Is Lateral Spine Densitometric Assessment Equivalent to Lateral Radiography?

Authors


What is unsought will go undetected.

Sophocles (495-406 B.C.)

INTRODUCTION

EXCELLENT IMAGE quality and rapid scan time are features of current bone densitometers. These characteristics make it clinically feasible to obtain lateral spine images for vertebral fracture evaluation using lateral densitometry. Thus, this technique may improve care of people with osteoporosis, because its ability to detect moderate-to-severe vertebral compression fractures is similar to standard spine radiography.(1) However, whether lateral spine densitometric images subserve the role of the lateral spine radiograph with respect to other abnormalities remains unknown. We report a case in which X-ray absorptiometry and radiography obtain divergent results.

CASE REPORT

A 76-year-old white female participated in a research study evaluating the ability of X-ray absorptiometry to detect vertebral fractures. She had known osteoporosis; her L1-L4 bone mineral density had been measured previously using a GE Lunar DPX-IQ bone densitometer (GE Medical Systems, Madison, WI, USA) and was 0.750 g/cm2 (T score, −3.6). Additionally, she related a distant history of wrist fracture but no vertebral compression fracture, height loss, or back pain. In addition, past medical history included a 20-year smoking history with chronic obstructive pulmonary disease and a history of intermittent atrial fibrillation.

Lateral spine images were obtained using Lateral Vertebral Assessment software (v4.0) on a Prodigy densitometer (GE Medical Systems). No vertebral fractures were detected on this image (Fig. 1) and no other abnormalities were immediately perceived. However, on the same day, lateral thoracic and lumbar radiographs were obtained at a distance of 40 in using FUJIFILM Medical Systems (Stamford, CT, USA) HRG film. On review of these conventional spine radiographs, a probable abdominal aortic aneurysm was noted (Fig. 2). This information was conveyed to the patient's primary care provider who consequently obtained an abdominal computed tomography (CT) scan documenting a 4.0 cm × 3.4 cm abdominal aortic aneurysm located below the renal arteries and extending to the aortic bifurcation (image not shown). On subsequent review, it was apparent that the aneurysm could be seen on the DXA image (Fig. 3).

Figure FIG. 1.

Lateral DXA image. No vertebral fractures were identified.

Figure FIG. 2.

Lateral spine radiograph shows this patients abdominal aortic aneurysm (arrows).

Figure FIG. 3.

In retrospect, the aortic aneurysm observed on radiograph could be identified on the densitometric image (arrows). However, because the scan interpreter focused on fracture assessment, it was not detected initially.

DISCUSSION

Vertebral deformities often are clinically silent(2); however, the presence of these fractures increases risk for subsequent osteoporotic vertebral and nonvertebral fracture.(3,4) Such individuals may be identified using lateral densitometric imaging, which reproducibly permits detection of moderate-to-severe fracture, and does so rapidly, conveniently, and with low radiation exposure. Thus, this technique may provide improved care of patients by detecting silent osteoporotic fractures. However, the image resolution obtained with this technique is less than the image resolution achieved with conventional radiographs. Furthermore, physicians interpreting these images are likely to evaluate only vertebral fractures, making it possible that other pathology such as malignancy, osteomyelitis, gall stones, renal stones, and vascular abnormalities may go undetected. This case illustrates one such case in which an aortic aneurysm would not have been identified if only lateral absorptiometry were performed. This case history should serve as an important example in showing that “lateral imaging on a densitometer is not lateral radiography.”

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