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Hello and welcome to Case Crunch Rapid case review

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with a multiple choice question,

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link provided in the chat.

0:28

Today we are honored to welcome back Dr.

0:30

Mahesh for a physics board review in ct. Dr.

0:34

Mahesh is a professor of radiology

0:36

and cardiology at Johns Hopkins School

0:38

of Medicine in Baltimore, Maryland.

0:41

He is au the author of the textbook MDCT Physics,

0:45

the Basics Technology, image Quality, and Radiation Dose.

0:49

Additionally, he is chair

0:50

of the Radiation Control Committee, president

0:53

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

0:58

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

1:02

and I-I-C-R-P.

1:04

Questions will be covered at the end if time allows.

1:06

Please remember to use the q

1:08

and a feature to submit your questions.

1:10

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

1:13

Mahesh, please take it from here.

1:16

Wonderful. It's my great pleasure to talk about ct,

1:20

one of my favorite subject.

1:22

Um, before I dwell into my the session, I just want

1:25

to tell you guys who are joining us next, today

1:29

and tomorrow day after tomorrow,

1:30

you're gonna hear something about CT in the media

1:32

because there's a publication which came out today in JAMA

1:37

are projecting that CT will cause a lot more cancers in

1:40

future APM and the American College of Radiology.

1:43

We have put out a press release, uh,

1:47

talking about it pushing back and

1:49

because the estimations are not correct

1:51

and it's, it's basically scar people out.

1:54

So if you happen to see these papers, these media, uh,

1:59

tidbits, remember that's where we have, uh,

2:02

addressing the issue through both A A PM press release

2:06

and also a A CR press release release about this,

2:09

what the benefit of the ct.

2:10

Having said that, let me jump start

2:13

from, from the first question.

2:14

Okay. Here is one of the, uh, generic question

2:19

introductory, wonderful.

2:25

Um, welcome the, uh, attending physician and

2:28

and the resident and anybody taking the exam CT as you know,

2:33

where whether you are um, radiologist

2:36

or any type of physicians, you'll be using CT to some extent

2:39

because CT has been considered the one

2:41

of the most advanced technology innovation in medical

2:45

science for the last 50 years.

2:47

Uh, constantly related as high,

2:49

which changes the whole diagnosis for this one.

2:52

So be having that said that,

2:54

let me start with the first question.

2:55

I don't wanna delay too much time,

2:57

a lot of material to cover.

2:59

You see, this is my first question.

3:07

This is an easy question.

3:09

I'm glad everybody has uh, uh, said it's zero.

3:13

Why zero and what is u HU is known for hounsfield,

3:17

the person who first developed the C back in 1970s

3:23

probably would not surprise that he worked for um,

3:26

gramophone company, which also had Beetles record.

3:28

So the gramophone HMV kind of sold off the, uh, this,

3:32

this particular research part thinking

3:34

that it would not prevail, but that's a different story.

3:37

So the CT number of water is expressed in ounce field unit

3:41

and the number is zero.

3:43

The reason is like the CT numbers are normalized

3:47

to a al containing water

3:49

and we pegged the CT number of water to be zero

3:53

and every other tissue around this zero unit.

3:56

This is one of the reason in the clinic.

3:59

Every day morning a technologist will scan a water phantom

4:03

to make sure the CT number of the water

4:06

is is within zero plus or minus seven ounce per unit.

4:10

The reason is if the CT number is giving wrong

4:13

every other tissue which will displayed based on the whole

4:16

internet ho speed number will also be wrong.

4:19

That's why the most commonly done quality controlled

4:23

by the technologies on a daily basis is to do a,

4:27

a scan phantom, a water phantom or even a water bottle.

4:30

You can scan and make sure the scarcity number from the

4:33

system is coming out to zero because

4:36

otherwise the calibration is off that needs to be tested.

4:40

Here is the thing, the idea here is like the formulation

4:43

for CT number is as you can see here, this is the difference

4:47

of the tonation coefficient of a material with with that

4:52

of a ation of water diverted by the atian ion of water.

4:57

If we are scanning water,

4:58

water this minus this will be zero,

5:00

therefore the CT number is zero.

5:03

Everything else, depending on the tenian coefficient

5:06

and multiplied by a scaling factor, this wax

5:09

of image will can display the CT number anywhere from uh,

5:13

all these numbers and these numbers are then translated into

5:18

a pixel image and

5:19

that's why the CT image from a physicist standpoint

5:23

of view is a bunch of tonation values

5:26

displayed on a gray scale.

5:28

The way done is like the tissues are denser than water has

5:33

a positive CT number, um,

5:36

and anything less than water will have minus uh CT number.

5:40

That's why the fatty tissues are around minus 80

5:43

to one minus 100 bone

5:46

and muscles are all in the positive size

5:48

and the compact bone

5:50

and is the highest one lung which is the air has a

5:53

low atian coefficient.

5:54

That's why the lungs and the CT numbers are very low.

5:58

Then the other advantage

5:59

of CT is like we have window A level and windowing

6:03

and this is actually is the one which makes the CT image

6:07

look so nice to adjust a contrast.

6:11

Nowadays, all of you guys when you see the ct,

6:13

we have free fixed windowing window we call a bone window,

6:17

we have a lung window or a medicinal window.

6:21

Basically here's the cha chest city image

6:23

and the same image can be displayed both in a bone.

6:27

Bone window means you have a lo,

6:29

you have the window centering a thousand.

6:31

Everything else, the 2 56 gray scale is

6:34

distributed around this number.

6:36

That's why only the bone will show up white,

6:38

everything else become dark.

6:40

Whereas here in the lung window we want to look the lung,

6:44

lung aviles and other structure.

6:46

Everything else becomes uh, washed out except this area

6:50

that is the idea behind this unfilled scale

6:53

and the variation which allows us

6:56

to display different tissue which is not possible in

6:59

radiography and fluoroscopy

7:01

and that's the reason why CT images pops out

7:04

and it's going to pop out even more with the duality CT

7:08

and the photon counting CT

7:10

where we can even further distinguish between kidney

7:13

and pancreas blood and so forth.

7:15

That's the direction we are going.

7:18

Next question, what is the ratio of radiation dose

7:22

at skin surface compared to the center

7:25

of the abdomen in an abdominal ct?

7:35

Answer is all over the place.

