Two versions of DXA scanner were used in the study. All urban participants measured in MRCG Fajara (n=58), were scanned using a GE Lunar iDXA (enCORE™ software 2015, Version 15.20.002, GE Healthcare Lunar, Belgium). All rural participants were measured at MRCG Keneba, thirty-one rural participants were scanned on the iDXA (Figure 6:3, A) and fifty-one rural participants using a GE Lunar Prodigy Advance DXA (Figure 6:3, B) (enCORE™ software 2015, Version 15.20.002, GE Healthcare Lunar, Belgium). In order to reduce systematic difference in the two scanners, a cross calibration study was conducted (see Section 5.3).
Figure 6:3 GE Lunar DXA scanners (A) IDXA and (B) Prodigy (GE Medical Systems, 2014)
Image of the two GE Lunar DXA scanners used in the study removed for copyright reasons. Copyright holder is GE Medical Systems.
145 | P a g e
6.2.2 DXA scan acquisition
Standard operating procedures (SOPs) were adhered to for both acquisition and analysis of scans.
Two fully trained and experienced bone imaging staff, Mr Michael Mendy and Mr Mustapha Ceesay, from the CDBH group in The Gambia were responsible for the bone imaging of WMS study participants in both Keneba and Fajara; I was present for the acquisition of almost all new rural scans.
Scan acquisition mode is automatically determined by the software, and relates to trunk thickness.
There was no difference between the patient thickness limits of the two instruments (Prodigy and iDXA). During the cross-calibration scans, occasionally there were differences in scan mode selected, resulting in fewer subjects scanned in thin mode on the iDXA than Prodigy, particularly for lumbar spine and hip; and more for total body.
The identity of each participant was confirmed and their details (study ID, date of birth, height and weight) were entered into the software. Women were asked to confirm they were not pregnant. Participants could wear light clothing (without zips, metal fixings, jujus etc...), but all jewellery, or anything metal (e.g. keys, coins) had to be removed; these would attenuate the X-rays and affect bone density measurements. The scanning procedure was explained to the participant, highlighting the importance of staying as still as possible. The total effective dose of radiation that participants receives was less than 21 microsieverts.
6.2.2.1 Total body scan protocol
The participant had to lie in a supine ‘flat’ position on the DXA table; ankles (and knees if required) strapped together with a Velcro strap (Figure 6:5). Care was needed to ensure that head, arms, and feet were kept within the scanning field, particularly for large participants. Guidance lines on the table pad helped to keep the total body central, with arms pronated (palms face down), and kept separated from the thigh, except where this would result in extending beyond the scan field limits. In case of this, hands were placed just under the thighs. The implications of this were limitations on regional measures and on body composition.
Chapter 6: Methods
6.2.2.2 Proximal femur (hip) scan protocol
The participant had to be positioned flat on the bed. Feet were strapped to a positioning aid which assists in providing the correct rotation of the femur (Figure 6:6). The red ‘positioning’ light was moved to the correct place, indicated by using a hand span from the greater trochanter (approximately 7cm down the mid shaft of the femur). Arms were crossed over the chest, to ensure they were not within the ROI.
6.2.2.3 Lumbar spine scan protocol
To enable good positioning, i.e. straight spine alignment, a soft foam block was placed under the legs, supporting the knees, allowing for separation of the vertebrae and alignment with the X-ray beam (Figure 6:4). The positioning light was placed about 2cm (two-finger widths) below the navel. Identification of anatomical markers in the scan image should include the top of the pelvis and the 12th rib, L5 is usually characterised by its M or butterfly shape (Njeh and Shepherd, 2004).
Figure 6:4 Anterior posterior (AP) L1-4 lumbar spine positioning (GE Medical Systems, 2014).
Image of positioning for AP spine scan removed for copyright reasons.
Copyright holder is GE Medical Systems.
147 | P a g e
6.2.3 Quality Assurance (QA) and Quality Control (QC)
Throughout the study, the GE QA calibration block (Figure 6:7) was used to perform daily checks;
calibrating the machine’s functioning and assessing accuracy and precision of measurements. The software provided a detailed report, which indicated the pass/fail of the DXA. A printout of the QA report was checked and filed in the scanning room; any problems were reported. This process enabled monitoring of both the scanner’s performance and precision during the study.
Figure 6:5 Total body positioning (A) Space above head; (B) Hands placed flat, unless insufficient space in scan area, in which case they are placed flat slightly beneath the hips; (C) Velcro straps used to keep ankles and sometimes knees together (GE Medical Systems, 2014).
Figure 6:6 Hip scan positioning (GE Medical Systems, 2014).
Image of positioning for hip scan removed for copyright reasons.
Copyright holder is GE Medical Systems.
Image of positioning for total body scan removed for copyright reasons. Copyright holder is GE Medical Systems.
Chapter 6: Methods
In addition, an aluminium spine phantom (Figure 6:7) was scanned once or twice per week in Keneba and daily in Fajara. The aBMD measured was monitored over time to ensure there was no drift. Both QA and QC checks remained stable throughout the study for all DXAs. QA checks were also conducted immediately after scan acquisition, which involved inspecting the image to ensure good positioning, no movement and that there was no need to repeat the scan before the participant left.
The room temperatures were monitored in accordance with the manufacturers recommendation, QA checks were repeated if the temperature changed by more than 5°C.
6.2.4 DXA scan image analysis (after acquisition and participant left)
Firstly, sets of scans were crosschecked to ensure that the acquisition date was the same as the date recorded on their visit form. Discrepancies were investigated, and when resolved, scans were visually compared by a trained research assistant and myself at MRC EWL, (approximately 50%
each). This involved checking for appropriate positioning of both the subject and ROIs; and whether artefacts were present such as high-density areas (e.g. fracture, osteophytes), jewelry, jujus, or geophagy.
Figure 6:7 Calibration block consisting of tissue-equivalent material with three bone-simulating chambers of known mineral content (GE Medical Systems, 2014).
Image of QA block and spine phantom removed for copyright reasons. Copyright holder is GE Medical Systems.
149 | P a g e Automatic detection and positioning of ROIs by the software sometimes required adjustment to ensure inclusion of the appropriate area and consistency between pairs of scans (for the cross calibration). For hip scans, the neck box dimensions were 1.5 x 6 cm, except when subjects had a short femoral neck, or poor positioning resulted in insufficient rotation.
The software setting ‘enhanced analysis' (EA) was used for Prodigy total body scans; according to GE, this EA enables improvements in point typing (i.e. bone edge detection, particularly for younger and smaller individuals) and the soft tissue composition model is more comparable to iDXA, which uses EA.
Calcification of the aorta could lead to overestimation of BMD as it can lie within the Spine ROI, this was checked and density values checked. The software automatically labels the ROI L1-L4, but this must be checked, along with detection of bone edges and definition of tissue region. Spine BMD should increase progressively from L1 to L4. Table 6:1 details the grading protocol used at MRC EWL and MRCG Keneba. Scans were graded by trained researchers, including myself i.e. not by machine, from 1 to 3, with 1 being very good and 3 indicating that part or all of a scan was unusable.
Grade Scan quality Analysis quality
1 Good
Unusable area of interest (one or more) Area of interest outside scan field
Artefact over area of interest, affecting BMD Poor bone detection
Abnormal T-scores in spine (≥3) Femoral neck box ≤ 1cm
Grading not required
Source: MRC EWL DXA grading protocol Table 6:1 Grading criteria for DXA scans
Chapter 6: Methods