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Today we're honored to welcome back Dr.

0:30

Mahesh for a physics board review in MRI. Dr.

0:34

Mahesh is a professor of radiology

0:35

and cardiology at Johns Hopkins School

0:37

of Medicine in Baltimore, Maryland,

0:39

and he's chair of the Radiation Control Committee President

0:43

of A A PM board, member of a CR subject matter expert

0:47

for U-N-I-A-E-A and an elected member of NCRP

0:51

and ICRP questions will be covered at the end

0:54

of time allows, so please remember to use that q

0:56

and a feature to submit your questions.

0:59

With that, we're ready to begin today's board review. Dr.

1:02

Mahesh, please take it from here.

1:05

Wonderful. Welcome to all those who are preparing for exam

1:09

and um, I'm gonna try to cover as many po basic concept

1:14

as possible for MRI.

1:16

It's not something we can finish in one hour,

1:18

but I will try to do the best for this board review.

1:21

So let me start off with the first question.

1:24

First question is electromatic ways travel

1:27

at the speed of sound.

1:35

Good. It's false

1:36

because sound, uh, travels much slower electromagnetic ways.

1:40

Below, below is the diagram showing the energy spectrum,

1:44

which are different components

1:46

of the electromatic spectrum includes um, gamma rays,

1:50

x-rays, light waste, um, and so hold on second

1:54

and also infrared and microwave.

1:56

They all travel at the same speed

1:58

of light E equals mc square formula still

2:01

works for any of them.

2:03

They're only differentiated by the valent and the frequency

2:06

and you characterize different things here.

2:09

Next question. Hydrogen protons are used in MRI

2:13

because of their abundance

2:21

answer is true.

2:23

They are the most abundant found in the body

2:25

and that's what we use for all our MRI imaging.

2:29

Now the next question is which of these has the highest

2:33

gyromagnetic ratio?

2:41

The answer is majority 52 has percent hydrogen

2:45

and the other has fluorine, oxygen, sodium.

2:48

So, okay, here's the answer. Answer is hydrogen.

2:52

But now the question is what is tic ratio?

2:56

And that leads to the fundamental equation in MRI,

3:00

which is called the equation,

3:03

which basically tells the frequency of the precision

3:06

of the atoms or the proton in a presence

3:09

of a magnetic field is directly proportional

3:12

to the magnetic field strength.

3:15

And that the proportionality

3:17

constant is called a magnetic ratio.

3:21

And this in interesting pattern is RO ratio is unique

3:25

as a unique signature to each of the proton

3:29

or each of the enemies.

3:31

In fact, that is so essential.

3:33

One can compare similar

3:34

to atomic number in the periodic table,

3:37

which differentiate different elements.

3:39

The atomic number, similarly,

3:41

each element has its own unique matic ratio.

3:46

And if you look in here, hydrogen, one

3:48

of the most abundantly found element in the body,

3:51

which whose proton is what we are.

3:53

Magna imaging has about 42.6 megahertz

3:58

per per Tesla, which means for, for in a, in a,

4:02

if the magnetic field strength is measured in terms

4:06

of the Tesla, while it is not the uh, so

4:09

hydrogen has 42.6 megahertz per Tesla.

4:13

So therefore if it is,

4:15

if the hydrogen atoms are protons are placed in the 1.5

4:19

Tesla machines, that matic ratio is 64 megahertz.

4:23

Whereas if it is in a three Tesla lab,

4:25

that ratio doubles this 1 28.

4:28

This is important because the proton precision frequency is

4:33

what allows, how the resonance phenomena occur

4:36

to transfer the energy.

4:38

And that is first one of the basic aspect of um,

4:42

uh, MRI imaging.

4:44

So you can see here we also use a term called

4:47

angular precision frequency

4:49

or just frequency acquisition that is basically is equal

4:52

to gamma, the RO ratio divided by the two pi and,

4:56

and multiplied by this uh, um, V dot,

5:00

which is the main magnetic field.

5:02

That's why we are going into higher

5:04

and higher magnetic strength

5:05

because that will create greater

5:08

and greater strongest signals

5:09

because the matic ratio of any element, uh, which we want

5:14

to image is proportional to the

5:17

the magnetic static magnetic field.

5:20

So let me show you, here are an example.

5:22

These are the elements, different elements

5:25

and then natural abundance, um, in our body

5:30

or in uh, natural abundance in nature as you can see here.

5:33

And their matic rest ratio also as follows.

5:37

So you can see here these are fundamental elements of

5:40

of the human body like hydrogen, carbon, oxygen, fluorine,

5:44

sodium and phosphorus.

5:45

They all are different, uh,

5:47

and they have different amount found in nature,

5:50

natural abundance you can see.

5:52

But their matic ratio also is varies like this.

5:56

That's why you might see, you heard about um,

5:59

sodium imaging, phosphorus imaging,

6:01

but it becomes a little bit more trickier and challenging

6:04

because their matic ratio is

6:07

smaller compared to the hydrogen.

6:09

And that's where the proportionality majority

6:11

for MRI done is based on the hydrogen proton imaging,

6:15

the hydrogen proton, uh,

6:17

associated in the body and so forth.

6:21

This is a follow up question.

6:22

Is a precision frequency of elements decrease

6:26

with increasing mantic field strength,

6:34

okay?

6:35

Right, because the precision frequency decreases with

6:39

what is false because the precision frequency is directly

6:43

proportional to magnetic field strength

6:45

and it's proportional to the gamma matic ratio.

