Cardiac magnetic resonance imaging in the assessment of systemic and organ blood flow and the function of the developing heart

Circulatory physiology and assessment

All circulatory function relies on the interplay of preload, contractility, and afterload ( Chapter 1 ). The preterm transitional circulation is further complicated by the persistence of fetal shunt pathways (foramen ovale and ductus arteriosus), which may alter circulatory dynamics. A robust assessment of the newborn circulation therefore needs to quantify preload, contractility, afterload, and systemic and organ perfusion.

Unfortunately, the current methods fall short of this ideal; in particular, routine clinical monitoring of circulatory status in the neonatal unit still relies heavily on arterial blood pressure. ^, However, systemic arterial blood pressure is the product of systemic blood flow and systemic vascular resistance, but it cannot itself distinguish between the two. While clinicians presumably feel that monitoring systemic blood pressure is a screening tool for low systemic perfusion, in fact, blood pressure in itself is at best weakly predictive of volume of blood flow, and some studies have suggested no, or even an inverse, relationship between blood pressure and flow in newborn preterm infants. Other clinical assessments, such as capillary refill time or volume of urine output, also have limited value in indicating circulatory health.

A range of technologies are now entering “prime time” for hemodynamic assessment in the neonatal unit. Point of care cardiac ultrasound/echocardiography ( Chapter 9, Chapter 10, Chapter 11 ) leads the way, has produced important advances in the understanding of circulatory physiology, and has an increasing role in the assessment of circulatory status at the bedside. Established techniques are being improved upon ^, and newer modalities are emerging and undergoing optimization. ^, Bedside cardiac imaging currently provides only snapshots of hemodynamic status, so continuous monitoring techniques such as impedance monitoring ( Chapter 12 ) and near-infrared spectroscopy are also reaching the point of clinical utility. Finally, the evolving systems capable of real-time, comprehensive, and mostly noninvasive, continuous cardiorespiratory and neurocritical care monitoring and data acquisition at the bedside provide an approach with the potential of addressing many of the presently unanswered questions and unresolved controversies ( Chapter 14 ). CMR will again play a key role in helping to improve our understanding of the interplay among important hemodynamic factors during transition and in pathological conditions, as well as assessing the response to various treatment approaches in neonates with hemodynamic compromise.

A key step for all new modalities is robust validation in the population of interest. In the current absence of a gold-standard assessment of cardiac function in the premature infant, CMR has a role as a validation tool – its accuracy and repeatability in newborns are consistently outperforming that of cardiac ultrasound, near-infrared spectroscopy, and impedance monitoring. Outside the early newborn period (where transfer of a sick newborn to an MRI scanner, even one located on the NICU, is challenging), the accuracy of CMR in quantifying volume of blood flow and myocardial mass provides valuable insights into circulatory physiology and cardiac development.

CMR in adults

Cardiac magnetic resonance techniques have significantly advanced understanding of cardiovascular physiology and pathophysiology in adults and are now considered the gold-standard functional assessment tool. These noninvasive assessments of cardiac health are now being gained faster, in more detail, and with greater sophistication than ever before. The range of techniques available have been summarized in recent reviews. ^, The key benefits of CMR in the adult population are the ability to quantify the component factors of circulatory function and the improved repeatability over echocardiography for assessment of quantitative measures. Critically, improved repeatability also translates into a reduction in the patient numbers required to prove a hypothesis in research studies.

CMR in newborns

It is clear that functional CMR will not become a routine clinical assessment tool for the sick newborn infant in the foreseeable future – imaging acutely ill children and infants remains challenging. It is more feasible, especially for more stable newborns, when an MR scanner is located within the neonatal unit – a growing number of centers are installing either dedicated neonatal ^, or full-size adult scanners, bringing “point-of-care MRI” closer to reality. It is now well established that MR scans can safely be performed in the newborn population while maintaining respiratory, circulatory, and thermal stability. We have now performed cardiac MR examinations in over 350 newborn preterm and term infants without any adverse events. Functional CMR images have been successfully obtained in infants weighing under 600 g and at 25 weeks’ corrected gestation.

Our belief is that virtually all infants undergoing CMR imaging can lay still after they have been fed and allowed to fall into a natural sleep after a feed. A recent systematic review of this “feed and wrap” approach reports very high success rates. Infants should be scanned with oxygen saturation, heart rate, and continuous temperature monitoring; a pediatrician/neonatologist or trained neonatal nurse should be in attendance throughout each scan. Protection from acoustic noise is achieved by applying moldable dental putty or ear plugs and then covering with neonatal ear muffs. Application of an acoustic hood can further reduce disturbance from scanner noise. Scans can be performed free-breathing or, with the provision of mechanical ventilation, with nasal continuous positive airway pressure and high-flow or low-flow oxygen therapy as clinically indicated. In older subjects respiratory motion causes significant image degradation requiring breath-holds, use of image navigators to coordinate acquisition with diaphragmatic movement, or advanced post-processing to account for respiratory motion. In neonates relatively high-quality cardiac images are obtainable without the use of respiratory navigation, presumably due to the relatively small degrees of diaphragmatic excursion. However, the widespread implementation of free-breathing techniques in older subjects is making their application in newborns feasible (see below).

Gating of scan acquisitions to the infant’s cardiac cycle remains vital for adequate imaging. This is relatively easily achieved using a four-lead vector cardiogram (VCG) (standard three-lead ECGs tend to be significantly degraded by the magnetic fields in the MRI environment). VCG gating can be carried out prospectively or retrospectively. In general, retrospective gating is preferred as this technique allows imaging throughout the entire cardiac cycle and gives additional flexibility to adapt to the variable heart rate seen in the newborn. Novel approaches to cardiac self-gating and real-time sequences are emerging, which may have particular benefits in newborns with unpredictable heart rates and whole-body motion patterns.

Adult CMR studies are increasingly performed at higher (3.0 Tesla) field strengths. Discussion on the specific challenges faced when imaging at higher field strengths is outside the scope of this review. However, in brief, the main drive to higher field systems comes from the increased signal offered as field strength increases, which may confer a substantial benefit for imaging small neonates. The major challenges faced in performing CMR in neonates arise from the need to significantly increase image resolution, both spatially – due to the size of the heart – and temporally – due to the rapid heart rates. These requirements place significant increased demands on the scanner hardware. In addition, the standard imaging protocols applied to adults may not be directly transferable and thus require modification to be used for scanning small neonates. The initial process of image optimization which we have undergone to improve image quality has previously been described. ^,