0:00

Then came the direct digital radiography where the

0:04

rooms were completely built in, where the room

0:07

automatically synchronized with the X-ray tube and the digital

0:11

detector, and we call this as a direct digital radiography.

0:16

So with the direct digital radiography, what happened is this is a

0:19

generic term applied to an imaging system, which has

0:23

a digital detector,

0:25

which is to acquire the X-ray projection image, and then it has a digital

0:29

electronics to convert that signal to digital form.

0:33

Like the modern cameras, we no longer use the old Kodak

0:37

films. Now we have our cameras, which directly captures the

0:40

image and create a digital image. That's an exactly analogy

0:44

happened in our digital radiography also.

0:48

But if you look at the digital radiography, there are different process

0:52

that we done. So in this particular slide, I'm showing

0:56

three different way of capturing the image, and on the

1:00

left-hand side is the most analog system, the film screen

1:04

cassette.

1:05

And on the middle one is a indirect digital, which

1:09

uses a scintillator, and the last one is a direct

1:12

digital. What is the difference between the three things?

1:16

So if you look at the way the film screen system work, every film

1:20

cassette has a intensifying screen, and that

1:23

intensifying screen converts the X-rays into light

1:27

photons, and the light photons then was recorded

1:31

on the film cassette.

1:34

And this is the typical way it is done.

1:36

In a indirect digital, it's almost like this, where you

1:40

have a scintillator.

1:42

Typically, we use caesium iodide crystal scintillator.

1:47

And that scintillator is the first end, front end of

1:51

these digital detectors, which will capture the X-ray

1:54

photons. Immediately, X-ray photon coming out of the patient,

1:58

it hits the caesium scintillators.

2:01

It converts the X-rays into light photons.

2:04

The light photons will dance all over the place, and some of

2:08

it will reach the detector. And this is electronically,

2:12

it's called the thin film transistor array or a

2:16

CCD camera. So it is made up of amorphous

2:20

silicon thin film transistor.

2:23

That transistor will absorb the light photons,

2:26

converts into electrical signal, and that converts into an image.

2:31

And so therefore, there is always some

2:34

compromise with respect to the spatial resolution

2:38

because at the point it hits, the light spreads.

2:42

So almost there is a light, we call it a line spread

2:45

function for calculating or estimating the spatial

2:49

resolution. There is some diffusion because it

2:53

depends on how the light is reaching and which part of the lights are reaching,

2:57

light photons are reaching the detector.

2:59

Then there is a special category of digital detector called

3:03

direct digital. It converts X-ray directly

3:07

into electrical signal. There is no caesium iodide crystal,

3:12

and this is a special material. It's called amorphous

3:16

selenium semiconductor. So the amorphous selenium

3:19

material suits very well in the energy range of the

3:23

diagnostic X-rays coming out of the patient,

3:27

converts the X-ray photons directly into

3:31

electrical signal. Therefore, at any point it is

3:35

interact, it creates electrical signal, and that's captured by

3:38

the thin film transistor array. Therefore, that

3:43

line spread function is almost straight.

3:46

The possibility of getting a very high spatial resolution is

3:50

with this direct digital. So compared to analog,

3:54

the digital radiography is predominantly indirect

3:58

digital or direct digital.

4:02

So indirect capture, more detail if you look in here, there

4:06

is a two-step process where the scintillators such as

4:10

caesium iodide crystal will capture the energy

4:14

and converts to light photon. This caesium iodide crystal

4:18

also has a impurity built in, it's called thallium

4:21

impurities built in, so that the light conversion is

4:25

not overflood the image. Some of it is captured by the

4:28

thallium, and that's to control the light conversion and the

4:32

amount of light hitting the photon.

4:34

So the array of thin film transistors, in the bottom

4:39

are like a CCD cameras, converts the light

4:42

photons to electronic signal.

