0:01

So the design itself has a challenge.

0:03

We can't simply use a radiography tube for doing a mammography.

0:08

So the mammography tube design is as follows.

0:11

Because here in this case, we are utilizing characteristic

0:15

X-ray, not the Bremsstrahlung radiation.

0:18

So if we use a molybdenum target, if

0:22

any electrons are hitting through the target, this target will

0:26

create a Bremsstrahlung radiation X-ray spectrum like this.

0:30

It ranges from zero to 30 kVp. You can see a lot of X-rays are

0:34

produced at lower energy, higher energy.

0:37

Now, if the energy of the electrons is slightly higher to

0:40

create a characteristic X-ray, this molybdenum energy

0:44

also creates characteristic X-ray around this

0:47

19 or 18, 17, 18, 19 here in this area.

0:53

If you combine this one, this is what the spectrum looks like.

0:56

The composite spectrum of a molybdenum target looks like this.

1:00

It has a Bremsstrahlung radiation, but it also has characteristic

1:04

X-rays.

1:05

So monoenergetic X-rays are required to create

1:10

high subject contrast at low radiation.

1:13

Therefore, we utilize what is called as, we want to

1:17

enhance this characteristic X-ray and subtract all

1:20

this Bremsstrahlung radiation.

1:23

Typically, in the mammography range, we use between 17 and

1:27

25 kilo electron volt range. That translates to

1:31

anywhere from 25 to 40 kVp. That is

1:35

the tube voltage set on the machine.

1:37

And also because of the need for high spatial resolution,

1:41

we use a very small focal spot of the order of

1:45

0.1 to 0.3 millimeter,

1:48

and that's okay to use it because the breast does not have any bone,

1:52

and it can penetrate the soft tissue, and without damaging the X-ray tube.

1:56

That's why the lower, very fine focal spot sizes are used.

2:01

Now,

2:02

the challenge in the mammography is how we play

2:06

between the target and the filter material.

2:10

So what I'm trying to show you here is like, if we have an

2:14

unfiltered Bremsstrahlung spectrum for a Mo

2:17

target of the X-ray tube, this is how it looks.

2:21

The maximum tube voltage is 30 kV here, and you know

2:25

all the X-rays in Bremsstrahlung radiation are at the lower, spreads across

2:29

the energy range. There is also characteristic X-ray.

2:33

These X-rays coming out of the molybdenum target

2:37

passes through a filter. Let's say we have an aluminum

2:41

filter to lock off a lot of the low energy.

2:45

Let's say we want to use aluminum that has a very

2:49

high attenuation coefficient at the lower energy,

2:52

therefore it absorbs a lot of these Bremsstrahlung radiation, and the

2:56

resulting spectrum will look like this.

2:59

A lot of the Bremsstrahlung is gone.

3:01

Characteristic X-rays are predominant, but there is still lingering effect of

3:05

the high-energy spectrum of Bremsstrahlung.

3:08

That's why we use a combination of molybdenum with a

3:12

molybdenum filter

3:14

because the molybdenum filter has a very attenuation

3:17

coefficient, very high absorption between, and around this

3:21

area, it opens up very high because of characteristic X-rays.

3:26

Because of that, if you subtract these two spectrum, you'll get

3:30

like this.

3:31

The spectrum is more predominantly characteristic

3:36

X-rays, which is now interacting with respect to the patient.

3:41

There is also other targets used. Rhodium target is

3:45

used for slightly thicker breast or dense breast, which is

3:49

recommended. The system has the rhodium target, where the

3:52

characteristic X-rays is slightly higher, and again, with the

3:56

molybdenum target with Mo filtration, rhodium target with

4:00

rhodium filtration, you can see the K characteristic is slightly to the

4:04

left of the molybdenum target in Mo filtration.