7:36

We are going reasonably

7:39

and the reason is like here is the thing,

7:41

the idea here is the answer is one is two 0.5 means the

7:45

surface dose is one

7:46

and the center dose is about half of the dose.

7:49

The reason is this is how it looks.

7:51

If an object is small, the surface dose is same

7:56

as the central dose and that's true with the head

8:00

and I'm showing here a phantom which we physicist measure

8:04

and we use this to measure CT dose.

8:06

So the surface dose is same as the central dose.

8:09

Now on the larger abdomen, which we use a standard phantom

8:14

of 32 centimeter, the center dose is half

8:17

of the cent surface dose.

8:19

This has an implication how we do CT dose measurement

8:23

but if the saw object everything is same,

8:25

if a large object larger day,

8:27

you may have observed some time a large patient um,

8:32

CT scan of a large abdomen.

8:34

You see lot of very graininess in the center

8:38

and that we call it

8:39

as photon starvation artifact which means less amount

8:43

of x-rays are penetrating here, contributing the image.

8:46

Therefore that is having a lot of graininess.

8:49

That is what is called for salvation.

8:51

Another point is like if a pregnant woman is scanned in a

8:55

ct, if you want to calculate the fetus dose, we

8:58

as a physicist want to know where the fetus is located.

9:02

Is it located at the center?

9:05

That way then we can actually estimate the dose gonna be

9:07

much lesser than at the surface and so more a reason.

9:12

So that brings to the idea of how is CT dose measured.

9:16

The CT dose AAV measure is nothing to do with the patient.

9:20

It is measured using standard phantom.

9:24

These phantom you met a senior physicist drag

9:26

around this heavy material called lucite material with holds

9:30

to insert the chamber.

9:32

The physicists use these phantoms

9:34

and this has been standardized worldwide.

9:36

32 centimeter body phantom is about,

9:39

it's called body phantom 16 centimeter diameter is called

9:42

the head phantom and physicists measure at the center

9:45

of this one to calculate the CTDI

9:49

and once we calculate we nowadays we use

9:53

what is called A-C-T-D-I complete tomography

9:56

dose index weighted.

9:58

The reason is like since since the surface,

10:01

since the surface dose is different in the body compared

10:05

to the head, we had to weight that one

10:07

and we use a formulation like this.

10:10

I'm not expecting you to be uh,

10:11

follow the formulation right now

10:13

but it's good to know where this is arriving.

10:16

So CTDI is one of the dose description in CT

10:19

and it is called computer tomography dose index only.

10:24

Now next, next there's another dose metrics

10:26

called dose length product.

10:29

So let's try this math, try to keep you awake.

10:32

What is the dose length product of a 20 milligram

10:37

CTDA weighted pitch of two and a scan length of 100.

10:48

So 50%

10:49

of you have correct which is skew 50,000 milligrams

10:52

centimeter and how is it calculated?

10:55

This is calculated as follows, the DLP,

10:58

the dose length product is calculated

11:00

by the CTDI weighted divided by the pitch factor.

11:05

I'll tell you what more about pitch as we go along

11:07

multiplied by the scan length from this way the answer is

11:11

1000 milligram.

11:13

Why are we interested in DLP?

11:15

Because the CTDI only measured the dose of one slice,

11:20

whereas the actual scan stone on a CT is not one slice.

11:25

We scan over a region of length, anatomy, chest, abdomen,

11:29

head, everything in order to accommodate the exposure

11:33

to the body we need to ac we use the term called DLP

11:37

dose length product and the unit is milli gray centimeters.

11:42

Very important. Therefore between the two of them here

11:46

if you everything keeping same,

11:49

the CTDI will be the same in both the cases.

11:51

Whereas here the patient is twice scanned twice the length

11:55

therefore that is reflected in the DLP

11:58

and this is very important to estimate the risk to the body

12:02

or stochastic risk and other fall.

12:04

That's why this is DLP

12:07

and the CTDA volume are the two dose metrics in the ct.

12:12

Next question. What is the unit of effective dose in ct?

12:23

Correct and why do we say what is the,

12:26

what are the all the, what is the other things?

12:28

Let's say milli gray

12:29

and milli seaver are the standard international unit.

12:33

However, in the US

12:35

our radiology still are stuck in the older units and

12:39

because of that we some used milli rats

12:41

and Rankin ranking is the unit of exposure.

12:46

RAD is the unit of observed dose that is the same

12:49

as milli gray and milli seaver is the unit

12:52

of effective dose in ct.

12:54

What is effective dose?

12:56

Effective dose is a terminology we use

12:59

to estimate the risk to the whole body from exposure

13:04

to any part of the body

13:05

and there's a whole sort of calculation we do

13:08

and I think probably in my radiation protection review

13:11

class I'll go into the details.

13:13

Okay, the other thing is like in order

13:16

to calculate those affected dose is a very complicated way

13:19

but for simplicity we also have

13:22

what is called a dose coefficients, RAK factors in CT

13:27

and there they are published by various groups

13:30

and this is the K factors we currently use.

13:34

As you can see here, you simply take the DLP from the

13:37

export, from the scanner, from every study.

13:41

Now and according to the international standard,

13:44

every CT scanner, every patient scan will include

13:48

or display DLP and CTDI volume used.

13:53

Now you can take the K factor

13:55

and multiply to get an effect dose value.

13:58

So why is effect dose important If you're communicating

14:01

with patient about the risk about radiation risk

14:04

and you want to compare the risk to other type of radiation

14:08

exposure, the effective dose terminology

14:11

is useful to do that.

14:13

So I can explain to my patient

14:15

what seven milli CT really means.

14:17

It's like the annual radiation exposure

14:20

to US population from natural background radiation is three

14:23

milli ct so

14:24

therefore I can say your particular CT got is into about two

14:29

years worth of natural background radiation.

14:32

So this gives us an conversation, a point of conversation

14:36

or communication to our patient.

14:39

So this is how one example of a CT information,

14:42

dose information saved with each patient.

14:45

Here's an example of a screenshot of a Siemens scanner.

14:50

I would advise you to take a look if you're reading any CT

14:53

images next few days, look at any of the CT vendor.

14:56

One of the image in a patient study will be dose page

15:00

and it'll be something like this

15:02

and it basically tells

15:04

what technique was utilized in the scan

15:07

and what CTD volume was imp uh that relates to

15:11

and what DLP using this weakened compute effect two dose

15:16

and then compare and so forth.

