Functional MR imaging: comparison of BOLD signal intensity changes in fetal organs with fetal and maternal oxyhemoglobin saturation during hypoxia in sheep.
ABSTRACT To compare relative changes in blood oxygen level-dependent (BOLD) signal intensity in the fetal brain, liver, heart, lungs, and cotyledon with maternal and fetal blood oxygenation during maternal hypoxia in sheep.
All experimental protocols were reviewed and approved by local authorities on animal protection. Six anesthetized ewes carrying singleton fetuses underwent magnetic resonance (MR) imaging with rapid single-shot echo-planar imaging BOLD sequence. BOLD imaging of the fetal brain, lungs, liver, heart, and cotyledon was performed during a control phase (ie, normoxia) and a hypoxic phase. Maternal oxyhemoglobin saturation was recorded continuously with pulse oximetry. Fetal blood samples were obtained with a carotid catheter at each phase. Regions of interest were placed in fetal organs. Normalized BOLD signal intensity was calculated with mean values of control and hypoxic plateaus. BOLD signal intensity was correlated with maternal oxyhemoglobin saturation and fetal oxyhemoglobin saturation; linear regression analysis was performed.
Control maternal and fetal oxyhemoglobin saturation values were 97% (95% confidence interval [CI]: 95%, 100%) and 62% (95% CI: 51%, 73%), respectively. During hypoxia, maternal and fetal oxyhemoglobin saturation values decreased to 75% (95% CI: 65%, 85%) and 23% (95% CI: 17%, 29%), respectively. Fetal BOLD signal intensity decreased to 81% (95% CI: 73%, 88%) in the cerebrum, 78% (95% CI: 67%, 89%) in the cerebellum, 83% (95% CI: 80%, 86%) in the lungs, 58% (95% CI: 33%, 84%) in the liver, 53% (95% CI: 43%, 64%) in the heart, and 71% (95% CI: 48%, 94%) in the cotyledon. Correlation of fetal BOLD signal intensity was stronger with fetal (r = 0.91) than with maternal (r = 0.68) oxyhemoglobin saturation; however, the difference was not significant. The highest slope values were obtained for the heart: 1.68% BOLD signal intensity increase per 1% maternal oxyhemoglobin saturation (95% CI: 1.58, 1.77) and 1.04% BOLD signal intensity increase per 1% fetal oxyhemoglobin saturation (95% CI: 0.94, 1.13).
BOLD MR imaging can be used to measure changes of oxyhemoglobin saturation in fetal organs during hypoxia. The liver and heart demonstrated the greatest signal intensity decreases during hypoxia.
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ABSTRACT: The purpose of this study was to assess the feasibility to determine fetal blood oxygen saturation (sO(2)) with T(2)-weighted MR sequences using a fetal sheep model. T(2) measurements were performed on a 1.5-T scanner using a T(2) preparation pulse in combination with a three-dimensional balanced steady-state free precession sequence repeated at different echo times. Eight sheep fetuses were examined during a control, hypoxic, and recovery phase to perform T(2)-weighted scans of the fetal blood in the heart. Signal intensities in the left and right ventricle were measured to calculate the MR blood sO(2). During each phase, fetal carotid artery sO(2) was directly measured and correlated with MR sO(2). A Bland-Altman plot was performed. Fetal carotid artery sO(2) was 69% sO(2) during control, 16% sO(2) during hypoxemia, and 67% sO(2) during recovery. Mean values of the MR sO(2) were 49% sO(2) and 40% sO(2) for control, 6% sO(2) and 3% sO(2) for hypoxemia, and 51% sO(2) and 43% sO(2) for recovery in left ventricle and right ventricle, respectively. Mean values of fetal carotid artery sO(2) and MR sO(2) were highly correlated (left ventricle: r = 0.87, right ventricle: r = 0.89). According to the Bland-Altman plot, MR sO(2) was lower compared to fetal carotid artery sO(2) (left ventricle: 15%, right ventricle: 20%). Based on our preliminary results, it seems to be possible to assess fetal sO(2) with MR oximetry.Magnetic Resonance in Medicine 07/2010; 64(1):32-41. · 3.27 Impact Factor
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ABSTRACT: The aim of this study was to perform fetal cardiac magnetic resonance imaging (MRI) with triggering of the fetal heart beat in utero in a sheep model. All experimental protocols were reviewed and the usage of ewes and fetuses was approved by the local animal protection authorities. Images of the hearts of six pregnant ewes were obtained by using a 1.5-T MR system (Philips Medical Systems, Best, Netherlands). The fetuses were chronically instrumented with a carotid catheter to measure the fetal heart frequency for the cardiac triggering. Pulse wave triggered, breath-hold cine-MRI with steady-state free precession (SSFP) was achieved in short axis, two-, four- and three-chamber views. The left ventricular volume and thus the function were measured from the short axis. The fetal heart frequencies ranged between 130 and 160 bpm. The mitral, tricuspid, aortic, and pulmonary valves could be clearly observed. The foramen ovale could be visualized. Myocardial contraction was shown in cine sequences. The average blood volume at the end systole was 3.4 + or - 0.2 ml (+ or - SD). The average volume at end diastole was 5.2 + or - 0.2 ml; thus the stroke volumes of the left ventricle in the systole were between 1.7 and 1.9 ml with ejection fractions of 38.6% and 39%, respectively. The pulse wave triggered cardiac MRI of the fetal heart allowed evaluation of anatomical structures and functional information. This feasibility study demonstrates the applicability of MRI for future evaluation of fetuses with complex congenital heart defects, once a noninvasive method has been developed to perform fetal cardiac triggering.European Radiology 06/2009; 19(10):2383-90. · 3.55 Impact Factor
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ABSTRACT: The purpose of this descriptive study was to correlate changes in the blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) signal with direct measurements of fetal tissue oxygenation. Seven anesthetized ewes carrying singleton fetuses at 125 days' gestation (term 145 days) underwent BOLD MRI, covering the entire fetus in a multislice approach. The fetuses were subjected to normoxic, hypoxic and hyperoxic conditions by changing the O(2)/N(2)O ratio in the maternal ventilated gas supply. The partial pressure of oxygen (pO2) in the fetal liver was measured using an oxygen-sensitive optode. Maternal arterial blood samples were simultaneously withdrawn for blood gas analysis. These measurements were compared with BOLD MRI signals in the fetal liver, kidney, spleen and brain. We demonstrated a consistent increase in the BOLD MRI signal with increasing tissue pO(2). For the fetal liver, spleen and kidney we observed a clear association between changes in maternal arterial blood pO2 and changes in BOLD MRI signal. Interestingly, we found that the BOLD signal of the fetal brain remained unchanged during hypoxic, normoxic and hyperoxic conditions. This experimental study demonstrated that BOLD MRI is a reliable non-invasive method for measuring changes in tissue oxygenation in fetal sheep. The unchanged signal in the fetal brain during altered maternal oxygen conditions is probably explained by the brain-sparing mechanism.Ultrasound in Obstetrics and Gynecology 09/2009; 34(6):687-92. · 3.56 Impact Factor