Purpose or Case Report: Fetal intracranial volume is one of the most important parameters used in assessing normal fetal growth. Based on volumetric ultrasound (US), Virtual Organ Computer-Aided-anaLysis (VOCAL^TM) is a commonly used technique for such measurement. Magnetic resonance imaging (MRI) offers better soft tissue contrast for fetal brain imaging and is considered as one of the best tools for tissue volumetry, and as a standard for comparison. In this work we compare fetal intracranial volumes measured from MRI with those obtained from VOCAL^TM measurements in the same fetuses.
Methods & Materials: Forty Nine fetuses from uncomplicated pregnancies at a median gestational age 31 weeks (1^st quartile = 27.1, 3^rd quartile = 35.6, range = 20-39) were evaluated. Intracranial volumetric US was performed using a Voluson E-8 (GE Healthcare, Milwaukee, WI, USA) system and the VOCAL^TM tool using a rotational method with 30^o separation between each image. Volume calculation was performed by delineating the intracranial perimeter in each of the 12 images obtained from the US volume. Fetal MRI was performed on a 3.0T Siemens Verio system within 1 week of the US assessment. HASTE (Half-Fourier Acquisition Single-shot Turbo spin Echo imaging) images of the fetal brain were acquired in 3 orthogonal orientations. MRI images were systematically assessed by an experienced pediatric neuro radiologist for fetal motion and artifacts and image volumes suitable for intracranial volumetry were selected. Intracranial (IC) volume assessment was carried out through manual tracing. The correlation between volumes obtained from US and MRI was estimated using Pearson's (R) and determination coefficients (R²), and their association using linear regression.
Results: Excellent correlation (R² = 0.98; p<0.05) between MRI and US fetal intracranial volumes was found with the equation relating the US and MRI volumes as Y = 0.73X. MRI-IC volumes were slightly larger than those measured from US. A quadratic increase in the MRI-IC volumes as a function of gestational age was observed, in good agreement with previously published reports.
Conclusions: MRI and US measures of fetal intracranial volumes are in close agreement with each other. The increased volume estimation by MRI may be important to consider while interpreting fetal intracranial volumes in absolute terms.