Kathryn M. Broadhouse

Motion-Compensation Techniques in Neonatal and Fetal MR Imaging

SUMMARY: Fetal and neonatal MR imaging is increasingly used as a complementary diagnostic tool to sonography. MR imaging is an ideal technique for imaging fetuses and neonates because of the absence of ionizing radiation, the superior contrast of soft tissues compared with sonography, the availability of different contrast options, and the increased FOV. Motion in the normally mobile fetus and the unsettled, sleeping, or sedated neonate during a long acquisition will decrease image quality in the form of motion artifacts, hamper image interpretation, and often necessitate a repeat MR imaging to establish a diagnosis. This article reviews current techniques of motion compensation in fetal and neonatal MR imaging, including the following: 1) motion-prevention strategies (such as adequate patient preparation, patient coaching, and sedation, when required), 2) motion-artifacts minimization methods (such as fast imaging protocols, data undersampling, and motion-resistant sequences), and 3) motion-detection/correction schemes (such as navigators and self-navigated sequences, external motion-tracking devices, and postprocessing approaches) and their application in fetal and neonatal brain MR imaging. Additionally some background on the repertoire of motion of the fetal and neonatal patient and the resulting artifacts will be presented, as well as insights into future developments and emerging techniques of motion compensation.

Validation Study of the Accuracy of Echocardiographic Measurements of Systemic Blood Flow Volume in Newborn Infants

Background The echocardiographic assessment of circulatory function in sick newborn infants has the potential to improve patient care. However, measurements are prone to error and have not been sufficiently validated. Phase-contrast magnetic resonance imaging (MRI) provides highly validated measures of blood flow and has recently been applied to the newborn population. The aim of this study was to validate measures of left ventricular output and superior vena caval flow volume in newborn infants. Methods Echocardiographic and MRI assessments were performed within 1 working day of each other in a cohort of newborn infants. Results Examinations were performed in 49 infants with a median corrected gestational age at scan of 34.43 weeks (range, 27.43–40 weeks) and a median weight at scan of 1,880 g (range, 660–3,760 g). Echocardiographic assessment of left ventricular output showed a strong correlation with MRI assessment (R2 = 0.83; mean bias, −9.6 mL/kg/min; limits of agreement, −79.6 to +60.0 mL/kg/min; repeatability index, 28.2%). Echocardiographic assessment of superior vena caval flow showed a poor correlation with MRI assessment (R2 = 0.22; mean bias, −13.7 mL/kg/min; limits of agreement, −89.1 to +61.7 mL/kg/min; repeatability index, 68.0%). Calculating superior vena caval flow volume from an axial area measurement and applying a 50% reduction to stroke distance to compensate for overestimation gave a slightly improved correlation with MRI (R2 = 0.29; mean bias, 2.6 mL/kg/min; limits of agreement, −53.4 to +58.6 mL/kg/min; repeatability index, 54.5%). Conclusions Echocardiographic assessment of left ventricular output appears relatively robust in newborn infant. Echocardiographic assessment of superior vena caval flow is of limited accuracy in this population, casting doubt on the utility of the measurement for diagnostic decision making.

A modified echocardiographic approach improves reliability of superior vena caval flow quantification

Objective To assess accuracy and repeatability of a modified echocardiographic approach to quantify superior vena cava (SVC) flow volume that uses a short-axis view to directly measure SVC area and a suprasternal view to measure flow velocity, both at the level of the right pulmonary artery. Setting Three tertiary-level neonatal intensive care units. Design This was a multicentre, prospective, observational study. Accuracy of the traditional and modified approach was first assessed by comparing echo measurements according to both techniques with Phase contrast MRI (PCMRI) assessments, in a cohort of 10 neonates. In a second cohort of 40 neonates, intraobserver scan–rescan repeatability and interobserver analysis–reanalysis repeatability were assessed by repeated SVC flow echo measurements, according to both techniques. Results The traditional echocardiographic approach to assessment of SVC flow had a moderate agreement with PCMRI (r2 0.259), a scan–rescan intraobserver repeatability index (RI) of 37% (limits of agreement (LOA) −47/+51 mL/kg/min) and an interobserver analysis–reanalysis RI of 31% (LOA −38/+40 mL/kg/min). The modified approach showed a stronger agreement with PCMRI (r2 0.775), an improved intraobserver scan–rescan repeatability (RI 22%, LOA −24/+18 mL/kg/min) and improved interobserver analysis–reanalysis repeatability (RI 18%, LOA −18/+20 mL/kg/min). Conclusions Echocardiographic assessment of SVC flow volume by tracing area from a short-axis view and measuring velocity–time integral from a suprasternal view offered an improvement in accuracy and repeatability, building on the traditional approach previously described.