15:17

That is the idea behind this one.

15:21

So here's the next question.

15:24

An MDCT protocol has beam width of one point 16 millimeter

15:29

and a table movement of 24 millimeter per rotation.

15:33

What is the pitch?

15:41

The pitch is 1.5 majority has said that's correct.

15:46

I think your preparation is going very well. Thank you.

15:48

And so that the formulation is pitch is table feed divided

15:53

by beam width and the concept

15:55

of pitch only applies when you're doing a helical scan.

15:58

Helical scan is when the table is transported

16:01

and you're still collect

16:02

and the XLE tube goes around and round.

16:05

So at any one plane the images,

16:07

there's always a heli created like a slinky in and out.

16:11

If you collapse the slinky there's a lot more overlap.

16:14

If we extend the slinky there's a lot of gap

16:17

and that's what pitch it goes to.

16:18

1.5. What does it mean?

16:21

We have a relationship between the pitch

16:23

and the radiation dose to the patient

16:25

and that is related as follows.

16:27

The radiation dose is inversely proportionate

16:31

to pitch value, which means if the pitch is greater than

16:34

one, the radiation will be less than one.

16:37

If the pitch is less than one,

16:39

the radiation dose will be higher.

16:42

So concept while conceptually you can think as follows,

16:46

if a pitch equals one means

16:49

you are scanning the patient continuously

16:51

without leaving any anatomical gap.

16:54

It's like if like aligning a Lego table, Lego blocks one

16:58

after the other, there is no air gap that's equal

17:01

to pitch equal to one.

17:03

If a pitch greater than one as in the formulation

17:06

of 1.5 means the table is traveling faster than the

17:11

and than the beam width.

17:13

Because of that there is a gap which that means there is a,

17:17

there is some part of the anatomy is safe is not radiated,

17:22

therefore the patient dose will become lesser

17:25

and then you can one minute ask why don't we,

17:29

how far can you push this linky out the, there is some point

17:33

where the, the way the images are reconstructed is

17:37

what we call it as the image requests

17:39

all to be in one plane.

17:41

In order to do that the data from the helix is interpolated,

17:46

it's called interpolation algorithms.

17:49

In order for the accuracy of the interpolation algorithm,

17:52

the pitch can go as high as 1.4

17:56

or 1.45.

17:58

Not beyond that, that's when the inaccuracy sets in

18:01

and we don't use a pitch less than one means

18:05

you are overlapping the anatomy

18:08

and that's why the radiation dose will be higher.

18:11

Generally in abdominal CT we can go a pitch greater than one

18:16

because there is a lot of this organ are not moving

18:19

so much whereas in cardiac ct,

18:22

since the heart is moving beating so fast we need to overlap

18:27

or we scan that same area because of that.

18:30

The pitch is the the in the protocol we use a pitch of one.

18:34

So conceptually equals one means there is no anatomical gap.

18:39

A pitch greater than one means there is a gap

18:41

therefore there is a radiation dose saving

18:43

and a pitch less than one means there's a overlapping,

18:46

therefore the radiation dose is higher.

18:48

That's the idea behind it.

18:51

We have, in order to accommodate the pitch,

18:54

we introduce a term called CTDI volume.

18:58

This is basically the CTDI weighted divided by the pitch.

19:02

In fact now the CTDA volume, the unit is also milli gray

19:06

and the DLP are the two main dose descriptors in ct.

19:12

What happens during CT fluoroscopy?

19:21

Okay, I see the answer all over the place here

19:25

and I'm not sure how many of you have done CT fluoroscopy.

19:29

On the other hand these days you may be involved

19:32

what is called a CT biopsy.

19:34

I know more and more radiologists

19:36

or interventional radiologists are involved in CT biopsy

19:39

which means at that time the table is kept stationary while

19:43

the tube moves continuously.

19:46

For example, if you're trying to insert a tube

19:48

to do a biopsy and you're injecting inserting the tube in a,

19:53

so you want to make sure whether the payer you are

19:55

injecting, uh, you are pinning is entering the right region,

20:00

you scan and get an images quickly and see

20:03

and then move the needle in the right position

20:05

and that is the purpose of the CT fluoroscopy.

20:07

There the table is kept stationary while the tube moves

20:11

continuously and create generates images

20:14

and you can see how the needle is inserted

20:17

to get the biopsy, the tissue and so forth.

20:19

That's the idea of the CT fluoroscopy

20:22

and this is the only type where the radiologist

20:26

or the interventional fluoroscopist are inside a CT scanner

20:31

every other time everybody's outside the CT scanner.

20:35

Therefore in CT fluoroscopy radiation protection

20:38

also becomes very critical.

20:40

That's why you're seeing them everybody wearing lead

20:43

and also lead apron

20:44

and also monitored by the radiation batch and so forth.

20:49

Next question. Low radiation dose CT studies

20:53

yield higher image noise.

20:56

What type of exams are relatively acceptable

20:59

with high image noise?

21:08

Okay, lung scan, abdominal scan

21:11

and the correct answer is lung cat lung scan.

21:15

The reason is like the lung structure

21:18

or the nodules can be seen even when the background is noisy

21:24

and that's the rational behind why we use a low dose CT

21:29

as a way to do the lung cancer screening.

21:33

In fact, the lung cancer screening right now is defined

21:38

in the clinic to set the protocol such that the CTDI volume

21:43

does not exceed three milligram.

21:45

The reason is like the the,

21:47

the effective dose from the low dose CT screening should be

21:51

almost same as the mammography screening program.

21:54

That's the rational behind it

21:56

and with the hope that lung cancer screening will become

22:00

universally accepted

22:01

for asymptomatic subject right now it's very much defined

22:05

patient graphic especially for smokers,

22:09

somebody more than 55 years and they can scan

22:12

and detect this lung cancer

22:14

and there is also definition that's why we can get away

22:17

with the noisy image in terms of the lung scans,

22:21

not in other type of scans.

22:27

Which factor contributes the most surface radiation dose

22:31

to breast during chest CT scan?

22:41

Okay, actually it is interesting

22:44

because the answer is, let me come back here.

22:47

The answer is low tube voltage.

22:49

This is reflecting back to radiography and fluoroscopy.

22:52

Lower the tube voltage means more radiation does not

22:55

penetrate and they deposit more energy on the surface

22:59

and that can provide more dose to the breast.

