Serial or compare bone mineral densitometry: how to do it step by step

Bone mineral densitometry (BMD) is the most valuable method for assessing bone and calculating fracture risk. Serial or comparative bone densitometry is important in rheumatologists' work on osteoporosis management. The response or lack of response to osteoporosis treatment based on densitometry scans is crucial. This paper examines the timing of scan requests concerning the history of glucocorticoid use, renal or other solid organ transplantation, malignancy, and other situations discussed. We encountered four types of compared scans based on the centers where BMD was performed and the precision of the devices used for this survey: Same Center, Same Device (SSSD), Same Center Different Devices (SCDD), Different Centers Same Devices (DCSD), and Different Centers Different Devices (DCDD). We discussed the principles of comparison and the key indicators.

Step II: Control of good scan criteria This step has 2 phases: I. Checking the similarity of scan image (straightness, top, bottom, both sides) II.Checking the good scan criteria in each scan It's very important that takes the new scan looks as similar as possible to the previous scan (except artifacts).The straightness of limbs (for example spine or hip) is very important at this step (Figures 5, 6).Since the first (old) scan is the basis for the second (last) scan, it is important to teach the technician the criteria for a good scan as shown in Figures 7 and 8 [9][10][11][12][13].
Spine good scan criteria (Figure 7) are: 1-Lack of artifact 2-Spine should be straight (S) 3-Upper border: at least ½ of T12 (A) 4-Lower border: at least ½ of L5 (B) 5-Each side: at least 2 cm at each side of spine (C) 4. Lower border: at least 1.5 cm below ramus of pubis (B) 5. Inner border: a. no or small size of lesser trochanter (E) b. visibility of a part of obturator foramen (D) c. distance between ramus and neck 1cm(C) 6. Outer border: at least 1 cm soft tissue (OB) When this step successfully passed (similarity of each scan in previous and recent results such as Figure 4 for spine scans and Figure 5 for hip region), we can go to step III.In should be emphasized that the technician must be seen the previous scan and take the new one with maximum similarity to earlier scan [14][15][16][17][18][19][20][21][22][23][24][25][26].

Step III: Unification of Region of Interest (ROI) insertion
During this step, the ROI or area of both scans should be the same.In spine region it's necessary to uniform labeling of vertebrate in scans.For this purpose, the vertebrates should be labeled or numbered.Technically two ways for labeling exist: Serial or compare bone mineral densitometry Rajaei 1. Shape of vertebrae: as a rule, vertebrae of L1, L2 and L3 are U shape and L4 is "X" or "H" shape.L5 has "wm" so the above border of L5 is same "w" and below borders as "m" (Figures 9, 10).
2. Landmark use: iliac crest bone is parallel of L5 and rib connects to T12 and the third landmark is long transverse process that see on L3 (Figures 11  and 12).In compared BMD, unifying of both spine scan is very important and need to use of similar labeling of vertebrae correct passing of this step is obligatory for going to next step.
In the pelvic area, it is important to have the neck box (rectangular box) in the same location in both scans (Figure 13).For this purpose, four characters should be assessed:   17) and should not be separated (Figure 18).If all the characters in the two images are similar, you can go to the next step.It should be mentioned again, we or technician can correct this step without repeating of scan [27][28][29].If all the characters in the two images are similar, you can go to the next step.It should be mentioned again, we or technician can correct this step without repeating of scan [27][28][29].

Step IV: Control of area & BMD in both scan
This step has 3 phases for spine region and two steps for hip and forearm regions.In the spine scan 3 phases should be considered:

Phase 1: selection of best region
For selection of best region, area and BMD rules should be considered.According to the area rule, the lumbar spine area gradually increases from L1 to L4 (L1 < L2 < L3 < L4) (Figure 19).The border of the vertebral region is determined by the technician, so it requires great care to clarify the top, bottom and both sides.On the other hand, the area rules are the most important duty of the technician, and compliance with the rules of the area in each scan (spine, hip, forearm and whole) is the main duty of the technician to reduce reporting errors.

Phase 2: selection of the common best region for both scan
If two scans have the same best region based of area and bone density, the same region is selected as the common best region.But if in the first scan L1-3 is suitable and in the second L2-4 is suitable, then L2-3 is the common best region (Figure 23).

Phase 3: Controlling the area difference between two scans
At this phase, it is necessary to compare the common best region between the two scans and this difference should not be more than 2 square centimeters (Figure 24).It should be noted that if there is no problem in performing & analysis the scans, the device itself compares the L1-4 region (Figure 25).
In the hip scan 2 phases should be considered:

Phase 1: Choosing the total area as the common best region
For comparison in hip scan, we should be used total region.If the total region cannot be used in decreasing frequency greater trochanter, inter trochanter, and finally neck regions may be used.

