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DOI: 10.1148/radiol.2393050583
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(Radiology 2006;240:187-193.)
© RSNA, 2006

Obstetric Imaging

Fetal Lung Volume Measurements: Determination with MR Imaging—Effect of Various Factors1

Valerie L. Ward, MD, MPH, Mizuki Nishino, MD, Hiroto Hatabu, MD, PhD, Judy A. Estroff, MD, Carol E. Barnewolt, MD, Henry A. Feldman, PhD and Deborah Levine, MD

1 From the Department of Radiology (V.L.W., J.A.E., C.E.B.), Advanced Fetal Care Center (V.L.W., J.A.E., C.E.B.), and Clinical Research Program (H.A.F.), Children's Hospital, and Department of Radiology, Beth Israel Deaconess Medical Center (M.N., H.H., D.L.), Harvard Medical School, 300 Longwood Ave, Boston, MA 02115. From the 2003 RSNA Annual Meeting. Received April 7, 2005; revision requested June 3; revision received August 29; accepted September 22; final version accepted November 1. Address correspondence to V.L.W. (e-mail: valerie.ward{at}childrens.harvard.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
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.

© RSNA, 2006


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Fetal biometric measurements are noninvasive quantitative indicators of normal growth and overall well-being of pregnancy. Measurements obtained routinely with ultrasonography (US) include biparietal diameter, head circumference, crown-rump length, abdominal circumference, and femur length. These measurements of fetal growth are important predictors of birth size and neonatal outcome (1). Additional biometric measurements, such as lung-to-head ratio (2) and lung volumes (3,4), are sometimes obtained in pregnancies with complications caused by congenital thoracic abnormalities.

Despite the growing body of literature in regard to fetal lung volume (FLV) measurements obtained with magnetic resonance (MR) imaging and the potential advantages of MR imaging over US in the determination of FLV, most of the MR measurements have been performed by one observer (5–8). To our knowledge, few studies have included an evaluation of interobserver variability. Researchers in those few existing studies in which such an evaluation has been included reported excellent interobserver agreement (3,9). Investigators in even fewer studies have evaluated intraobserver variability. In one investigation, intraobserver errors were 2% for FLV measurements on MR images (7). In addition, the actual performance of FLV measurements on MR images can be a time-consuming process. The time to obtain a single FLV measurement on an MR image has been reported to range from approximately 2 or 3 minutes (10) to 30 minutes (8). Thus, the purpose of our study was to retrospectively determine the effect of gestational age (GA), imaging plane, section thickness, and inter- and intraobserver variability on FLV measurements obtained with MR imaging in a cohort of fetuses without thoracic abnormalities.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Study Population
Findings in consecutive patients referred to the Beth Israel Deaconess Medical Center, Boston, Mass, for prenatal MR imaging between July 30, 2002, and March 28, 2003, were reviewed. Patients were selected if their examinations were performed with a particular MR unit (Twinspeed; GE Medical Systems, Milwaukee, Wis) and a four-element phased-array surface coil, with adequate thoracic images acquired in all three planes, and if they were referred for evaluation because of either maternal indications or a fetal anomaly that did not involve the fetal thorax. In all fetuses, US was performed within 24 hours of the MR examination at our institution. If the age according to US biometric data was greater than 2 weeks less than the age according to the date of the last menstrual period, then the fetus was excluded from data analysis. We excluded one fetus as a result of this determination. Hence, the study group consisted of 30 fetuses in 29 pregnant women who were referred for consultation. One set of twins was included. The indications for referral were central nervous system anomaly (n = 13); screening for central nervous system anomaly (n = 5); abdominal wall defect (n = 2); cleft lip with or without cleft palate (n = 2); and unilateral absent kidney, abdominopelvic cyst or absent stomach, and maternal abdominopelvic abnormalities (n = 4). The range for GA of the fetuses was 17–36 weeks. Institutional review board approval was obtained to perform this investigation. Informed consent for this retrospective cohort study was waived, and the conduct of this study complied with the Health Insurance Portability and Accountability Act.

