쉐도잉 연습: MRI Brain Sequences - radiology video tutorial - 영상으로 영어 말하기 배우기

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Hello and welcome to the Radiopaedia Adult Brain MRI Review Course.

I'm Frank Gaylord and I'll be presenting the majority of the content during this course.

This pre-course video is really an introduction to make sure

that we're all on the same page as we'll be covering

a great deal of material during the actual live presentation and during the remainder of the videos.

A degree of familiarity with MRI sequences is expected.

And so this talk is really just to make sure that we're all on the same page.

I'm not going to be going into a great deal of detail into any of the sequences, and in fact we won't be touching on any of the physics,

but just to ensure that we understand the appearance of all of the main ones and the terminology used.

There are a great number of MRI sequences, so much so that I think in many ways it's easier to consider MRI.

a collection of modalities because the differences between different sequences is almost as large, if not larger, than between some different modalities.

So when we talk about sequences, there's no single way of grouping them or of discussing them.

But a useful one is to think about what the dominant weighting or the dominant characteristic that's been examined.

And then there's a number of modifiers applied to each of these.

So let's start with the main two that we are used to thinking about.

T1 weighted and T2 weighted.

These are the two fundamental anatomical sequences upon which many other sequences are built.

And different tissues have different appearances depending on which sequence is chosen.

They are characterized by specific TR and TE intervals, with T1 being short and short and T2 being long and long.

And this is their typical appearance.

The easiest way to identify a T1-weighted sequence is to examine the relationship between white matter and grey matter.

where the white matter is whiter than the grey matter.

So they match.

The white matter is white and the grey matter is grey.

In contrast, T2-weighted sequences have the opposite relationship with the grey matter being more hyper-intense compared to the white matter,

which is really quite dark.

And yes, of course, the CSF is bright on T2 and dark on T1, but as we'll see shortly, that's not always the safest way of identifying them.

A number of substances are hyper intense on T1, with the ones most relevant to the brain being blood product,

fat, some protonaceous material and contrast enhancement.

Proton density is an intermediate sequence characterized by a long TR

but a short TE and has appearances that were very useful in the early era of MRI in examining white matter

making white matter lesions easier to identify

than T2 or T1 The role of proton density has largely been superseded by flare imaging or fluid attenuated inversion recovery

to the point that very rarely is a proton density performed in modern day protocols.

So essentially we can forget about proton density, brain imaging, as of being only of historical value.

It's important to note however that proton density as a sequence is still alive

and well in other parts of the body and is particularly useful in musculoskeletal.

So if we take T1 and T2 weighted imaging we can see the first ways that we can modify these images.

The first is to add gadolinium and then perform T1 weighted sequences.

In this case where we have a thalamic primary brain tumor with some hemorrhagic change which has intrinsic high T1 signal,

we can see that another component of it demonstrates contrast enhancement.

The most common modifier for T2 weighted images is fluid attenuation,

usually referred to as FLARE for fluid attenuated inversion recovery.

And here we see this same tumor standing out more brightly against the white matter

and adjacent brain parenchyma now that the CSF within the ventricles has been attenuated.

Note that despite the attenuation of a CSF such that it is dark, similar to the appearance on T1,

the relationship of the white matter to the grey matter has remained that of a T2 weighted sequence.

And this is why this relationship is more important and an easier way to identify a T1 versus a T2 weighted sequence.

Otherwise, a flare sequence can be mistaken for a T1.

The next most important way that T1 images can be manipulated is to apply fat suppression.

This can be done both on pre-contrast or on post-contrast images.

Here we have a case of a pericolosal lipoma with a large amount of very hyper-intense fat seen in the midline.

When we perform contrast, it is a little bit difficult to tell whether there is contrast enhancement at the margins

or whether this represents merely fat.

the slice position is not always the same

so exact comparison is difficult one could assume that this is enhancement

but one may be just seeing the result of slightly different positioning

so what we can perform is fat saturation or fat suppression

where only fat loses signal and contrast remains high signal

and here we can see

that at the margins of this tumor there's only very minimal contrast enhancement with the bulk of the tumor completely attenuating out.

Note that in the scalp it's a subcutaneous fat

that becomes dark with the scalp remaining hyper-intense post-contrast due to the administration of gadolinium.

Fat-saturated post-contrast T1 sequences are routine in most parts of the body because of the presence of significant amounts of fat.

This is not the case in the brain where fat is an abnormal substance

and as such for purely intraparenchymal lesions fat saturation is usually not performed.

The exception to this is intracranial masses that are involving the skull

or skull base where an extra perennial extension is being sought.

So the most common situation for this to be performed would be a base of skull meningiomas

or cerebellopontine angle masses where potential for extracranial spread is present.

Moving on to T2 weighted sequences, fat suppression can be performed as part of a gradient echo

sequence where the intention is not particularly to fat suppress but to make it more susceptible to paramagnetic effects.

