SULCI AND GYRI

Sonographic appearance of the sulci and gyri depend on the higher echogenicity of the pia-mater and pia-arachnoid complex (leptomeninx).

Sulcus = indentation at the expected location of the initially smooth cortical surface.

Insula assessed by evaluating the age at which opercular overfolding changed from obtuse to acute.

 

ULTRASOUND

 

-          Early in the second trimester the normal brain is still quite smooth and abnormal cortical development should not be diagnosed before 20 weeks of gestation.

-          Left-right symmetry of time of appearance of sulci is the rule in anatomical studies (1-4) but a few exceptions are reported.

o       Dorovini-Zis and Dolman evaluated symmetry in 23/80 brain samples and reported that 5/23 showed left-right differences (2). In three cases the right superior temporal sulcus was evident earlier. In two cases the central sulcus appeared earlier, once on the right and once on the left.

o       Chi et al. reported that the right superior frontal and superior temporal sulci, secondary sulci and right insular sulci were visible 1-2 weeks earlier than the left (1).

-          Sulci are easier to detect enface in a direction perpendicular to their plane of orientation.

o       Sulci on the hemisphere farther from the transducer are seen more clearly than those in the near field.

o       Sulci on the medial surfaces of the hemispheres, specifically the parieto-occipital fissure, calcarine sulcus and cingulate sulcus, appeared earlier and are more confidently seen than convexity sulci.

o       The earliest appearance of a sulcus was as a small dot in the expected site of the sulcus.

o       Later, the sulci formed an obvious V indentation.

o       Finally, the sulci became deeper and were visible as a surface notch and an echogenic line extending into the brain matter in a Y configuration

-          When imaged in a direction parallel to their plane of orientation, sulci appear as an echogenic plate that should not be mistaken for a disorder of the brain parenchyma. This is especially true of the calcarine sulcus, which on axial views can be seen as an echogenic band on the medial surface of the occipital lobe just medial to the occipital horn.

-          Anatomical reports of sulcal appearance times differ by as much as 4-6 weeks (1,2).

-          Generally, sulcal detection by imaging studies lags behind their anatomical appearance.

o       The identification of the parieto-occipital fissure and calcarine sulcus by imaging lags behind anatomical identification by about 2 weeks (anatomy 16 weeks vs. ultrasound and MRI 18 weeks).

o       The cingulate sulcus is more difficult to see and its appearance on ultrasound and MRI lags anatomical descriptions by about 7 weeks (anatomy 16 weeks vs. ultrasound 23 weeks and MRI 24 weeks).

 

Sulcus

Threshold (first seen) (wks)

Sulcus always seen  (wks)

Parieto – occipital

18.5

20.5

Best imaged axially in a plane near the upper margin of the occipital horns of the lateral ventricles.

It first appeared at 18.5 weeks, and was always visible after 20.5 weeks.

 

 

Calcarine

18.5

21.9

Best imaged in a coronal plane through the occipital lobes.

It could be seen as early as 18.5 weeks and was always visible after 21.9 weeks.

 

 

 

 

 

Cingulate sulcus (click

here for link).

 

23.2

24.9

·         The cingulate sulcus was generally not as confidently seen. It was best imaged in a coronal plane above the region of the thalami. It became visible by 23.2 weeks in some fetuses and was always seen after 24.3 weeks

 

White = cingulated sulcus

 

Red = parieto-occiptal sulcus

Insula / Sylvian

First acute

Always acute

Insula – axial view

23.5

       24.4

Insula – coronal view

23.2

       24.9

 

-          Has a characteristic pattern of development.

o       In early pregnancy the Sylvian fossa is a smoothly margined indentation.

o       By 18-22 weeks' gestation the smooth Sylvian fossa indentation developed angular margins at the site of the developing circular sulcus. This resulted in a plateau-like appearance with angularity at the margins (the circular sulcus) where the insula meets the frontal, parietal and temporal opercula anteriorly, superiorly and posteriorly. These angles were initially obtuse but became acute as the opercula progressively overgrew the insula and eventually met to form the closed Sylvian fissure. As the temporal and parietal lobes enlarge they overgrow the insula (operculization).

-          Acute insula / operculum angles are seen as early as 23.2 weeks in some fetuses.

