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hello and welcome to the radiopaedia

00:04

adult brain MRI review course I'm Frank Galen and I'll be

00:06

presenting the majority of the content

00:08

during this course this pre course video

00:12

is really as an introduction to make

00:15

sure that we're all on the same page as

00:17

we'll be covering a great deal of

00:18

material during the actual live

00:20

presentation during the remainder of the

00:22

videos a degree of familiarity with MRI

00:26

sequences is expected and so this talk

00:29

is really just to make sure that we're

00:31

all on the same page I'm not going to be

00:33

going into a great deal of detail into

00:36

any of the sequences and in fact we

00:37

won't be touching on any of the physics

00:39

but just to ensure that we understand

00:41

the appearance of all of the main ones

00:43

and the terminology used there are a

00:46

great number of MRI sequences so much so

00:49

that I think in many ways it's easier to

00:50

consider MRI a collection of modalities

00:54

because the differences between

00:55

different sequences is almost as large

00:58

if not larger than between some

01:00

different modalities so when we talk

01:03

about sequences there's no single way of

01:05

grouping them or of discussing them but

01:07

a useful one is to think about what the

01:09

dominant weighting or the dominant

01:11

characteristic that's been examined and

01:13

then there's a number of modifiers

01:14

applied to each of these so let's start

01:17

with the main two that we are used to

01:19

thinking about t1 weighted and t2

01:21

weighted these are the two fundamental

01:23

anatomical sequences upon which many

01:26

other sequences are built and different

01:29

tissues have different appearances

01:30

depending on which sequence is chosen

01:32

they are characterized by specific TR

01:36

and T II intervals with t1 being shorter

01:39

and short and t2 being long and long and

01:43

this is their typical appearance the

01:45

easiest way to identify a t1 weighted

01:47

sequence is to examine the relationship

01:50

between white matter and gray matter

01:52

where the white matter is whiter than

01:54

the gray matter so they match the white

01:57

matter is white and the gray matter is

01:59

gray

01:59

in contrast t2-weighted sequences have

02:03

the opposite relationship with the gray

02:04

matter being more hyper intense compared

02:07

to the white matter which is really

02:09

quite dark

02:10

and yes of course the CSF is bright on

02:14

t2 and dark on T one but as we'll see

02:16

shortly that's not always the safest way

02:18

of identifying them a number of

02:20

substances are hyper intense on t1 with

02:24

the ones most relevant to the brain

02:25

being blood product fat some

02:29

proteinaceous material and contrast

02:32

enhancement proton density is an

02:35

intermediate sequence characterized by a

02:36

long TR but a short te and has

02:40

appearances that were very useful in the

02:42

early era of MRI in examining white

02:46

matter making white matter lesions

02:48

easier to identify then t2 or t1 the

02:54

role of proton density has largely been

02:56

superseded by flare imaging or fluid

02:59

attenuated inversion recovery to the

03:03

point that very rarely is a proton

03:05

density performed in modern day

03:06

protocols so essentially we can forget

03:10

about proton density brain imaging as of

03:13

being only of historical value it's

03:15

important to note however that proton

03:17

density as a sequence is still alive and

03:19

well in other parts of the body and is

03:21

particularly useful in musculoskeletal

03:24

so if we take t1 and t2 weighted imaging

03:27

we can see the first ways that we can

03:29

modify these images the first is to add

03:32

gadolinium and then perform t1 weighted

03:35

sequences in this case where we have a

03:37

thalamic primary brain tumor with some

03:40

hemorrhagic change which has intrinsic

03:42

high t1 signal we can see that another

03:45

component of it demonstrates contrast

03:47

enhancement the most common modifier for

03:50

t2-weighted images is fluid attenuation

03:54

we usually referred to as flare for

03:57

fluid attenuated inversion recovery and

04:00

here we see this same tumor standing out

04:03

more brightly against the white matter

04:05

and adjacent brain parenchyma now that

04:08

the CSF within the ventricles has been

04:10

attenuated note that despite the

04:13

attenuation of CSF such that it is dark

04:17

similar to the appearance on t1 the

04:19

relationship of the white matter the

04:21

gray matter has remained that of a