4:45

This is a schematic diagram of a digital detector

4:49

where you have, which is placed under the patient to

4:53

capture the image. So on the top is this layer of thin film

4:57

caesium iodide crystal, which converts the X-rays into light

5:01

photon. The light photon, when they hit these thin film

5:05

transistors here, they converts that into electrical

5:08

signal, and this is made up of amorphous

5:12

silicon-based system. That's why it's called amorphous silicon

5:16

detector, which is an indirect digital

5:19

because this is what this X-rays light hits these buckets here,

5:24

and that's how it converts it. And this is an electronic built into each of these

5:27

pixel and converts. Smaller the pixel size

5:31

here, more expensive the digital detector becomes

5:35

because this pixel is, this particular diagram is the

5:38

pixel size, and part of it is taken away by the electronic

5:42

itself. So there is a limitation how small it can make, and

5:46

that's with the amorphous silicon system.

5:49

Why do we use caesium iodide crystal?

5:52

Because the caesium iodide crystal can be grown

5:56

in the lab like a linear structure.

5:59

This is an electron microscopy image

6:02

The pixel length is 143 micrometer.

6:06

This is the distance, is about less than 20 micrometer, is so many fibers.

6:10

So each of these cesium iodide crystals, imagine the

6:14

X-rays hitting them, the cesium iodide will convert into light,

6:18

and the light doesn't spread too much because of the linearity of the

6:22

crystal, and they are funneled towards and hit the thin film

6:26

transistor to create an electrical signal.

6:29

This is a direct capture mechanism, and that is the material

6:33

used is amorphous selenium. It is a

6:38

special type of material, and it is a direct photoconductor.

6:43

It absorbs the X-rays, and that X-ray is immediately

6:47

converted into electron pairs. The electron pairs

6:50

then drift towards the polarized electrodes and is collected by

6:54

pixel capacitor.

6:56

Especially, it has a very nice narrow line spread

7:00

function because there is no diffusion of the light.

7:03

There's no light conversion. The X-rays directly converts the

7:07

electrical signal.

7:09

So just to recap it, an X-ray image can be

7:13

captured either analog process using the film screen cassette, which

7:17

is what it used to be done from the time of

7:21

1895 when X-rays were discovered, to all the way up to

7:24

1970s and '80s. Only in the '90s and

7:28

2000s onwards, we started the influx of the digital

7:32

radiography coming into picture.

7:34

Among the digital radiography, the first process most hospital

7:37

adapted was the indirect digital method using the CR

7:41

plate so that they don't have to replace their X-rays rooms

7:45

immediately. But over the period of last 20 years, we are now

7:49

seeing most of the X-ray rooms are now

7:53

with digital detector system, which has a built-in X-ray tube

7:57

with the detector itself. And the digital detectors

8:00

have two ways of doing it. One is indirect capture,

8:04

which is an amorphous silicon-based detector, direct

8:08

capture, amorphous selenium-based system.

8:11

As of now, most of the radiography done

8:16

in the US is all indirect capture method,

8:19

amorphous silicon detector.

8:22

Except in mammography. There we use what is called as the direct

8:26

capture, also amorphous selenium.

8:30

The digital detector used in interventional fluoroscopy or

8:33

anything are all amorphous silicon based.

8:36

We are not being able to get the amorphous selenium to do the

8:40

fluoroscopy because of there are a lot of physics inherent

8:43

limitation. Having said that, to just to give you an

8:47

overview of the status, now, most hospital in the

8:50

US or developed countries are digital radiography

8:54

rooms.

8:55

You'll be surprised many of the LMIC country still has the

8:59

analog system. Just to give a caveat, the

9:03

spatial resolution obtained by the digital detector

9:07

still is striving to match the spatial resolution

9:11

of that of a film screen system.

9:14

Film screen system still holds the highest spatial resolution

9:18

system.

9:19

But the other advantage of the digital radiography is the one which has

9:23

impetus to make this towards moving towards digital radiography.

9:28

With the basic information of the radiography system,

9:32

how it is acquired, and so forth, now I move into more

9:36

specialized aspect called mammography physics.