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An Ultrasound Journey into the Normal Fetal Brain No disclosures

Ana Monteagudo, MD

Fetal Brain Scan Introduction • Many, find the brain anatomy challenging. • The ‘basic’ or ‘every day’ brain scan is performed by • It is true, that in order to understand what is seen in TAS using 3 axial planes: the transventricular, the ultrasound screen a basic understanding of the transthalamic and transcerebellar planes. developmental embryology of the brain is necessary. • The ‘expert’ or targeted fetal brain scan or • In this lecture, will review the pertinent ‘highlights’ of neurosonography is performed by 2D and 3D TAS the brain anatomy in the first trimester, but the main and TVS adding the coronal and sagittal planes to the focus is the anatomy during the second trimester and axial planes of the ‘basic scan’ . beyond.

The Rhombencephalon: 8+ weeks The First Trimester Sagittal Axial • The first brain structure that is easy to evaluate sonographically is the • Anechoic Rhombencephalon. structure • It represents the : medulla, pons and cerebellum posterior embryonic • Appears as an anechoic structure, in the Coronal posterior region of the head/brain, measuring brain approximately 3-4 mm. • Marker of the ~8 weeks US brain scan

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Fetal Brain Scan 11 to 13 6/7 weeks The First Trimester Cranium Choroid Brain Nasal bone Plexus • The next brain structure that is easy to • The correct plane to adequately evaluate sonographically is the midline measure the NT is the median falx; seen at 9+ weeks. plane of the fetus. • It divides the brain into the right and • This plane, provides more than left hemispheres with the choroid just an opportunity to measure plexus seen to each side of the falx. the NT thickness and assess for the presence of the nasal bone. • The CP is brightly echogenic, Falx • It allows evaluation of the symmetric and fills 2/3 the cavity of developing fetal head and brain. the lateral ventricle Nuchal ‘Butterfly sign’ Marker of the Normal Midline translucency 12 2/7 weeks

Cranium Frontal horn 6/7 Choroid Fetal Brain Scan 11 to 13 6/7 weeks Fetal Brain Scan 11 to 13 weeks plexus Thalamus Posterior fossa in the median plane • Structures seen using the during the 1st trimester median plane are the: • ‘3 sonolucent spaces’ • Cranium Brainstem • Frontal horn Intracranial translucency • Choroid plexus Cisterna magna Posterior • Thalamus fossa • Brainstem is the widest and the • Posterior fossa cisterna magna is the thinnest • Occipital bone • Non-visualization of these 3 spaces may be a clue to the presence of 12 2/7 weeks Occipital bone ONTD- spina bifida.

Fetal Brain Scan Second Trimester and Beyond Transventricular plane • The ‘basic’ or ‘every day’ brain scan is performed by TAS • Landmarks using 3 axial planes: the transventricular, transthalamic • Cavum septi pellucidi (1) SF and transcerebellar planes. • Frontal horns (2) 3 4 • Choroid plexus (3) 2 • Posterior horn (4) 1 • Parieto-occipital fissure (POF) POF 2 4 • Sylvian Fissure (SF) 3

• Measure LV SF

Transventricular Transthalamic Transcerebellar Drawings from Bethune, et al.AJUM August 2013 16 (3)

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Transventricular plane Transventricular plane Criteria for appropriate measurement of Lateral Ventricle Where to Measure the Lateral Ventricle • Measuring of the LV is part • Normal < 10mm- accepted of the 2nd trimester anatomic survey FH

CSP

PH + CP +

20 wks SMFM Consult Series July 2018

Transventricular plane Asses for the Normal Shape of Anterior & Posterior Brain Complexes • Recently, in an effort to standardize and subsequently Back-to-back letters ‘C’ • Falx (4) • Frontal horns (2) improve diagnosis of fetal midline brain anomalies • Choroid plexus (6) the terms anterior and posterior brain complexes has been introduced. Anterior Posterior • Normal appearance of the complex complex complexes is reassuring while abnormal appearance can aid in the detection of midline brain anomalies such as: HPE and AGCC .