6:48

Therefore, when you increase the magnetic field strength

6:52

corresponding elements,

6:53

precision frequency will also increase.

6:56

And as shown here is like hydrogen has a precision frequency

7:00

of 21.29 megahertz in,

7:04

in a 0.5 Tesla field compared to three tesla

7:08

is almost six times more.

7:10

More number, more number greater of precision frequency.

7:14

Precision frequency it's is rotating around

7:17

and that's in three Tesla lab

7:18

and that has an implication of creating the images.

7:21

Similarly for phosphorus shown here, all these elements

7:25

proportionally they increase

7:27

with higher magnetic field strength.

7:30

Next question, which of the tissues

7:34

among the following has the shortest T one relaxation time?

7:45

The answer is white matter, majority

7:47

of you got the right answer.

7:49

So now what is T one?

7:51

T one is called spin lattice relaxation time

7:54

and that is defined as follows

7:56

and T one are relaxation constant is very specific

8:01

to each tissue square image.

8:03

So imagine like this, this is the growth

8:06

of the magnetization versus the time

8:09

and how it grows back to the normal.

8:12

So imagine this is the best analogy I can tell is like,

8:15

let's imagine a a kindergarten

8:17

or elementary school running race.

8:20

So you have 10 kids lined up in the starting point

8:23

and they had to go to one end point and come back.

8:26

That's about 50 meter come back.

8:28

This is exactly the same thing you can do when you place the

8:32

body in a magnetic field.

8:34

All the hydrogen proton, all the are lining out, lining up

8:38

with a man magnetic field.

8:41

So now you,

8:42

you push this into the opposite hundred 80 degrees

8:45

that is called we had the radio frequency pulse sequence.

8:48

You push this to the 80 degree

8:50

and then wait for them to come back the relaxation

8:53

of these protons to come back to the normal magnetic field

8:57

that is same as I ask the kids to run

9:00

for a distance and come back.

9:02

So it's almost like you're pushing

9:04

and R frequency is like a turning on the race

9:07

and having the kids run to some point and then coming back.

9:10

While coming back.

9:12

You see that each

9:13

of the kids will reach the undercover the starting point

9:17

differently depending on how strong they're running race

9:20

or how strong their strength is to reach back.

9:23

That's exactly the same thing.

9:25

Different proton of different tissues have different T one

9:29

will relax differently

9:30

and that's exactly how this growth of this T one happened

9:35

and that is very much characteristic of a specific tissue.

9:38

So the T one relaxation constant is defined

9:42

as the time it takes for the longitudinal magnetization

9:47

to grow back about 63% of its funnel value.

9:51

That is 63% comes because of the one hour E principle.

9:55

So the formula is given here, this is the main magnet field

9:59

and this is a magnetization in the Z direction

10:02

how it grows back and depends on the main magnet field

10:06

multiplied by the one minus the exponential

10:09

and this exponential term is called a T one

10:12

and that's what this exponential 1 33 comes to.

10:16

So, so if you picture in your mind the running race of kids

10:19

and coming back from end point

10:21

and you can see different kids are running are either the

10:24

running race, are the swimming pool,

10:26

how they come back depending on the strength

10:29

and that's exactly how the tissues relaxes back means each

10:33

of these tissues have different T one state

10:35

and you want to exploit that information to create an image

10:40

what the fundamental of MRI imaging.

10:43

So here is an example.

10:45

So here images obtained at a time when the relaxation curs

10:49

are widely separated in this curve.

10:52

This is a longitudal magnitude magnetization thus in the

10:56

direction of the magnetic field strength.

10:58

This is a time and if only in the, if at this point,

11:01

if you lie at run across this at this time point,

11:06

if you capture each of the signal that is

11:08

what is called the T one weighted contrast

11:11

because that has the maximum difference

11:13

between different tissue.

11:15

If you capture the signal at this point

11:17

they almost all the same.

11:19

That doesn't really matter.

11:20

You can't differentiate

11:21

between the gray matter white matter or the CSF.

11:25

Historically what we are natural naturally we see white

11:28

matter has a very short T one means they come back very fast

11:33

and that's called that and result in a wider lighter pixel.

11:37

In this image the CSF has a longer T one means there's slow

11:42

runner, they're coming slowly coming back to the end point.

11:45

Therefore if you point this area here,

11:47

the C one has a darker pixel

11:50

and the gray matter lies in between.

11:52

So it all depends on

11:54

what time point we are collecting the signals in the

11:57

receiver coil and create an image.

11:59

And the ideal place is when the, when the difference

12:02

between the tissues is the greatest one.

12:04

And that's exactly

12:05

how they define T one weighted contrast is maximized

12:10

depending on what time we're differentiating.

12:12

If you are collecting the information,

12:14

creating an image at this point,

12:16

one can easily differentiate between the white matter

12:19

way matter and the CSF.

12:21

So in this question CT one relaxation um, is CSF.

12:26

Since CSF has a longer T one, it creates a darker signals

12:31

compared to the gray matter and white matter.

12:34

Now here is a table showing you a bunch of tissues,

12:38

which we are interested in imaging

12:41

and they corresponding T one value

12:44

that a larger relaxation time

12:46

and how does that change with respiratory field strength?

12:50

And this is an example I given 0.5 tesla versus 1.5 Tesla

12:55

and now the three test is even more greater.

12:57

So each tissues have a longer relaxation time when the

13:03

magnitude field is stronger and stronger.