23:01

Now the other question regarding slice thickness thinner

23:04

slice can contribute patient size can also begin the

23:08

patient size larger.

23:10

The patient size automatically required more dose

23:12

and reconstruction algorithm does not

23:14

because this is basically a a post-processing technique,

23:19

it does not have any impact on the way we acquire the image.

23:22

So the correct answer is low tube voltage.

23:28

What does tube current modulation reference to to

23:38

tube output changes?

23:40

As it rotates the answer is correct.

23:44

Tube output changes as it rotates,

23:46

which means the tube current is changes

23:49

as it goes around it.

23:51

So now next other question I want to follow

23:54

before I explain about the modulation

23:56

here is another question regarding cardiac CT compared

24:00

to retrospective ECG gated scans,

24:03

prospective ECG triggered scans yield dash dose,

24:14

okay, um, I'm glad majority are right right answer

24:18

but again it can be a tricky, a tricky way

24:21

or we can pick one of the other one

24:23

but I hope you understand what cardiac CT is

24:26

and what are the two different type of trans scanning is.

24:29

Now let me come back to what is, oh, one more question

24:33

generally on what basis

24:36

does the automatic tube current modulation

24:39

varies the tube current in CT

24:47

patient thickness body size,

24:49

correct The answer is patient thickness.

24:53

Now let me spend some time on

24:54

what dose modulation is this is

24:56

because this is an important concept which has now kind

24:59

of become a defacto standard while we are

25:02

doing a CT protocol.

25:04

When you say tube, current dose modulation means we are

25:07

changing the tube current as the scanner is acquiring data

25:12

rotating around the patient.

25:14

There are two type of dose modulation.

25:16

One is called the spatial dose modulation which is

25:20

what generally every radiologist will tell you

25:23

because radiologists who doesn't do cardiac CT

25:26

only are aware of the spatial dose modulation which applies

25:30

to body protocol.

25:32

CT had everything whereas cardiac, if you talk

25:35

to a cardiologist

25:36

or a cardiologist who does cardiac ct,

25:39

they would not know spatial dose modulation

25:41

but they will only know what is temporal dose modulation

25:45

and that is what is used in cardiac protocol.

25:48

Now let me explain why we had to do this one.

25:51

Prior to the dose modulation we were acquiring the same

25:56

output of x-rays that tube current was used while going

25:59

around the patient even though assuming

26:02

the patient is circular.

26:04

In general the patient is more like elliptical

26:07

therefore you don't need the same tube current

26:10

to penetrate this thin portion of the body compared

26:13

to the thicker portion of the body.

26:16

That principle what utilized in by various vendor

26:19

to fine tune their x-ray tubes such that it'll

26:23

automatically change the tube current while acquiring the

26:27

thicker portion and lowering around the thin portion.

26:31

And that's done because every CT scan a scout image taken

26:36

also called as a topographic image.

26:38

When they do that they can automatically scanner can sense

26:42

the patient thickness to some extent.

26:44

Therefore the two type

26:46

of modulation is one is called the spatial modulation.

26:49

This is based on modulating the tube current,

26:52

a different spatial projection

26:55

and this is how it is mostly used in p body CT protocol.

26:59

In fact now if if somebody is doing a CT without dose

27:03

modulation, they're not doing it correctly.

27:06

Optimally every CT scanner should automatically do the dose

27:10

tube current dose modulation.

27:13

Now here is the plan. In the past when we did not had the

27:17

um, when the dose modulation for a for a chest CT

27:21

or a abdominal ct, the tube current was set constant

27:25

and it scanned it utilized the same tube

27:27

current through the whole thing.

27:29

Whereas ideally

27:30

for a certain image quality you don't need the same tube

27:33

current around the thin area which is the lung area which a

27:37

lot of air or

27:38

around the abdominal area compared to the shoulder area.

27:42

So the scanner will automatically modulate the tube current

27:46

which automatically protects or optimize the dose

27:49

and reduces the dose overall to the patient.

27:52

In cardiac cts even more trickier there we don't change

27:56

based on the patient thickness,

27:58

we change the tube current based on the location

28:01

of R to R cycle.

28:03

For example in this sec this is why R

28:06

to R peak the area which we need data to collect

28:10

to create a cardiac CT image is the best area is the

28:14

diastolic area which is the quant part

28:16

of the car or the heart cycle.

28:19

Earlier in the retrospective gated helical scanning we used

28:23

to acquire data through multiple heartbeats

28:25

without changing anything.

28:27

That's why the retrospective gated helical scan

28:31

resulted in a higher patient dose.

28:33

When you use the temporal dose modulation,

28:36

we lower the tube current around the systolic area,

28:39

therefore all the wide space you see in this drawing is

28:43

actually the radiation dose saving.

28:46

Now the other part which is the, which most

28:48

of you answered correctly, the

28:50

prospect you triggering axi scan.

28:53

Instead of acquiring data through multiple heartbeat,

28:56

the scanner will wait

28:58

and collect only the diastolic area part

29:01

and then waits until the heartbeat is ques

29:04

and collects then the next location

29:06

and next location and so forth.

29:08

Therefore, the prospect triggering method

29:11

is much lower dose than retro respirating now come

29:15

to the most advanced scanner such as the dual energy CT

29:18

or the photon counting or the three 20 detector CT which we

29:22

have uh from Canon and the Siemens and uh Bernard from Canon

29:26

and also from Phillips and ge.

29:29

You don't need to scan multiple heartbeat,

29:32

you can only scan in one heartbeat.

29:35

So therefore the advancement in technology

29:38

and the way we do has lowered the radiation dose quite

29:42

remarkably over the past 15 years

29:44

and now we are at this stage doing a much lower dose

29:48

for cardiac CT and geography procedure.

29:52

Next question, what slice thickness is preferred

29:55

by radiologist to observe subtle bones fracture?

30:06

The correct answer is the thinnest slice possible 0.625

30:10

is what among the choice.

30:12

The reason is we want very high spatial resolution in the

30:16

images to create this bone

30:18

to subtly diagnose this bone structure

30:20

and that is one of the driving factor in a multi detector CT

30:25

to become thinner and thinner ISIS

30:27

or the thinner and thinner detector.

30:29

Therefore, when we reconstruct the image in the coronal

30:32

or sagittal or anything, we have what is called

30:36

as a isotropic spatial resolution and that is possible

30:39

because of the thinner strain

30:41

and that will allow to to diagnose much easier

30:45

the subtle bone fracture.

