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AJR 2003; 180:1155-1158
© American Roentgen Ray Society

Fetal Central Nervous System Biometry on MR Imaging

Taylor F. Reichel1, Ronald M. Ramus2, Jacqueline T. Caire1, Linda S. Hynan3, Kevin P. Magee4 and Diane M. Twickler1,2

1 Department of Radiology, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-8896.
2 Department of Obstetrics and Gynecology, University of Texas Southwestern Medical Center, Dallas, TX 75390-8896.
3 Department of Psychiatry and Academic Computing Services, University of Texas Southwestern Medical Center, Dallas, TX 75390-8896.
4 Perinatal Associates of Texas, 8160 Walnut Hill La., Dallas, TX 75231.

Received July 23, 2002; accepted after revision September 17, 2002.

 
Address correspondence to D. M. Twickler.


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. We sought to compare the biometry of the fetal head on MR imaging with sonographic measurements in fetuses with and without suspected central nervous system abnormalities.

MATERIALS AND METHODS. Blinded retrospective measurements of biparietal diameter, head circumference, and cerebellar width obtained on MR imaging were assigned a gestational age on the basis of median sonographic measurements and compared with sonographic and clinical assignment of gestational age in fetuses with no central nervous system abnormalities. In fetuses with central nervous system abnormalities, the same MR measurements were compared with sonographic measurements obtained within 1 week. Single-shot fast spin-echo sequences were obtained. Pearson's product moment correlation coefficients and paired sample t tests were performed.

RESULTS. In 22 fetuses with no suspected central nervous system abnormalities, significant correlation was seen in the assignment of gestational age by MR measurements and sonographic gestational age. In 25 fetuses with central nervous system abnormalities, significant correlation was also seen between biparietal diameter and head circumference measurements. The mean biparietal diameter on MR imaging was greater than on sonography in those fetuses with central nervous system abnormalities (p = 0.038).

CONCLUSION. MR imaging measurements of biparietal diameter, head circumference, and cerebellar width are strongly correlated to gestational age in fetuses without central nervous system abnormalities. Significant correlation is found between MR imaging and sonographic measurements of biparietal diameter and head circumference in fetuses with central nervous system abnormalities. Larger biparietal diameter measurements were seen with MR imaging than with sonography in the abnormal group. Fetal central nervous system biometry can be performed as part of the MR imaging evaluation of the fetal central nervous system.


Introduction
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 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Fetal MR imaging is being increasingly used for suspected fetal abnormalities, particularly those of the central nervous system [1, 2, 3, 4]. Ultrafast acquisition sequences such as half-Fourier acquisition single-shot turbo spin echo (HASTE) and single-shot fast spin echo have allowed excellent resolution and anatomic detail of the fetal brain [1, 2, 3, 4]. The average acquisition time for these ultrafast sequences is less than 90 sec for 40 images.

In addition to evaluation of fetal anatomy and dysmorphology, volumes of both fetal organ systems and the entire fetus have been attempted, with promising preliminary findings [5, 6, 7, 8, 9]. Central nervous system measurements of the cisterna magna and ventricular atria of the fetal brain have been performed on MR imaging (Twickler DM et al., presented at the Society of Maternal Fetal Medicine, January 2002). However, to our knowledge, biometric parameters of biparietal diameter, head circumference, and cerebellar width, which are routinely measured as part of the fetal sonographic evaluation, have not yet been published [10, 11]. Fetal MR imaging should allow accurate biometry given its potential for superior resolution compared with sonography. The fetal head size becomes important clinically, especially in the setting of central nervous system abnormalities, because the head size may dictate the mode of delivery when macrocrania is present. MR imaging head measurements and pelvimetry may identify those anomalous fetuses at risk for dystocia. [12] We therefore sought to compare fetal head measurements on MR imaging with those on sonography in fetuses with and without central nervous system abnormalities.


