How To Read A Brain MRI - Neuroradiology Made Easy (Maybe?)

Introduction

In this lecture, the speaker discusses how to approach a brand MRI. The lecture is aimed at radiology residents on their first neuro rotation who may not have much experience reading brain MRIs.

Understanding the Patient's Information

• Before interpreting imaging, it is important to know the patient's information and what they are looking for.

• Knowing clinical history is crucial as interpretation of imaging in vacuum without knowing clinical history can be dangerous.

• It is also important to know whether or not there's any type of comparison study; whether it's prior MRI, prior CT, or ultrasound.

Common Sequences in Brain MRI

This section covers common sequences that are usually present in a typical brain MRI.

Common Sequences

• A typical brain MRI should have an axial DWI (diffusion-weighted sequence), ADC (apparent diffusion coefficient), some type of sagittal image (usually T1, sometimes T2), axial T1, axial T2, axial T2 FLAIR.

• Axial SWI or gradient echo (GRE) may also be present.

• Post contrast sequence such as post contrast T1 typically in three planes may also be present.

Axial DWI and ADC Sequences

This section covers how to interpret the axial DWI and ADC sequences in a brain MRI.

Interpreting Axial DWI and ADC Sequences

• Axial DWI and ADC sequences are very useful for detecting acute infarction.

• When interpreting these sequences, always look at them together.

• Abnormal bright signal on DWI with corresponding hypointensity on ADC map indicates restricted diffusion which is characteristic of acute infarction.

• For acute infarction:

• First 10 days: DWI is bright and ADC is dark (restricted diffusion).

• Second week: DWI is bright, ADC is iso-intense (pseudo-normalization).

• 3 weeks and out: DWI remains bright but now ADC is bright (T2 shine through).

Other Lesions That Show Restricted Diffusion

This section covers other lesions that can show restricted diffusion in a brain MRI.

Other Lesions That Show Restricted Diffusion

• Hypercellular tumor, infection, viral encephalitis, meningitis, trauma, hemorrhage, epidermoid cysts, abscesses, active demyelination or central pontine myelinolysis can all show restricted diffusion.

• Matching signal intensity can help determine the age of the infarction.

Conclusion

This section summarizes the key points covered in the lecture.

Key Takeaways

• Always look at clinical history before interpreting imaging.

• Axial DWI and ADC sequences are very useful for detecting acute infarction.

• Abnormal bright signal on DWI with corresponding hypointensity on ADC map indicates restricted diffusion which is characteristic of acute infarction.

• Other lesions such as hypercellular tumors, infections, etc. can also show restricted diffusion.

Importance of Sagittal Images

In this section, the speaker explains why they prefer to review sagittal images early on in their analysis.

Paying Attention to Areas Not Visible on Axial Views

• The speaker pays particular attention to areas that won't be visible again on axial views.

• Examples include tongue lesions, oral cavity lesions, nasopharyngeal lesions, and oropharyngeal lesions.

• Lesions that are not detected on sagittal views may not be seen again on axial views.

Detecting Lesions in Lateral Neck Mass or Lymphadenopathy

• The speaker also tries to detect any lesion in the lateral part of the neck.

Paying Attention to Midline Structures

• The speaker pays good attention to midline structures because they can be seen very well on a midline view.

• They draw a line at the level of foramen magnum and make sure that cerebellar tonsils are positioned above it.

• Corpus callosum is an important midline structure with four parts: rostrum, genu, body, and splenium.

Other Important Commissural Fibers

• Anterior commissure and posterior commissure are other important commissural fibers that should be known when viewing midline sagittal images.

• Posterior commissure is harder to see but can be found between the pineal gland and tectal plate or quadrigeminal plate.

Anatomy of the Brain

In this section, the speaker discusses the anatomy of the brain and its different structures.

Fornix and Mammillary Body

• The fornix is located underneath the corpus callosum.

• The mammillary body sits in front of the midbrain and is connected to hippocampus via fornix.

• Ipsilateral atrophy of mammillary body may occur with hippocampal sclerosis.

Midbrain, Pons, Medulla, and Cerebral Aqueduct

• The midbrain, pons, and medulla look like a bird on sagittal view.

• The size of pons should be about twice that of midbrain and medulla on mid-sagittal view.

• Cerebral aqueduct is covered by tectal plate and connects third ventricle into fourth ventricle. Pineal gland is located above it. Obstructive hydrocephalus may occur with pineal mass.

Fourth Ventricle and Pituitary Gland

• Fourth ventricle looks like a triangle on sagittal view with roof resembling an A-frame house and floor being a straight line across. Foramen Magendie is medial outlet for CSF while paired foramina Lushka are lateral outlets. Obex marks boundary between medulla and spinal cord.

• Pituitary gland has an anterior pituitary gland and bright posterior pituitary gland called posterior pituitary bright spot on T1 weighted sequence or neurohypophysis. Optic chiasm sits right above posterior pituitary gland which can be ectopic in some cases causing visual issues due to compression of optic chiasm.

