Brain





GROSS ANATOMY


Supratentorial Structures




  • •

    Gyri : Complex convolutions of brain cortex; hypoechoic on US


  • •

    Sulci (fissure): Cerebral spinal fluid (CSF)-filled grooves or clefts that separate gyri; echogenic on US



    • ○

      Sulci separate gyri, fissures separate hemispheres/lobes



  • •

    Frontal lobe



    • ○

      Central sulcus separates frontal, parietal lobes


    • ○

      Precentral gyrus contains primary motor cortex



  • •

    Parietal lobe



    • ○

      Posterior to central sulcus


    • ○

      Separated from occipital lobe by parietooccipital sulcus (medial surface)


    • ○

      Postcentral gyrus: Primary somatosensory cortex



  • •

    Occipital lobe



    • ○

      Posterior to parietooccipital sulcus


    • ○

      Primary visual cortex



  • •

    Temporal lobe



    • ○

      Inferior to sylvian fissure


    • ○

      Primary auditory cortex


    • ○

      Middle temporal gyrus: Connects with auditory, somatosensory, visual association pathways



  • •

    Insula



    • ○

      Lies deep in floor of sylvian fissure, overlapped by portions of frontal, temporal, & parietal lobes, called the opercula



  • •

    Limbicsystem



    • ○

      Includes amygdala, hippocampus, thalamus, hypothalamus, basal ganglia, & cingulate gyrus



      • –

        Cingulate gyrus is directly above & parallels the corpus callosum



    • ○

      Important role in emotion, behavior, & long-term memory



  • •

    Corpus callosum is white matter tract, which links cerebral hemispheres



    • ○

      Genu is front-most portion; the body arches over the cavum septi pellucidi, ending at the splenium above vermis



      • –

        Rostrum is short posterior extension from inferior portion of genu




  • •

    Basal ganglia are paired deep gray matter structures, including the caudatenuclei


  • •

    Thalami are paired, large nuclear complexes, which act as relay stations for most sensory pathways



Posterior Fossa (Infratentorial) Structures




  • •

    Protected space surrounded by calvarium



    • ○

      Bounded by tentorium cerebelli superiorly (an extension of the dura mater similar to falx) & foramen magnum inferiorly (where spinal cord exits skull)



  • •

    Posterior fossa contents



    • ○

      Brainstem (midbrain, pons, & medulla oblongata) anteriorly, cerebellum posteriorly


    • ○

      Cerebral aqueduct & 4th ventricle


    • ○

      CSF cisterns containing cranial nerves, vertebrobasilar arterial system, & veins



  • •

    Cerebellum



    • ○

      Integrates coordination & fine-tuning of movement & regulation of muscle tone


    • ○

      2 hemispheres & midline vermis



      • –

        Vermis divided into lobes & lobules by multiple fissures



        • □

          Appears highly echogenic on US



      • –

        Hemispheres have thin, curved gyri called folia



    • ○

      Connected to brainstem by 3 paired peduncles



  • •

    Brainstem



    • ○

      3 anatomic divisions



      • –

        Midbrain (mesencephalon) : Upper brainstem; connects pons & cerebellum with forebrain


      • –

        Pons : Bulbous midportion of brainstem; relays information from brain to cerebellum


      • –

        Medulla : Caudal (inferior) brainstem; relays information from spinal cord to brain





Ventricular System & Subarachnoid Space




  • •

    Cerebral ventricles consist of paired, lateral, midline 3rd & 4th ventricles


  • •

    Communicate with each other as well as central canal of spinal cord & subarachnoid space


  • •

    Direction of CSF flow



    • ○

      Lateral ventricles → foramen of Monro → 3rd ventricle → cerebral aqueduct (of Sylvius) → 4th ventricle → foramina of Luschka & Magendie → subarachnoid space


    • ○

      Bulk of CSF resorption through arachnoid granulations in superior sagittal sinus



  • •

    Lateral ventricles



    • ○

      Paired, C-shaped structures, which arch around/above thalami


    • ○

      Each has body, atrium, 3 horns (frontal, temporal, & occipital)



      • –

        Occipital horn typically largest


      • –

        Asymmetry is common, often L > R


      • –

        Sizes change with maturity, more prominent in preterm infants



    • ○

      Atrium/trigone : Confluence of horns



      • –

        Contains glomus (thickened portion) of choroid plexus



    • ○

      Lateral ventricles communicate with each other & 3rd ventricle via Y-shaped foramen of Monro