Materials and Methods
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 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
This retrospective study reviewed our fetal MR imaging database from January 2001 to January 2002 and selected those studies that had adequate images for central nervous system biometry measurements. Images were defined as adequate when the thalami and cavum septum pellucidum were visualized and situated midline in the axial plane for biparietal diameter and head circumference and an adequate posterior fossa axial view was seen for cerebellar width (Figs. 1A, 1B, 1C and 2A, 2B). Studies were divided into two groups. One group consisted of fetuses with abnormal sonographic central nervous system findings that were confirmed on MR evaluation. Abnormal findings included Chiari II malformations, varying degrees of ventriculomegaly, agenesis of the corpus callosum, posterior fossa cysts and Dandy-Walker malformations, aqueductal stenosis, and hydranencephaly, as well as less frequent findings. Because these fetuses had targeted MR imaging evaluations of the neural axis, the acquisition included axial, sagittal, and coronal images of the brain; 25 of 25 cases reviewed were judged to be adequate for obtaining retrospective measurements.



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  		Fig. 1A. —MR images of head biometry in fetus of 31 weeks' gestation (based on sonography and clinical findings) with no central nervous system abnormalities. (1 indicates measurement number 1.) Biparietal diameter is 77 mm.

 


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  		Fig. 1B. —MR images of head biometry in fetus of 31 weeks' gestation (based on sonography and clinical findings) with no central nervous system abnormalities. (1 indicates measurement number 1.) Occipital frontal diameter is 100 mm (head circumference = [biparietal diameter + occipital frontal diameter] x 1.57).

 


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  		Fig. 1C. —MR images of head biometry in fetus of 31 weeks' gestation (based on sonography and clinical findings) with no central nervous system abnormalities. (1 indicates measurement number 1.) Cerebellar width is 39 mm.

 


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  		Fig. 2A. —MR images of head biometry in fetus at 20 weeks' gestation (based on sonography and clinical findings) with central nervous system abnormalities. Biparietal diameter (61 mm) and occipital frontal diameter (82 mm) are shown in fetus of 00 weeks' gestation with Chiari II malformation and meningomyelocele. (1, 2 indicate measurement numbers.)

 


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  		Fig. 2B. —MR images of head biometry in fetus at 20 weeks' gestation (based on sonography and clinical findings) with central nervous system abnormalities. Sonographic biparietal diameter and occipital frontal diameter in Chiari II malformation.

 

The second group, those fetuses with no central nervous system abnormalities, consisted of either fetuses with other organ system findings (renal [n = 8], thoracic [n = 4], abdominal [n = 2]) or a maternal history of preterm labor to evaluate the cervix as part of an institutional review board protocol (n = 8). Of 22 cases reviewed, 22 were deemed adequate to obtain head measurements from the axial plane of the fetal brain. All 22 fetuses had sonographic evaluation that revealed no abnormalities of the central nervous system, although not all studies were contemporaneous with the MR imaging.

Measurements of biparietal diameter, head circumference, and cerebellar width obtained on sonography and MR imaging were compared in the abnormal group. A sonographic evaluation within 7 days of the MR imaging examination was required in cases of an abnormal central nervous system for inclusion in the study. At the time of the MR study, documentation of gestational age based on sonographic and clinical criteria was required in the cases of normal central nervous system findings for inclusion in the study. In fetuses with no central nervous system abnormalities, head biometric measurements obtained on MR imaging were assigned a gestational age from median sonographic measurements and compared with the gestational age based on sonography and clinical criteria [10, 11].

Sonography was performed using an XP/10 or Sequoia scanner (Acuson, Mountain View, CA) or an Elegra scanner with software version 6 (Siemens, Issaquah, WA) using 3- or 5-MHz curved linear and occasionally 5- or 7-Hz transvaginal probes. All studies were interpreted by a radiologist (n = 1) or maternal fetal medicine sonologists (n = 5). Any one of eight experienced registered diagnostic medical sonographers performed the study. Interobserver error was not evaluated.

MR imaging was performed using a 1.5-T Signa magnet (General Electric Medical Systems, Milwaukee, WI). Before scanning, each maternal patient was counseled with regard to fetal safety issues, and written informed consent was obtained, either as part of an internal review board protocol or as an indicated study.