MR Examinations and Sequence Selection
A single-shot fast spin-echo MR imaging sequence was performed in the transverse, sagittal, and coronal planes (effective echo time, 90 msec; section thickness, 4–6 mm; matrix, 128 x 256 or 256 x 512; and scanning time per section, 300–900 msec). Observer A (M.N., with 3 years of experience in fetal MR imaging) evaluated each image obtained in the sequence that was performed at each examination for motion. If a particular sequence had too much motion, then it was not used for review. Specifically, a sequence was excluded when the lung margins could not be traced because of the subjective impression that too much motion was present. No examinations were excluded from review due to motion in all sequences. If a particular imaging plane was used more than once during a single examination, then the sequence with less motion was chosen for review. There was a time interval of 2 weeks from the selection of images by observer A to the time when the FLV measurements were performed to mitigate recall. In addition to the 2-week interval to mitigate recall, the images were presented to observers A and B 2 weeks later in a different order than that which was used when they were initially selected by observer A. Observer B (V.L.W., with 4 years of experience in fetal MR imaging) used the same sequences chosen by observer A for data analysis.

FLV Measurements
FLV measurements were determined by two independent observers, observers A and B, who were aware of the estimated GAs of the fetuses, by using a workstation (Advantage, version 2.0; GE Medical Systems) and tracing a single free-form region of interest around the visible portions of both the right and left lungs manually on individual consecutive sections in the transverse, sagittal, and coronal planes (Fig 1). The area of each region of interest obtained per image was calculated (based on the cross-sectional area and section thickness), and these calculated areas were summed to determine the total volume of both lungs. Both observers were careful to include only the lung parenchyma in each region of interest and, hence, to exclude the pulmonary hila and thoracic vascular structures.


Figure 1
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  		Figure 1: FLV measurement with MR imaging. Coronal half-Fourier single-shot turbo spin-echo MR image (echo time, 92 msec; section thickness, 5 mm; field of view, 32 x 32 cm; matrix, 256 x 256) of a fetus at 32 weeks GA in cephalic presentation shows a manually drawn free-form region of interest tracing around the visible portions of right and left lungs.

 
In each imaging plane, FLV measurements were performed twice by each of the two observers independently and at separate times. Specifically, FLV measurements were performed at an interval of 1 day to 1 week apart. Repeat measurements were performed without knowledge of prior measurements. The time required for an FLV measurement by both observers in all three planes was compared for eight of 30 fetuses, specifically for four of the fetuses with 21-week GA and four of those with 29- to 30-week GA. We used four of five randomly chosen fetuses with 21-week GA and all four of those with 29–30 week GA. We chose these two groups since they are typical periods when fetuses undergo MR imaging in the second and third trimester. Observer B provided time measurements for each of these eight fetuses twice (ie, two replicate measurements in each fetus and plane). Time was measured in seconds, and the two observers monitored their own time to perform the measurements.

Statistical Analysis
To allow acceleration of growth, FLV was modeled as a quadratic function of GA. Data from both observers and all three imaging planes of measurement were analyzed by using a mixed-effects regression model with a fixed effect for measurement plane; linear and quadratic effects of GA; linear adjustment for section thickness; and random effects for variation among fetuses, between observers, and within observers (residual error). We tested an additional random effect for variation between twins, a single set of which was included in the sample, but found that it had a negligible effect and omitted it from subsequent analyses. To determine whether the plane of measurement influenced the fetal lung growth curve, tests were performed for interaction between imaging plane and the linear and quadratic effects of GA. Fetus standard deviation (SD) was constrained to be independent of plane of measurement. Observer and residual SD were allowed to differ according to plane. The derivative of the fitted regression model with respect to GA provided an estimate of lung growth rate.

