Christopher K. Macgowan

Reduced Fetal Cerebral Oxygen Consumption is Associated With Smaller Brain Size in Fetuses With Congenital Heart Disease

Background— Fetal hypoxia has been implicated in the abnormal brain development seen in newborns with congenital heart disease (CHD). New magnetic resonance imaging technology now offers the potential to investigate the relationship between fetal hemodynamics and brain dysmaturation. Methods and Results— We measured fetal brain size, oxygen saturation, and blood flow in the major vessels of the fetal circulation in 30 late-gestation fetuses with CHD and 30 normal controls using phase-contrast magnetic resonance imaging and T2 mapping. Fetal hemodynamic parameters were calculated from a combination of magnetic resonance imaging flow and oximetry data and fetal hemoglobin concentrations estimated from population averages. In fetuses with CHD, reductions in umbilical vein oxygen content (P<0.001) and failure of the normal streaming of oxygenated blood from the placenta to the ascending aorta were associated with a mean reduction in ascending aortic saturation of 10% (P<0.001), whereas cerebral blood flow and cerebral oxygen extraction were no different from those in controls. This accounted for the mean 15% reduction in cerebral oxygen delivery (P=0.08) and 32% reduction cerebral VO2 in CHD fetuses (P<0.001), which were associated with a 13% reduction in fetal brain volume (P<0.001). Fetal brain size correlated with ascending aortic oxygen saturation and cerebral VO2 (r=0.37, P=0.004). Conclusions— This study supports a direct link between reduced cerebral oxygenation and impaired brain growth in fetuses with CHD and raises the possibility that in utero brain development could be improved with maternal oxygen therapy.

Differential Regurgitation in Branch Pulmonary Arteries After Repair of Tetralogy of Fallot. A Phase-Contrast Cine Magnetic Resonance Study

Background The importance of pulmonary regurgitation (PR) after repair of tetralogy of Fallot (TOF) is increasingly recognized, but little is known regarding its underlying mechanisms. This phase‐contrast cine magnetic resonance (PC MR) study was performed to evaluate the relative contribution of each lung to total regurgitant volume. Methods and Results Twenty‐two patients with significant PR underwent a PC MR 3 to 16 years after repair of TOF. Regurgitant fraction of the main pulmonary artery was 39±10%. Regurgitant fraction of the left pulmonary artery (LPA; 46±18%) was greater than that of the right pulmonary artery (34±16%; P=0.002). The average contribution of the LPA to the total regurgitant flow volume was 54±19%, whereas its average contribution to the total forward flow volume was 44±13% (P=0.002). In 4 patients, regurgitant flow in the LPA accounted for 75% to 100% of the total regurgitant flow. There was a linear relationship between regurgitant fraction and fraction of the regurgitant flow duration in the main pulmonary artery (P<0.001) and right pulmonary artery (P=0.001) but not in the LPA (P=0.129). Conclusions PR after repair of TOF is commonly associated with differential regurgitation in the branch pulmonary arteries, which is usually greater in the LPA. Although the cause of this disparity requires further investigation, those patients with a significant unilateral contribution to total PR may be amenable to localized techniques to reduce regurgitation. (Circulation. 2003;107:2938‐2943.)

Phase-contrast magnetic resonance quantification of normal pulmonary venous return.

To assess the feasibility of phase‐contrast magnetic resonance (PCMR) in quantifying the pulmonary venous return in normal subjects.

Metric optimized gating for fetal cardiac MRI

Phase‐contrast magnetic resonance imaging can be used to complement echocardiography for the evaluation of the fetal heart. Cardiac imaging typically requires gating with peripheral hardware; however, a gating signal is not readily available in utero. No successful application of existing technologies to human fetal phase‐contrast magnetic resonance imaging has been reported to date in the literature. The purpose of this work is to develop a technique for phase‐contrast magnetic resonance imaging of the fetal heart that does not require measurement of a gating signal. Metric optimized gating involves acquiring data without gating and retrospectively determining the proper reconstruction by optimizing an image metric. The effects of incorrect gating on phase contrast images were investigated, and the time‐entropy of the series of images was found to provide a good measure of the level of corruption. The technique was validated with a pulsatile flow phantom, experiments with adult volunteers, and in vivo application in the fetal population. Images and flow curves from these measurements are presented. Additionally, numerical simulations were used to investigate the degree to which heart rate variability affects the reconstruction process. Metric optimized gating enables imaging with conventional phase‐contrast magnetic resonance imaging sequences in the absence of a gating signal, permitting flow measurements in the great vessels in utero. Magn Reson Med, 2010.