30:47

Next question. Which

30:52

of the following changes will reduce noise in ct?

31:01

Okay, so basically you're telling is like slice,

31:04

the correct answer is slice thickness from 1.525

31:08

to five, let me go back here.

31:10

So why is that important?

31:12

Because when we have very thin slice, less number

31:16

of x-ray photon are contributing to the image

31:19

therefore the image become very noisier.

31:21

So when the noisier image means there is a trade off

31:24

thinner, the slice means higher spatial resolution,

31:27

it's good for our LER bone fracture

31:29

but the image noise can be too much.

31:31

You might miss some low contrast object.

31:34

That's why we can reconstruct the same thin slices into

31:37

thicker slice to reduce the image noise

31:40

and that's what we do with this.

31:41

These other factor do not count directly to the irre noise,

31:45

noise, image noise

31:47

but tube current will definitely not reduce actually

31:49

it'll increase the tube noise.

31:51

Uh, increase the noise

31:53

because the lower the tube current means more image noise.

31:56

So that is not the right answer.

31:58

So just think through this, this type of question what

32:02

that each of these parameter impacts

32:04

and that's what it helps.

32:07

What is the primary motive

32:09

for utilizing iterary reconstruction in ct?

32:19

Yes, the primary motive

32:21

for utilizing iterative reconstruction is

32:23

to decrease image noise, which means it allows us

32:27

to acquire the image at a very low technique means at a low

32:32

radiation dose to the patient

32:33

and which the images are very noisy.

32:36

Then we use iterative reconstruction

32:38

to improve the image noise and reduce the image noise.

32:41

This particular publication I cited here is um,

32:44

freely available on radiology.

32:46

There we go and discuss majority

32:48

of the things which involved with CT and how I do it.

32:51

Article managing radiation dose in ct.

32:54

I welcome anybody to take a look at it.

32:56

So now iterative reconstruction are now being replaced by

33:00

what is called a deep learning algorithm like ai.

33:03

So we are using increasingly using more

33:05

and more AI to reduce the image noise

33:08

but we also have to be careful

33:10

we cannot get a CT image without any x-rays.

33:15

So you can't keep reducing all the way down

33:18

until we are gonna use deep uh AI to create an image

33:22

actually that can create a lot more nuisance to the image.

33:25

There are studies which showed if you reduce the dose too

33:29

much uh or the tube current too much

33:31

or the radiation dose too much

33:32

and then trying to correct it by using

33:35

iterative reconstruction

33:36

or the deep learning algorithm it actually create more

33:40

of false low contrast object in the

33:43

image and there is a trade off.

33:44

That's why I think this is already I seen

33:49

that which slice thickness is preferred

33:51

to visualize subtle bone fracturing in ct.

33:54

I think this is a repeat question so let me go

33:56

and skip this one now.

33:59

What are the typical range of tube voltages used in ct?

34:09

Correct, I'm glad.

34:11

I also would encourage you when you're doing a reading,

34:14

reading a CT image of a patient,

34:16

take a look at the technique being used there.

34:18

It'll tell you so nowadays that typically we have range

34:23

of 80 to one 40 kv.

34:25

Now some of the advanced scanner they also calibrated

34:27

to 70 also

34:29

but historically 90% of all the CT protocol

34:34

uses one 20 kv.

34:36

And you may ask why

34:38

because the reason is like the energy created

34:41

by 1 22 voltage x-ray two x-rays are quite sufficient

34:46

to penetrate even the thick portion of the body

34:49

of an average patient and also thin patient of the body.

34:53

Because of that sometime in the patient is really obese,

34:56

we jack up go up to one 40

34:59

these days when the patients have thinner pediatric cases

35:03

there is no point in doing it one 20 we can go down to 100

35:08

or 180 which has a double advantage of reducing the dose

35:13

and also improving the image contrast.

35:15

So that is the range but

35:17

otherwise there is also some of the scanners now do

35:20

what is called a tube voltage modulation.

35:23

They don't do tube current modulation exactly like a tube

35:27

current modulation but the scanner can suggest

35:30

based on the scout image what tube voltage to use.

35:34

If you don't like you can always override as that is.

35:37

We call it Siemens, we call it care kv.

35:39

Different vendors call different names

35:41

that is also available.

35:42

Now how does radiation dose

35:47

in CT varies with tube current?

35:57

The answer is correct.

35:59

Um, or hold on a second is linear.

36:04

So the radiation dose in patient varies

36:08

linearly with tube current.

36:10

This is an important factor in fact for this reason I wanted

36:13

to, this is another example

36:14

before I want to explain some aspect.

36:17

What scan technique improves temporal resolution in cardiac

36:29

so it's all over the place.

36:30

I'll come back to this one detail.

36:32

So now let me see, it's,

36:34

it's actually the faster ganter rotation is

36:37

what will improve the temporal resolution.

36:40

Okay, now I have this question. So what is scan time?

36:45

Scan time means the amount of time an x-ray tube needs to go

36:48

around takes to go around the subject between 360 degrees.

36:53

That's called scan time.

36:55

Sometime when people mistake scan time

36:57

to the overall CT scan scan study time,

37:00

but this is how it's defined.

37:02

There are three type of scan time in a protocol in a city

37:07

when when the tube is turned on

37:10

complete 360 degree collecting data,

37:13

that's called full rotation time

37:16

and that is what is drive the technology now

37:18

to commit faster and faster the advanced scanner.

37:22

Now can you can the scan time can be less than

37:27

300 millisecond.

37:29

That is 0.3 second by 0.3 second the x-ray tube goes

37:33

around the subject 360 degree rotation

37:36

and that's so typically done

37:39

the temporal resolution is defined as the the capability

37:43

to freeze the motion on how faster you can go

37:47

to collect the data in radiography.

37:50

For example, the scan time can be 10 millisecond,

37:54

you snap take a snapshot X-rays then

37:56

because of that in temporal resolution is highest in

38:00

radiography but radiography doesn't provide the value as ct.

38:04

But in CT the temporal resolution is defined by the rot.

38:09

The data collected rotation time

38:11

so the best temporal resolution

38:13

to get a temp best resolution with one x-ray tube ct you can

38:18

actually get away with only half the rotation,

38:21

which means partial scan time.

38:24

That is, that is the minimum data you require

38:27

to reconstruct the image.