A post hoc power analysis was conducted to estimate what magnitude of influence on lung volume would have been detectable with a high likelihood, had it been present, as follows: Effdet = (z[0.05/2] + z[0.10]) · Errst, where Effdet is the detectable effect, z indicates Gaussian deviates corresponding to 90% power and 5% two-sided critical P value, and Errst is the standard error of the estimated effect. The standard error of each effect of interest (plane of measurement, section thickness) was obtained from the data.

Inter- and intraobserver variability values of each FLV measurement were calculated for each imaging plane. Correlations were assessed between observers with this calculation: [fetus SD2]/[fetus SD2 + observer SD2]. Correlations were assessed within observers with this calculation: [fetus SD2 + observer SD2]/[fetus SD2 + observer SD2 + residual SD2].

To examine the influence of imaging plane and GA on the time required for an FLV measurement, a factorial analysis of variance was used, with fixed effects for plane of measurement and GA (21 weeks vs 29–30 weeks) and random effects for variation among fetuses of a given age, variation between observers, and residual variation. We also tested for interaction between GA and plane of measurement (ie, whether the difference among planes in time required to measure FLV varied with GA). Because the time data (in contrast to the volume data) were relatively sparse, separate variance parameters for the three planes of measurement could not be reliably obtained, and, instead, uniform variance estimates for all three random effects were fitted. Statistical software (SAS, version 9.0, 2002; SAS, Cary, NC) was used for all computations. We used P = .05 as the critical level for statistical significance in all models. The statistical analyses were performed primarily by one individual (H.A.F.), with statistical contributions from another (D.L.).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Correlation between FLV and GA, Section Thickness, and Imaging Plane
Total FLV ranged from 2 to 110 mL. Mean FLV measurements obtained with MR imaging ranged from 9.0 mL at 17 weeks GA to 100.0 mL at 36 weeks GA. Measured FLV showed a strong, statistically significant dependence on GA (Fig 2a–2c) in both the linear and the quadratic terms (r = 0.89–0.91, P < .001). The strength of correlation of FLV with GA was comparable in the three imaging planes. There was no statistically significant effect for the section thickness (P = .23) or imaging plane of measurement (P = .82) on the FLV measurements. Addition of the interactions of plane to the linear and quadratic effects of GA (in essence fitting a separate quadratic function for each plane of measurement) produced an improvement in the fit that was statistically significant (F = 3.76, df = 4, 320, P < .01), but negligible in magnitude with respect to the FLV measurement (Fig 2d). The fitted dependency of lung volume (V) in milliliters on GA was as follows: V = 26.3 + (3.3 · [GA – 25 weeks]) + (0.167 · [GA – 25 weeks]2), adjusted to mean section thickness (4.5 mm) and averaged over the three imaging planes of measurement. The rate of growth in millimeters per week (derivative of volume with respect to GA as shown in Table 1) was thus: dV/dGA = 3.3 + (0.334 · [GA – 25 weeks]).


Figure 2
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  		Figure 2a: Graphs show measurements of FLV with MR imaging as a function of GA. MR imaging–based FLV measurements from 30 fetuses with GA of 17–36 weeks. For each fetus, measurements were obtained in duplicate by two observers in the (a) transverse, (b) sagittal, and (c) coronal planes. bullet = observer A, * = observer B. (d) Fitted quadratic functions for three MR imaging planes of measurement differed negligibly.

 

Figure 2
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  		Figure 2b: Graphs show measurements of FLV with MR imaging as a function of GA. MR imaging–based FLV measurements from 30 fetuses with GA of 17–36 weeks. For each fetus, measurements were obtained in duplicate by two observers in the (a) transverse, (b) sagittal, and (c) coronal planes. bullet = observer A, * = observer B. (d) Fitted quadratic functions for three MR imaging planes of measurement differed negligibly.