  • •

    3rd ventricle



    • ○

      Thin, usually slit-like, between thalami



      • –

        May not see fluid, just bright echogenic line on US



    • ○

      80% have central adhesion between thalami ( massa intermedia )


    • ○

      There are 4 small recesses: 2 projecting anteriorly (optic & infundibular recesses) & 2 projecting posteriorly (suprapineal & pineal)


    • ○

      Communicates with 4th ventricle via cerebral aqueduct (of Sylvius), passing through dorsal midbrain



  • •

    4th ventricle



    • ○

      Infratentorial, diamond-shaped cavity between brainstem & vermis


    • ○

      Fastigial point : Blind-ending, dorsally pointed midline outpouching from body of 4th ventricle



      • –

        Important marker for true midline vermian plane on US



    • ○

      CSF exits 4th ventricle into subarachnoid space via foramina of Magendie (midline) & Luschka (lateral)


    • ○

      Inferiorly communicates with central canal of spinal cord



  • •

    Choroid plexus



    • ○

      Produces CSF


    • ○

      Glomus (enlargement of choroid plexus in atrium) thickest area


    • ○

      Tapers & extends anteriorly to foramen of Monro & roof of 3rd & 4th ventricles


    • ○

      Tapers laterally into roof of temporal horns


    • ○

      Never extends into frontal or occipital horns



  • •

    Subarachnoid space/cisterns



    • ○

      CSF-containing spaces around brain


    • ○

      Numerous trabeculae, septa, membranes cross subarachnoid space & create smaller compartments termed cisterns



      • –

        Cisterna magna is large cistern in posterior fossa



    • ○

      All cisterns communicate with each other & with ventricular system



  • •

    Midline cystic structures (normal variants)



    • ○

      Cavum septi pellucidi



      • –

        Anterior to foramen of Monro, between anterior horns of lateral ventricles


      • –

        85% closed by 3-6 months after birth, but some remain open into adulthood



        • □

          Once closed called septum pellucidum




    • ○

      Cavum vergae



      • –

        Posterior to foramen of Monro, interposed between bodies of lateral ventricles


      • –

        Posterior extension of cavum septi pellucidi ( cavum septi pellucidi et vergae )


      • –

        Begins to close from posterior to anterior from 6-months gestation; 97% closed by full term



    • ○

      Cavum velum interpositum



      • –

        Potential space, which may accumulate CSF, above choroid in roof of 3rd ventricle & below fornices


      • –

        Typically seen in premature infants





ANATOMY IMAGING ISSUES


Imaging Approaches




  • •

    All scanning should be performed keeping the exposure as low as reasonably achievable, the ALARA principle


  • •

    Scans should be performed using a small footprint, high-frequency transducer, which allows sufficient penetration to see deep structures



    • ○

      Use linear high-frequency transducer for evaluating superficial structures, such as extraaxial fluid spaces (e.g., subdural hematoma) or superior sagittal sinus (e.g., thrombosis)



  • •

    Use Doppler (color, spectral, &/or power) as needed to evaluate vasculature structures



    • ○

      Always use to confirm an anechoic structure is truly a cyst & not a vascular malformation



  • •

    Anterior fontanelle most commonly used approach


  • •

    Coronal scans



    • ○

      Complete sweep from front to back documenting key landmarks



      • –

        Begin anterior to frontal horns & extend to posterior to occipital horns, ensuring entire brain has been covered



        • □

          May need to tilt transducer laterally to include superficial peripheral surfaces



      • –

        Adjust transducer position as needed to keep image symmetric side to side


      • –

        Adjust depth to include posterior fossa, including cerebellar hemispheres & cisterna magna



    • ○

      Symmetrical structures (from anterior to posterior) include frontal horns, bodies, trigones & occipital horns of lateral ventricles; caudate nuclei & thalami



      • –

        Foramina of Monro seen extending inferomedially into 3rd ventricle creating a Y shape



        • □

          Just posterior to this will be 3-dot sign : Choroid plexus on floor of lateral ventricles & roof of 3rd ventricle


