The Spine in the Lateral Projection

The effect on BMD measured in the AP or PA projection from aortic calcification, facet sclerosis, osteophytes, and other degenerative changes in the spine can be nullified by quantifying the bone density of the spine in the lateral projection as shown in Fig. 2-15B. In addition, the highly cortical posterior elements and a portion of the cortical shell of the vertebral body can be eliminated from the measurement, resulting in a more trabecular measure of bone density in the spine. The measurement is not a 100% trabecular measure as portions of the cortical vertebral body shell will still be included in the measurement. In addition to the elimination of artifact or confounding degenerative changes, the lateral spine BMD measurement is desirable in those circumstances in which a trabecular measure of bone density is indicated and particularly in circumstances in which changes in trabecular bone are being followed over time. The higher metabolic rate of trabecular bone compared to cortical bone should result in a much larger magnitude of change in this more trabecular measure of bone density compared to the mixed cortical-trabecular measure of bone density in the PA spine.

Vertebral identification in the lateral projection can be difficult. The lumbar vertebrae are generally identified by the relative position of the overlapping pelvis and the position of the lowest set of ribs. The position of the pelvis tends to differ however, when the study is performed in the left lateral decubitus position compared to the supine position. Rupich et al. (29) found that the pelvis overlapped L4 in only 15% of individuals when studied in the supine position. Jergas et al. (30) reported a figure of 19.7% for

Dxa Lumbar Spine
Fig. 2-20. A DXA PA spine study acquired on the Lunar DPX. The image is unusual at L4, with what appears to be an absence of part of the posterior elements. This was confirmed with plain films. This should decrease the BMD at L4.
Bone Cancer

Fig. 2-21. A DXA PA spine study acquired on the Lunar DPX. The image suggests a marked sclerotic reaction at L4 and L5. There is also a marked increase in the BMD at L4, compared to L3. This sclerotic process was thought to be the result of an episode of childhood discitis. The patient was asymptomatic.

Fig. 2-21. A DXA PA spine study acquired on the Lunar DPX. The image suggests a marked sclerotic reaction at L4 and L5. There is also a marked increase in the BMD at L4, compared to L3. This sclerotic process was thought to be the result of an episode of childhood discitis. The patient was asymptomatic.

L4 overlap for individuals studied in the supine position. In DXA studies performed in the left lateral decubitus position, pelvic overlap of L4 occurred in 88% of individuals in the study by Peel et al. (12). In the other 12%, the pelvis overlapped L5 in 5% and the L3-4 disc space or L3 itself in 7%. As a consequence, although the position of the pelvis tends to identify L4 in most individuals scanned in the left lateral decubitus position, it also eliminates the ability to accurately measure the BMD at L4 in those individuals. The ribs are less useful than the pelvis in identifying the lumbar vertebrae. Rib overlap of L1 can be expected in the majority of individuals whether they are studied in the supine or left lateral decubitus position (12). This may not be seen, however, in the 12.5% of individuals whose lowest set of ribs in on T11.

Although the location of the pelvis and the presence of rib overlap aid in identification of the vertebrae, they also limit the available vertebrae for analysis. When a lateral spine DXA study is performed in the left lateral decubitus position, L4 cannot be analyzed in

Proximal Femur True

Fig. 2-22. (A) The proximal femur as viewed from the front. The lesser trochanter is behind the shaft of the femur. (B) The proximal femur as viewed from behind. The lesser trochanter is clearly seen to be a posterior structure. (Adapted from McMinn RMH, Hutchings RT, Pegington J, and Abrahams PH. [1993] Colour Atlas of Human Anatomy, 3rd edition, p. 267-268. By permission of the publisher Mosby.)

Fig. 2-22. (A) The proximal femur as viewed from the front. The lesser trochanter is behind the shaft of the femur. (B) The proximal femur as viewed from behind. The lesser trochanter is clearly seen to be a posterior structure. (Adapted from McMinn RMH, Hutchings RT, Pegington J, and Abrahams PH. [1993] Colour Atlas of Human Anatomy, 3rd edition, p. 267-268. By permission of the publisher Mosby.)

the majority of individuals because of pelvic overlap. L1 is generally not analyzed because of rib overlap, regardless of whether the study is performed supine or in the left lateral decubitus position. Rupich et al. (29) also found that rib overlay L2 in 90% of individuals studied in the supine position. It was estimated that rib BMC added 10.4% to the L2 BMC. As a consequence, when lateral DXA studies are performed in the left lateral decubitus position, L3 may be the only vertebra that is not affected by either pelvic or rib overlap. In the supine position, L3 and L4 are generally unaffected. This means that depending on the positioning required by the technique, the value from a single vertebra or from only a two-vertebrae average may have to be used. This is undesirable, although sometimes unavoidable, from the standpoint of statistical accuracy and precision.

If the vertebrae are misidentified in the lateral projection, the effect on BMD can be significant. In the study by Peel et al. (12), misidentification of the vertebral levels would have occurred in 12% of individuals in which the pelvis did not overlap L4 in the left lateral decubitus position. If L2 was misidentified as L3, the BMD of L3 was underestimated by an average of 5.7%. When L4 was misidentified as L3, the BMD at L3 was overestimated by an average of 3.1%. Although spine X-rays are rarely justified for the sole purpose of vertebral identification on a DXA study performed in the PA or AP projection, this may occasionally be required for DXA lumbar spine studies performed in the lateral projection. Analysis may be restricted to only one or two vertebrae because of rib and pelvic overlap. This reduces the statistical accuracy and precision of the measurement. Because of this reduction in accuracy, consideration should be given to combining lateral DXA spine studies with bone density assessments of other sites for diagnostic purposes.

Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • belba
    How to tell where l4 and l5 is on a dexa scan?
    6 years ago
  • BRIFFO GALBASSI
    What is a lateral spine bone density?
    6 years ago
  • brian
    What is the lateral projection of spinal bone call?
    6 years ago
  • Niall
    How to identify L5 in a DXA lumbar spine scan?
    5 years ago
  • RENZO
    HOW LATERAL LUMBERBone Density?
    4 years ago
  • Hiwet
    What is the lateral.projection of a vertebra?
    1 year ago
  • lisa
    Is supine lateral BMD better than latera decubitus BMD?
    12 months ago
  • Zewdi Massawa
    What is lateral projecting?
    5 months ago

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