Feasibility of quantification of the distribution of blood flow in the normal human fetal circulation using CMR: a cross-sectional study

BackgroundWe present the first phase contrast (PC) cardiovascular magnetic resonance (CMR) measurements of the distribution of blood flow in twelve late gestation human fetuses. These were obtained using a retrospective gating technique known as metric optimised gating (MOG).MethodsA validation experiment was performed in five adult volunteers where conventional cardiac gating was compared with MOG. Linear regression and Bland Altman plots were used to compare MOG with the gold standard of conventional gating. Measurements using MOG were then made in twelve normal fetuses at a median gestational age of 37 weeks (range 30–39 weeks). Flow was measured in the major fetal vessels and indexed to the fetal weight.ResultsThere was good correlation between the conventional gated and MOG measurements in the adult validation experiment (R=0.96). Mean flows in ml/min/kg with standard deviations in the major fetal vessels were as follows: combined ventricular output (CVO) 540±101, main pulmonary artery (MPA) 327±68, ascending aorta (AAo) 198±38, superior vena cava (SVC) 147±46, ductus arteriosus (DA) 220±39,pulmonary blood flow (PBF) 106±59,descending aorta (DAo) 273±85, umbilical vein (UV) 160±62, foramen ovale (FO)107±54. Results expressed as mean percentages of the CVO with standard deviations were as follows: MPA 60±4, AAo37±4, SVC 28±7, DA 41±8, PBF 19±10, DAo50±12, UV 30±9, FO 21±12.ConclusionThis study demonstrates how PC CMR with MOG is a feasible technique for measuring the distribution of the normal human fetal circulation in late pregnancy. Our preliminary results are in keeping with findings from previous experimental work in fetal lambs.

Reference Ranges of Blood Flow in the Major Vessels of the Normal Human Fetal Circulation at Term by Phase Contrast Magnetic Resonance Imaging

Background—Phase-contrast MRI with metric-optimized gating is a promising new technique for studying the distribution of the fetal circulation. However, mean and reference ranges for blood flow measurements made in the major fetal vessels using this technique are yet to be established. Methods and Results—We measured flow in the major vessels of the fetal circulation in 40 late-gestation normal human fetuses using phase-contrast MRI (mean gestational age, 37 [SD=1.1] weeks). Flows were indexed to the fetal weight, which was estimated from the fetal volume calculated by MRI segmentation. The following mean flows (in mL/min per kilogram; ±2SD) were obtained: combined ventricular output, 465 (351, 579); main pulmonary artery, 261 (169, 353); ascending aorta, 191 (121, 261); superior vena cava, 137 (77, 197); ductus arteriosus, 187 (109, 265); descending aorta, 252 (160, 344); pulmonary blood flow, 77 (0, 160); umbilical vein, 134 (62, 206); and foramen ovale, 135 (37, 233). Expressed as percentages of the combined ventricular output, the mean flows±2 SD were as follows: main pulmonary artery, 56 (44, 68); ascending aorta, 41 (29, 53); superior vena cava, 29 (15, 43); ductus arteriosus, 41 (25, 57); descending aorta, 55 (35, 75); pulmonary blood flow, 16 (0, 34); umbilical vein, 29 (11, 47); and foramen ovale, 29 (7, 51). A strong inverse relationship between foramen ovale shunt and pulmonary blood flow was noted (r=−0.64; P<0.0001). Conclusions—Although too small a sample size to provide normal ranges, these results are in keeping with those predicted in humans based on measurements made in fetal lambs using radioactive microspheres and provide preliminary reference ranges for the late-gestation human fetuses. The wide range we found in foramen ovale shunting suggests a degree of variability in the way blood is streamed through the fetal circulation.

Phase-contrast MR assessment of pulmonary venous blood flow in children with surgically repaired pulmonary veins

BackgroundPulmonary venous (PV) obstruction may complicate surgical repair of PV abnormalities. By combining phase-contrast cine (PC) imaging and contrast-enhanced angiography, magnetic resonance (MR) imaging can provide physiological information complementing anatomical diagnosis.ObjectivesTo compare the PV flow pattern observed after surgical repair of PV abnormalities with normal PV flow pattern and to investigate the changes occurring in the presence of PV stenosis by using PC MR in children.Materials and methodsBy using PC MR, PV flow was evaluated in 14 patients (3 months-14 years) who underwent surgical repair for PV abnormalities. Eleven children (8–18 years) were studied as normal controls. Peak flow velocities and patterns were compared among three groups: normal veins (n=23), surgically repaired veins without (n=44) and with stenosis (n=10).ResultsNormal and unobstructed pulmonary veins after surgery showed a biphasic or triphasic flow pattern with one or two systolic peaks and a diastolic peak. Unobstructed surgically repaired veins showed decreased peak systolic velocity (P =0.001) and an increased peak diastolic velocity (P=0.005) when compared to normal values. Obstructed veins showed decreased systolic and diastolic velocities when measured upstream from the stenosis.ConclusionPC MR shows different flow patterns among normal, surgically repaired pulmonary veins with and without stenosis.

