Fusion imaging (also called image fusion or navigation ultrasound) overlays or synchronizes a live ultrasound image with a pre-acquired cross-sectional dataset — typically CT or MRI — using electromagnetic or optical tracking systems. As the sonographer moves the transducer, the corresponding plane in the CT or MRI dataset is displayed simultaneously. This allows precise targeting of lesions that are visible on CT or MRI but poorly seen or invisible on ultrasound alone. Fusion imaging is used in liver biopsy guidance, tumor ablation planning, and follow-up of treated lesions.