osteoporosis is characterized by low bone mass and microarchitectural deterioration of bone tissue , leading to increased bone fragility and a consequent increase in fracture risk .
consequently , noninvasive techniques for measuring bone mineral density ( bmd ) play an important role in the clinical diagnosis of osteoporosis and in monitoring its progression .
dual x - ray absorptiometry ( dxa ) and quantitative computed tomography ( qct ) are the most common tools for measuring bmd .
dxa determines bmd in two dimensions , including both trabecular and cortical bone , with the results expressed as areal density ( grams per square centimeter ) .
qct allows measurement of volumetric trabecular bone density without superimposition of cortical bone and other tissues , with the results expressed in milligrams per cubic centimeter of calcium hydroxyapatite .
dxa and qct findings can not be compared directly , and sometimes the diagnosis indicated by bmd findings differs between the two modalities .
therefore , we compared the detection rate of osteoporosis for posteroanterior dxas ( pa - dxa ) with the rate for qct and analyzed the reasons for the differences between the two .
between february 2010 and february 2013 , we reviewed data , for our study , for 194 postmenopausal women who underwent qct and areal spinal and hip dxa in our department with an interval between qct and dxa scans of two months
. study exclusion criteria included a history of multiple myeloma , rheumatoid arthritis , ankylosing spondylitis , systemic lupus erythematosus ( sle ) , connective tissue disease , metabolic and endocrine diseases , or bone tumors .
dxa measurements were obtained using a prodigy dxa scanner ( ge , lunar , madison , wi , usa ) and were analyzed using the manufacturer 's software . the dxa t - score
vertebrae from l1 to l4 and the left hip were scanned in the supine position using posteroanterior projections .
the t - score for l1l4 and for the femoral neck plus the total hip measurement by dxa were used to diagnose osteoporosis .
we used the diagnostic criteria established by the world health organization ( who ) in 1994 .
qct measurements were obtained with an aquilion 64-slice ct scanner ( toshiba , tokyo , japan ) with a solid mindways qct phantom ( mindways software inc .
elliptical regions of interest were put in the midplane of three vertebral bodies ( l2l4 ) in the trabecular bone automatically , avoiding the cortical bone of the vertebrae .
for the bmd of spinal trabecular bone , thresholds of 120  mg / cm for osteopenia ( equivalent to a dxa t - score of 1.0  sd ) and 80  mg / cm for osteoporosis ( equivalent to a dxa t - score of 2.5  sd ) were suggested by the international society for clinical densitometry in 2007   and by the american college of radiology in 2008 . to estimate the degree of spinal degeneration and abdominal aortic calcification ( aac ) , two radiologists who were blinded to the bmd results independently reviewed lumbar ct images .
the difference between the osteoporosis detection rates for dxa versus qct was analyzed using the chi - square test .
the 140 study participants ranged in age from 47.1 to 85.9 years ( mean : 63.2  8.1 years ) .
the interval time between dxa and qct scans ranged from 0 to 43 days ( mean : 4.3  9.7 days ) .
the bmd of l1l4 as measured by dxa ranged from 0.575 to 1.621  g / cm ( mean : 0.973  0.169 
the trabecular bmd of the lumbar spine as measured by qct ranged from 5.0 to 199.4  mg / cm ( mean : 81.7  34.8  mg / cm ) .
the osteoporosis detection rates for lumbar pa - dxa , lowest bmd of the femoral neck and total hip , lowest bmd for spinal and hip dxa , and lumbar qct were 17.1% , 12.9% , 20.0% , and 46.4% , respectively ( table 1 ) .
the intragroup detection rates for qct versus dxa were significantly different ( p < 0.01 ) , with qct detecting osteoporosis more frequently than spinal and hip dxa , did .
of the 140 participants , 41 ( 29.3% ) were found by qct but not by dxa to have osteoporosis . of these , 7 ( 17.1% ) had single or multiple vertebral fractures ( figure 1 ) .
all 41 ( 100% ) had vertebral osteophytes or end plate sclerosis , 33 ( 80.5% ) had facet joint osteoarthritis , 14 ( 34.1% ) had spinous process osteophytes , and 25 ( 61.0% ) had aac .
in addition , 3 participants had bone islands or focal sclerosis ( figure 2 ) .
