Fetal Sheep with Tracheal Occlusion: Monitoring Lung Development with MR Imaging and B-Mode US

Assessment of normal pulmonary development using functional magnetic resonance imaging techniques

BACKGROUND

The mainstay of assessment of the fetal lungs in clinical practice is via evaluation of pulmonary size, primarily using 2D ultrasound and more recently with anatomical magnetic resonance imaging. The emergence of advanced magnetic resonance techniques such as T2* relaxometry in combination with the latest motion correction post-processing tools now facilitates assessment of the metabolic activity or perfusion of fetal pulmonary tissue in vivo.

OBJECTIVE

This study aimed to characterize normal pulmonary development using T2* relaxometry, accounting for fetal motion across gestation.

METHODS

Datasets from women with uncomplicated pregnancies that delivered at term, were analyzed. All subjects had undergone T2-weighted imaging and T2* relaxometry on a Phillips 3T magnetic resonance imaging system antenatally. T2* relaxometry of the fetal thorax was performed using a gradient echo single-shot echo planar imaging sequence. Following correction for fetal motion using slice-to-volume reconstruction, T2* maps were generated using in-house pipelines. Lungs were manually segmented and mean T2* values calculated for the right and left lungs individually, and for both lungs combined. Lung volumes were generated from the segmented images, and the right and left lungs, as well as both lungs combined were assessed.

RESULTS

Eighty-seven datasets were suitable for analysis. The mean gestation at scan was 29.9±4.3 weeks (range: 20.6-38.3) and mean gestation at delivery was 40±1.2 weeks (range: 37.1-42.4). Mean T2* values of the lungs increased over gestation for right and left lungs individually and for both lungs assessed together (P=.003; P=.04; P=.003, respectively). Right, left, and total lung volumes were also strongly correlated with increasing gestational age (P<.001 in all cases).

CONCLUSION

This large study assessed developing lungs using T2* imaging across a wide gestational age range. Mean T2* values increased with gestational age, which may reflect increasing perfusion and metabolic requirements and alterations in tissue composition as gestation advances. In the future, evaluation of findings in fetuses with conditions known to be associated with pulmonary morbidity may lead to enhanced prognostication antenatally, consequently improving counseling and perinatal care planning.

Correlation between US and MRI for prenatal lung volumetry in diaphragmatic hernia, and use of Doppler to identify the ipsilateral lung cap

BACKGROUND

Pulmonary hypoplasia is a common cause of neonatal death.

OBJECTIVE

To describe the correlation between relative fetal lung volume (RFLV) and lung-to-head ratio (LHR) in fetuses with unilateral diaphragmatic hernia. Additionally, to describe identification of the ipsilateral lung cap by power Doppler.

MATERIALS AND METHODS

Single-institution study of consecutive fetuses with diaphragmatic hernia. LHR (by US) and RFLV (by MRI) were correlated in fetuses with and without an ipsilateral lung cap seen at MRI. In four, color/power Doppler was used to follow the pulmonary artery of the ipsilateral lung to identify the compressed cap.

RESULTS

The study included 48 fetuses of 20-38 weeks' gestational age (mean, 26 weeks). Mean LHR was 1.52 (range, 0.6-3) in fetuses with a lung cap and 1.15 (range, 0.6-2.58) in fetuses without (P = 0.043). Mean RFLV was 47.4% (range, 18-80%) in fetuses with and 32.9% (range, 14-57%) in fetuses without a lung cap (P = 0.005). RFLV and LHR correlated (r = 0.41, P = 0.01 in those with a cap; r = 0.50, P = 0.05 in those without). Power Doppler identified the ipsilateral lung cap and pulsed Doppler confirmed pulmonary vascularization in four of four fetuses.

CONCLUSION

LHR underestimates lung volume in fetuses with an ipsilateral lung cap. Power Doppler may be useful for identifying the cap.

Influence of different rotation angles in assessment of lung volumes by 3-dimensional sonography in comparison to magnetic resonance imaging in healthy fetuses

OBJECTIVES

Three-dimensional (3D) sonographic volumetry is established in gynecology and obstetrics. Assessment of the fetal lung volume by magnetic resonance imaging (MRI) in congenital diaphragmatic hernias has become a routine examination. In vitro studies have shown a good correlation between 3D sonographic measurements and MRI. The aim of this study was to compare the lung volumes of healthy fetuses assessed by 3D sonography to MRI measurements and to investigate the impact of different rotation angles.