13:05

So it'll give a little bit more time to collect

13:07

and create any signal properly.

13:09

Whereas the T two is a spin,

13:11

spin relaxation I will tell you a little bit more.

13:14

Next one is much shorter, smaller here.

13:17

So this is the swimming pool

13:20

or the kids running back from the end point

13:22

how they differentiate because the strength

13:24

and that's how we image different tissues are labeled into

13:28

different gray scale.

13:30

And you can see here BR CSF has hundred

13:34

1800 millisecond.

13:36

That's wow, that's about 1.8 seconds of time compared

13:40

to adipose tissue, which is about a very short one

13:44

or a white matter is about 500 and so forth.

13:47

Now, which tissue among the following has the longest

13:52

T two relaxation type.

13:54

Now it's T two we're talking about.

14:02

So now again it's mentioned is here as CSF,

14:05

which is a longer T two.

14:06

So answer is TCSF is correct.

14:09

And now let's define what is T two

14:12

T two weighted contrast is a T two is it is it is a spin,

14:16

spin relaxation when the proton are spinning

14:19

around the matter how the D phase is

14:21

what is defined as the T two.

14:23

So the T two will always be shorter than T one.

14:26

That's by guarantee of any T tissue.

14:28

So here you can see here

14:30

with your time point it is almost opposite.

14:33

CSF goes back very so uh, CSF has a long

14:38

T two, it defaces defaces slowly compared, sorry,

14:45

yes, I apologize.

14:49

So here this is called the transverse magnet magnetization,

14:53

whereas in this one we talked, uh,

14:56

longitudinal magnetization.

14:58

Longitudinal magnetization took C one longer T one.

15:01

That's why 'cause darker whereas in transfer to T two,

15:04

if you look at here, T two in the transverse magnetization

15:09

CSF takes much longer to um, uh, reface

15:14

compared to gray matter and white matter.

15:16

And because it's going slow longer,

15:19

that CSF has a longer T two and defaces slowly

15:23

and whereas white matter short T two and defaces quickly

15:28

and so forth, as you can see here.

15:30

So the image obtained at a point when the relaxation curves

15:33

or widest is called the T two weighted.

15:36

We contrast, here's the next question.

15:41

T two star will be same as T two.

15:43

If there is no genome ity in the external magnetic field,

15:54

the answer is true and the answer is also true.

15:58

The reason is T two star is as follows.

16:02

This is T two star is called.

16:04

It is is a, um, is a is similar to T two,

16:08

but it also adds other things.

16:10

There are other factors which can influence T two

16:13

or to start such as magnetic field homogeneity,

16:18

magnetic susceptibility and chemical shifted.

16:21

When all these things are not there,

16:23

then T two star will be same as T two as you can see here.

16:27

So if because the homogeneity, the T two star will,

16:32

will go, will relax, will, uh, will, will grow back so fast,

16:36

it can grow, lose the signal.

16:38

And that's why the maintaining

16:41

magnetic field homogeneity is very critical

16:44

for imaging T two, T two weight and imaging.

16:47

And that's why the magnetic field

16:49

homogeneity is very critical.

16:51

And the magnetic field homogeneity is challenged

16:54

by the strength of the magnetic field, greater the strength

16:57

of the magnetic field, the homogeneity, uh, chances

17:01

that grow stronger, longer and on on those things.

17:04

So T two star will become same

17:07

as T two when there is no human,

17:10

but in the presence of T two star will deface quicker.

17:17

Which of the following is a nonag,

17:39

Some asset still some said mag, uh, magnesium.

17:43

Actually the answer is magnesium is considered the

17:46

nonromantic still to some extent is material.

17:50

So now the next question is like,

17:52

here are the characteristic of different elements

17:55

we classify into two group.

17:57

One is called paramagnetic metal, which these are the,

18:01

these are the, uh, tissues

18:02

or elements which are unaired electron.

18:05

That ATO automatically becomes augments in the presence

18:09

of the magnetic field.

18:11

Paramagnetic means the enhances when they're in the presence

18:16

of the magnetic field.

18:17

And my best example is gadolinium

18:20

and dysphoria among these two gadolinium is widely used

18:24

as a contrast agent because it's a para uh, uh, uh, element.

18:29

So when it is in the presence of a magnetic field,

18:32

the gadolinium, wherever it is uptake,

18:34

it provides a greater signal.

18:36

That's the idea behind this.

18:38

Using gadolinium contrast as a contrast agent.

18:41

Then there is what is called a dia magnetism.

18:44

These are built electron, almost like a neutral elements,

18:49

which actually, um, depletes magnetic field.

18:53

When there is any magnetic in the presence

18:55

of any magnetic field, they don't create any signal.

18:58

In fact, they create almost a negative susceptibility though

19:02

therefore we see vast majority

19:04

of the tissues in our body are dia magnet.

19:07

But the most common one which we are using is the phag.

19:11

They produces magnetic field due to molecular structure.

19:14

Best example is iron, cobalt, nickel.

19:17

And that's why in the olden time,

19:20

aneurysm clips were very much no go for any of these MR MRI

19:24

because these clips are, were ma ing.

19:28

So patients with aneurysm clips can really

19:32

dislodge the clips and create havoc in the body.

19:34

That's why any patients with any

19:36

of the surgical elements were not allowed in the MRI

19:40

and we'll talk about more about later,

19:42

but we also use a concept called susceptibility.

19:45

Susceptibility means the extent

19:48

of material magnetization in the manip pool.