A surface coil was wrapped around the mother's pelvis and centered over the fetal region of greatest interest. No maternal sedation was given in an attempt to sedate the fetus, because with acquisition times of 1–1.5 sec per slice, sedation was not necessary. A 15-sec localizer three-plane gradient-echo T2*-weighted sequence was obtained to plan the orthogonal planes relative to fetal lie. A singleshot fast spin-echo sequence with the following parameters was used to obtain images: effective TE, 50–100; field of view, 12–36 cm; matrix, 256 x 128 or 512 x 256; bandwidth, 31.2 or 62.5 kHz or 0.5 averages (number of excitations, 0.5); and slice thickness, 3–8 mm. The TR was as long as necessary to virtually eliminate T1 weighting and to complete the required acquisition. Scanning acquisition times varied from 40 to 90 sec depending on image quantity. An average of 7 acquisitions (range, 5–9) were performed with an average scanning time of 9–12 min. The entire MR study, including setup, was completed within 30 min.

All biometric measurements were obtained at an Advantage Windows postprocessing workstation (General Electric Medical Systems). For purposes of consistency, the biparietal diameter measurements were obtained from the inner table of the skull on one side and from the outer table of the skull on the contralateral side. Head circumference and cerebellar width measurements were obtained in a manner similar to the sonographic technique (Figs. 1A, 1B, 1C and 2A, 2B). One radiologist obtained all the measurements and was not aware of sonographic or gestational age measurements.

The data were analyzed using paired sample t tests and Pearson's product moment correlation coefficients. Analyses were performed using statistical software (version 10.2; SPSS, Chicago, IL), and statistical significance was defined as a p value of less than 0.05.


Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
We reviewed 47 of 57 cases from a 1-year period in our MR imaging database that were judged to have adequate MR images and sonographic and gestational age criteria for inclusion in our study. The mean fetal gestational age was 26 weeks (range, 14–36 weeks). In fetuses without central nervous system abnormalities (n = 22), a significant correlation was seen between the MR imaging predictors of gestational age from sonographic data and gestational age assignment for biparietal diameter, head circumference, and cerebellar width (Table 1). The relationship of gestational age and MR imaging measurements of biparietal diameter, head circumference, and cerebellar width, is presented in Figure 3. In fetuses with central nervous system abnormalities (n = 25), correlation was seen between measurements on MR imaging and sonography for biparietal diameter and head circumference (Table 2). The cerebellar width was not analyzed in this group because the width was measured in only four fetuses with central nervous system abnormalities.


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  		TABLE 1 Fetal Head Biometry in 22 Fetuses with No Central Nervous System Abnormalities

 


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  		Fig. 3. —Graph shows MR imaging estimates of biparietal diameter (•, BPD) (r = 0.960), head circumference ({blacksquare}, HC) (r = 0.972), and cerebellar width ({blacktriangledown}, CW) (r = 0.849) that were assigned a gestational age on basis of median sonographic measurements (y-axis) as function of gestational age based on sonographic and clinical assessment (x-axis). Pearson's correlation analysis was performed. For all measurements, p is less than 0.001.

 

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  		TABLE 2 Fetal Head Biometry in 25 Fetuses with Central Nervous System Abnormalities

 

In spite of a positive correlation, biparietal diameter measurements were consistently larger on MR imaging (7.4 ± 18.1 cm) than on sonography (6.6 ± 20.2 cm) in the abnormal central nervous system group (p = 0.038). However, such was not the case for head circumference, for which no difference was seen (p = 0.532).


Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
To our knowledge, ours is the first fetal MR biometry study of head parameters routinely obtained on sonography that include biparietal diameter, head circumference, and cerebellar width. In our retrospective series, we found an excellent correlation between established fetal gestational age and biparietal diameter, head circumference, and cerebellar width measurements on MR imaging in fetuses with no central nervous system abnormalities. Correlation was also significant for biparietal diameter and head circumference in fetuses with central nervous system abnormalities compared with sonographic measurements. Cerebellar width was not compared because there were so many abnormalities of this region in the abnormal group that measurements were obtained on sonography in only four cases. The cerebellar width should not be a difficult measurement to obtain with MR imaging, however, because of excellent visualization of the posterior fossa, even at later gestational ages when sonographic evaluation of the posterior fossa is problematic [13]. Gestational age did not appear to affect the ability to obtain measurements.