04:23

t2-weighted

04:24

sequence and this is why this

04:26

relationship is more important an easier

04:29

way to identify a t1 versus a

04:31

t2-weighted sequence otherwise a flare

04:34

sequence can be mistaken for a t1 the

04:36

next most important a way that t1 images

04:39

can be manipulated is to apply fat

04:42

suppression and this can be done both on

04:44

pre contrast or on post contrast images

04:47

here we have a case of a pure colossal

04:49

lipoma with a large amount of very hyper

04:52

intense fat seen in the midline when we

04:55

perform contrast it is a little bit

04:59

difficult to tell whether there is

05:01

contrast enhancement at the margins or

05:02

whether this represents merely fat this

05:06

last position is not always the same so

05:08

exact comparison is difficult one could

05:11

assume that this is enhancement but one

05:13

may be just seeing the result of

05:15

slightly different positioning so what

05:17

we can perform is perform fat saturation

05:20

or fat suppression where only fat lose a

05:24

signal and contrast remains high signal

05:27

and here we can see that at the margins

05:29

of this jima there's only very minimal

05:32

contrast enhancement with the bulk of

05:34

the chima completely attenuating out

05:36

note that in the scalp it's a

05:38

subcutaneous fat that becomes dark with

05:40

the scalp remaining hyper-intense post

05:43

contrast due to the administration of

05:45

gadolinium a fat saturated post contrast

05:50

t1 sequences are routine in most parts

05:53

of the body because of the presence of

05:56

significant amounts of fat this is not

05:58

the case in the brain where fat is an

06:00

abnormal substance and as such for

06:03

purely intraparenchymal lesions fat

06:05

saturation is usually not performed the

06:07

exception to this is intracranial masses

06:09

that are involving the skull or skull

06:11

base where an extra cranial extension is

06:16

being sought so the most common

06:18

situation for this to be performed would

06:20

be a base of scalp meningiomas

06:22

or cerebral upon tyne angle masses where

06:25

potential for extra cranial spread is

06:27

present

06:28

moving onto t2-weighted sequences fat

06:32

suppression is also can be performed

06:35

this can be performed as part of great

06:37

Eco sequence where the intention is not

06:40

particularly the fat suppress but to

06:42

make it more susceptible to paramagnetic

06:44

effects and we'll talk about that more

06:46

in a second or more commonly in the

06:50

orbit or base of skull to examine

06:54

structures that are otherwise closely

06:56

related to fat this would be commonly

06:59

the case in the orbits where the

07:02

extraocular muscles and optic nerve are

07:05

surrounded by fat and thus examining for

07:07

abnormal signal within either of those

07:09

structures as much more easily performed

07:11

with the attenuation of fat the

07:14

situation in intracranial imaging that

07:17

this would be useful for would be to

07:19

look for a CSF leak for example

07:22

susceptibility weighted sequences

07:24

represent a number of different

07:26

sequences that share the propensity to

07:29

have signal loss due to paramagnetic or

07:31

diamagnetic effects so calcium or blood

07:34

product will result in dark signal or

07:36

black signal the newer sequences such as

07:40

SWI susceptibility weighted imaging are

07:42

exquisitely sensitive to very small

07:44

amounts of such materials in this

07:46

example of a patient that has familial

07:49

autosomal dominant multiple Kevin OMA

07:51

syndrome we can see numerous large black

07:55

areas if you were to look at t1 or t2

07:58

weighted sequences these abnormalities

08:00

will be much smaller than the ones here

08:01

and this phenomenon is called blooming

08:04

where the signal loss extends beyond the

08:06

anatomical confines of the lesion and

08:08

this is due to the fact that

08:10

paramagnetic or diamagnetic materials

08:13

distort the magnetic field locally

08:15

beyond their margins susceptibility

08:18

weighted imaging at higher field

08:20

strengths is particularly sensitive and

08:22

earlier and still widely used

08:25

susceptibility sensitive sequences

08:28

gradient echo imaging but there are a

08:30

number of others they probably deserve

08:32

their own column but as I'm going to run

08:34

out of room we're going to put them

08:35

under the t2-weighted column as many of

08:37

these are t2 star weighted the next

08:41

column is diffusion weighted imaging

08:43

which really encompasses two main

08:46

sequences what we commonly refer to as

08:48