-          After 24.5 weeks the angles were always acute.

-          Anatomically, operculization can be seen by about 22-24 weeks (2,5,6). It begins at the posterior pointed end of the insula and proceeds anteriorly (2,7).

-          By 28-35 weeks, most of the insula is covered (1,7,8), but full closure of the most anterior part is not achieved until birth to 2 years (9).

-          The insular plate remains smooth until insular sulci appear anatomically starting about 32-35 weeks (1,9) at which time they can also be recognized on MRI (10). This pattern of development was also seen in the present study. As temporal and parietal operculization progressed, the angle between the insula and overgrowing brain changed from obtuse to acute. An acute angle could be seen in some fetuses by 23.2 weeks and in every fetus older than 24.5 weeks.

 

 

Progression from obtuse to acute insula / operculum angle

 

Obtuse Angle

Acute angle

 

 

 

 

 

Earliest possible

Latest not visible                  Distinctly visible

Convexity sulci

23.2

             25.5                                      27.8

 

 

26

Deepening of fissures and sulci.

28-30

Growth spurt.

Sulci and gyri
    - Deepen.
    - Become more branched.

  • More difficult to visualize.
  • Best imaged semi-axially by employing the window through the proximal squamosal bony suture and then angling the plane of the scan on the farther brain surface from the approximate level of the insula superiorly.
  • The late detection of convexity sulci is likely to be due to their initial development in the high parietal regions where ultrasound access is obstructed by cranial bones.
  • Convexity sulci sre occasionally seen in some fetuses by 23.2 weeks in the parietal and temporal regions but are only  confidently seen only in fetuses older than 27.9 weeks.
  • The earliest detected convexity sulci are on the outer convex peripheral hemispheric surface posterior or superior to the Sylvian fissure and were likely to be the superior temporal, central and postcentral sulci.

 

 

Ref: Toi A, Lister WS. How early are fetal cerebral sulci visible and when can lissencephaly be suspected. ISUOG conference, Paris Sept 2003.

 

 

 

REFERENCES

 

  1. Chi JG, Dooling EC, Gilles FH. Gyral development of the human brain. Ann Neurol 1977; 1: 86-93
  2. Dorovini-Zis K, Dolman CL. Gestational development of brain. Arch Pathol Lab Med 1977; 101: 192-195.
  3. Bernard C, Droulle P, Didier F, Gerard H, Larroche JC, Plenat F, Bomsel F, Roland J, Hoeffel JC. Echographic aspects of cerebral sulci in the ante- and perinatal period [in French]. J Radiol 1988; 69: 521-532
  4. Hadi HA. Fetal cerebral maturation in hypertensive disorders of pregnancy. Obstet Gynecol 1984; 63: 214-219.
  5. Ruoss K, Lovblad K, Schroth G, Moessinger AC, Fusch C. Brain development (sulci and gyri) as assessed by early postnatal MR imaging in preterm and term newborn infants. Neuropediatrics 2001; 32: 69-74
  6. Patriquin H, Fontaine S, Michaud J, Lafortune M, Boisvert J. Development of the fetal brain in the second trimester: an anatomic and ultrasonographic demonstration. Can Assoc Radiol J 1992; 43: 131-137.
  7. Monteagudo A, Timor-Tritsch IE. Development of fetal gyri, sulci and fissures: a transvaginal sonographic study. Ultrasound Obstet Gynecol 1997; 9: 222-228
  8. Chen CY, Zimmerman RA, Faro S, Parrish B, Wang Z, Bilaniuk LT, Chou TY. MR of the cerebral operculum: abnormal opercular formation in infants and children. AJNR Am J Neuroradiol 1996; 17: 1303-1311.
  9. Naidich TP, Grant JL, Altman N, Zimmerman RA, Birchansky SB, Braffman B, Daniel JL. The developing cerebral surface. Preliminary report on the patterns of sulcal and gyral maturation - anatomy, ultrasound, and magnetic resonance imaging. Neuroimaging Clin N Am 1994; 4: 201-240.
  10. Levine D, Barnes PD. Cortical maturation in normal and abnormal fetuses as assessed with prenatal MR imaging. Radiology 1999; 210: 751-758.