0:01

How do we understand the CT dose display?

0:03

These days, all CT scanner provides both pre and

0:07

post displays.

0:09

In the pre-display, if we change the technique, it'll

0:13

automatically show what is an estimated CTDI volume.

0:17

Again, as I mentioned, we don't measure anything dose on patient

0:21

directly. We estimate the patient down indirectly.

0:25

Therefore, by changing any of the parameter here on the scanner, even

0:29

prior to scanner, you can see what is the CTDI will be, and then

0:33

you can verify afterwards the scan is done.

0:36

On the other hand, if you're seeing a patient image these days, any

0:40

CT images will have an exam, will have

0:43

one image which has the detail of the scan.

0:48

There are other files available called the DICOM header information.

0:52

For this discussion, it is suffice to say this post-scan display

0:56

will display the details of the scan, the scan parameter

1:00

used, and how do we can estimate from here effective dose is an important

1:04

one.

1:05

Because we don't care about the numbers, but we would rather care

1:09

about how that radiation impacts me.

1:12

And we have lot of example why people are worried about radiation,

1:16

because it's very difficult to quantify, see, or touch.

1:20

The way we do is, like in CT, we estimate what is called as the

1:24

effective dose.

1:26

So the effective dose was a concept developed in '50s and

1:30

'60s for

1:32

estimating risk to the whole body for nuclear power planters or

1:35

occupationally employed workers.

1:38

Now, effective dose can be calculated with this

1:41

formulation, where H is a dose equivalent

1:45

multiplied by the tissue waiting factor.

1:48

Some organs in our body has more sensitivity to radiation

1:52

compared to the other organs. Right now, the most

1:56

sensitive organs are breast, GI tract, lung, and so forth.

2:01

Gonads are considered no longer the most sensitive organ in our

2:05

body.

2:06

And this effective dose is measured expressed in the sievert

2:10

for the international unit and rem for the local unit.

2:15

The way effective dose is calculated is if this particular area is

2:19

exposed in the image, we need to take into account the organ dose to the

2:22

organ, multiplied by a waiting factor and so forth

2:26

until the entire region is covered.

2:29

For simplicity case, there is also a quick conversion factor

2:33

called as k-factor or dose coefficients.

2:38

And these are given established numbers for a variety of

2:42

most commonly done CTs. And you take this number and

2:45

multiply with the DLP available in a scanner, you

2:49

can get some idea of what the effective dose is going to

2:53

be. This is exactly what's done here in this case,

2:56

where this particular patient had four of these series,

3:01

calcium score, test bolus, and coronary CT geography.

3:05

And having this information, now a medical physicist can

3:09

calculate what is the effective dose to the patient based on this particular

3:13

information.

3:15

Here's a table of adult effective dose for various CT procedures.

3:18

And this was we published back in 2008, giving you an

3:22

average effective dose for the different CT exams

3:26

and the range we found in the literature.

3:30

We also use some of this information, what was calculated,

3:34

is now in the radiologyinfo.org website.

3:37

This is a website which has information for patient over 240

3:41

procedures. Along with it, we also have a radiation

3:45

safety section where the radiation dose from

3:49

different procedure are given with the approximation value and

3:53

how does that equal to the number of background radiation level in the

3:57

US. Because in the US, the natural background

4:00

radiation level is about 0.3

4:04

rem or 3 millisievert, and that's how we estimate this

4:07

comparison.

4:09

I like to demonstrate or show this importance of image quality because

4:13

what happening these days is sometime the clinic are driven

4:18

to lower the radiation dose, but without evaluating

4:22

the image quality can actually jeopardize the

4:25

diagnosis. So generally, the imaging should

4:29

drive CT dose, not the CT dose driving the imaging.