        • □

          Choroid plexus does not extend anterior to this point; echogenic material anterior to this would represent hemorrhage




    • ○

      Midline structures include interhemispheric fissure, genu & body of corpus callosum, cavum septi pellucidi, 3rd ventricle, vermis, & 4th ventricle



  • •

    Sagittal scans



    • ○

      Midline scan: Best view for corpus callosum, cerebellar vermis, & 4th ventricle


    • ○

      Sweep side to side from this position documenting key areas



      • –

        Caudothalamic groove : Site of germinal matrix (highly vascular area from which cells migrate during brain development)



        • □

          Most common site of hemorrhage in premature infants



      • –

        Size of lateral ventricle


      • –

        Far lateral to assess degree of sulcal development




  • •

    Posterior fontanelle



    • ○

      Best view to evaluate occipital horns for intraventricular hemorrhage



      • –

        Can misinterpret clot adherent to choroid plexus from anterior fontanelle approach alone


      • –

        Use color Doppler to confirm flow in choroid plexus




  • •

    Mastoid fontanelle



    • ○

      Located at junction of squamosal, lambdoidal, occipital sutures


    • ○

      Transducer placed about 1 cm behind helix of ear & 1 cm above tragus


    • ○

      Allows assessment of brainstem & posterior fossa


    • ○

      Best view for 4th ventricle, posterior cerebellar vermis, cerebellar hemispheres, & cisterna magna



  • •

    Transtemporal



    • ○

      Temporal bone anterior to ear is thin, allowing imaging of brainstem even after sutural closure


    • ○

      Best view for cerebral peduncles & 3rd ventricle




Imaging Pitfalls




  • •

    Need to understand the normal development & changing appearance of the brain as it matures


  • •

    Must know not only age in days/weeks of infant but gestational age at birth



    • ○

      Normal gyral pattern in 26-week preterm infant would be abnormal in term infant



  • •

    Slit-like lateral ventricles common in infants, not to be mistaken for cerebral edema


  • •

    Glomus of choroid plexus can be bulbous & irregular, not to be mistaken for blood clot



    • ○

      Evaluate with color Doppler & posterior fontanelle view



  • •

    Echogenic material in frontal or occipital horns is clot; choroid does not extend into these horns



BRAIN






Graphic of the brain viewed from the side shows the 4 lobes. The frontal lobe (pink) is separated from the parietal lobe (yellow) by the central sulcus. The occipital lobe (green) is the most posterior portion of the brain, and the temporal lobe (blue) lies below the sylvian fissure. The insula lies deep to the sylvian fissure and is covered by portions of the frontal, parietal, and temporal lobes, which are referred to as the opercula.








Midline sagittal graphic shows the major structures, which should be seen on a midline US. The corpus callosum forms an arc over the cavum septi pellucidi. The cingulate gyrus has a similar arc-like configuration on top of the corpus callosum. In cases of agenesis of the corpus callosum, this configuration will be absent, and the gyri will fan out in a radial configuration, giving a sunray appearance. Several major sulci are shown, including the central, parietooccipital, calcarine, and cingulate sulci.




VENTRICULAR SYSTEM






Schematic 3D representation of the ventricular system, viewed from the side, demonstrates the normal appearance and communicating pathways of the cerebral ventricles. Cerebral spinal fluid (CSF) flows from the lateral ventricles through the foramen of Monro into the 3rd ventricle and from there through the cerebral aqueduct into the 4th ventricle. CSF exits the 4th ventricle through the foramina of Luschka and Magendie to the subarachnoid space. There are 4 small recesses (2 anterior and 2 posterior) off the 3rd ventricle that are usually too small to be resolved on US.




STANDARD US PLANES VIA ANTERIOR FONTANELLE






Graphic shows the common coronal planes used in US brain scanning. The scan should include a complete sweep from front to back and side to side with still images as indicated [cerebral cortex (CC), body of lateral ventricle (BV), frontal horn (FH), occipital horn (OH), massa intermedia (M), 3rd ventricle (3), temporal horn (TH), 4th ventricle (4), cerebellum (CB)].








Graphic shows the common sagittal planes used in US brain scanning: Planes A-C from midline to lateral [cerebellum (CB), cerebral cortex (CC), corpus callosum (Coc), cavum septi pellucidi (CSP), frontal horn (FH), foramen of Monro (FM), occipital horn (OH), temporal horn (T), 3rd ventricle (3), 4th ventricle (4)].




