The influences of tube voltage and scan direction on combined tube current modulation: a phantom study

For the optimum application of tube current modulation in paediatric CT it is important to know the influences of different imaging parameters on it.

This phantom study was performed to evaluate the influences of tube voltage and scan direction on combined tube current modulation, which is a combination of angular and z-axis modulation.

Sixteen-slice spiral CT with combined tube current modulation was performed using four different phantoms (one adult-size anthropomorphic phantom and three cone-shaped acryl phantoms) at three different tube voltages (80, 100 and 120 kVp) and two scan directions (craniocaudal and caudocranial). Effective tube current-time product (mAs) was chosen to maintain a consistent CTDIvol for each phantom. Other parameters, including detector collimation, slice width, pitch, reconstruction algorithm, scan field of view, and scan range were identical for each phantom. Changes in CTDIvol and mAs resulting from the modulation were calculated and compared.

Changes in CTDIvol and mAs resulting from the modulation were different among the three tube voltages. In larger phantoms the greatest reductions (-6.1+/-3.9% in CTDIvol and -12.5+/-4.0% in mAs) were obtained with 120 kVp, whereas in smaller phantoms they were obtained with 80 kVp (-2.8+/-0.9% in CTDIvol and -2.5+/-4.9% in mAs). Smaller CTDIvol (2.4+/-0.9 mGy vs. 2.5+/-1.0 mGy, P=0.017) and mAs (57.2+/-40.4 mAs vs. 61.4+/-43.2 mAs, P=0.002) were used in caudocranial scans than in craniocaudal scans with the modulation.

Combined tube current modulation is influenced by tube voltage and scan direction depending on phantom profile.