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## Main Question or Discussion Point

Sorry for having to ask another question, but there is just one more thing that I want to get my head around.

Let's say you have this protocol:

Standard Resolution

64/40 x 0.625 collimation

2.5mm thickness

40/25 mm increment

240 scan angle

0.4s rotation

120 kv

55-165 mas

512 matrix

75% prospective phase

220 DFOV

approx 120 mm scan length

Filter CB

100 mAs

OK now here is my issue. I've been told that you can estimate effective dose by using the following formula:

[tex] Effective dose = DLP * k[/tex]

Now the k-factor is a normalised conversion factor and can be found at this site:

http://www.drs.dk/guidelines/ct/quality/mainindex.htm ------- Appendix 1 of Chapter 1

What I don't understand is the scan angle. I thought that the CTDI and therefore the DLP were given for 360 degree scans. So therefore your estimation of effective dose (if you only use a 240 degree scan angle) wouldn't be correct if you used the above formula.

For example, let's say that you performed 2 scans of the chest. For both scans you have the EXACT same protocol (same kVp and mAs) except for the scan angle. One has a scan angle of 180 degrees and one has a full 360 degree scan angle. I'm just not understanding how you can use the same formula for both these situations. I mean if you are scanning 180 degrees that is centred beneath the patient then you aren't uniformly irradiating all the organs in the chest region are you? I'm really confused about this because I just don't think that you can use that formula yet I've been told that the dose isn't dependent on the total scan angle?

I hope this hasn't been to muddled of an explanation, but I'd appreciate any assistance.

Let's say you have this protocol:

Standard Resolution

64/40 x 0.625 collimation

2.5mm thickness

40/25 mm increment

240 scan angle

0.4s rotation

120 kv

55-165 mas

512 matrix

75% prospective phase

220 DFOV

approx 120 mm scan length

Filter CB

100 mAs

OK now here is my issue. I've been told that you can estimate effective dose by using the following formula:

[tex] Effective dose = DLP * k[/tex]

Now the k-factor is a normalised conversion factor and can be found at this site:

http://www.drs.dk/guidelines/ct/quality/mainindex.htm ------- Appendix 1 of Chapter 1

What I don't understand is the scan angle. I thought that the CTDI and therefore the DLP were given for 360 degree scans. So therefore your estimation of effective dose (if you only use a 240 degree scan angle) wouldn't be correct if you used the above formula.

For example, let's say that you performed 2 scans of the chest. For both scans you have the EXACT same protocol (same kVp and mAs) except for the scan angle. One has a scan angle of 180 degrees and one has a full 360 degree scan angle. I'm just not understanding how you can use the same formula for both these situations. I mean if you are scanning 180 degrees that is centred beneath the patient then you aren't uniformly irradiating all the organs in the chest region are you? I'm really confused about this because I just don't think that you can use that formula yet I've been told that the dose isn't dependent on the total scan angle?

I hope this hasn't been to muddled of an explanation, but I'd appreciate any assistance.