Anterior (AC) & Posterior (PC) Anterior Complex Complexes of the Brain Interhemispheric fissure (1) Callosal sulcus (2) • Best plane for the Genu of the corpus callosum (3) AC is the Cavum septi pellucidi (4) transventricular Anterior horns (AH) • Best for the PC is Normal CSP ‘shape’ variations 2-3 mm above the transventricular plane

Graphic from : Hormazabal L et al Prenat Diagn 2020;40:596-604 Graphics From Viňales et al. UOG 2015; 46: 585–594

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Transventricular plane Posterior Complex Asses for the Normal Sylvian Fissure aka Lateral Sulcus

Oblique to midline Choroid plexus (CP) disposition of the LV Parieto-occipital fissure (POF) • With increasing GA; the fissure becomes more angular Splenium corpus callosum (arrow and S) and deeper Callosal sulcus (CS) • Abnormal S. fissure may be a marker of an underlying Interhemispheric fissure (IHF) From Viňales et al. UOG 2015; 46: 585–594 CNS anomaly Diagram from Ultrasound Obstet Gynecol 2008; 32: 50–60

Transthalamic plane Transthalamic plane • Landmarks • Landmarks • Cranium SF • Cavum septi pellucidi (1) • Falx • Frontal horns (2) 7 • Frontal horns • Thalami (3) • Cavum septi pellucidi • Choroid plexus (6) • Thalami • Posterior horn (7) 7 • Cerebellar pedunculi • Sylvian Fissure (SF) • Hippocampal gyrus • Measure BPD and HC SF • Sylvian fissure • Measure BPD, HC, OFD

Transthalamic plane Transthalamic plane Where to measure the BPD Where to measure the HC Leading edge-to leading edge Around the outside of the skull bones • Transducer must be + • Transducer must be perpendicular & hemispheres perpendicular & and head should be hemispheres and head symmetric. should be symmetric. • Calipers should be placed at the: outer edge of the near • Ellipse should be place calvarial wall inner edge of the around the outer table of far calvarial wall the calvarium • Orbits, ears or cerebellar • Orbits, ears or cerebellar hemispheres should not be in hemispheres should not be the plane + in the plane

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Transthalamic plane Transthalamic plane Asses for the Normal CSP Leave of septum Asses for the Normal Shape of Anterior Horns Anterior Horn pellucidum Shaped like a “V” SF • A fluid filled structure • Upward & laterally Corpus callosum Cavum diverging anterior horns between the leaves of 7 the • If a central ‘third-line’ is 7 seen this is NOT the cavum, but the fornix SF

Transthalamic plane Transthalamic plane What is the Fornix? Reflections of the medial wall Echogenic Central Line of the lateral ventricle Fornix

• If a central ‘third-line’ is seen this is NOT the cavum, but the fornix

Located inferiorly to the CSP • By US can be recognized as the central ‘third’ line*

• Another ‘clue’ the non-visualization of the frontal horns *J Ultrasound Med 2008;27:25-31.

Transthalamic plane Normal CSP and Fornix Transcerebellar plane CSP Fornix • Landmarks • Frontal horns (1) • Frontal lobes (4) • Cavum septi pellucidi (6) • Cerebellum (7) • Vermis (V) V • Cisterna magna (8) • Cerebellar pedunculi (9) • Parietal lobes (10) Frontal horns • Sylvian fissure (SF) A fluid filled structure between the Fornix Central ‘third-line’ • Measure the transcerebellar leaves of the septum pellucidum Non-visualization of the fluid filled FH diameter and cisterna magna

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Transcerebellar plane Transcerebellar plane Where to measure the transcerebellar diameter (TCD) Where to measure the Cisterna Magna (CM) • The TCD is measured at the • The CM is measured from widest part of the cerebellum, the posterior margin of the perpendicular to the falx vermis to the inside of the • TCD in mm correlates with GA Falx occipital bone in the up to 20 weeks. After 20 weeks midline. is larger than GA. • Measurement of 2-10 mm • A TCD ≤ 2mm than the is normal during 2nd to the estimated GA or < 5% is a 3rd trimesters. concerning finding.

Transcerebellar plane Transcerebellar plane Cisterna Magna Septa • Cerebellar • Are NL anatomic structures; seen during the NL embryologic hemispheres (1) 5 1 development of the posterior fossa*. 5 • (2) 1 4 • Usually, 2 septae, are imaged inferior 4 • Vermis (3) 3 3 22 & posterior from the vermis forming • Blake’s pouch (4) 1 a cyst-like structure. 5 5 • Cisterna magna (5) 1 • Present in most fetuses (84-92%) • In actuality, they represent the walls of Blake’s pouch * Pretorius DH et al, JUM1992;11:125; * Knutzon RK et al, Radiology 1991;190:70 ** Robinson AJ & Goldstein R, JUM2007;26:83

