1. Cardiac function assessment: The conventional method for assessing heart function (in terms of 'ejection fraction') in patients being treated for cancer uses nuclear medicine (MUGA scan). MRI can also provide this information but it is not typically used for cardiac function assessment in these patients. MUGA scans are typically performed before chemotherapy in selected patients at higher risk of cardiotoxicity and in all cases before starting anti-HER2 antibody treatment (trastuzumab, pertuzumab). Patients receiving anti-HER2 antibody treatment receive repeat cardiac assessments at 3-4 month intervals until the completion of treatment - it is not practical or cost-effective to scan more frequently. This study will determine whether an alternative and inexpensive method for monitoring heart function (Impedance Cardiography, ICG) can adequately quantify changes in ejection fraction during treatment. If it can, then this technique could be used more frequently during and following treatment, with the aim of determining the treatment stages during which the heart is most at risk of damage. The ability of ICG to dynamically characterise a relatively rapid decline in cardiac function in breast cancer patients has not been established. Neither has its performance been compared with MRI or MUGA in this population. This study will investigate whether ICG is a sufficiently accurate and reliable alternative to MRI/MUGA for determining cardiac dysfunction during and following cancer treatment.