METHODS

A total of 126 fetuses between 20 and 40 weeks' gestation were measured by 3D sonography, and 27 of them were also assessed by MRI. The sonographic volumes were calculated by the rotational technique (virtual organ computer-aided analysis) with rotation angles of 6° and 30°. To evaluate the accuracy of 3D sonographic volumetry, percentage error and absolute percentage error values were calculated using MRI volumes as reference points. Formulas to calculate total, right, and left fetal lung volumes according to gestational age and biometric parameters were derived by stepwise regression analysis.

RESULTS

Three-dimensional sonographic volumetry showed a high correlation compared to MRI (6° angle, R(2) = 0.971; 30° angle, R(2) = 0.917) with no systematic error for the 6° angle. Moreover, using the 6° rotation angle, the median absolute percentage error was significantly lower compared to the 30° angle (P < .001). The new formulas to calculate total lung volume in healthy fetuses only included gestational age and no biometric parameters (R(2) = 0.853).

CONCLUSIONS

Three-dimensional sonographic volumetry of lung volumes in healthy fetuses showed a good correlation with MRI. We recommend using an angle of 6° because it assessed the lung volume more accurately. The specifically designed equations help estimate lung volumes in healthy fetuses.

Reliability of the lung-to-head ratio as a predictor of outcome in fetuses with isolated left congenital diaphragmatic hernia at gestation outside 24-26 weeks

OBJECTIVE

The purpose of this study was to investigate the relationship between lung-to-head ratio (LHR) and gestational age (GA) in fetuses with isolated left congenital diaphragmatic hernia and to determine the applicability and reliability of LHR to predict postnatal outcome beyond 24-26 weeks of gestation.

STUDY DESIGN

The institutional review board approved this retrospective review of the University of California, San Francisco, Fetal Treatment Center database for cases with left congenital diaphragmatic hernia who were referred between March 1995 and June 2004. LHR was determined at the initial evaluation. One hundred seven live-born fetuses at 20-34 weeks of gestation (excluding cases that were lost to follow-up, with factors that potentially could influence the LHR measurement or postnatal outcome, or that were terminated electively).

RESULTS

The median GA at LHR measurement was 25.6 weeks; the median LHR was 1.01; the median GA at birth was 37.7 weeks; and the overall survival rate was 59% (64/107). The median LHR of nonsurvivors was significantly lower than that of survivors, but neither GA at LHR measurement nor at delivery was significantly different between the groups. Multiple logistic regression analysis confirmed LHR to be an independent predictor of postnatal survival, and receiver-operator characteristic curve analysis demonstrated that an LHR of > or = 0.97 has the highest performance in predicting postnatal survival. When fetuses were grouped by GA at initial LHR measurement to determine reliability of LHR, specifically with respect to GA, in the 26-34 and 24-26 weeks of gestation groups, median LHR of survivors was significantly higher than that of nonsurvivors, and receiver-operator characteristic curve analysis confirmed LHR to be a reliable predictor of postnatal survival. However, for fetuses at 20-24 weeks of gestation, there was a trend toward a higher LHR in survivors, although this did not reach statistical significance.

CONCLUSION

A significant positive linear relationship exists between LHR and GA at the time of measurement, such that LHR reliably predicts postnatal survival in fetuses with left congenital diaphragmatic hernia at 24-34 weeks of gestation and less reliable at 20-24 weeks. However, given the limitations of a retrospective, cross-sectional study, further prospective longitudinal studies that will investigate the change of LHR with GA and its association with fetal outcome are necessary.

Antenatal magnetic resonance imaging is useful in providing predictive values for surgical procedures in abdominal wall defects

BACKGROUND

Antenatal magnetic resonance imaging (MRI) is useful for the diagnosis of abdominal wall defects. Its predictive value concerning the possibility of primary closure of the abdominal wall, however, has so far not been reported.

METHODS

Between August 2001 and November 2004, antenatal MRI was performed on 9 patients with abdominal wall defects in whom surgical repair was performed immediately after birth. Areas of the abdominal cavity and exteriorized viscera were manually traced from both sagittal and axial MR images, and the data were further transmitted to a Workstation for MRI Volumetry (Advantage Windows 4.1, General Electric Medical Systems, Milwaukee, Wis). We examined the exteriorized ratio (ER), which is calculated by dividing the absolute volume of the abdominal cavity by that of the exteriorized viscera, and evaluated the predictive value by a retrospective comparison with surgical procedure.