the diagnostic criteria for dxa established by who in 1994 have long been used as the gold standard in the clinical diagnosis of osteoporosis . the sites most commonly measured are the lumbar spine and hip .
our study showed a significant difference in osteoporosis detection rates between dxa and qct , providing clinical evidence that qct has a greater diagnostic sensitivity than dxa .
osteoporosis , spinal degeneration , and aac are most commonly seen in the elderly , and the consequences of these conditions are more serious with increased age .
this is why it can be problematic to use dxa rather than qct ; even though clinical findings may indicate osteoporosis , dxa may still indicate that bmd is normal . a possible explanation for the superior performance of qct may be spinal degeneration and calcinations in the soft tissue around the spine .
yu et al . reported that bmd measured by pa - dxa was significantly higher in patients with spinal degenerative joint disease changes than in those without such changes , particularly when osteophytes were present at the vertebral bodies and facet joints .
indicated that the presence of osteophytes is associated with higher bmd when measured with dxa .
our results show that spinal degeneration and aac may be associated with the overestimation of bmd and the underestimation of osteoporosis by dxa .
several clinical techniques for bmd measurement are available , including dxa , qct , and ultrasound , each with its own advantages and shortcomings .
appropriate choice of technique and measurement site is important for the accurate diagnosis of osteoporosis .
found that , under the who classification , women with spinal osteoarthritis were more likely to be given a diagnosis of osteoporosis of the femoral neck and hip than those without spinal osteoarthritis but less likely to receive such a diagnosis when bmd was based on the pa spine ( 14.4% versus 24.5% ) .
recommended that , in women aged 65 years and older who are likely to have spinal osteoarthritis , dxa of the hip be used for identification of osteoporosis .
however , dxa of the hip still includes cortical bone , so , findings can be influenced by degenerative changes , leading to a decrease in the ability to detect osteoporosis . in our study ,
the detection rate of osteoporosis at any spinal or femoral site by dxa was significantly lower than the rate for qct , and no femoral site was superior to the pa spinal site .
it may measure bmd more accurately and reproducibly , especially in patients with spinal deformity , severe degenerative changes , extreme obesity , or low body mass index .
qct may be particularly useful in china , where dxa scanners are not available in most areas .
found that vertebral fractures were present in 18.3% of asymptomatic postmenopausal women and that 11.0% to 18.7% of individuals with clinical osteoporosis would have been classified as having normal bone by bmd criteria alone .
ling et al . reported that vertebral fractures were present in 15% of women aged 50 years or older in beijing . in our study , we found vertebral fractures in 17.1% of women in whom qct but not dxa showed osteoporosis
as our study demonstrated , qct may avoid the overestimation of bmd by dxa associated with spinal degeneration , aac , and other sclerosis lesions , such as bone islands .
qct may be more sensitive for detecting osteoporosis , but this must be validated in a larger population .
 
 objective . to compare the osteoporosis detection rates in postmenopausal women when measuring bone mineral density ( bmd ) with quantitative computed tomography ( qct ) in the spine versus dual x - ray absorptiometry ( dxa ) in the spine and hip and to investigate the reasons for the discrepancy between the two techniques 
 . methods . 
 spinal volumetric bmd was measured with qct , and areal spinal and hip bmds were measured with dxa in 140 postmenopausal women . 
 we calculated the osteoporosis detection rate for the two methods . 
 lumbar ct images of patients who had a discrepancy between qct and dxa findings were reviewed to evaluate vertebral fractures , spinal degeneration , and abdominal aortic calcification . 
 results . for the entire 140 patients , 
 the detection rate was 17.1% for dxa and 46.4% for qct , a significant difference ( p < 0.01 ) . 
 of the 41 patients with conflicting diagnoses , 7 whose diagnosis by qct was osteoporosis had vertebral fractures even though their dxa findings did not indicate osteoporosis . 
 varying degrees of spinal degeneration were seen in all of the 41 patients . conclusion . 
 qct may avoid the overestimation of bmd by dxa associated with spinal degeneration , abdominal aortic calcification , and other sclerotic lesions . 
 it may be more sensitive than dxa for detecting osteoporosis in postmenopausal women . 
1. Introduction
2. Materials and Methods
3. Results
4. Discussion
5. Conclusions