• Large tumors like pituitary macroadenoma can cause compression of optic chiasm leading to visual issues.

T1 Weighted Image

• T1 weighted image provides anatomical detail with gray matter appearing gray and white matter appearing white. CSF is dark and fat is bright on T1 weighted sequence.

• Use CSF and fat as internal references for both CT and MRI scans.

T2 Weighted Sequence

• Gray/white junctions or the gray/white matter intensity is reversed on T2 weighted sequence with gray matter appearing white and white matter appearing darker than the gray matter.

T1 and Anatomy Overview

In this section, the speaker discusses the importance of T1 in identifying certain pathologies and structures. They also provide an overview of key anatomical structures that are important to know.

T1 Pathology Identification

• Fat is bright on T1, while blood (specifically methemoglobin), melanin, and gadolinium are also bright on T1.

• Knowing what lesions or structures are bright on T1 can help with differential diagnosis.

• A cyst that is bright on T1 may be hemorrhagic or have high protein content. Calcium/mineralization in the basal ganglia is also bright on T1.

Anatomy Overview

• The centrum semiovale is a large chunk of white matter sitting above the ventricle and above the corona radiata. Below that, at the level of ventricles and below, you'd see corona radiata, that's the fan-shaped white metal track that funnels into the internal capsule.

• The caudate nucleus is gray matter with similar signal intensity as an overlying cortex. Further down you'd see the head of caudate nucleus or caudate head which is separated from putamen and globus pallidus by internal capsule. Caudate head, putamen, and globus pallidus are collectively known as basal ganglia; putamen and globus pallidus are known as lentiform nucleus.

• Other important structures include insular cortex, genu and splenium of corpus callosum, anterior commissure and posterior commissure, interthalamic adhesion, and massa intermedia.

• Important cisterns to know include the suprasellar cistern, interpeduncular fossa, mammillary bodies, quadrigeminal cistern, and ambient cisterns.

Understanding Cisterns and Basic Anatomy

In this section, the speaker discusses the components of cisterns and their importance. They also cover basic anatomy related to the optic tract, oculomotor nerve, trigeminal nerves, cerebellum, and cerebellar tonsils.

Components of Cisterns

• The ambient cistern can be compressed by transtentorial herniation.

• It is important to know the components of cisterns and what they look like.

• The pre-chiasmatic optic nerve crosses at the optic chiasm.

• The optic tract extends beyond the chiasm.

Basic Anatomy

• The oculomotor or third cranial nerve comes from the interpeduncular fossa and goes towards the orbit.

• Compression of the third nerve due to uncal herniation can cause a blown pupil.

• The trigeminal nerves are located in the pre-pontine cistern at the level of pons.

• Meckel's cave is where trigeminal nerves extend into CSF space.

• CAP or cerebellopontine angle cistern contains internal auditory canals for seventh and eighth cranial nerves.

• Cerebellar tonsils should sit above foramen magnum.

Posterior Fossa Stuff

This section covers basic anatomy related to posterior fossa structures such as vermis, cerebellum, superior/middle/inferior cerebellar peduncles, foramen Lushka/Magendie, and cerebellar tonsils.

Posterior Fossa Anatomy

• Vermis is located at midline while cerebellar hemispheres are on each side.

• Superior/middle/inferior cerebellar peduncles connect brainstem and cerebellum.

• Foramen Lushka/Magendie allow CSF to escape from ventricles into subarachnoid space.

• Cerebellar tonsils should sit above foramen magnum.

Understanding FLAIR and T2 Sequences

This section covers the basics of FLAIR and T2 sequences, including their differences and how to interpret them.

FLAIR and T2 Sequences

• FLAIR stands for fluid attenuated inversion recovery, which is a T2-weighted sequence with CSF suppression.

• Gray matter is gray and white matter is white in T1 while gray matter is white and white matter is darker in FLAIR.

• T1 weighted FLAIR can exist but it's typically referred to as "T1".

• When people refer to FLAIR, they typically mean T2/FLAIR.

FLAIR and T2 Sequences

In this section, the speaker explains the importance of FLAIR and T2 sequences in detecting pathology in MRI scans.

FLAIR Sequence

• FLAIR is extremely sensitive to pathology. It can be thought of as the "pathology sequence".

• The reason for using FLAIR is that CSF is everywhere, not just in the ventricles but also in the sulci. Therefore, detecting a bright lesion on a bright background (T2) can be difficult.

• Any lesion that's in the background of CSF or close to ventricle is much better seen on FLAIR. However, it's not very specific.

T2 Sequence

• The first thing to look for using T2 would be any space that's filled with CSF such as ventricles and cisterns.

• Other important CSF spaces include star-shaped suprasellar cistern and cerebral aqueduct covered by tectal plate into fourth ventricle.