38:29

Therefore the partial scan time is the best temporal

38:33

solution you can acquire in any C CT scanner.

38:36

So if you happen to, if you're buying a CT scanner let's say

38:39

and you're interested in temporal resolution, you can ask

38:42

how fast is the gait rotation time In a scanner they say 300

38:46

millisecond means the best temporal resolution you can get

38:49

is half of that, about 170 hundred 50 millisecond.

38:54

Now one vendor inserted two x-ray tube,

38:57

it's called the dual source IT scanner

39:00

and be the reason why they did is in

39:03

to enhance the temporal resolution even further.

39:06

Now with the dual energy dual source it you can

39:10

actually collect the data from one quarter

39:13

of the rotation time

39:14

and combined with the other quarter of the rotation time

39:17

and reconstruct animation.

39:19

Therefore with this type of setup

39:22

that the best temporal resolution is one fourth

39:24

of the canty rotation time.

39:26

The reason why they did is like they want

39:28

to capture the heart as fast as possible

39:31

and in past one

39:33

of the technological advances in CT was driving

39:36

to acquire the scan as fast as possible

39:39

and eventually they came up with two tubes.

39:42

There are also other plans on the drawing

39:44

to have three tubes and that has not become possible yet

39:47

but that is a concept like a concept car.

39:50

It's in the concept tubes area.

39:53

Now this one slide I like to show this slide

39:57

because this will tell you what are the scanned parameter

40:02

in CT which impact both radiation dose

40:05

and image quality in ct.

40:07

I like to compare this like

40:08

for a chf we have different ingredients.

40:11

What are the primary ingredients, secondary ingredients

40:14

and how you mix

40:15

and match the ingredient by the CHF will create a dish

40:19

and the quality of the dish.

40:20

Depending on how you integrate

40:22

or mix these ingredients, you can think

40:25

of the same analogy for ct.

40:27

Also how you utilize these parameter will impact

40:31

of the quality of the image of the CT coming out

40:34

among them the most primary factors are these three,

40:38

these five which is important.

40:40

I would encourage all of you to understand what each

40:43

of these factors imply

40:45

because that has an impact both on the impact quality

40:50

but also on the patient dose.

40:52

As you already said, tube current is varies linearly

40:55

with patient I uh, radiation dose.

40:58

If you increase the tube current radiation dose is higher,

41:02

image quality is also higher,

41:04

but if the tube current is lower, radiation is lower

41:07

but the image noise can be higher so there is a trade off.

41:10

So these are primary factor scan time only applies

41:14

for cardiac ct, which you already answered.

41:17

The prospect triggering method is much lower dose than the

41:20

retrospective ECG getting.

41:23

These secondary factors are post-processing,

41:27

the data is already acquired, the the radiation exposure

41:29

to the patient is already done.

41:31

Now you can reconstruct into slice thickness, thinner slice

41:35

for better spatial resolution, thicker slice

41:38

for better image, lower image noise and so forth.

41:42

The other factors are something which we don't have control

41:45

and one of them is the training

41:46

and experience which we can we insist on the training

41:50

and experience of the radiologist or the, or the technology.

41:53

So how they use the scanner.

41:55

So now the question is what is the effect of lowering

42:00

KVP or two voltage on radiation dose when all other scan

42:05

parameters remain unchanged,

42:14

decrease increase.

42:15

This is like taking a coin and flipping.

42:17

So majority I said decrease is good,

42:19

how does it decrease is an important factor tube current

42:24

impact linearly whereas tube voltage has a different

42:27

relationship and that relationship is as follows.

42:31

If you decrease the tube voltage significantly,

42:35

it reduces dose typically in the ratio of KV square.

42:40

So here is example for a one 20 kv,

42:43

let's say the effective dose

42:45

of a particular protocol is 1.6 milli going down

42:49

to a hundred kv it went down by 60%.

42:53

If you go down to 80 kv it goes almost half of this one.

42:57

So what I'm trying to convey here is like the relationship

43:01

of the two voltage and the patient dose is lot more

43:04

complicated and it's not linear but even more KV square.

43:10

That's why by for patients of small size a thin slice,

43:14

if we go from one 20 to 100

43:17

it not only increase the image quality

43:19

but also reduce the radiation dose quite remarkably.

43:23

And that's what is process in the process

43:25

of dose optimization of the protocol implies.

43:29

So KBS goes down, dose also goes down,

43:32

but it's more like KB square

43:34

and we are for approximation we use a KB square

43:37

but it actually it's KV to the power

43:39

of NN can change from 1.8 to 2.4.

43:43

Since we are not arguing with another physicist

43:46

for radiologist it's okay for our range to use KB squire,

43:52

which of the following has the least impact on

43:55

the CT image quality?

44:03

That's correct. The quality

44:05

of the display monitor does not matter

44:07

because whereas the focal spot max ma ma matters a lot

44:11

because that will impact on the image quality plus the

44:14

spatial resolution and so, so forth.

44:16

But quality of the display monitor does not matter.

44:19

It'll look bad but you can always use a different

44:21

quality display monitor.

44:23

But the CT image quality is inherently dependent,

44:26

is not dependent on the quality

44:27

of the image display monitor.

44:30

Now before I end there, a couple

44:32

of FA fundamental factors I want to share with you.

44:35

One is if you look at the CT gantry, the PA

44:39

and sometime you may have seen the images

44:41

with the shoulder cutoff.

44:43

Why is the shoulder or the pelvic uh,

44:45

the uh, pelvis area cutoff?

44:47

Because we have what is called as a reconstruction

44:52

MA maximum sampling region.

44:55

It is a little bit less than the actual gantry.

44:58

That's why we can push a, a large patient in

45:01

and out of the gantry.

45:03

But when it comes to images, the shoulder can be chopped off

45:06

because the image reconstruction is validated up to

45:10

what is called the sampling region.

45:12

And that's typically, sorry,

45:14

that's typically 50 centimeter in diameter

45:18

and the blue scan is called the user defined scan field

45:22

of view and

45:23

and that will also impact on the display of the images.

45:27

So if you look in the, if you imagine a pet CT images,

45:31

if you look at the pet CT images where we do the CT portion

45:34

for aian correction, the images are reconstructed

45:38

with one field of view.

45:39

Because of that the head looks

45:41

so small whereas the body is accommodated.

45:44

Whereas if you look at the head ct, the display is almost,

45:48

the head is very big

45:49

because that is impacted

45:50

by the user defined display field of field.