Phase 2: Controlling the area difference between two scans
In hip scan, the difference area should not be above 2 cm² (Figure 26).If the difference in the total area of two scans is more than 2 square centimeters, then it is necessary to correct this difference by increasing or decreasing the area.
Figure 27 shows the compared scan in hip region with high area difference and how to correct this problem [30,31].
Step V: interpretation This step has 3 phases: Phase 1: report each scan separately In this phase based on the results (best selected region in step IV for spine, lower result between total and neck in hip scan and 1/3 of radius for forearm) of bone density are reported (Figure 28).It should be mentioned here that the final conclusionis the lowest result between spine, total and neck in hip, so that if, for example the spine is osteoporotic and the neck is normal, the final conclusion is osteoporosis (Figure 29).Phase 2: determine percentage change between two scans: In spine, after finding of best region or best vertebrae in previous step we can take percentage change between scans based this formula: Cbmd= new the common selected region bmdprevious same the common selected region bmd/ previous same the common selected region bmd ×  Serial or compare bone mineral densitometry… Rajaie Another way to measure Cbmd is to use standardized bmd or sBMD calculator.To interpret densitometry with two devices, it is necessary to first convert BMD into sBMD (Figure 30) which shows how to convert in three different devices used (we will fully discuss the use of SBMD and its value later in DCDD, SCDD, DCSD topics).When the information is put in the standardized BMD calculator, the result of the Figure 31 is obtained.In hip region, the best region for comparison is total hip and we can take percentage change between scans based on this formula: Calculation of compared hip scan (based formula) = total hip bmd (new) -total hip bmd (previous) / total hip bmd (previous) × 100 For example, the calculation of compared scan in the hip scan based on the standardized BMD calculator for case Figure 32 is as follows: = total hip (2020) -total hip (2018) / total hip (2018) × 100 = 0.887 -0.864 / 0.864 × 100 = 2.7% ~ 3% The bone density change percentage in the hip scan is often presented as a graph (Figure 33).Phase 3: Review of response to treatment: To check the response to treatment, we need at least two information: 1. Precision of the device 2. The Least Significance change (LSC) of the It should be noted that the precision of the device is constant, but the LSC of the center is often variable, because the technicians and their ability can change over time.How to calculate the summary LSC (sLSC) is as follows: sLSC = (LSC C1 + LSC C2) × (precision C1 + precision C2) Based on this, LSC C1 is the LSC of the first center, LSC C2 is the LSC of the second center, and precision C1 & C2 is the precision of the first and second centers.For example, with the following information, the result of the sLSC for hip & spine region is as follows: Spine: LSC C1=1.8%,LSC C2= 2%, precision C1= 1%, precision C2= 1.2% sLSC For SCSD, the precision of the device should be considered constant, and often LSC is often constant, so the formula changes as follows: sLSC= LSC × precision (if LSC of first & second   There are other definitions for examining response to treatment, an example of which can be seen in Figure 34.Based on the mentioned rules, for the case, the spine changes -5% and hip +3% were calculated.The final response is "partial response", which means that is not necessary to change the treatment [31][32][33][34][35][36].[33]

Figure 1 .
Figure 1.Algorithm for repeat BMD in some conditions

Figure 2 .
Figure 2. Algorithm for serial BMD based of BMD result

Figure 3 .
Figure 3. Attention to ID characters in the first scan

Figure 4 .
Figure 4. Attention to ID characters in the second scan

Figure 5 .Figure 6 .Figure 7 . 8 .
Figure 5. Special attention to spine image for similarity of straightness, above, below & sides of both scan

Figure 11 .
Figure 11.labeling of vertebrae based of landmarks

4 .
The ward triangle (the square box) should be near or attached to neck box (WBNB at Figure

Figure 13 .Figure 14 .Figure 15 .Figure 16 .Figure 17 .Figure 18 :
Figure 13.Neck box (green box) Figure 14.Neck box between head (H) and greater trochanter (GT) Thus, if both rules (area & BMD) are kept, L1-4 selected and based on the rules three or two consecutive vertebrae should be elected (Figure 20 area error & Figure 21 BMD error).The best region to scan the spine for a patient is shown in

Figure 22 .
Figure 22.Phase 2: selection of the common best region for both scanIf two scans have the same best region based of area and bone density, the same region is selected as the common best region.But if in the first scan L1-3 is suitable and in the second L2-4 is suitable, then L2-3 is the common best region (Figure23).Phase 3: Controlling the area difference between two scansAt this phase, it is necessary to compare the common best region between the two scans and this difference should not be more than 2 square centimeters (Figure24).It should be noted that if there is no problem in performing & analysis the scans, the device itself compares the L1-4 region (Figure25).In the hip scan 2 phases should be considered:Phase 1: Choosing the total area as the common best regionFor comparison in hip scan, we should be used total region.If the total region cannot be used in decreasing frequency greater trochanter, inter trochanter, and finally neck regions may be used.Phase 2: Controlling the area difference between two scansIn hip scan, the difference area should not be above 2 cm² (Figure26).If the difference in the total area of two scans is more than 2 square centimeters, then it is necessary to correct this

Figure 19 :Figure 20 .Figure 21 .Figure 23 .Figure 24 .
Figure 19: keeping of rules in area and BMD in all vertebrae from L1 to L4 and election of L1-4 for report

Figure 28 .
Figure 28.Algorithm of BMD report based the results of best region of spine and hip(total or neck), forearm(1/3 or 33%) & whole body (subtotal or TBLH)

Figure 32 .
Hip scan compared BMD calculator

Figure 31 .
Figure 31.Use of standardized BMD for case of Figure 22