Larger X-ray dose in traditional X-ray of head vs. Head CT?

[Albertto]

Does anyone have experience with radiation doses and measurement units in radiology?

I was looking at some DICOM metadata...
X-ray image of sinuses that include forehead sinus using Philips DigitalDiagnost C50

(0018,0015) Body Part Examined SKULL
(0018,0060) KVP 77
(0008,1030) Study Description Sinuses
(0008,103E) Series Description Open Mouth PA
(0018,1150) Exposure Time 12 ms
(0018,1151) X Ray Tube Current 581 mA
(0018,1152) Exposure 7 mAs
(0018,1153) Exposure in µAs 6680
(0018,115E) Image And Fluoroscopy Area Dose Product 1.673

"Image And Fluoroscopy Area Dose Product" stood out to me since it is measured in dGy - deciGray

Image and Fluoroscopy Area Dose Product Attribute - X-Ray dose, measured in dGy*cm*cm, to which the patient was exposed for the acquisition of this image plus any non-digitally recorded fluoroscopy that may have been performed to prepare for the acquisition of this image.

​

1 dGy = 100 mGy,​

so 1.673 (dGy × cm2) = 167.3 mGy​

When converting miliGray (mGy) to miliSievert, that would be 163.7 mSv?

1 milligray [mGy] = 0.001 sievert [Sv]
167.3 milligray = 0.1673 sievert or 163.7 mSv

VS.

Cranium/Head CT radiation dose. Using GE Revolution Maxima

Dose Report​

Series	Type	Scan Range (mm)	CTDIvol (mGy)	DLP (mGy-cm)	Phantom cm
1	Scout	S160.000-l100.000	0.07	1.85	Body 32
2	Scout	S160.000-I100.000	0.07	1.85	Body 32
3	Axial	l21.000-S56.500	45.75	366.03	Head 16
4	Axial	S58.000-S153.000	43.03	430.27	Head 16​
			Total Exam DLP:	800.01

This CT study is around 1.680-1.9 mSv according to this DLP calculator, in some other places on the internet, it is mentioned head CT is typically rounded to 2 mSv, which is 8 months of natural background radiation.

DICOM from CT shows much lower X-ray tube current (only 10 mA compared to 581 mA in traditional xray), but longer exposure time in msec (there are various parameters, several exposure times) and therefore larger 'exposure', which is just a pure calculation in mAs.

(0018,1150) Exposure Time 8000
(0018,1151) X Ray Tube Current 10
(0018,1152) Exposure 26
...
(0018,0015) Body Part Examined HEAD
(0018,0060) KVP 120
(0018,1150) Exposure Time 8000
(0018,8151) X Ray Tube Current in µA 127770.74
...
(0018,0015) Body Part Examined HEAD
(0018,0060) KVP 120
(0018,1150) Exposure Time 10000
(0018,8151) X Ray Tube Current in µA 120156.83

It seems that according to some DICOM info, "Image And Fluoroscopy Area Dose Product" is automatically measured with a specific capturing device built in/behind the surface where you are lying or standing in front of..

But could it really be that you receive a higher X-ray dose from a traditional head or sinus X-ray than from a full head CT?

 

[mfb]

so 1.673 (dGy × cm2) = 167.3 mGy

That equation has mismatching units.

I don't find the source of your quote, but it looks like you need to divide by the exposed area to get an answer in Gray. A larger dose for a smaller volume can have the same average dose - it doesn't have to be worse, even though you get very large numbers.

 

[Albertto]

That equation has mismatching units.

I don't find the source of your quote, but it looks like you need to divide by the exposed area to get an answer in Gray. A larger dose for a smaller volume can have the same average dose - it doesn't have to be worse, even though you get very large numbers.

It's per square centimeter, yes, and if picture is taken, for example 15x20 cm, according to this, then you would even need to multiply both horizontal and vertical sizes of field with entrance dose:

For example, a 5 cm × 5 cm X-ray field with an entrance dose of 1 mGy will yield a 25 mGy·cm2 DAP value. When the field is increased to 10 cm × 10 cm with the same entrance dose, the DAP increases to 100 mGy·cm2, which is four times the previous value.