Transcerebellar plane Transcerebellar plane “Absent -vermis” Blake’s Pouch The Vermis < 20 weeksLow posterior axial section of the posterior fossa • The cisterna magna septa are the walls of Blake’s pouch • Blake’s pouch is a normal fingerlike appendage of the 4th ventricle. • ‘Potential marker’ for normal development* *Pretorius DH et al, JUM1992;11:125 Knutzon RK et al, Radiology 1991;190:70 Robinson AJ & Goldstein R, JUM2007;26:83 Volpe P et al. UOG 2012:39:632

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Transcerebellar plane Fetal Brain Scan Second Trimester and Beyond The Vermis < 20 weeks The “Absent-Vermis” If your scanning plane is too •Up to this point we have low… it can be confused with concentrated in the normal posterior fossa abnormality anatomy using the 3 classic (abnormal vermis) transabdominal (TAS) axial views. •Let’s go beyond and look at the anatomy in coronal and median (sagittal) planes.

15 6/7 weeks Normal

The Normal Fetal Brain- Beyond the Basics Coronal Coronal • The ‘expert’ or targeted fetal brain Plane scan or neurosonography is performed by adding the coronal and • Numerous sagittal planes, typically by TVS, to tomographic the axial planes of the ‘basic scan’ . coronal sections of the fetal brain Sagittal suture Sagittal is possible. Coronal suture • However, will concentrate on the mid-coronal Anterior fontanelle Metopic plane suture Anterior fontanelle provides an acoustic window

Mid-Coronal (or Transthalamic plane) Mid-Coronal Plane Falx Cerebri • Cavum septi pellucidi (1) • Midline • Anterior horns (2) • Superior sagittal sinus • Thalami (3) 2 2 • Subarachnoid space • Falx (4) 5 6 SF * * • Falx • Choroid plexus (5) • • Interhemispheric Body of corpus callosum (6) fissure • Interventricular foramina (*) • Body of the corpus • Sylvian fissure (SF) callosum • Cavum septi pellucidi

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Mid-Coronal Plane Mid-Coronal Plane Interhemispheric Fissure Interhemispheric fissure Cavum Septi Pellucidi Interhemispheric fissure Cingulate sulcus and gyrus Sidewall of septum Cingulate sulcus pellucidum and gyrus • A fluid filled structure Corpus callosum between the sidewalls of the septum pellucidum

Anterior Horn

Cavum Smooth “Squiggly‘

EGA 20 weeks EGA 28 weeks EGA 34 weeks

Mid-Coronal Plane Mid-Coronal Plane Anterior Horns • Slit-like structure seen between the thalami • Typical width is about 1 mm up to 28 weeks; afterwards may enlarged to reach a width of 1.9 mm • If the width measures greater than 3.5 mm anytime during the pregnancy, it should be considered abnormal Anterior horns diverge up and laterally on this coronal plane Radiology. 1997 Jun;203(3):641-4

median plane

MANY consecutive sections can be generated in the sagittal plane. SagittalThe most important one is the MEDIAN plane (Mid-sagittal) Sagittal Median Plane Plane • Corpus Callosum • CSP ( & Vergae) • Numerous • Thalamus (3rd ventricle) tomographic • Tela chroidea sagittal sections • Tectum (corpora of the fetal brain quadrigemina) is possible. • Posterior fossa • However, will • Cerebellar vermis concentrate on • Cisterna magna the median • 4th ventricle plane • Fastigium

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Median Plane Corpus callosum & Cavum Septi Pellucidi Corpus Callosum & Cavum Septi Pellucidi • The rostrum (beak), genu (knee), corpus (body) and

Corpus Callosum the splenium (tail) CC develops in a anterior to posterior fashion • • Exception: Rostrum & anterior genu which Hypoechoic Cavum septi pellucidi & vergae develop last • C-shaped Body Genu CSP Splenium The cava does • Fluid filled space not • Develops together communicate with the CC with the Rostrum ventricular • Continues posteriorly system as the cavum vergae Cavum septi pellucidi Cavum Vergae 22 w

Median Plane Median Plane Corpus Callosum & Cavum Septi Pellucidi Nomograms Corpus Callosum • Splenium (tail) extends to quadrigeminal cistern (tectal plate) • If it doesn’t this is suspicious for partial AGCC

Malinger et al AJR 1993 22 w J Ultrasound Med 2014; 33:1065–1078

Median Plane Median Plane The tectal plate (Quadrigeminal Plate) Aqueduct of Sylvius rd • It is the portion of the midbrain (tectum) upon which the • Connects the 3 th superior and inferior colliculi sit. ventricle with the 4 • When fully formed, the tail of the corpus callosum reaches it ventricle • Site for pathologies such as: Cysts, Lipomas • It appears as two parallel echoic lines 3V