Examples of Pathologies Detected by FLAIR

In this section, the speaker provides examples of pathologies detected by FLAIR.

• Acute edema, old infarction with gliosis, and tumors are all bright on FLAIR but not very specific.

• Sulcal abnormality on FLAIR signal should be carefully examined as it could indicate blood, pus or cells. However, incomplete suppression could also be due to oxygen or general anesthesia during the scan.

Examples of Abnormal Sulcal FLAIR Signal

In this section, the speaker provides examples of abnormal sulcal FLAIR signal and their corresponding pathologies.

• Trauma patients with subarachnoid hemorrhage have incomplete FLAIR suppression in some sulci.

• Patients with meningitis have multiple sulci that have incomplete FLAIR suppression.

• Patients with leptomeningeal metastasis also show abnormal sulcal FLAIR signal.

Conclusion

In this section, the speaker concludes by summarizing the importance of using both FLAIR and T2 sequences in detecting pathology in MRI scans.

• The use of both FLAIR and T2 sequences is important to detect pathology in MRI scans. While FLAIR is very sensitive, it's not specific at all. On the other hand, T2 can be used to detect spaces filled with CSF such as ventricles and cisterns.

MRI Sequences and Interpretation

In this section, the speaker discusses how to interpret MRI sequences and what to look for in each sequence.

Interpreting T2-Weighted Sequence

• Look for flow void of arteries

• Check for central herniation

• Follow the flow void of vertebral arteries joined into basilar artery

• Look for M1, A1, and A2 anterior cerebral artery

• Ensure all cisterns are widely patented

Interpreting SWI or GRE Sequence

• Use SWI susceptibility weighted sequence or GRE gradient echo to look for blood product

• SWI is a more robust version of GRE

• Detect blood products but cannot determine chronicity or acuity of blood

Other Considerations

• Look at sinuses and air-filled structures such as paranasal sinuses

• Fluid should be very bright on dark background

• Loss of flow void could be due to proximal stenosis and slow flow or complete occlusion

• Look for large aneurysms

Brain Imaging Techniques

In this section, the speaker discusses different brain imaging techniques and how they can be used to detect various pathologies.

T2-Weighted Sequence Spin Echo

• This sequence is difficult to use for detecting pathology.

• Generalized cerebral atrophy and T2 signal in the white matter can be seen.

• SWI is better for detecting microhemorrhages.

Susceptibility Artifact

• Blooming artifact occurs around areas filled with air.

• Sustainability artifact is visible on SWI but not on spin echo T2.

Venous Sinus Thrombosis

• Loss of flow void indicates a problem on one side where the pathology is located.

• Abnormal signal dropout along the transverse sinus can be seen on SWI.

• The patient has venous sinus thrombosis of the transverse sinus leading into venous infarction presenting with parenchymal hemorrhage.

Post Contrast Sequences

• Post contrast T1-weighted sequence is considered the "diagnostic sequence."

• Lesions that are bright on T1 can be detected using gadolinium as part of the bright T1.

Differential Diagnosis

• Rim enhancing lesions in temporal lobe and inferior frontal lobe with surrounding edema could indicate metastasis or abscess if internal restricted diffusion is present.

• A bright lesion before contrast could lead to a mistaken diagnosis of an enhancing lesion.

Brain MRI Sequences Overview

In this section, the speaker provides an overview of how to approach brain MRI and which sequences to use.

Approach to Brain MRI

• Start with a diffusion-weighted sequence and ADC.

• Scroll through to B1000, not B0. Look for CSF that's dark. Look for any diffusion restriction that is bright on DWI and dark on ADC.

• Go to a sagittal view. Draw a line across the foramen magnum, ensure cerebellar tonsils are positioned above it, look at all midline structures, pay attention to spinal canal and spinal cord.

• Use FLAIR sequence to look for pathology and general overview. Ensure everything stays at the midline, make sure everything is symmetric; ventricles are roughly symmetric and normal in size.

• Use T2 weighted sequence to look for flow void problems such as large aneurysm or absence of flow void that could represent proximal stenosis or occlusion. Use T2 to look for paranasal sinuses, effusion, mastoid effusions and globes.

• Use SWI or GRE sequence to look for abnormal signal dropout; look for blood product.

• Look at T1 weighted sequence. Look at general anatomy, symmetry etc., pay particular attention to the brain and look for lesion that's bright on T1 before contrast so that will give you a different differential diagnosis.

Post Contrast Imaging

• If post contrast exam is done then examine images in all three planes. Look for pattern of enhancement such as parenchymal enhancement, leptomeningeal enhancement or dural enhancement. Also look for enhancement around the calvarium and outside of the brain.

• If post contrast is done in a 3D sequence such as 3D IRSPGR, this is also a very good sequence to look for venous sinuses. Make sure you do not have a filling defect.

Conclusion

• This is the speaker's general approach to brain MRI. The speaker hopes that this provides a sense or direction of where to start.