The purpose of this study is to determine if detailed scanning of the heart before biventricular device insertion will improve outcomes for heart failure patients.

Inclusion Criteria:

1. Age > 18, <90
2. Patients with moderate or severe heart failure ( NYHA functional class II-IV) due to ischaemic or non-ischaemic cardiomyopathy.
3. QRS duration >120ms on 12 lead ECG
4. LVEF < 35%
5. LVEDD >55 mm
6. Patients on optimum medical treatment for heart failure. The doses of these medications have been stable for 3 months.

Exclusion Criteria:

1. Patients with a cardiac or cerebral ischemic event within the previous three months prior to recruitment
2. Patients who had had an atrial arrhythmia within one month prior to recruitment
3. Patients with a systolic blood pressure of more than 170 or less than 80 mm Hg
4. Patients with a heart rate of more than 140 beats per minute
5. Patients with severe renal failure (eGFR < 30)
6. Patients with a history of allergy to iodine based contrast agents
7. Predicted life expectancy < 1 year
8. Pregnancy