19:51

It is almost like, uh, these days the younger gay,

19:53

younger kids are more susceptible to Instagram

19:57

or influencers.

19:58

Similarly, we can think here,

20:00

influencers influencing the teenagers.

20:02

Similarly here, these material were susceptible in

20:05

the presence of magnetic field.

20:07

They can, they will create, they get attracted,

20:10

they change their features

20:11

and that's why it's called magnetic susceptibility

20:13

in the phenomenon.

20:15

But use for example, in the presence of a magnetic line.

20:19

This is, imagine this is the board of the MRI scanner

20:22

where the patient is placed inside.

20:25

Once you are in the field of magnetic field,

20:28

if there are any dire magnetic materials

20:31

and tissues in the body, they create a reflection.

20:34

They, they divergent

20:36

because they are, they are not, uh,

20:38

making the signals great.

20:40

They become almost hollow it out.

20:42

So the signal becomes darker.

20:44

Whereas is a paramagnetic such as a gang gadolinium

20:48

or one molecule, they attract the magnetic field even more.

20:51

It enhances the signal.

20:54

So therefore a diamagnetic agent

20:57

depletes local magnetic field,

20:58

whereas a paramagnetic agents augments local magnetic field.

21:02

And that's why gadolinium is heavily used

21:04

as a contrast agent.

21:08

Now there are types of magnets, different type of magnet

21:11

and we we group into

21:13

as follows based on the magnetic strength.

21:16

So ultra low field is less than 0.1 Tesla.

21:21

And now we are seeing even ultra low, low field such

21:24

as even less than 0.01 Tesla,

21:27

which you may have heard about in used in a mobile

21:31

MR scanner, which has a limited application and so forth.

21:34

That's a different discussion.

21:36

Then you have this low field

21:38

come at midfield once you are coming into one Tesla

21:41

to Tesla, and that's where we start using

21:44

what is called a superconducting.

21:46

Magnets are required to create the high field, low field,

21:50

and ultra low field can be created

21:52

by placing a magnetic bricks and create these fields.

21:56

But high field are, are ultra high field are always only

22:00

because of the, um, superconducting magnets.

22:04

So based on the design, the permanent magnets which can use

22:09

for creating a low field

22:10

and low field, they're very low cost,

22:12

they're low maintenance.

22:14

Once you have the magnets lined up properly,

22:16

you don't have anything repair any

22:18

more expenses of these things.

22:19

Best example is Hitachi

22:21

and uh, Fuji has very low magnetic field, magnetic fields.

22:25

And MRI scans 0.3 Tesla 0.1 Tesla

22:29

Phon is one unique company which has, uh,

22:33

a very unique design where the patient has to sit in

22:36

between two, two surfaces of the, um, magnetic field.

22:40

It's almost like, uh, uh, imagine a a a bagel

22:44

or a donut separated and you can place a patient in between.

22:48

That's the four and that's a standing alarm.

22:51

Then you have what is called a superconducting magnet.

22:54

This, uh, this is possible with the inclusion

22:58

of helium in the superconducting magnet with the presence

23:02

of the helium, the ma the the, the,

23:05

the field will become super conducted

23:07

and that's why you have associated with, um, helium crisis

23:11

and superconducting magnets

23:12

and for, that's a different discussion,

23:14

but that's why we need helium so much worldwide these days

23:18

for utilizing in a super these Mr magnet mam,

23:22

RI scanners, high field strand.

23:26

Now we are already seeing seven Tesla machine.

23:29

We have at Hopkins, we have have a seven Tesla machine

23:31

for almost 20 years now purely used for research for brain,

23:34

but now we are also beginning to use for the clinical area.

23:37

But there are other centers who are already experimenting

23:40

with 14 Tesla machine, LA Tesla machine.

23:42

That's the direction's point.

23:44

So now the question is, the next question is T one

23:47

of the tissues larger with the stronger magnetic field?

23:56

Good. And the answer is true.

23:59

Um, T one larger the magnetic field,

24:02

the T one relaxation time is also I showed you in the table

24:05

where the T one relaxation time,

24:08

how will it grade from 0.5 Tesla to 1.5 Tesla.

24:11

And this was uniformly across all the element created the

24:15

magnetic field create a larger,

24:17

longer be the T one relaxation time.

24:20

Next question rate which protons are magnetized

24:25

when placed in a magnetic environment is the same as rate

24:29

of recovery of long-term magnetization.

24:39

The answer is true. So basically

24:42

what happens is like when you place the body

24:43

or pros in the main magnetic field, they're all aligned.

24:47

Then we use a radio frequency RF pulse

24:50

to push this proton 80 degrees or 90 degrees

24:54

and then allow it to align back to the manic field.

24:57

So the recovery is almost same,

24:59

it's almost like random pros placed in the magnetic field RT

25:03

one coming back and that's the answer.

25:05

It's the same rate of recovery for a large magnetization.

25:10

Next question. US FDA mandates that patients

25:14

with electrical implants be kept away from magnetic field in

25:19

excess of one G two G 5G and 10 g.

25:29

And the answer is majority are chosen 5G

25:33

and the answer is correct 5G imagine this is exact,

25:36

this is not only patients, but anybody else.

25:39

That's why we have very costly line of walked

25:43

outside the MRI scanner room, which can sometime often show

25:48

where the 5G gause line lineup.

25:51

So anything in in

25:52

inside the 5G gause line is considered

25:55

manic to be very careful.

