0:01

So now the third factor, pitch, the scan time.

0:04

So when I say scan time, that is the amount of time the X-ray

0:08

tube takes to rotate around the object one time,

0:13

and that's usually measured in second or millisecond.

0:17

And this is typically is the gantry

0:21

rotation time, which will affect the total study time.

0:26

The scan time will also impact the temporal resolution

0:30

because unlike only in X-rays, when you

0:34

take an X-ray, it is done in a fraction of a millisecond, means

0:38

you freeze any physiological motion.

0:42

Not so in other areas, and that is defined by what is called as a temporal

0:46

resolution. We want very high temporal resolution

0:50

to freeze any physiological motion.

0:54

In general routine scanning of the abdomen or extremity,

0:58

there is hardly any physiological changes.

1:00

Whereas if you are trying to image in heart, you are

1:04

challenged because of the fast movement, therefore, you need a

1:08

technology which has a faster and faster, means like a

1:12

lower scan time in order to get a better temporal

1:15

resolution. And this also have an impact on the radiation

1:20

dose. The scan time will also have an impact on the IV

1:23

contrast timing that needs to be adjusted, and so forth.

1:28

Now,

1:30

these three pictures I'm going to show you to show different scan time

1:34

we talk about in CT.

1:36

Generally, when we say scan time, it is the amount of time

1:40

the X-ray tube take to rotate one time around the

1:44

object. That is called the full rotation time,

1:47

where X-ray tube is on during the entire

1:51

360-degree rotation.

1:54

Partial scan time can be done

1:57

because a minimum amount of data required for image

2:01

reconstruction is at least half of the gantry rotation

2:04

time.

2:06

So sometimes for cardiac CT, we get away with what is called as a

2:09

partial scan time. The X-ray tubes are only turned half of the

2:13

rotation time.

2:15

That will define the temporal resolution.

2:18

Means faster the scanner. Now, there is a limitation on

2:22

how fast the X-ray tube can rotate.

2:24

We are pushing the boundaries to the point where the

2:28

G-force created in this fast rotation is several

2:32

G-forces, similar to what the fighter jet pilots

2:35

experience with the high G. Similarly, we see similar

2:39

G-forces created with this faster rotation time.

2:43

We have moved from five-second rotation time now to

2:47

0.3-second rotation time or even

2:50

0.27-second rotation time, also called as

2:53

270 millisecond.

2:55

Partial scan time is half of it, plus the half angle.

2:59

Now, with the technology having a two X-ray tube,

3:04

we can further call what is called as acquire the data

3:07

in one quarter of the rotation time.

3:10

That is called the quarter scan time, which is achievable with

3:14

dual X-ray tube.

3:16

So for completion's sake, these are the three different scan time, but the

3:19

most common one we associate with CT is the full rotation

3:23

time.