3/27/2021 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 hindbrain: 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 1 3/27/2021 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) 2 3/27/2021 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 Lateral Ventricles 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 3 3/27/2021 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 4 3/27/2021 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 septum pellucidum • 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 5 3/27/2021 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 • Fourth ventricle (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 6 3/27/2021 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