 

Figure 2
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  		Figure 2c: Graphs show measurements of FLV with MR imaging as a function of GA. MR imaging–based FLV measurements from 30 fetuses with GA of 17–36 weeks. For each fetus, measurements were obtained in duplicate by two observers in the (a) transverse, (b) sagittal, and (c) coronal planes. bullet = observer A, * = observer B. (d) Fitted quadratic functions for three MR imaging planes of measurement differed negligibly.

 

Figure 2
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  		Figure 2d: Graphs show measurements of FLV with MR imaging as a function of GA. MR imaging–based FLV measurements from 30 fetuses with GA of 17–36 weeks. For each fetus, measurements were obtained in duplicate by two observers in the (a) transverse, (b) sagittal, and (c) coronal planes. bullet = observer A, * = observer B. (d) Fitted quadratic functions for three MR imaging planes of measurement differed negligibly.

 

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  		Table 1. Estimated Mean FLV and Growth Rate for GA of 17–36 Weeks

 
Addition of a random effect for variation within the single set of twins had a negligible effect on the model parameters.

Post hoc power analysis indicated that the regression model was unlikely to miss any substantial influences on lung volume measurement. By using standard errors for the effects that did not show a significant difference, we calculated that we had 90% power to detect a variation among planes of measurement so long as it amounted to anything larger than a systematic difference of 13.6 mL between two given planes. Similarly, we had 90% power to detect any effect of section thickness larger than 4 mL/mm.

Assessments of Inter- and Intraobserver Variability
Variance parameters are shown in Table 2. Estimated SD of FLV among fetuses at a given GA was 6.7 mL. Difference between observers for a given fetus was least for the sagittal plane (SD, 3.7 mL; r = 0.76) and slightly greater for the transverse and coronal planes (Table 2). Agreement between replicate measurements made by a given observer was best for the transverse plane (SD, 3.6 mL; r = 0.83).


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  		Table 2. Variability of FLV Measurements among MR Imaging Planes and Observers

 
Time for Performance of FLV Measurements
The mean time required for performance of FLV measurements was 62 seconds for all fetuses and was significantly longer for fetuses with a 29–30-week GA than it was for fetuses with a 21-week GA (Table 3). Mean time also varied significantly with the imaging plane of measurement; transverse measurements took the longest time to obtain (mean, 70 seconds), and sagittal measurements were obtained most rapidly (mean, 53 seconds). The difference among planes did not vary significantly with GA (P > .1). Among fetuses of a given GA, the mean time of measurement varied by 16.8% (10.5 of 62.4), with a similar variation for replicate measurements of a given fetus and plane according to the observer who performed them (SD = 13.5 seconds). The two observers' mean times for a given fetus and plane did not vary enough in this small sample to be measurable.


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  		Table 3. Time Required for FLV Measurement on MR Images

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCES IN KNOWLEDGE
 References
 
Our data indicate that, in fetuses at 17–36 weeks of gestation, use of different section thicknesses and imaging planes did not result in statistically significant differences in FLV measurements with MR imaging and that these measurements can be obtained rapidly (in <1 minute at 21 weeks gestation and in <2 minutes at 29–30 weeks gestation). This finding is different from those of prior investigators, who reported that performance of each FLV measurement required as short a time as 2–3 minutes or as long as 30 minutes (8,10).

We showed that the MR imaging plane did not affect the determination of the FLV measurement. Similar findings were reported in an in vivo fetal sheep model of tracheal occlusion. These animal model MR imaging data showed that the FLV measurements were highly correlated with postmortem FLV measurements obtained within 24 hours, and there were no significant differences between FLV measurements obtained in the coronal plane and those obtained in the transverse plane (11). This finding is important in clinical use of FLVs because it means that imaging does not need to be repeated for measurement of volume if motion affects a single imaging plane.

