Medical CT Dose Info: Understanding CTDI & DLP Values

[big man]

I've recently started a mini project on typical doses that patients receive when having particular CT examinations and there is just something that I don't really understand that I was hoping someone here might be able to help out with.

Now when a doctor takes some CT images the CT machine will give him dose information. That is, it will provide him with a CTDI (computed tomography dose index) value and a DLP (dose length product) value.

From all the material I've read so far on this the CTDI value is something that is determined using phantoms (head or body) with pencil ionisation chambers. So how does the CT machine estimate CTDI and DLP for patients?

It's just confusing because I thought to get a CTDI value you had to perform certain measurements with a head or body phantom and I'm pretty sure the doctors wouldn't do that before processing the patient.

Any help would be appreciated.

 

[imabug]

So how does the CT machine estimate CTDI and DLP for patients?

The CTDI and DLP displayed for patients are based on factory calibrations done prior to the machine being shipped and installed. The CT manufacturer measures the CTDI for a variety of phantom sizes and kVp. These values are scaled by the mAs used during a patient scan to obtain the patient specific values.

 

[Tsu]

Just in case anyone is interested - if you have a CT scan and want to know what dose you received, most new scanners can give you that info immediately. Many of the scanners are now dose modulated. It gives less radiation in the smaller areas of your body and more in the thicker parts where it is needed. Just ask your technologist.

 

[big man]

Thanks for the information imabug. I have to say that this topic, although interesting, is also quite confusing at the same time. So I really appreciate you straightening that point out for me.