Congenital diaphragmatic hernia (CDH) is a defect of the diaphragm in utero which allows the abdominal organs to herniate into the thoracic cavity. Compression of the developing lungs leads to pulmonary hypoplasia and altered pulmonary vascular development. Pulmonary arterial hypertension (PAH) is very common in CDH and is a major contributor to morbidity and mortality in these infants. Echocardiography and cardiac catheterization are the current standard for evaluating pulmonary hemodynamics in CDH infants, but these modalities have significant limitations and/or risks. Phase contrast (PC) MRI can provide quantitative information about velocity and flow; however, application of MRI in the Neonatal Intensive Care Unit (NICU) patient population is limited to date, primarily due to the medical risks and logistical challenges of transporting the infants out of the unit for the exam. These challenges have been mitigated at our institution by the installation of a unique, neonatal-sized 1.5T MRI scanner within our NICU ^{1, 2}. Furthermore, using the “feed and swaddle” approach, the NICU MR exams can be performed without sedation². The objective of the present study was to evaluate the feasibility of PC MRI within the NICU in free-breathing, non-sedated infants with CDH, with the long-term goal to establish imaging biomarkers to predict PAH and assess response to therapies.IRB approval and written informed consent was obtained from the parents prior to imaging on a unique small footprint 1.5T MR system installed within the NICU at our institution1,2. 8 (5M/3F) infants with left-sided CDH, ranging from mild to moderate in severity, 3wks±2wks post-surgical repair were imaged free breathing without sedation (via feed and swaddle) or intravenous contrast. One CDH patient was also imaged pre-surgical repair, in addition to one control infant. The infants were imaged at an average age of 40±4wks adjusted gestational age with weight and heart rate 3.1±0.5kg and 158.8±12.5bpm respectively. 2D black blood imaging was used to localize the aorta, main and branch pulmonary arteries. Retrospective ECG gated 2D PC images were acquired in planes perpendicular to each of these vessels (TR=12.0-12.8ms; TE=4.5-5.1msec; FA=20°; in plane resolution 0.9375x1.875mm2; slice thickness=5mm, velocity encoding (VENC)=150-200cm/sec; 2 views per segment; receiver BW = 122Hz/pixel). Standard commercially available software (QFlow; Medis Medical Imaging Systems; Leiden, the Netherlands) was used to analyze the PC data to obtain quantitative flow measurements for the main (MPA), right (RPA) and left (LPA) pulmonary arteries. 3D Fast Gradient Recalled Echo (FGRE) (TR/TE/FA=6.3ms/1.8ms/4°; FOV=18-20cm; matrix=256x256; 28-32 3mm partitions) and 2D Time-of-Flight (TOF) (TR/TE/FA= 6.6-10.5ms/3.5-4.6ms/45-60°; FOV=16-17cm; matrix=192x320; 1.5 - 3mm thick slices; acquired sequentially inferior to superior; 2 averages) data sets were also obtained to provide an overview of the pulmonary anatomy and vasculature respectively. To quantify the impact of CDH on the vascular development in the affected lung, the number of vessels identified at 50% of the maximum diameter of the left lung was determined from axial maximum intensity projections (MIP’s) of the 2D TOF data. PC, 3D FGRE, and 2D TOF data were obtained during free breathing and without sedation in all 10 infants (Table 1). Hypoplasia and reduced vascularization (Table 1) of the left lung were well visualized in the 3D FGRE and 2D TOF MIP images of the CDH infants (Figure 1). The values for total pulmonary flow were 361.6 mL/kg/min for the control infant, 207.3mL/kg/min for the pre-surgical repair CDH infant, and 344.7±110.6 mL/kg/min [range 208.5-526.3 mL/kg/min] for the post-surgical repair CDH infants --well within the expected range ³. As expected, the flow from the MPA was fairly evenly distributed between the RPA and LPA (%LPA/MPA = 42.5%; %RPA/MPA =54.8%) in the control infant; however the relative proportion of MPA flow directed to the LPA versus the RPA was reduced in the CDH infants. In the pre-surgical repair CDH infant, the % LPA/MPA flow ratio was 12.7%. Similarly, in the post-surgical repair CDH infants, the %LPA/MPA flow ratio was 31.6±15.5%. This is consistent with what has been reported for older pediatric patients with a history of left sided CDH repair⁴.PC MRI is a viable method to quantitatively assess pulmonary blood flow in CDH infants within the NICU during free-breathing and without sedation, with the potential to safely and longitudinally provide information about PAH and response to therapies.