Our data confirm the results of prior investigations that included an assessment of interobserver variability (3,9). Specifically, FLV measurements on MR images are reproducible, with good agreement between independent observers. Hence, FLV measurements on MR images can be quickly and reproducibly performed and correlate well with GA. The imaging plane of measurement and section thickness do not affect measured FLV.

Results of our investigation also provide estimations of the mean FLV and interval growth rate of the normal fetal lung in utero. The idea that the macroscopic size of fetal lungs might have relevance in the determination of the pulmonary well-being of the resulting newborn seems logical. This interval growth data has particular bearing on neonatal health outcomes for fetuses at risk for pulmonary hypoplasia, such as those fetuses in pregnancies with oligohydramnios and congenital diaphragmatic hernia.

Both direct and indirect measurements of FLV have been proposed for use with US. Indirect measurements include the lung-to-head ratio. This is the ratio of the right lung area to the GA-corrected head circumference and has been shown to be a useful US predictor of outcome in fetuses with left congenital diaphragmatic hernia (2,12). However, lung-to-head ratio measurements may be operator dependent (2) and technically difficult. Lung-to-head ratio measurements do not provide quantitative information about the ipsilateral lung in congenital diaphragmatic hernia, are often in the indeterminate range, and are only an indirect measurement of lung volume (3,8). Another US index of pulmonary hypoplasia is the comparison of fetal chest circumference with GA and femur length. This US index, however, is sometimes inaccurate in the presence of fetal thoracic or abdominal abnormalities (13,14).

More recently, three-dimensional US methods have been used to measure FLV in normal pregnancies (15–17). Yet, FLV measurements obtained with three-dimensional US have been reported to be influenced by shadowing of the spine, scapula, and ribs; suboptimal image quality; and fetal respiratory, cardiac, and body movements (16). Other limitations of three-dimensional US have included inadvertent inclusion of mediastinal structures into the FLV measurement (15). By using the Bland-Altman method, good agreement was shown between FLV measurements obtained with three-dimensional US and those obtained with MR imaging in a small cohort of patients with congenital diaphramatic hernia (18).

MR imaging frequently is used in some centers to assess complex fetal thoracic abnormalities that cannot be adequately assessed with US alone (19–23). MR imaging–based measurements of FLV may be superior to those obtained with US because fetal lungs are conspicuous on MR images due to their high signal intensity on T2-weighted images (20,22,23). Advanced GA, fetal position, maternal body habitus, and oligohydramnios can limit the acoustical window at US. Unlike FLV measurements obtained with US, those obtained with MR imaging are not as limited by these fetal and maternal factors (7). Rapid MR imaging sequences have made detailed evaluations of fetal anatomy feasible and reproducible (24–26). MR imaging–based measurements of FLV have been shown to be feasible (7,8,10) and accurate in investigations in both animal models (11) and humans (5). Normative references for FLV measurements on MR images have been established (9,27). FLV measurements obtained at midgestation on MR images have been demonstrated to be more strongly correlated with biometric measurements (ie, fetal size) than with GA (8).

MR imaging can be used as a noninvasive method to confirm and quantify fetal pulmonary hypoplasia (8). In a study of 11 fetuses with left congenital diaphramatic hernia, FLV measurements on MR images correlated with US measurements of lung-to-head ratio and were predictive of neonatal outcome (3). In a study of 13 fetuses with congenital diaphragmatic hernia and 74 control fetuses, a ratio of observed FLV on MR images to expected FLV on MR images was significantly lower in fetuses with congenital diaphragmatic hernia, and these investigators concluded that this observed-expected FLV ratio on MR images was a potential predictor of pulmonary hypoplasia and neonatal outcome (4). Findings in a recent investigation suggested that the ratio of FLV on MR images to fetal body weight on US images may be more useful for prediction of pulmonary hypoplasia than any single US parameter (28). Although a great deal has been written about the potential use of FLV as measured with MR imaging in this setting, not much information is known about the best approach to the process of measurement, timing of assessment (ie, GA at determination), or reproducibility of measurements between observers.

