A high-resolution MRI study of linear growth of the human fetal skull ...

364Fig. 7a,b. Sketches summarisingtemporal variations. a Axialview of the posterior cranialfossa with shaded increments of2 weeks showing that widthincreases in relation to length.b Sagittal view demonstratingincrements of skull base lengthat 2-week intervals. Plus signsare used to illustrate thatgrowth of the anterior skullbase is almost twice that alongthe posterior portionFig. 8. Bivariate plot of total, anterior and posterior skull baselengths against gestational ageFig. 9. Bivariate plot of anterior against posterior skull base lengthwith previous key investigations [10, 11, 12, 13, 38, 39,40, 52]. In particular, Burdi’s investigations [10, 13] ofhemisectioned specimens show that the slope of anteriorskull base length against crown-rump length is almosttwice that of the posterior skull base. The anterior baseconsistently contributes more than 50% to increases ofoverall base length. Birch [38] showed that the rate oflinear increase along the anterior skull base is twice thatof the posterior segment in sectioned material from10–27 weeks gestation. Overall linear increases were24.5 mm for the anterior base and 12 mm for the posteriorbase with growth rates of 1.44 and 0.71 mm/week,respectively. Kvinnsland [14] reported a linear increaseof 21.7 mm for the anterior skull base and 11.5 mm forthe posterior skull base based on a sample of 159 sectionedfetuses from 10–32.5 weeks. Analysis of the Table4 in the paper by Ford [11] shows that linear growthof the anterior base is approximately twice that of theposterior skull base.Radiographic studies also demonstrate differences ofgrowth rate between the anterior and posterior skullbase [12, 17, 39, 40]. Levihn [12] noted a linear increaseof 17 mm for the anterior and 12 mm for the posteriorbase from 12–24 weeks. The former is considerably lessthan that observed for the anterior skull base in sectionedmaterial using identical landmarks, i.e., Na-S[10, 38]. In particular, Birch [38] observed an increaseof 24.5 mm for the anterior segment, which is 44%greater than that measured by Levihn for a comparablesample. Elongation of the posterior skull base asmeasured radiographically is, however, broadly consistentwith that reported for sectioned material and myfindings [10, 12, 14, 38, 39]. Presumably, this discrepancyarises because the posterior skull base ossifiesearly in fetal life, around 11 weeks, and is therefore aseasily quantified on radiographs as it on sectionedmaterial and hrMRI. In contrast, the anterior skullbase ossifies much later and is not so readily shownwith radiographs. More generally, rates of increase ofboth the anterior and posterior skull base given in the