Neuroblastoma is the most common tumour of childhood after brain tumours. Approximately half of cases are high risk and despite extensive treatments outcome is very poor. More than 60% of high risk patients suffer relapse or further spread of their disease and long-term survival is below 10%. Existing imaging techniques are not sensitive enough to accurately assess the level of risk which is critical in determining the best choice of treatment. This study will compare a new type of imaging against the existing imaging techniques. The new scans use a new tracer called [124I]mIBG which is taken up by the cancer tissue much more than by normal tissues. This tracer can be used with a 3D imaging technique called PET/CT to pinpoint where the disease has spread and quantify the amount of disease. Patients will be those scheduled to have an [123I]mIBG scan for routine care during a planned break in treatment.