4V

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Median Plane Median Plane The tectal plate (Quadrigeminal Plate) Gyri & Sulci • Splenium Brain changes from smooth; to having multiple The choroid plexus of hyperechoic sulci the 3rd ventricle covering • Cingulate gyrus from about 24 weeks the thalamus and the choroid plexus of the quadrigeminal plate Thalamus (generates a figure of 3 appearance. Tail of CC reaches the middle of the 3)

16 weeks 22 weeks 34 weeks

Median Plane Median Plane Gyri & Sulci- Adult-like appearance Gyri & Sulci- Adult-like appearance

Cingulate sulcus (hyperechoic line)

Sulcus of the corpus Cingulate gyrus callosum (hyperechoic line) (hypoechoic stripe)

Fissura calcarina or parieto- Corpus callosum occipital fissure (calcar avis) (hypoechoic line) MONTEAGUDO A, TIMOR-TRITSCH IE. Development of fetal gyri, sulci and fissures: a transvaginal sonographic 34 weeks study. Ultrasound Obstet Gynecol 1997;9:222-8.

Median Plane Median Plane Corpus Callosum & Cavum Septi Pellucidi Corpus Callosum & Cavum Septi Pellucidi

• The development of corpus callosum is closely associated with that of the CSP 17 weeks • There cannot be a CSP without a 11 weeks 16 weeks covering corpus callosum • However, a corpus callosum can be present in the absence of the CSP such as in septal agenesis as the result of SOD 22 weeks 25 weeks 34 weeks Best time to image is after 20 weeks

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Median Plane Median Plane Corpus Callosum & Cavum Septi Pellucidi Arteries, Veins & Sinuses Corpus callosum 22 w • The CC reaches its final adult-like • Best plane to image vessels that are in the midline appearance at around the 28 wks. • Color/power Doppler reveals both the arterial and the venous • By the 3rd trimester the CSP begins system to narrow in an occipital-to-rostral Cavum septi pellucidi

fashion; closure of the posterior 34 w part will occurs in most fetuses by Cingulate sulcus term Corpus callosum • The cingulate sulcus is seen from 24-26 wks on Cavum septi pellucidi ‘Color-Doppler’ ‘Slow-flow”

Median Plane Median Plane Corpus Callosum & Pericallosal Arteries Corpus Callosum & Pericallosal Arteries Sagittal Coronal • The presence of normal pericallosal arteries predicts normal development of the corpus callosum

Right Left PCA PCA Right PCA

Left 2/7 PCA Axial 25 weeks Paired vessels

Median Plane Median Plane Corpus Callosum & Pericallosal Arteries Pericallosal arteries at 11-13 weeks

• Before 18 weeks, using only gray scale, the corpus callosum • Several authors have may not be evident. demonstrated the presence of • However, when using color Doppler if the pericallosal artery the pericallosal arteries in the 1st is present is proof that the CC is developing trimester using 2D and 3D Power Doppler • It may be used to screen pregnancies at increased risk of AGCC Pati M et al. J Ultrasound Med 2012;31:231–237. 7 Diaz-Guerrero L et al. Fetal Diagn Ther 2013; 34:85–89. 15 6/7 weeks Conturso R et al. Fetal Diagn Ther 2015;37:305–309

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Median Plane Median Plane Pericallosal arteries at 12-25 weeks Beyond Pericallosal Arteries

5/7 6/7 12 weeks 15 17 1/7 weeks weeks

1/7 20 3/7 22 2/7 25 weeks weeks weeks

Median Plane Superior Median Plane Beyond Pericallosal Arteries- Veins sagittal& Sinuses sinus The Posterior Fossa

Internal Tentorium/Torcular cerebral v

Vermis

Fastigium 4th Ventricle

Cisterna magna Confluence Pons of the sinuses Great vein of Galen Straight sinus Transverse sinus

In Summary……. In Summary……. • Becoming comfortable with the normal anatomy of the • If an anomaly is suspected the next step is adding the brain is the key to diagnose complex brain malformations. coronal and sagittal planes; ideally obtained by TVS. • Always start with the axial planes using TAS. • The mid-coronal and the median plane are the most valuable. • Don’t hesitate to add color Doppler

Transventricular Transthalamic Transcerebellar Drawings from Bethune, et al.AJUM August 2013 16 (3)

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