25:57

So the patients

25:58

with analytical upgrade should be kept away from manic

26:02

in excess of 5G.

26:03

So if we go closer and closer,

26:05

the 5G will increase to 10 G, 12 g.

26:07

Another things and one test line is 10,000 Gauss line gause

26:11

is a physicist, uh, is a unit based,

26:16

uh, after a, uh, a German physicist

26:19

who discover this phenomena.

26:21

So that's why the shielding up the rooms are well shielded

26:25

such that the Phi ghost line is pretty much kept

26:28

inside the shielding.

26:30

Sometime in spite of that,

26:31

when the room is facility is very small,

26:33

the five ghostland can extend outside the room

26:36

and that's where the cautionary, um, precautions are came

26:40

with all the warnings for anybody to not

26:42

to cross these 5G especially patients

26:45

with electrical implants and so forth.

26:48

So in, in right now we also have,

26:51

and I I think I'm gonna come back later, uh, about this uh,

26:55

MR labeling criteria later.

26:58

So now, which of the following items is not affected by

27:03

a nearby MRI system,

27:11

right?

27:12

Optical dense diameter does not, is not affected

27:16

by magnetic field and

27:17

because it is just an, it just uses the optical light

27:21

to measure the density of a x-ray film.

27:24

Whereas the other things which have electronic inserted

27:28

electronic circuits is heavily implanted, heavily affected

27:31

by the MRI.

27:33

For example, in one, one of the PLA in our older hospital

27:36

when we had an x-ray room next to an MRI scanner, they,

27:40

especially the x-ray fluoroscopy system, which had

27:43

a image intensifier, we could see the signal going

27:49

whenever as MRI was brought closer

27:53

to the MRI system.

27:54

So the image intensifier or a credit card

27:57

or a cardiac pacemaker have an impact on cause

28:01

of the MRI system for a long time.

28:05

Uh, patients are well screened when they come for MRI scan,

28:09

probably have observed in any MRI suite,

28:11

there is almost a book of work

28:13

or a large thickness book with lot

28:16

of these materials in informing which of them are safe for

28:20

or not these days.

28:23

FDA has created these type of a labeling criteria

28:27

so the manufacturing companies can, can get certified

28:31

for their um, uh, objects, which may be used in an MRI suite

28:36

and if they, if they meet the criteria, they can get

28:39

what is called aqui green.

28:41

It's called the MR safe.

28:43

Now we also have MR conditional equipment and not MR. Safe.

28:48

Why? The reason why I'm saying here is like for a long time

28:52

patients with car pacemaker were not alone in MR scan

28:55

because the pacemaker could, could bely affected

28:59

by the magnetic field and can create HAC in the pacemaker.

29:03

But these days advanced manufacturing

29:06

companies are making cardiac pacemaker which have been

29:10

deemed RC.

29:12

So those patient with that type of a pacemaker can

29:16

actually get a scan these days.

29:18

So a lot of things are happening with respect

29:20

to these things, but this is the criteria which has

29:22

been normally used.

29:25

Next question, what does longer TR yield with regard

29:30

to MR images?

29:38

The answer is reduces T one waiting.

29:41

Let's see, the answer is reduces T one waiting

29:46

here is the here, this is the one I wanted to explain to

29:50

the fundamental, um, description of what is tr

29:53

what is T and so forth.

29:55

So here's the pulse sequence. This is the pulse sequence.

29:59

After the, the subject is placed in a magnetic field,

30:03

you create what is called a transmit calls

30:06

to provide RF pulse

30:07

of 90 degree means you're pushing all the proton aligned in

30:11

the direction of the magnetic field,

30:13

push them about 90 degree and allow them to relax back.

30:18

And that's what the T one relaxation time is that

30:21

during the T one relaxation time, the spin, the spinning

30:24

of electrons D phase it, that will create a T two phase.

30:28

But in addition to that, we also have what is called a say,

30:32

um, gradient to create different slices.

30:35

Then we have what is called

30:37

AST is the first at which the first echo is collected.

30:41

That's called a time, time between the peak 90 degree pulse.

30:45

And the peak of the path of the first echo is what is called

30:49

as the te

30:50

and tr is the actual, the total time between takes it time

30:55

to run through the pulse sequence one time,

30:58

one time it's like completely

31:00

until the signal dies out slowly, this echo goes down

31:03

and down one time promotes one row of data.

31:07

That's also important to understand.

31:09

Therefore, pulse sequence is repeated as many times

31:14

as necessary to provide many rows of data required

31:17

to reconstruct the image.

31:19

That's why sometimes the MRI scans takes so much longer time

31:24

compared to CT and so forth.

31:26

Now here is the thing,

31:28

if the image metrics you design is 2 56 by 2 56,

31:32

that is like a pattern,

31:33

2 56 rows versus 2 56 column of data.

31:37

In order to do that, it requires pulse sequence

31:40

to run 256 times.

31:43

Therefore a scan time depends on the total reputation time,

31:48

which tr multiplied by the image metrics.

31:52

That is, that is what will determine the scan type.

31:56

So here is the parameter for T two 80. We saw that one.

32:01

The, the, this is the magnetization in the XY plane

32:05

and that depends on defacing of T

32:08

of the T two and that's that.

32:10

And if you create, this is the,

32:12

if you collect the information here, the time

32:14

to echo at this point we have the maximum differentiation

32:17

between the tissue and this is called the magnetization,

32:21

the Z direction where all the proton which are thrown 90

32:25

degree come back to the main field

32:27

and that's what is called the tr, the total reputation time.