This, again doesn't make much sense regarding the exposure in mSv and the 1.673 dGy cm2 result, which would result in excessively large DAP if multiplied by horizontal x vertical sizes.

And if considering what you mentioned about diving it by the area, it makes a bit more sense, for example area 15x20=300, 1.673/300=0.00557 dGy, although, still it would be around 0.557 mSv, which isn't as low as typically noted everywhere and would result in 2 months of natural background radiation equivalent.

Edit: according to this, detector size mentioned as 24x30 cm (it's probably slightly different everywhere and for each machine). So, it would be 1.673/720 = 0.00232 dGy = > 0.232 mSv, which looks more within the range.

 

[Vanadium 50]

167 mGy cm² divided by the area of 300 cm² is 560 μGy. For x-rays, that's also 560 μSv.

Whether that's "large" or "small" depends on your perspective. It's about two months background radiation.

 

[gleem]

@Albertto could you please tell us your background or interest in radiation doses so that I may provide a better response to your OP?

 

[Albertto]

@Albertto could you please tell us your background or interest in radiation doses so that I may provide a better response to your OP?

My interest was basically to understand how to find out the approximate equivalent effective dosage in Sieverts (particularly mSv) from DICOM metadata of xray imaging that didn't give direct DLP values (like CT do), if for example 1.673 dGy*cm*cm (deciGrey per cm²) were given. Apparently it must be divided by the area of detector size, so it turns out to be less.

So, @Vanadium 50 is probably right about the 560 μSv value, which is comparatively 0.5 mSv for a skull x-ray and about 1.7 mSv for a head CT scan.

 

[gleem]

Instead of answering your question(s) I direct you to a few articles that can give an idea of the issues involved in converting DAP or DLP readings to effective dose. As you will see things are not simple. If you have any questions about these articles I will gladly try and answer them.

For an explanation of the distinction between absorbed dose, effective dose and their determination see
https://www.google.com/search?q=cal...ve&ip=1&vld=cid:6b8582c5,vid:3B-7GFQ7GKc,st:0

For determining CT effective dose from DLP see:
https://www.advocatehealth.com/assets/documents/2015ct-vino-understandingctdose.pdf

for an application of DAP to pediatric cardiac cath see
https://www.ahajournals.org/doi/full/10.1161/01.CIR.0000151098.52656.3A

 

[Sargon38]

The DAP (dose-area-product) - of which I admit not having known the existance until I read this thread - is defined here:
https://radiopaedia.org/articles/dose-area-product-1
and is some kind of medics way of estimating the skin exposure to medical irradiation.

One shouldn't forget that the standard radiation dose units (Gy and Sv) are PER VOLUME units. The Gray is defined as the radiation dose that deposits one Joule of ionising energy into a volume of 1 kg which is close to 1 liter of material if the density is close to that of water (assuming so for human tissue). The Sv is simply a biological-damage corrected Gray, so it is also a per-volume unit. Whether just your little finger was exposed to a beam, or your entire body, if the flux density was uniform, you get the same dose, essentially. Your little finger could have gotten 1 Sv, or your entire body could have gotten 1 Sv. The last case is worse than the first, because in the last case, your entire body has undergone radiation damage (but the same amount of damage per unit of volume) and in the first case, only a part of your little finger.

In order to take into account the "amount" of exposed skin, medics invented this unit of dose-area.
But you cannot determine the dose from the dose-area if you don't know the area.

I do have a feeling for "dangerous" doses, but I have no experience with these dose-area values. Essentially, a few mSv are not really a problem, a few hundred start to be problematic. Medical examinations such as CT scans are of the order of a few mSv at most. Standard radiography is less than a mSv. But of course, the exposed body part can be larger or smaller. The dentist's radiography will only expose a small part of your head, and a body CT scan will expose your entire body. To take account for that, medics use this dose-area unit.