Because of the difficulty in identifying vertebral fractures clinically, and the practical difficulties preventing routine radiographic assessment at the point of care, vertebral fracture status is frequently unknown at the time of patient evaluation for BMD . Hence the interest in morpho-metric assessment from dual X-ray absorptiometry (DXA) images was a natural consequence of the need for quantitative fracture evaluation in pharmaceutical trials. The main advantage of the morphometric X-ray absorptiome-
try technique is that the radiation dose to the patient is substantially reduced compared with conventional radiography. The use of "high-resolution" lateral spine images, obtained with fan-beam X-ray bone densitometry systems (Fig. 5), offers a potential practical alternative to radiographs for clinical vertebral fracture analysis. "High-resolution" fan-beam DXA systems, utilizing technology similar to that used by computed tomography (CT) systems, can image the lateral spine in as little as 10 s. In fact CT scout scans, with about the same image resolution as fan-beam DXA scans, have been used for vertebral fracture identification [24, 43, 48].
As with radiographs, however, CT images are expensive and are not available clinically without referral. Consequently CT is not generally an option unless performed in conjunction with quantitative CT for BMD assessment. In contrast, DXA images can be performed at the point of care, in conjunction with standard BMD determination, with a radiation dose as much as 100 times lower than that of conventional radiographs. The most notable strength of radiographs, of course, is image resolution, which is superior to that of DXA images.
DXA images provide several advantages. The digital nature allows for electronic data storage, digital image enhancement and processing, as with magnification and contrast adjustment, which is not possible with conventional radiographic techniques. Cone-beam distortion, inherent in the radiographic technique, is not present when using the scanning fan-beam geometry of DXA devices. Low-dose, single-energy acquisition modes are substantially faster than dual energy scan modes due to substantially lower signal to noise in the images and can be performed during suspended respiration. High-dose, dual-energy acquisitions, while slower, generally provide higher bone contrast images and sometimes reduce artifacts.
The use of fan-beam DXA images for quantitative (morphometric) assessment of spinal fractures has been reported in both research applications and pharmaceutical trials [4, 11, 19, 21, 28, 37, 38, 46]. Clinical studies demonstrated the feasibility of visual evaluation of fan-beam lateral DXA spine images compared to conventional lateral spine radiographs in postmenopausal women, with a strong overall agreement of 96.3% [37, 38]. This agreement was approximately as strong as that found among different morphometric techniques [15, 21]. The images permitted visual assessment of about 90% of all vertebrae. The main shortcoming of the MXA scans in comparison with conventional radiographs is the inferior image quality that limits the evaluation of vertebrae in the upper thoracic spine. This is less of a concern if MXA is used as a screening tool for conventional radiography and this approach may help reduce the radiation dose in the diagnosis and monitoring of osteoporosis.
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