I've been looking at the ancillary data for my DEXAs and I have a lot of questions. Generally, all anyone talks about on this website are T-scores, which is fine in terms of having points of comparison, but I am look more at the numbers that go into calculating BMD.
BMD = g/cm sq.
g = BMC (bone mineral content)
cm sq = area
area = width (cm) X height (cm)
g = grams
What I am specifically interested in is how does a DEXA machine determine the grams or BMC.
I'm looking at the numbers they have reported for the area of my vertebrae and they vary a good deal. I get that it is hard to measure the area of each vertebra (they're not blocks) but it seems to me that the area shouldn't be changing that much unless you've grown a ton of new cortical bone, which is unlikely but, if it does happen, should mean your bone density is increasing, unless the area was measured incorrectly on a previous DEXA.
Below is the ancillary data for my L1 - L4 for 4 DEXAs all done on a GE Lunar scanner.
Note: 2019, 2021, and 2022 DEXAs were done on the same GE LUNAR scanner. That scanner broke and my 2023 DEXA was done on a different GE LUNAR scanner.
As you can see, on the 2023 DEXA, the area of my L1 increased over 1 cm sq. LOL.
The reason I want to know how a DEXA determines the BMC or grams is that if the area of a vertebra were to stay constant and BMC were to increase then BMD would also increase. Given that I feel somewhat certain that my L1 vertebra did not increase by 1.33 cm sq, if I use the grams from my 2023 scan and the area from my 2022 scan (which is also approximately the average of my 2019, 2021, and 2022 scans), I might be able to calculate a more realistic BMD. But if the grams are determined by some goofball equation that isn't shown, then what I am trying to see won't make sense.
The question I am trying to have answered is how does a DEXA scanner determine (calculate) the BMC or grams.
Written by
FearFracture
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What an interesting and pertinent question. I can see why you would question the apparent changing width of L1! I gather the scan operator has to manually draw a box round each vertebra and should ensure that this matches each time, but beyond that I have no idea how it's done!
There seem to be a lot of odd things in the ancillary data and it's not because the scans were done on two different machines. Something is just OFF. Here is another example.
2022 L1 Width 3.1 cm ------- 2023 L1 Width 3.7 cm
2022 L2 Width 3.3 cm ------- 2023 L2 Width 3.7 cm
2022 L3 Width 3.4 cm ------- 2023 L3 Width 3.7 cm
2022 L4 Width 3.7 cm ------- 2023 L4 Width 3.8 cm
Maybe the 2023 technician was just really sloppy but that can't be right.
Also the math is off on 2023 report. My L1 shows width 3.7 cm, height 2.84 cm, area 10.36 cm sq, but area = width x height and if you multiply 3.7 x 2.84 you get 10.508 cm sq.
What if the area they reported in 2023, which for my L1 is 10.36 cm sq,, what if that (the area) is correct and just the width was listed incorrectly (all the heights vary). For my L1 they have the height listed as 2.84 cm. If I divide the area they listed by the height they listed, so 10.36/2.84, that would make the "true" width 3.6478 cm. But again, if I look back at my 2022 DEXA ancillary data, which reports my L1 width as 3.1 cm, there is just no way that the width of any of my vertebrae increased by .5478 cm in one year--that's more than 1/2 a centimeter.
I think I am going to have to go talk to someone at the imaging center to find out what is going on because who knows.
Of course after going through all of this I got a measuring tape and measured out 3.1 cm and now I wonder how I can even stand up. LOL.
Hi FearFracture, I remember a while ago I went to a fraction liaison service webinar. They mentioned that different machines will give different results. I could not find the notes or recording but I found some links below.
For your 2023 results on the new scanner as your scanner from the previous years broke, the 2023 results cannot be compared to the old results.
From what I understood there are differences between machines even if they are the same make and model. So there will be an error in the scores which is maybe why the 2023 scores are higher.
For how the Bone mineral content (BMC) is calculated the machine uses this equation:
BMC(g) = total mass (g) -fat free mass (g) -muscle mass (g)
The x-rays used in the dexa scan are absorbed by soft tissue (fat free mass and muscle mass) and your bone.
This difference in absorption between the different types of tissue is what helps to identify the BMC.
While BMC is the weight of the bone the BMD is a measure of the density of the bone. I.e. how much bone in grams is in this area e.g vertebrae.
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Big improvement in dexa - could they have missed a fracture? 
DEXA results error due to incorrect positioning
Is it really necessary to take Alendronic Acid with these results?
Need to get incorrect diagnosis changed