Neonatal cardiac MRI using prolonged balanced SSFP imaging at 3T with active frequency stabilization

Cardiac MRI in neonates holds promise as a tool that can provide detailed functional information in this vulnerable group. However, their small size, rapid heart rate, and inability to breath‐hold, pose particular challenges that require prolonged high‐contrast and high‐SNR methods. Balanced‐steady state free precession (SSFP) offers high SNR efficiency and excellent contrast, but is vulnerable to off‐resonance effects that cause banding artifacts. This is particularly problematic in the blood‐pool, where off‐resonance flow artifacts severely degrade image quality. Methods: In this article, we explore active frequency stabilization, combined with image‐based shimming, to achieve prolonged SSFP imaging free of banding artifacts. The method was tested using 2D multislice SSFP cine acquisitions on 18 preterm infants, and the functional measures derived were validated against phase‐contrast flow assessment. Results: Significant drifts in the resonant frequency (165 ± 23Hz) were observed during 10‐min SSFP examinations. However, full short‐axis stacks free of banding artifacts were achieved in 16 subjects with stabilization; the cardiac output obtained revealed a mean difference of 9.0 ± 8.5% compared to phase‐contrast flow measurements. Conclusion: Active frequency stabilization has enabled the use of prolonged SSFP acquisitions for neonatal cardiac imaging at 3T. The findings presented could have broader implications for other applications using prolong SSFP acquisitions. Magn Reson Med 70:776–784, 2013.

Reversible magnetogenic cobalt complexes

Cobalt complexes have been extensively used for their catalytic and solid-state magnetic properties, but the solution-state magnetic properties have not yet been widely exploited. Two versatile cobalt ligand scaffolds were investigated for their magnetic properties, which both demonstrated a diamagnetic to paramagnetic transition upon reduction. Notably, one of these complexes, cobalt tris(2-pyridylmethyl)amine (Co-TPA), was capable of cycling between a stable Co(II) species that could induce longitudinal and transverse relaxation of surrounding water protons, and a stable diamagnetic Co(III) species, exhibiting negligible relaxivity effects. Therefore, we propose Co-TPA as a dynamic redox responsive contrast agent for the magnetic resonance imaging (MRI) of hypoxia.

Cardiovascular magnetic resonance of cardiac function and myocardial mass in preterm infants: a preliminary study of the impact of patent ductus arteriosus

BackgroundMany pathologies seen in the preterm population are associated with abnormal blood supply, yet robust evaluation of preterm cardiac function is scarce and consequently normative ranges in this population are limited. The aim of this study was to quantify and validate left ventricular dimension and function in preterm infants using cardiovascular magnetic resonance (CMR). An initial investigation of the impact of the common congenital defect patent ductus arteriosus (PDA) was then carried out.MethodsSteady State Free Procession short axis stacks were acquired. Normative ranges of left ventricular end diastolic volume (EDV), stroke volume (SV), left ventricular output (LVO), ejection fraction (EF), left ventricular (LV) mass, wall thickness and fractional thickening were determined in “healthy” (control) neonates. Left ventricular parameters were then investigated in PDA infants. Unpaired student t-tests compared the 2 groups. Multiple linear regression analysis assessed impact of shunt volume in PDA infants, p-value ≤ 0.05 being significant.Results29 control infants median (range) corrected gestational age at scan 34+6(31+1-39+3) weeks were scanned. EDV, SV, LVO, LV mass normalized by weight and EF were shown to decrease with increasing corrected gestational age (cGA) in controls. In 16 PDA infants (cGA 30+3(27+3-36+1) weeks) left ventricular dimension and output were significantly increased, yet there was no significant difference in ejection fraction and fractional thickening between the two groups. A significant association between shunt volume and increased left ventricular mass correcting for postnatal age and corrected gestational age existed.ConclusionCMR assessment of left ventricular function has been validated in neonates, providing more robust normative ranges of left ventricular dimension and function in this population. Initial investigation of PDA infants would suggest that function is relatively maintained.