45:54

So nowadays the table can withstand quite a weight.

45:58

Uh, they are, they're quoted almost as much

46:00

as 500 point patient.

46:02

The cant reopening is about 70 centimeter and so forth.

46:06

Let me go to the next question

46:07

before we end this session to answer the questions.

46:11

What is the main advantage of lowering the tube voltage

46:14

in cardiac CT for small patient?

46:23

The answer is correct reduces radiation dose.

46:26

So that's the idea, especially for the tube current.

46:29

So one more question.

46:32

What is the primary source of exposure

46:34

to fetus when a pregnant patient undergo head ct?

46:45

That's also correct, internal scatter radiation,

46:48

not the external scatter radiation

46:49

but the internal scatter radiation.

46:51

This is a very important practical question whether you are

46:55

taking the exam or past

46:57

after finishing if you're forgetting.

46:59

But this is important. The reason is there is

47:02

so much misunderstanding about radiation

47:05

because of that you may face other physician in the

47:08

hospital, um,

47:09

alarming the pregnant patient if they're a asked

47:12

to get a CT scan of a head or chest CT

47:15

because they're worried about the radiation going

47:18

to impact the per fetus, the fetus is impacted

47:21

or at least get exposed to radiation

47:24

only when you do an abdominal or pelvic abdominal

47:27

or pelvic CT where the fetus is directly

47:30

that impacted by the primary radiation.

47:33

Even in that case

47:34

what we have just shown is like if the benefit

47:37

to the patient is very great, it's okay to do a CT

47:41

and the CT to the fetus dose is much lesser than what

47:44

what we use it as a landmark to be concerned about.

47:48

Therefore the interal scatter radiation get absorbed

47:52

therefore from the head ct,

47:54

the inal scatter radiation almost does not reach the fetus,

47:57

even the chest C the internet radiation is

48:00

immeasurable to the fetus.

48:01

Therefore, if a pregnant patient is brought into the

48:05

hospital for emergency or scanning, it needs to be done.

48:08

One should not decline CT purely because of radiation,

48:13

but these are important to understand.

48:16

So this is one of my books.

48:19

If somebody is interested in Japanese,

48:21

it's also translated in Japanese.

48:22

In addition, I have a number of podcasts, a very short, uh,

48:27

what is called it a podcast

48:28

or a video cast on variety of topics on ct,

48:33

about 20 of them on this website.

48:35

My colleagues um, run this website.

48:37

Elliot Fishman is a world famous radiologist.

48:40

His website we have put in this couple

48:42

of physics, uh, short one.

48:44

Welcome to take a look at it. Uh, hear it.

48:47

It's a lot of these topics are 15 minutes only for easy.

48:51

So now let me answer some of the question I'm seeing here.

48:55

There are a couple of questions.

48:56

One is for question for question number 13.

48:58

Let me go back to the question number 13.

49:04

Let me, let me see. Question number 13.

49:11

So here's the question which tells decreasing motive

49:14

for utilizing ity.

49:15

Why not decrease? Uh, let me see, what is the question here?

49:19

Forgot here. Lemme see.

49:23

So the question is why not two voltage?

49:27

So if you, if you're talking about question 13, let me see,

49:33

two voltage will not have imp not that that have same impact

49:38

as the um, uh,

49:41

the reducing dose from going from one 20 to 80.

49:44

In fact, that is only going to impact

49:46

dec they in decrease the radiation dose

49:49

but not the image noise.

49:51

So that's why there is a choice.

49:53

Um, the other question asked is like will increased tube

49:57

current increases or decreased

49:58

of increasing the tube current decreases the image noise.

50:02

They go in opposite.

50:04

Imagine if you're taking a portrait of your face, uh,

50:07

with a light less li less light hitting your face

50:10

that usually the portray looks very creamy.

50:13

That is the same analogy for the city.

50:16

More x-ray photon tries to create an image.

50:19

The image noise decreases so they go in opposite direction,

50:22

but the image noise changes by its square root

50:25

of n the peal statistics and

50:27

therefore, um, so there is 1D tube,

50:32

current tube current going from two 50

50:37

to 100 will actually increase the noise

50:40

but not the reduced dance.

50:41

So therefore the D will not be current.

50:44

It's only the point B is if I told increased it'll

50:49

it'll not reduce dose.

50:50

Actually it'll increase the dose if you

50:52

go down the tube correct.

50:53

So that's why I said D is not the correct.

50:56

Wouldn't lowering KV cost less penetration

50:59

therefore more dose deposit in the patient?

51:02

That's correct. Generally KV going

51:04

down will less penetration.

51:06

However, depending on the patient size,

51:08

if the patient size is small

51:10

or a pediatric space, you don't need the same amount

51:13

of penetration, therefore you can go down in KV

51:17

and that will actually decrease the patient dose,

51:19

but also now decrease the patient dose plus it's also

51:23

increases the image contrast.

51:25

That is the tradeoff we have in the radiography

51:27

or in x-ray lowering tube voltage means higher image

51:31

contrast, especially if you're giving a contrast

51:34

to the patient iod, in contrast there's a KKH around 70

51:38

to 80 KP.

51:40

Therefore if you lower the tube voltage to a hundred

51:42

or 80 kv, we are going

51:44

to gain the image quality plus the radiation dose in case

51:48

this is the only opportunity where you have

51:50

twice the victory of going down in the K voltage.

51:55

How to improve image quality in patient

51:57

with corona STR stents.

51:59

So this is this, the question can be thought

52:02

of like this one is corona coronary stents can cause lot

52:06

of blooming blooming artifacts.

52:08

When you do blooming artifact, the the structure around

52:11

that becomes almost impossible to penetrate because of that.

52:15

Now there are some, uh,

52:16

metal art blooming artifact reduction softwares are

52:19

available that can improve image quality,

52:22

but nowadays we are seeing the photon counting C can be even

52:26

better in terms of reducing this, uh, uh, uh,

52:29

this blooming artifact thereby improving the corona stents.

52:33

There is some excellent publication showing the advantage,

52:36

the photon counting city

52:37

or coronary stent, which will also have a long-term impact

52:41

on the way we are going to treat coronary

52:43

diseases in the future.

52:46

The other question is can you please comment

52:48

on DLS and the use?

52:51

I'm glad you asked that. DRL stand

52:53

for diagnostic reference level.