CORONAL US VIA ANTERIOR FONTANELLE






The 1st of 9 coronal US of the brain through the anterior fontanelle in a term infant shows the frontal lobes lie in the anterior cranial fossa with orbital cavities deep to the floor of the skull base. The sulci are echogenic, and the gyri are hypoechoic. CSF is seen in the interhemispheric fissure.








US centered more posteriorly demonstrates a slightly more echogenic white matter region of the brain parenchyma known as the centrum semiovale. Parts of the skull base, including the sella turcica where the pituitary gland is located, can be seen.








US acquired just anterior to the foramen of Monro is shown. The frontal horns of lateral ventricles are now seen. No choroid plexus should be present in the frontal horns. Any intraventricular echogenic material seen at this level should raise the suspicion of blood clot. The head of the caudate nucleus is inferior and lateral to the respective frontal horn.




CORONAL US VIA ANTERIOR FONTANELLE






US is taken at the level of the foramina of Monro, which connects the lateral ventricles to the 3rd ventricle, forming a Y shape. The caudate nucleus is slightly hypoechoic and located along the lateral inferior margin of the lateral ventricle. Some asymmetry, as seen here, is normal.








Just slightly more posterior, the choroid plexus is seen in the lateral and 3rd ventricles. They create 3 echogenic foci, 1 on the roof of the 3rd ventricle and 2 located bilaterally on the floor of the lateral ventricles, and are known as the 3-dot sign.








A more posterior coronal US shows the thalami on either side of the midline. The quadrigeminal cistern forms a star-shaped CSF structure just above the vermis in this view.




CORONAL US VIA ANTERIOR FONTANELLE






US is taken at the trigone of the lateral ventricles. The glomus of the choroid plexus appears highly echogenic, nearly occupying the whole trigone. The splenium, the most posterior portion of the corpus callosum, is seen crossing the midline and connecting the 2 cerebral hemispheres.








US, slightly posterior to the trigone, shows mildly echogenic white matter regions lateral and parallel to both trigones of the lateral ventricles. These regions are known as the periventricular halo, a normal finding, present in almost all normal mature and premature neonates. The echogenicity of the halo should be less than that of the choroid plexus and symmetrical in appearance.








The most posterior coronal US shows the cortex of the occipital lobe with multiple echogenic sulci extending medially from the lateral margin of the brain. The falx is in midline.




SAGITTAL US VIA ANTERIOR FONTANELLE






The 1st of 6 sagittal US of the brain through the anterior fontanelle in a term infant is shown. This US, obtained in the midline, shows the corpus callosum as a hypoechoic, curving line. The cingulate gyrus is above and parallels the corpus callosum. The midline also allows evaluation of the posterior fossa structures, including the brainstem anteriorly and the vermis posteriorly. The 4th ventricle is well seen in this plane and appears as a triangular, CSF-filled structure at the level of the midvermis.








Slightly lateral to midline, the thalamus is more clearly seen.








Parasagittal US obtained by angling more laterally shows the caudothalamic groove, the junction between the caudate nucleus and the thalamus. This is the area of the vascular germinal matrix, which is vulnerable to hemorrhage in preterm infants. This is a very important image to obtain when performing an exam.




SAGITTAL US VIA ANTERIOR FONTANELLE






Parasagittal US shows the glomus of the choroid plexus in the trigone. The glomus tapers anteriorly as it courses along the floor of the lateral ventricle to the foramen of Monro and continues along the roof of the 3rd ventricle. It also tapers posteriorly from the trigone into the roof of the temporal horn of each lateral ventricle. Glomus may appear bulbous and irregular at the trigone and should not be mistaken as a blood clot.








Parasagittal US is obtained just lateral to the lateral ventricle. The echogenic white matter of the brain just posterior and superior to the ventricular trigone is known as the peritrigonal blush or halo, representing radiating white fiber tracts (corona radiata). The peritrigonal blush is more prominent in premature than in term neonates.








This is the last and most lateral sagittal US, showing the mature sulcal pattern with hyperechoic sulci and hypoechoic gyri.


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Nov 10, 2024 | Posted by in ULTRASONOGRAPHY | Comments Off on Brain

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