Original article measures appear robust when subjected to ﬂow phantom validation. While we do not envisage CMR becoming a wide- spread tool in clinical practice, we believe that it could prove to be a useful research technique to study small numbers of newborn infants in specialist centres; providing insights into the pathophysiology of circulatory failure; acting as an out- come measure in clinical trials of inotropic intervention and so guiding clinical practice in the wider neonatal community. Table 3 Left ventricular indices taken from stacks of cine images in 75 stable newborn infants Mean SD LV EDV (ml/kg) 2.51 0.40 LV ESV (ml/kg) 0.78 0.21 LV SV (ml/kg) 1.73 0.28 LV EF (%) 69.0 5.7 HR (bpm) 142 17.1 LVO (ml/kg/min) 245 47.1 EDV, end-diastolic volume; EF, ejection fraction; ESV, end-systolic volume; HR, heart rate; LV, left ventricle; LVO, left ventricular output; SV, stroke volume. Table 4 Repeatability of left ventricular indices in 22 newborn infants EDV ESV SV EF LVO Limits of agreement Repeatability index 1.07 ml 0.48 ml 0.88 ml 7.4% 19.4% 27.7% 23.1% 10.8% 58.3 (ml/kg/min) 22.1% EDV, end-diastolic volume; EF, ejection fraction; ESV, end-systolic volume; LVO, left ventricular output; SV, stroke volume. Current understanding of the neonatal circulation is heav- ily based on studies of blood pressure, which has a weak correlation with tissue perfusion.24 Intervention to increase blood pressure may not increase tissue perfusion25 or improve outcome.26 While echocardiography has value in providing functional circulatory assessments27 its accuracy is limited with studies suggesting that echo cannot reli- ably detect changes in neonatal cardiac output,9 or systemic perfusion10 11 of below 30–40%, consistent with our ﬁnd- ings of a RI of 49%. Repeatability of echo measures may be improved if only changes in stroke distance are used.28 However, this approach does not assist with provision of absolute ﬂow data; nor can it be used in vessels where diam- eter is variable such as the SVC. Novel circulatory biomark- ers could improve understanding of the pathophysiology of neonatal circulatory failure and allow assessment of efﬁcacy of inotropic intervention. PC CMR is known to reliably quantify ﬂow in adults.18 We report optimised PC parameters for newborn infant imaging at 3 T. A voxel size of <1 mm and scan duration of <60 s allow quantiﬁcation of ﬂow at multiple sites. A RI of 19–22% was consistently seen in preterm and term newborns. While this variability is higher than seen in adults, it is signiﬁcantly lower than prior echocardiographic studies in newborns9–11 and will be partly due to the increase in spontaneous ﬂuctuation of cir- culatory parameters seen in the newborn.11 Figure 5 Normal ranges by corrected gestational age at scan for bFFE assessment of left ventricular output (A), end-diastolic volume (B), end- systolic volume (C) and ejection fraction (D). F90 Arch Dis Child Fetal Neonatal Ed 2011;96:F86–F91. doi:10.1136/adc.2010.189142