32:31

It'll come back and then again you push back another 90

32:33

degrees and then wait, push another one, wait

32:37

and so forth until you collect all the data.

32:39

Therefore, therefore a longer

32:43

te produces maximum T two waiting time depending on here

32:49

and a longer tr will produce minimal t

32:53

and t one waiting time will result in T two waiting images.

32:56

That's the, that's the principle

32:58

behind these two, two phenomena.

33:02

What does shorter t yield with like respect to images

33:14

is increases.

33:15

T one waiting decreases TO two waiting.

33:18

Let's see the answer here. The answer is reduces.

33:21

T two waiting time when you have shorter T

33:25

is respect T two waiting and it'll provide D two image.

33:30

So here is one. So shorter T will

33:33

reduce RT two waiting.

33:35

So short T produces minimal T two waiting images

33:39

and longer tr produces minimally T one waiting images.

33:45

Which of the following is correct?

33:52

Sorry, I clicked the answer

33:54

and I think the answer is gonna come soon.

33:58

I'm glad. Can I take, so

34:01

the answer is T one is greater than T two and T two start.

34:04

Remember, T one is always longer than T two

34:08

because that's the time it takes for the mag for the protons

34:11

to align back to the, uh, back

34:14

to the main magnetic fielder magnet.

34:16

Longin magnetization T two is simply the, the, the protons

34:23

phasing out, they try to regroup in the XY plane.

34:26

So there always are limited by the, the, the time alone.

34:30

So T one is always greater than T two,

34:33

T two star is even smaller

34:34

because especially if there's a presence of inner virginity,

34:38

that relaxation goes very even faster.

34:40

Therefore, T one is always greater than T two

34:44

and T two is always greater than T two star.

34:47

If the in and all those things are gone,

34:49

then T two will be the same as T two star,

34:52

which we see the earlier question

34:58

in spin echo imaging, when is echo signal normally measured

35:10

after time?

35:11

T That's correct.

35:13

After time t this is the best signal produced here.

35:18

The second echo and third echo,

35:19

the signal strength is smaller

35:21

and smaller until it becomes almost zero.

35:24

That's when you throw it again another pulse sequence.

35:26

That's when the, that it reaches the whole t

35:29

reputation time starts.

35:31

So that in spin echo, especially the,

35:34

this echo signal is normally measured here on the first one.

35:37

When you have the maximum, uh, echo signal here at this one

35:41

where the T two decay is happening here, T two decay,

35:44

T two star decay is happening.

35:49

So now the, the fundamental, um, description

35:51

of a pulse sequence, so the pulse sequence diagram can be

35:55

used to show the relative timing of certain events

35:59

during an imaging acquisition.

36:01

So time T is called the time to echo here

36:05

and tr is the time for reputation.

36:08

It, it goes on collecting multiple signals

36:11

and by the time they echo die out,

36:13

that's when it starts again as the single,

36:15

that's called the T type.

36:17

A DC is basically analog digital converter.

36:20

That's electronic component.

36:21

And these things, G slice, G read

36:24

and G phase are a gradient call in different direction,

36:28

which will differentiate,

36:29

provide different slice thick the slices

36:32

and different phase coding and so forth.

36:34

So the repre reputation time tr is the time time it takes

36:39

to go through pulse sequence.

36:40

Once now in spin echo sequences users

36:45

90 degree pulse with and hundred

36:48

and 80 degree pulse to face spins to form an echo.

36:52

The the, the idea here is like the,

36:56

the pulse from the main magnetic field providing an RF pulse

37:01

is pushing all the product to 90 degree and they will deface

37:05

therefore immediately they use

37:07

what is called let's say 180 degree

37:09

to put it in the opposite direction.

37:11

That way they always reface towards the same direction.

37:15

And when they are changing this direction,

37:17

that's when you collect the signal in the echo area here,

37:21

measure the signal

37:22

to get the maximum differentiation of different tissues.

37:26

In a spin echo signal, T one weighting is obtained with

37:32

long T or shortt or sharp along with shortt or long tier.

37:43

And the answer is short T and short tier

37:46

and the answer is correct.

37:48

Short T and short tier will provide the mag.

37:52

The T one weighted images are obtained in spinco.

37:56

What produces the loud noise heard during an MRI scanning?

38:02

Is it arising from shim coil gradient coil

38:05

transmit coil on just the magnets gold head

38:15

and the answer is gradient coil majority OFin.

38:18

That's correct. In fact, that is one

38:19

of the things which really bothers our patients

38:22

because the gradient coils you produce different

38:26

currents at different time.

38:27

So that that, that is used to pro separate different slices

38:32

and different face or coating.

38:34

And so you create, you pa pass through time

38:37

through this cranium coil and that produces lot of noise

38:41

and that's why every patient is given a, um, something

38:44

to put in their ear to block up this, uh, loud noise

38:48

and that becomes even louder when the manic

38:50

field stent is even larger.

38:52

So that's where this uh, uh, origin

38:54

of the loud noise heard is from the gradient calls.

39:00

EMR signal to noise ratio cannot be improved

39:05

by increasing the following.

39:13

So, so the answer is all over the body.

39:16

The s signal to noise ratio is actually not,

39:20

cannot be improved by increasing the matrix error.

39:23

If for example, if you, sorry, if you increase the number

39:27

of acquisition, you can increase the signal to noise ratio.

39:30

That's where we increase the number of acquisition.

39:32

It'll also increase the scan time.

