Artifacts in PA or AP Spine Densitometry

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The PA lumbar spine has been, and continues to be, used extensively in densitometry for diagnosis, fracture prediction, and monitoring. Unfortunately, it is also the skeletal site most often affected by structural changes and artifacts that may limit its utility.

Vertebral Fractures

The BMD of a fractured vertebra will be increased because of the fracture itself. This increase in density could erroneously lead the physician to conclude that the bone strength is better and the risk for fracture, lower, than is the case. Vertebral fractures in osteoporosis frequently occur in the T7-T9 region and in the T12-L2 region (14,15). Because DXA measurements of the lumbar spine are often employed in patients with osteoporosis, osteoporotic fractures in the lumbar spine, particularly at L1 and L2, are a common problem, rendering the measurement of BMD inaccurate if the fractured vertebrae are included. An increased precision error would also be expected if the fractured vertebrae were included in BMD measurements performed as part of a serial evaluation of BMD. Although a fractured lumbar vertebra can be excluded from consideration in the analysis of the data, this reduces the maximum number of contiguous vertebrae in the lumbar spine available for analysis. For reasons of statistical accuracy and precision, the BMD for three or four contiguous vertebrae is preferred over two-vertebrae averages or the BMD of a

Cusum Charts Dexa

Fig. 2-8. A DXA PA spine study acquired on the Norland XR-36. The image suggests a loss of vertebral height and increased sclerosis at L3. Although the BMD at L3 is expected to be higher than at L2, the BMD at L3 here is markedly higher. These findings suggest a fracture at this level but this must be confirmed. In any case, the L2-L4 BMD will be increased by this structural change. Case courtesy of Norland, a CooperSurgical Company, Ft. Atkinson, WI.

Fig. 2-8. A DXA PA spine study acquired on the Norland XR-36. The image suggests a loss of vertebral height and increased sclerosis at L3. Although the BMD at L3 is expected to be higher than at L2, the BMD at L3 here is markedly higher. These findings suggest a fracture at this level but this must be confirmed. In any case, the L2-L4 BMD will be increased by this structural change. Case courtesy of Norland, a CooperSurgical Company, Ft. Atkinson, WI.

single vertebra. Figure 2-8 illustrates a PA lumbar spine study in which a fracture was apparent at L3. Although the BMD at L3 is expected to be higher than either L2 or L4, it is disproportionately higher. The L2-4 BMD will be increased because of the effect of the fracture on the BMD at L3. In the DXA PA lumbar spine study shown in Fig. 2-7, the image does not as readily suggest a fracture. The BMD at L1 however is higher than the BMD at L2, which is unusual. A plain lateral film of the lumbar spine of this patient, shown in Fig. 2-9, confirmed a fracture at L1.

Degenerative Changes and Dystrophic Calcification

Other structural changes within the spine can affect BMD measurements. Osteophytes and facet sclerosis can increase the BMD when measured in the AP or PA projection. Aortic calcification will also potentially affect the BMD when measured in the AP or PA spine because the X-ray beam will detect the calcium in the aorta as it passes through the body on an anterior to posterior or posterior to anterior path. It is therefore useful to note how often these types of changes are expected in the general population and the potential magnitude of the effect these changes may have on the measured BMD in the lumbar spine.

Effect of Osteophytes on BMD. In 1982, Krolner et al. (16) observed that osteophytes caused a statistically significant increase in the BMD in the AP spine when compared to controls without osteophytes. More recently, Rand et al. (17) evaluated a population of 144 postmenopausal women, aged 40 to 84 years, with an average age of 63.3 years, for the presence of osteophytes, scoliosis, and aortic calcification. These women were generally healthy women referred for the evaluation of BMD because of suspected postmenopausal osteoporosis. Table 2-5 lists the percentages of these women

Osteoporosis Lumbar Spine
Fig. 2-9. The lateral lumbar spine X ray of the patient whose DXA study is shown in Fig. 2-7. A fracture at L1 is indicated by the arrow.

Table 2-5

Frequency of Specific Types of Degenerative Changes in the Spines of 144 Women Aged 40-84

Type of Degenerative Change % with Change (n)

Osteophytes 45.8 (66)

Osteochondrosis 21.5 (31)

Vascular Calcification 24.3 (35)

Scoliosis 22.2 (32)

Adapted with permission of the publisher from ref. 17.

found to have these types of degenerative changes. Based on these findings, Rand et al. estimated the likelihood of degenerative changes in the spine as being less than 10% in women under the age of 50. In 55-year-old women, however, the likelihood jumped to 40% and in 70-year-old-women, to 85%. Of these types of degenerative changes, however, only the presence of osteophytes significantly increased the BMD. The magnitude of the increase caused by the osteophytes ranged from 9.5% at L4 to 13.9% at L1. Cann et al. (18) also estimated the increase in BMD from osteophytes in the spine at 11%. In 1997, Liu et al. (19) studied 120 men and 314 women, aged 60 to 99 years.

Sub Endplate Sclerosis And Osteophytes
Fig. 2-10. A lateral lumbar spine X ray of the patient whose DXA study is shown in Fig. 2-11. The arrow indicates a region of endplate sclerosis and osteophyte formation.
Spine Bone Density

Fig. 2-11. A DXA PA spine study acquired on the Lunar DPX. A sclerotic process is suggested at L2 by the image. The BMD is also increased more than expected in comparison to L1 and is higher than L3, which is unusual. These findings are compatible with the endplate sclerosis and osteophytes seen in Fig. 2-10.

Fig. 2-11. A DXA PA spine study acquired on the Lunar DPX. A sclerotic process is suggested at L2 by the image. The BMD is also increased more than expected in comparison to L1 and is higher than L3, which is unusual. These findings are compatible with the endplate sclerosis and osteophytes seen in Fig. 2-10.

Lumbar spine osteophytes were found in 75% of the men and 61.1% of the women. The effect of osteophytes on the BMD was sufficiently great to cause 50% of the men and 25% of the women with osteopenia to be misdiagnosed. About 20% of the men and 10% of the women with osteoporosis were misdiagnosed because of the effect of osteophytes on the BMD. In Fig. 2-10, osteophytes are clearly visible at L2 on the lateral lumbar

Osteoporosis Lumbar Spine Ray Images
Fig. 2-12. A lateral lumbar spine X ray of the patient whose DXA study is shown in Fig. 2-13. The arrow indicates a region of marked endplate sclerosis.
Norland Dxa
Fig. 2-13. A DXA PA spine study acquired on the Lunar DPX. The image dramatically suggests the sclerotic process seen on the X ray in Fig. 2-12. There is a marked increase in the BMD at L2 and L3 compared to L1 and L4.

radiograph. The appearance of this region on the DXA PA lumbar spine study in Fig. 2-11 suggests a sclerotic process at this level. Osteophytes and end-plate sclerosis are also seen on the plain film in Fig. 2-12. The effect on the DXA image of the lumbar spine, shown in Figure 2-13 is dramatic. There is also a disproportionate increase in the BMD at L2 and L3 compared to L1 and L4.

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