Effect of radiation exposure on quantitative evaluation of cerebral CT perfusion maps: results from a hybrid digital phantom

Purpose: To use a hybrid digital phantom to study the effect of mAs on quantitative evaluation of cerebral CT perfusion scans. Material and Methods: The hybrid digital phantom consists of scans of a homogeneous skull phantom on which circular objects (10-15-20mm) are digitally superimposed. Tissue perfusion curves derived from patient data were superimposed on the background (white matter) and on the objects (grey matter). We tested a CTP sequence using 30 scans every 2s over 60s (80kV, 5mm thickness) derived from a 320-row CT scanner. The mAs values per CTP sequence were varied per scan (10-230mAs). A synthetic dataset without noise was the gold standard. CBF maps for each mAs setting and for the synthetic dataset were calculated using the ASIST CT program. Noise in CBF maps was measured in the background; contrast between background and objects was measured in circular ROIs with a diameter of half the object diameter. Size- and dose-dependence of absolute values and CNR in CBF maps was calculated. Results: Above 55 mAs per scan, the CBV values for the 20mm objects were within 12% of the gold standard. At 10 mAs CBF was overestimated by >100%. With smaller objects the estimation of CBF became less precise. CNR for CBF in 20mm decreased rapidly: CNR at 230 mAs was 4.2 and decreased to 2.4 at 100 mAs and 0.4 at 10 mAs. Conclusion: Absolute CBF values require sufficient dose to be correct: values at low dose are overestimated. In addition, CNR rapidly decreases with lower dose.