39:35

It has a negative aspect,

39:36

but still that's the reason magnetic field spread definitely

39:39

will increase the signal to noise ratio.

39:41

That's the direction why we are going

39:43

to higher an iron magnetic field.

39:45

Call sensitivity is also improve the signal choice,

39:49

but matrix size does not have impact on

39:51

the signal to noise ratio.

39:52

Uh, and that's the answer. This one is matrix size.

39:57

The means for reducing SAR is called specific absorption

40:02

ratio include all of the following except,

40:12

and the answer is again, all over the place.

40:15

The answer is reducing the tr will

40:19

actually reduces the SAR including all except so all

40:24

of the other will reduce the, sorry, will reduce the SAR,

40:29

but reducing TR would not make any difference

40:32

because it's actually need more trs, more number

40:34

of reputation is required.

40:35

So reducing TR is not, does not include

40:39

for reducing specific absorption radio.

40:41

So what is specific absorption ratio?

40:44

Um, this is the amount of, um, resistance

40:48

and heat produced during the MRIs.

40:51

MRI aspect. That's why there FDA mandates on a specific

40:55

absorption ratio limit on using humans

40:58

because that can actually create a burning sensation

41:02

or even cause burns in the tissue if it,

41:06

if the pulse sequence exceeds the spec

41:08

specific absorption ratio.

41:11

What is the best way to reduce coasting artifact?

41:21

The answer is shorter tear time, shorter requisition.

41:24

The best way to reduce good artifact is actually

41:28

by having a shortening acquisition time

41:31

ghosting artifact result because of the motion

41:34

and any type of motion.

41:35

So in order to minimize motion, if you,

41:38

if you acquire an image faster and faster, you'll minimize.

41:42

So here's an example here is a

41:45

T two weighted spin echo sequence with a 12 minutes

41:48

and 48 second acquisition time.

41:51

And you can see some of this ation

41:54

and motion artifact caused here in this area.

41:57

If you reduce the acquisition time, it, uh, improves image

42:01

and also reduce motion here from 12

42:04

minute to two minute signal.

42:06

You can see this much more than cell uh, uh,

42:09

the coasting artifact is minimized

42:12

and this is, this is a fast pin echo

42:14

or this pin echo where the fast pin echo which has a minimal

42:19

um, reduced acquisition time

42:23

magnetic fields produced

42:24

by an MRI scanner is a sighting issue safety issue

42:29

Both and neither

42:37

answer is correct because both this, uh, both the siting

42:42

and the safety issue is very important.

42:44

Um, that's why this, uh, both

42:46

of the issues are very important with respect to TIC field

42:49

and MRIS scan, installation A-C-R-M-R MRI accreditation.

42:54

Phantom can be used to test all of the following, except

43:05

that's correct because the a CR accreditation, phantom is

43:09

for the quality control

43:10

or quality evaluating the quality of the protocols,

43:13

but that does not have any measurement

43:15

for the PHI Gauss line location.

43:18

In fact, physicist, we use a gause meter

43:21

to measure this gause line, uh, around the MR magnetic, uh,

43:25

MR MRI scan, which

43:29

of the following is not an MR artifact?

43:38

Correct? You have heard about chemical, um,

43:41

chemical shift artifact, zipper artifact ation is

43:46

actually is an artifact we see in ultrasound.

43:49

I think probably in the next large next

43:51

board review on ultrasound.

43:52

You, you might see a question about this one,

43:54

but it's not an MRI, um, uh, MRI artifact.

44:00

So one final question.

44:05

Which highest zone into which federal magnetic objects

44:09

and equipment may be safely taken?

44:18

Zone two. So what's happening?

44:20

What are the, the point here is like is important,

44:22

zone two is the right answer.

44:24

So what happened now is like a CR safety,

44:27

MR safety manuals are there

44:30

and it's upgrading on a routine basis.

44:32

So for safety principle, um, we created

44:35

what is called as the zones.

44:38

These are the different MRI facility zones.

44:41

So if you can look in here,

44:43

it is required now the joint commission

44:45

and any regulation require the hospital

44:48

or the clinic with have an MR scanner

44:51

to clearly mark the zones.

44:53

The idea is to to eliminate

44:56

or stop having any MR accidents

44:59

because there has been accidents which are really resulted

45:03

in a very damaging area when people with no understanding

45:07

bring thematic into zone three or zone four.

45:11

So therefore here if shown here, zone one is an outside

45:16

reception area, it's a general public area, it is uh,

45:20

accessable free to public, anybody can come in.

45:22

That's why the are scanner rooms have a well controlled

45:26

reception area to monitor who goes into the zone two.

45:30

The zone two area is the one which interface

45:33

between the public accessible uncontrolled zone one

45:37

and strictly controlled zone three.

45:40

Zone four is easy, that is inside the MR scan.

45:43

Zone three becomes very important.

45:45

That is the area of the patient prep area

45:48

and also where you access for by unscreen non MR personals.

45:54

And if you have any pH manic object can result in a serious

45:57

injury, either if they bring it work closer,

45:59

they get sucked into the magnet.

46:01

Therefore the anything if cannot pass beyond zone two.

46:07

In these past few years, we have been seeing a number

46:09

of incidents in a MS accidents in the respir, MR people

46:15

annoyingly, annoyingly bringing some stuff

46:17

and then creating the then causing damage.

