Effect of duration of scan acquisition on CT perfusion parameter values in primary and metastatic tumors in the lung

Objectives: To assess the effect of acquisition duration (T_acq) and pre-enhancement set points (T₁) on computer tomography perfusion (CT_p) parameter values in primary and metastatic tumors in the lung. Materials and methods: 24 lung CT_pdatasets (10 primary; 14 metastatic), acquired using a two phase protocol spanning 125 s, in 12 patients with lung tumors, were analyzed by deconvolution modeling to yield tumor blood flow (BF), blood volume (BV), mean transit time (MTT), and permeability (PS) values. CT_p analyses were undertaken for the reference dataset (i.e., T₁ = t₀) with varying T_acq from 12 to 125 s. This was repeated for shifts in T₁ (±0.5 s, ±1.0 s, ±2.0 s relative to the reference at t₀). Resultant CT _p values were plotted against T_acq; values at 30 s, 50 s, 65 s and 125 s were compared using linear mixed model. Results: All CT _p parameter values were noticeably influenced by Tacq, with generally less marked changes beyond 50 s, and with no difference in behavior between primary and secondary tumors. Apart from BV, which attained a plateau at approximately 50 s, the other three CT_p parameters did not reach steadystate values within the available 125 s of data, with values at 30 s, 50 s and 65 s significantly different from 125 s (p < 0.004). Shifts in T ₁ also affected the CT_p parameters values, with positive shifts having greater impact on CT_p values than negative shifts. Conclusion: CT_p parameter values derived from deconvolution modeling can be markedly affected by T_acq, and pre-enhancement set-points. 50 s acquisition may be adequate for BV, but longer than 125 s is probably required for reliable characterization of the other three CT_p parameters.