And we'll talk about that more in a second.

Or more commonly in the orbit or base of skull, to examine structures that are otherwise closely related to fat.

This would be commonly the case in the orbits where the extraocular muscles and optic nerve are surrounded by fat.

and thus examining for abnormal signal within either of those structures is much more easily performed with the attenuation of fat.

The situation in intracranial imaging that this would be useful for would be to look for a CSF leak, for example.

Susceptibility-weighted sequences represent a number of different sequences that share the propensity to have signal loss due to paramagnetic or diamagnetic effects.

So calcium or blood product will result in dark signal or black signal.

The newer sequences such as SWI, susceptibility weighted imaging, are exquisitely sensitive to very small amounts of such materials.

In this example of a patient that has familiar autosomal dominant multiple cavernoma syndrome,

we can see numerous large black areas.

If you were to look at T1 or T2 weighted sequences, these abnormalities would be much smaller than the ones here.

And this phenomenon is called blooming, where the signal loss extends beyond the anatomical confines of the lesion.

And this is due to the fact that paramagnetic or diamagnetic materials distort the magnetic field locally beyond their margins.

Susceptibility-weighted imaging at higher field strengths is particularly sensitive.

An earlier and still widely used susceptibility-sensitive sequence is gradient echo imaging, but there are a number of others.

They probably deserve their own column, but as I'm going to run out of room, we're going to put them under the T2 weighted column, as many of these are T2 star weighted.

The next column is diffusion weighted imaging, which really encompasses two main sequences.

What we commonly refer to as DWI, or diffusion weighted imaging, or isotropic imaging, or T2 weighted imaging are different synonyms for this,

and ADC, or apparent diffusion coefficient.

The DWI sequence is really quite a dirty sequence made up of both true diffusion information and T2 information.

And examining the DWI on its own can lead you to erroneously interpret high signal as representing true abnormal restricted diffusion,

when in fact what you're seeing is so-called T2 shine-through.

The underlying principles as to why T2 shine-through exists and the exact relationship between DWI and ADC is well beyond this talk,

but I will be creating a separate talk to just examine DWI

because I think it's a very important and often misunderstood sequence.

An extension of diffusion is diffusion tensor imaging which allows tractography

whereby the fact

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that water molecules' motion is restricted along white matter tracks enables

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the tracking of white matter tracks from one part of the brain to the other

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and here we can see white matter tracts crossing from one

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frontal lobe across the corpus callosum to the other frontal lobe as well as extending posteriorly.

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Although tractography is relatively frequently performed in academic institutions, it's still not really mainstream.

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Its role is largely research based but it is finding increasing roles in operative planning.

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Moving on to our next column of types of sequences, we're moving to flow sensitive sequences.

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These encompass MR angiography which is usually performed without intravenous contrast and relies on blood bringing with it signal.

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These can be shown just as a stack of very thin images and

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because there is little background for the vessels to be localized

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against it can be difficult to know exactly where one is on a single image.

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Thus often these images are shown as MIPS in this case of a young patient with a vein of galen malformation, and these can of course be surface shaded.

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MR venography can be performed similarly to MR angiography or also by other techniques such as phase contrast,

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and can be used to image dural venous sciases and cerebral veins.

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Here we can see thrombosis or occlusion of the posterior part of the superior sagittal sinus.

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And the same principles as phase contrast venography can be employed

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to look at the pulsatile flow of CSF in the cisterns and the aqueduct as shown here.

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This can be particularly useful if aqueduct stenosis needs to be excluded

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or if hyperdynamic flow of normal pressure hydrocephalus needs to be evaluated.

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And on to the last column of miscellaneous sequences which includes MR spectroscopy

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performed very routinely as part of particularly brain tumour or mass workup.

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A functional MRI where we can image specific parts of the brain being activated during motor or verbal or memory tasks.

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And lastly, MR perfusion, which has really become now routine in assessment of tumours and neurodegenerative conditions.

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So that rounds out the different sequences that we have available to us in performing imaging of the brain and intracranial content.

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Some of these, however, are not used terribly frequently and have only very specific applications,

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particularly fat-suppressed T2, where most of the time this is performed for extracranial pathology,

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DTI tractography, CSF flow studies,

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and functional MRI, which only have very specific indications and most frequently performed in academic centres only.

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If you want to get into a little bit more about different types of sequences, this is an excellent radiographics article which you can access by following this bit.ly link.

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Thanks for your attention and I look forward to meeting many of you in person and many more of you online shortly.

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We believe very strongly that access to quality medical education

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should not be restricted by your personal wealth or that of your institution or that of your country.

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And so that is the reason why we have created these courses and are making them available for free in 115 countries.

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So I would like to thank all of the rest of you for supporting us and for making these viable.

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