versus 17.7%).
Build-up factor (BF) is one of the radiation interaction parameters, which is important in radiology and power sector to mitigate the radiation risks as low as possible. In this paper, the BF values for water and soft tissue have been computed for gamma rays within the energy range 0.2–2 MeV for penetration depths up to 10 mean free path (mfp), using MCNP simulation code by introducing a co-centric multilayer model. The results were compared with previously published data and standards. The employed model improved prediction of the BFs at 7 mfp and 10 mfp for 0.2 and 2 MeV compared to slab geometry model of MCNP. Besides, for the most part, the BFs calculated by simulation had a good agreement (<10%) with those of the ANSI/ANS-6.4.3 standard. There was a maximum difference at 10 mfp and 0.5 MeV and the minimum difference was in exact agreement. The uncertainties for the simulation were always below 5%, which is a partial explanation of the differences between the BFs given by the ANS standard and simulation. There were some improvements in prediction of BF values with reference to the mentioned standard, mostly for lower energies. The maximum improvement was 2%. It was also observed that the soft tissue (4 component) and water behave as the equivalent materials compared to soft tissue (ICRU-44) and A-150 plastic. It is concluded that the proposed model has such a potency for further extension of investigation for higher photon energies and penetration depths, to be used for calculation of the BFs.
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