Measurements of the biparietal diameter were larger with MR imaging than with sonography in the abnormal group. This finding may be the result of better resolution that allows better measurements with MR imaging, ascertainment bias, or the differences in software between the MR imaging and sonographic biometry packages. This finding may suggest that MR biometry needs to have its own nomograms established because, at least in the case of the biparietal diameter, actual measurements were greater than those of sonography in the abnormal groups.

The limitations of our study include its retrospective review and small study population. Because this was a retrospective review, fetuses with normal brain findings with an indication for MR imaging who had other maternal or fetal indications cannot be considered a truly normal population. Another limitation is that neonatal findings were reviewed only in the abnormal group. Finally, in the normal group, median measurements of sonographic biometry for gestational age were used rather than contemporaneous sonographic measurements.

The significance of our finding that head biometry is feasible and corresponds to sonographic measurements should be addressed in the context of the present MR fetal studies. Recent studies suggest the usefulness of fetal MR imaging in various areas of fetal dysmorphology, especially central nervous system abnormalities [1, 2, 3, 4]. Fetal volume and weight have also been addressed, both as a whole and in relation to certain organs, including brain and lungs [5, 6, 7, 8, 9]. Preliminary data also suggest that most of the components of the nonbiometric fetal survey can be evaluated with a single 90-sec acquisition obtained axial to the fetal lie (Zaretsky M et al., Society of Gynecologic Investigation meeting, April 2002). Measurements of the cisterna magna and the atria have been addressed in the fetus (Twickler DM et al., presented at the SMFM meeting, January 2002). The combination of these preliminary studies suggests that MR imaging has the potential to assess almost all of the parameters recommended by the American College of Radiology for fetal sonography in the second and third trimesters [14]. Whether MR imaging can replace sonography in certain clinical settings because of its hypothetic superior resolution, needs to be explored.

Future MR imaging studies should include prospective biometry in normal fetuses to establish nomograms, comparison with contemporaneous sonographic studies performed within hours instead of days, and verification of gestational age estimate and normal brain based on neonatal outcome. Cost analysis of the modalities and analysis of interobserver error could also be explored.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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  3. Lan LM, Yamashita Y, Rang Y, et al. Normal fetal brain development: MR imaging with a half-Fourier rapid acquisition with relaxation enhancement sequence. Radiology 2000;215:205 –210[Abstract/Free Full Text]
  4. Levine D, Barnes PD. Cortical maturation in normal and abnormal fetuses as assessed with prenatal MR imaging. Radiology 1999;210:751 –758[Abstract/Free Full Text]
  5. Kubik-Huch RA, Wildermuth S, Cettuzzi L, et al. Fetus and uteroplacental unit: fast MR imaging with three-dimensional reconstruction and volumetry— feasibility study. Radiology 2001;219:567 –573[Abstract/Free Full Text]
  6. Walsh DS, Hubbard AM, Olutoye OO, et al. Assessment of fetal lung volumes and liver herniation with magnetic resonance imaging in congenital diaphragmatic hernia. Am J Obstet Gynecol 2000;183:1067 –1069[Medline]
  7. Gong QY, Roberts N, Garden AS, Whitehouse GH. Fetal and fetal brain volume estimation in the third trimester of human pregnancy using gradient-echo MR imaging. Magn Reson Imaging 1998;16:235 –240[Medline]
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  9. Baker PN, Johnson IR, Gowland PA, et al. Fetalweight estimation by echo-planar magnetic resonance imaging. Lancet 1994;343:644 –645[Medline]
  10. Hadlock FP, Deter RL, Harrist RB, Park SK. Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology 1984;152:497 –501[Abstract/Free Full Text]
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  12. Sporri S, Thoeny HC, Raio L, Lachat R, Vock P, Schneider H. MR imaging pelvimetry: a useful adjunct in the treatment of women at risk for dystocia? AJR 2002;179:137 –144[Abstract/Free Full Text]
  13. Stazzone MM, Hubbard AM, Bilaniuk LT, et al. Ultrafast MR imaging of the normal posterior fossa in fetuses. AJR 2000;175:835 –839[Abstract/Free Full Text]
  14. American College of Radiology. ACR standard for the performance of antepartum obstetrical ultrasound. In: Standards, 2000–2001, Reston, VA: American College of Radiology, 2000: 401–406

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