Fusion imaging in brain structure measurements on a fetus phantom, combining real-time ultrasound with magnetic resonance imaging

Objectives: To assess synchronisation of MRI and US in measuring foetus phantom head structures; inter-method, intra- and inter-observer differences on biparietal diameter (BPD), head diameter, anterio-posterior head diameter (HAP) and lateral ventricle structures (VS). Methods: Fusion Imaging (FI) has been performed by combining MRI and US simultaneously. Axial scans of 1.5 Tesla MRI on a foetus phantom were acquired and uploaded on a US machine (EPIQ 7G, Philips). A PercuNav US tracker allowed the system to recognise and display the position of the transducer. A fetal phantom tracker was used as a phantom reference. Real-time US of the phantom head was performed by synchronising the uploaded MRI images using different landmarks. Synchronisation has been assessed by taking measurements after rotating the US probe by 90. Measurements were taken by three different observers twice. Differences in measurements between MRI and US, inter-, intra-observer differences in all measurements were assessed. Results: BPD, HAP and VS measurements before rotation were 0.13 ± 0.06 cm, 0.46 ± 0.09 cm and 0.4 ± 0.23 cm (width) and mean 0.6 ± 0.25 cm (length) larger at MRI than at US using any number of landmarks. After US probe rotation VS were 0.3 ± 0.24 cm in width and 0.3 ± 0.27 cm in length. Intra- and inter-observer differences in all measurements were small. Conclusions: FI showed good synchronisation in measurements. BPD, HAP and VS were larger at MRI than US, likely a result of the way images are generated. Intra-, inter-observer differences between measurements were small. This can be important when reporting geometric measures from FI.