4D flow magnetic resonance imaging: role in pediatric congenital heart disease:

Imaging-based evaluation of cardiac structure and function remains paramount in the diagnosis and monitoring of congenital heart disease in childhood. Accurate measurements of intra- and extracardiac hemodynamics are required to inform decision making, allowing planned timing of interventions prior to deterioration of cardiac function. Four-dimensional flow magnetic resonance imaging is a nonionizing noninvasive technology that allows accurate and reproducible delineation of blood flow at any anatomical location within the imaging volume of interest, and also permits derivation of physiological parameters such as kinetic energy and wall shear stress. Four-dimensional flow is the focus of a great deal of attention in adult medicine, however, the translation of this imaging technique into the pediatric population has been limited to date. A more broad-scaled application of 4-dimensional flow in pediatric congenital heart disease stands to increase our fundamental understanding of the cause and significance of abnormal blood flow patterns, may improve risk stratification, and inform the design and use of surgical and percutaneous correction techniques. This paper seeks to outline the application of 4-dimensional flow in the assessment and management of the pediatric population affected by congenital heart disease.

4D phase contrast MRI in the preterm infant: visualisation of patent ductus arteriosus

Persistently patent ductus arteriosus (PDA) is correlated with multiple adverse outcomes; however, whether this association is causal or casual remains unclear.1 Echocardiography offers high sensitivity for detection of PDA, but has limited ability to quantify flow within the duct and surrounding vessels. Consequently, the haemodynamic significance can only be inferred.2 Four-dimensional (4D) phase contrast MRI (PCMRI) allows visualisation and quantitative analysis of haemodynamics at almost any anatomical location throughout the cardiac cycle.3 Should …

Quantification of aortic pulse wave velocity in preterm infants using 4D phase contrast MRI

Background 4D phase contrast (PC) MRI sequences providing full coverage of the aortic arch were acquired in neonates. Aortic pulse wave velocity (PWV) was then calculated from flow measurements taken at 5 to 8 locations along the arch and the aortic length between each location. Neonatal PWV values were compared with previously published adult values. Mechanical compliance within the healthy and diseased aorta has been well documented in adults and paediatrics [1,2]. Metafratzi et al [1] reported a PWV range from 4 to 10 ms-1 in the healthy adult aorta, whilst Vulliemoz et al found a mean PWV of 4.4 ms-1 [3]. PWV is an inverse measure of vessel compliance and marker for vessel stiffness. A significant increase in PWV has been found in paediatric subjects born prematurely at low birthweight when studied at ~8yrs and may explain the increase in cardiac disease in this population[2]. However PWV models do not extend back to preterm infants. Recent studies in adults have used PC MRI flow data to determine PWV [4].

Frequency drift during intensive SSFP scanning: implications and solution for 3T neonatal CMR

Background Balanced-SSFP is widely used because of its inherent highcontrast and high-SNR efficiency, and therefore is an obvious choice for neonatal cardiac applications. Typically, multiple averages are needed due to the high-spatial and temporal resolutions required, especially for the smallest preterm infants. Therefore, prolonged intensive scans are required placing high demands on scanner hardware. Consequently, one of the two critical prerequisites for successful SSFP sufficient B0 shimming and stable scanner frequency is often not met using standard protocols.