46:21

One example recently was a patient had, uh, a, a,

46:25

a FA family member accompanied the patient to the MR scanner

46:30

and the family member is a policeman

46:33

and he was outside in the waiting room,

46:35

but when the magnitude was going on, something happened.

46:38

So he rushed inside the MR scanner.

46:40

We thought doing that it was scanning a loaded gun.

46:43

Unfortunately the gun triggered and, and and ran out

46:48

and it shot and actually the person actually died

46:51

because the gun shot.

46:53

Similarly, another incident was when there was a fire in the

46:56

inside, uh, there's an accidental fire happened

46:59

and immediately firefighters came

47:01

inside without knowing what Mr.

47:04

Magnetic feelings and they got a lot

47:06

of their damage equipment got sucked into MR Scan

47:10

because of that, the joint commission

47:12

and also the uh,

47:13

MR safety aspect has been heightened up

47:16

these days with them.

47:18

The other incident which human has seen is like, you know,

47:20

for the MR scanner, the bag, the table is often detachable

47:25

and brought out and prepare the patient

47:27

before they bring the table inside.

47:30

So these table magnetic table, they're called

47:32

MR Safe tables, they're tested,

47:35

but in one hospital recently they brought a wrong table into

47:38

the MR and the whole table got whole patient table got

47:41

sucked into the MR scan.

47:43

So therefore if you are working in MR area

47:46

to very careful on how you damage, how you maintain this

47:49

and respect these zone areas for safety purpose.

47:54

So what do I say is like in a CT or X-rays

47:58

or fluoroscopy, x-rays are not always on it,

48:02

only on when you turn on the machine.

48:04

Whereas in an MRI magnet is always on, uh,

48:08

whether you're running a scanner, scan a protocol or not.

48:11

Magnet is always on.

48:13

Let me stop here and uh, let's take some questions.

48:16

There are some questions here.

48:18

Uh, I'll try to see what I can do.

48:20

So here is, uh, let me see

48:26

even what, uh, thank you.

48:28

Um, that's what my daughter tell, tell me also.

48:30

So, so is it reason

48:32

to choose hydrogenous abundance at the current?

48:35

It is both of them it's readily available plus it's also one

48:38

of the higher chiropractic issues.

48:40

So both of them are, is answered.

48:43

Was that at Bayview Hospital? I'm not sure.

48:46

Question 12, um, let see, I'm not sure how

48:50

to answer this question, but, uh, not

48:53

that the Bayview Hospital here at Hopkins, um,

48:57

will a cell phone get arranged in a 5G net?

49:00

So the cell phone in a 5G line, maybe not,

49:04

but if you walk inside

49:05

that can really damage your cell phone.

49:07

Uh, let me tell you an example.

49:09

I was having this, I was wearing a regular

49:11

watch, not an apple.

49:12

I, um, watch because Apple watch can kind damage completely.

49:16

You're very close to the hyper 1.5

49:18

or three Tesla missionary.

49:19

You don't wanna take chances.

49:21

But I was wearing a regular watch

49:23

and I was working for some experimenting

49:26

and then the next few days I was running late

49:28

for all my appointment.

49:30

Then I realized it,

49:31

since the regular watches has all these nice wheels which

49:34

are magnetic, which is now very magnetic,

49:37

they got attracted, impacted by the magnetic field.

49:40

It's slow down the rotation automatically setting the time

49:44

zone less, lower and slower.

49:46

So tic fields have a very strange aspect of mine.

49:50

So how to increase contrast to noise ratio?

49:53

That's the, that's a, that's a big loaded question.

49:56

Again, it depends on the pulse sequences we are using

49:59

and what time we are picking up the signal.

50:02

So it's, it, there's one, there's no one answer,

50:06

but the point is that's why we use contrast agents

50:09

to increase the contrast and I ratio.

50:11

But again, depending on the, on the pulse sequences,

50:14

there are variety of pulse sequences design, uh,

50:17

which are specifically done forin ratio.

50:22

I had a practice question where it said 5G

50:24

or 0.5 was changed to 0.9 G

50:28

and nine G recently I had a practical question

50:31

where it said 5G was changed to point,

50:34

I'm not understanding this question correct correctly.

50:38

5G line is what is normally generally we are recognizing,

50:41

um, higher strength magnet increase.

50:45

Yes, higher manually study do increases

50:47

because again, it has directly impact on the T one

50:50

and T two, uh, and other things.

50:52

So therefore it increases fat saturation

50:55

fail during frequency.

50:56

How to mitigate, I don't know the answer

50:59

to this question right after the back,

51:00

but again, um, there are, there are different ways

51:05

around the pulse sequences to mitigate this, uh,

51:08

fa fat saturation

51:09

because as, as you say, the fat saturation

51:12

produces the D two weighted images

51:14

and that can have some havoc.

51:16

I don't know that that answer to this question.

51:19

Any other question? Otherwise,

51:21

I wish you all the best and good luck.

51:25

Thank you Dr. Mahesh for

51:26

that awesome case review. Appreciate it.

51:28

You are welcome And thank you so much

51:31

for everyone else for participating.

51:33

You can access a replay of this, um, board review,

51:36

excuse me, by creating a free account.

51:38

We'll also email out a link to the replay later tomorrow.

51:43

Be sure to join us next Monday, April 28th with Dr. Mahesh.

51:47

He'll be back to lead us in a physics ultrasound board

51:50

review and you can register for that at the link

51:53

provided in the chat and follow us on social media

51:55

for updates on future meetings.

51:57

Thanks again for learning with us and we will see you soon.

52:00

Thank you.