DWI or diffusion weighted imaging or ice

08:51

tropic imaging or t2 weighted imaging

08:55

different synonyms for this and ADC or

08:58

apparent diffusion coefficient the DWI

09:01

sequence is really quite a dirty

09:03

sequence made up of both true diffusion

09:05

information and t2 information and

09:08

examining the DWI on its own can lead

09:11

you to erroneously interpret a high

09:14

signal as representing true abnormal

09:16

restricted diffusion when in fact what

09:18

you're seeing is so-called t2 shine

09:20

through the underlying principles is to

09:22

why t2 shine through exists and the

09:25

exact relationship between DWI and ADC

09:28

is well beyond this talk but I will be

09:30

creating a separate talk to just examine

09:33

DWI because I think it's a very

09:34

important and often misunderstood

09:36

sequence an extension of diffusion is

09:41

diffusion tensor imaging which allows

09:43

tractography whereby the fact that water

09:47

molecules motion is restricted a long

09:51

white matter tracks enables the tracking

09:53

of white matter tracks from one part of

09:55

the brain to the other and here we can

09:57

see white matter tracks crossing from

10:00

one frontal lobe across the corpus

10:02

callosum to the other frontal lobe as

10:03

well as extending posteriorly

10:05

although tractography is relatively

10:08

frequently performed in academic

10:09

institutions is still not really

10:11

mainstream its role is largely

10:14

research-based but in is finding

10:16

increasing roles in operative planning

10:20

moving on to our next column of types of

10:23

sequences we're moving to flow sensitive

10:25

sequences and these encompass mr

10:29

angiography which is usually performed

10:31

without intravenous contrast and relies

10:33

on blood bringing with it signal these

10:37

can be shown just as a stack of very

10:40

thin images and because there is little

10:42

background for the vessels to be

10:45

localized against it can be difficult to

10:47

know exactly where one is on a single

10:50

image thus often these images are shown

10:53

as MIPS in this case of a young patient

10:56

with a vein of Galen malformation and

10:58

these can of course be surface shaded

11:00

there Marvin ah graffiti performed

11:02

similarly to mr angiography or

11:05

by other techniques such as

11:06

phase-contrast and can be used to image

11:10

dural venous sinuses and cerebral veins

11:13

here we can see thrombosis or occlusion

11:15

of the posterior part of the superior

11:17

sagittal sinus and the same principles

11:20

as phase-contrast in agra fee can be

11:22

employed to look at the pulsatile flow

11:24

of CSF in the cisterns and the aqueduct

11:29

as shown here this can be particularly

11:32

useful if aqueduct stenosis needs to be

11:34

excluded or if hyperdynamic flow of

11:36

normal pressure hydrocephalus and needs

11:38

to be evaluated and on to the last

11:40

column of miscellaneous sequences which

11:43

includes a mass spectroscopy performed

11:46

very routinely as part of particularly

11:49

brain tumor or mass workup a functional

11:53

MRI where we can image specific parts of

11:56

the brain being activated during motor

11:59

or verbal or memory tasks and lastly mr

12:04

perfusion which has really become now

12:07

routine in assessment of tumors and

12:12

neurodegenerative conditions so that

12:15

runs out the different sequences that we

12:18

have available to us in performing

12:21

imaging of the brain and intracranial

12:23

content some of these however are not

12:25

used terribly frequently and have only

12:27

very specific applications particularly

12:30

fat suppressed t2 where most of the time

12:33

this is performed for extra cranial

12:35

pathology DTI tractography CSF flow

12:39

studies and functional MRI which only

12:41

have very specific indications and most

12:44

frequently performed in academic centers

12:47

only if you want to get into a little

12:50

bit more about different types of

12:52

sequences this is an excellent radio

12:53

graphics article which you can access by

12:56

following this bitly link thanks for

12:59

attention and I look forward to meeting

13:01

many of you in person and many more of

13:04

you online shortly we believe very

13:06

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13:10

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13:18

and so that is the reason why we have

13:20

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13:22

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13:24

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13:27

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