52:55

The idea here is like you want to create a DRL

52:59

for a particular population to see so

53:02

that you can compare your procedures, your clinic to

53:06

that particular d, r and C, whether your doses

53:09

to the patient are higher.

53:11

The way the DRLs are calculated is we had to collect lot

53:14

of patient data and then averages out to create a,

53:19

create a difference level.

53:21

And the point is like if you compare your protocol dose

53:25

to the DRLs, it has to be less than 75 percentile.

53:29

It is used as a more like a quality control tool

53:32

in the us For example, American College of Radiology has

53:36

what is called as a dose index registry.

53:38

They have, they have millions of data point from thousands

53:43

of centers in the US who send their data

53:46

to the ACR r dose index registry

53:48

because they have millions of data points.

53:50

Now they have, they have published DLS for, for most

53:55

of the 10 most common adult protocol

53:58

and 10 most common pediatric protocol which is

54:01

published in radiology.

54:03

And those publication are freely available. Check it out.

54:06

The other cautious point

54:08

of DLS is like you cannot apply the same DRL from one

54:12

patient population to another population.

54:14

I'll tell you why. For example,

54:15

unfortunately in the US the obesity is very high,

54:18

therefore our DLS numbers as X amount let's say.

54:22

And if it's apply the same thing

54:24

to a southeastern population

54:26

where the average patient size is much lesser.

54:30

So there the the dose, you don't need the same dls,

54:33

same amount of dls, it, you need much lower dose

54:36

to get the same image quality.

54:38

So you have to be cautious of comparing the DS

54:41

of one region to another region.

54:43

All those things are,

54:44

but in general the DLS are very useful

54:47

for quality control purposes

54:48

and you can rectify that's

54:51

what those index registry is providing

54:52

for the, for the client.

54:54

If you are participating in the DS dose index registry,

54:57

you send the data to their A CR

55:00

and they will send you a report telling like

55:02

what is your protocols resulting patient dose

55:05

and how does it compare to the local, regional

55:09

or national reference.

55:11

And then you can correct your protocol

55:13

to see whether do you have to do the same level or not.

55:16

That is the purpose of it. Any other question?

55:24

I know the, that's a good question.

55:26

I know I, I fight all the time with, uh,

55:29

my colleagues in Australia, Europe,

55:31

outside the us The point is like they have misconception

55:35

that DLS is like a, we had to strive

55:38

to go down in the dls.

55:40

In fact, I'll tell you one thing in fact in the A CR when

55:43

they start the um, accreditation,

55:45

in the beginning we were all involved.

55:47

The DLS was the pass fail criteria for a CR CT accreditation

55:52

of the head was set a certain level,

55:56

every site started failing.

55:57

So we had to increase it

55:58

because the image quality was not sufficient.

56:01

So the DLS should not be a driving force

56:04

to go down rather than should be used as a quality measure

56:08

and then adjusted according to the image quality.

56:11

Unfortunately a lot of the country uses dls.

56:14

We thought that actually the best d its best concept is

56:19

clinical reference level should be set up.

56:21

We don't have it. So DLS are easy to set,

56:24

use the phantom bunch

56:26

of physicists set up phantom measure some of these protocol,

56:29

let's say just CT and they can set a DRL.

56:32

Then they will the other parties who are not involved

56:35

with CTR who doesn't know the clinical value,

56:38

they will start using to drive the demand.

56:42

This clinic to lower down

56:44

what they're doing in terms is like they're lowering down

56:47

and that can impact the image quality

56:50

and that can lead to misdiagnosis.

56:52

That can actually do a disservice to the patient.

56:56

Unfortunately the radiation phobia you guys

56:58

and we all had to join hands to fight against this one.

57:02

You had to say we had to acknowledge DR S as its use,

57:06

but it has also has its limitation

57:09

and we had to be very careful on how we utilize DRS

57:12

and I have that constant battle

57:14

and that's what this uh, JAMMA paper which came out today,

57:17

it appears so scare scaremongering.

57:19

Basically the paper is

57:21

and we know the data, we have it utilize it, we know that.

57:23

And, and we have also published on the world report

57:27

and also US report, which they cite,

57:30

but they don't, don't, don't tell the benefit.

57:32

And we, you, we tell in our press release the NCRP report,

57:36

which I was a co-chair published after 10 years

57:40

after the previous report, which I was a committee member,

57:42

we clearly showed in the last 10 years in the US the number

57:46

of CT procedure have increased by 20%

57:49

but the dose has gone down by 20%.

57:51

What does it tell? It tells basically

57:53

what we are doing is right

57:55

and the optimization is important.

57:57

Justification important. So it's going down.

58:00

So maybe the oiss are right.

58:03

I don't know what is that, what is

58:04

that question or is it re comment?

58:06

I'm not sure what it is. I don't want to uh,

58:09

go into the rabbit hole here.

58:11

Does decreasing noise improve image call? Yes.

58:16

That's why for a head ct for example,

58:19

we acquire thin images, thin slice

58:21

to improve for spatial resolution.

58:23

But in order to improve uh, reduce noise, we combine

58:26

to five millimeter immediately.

58:28

Image contrast will create uh, remarkably show up.

58:32

And that's why now with the advanced scanner, the rule

58:36

of thumb is like acquires the thinness slice possible

58:40

and then you can always reconstruct into thicker slices.

58:44

But if you acquire the thicker slices,

58:46

you cannot reconstruct in the thinner slices.

58:49

So the advantage of the advanced advanced CT scanner is

58:52

acquire it as thin as possible.

58:54

That way you can reconstruct

58:56

and create a very nice 3D images

58:58

and you can have a very high spatial resolution,

59:01

but you can also set up a next create dataset

59:04

with a thicker slice thickness

59:05

and make, improve the image, no

59:08

decrease the image noise and so forth.

59:11

Can you touch on the 180 interpretation of the,

59:13

in regard to contrast?

59:14

I, I think I'm running out of time.

59:16

You have to read my book. It is explained very nicely.

59:20

Alright, thank you so, so much Dr.

59:21

Mahesh for a wonderful review.

59:24

And thank you to everyone for participating

59:26

and asking such great questions.

59:29

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59:31

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59:33

Be sure to join us next week, Monday, April 21st with Dr.

59:36

Mahesh again, who will lead us in a physics

59:40

MRI board review.

59:41

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59:44

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59:45

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59:47

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