RESULTS

In the primary closure group (n = 5), mean values of ER were 0.33 +/- 0.31 from axial and 0.45 +/- 0.31 from sagittal MR images. In contrast, in the staged closure group (n = 4), mean values of ER were 1.39 +/- 0.40 from axial and 1.34 +/- 0.42 from sagittal MR images. There was a significant difference (P < .05) between the 2 groups for both sets of images. The ER obtained from antenatal MRI correlated well with surgical procedure.

CONCLUSIONS

The ER might be useful for antenatal counseling, planning for delivery, and prediction of the most likely surgical procedure.

Fetal Sheep Development on Ultrasound and Magnetic Resonance Imaging: A Standard for the in utero Assessment of Models of Congenital Abnormalities

OBJECTIVE

To establish normative data for the size, conspicuity, and imaging characteristics of normal developing fetal sheep organs on ultrasound (US) and magnetic resonance (MR) imaging.

METHODS

US and MR images of ten normal pregnant sheep, at 40, 65, 90, 115, and 140 gestational days (term = 145 days), were scored for organ conspicuity and imaging characteristics. Imaging biometry was correlated with specimens. Gestational age-based growth parameters were modeled using regression.

RESULTS

Imaging biometry showed excellent correlation with specimens. Kidney, bladder, stomach, lung, liver, and spine were seen well from 65 days to term by US. More organs were consistently visible from 90 days to term by MR than by US. Most organ imaging characteristics tended not to change throughout gestation.

CONCLUSION

Normal fetal sheep biometry, organ conspicuity, and imaging characteristics are established for US and MR and have potential use for the in utero assessment of sheep models of congenital abnormalities.

Fetal Lung Volume Measurements: Determination with MR Imaging—Effect of Various Factors

PURPOSE

To retrospectively determine the effect of gestational age (GA), imaging plane, section thickness, and inter- and intraobserver variability on fetal lung volume (FLV) measurements obtained with magnetic resonance (MR) imaging in a cohort of fetuses without thoracic abnormalities.

MATERIALS AND METHODS

Institutional review board approval was obtained. Informed consent for this retrospective cohort study was waived, and the conduct of this study was HIPAA compliant. FLV was measured in 30 fetuses (GA, 17-36 weeks) referred for MR imaging for indications other than pulmonary abnormalities. Measurements were made on single-shot fast spin-echo images by tracing free-form regions of interest on individual consecutive sections in the transverse, sagittal, and coronal planes. Measurements were performed twice by two observers independently. Correlations between FLV and GA, imaging plane, and section thickness were assessed, as were intra- and interobserver variability. Time to perform FLV was assessed in a subset of fetuses.

RESULTS

Total FLV ranged from 2 to 110 mL. Mixed-effects regression model showed significant quadratic trend in FLV with increasing GA, with comparable strength of correlation (r = 0.89-0.91) in the three imaging planes of measurement. Intraobserver agreement was good in all three planes (r = 0.65-0.83) and was highest in the transverse plane. Interobserver agreement was good in all three planes (r = 0.68-0.76). FLV showed no significant dependence on section thickness (P = .23) or imaging plane (P = .82). Mean time to obtain FLV measurements ranged from 48 seconds at GA of 21 weeks to 77 seconds at GA of 29-30 weeks.

CONCLUSION

GA-based FLV measurements obtained with MR images are independent of section thickness and imaging plane and can be performed with good inter- and intraobserver agreement in less than 2 minutes.

MRI of normal and pathological fetal lung development

Normal fetal lung development is a complex process influenced by mechanical and many biochemical factors. In addition to ultrasound, fetal magnetic resonance imaging (MRI) constitutes a new method to investigate this process in vivo during the second and third trimester. The techniques of MRI volumetry, assessment of signal intensities, and MRI spectroscopy of the fetal lung have been used to analyze this process and have already been applied clinically to identify abnormal fetal lung growth. Particularly in conditions such as oligohydramnios and congenital diaphragmatic hernia (CDH), pulmonary hypoplasia may be the cause of neonatal death. A precise diagnosis and quantification of compromised fetal lung development may improve post- and perinatal management. The main events in fetal lung development are reviewed and MR volumetric data from 106 normal fetuses, as well as different examples of pathological lung growth, are provided.