Effect of Propofol Anesthesia and Continuous Positive Airway Pressure on Upper Airway Size and Configuration in Infants

Background:Infants are prone to obstruction of the upper airway during general anesthesia. Continuous positive airway pressure (CPAP) is often used to prevent or treat anesthesia-induced airway obstruction. The authors studied the interaction of propofol anesthesia and CPAP on airway caliber in infants using magnetic resonance imaging. Methods:Nine infants undergoing elective magnetic resonance imaging of the brain were studied. Head position was standardized. Spin echo magnetic resonance images of the airway were acquired at the level of the soft palate, base of the tongue, and tip of the epiglottis. Four sets of images were acquired in sequence: (1) during light propofol anesthesia at an infusion rate of 80 &mgr;g · kg–1 · min–1, (2) after increasing the depth of propofol anesthesia by administering a bolus dose (2.0 mg/kg) and increasing the infusion rate to 240 &mgr;g · kg–1 · min–1, (3) during continued infusion of 240 &mgr;g · kg–1 · min–1 propofol and application of 10 cm H2O CPAP, and (4) after removal of CPAP and continued infusion of 240 &mgr;g · kg–1 · min–1 propofol. Results:Increasing depth of propofol anesthesia decreased airway caliber at each anatomical level, predominantly due to anteroposterior narrowing. Application of CPAP completely reversed the propofol-induced decrease in airway caliber, primarily by increasing the transverse dimension. Conclusions:Airway narrowing with increasing depth of propofol anesthesia results predominantly from a reduction in anteroposterior dimension, whereas CPAP acts primarily to increase the transverse dimension. Although airway caliber during deep propofol anesthesia and application of CPAP was similar to that during light propofol anesthesia, there were significant configurational differences.

Pulse-wave velocity measured in one heartbeat using MR tagging.

A noninvasive method for measuring the aortic pulse‐wave velocity (PWV) in a single heartbeat is introduced. The method sinusoidally tags a column of blood within the vessel, and rapidly acquires a series of 1D projections of the tags as they move (in practice, 64 projections at 4‐ms intervals). From these projections, the relative motion of blood at different positions along the vessel is measured. The PWV is obtained by fitting a mathematical model of blood flow to the tag trajectories. Tests of this method in a pulsatile flow phantom are presented using latex and polyurethane tubes. The PWV measured in these tubes was (mean ± standard deviation) 4.4 ± 0.5 m/s and 2.3 ± 0.2 m/s, respectively. The distensibility of each tube was calculated from the PWV (latex = (7 ± 2) 10−3 mm Hg−1, poly. = (25 ± 4) 10−3mmHg−1) and found to agree within error with distensibility measurements based on the change of tube area with pressure (latex = (6.3 ± 0.3) 10−3mmHg−1, poly. = (27 ± 1) 10−3 mmHg−1). To test its feasibility, the PWV measurement was applied to four normal volunteers. The measured PWV values were 3.9 ± 0.8 m/s, 3.6 ± 0.9 m/s, 3.9 ± 0.5 m/s, and 5.3 ± 0.8 m/s. By acquiring an independent PWV measurement each heartbeat, errors introduced by arrhythmia and trigger variability appear to be avoided with this method. Magn Reson Med 48:115–121, 2002.

Late Gadolinium Enhancement of the right ventricular myocardium: Is it really different from the left ?

It has been suggested that, in late gadolinium enhancement, the signal of right ventricular myocardium is nulled at a shorter inversion time than the left. While we initially made the same observation, we believe that the difference is not real, but results from artifacts.We present 7 cases as well as computer simulations to describe the nature of these artifacts and explain how they can create the impression of different inversion times for the right and left ventricle. At inversion times that are shorter than ideal for the myocardium a black rim can be seen at the border of the myocardium with blood on the inside and with fat on the outside. This is most likely a partial volume effect. The thin myocardium of the right ventricle is sandwiched between these black rims and, at a low spatial resolution, is no longer visible. In this case, the adjacent black rims may then be misinterpreted as myocardium. While black rims also occur on the left side, the myocardium is thicker and remains discernable as a separate layer. As a consequence, the optimal inversion time for the right ventricle only appears different from that for the left. In fact, in the presence of hypertrophy of the right ventricle or during systolic wall thickening we did not find a difference in inversion times between the left and right ventricle. We conclude that sufficient spatial resolution is important for adequate late gadolinium enhancement of the right ventricle.