Limitations of our study include the fact that FLV measurements may change with the cardiac cycle or with fetal breathing (8). A prospective study with respiratory and cardiac gating would be necessary to evaluate these variables, but MR imaging technology has not yet conquered these challenges (8). Although these factors may potentially have influenced FLV measurements, two observers obtained measurements at two time periods, and the interobserver correlation and the correlation with the fetal GA was very high. The fact that imaging plane and 4–6-mm section thickness had no affect on FLV suggests that these factors are not critical. Another limitation was that our GA-specific numbers are small, and the study could have benefited from larger numbers of fetuses with a broader range of GA. A final limitation was that we assessed fetuses with only normal lungs. To prove the reliability of MR imaging measurements of FLV, a similar study should be performed in fetuses with suspected pulmonary hypoplasia.

In summary, we established that FLV measurement with MR imaging correlates well with GA and can be reproducibly and rapidly performed independently of section orientation or 4–6-mm section thickness. This finding should aid future developments in the use of MR imaging–based FLV measurement for the prediction of pulmonary hypoplasia.


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
References
 


   ACKNOWLEDGMENTS
 
The cost of the majority of the fetal MR examinations used for this study was paid by National Institutes of Health grant NS37945.


   FOOTNOTES
 

Abbreviations: FLV = fetal lung volume • GA = gestational age • SD = standard deviation

Authors stated no financial relationship to disclose.

Author contributions: Guarantors of integrity of entire study, V.L.W., M.N., D.L.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, V.L.W., M.N., D.L.; clinical studies, V.L.W., M.N., D.L.; statistical analysis, H.A.F., D.L.; and manuscript editing, all authors


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 

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A. K. Kilian, T. Schaible, V. Hofmann, J. Brade, K. W. Neff, and K. A. Busing
Congenital Diaphragmatic Hernia: Predictive Value of MRI Relative Lung-to-Head Ratio Compared with MRI Fetal Lung Volume and Sonographic Lung-to-Head Ratio
Am. J. Roentgenol., January 1, 2009; 192(1): 153 - 158.
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 RadiologyHome page
K. A. Busing, A. K. Kilian, T. Schaible, D. J. Dinter, and K. W. Neff
MR Lung Volume in Fetal Congenital Diaphragmatic Hernia: Logistic Regression Analysis--Mortality and Extracorporeal Membrane Oxygenation
Radiology, July 1, 2008; 248(1): 233 - 239.
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 RadiologyHome page
K. A. Busing, A. K. Kilian, T. Schaible, C. Endler, R. Schaffelder, and K. W. Neff
MR Relative Fetal Lung Volume in Congenital Diaphragmatic Hernia: Survival and Need for Extracorporeal Membrane Oxygenation
Radiology, July 1, 2008; 248(1): 240 - 246.
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 Am. J. Roentgenol.Home page
K. W. Neff, A. K. Kilian, T. Schaible, E.-M. Schutz, and K. A. Busing
Prediction of Mortality and Need for Neonatal Extracorporeal Membrane Oxygenation in Fetuses with Congenital Diaphragmatic Hernia: Logistic Regression Analysis Based on MRI Fetal Lung Volume Measurements
Am. J. Roentgenol., December 1, 2007; 189(6): 1307 - 1311.
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 RadiologyHome page
K. A. Busing, A. K. Kilian, T. Schaible, A. Debus, C. Weiss, and K. W. Neff
Reliability and Validity of MR Image Lung Volume Measurement in Fetuses with Congenital Diaphragmatic Hernia and in Vitro Lung Models
Radiology, December 1, 2007; 246(2): 553 - 561.
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 NeoReviewsHome page
R. A. Barth and E. Rubesova
Fetal Magnetic Resonance Imaging: Anomalies of the Neck, Chest, and Abdomen
NeoReviews, August 1, 2007; 8(8): e313 - e335.
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