Fetale Lungenentwicklung in der MRT

A well-organized interplay between many molecular factors as well as mechanical forces influence fetal lung development. At the end of this complex process a sufficiently sized and structurally mature organ should ensure the postnatal survival of the newborn. Besides prenatal ultrasonography, magnetic resonance imaging (MRI) can now be used to investigate normal and pathological human lung growth in utero. Oligohydramnios, due to premature rupture of membranes (PROM), is an important risk factor for compromised fetal lung growth. In these situations MR volumetry can be used to measure the size of the fetal lung quite accurately. Together with the evaluation of lung signal intensities on T2-weighted sequences, fetuses with pulmonary hypoplasia can be readily detected.

Fetal MRI in experimental tracheal occlusion

Congenital diaphragmatic hernia (CDH) is associated with a high mortality, which is mainly due to pulmonary hypoplasia and secondary pulmonary hypertension. In severely affected fetuses, tracheal occlusion (TO) is performed prenatally to reverse pulmonary hypoplasia, because TO leads to accelerated lung growth. Prenatal imaging is important to identify fetuses with pulmonary hypoplasia, to diagnose high-risk fetuses who would benefit from TO, and to monitor the effect of TO after surgery. In fetal imaging, ultrasound (US) is the method of choice, because it is widely available, less expensive, and less time-consuming to perform than magnetic resonance imaging (MRI). However, there are some limitations for US in the evaluation of CDH fetuses. In those cases, MRI is helpful because of a better tissue contrast between liver and lung, which enables evaluation of liver herniation for the diagnosis of a high-risk fetus. MRI provides the ability to determine absolute lung volumes to detect lung hypoplasia. In fetal sheep with normal and hyperplastic lungs after TO, lung growth was assessed on the basis of cross-sectional US measurements, after initial lung volume determination by MRI. To monitor fetal lung growth after prenatal TO, both MRI and US seem to be useful methods.

Accuracy of fetal lung volume assessed by three-dimensional sonography

OBJECTIVE

To determine the accuracy and precision of prenatal three-dimensional (3D) ultrasound in estimating fetal lung volume using the rotational multiplanar technique (VOCAL) by comparing it to postmortem volume measurements.

METHODS

Fetal lung volume was measured during 3D ultrasound examination using a rotational multiplanar technique in eight cases of congenital diaphragmatic hernia (CDH) (six left and two right-sided) and in 25 controls without pulmonary malformation, immediately before termination. Prenatal 3D sonographic estimates of fetal lung volume were compared with postmortem measurement of fetal lung volume achieved by water displacement.

RESULTS

The intraclass correlation coefficient of fetal lung volume estimated by 3D ultrasound and measured at postmortem examination was 0.95 in CDH cases and 0.99 in controls. Based on Bland-Altman analysis, the bias, precision and limits of agreement were, respectively, 0.35 cm(3), 1.46 cm(3) and between -2.51 and + 3.21 cm(3) in cases with CDH and 0.08 cm(3), 2.80 cm(3) and between -5.41 and + 5.57 cm(3) in controls. The mean relative error of 3D ultrasound fetal lung volume measurement was -7.19% (from -42.70% to + 18.11%) in CDH cases and -0.72% (from -30.25% to + 19.22%) in controls, while the mean absolute error of 3D ultrasound fetal lung volume measurement was 1.40 (range, 0.71-2.52) cm(3) and 2.12 (range, 0.05-4.98) cm(3), respectively. Accuracy of 3D ultrasound for measuring fetal lung volumes was 84.86 (range, 57.30-99.48)% in cases with CDH and 91.38 (range, 69.75-99.45)% in controls. The mean intraobserver variability for lung volume estimated by 3D ultrasound was 0.28 cm(3) in controls and 0.17 cm(3) in CDH cases.

CONCLUSION

Prenatal 3D ultrasound can estimate accurately fetal lung volume using the rotational multiplanar technique for volume measurements (VOCAL), even in fetuses with very small lungs, such as cases with isolated CDH.

Fetal Lung-to-Liver Signal Intensity Ratio at MR Imaging: Development of a Normal Scale and Possible Role in Predicting Pulmonary Hypoplasia in Utero

PURPOSE

To define retrospectively a normal range for lung-to-liver signal intensity ratio (LLSIR) in fetuses of 16-40 weeks gestation by using half-Fourier single-shot turbo spin-echo magnetic resonance (MR) imaging.

MATERIALS AND METHODS

Approval from the regional ethics review board for retrospective evaluation was obtained, and informed consent was waived. Retrospective analysis and follow-up of 157 pregnant women who underwent MR imaging over the past 4 years were performed. Seventy-four fetuses were subsequently identified as having clinically normal lung function or normal lung morphologic features at autopsy. A total of 141 normal lungs were analyzed, and the LLSIR was calculated from images on an MR workstation. A mixed-effects statistical model was applied, and 95% prediction intervals were calculated. Ten fetuses with hypoplastic lungs at autopsy were also evaluated.

RESULTS

Plotting LLSIR against gestational age demonstrated that, according to the fitted mean curve, the signal intensity ratio was higher with more advanced gestational age. Statistical modeling suggests a quadratic relationship between gestational age and LLSIR. For fetuses in the normal population, the LLSIR ranged from 1.52 at 21 weeks gestation to 4.31 at 34 weeks gestation. For all hypoplastic lungs in fetuses at or beyond 25 weeks gestation, the LLSIR was outside the lower bound of the 95% prediction interval for the normal population. The distinction between hypoplastic lungs and normal lungs at less than 25 weeks gestation is less definitive.

CONCLUSION

This study provides a normal scale with a 95% prediction interval for LLSIR.

Pulmonary circulation of fetal sheep with lung hyperplasia following tracheal occlusion

OBJECTIVES

To determine the alteration of blood flow velocity profile in the pulmonary arteries (PAs) of fetal sheep after tracheal occlusion (TO).

METHODS

Doppler ultrasound investigations of the PAs, the pulmonary trunk and the ductus arteriosus were performed weekly in nine ewes (gestational age 92-98 days, term 145 days) with singleton pregnancies after TO (n = 5) and in control fetuses (n = 4). Histological examinations with morphometry of pulmonary arterial vessels and of airways were performed in both groups.

RESULTS

In the control group the experiments lasted 38 +/- 6 days (mean +/- SD), and in the TO group 25 +/- 7 days. Relative lung weight was significantly higher in the TO group compared with the control group (14.5 +/- 3.4% and 4.0 +/- 0.5% of body weight). There were no significant differences in the Doppler parameters (pulsatility index, resistance index, the systolic peak, the diastolic minimum, time averaged maximum velocity) between groups. When the Doppler values of PAs in TO and control fetuses were combined, no significant differences between the left and right PA could be detected. The external diameter of peripheral PAs was significantly higher in the TO group as compared to control group (105.7 +/- 2.5 microm vs. 96.9 +/- 1.3 microm).

CONCLUSIONS

We found a threefold increase of fetal lung volume after TO without significant changes of blood flow velocity profiles in the PAs of fetal sheep.

Fetal Relative Lung Volume: Quantification by Using Prenatal MR Imaging Lung Volumetry

PURPOSE

To retrospectively determine a biometric algorithm for calculating relative lung volume in fetuses with normal lungs and of a wide range of gestational ages by using proved independent variables and to retrospectively investigate the use of this algorithm in fetuses with pulmonary hypoplasia.

MATERIALS AND METHODS

Total lung volume (TLV) was measured by using planimetry on single-shot rapid acquisition with relaxation enhancement magnetic resonance (MR) images obtained in 91 fetuses with ultrasonographically (US) normal chests and 28 fetuses with US-determined pulmonary hypoplasia. All fetuses were aged between 18 and 38 weeks gestation. Analysis of covariance was used to identify parameters that were not different between the fetuses with US-determined normal and those with US-determined abnormal chests, and these variables were used to construct an algorithm for calculating predicted lung volume. The relative lung volume-that is, the observed lung volume expressed as a percentage of the predicted lung volume-was then calculated in fetuses with pulmonary hypoplasia.

RESULTS

There was no significant difference in mean maternal or gestational age between the two fetus groups. Stepwise regression analysis was used to generate the following equation for predicting fetal lung volume on the basis of independent biometric indexes, with a correlation coefficient of 0.93: TLV = (0.52 . LV) + (0.33 . BD) - (0.06 . FL) - 13.7, with TLV and liver volume (LV) in milliliters and biparietal diameter (BD) and femoral length (FL) in centimeters. In the fetuses with normal chests, relative lung volume varied between 51% and 134%. In the fetuses with pulmonary hypoplasia, relative lung volume varied between 6% and 70%.

CONCLUSION

The predicted lung volume in fetuses of a wide range of gestational ages can be calculated with a high degree of accuracy, enabling prenatal MR imaging lung volumetry in which relative lung volume is used to quantify fetal pulmonary hypoplasia.