MYTHS AND MISCONCEPTIONS ABOUT QCT

A higher cost for QCT over DXA may have been a valid objection in the days of older CT scanning technology, but modern multi-slice helical CT scanners can acquire QCT spine and hip scan data in just a few seconds. Because of their fast scan times, these CT machines frequently have spare time available, especially in a rural community hospital setting, where the CT scanner is installed for emergency cases. Compared to a QCT system, DXA requires a dedicated room and technician rather than utilizing existing CT infrastructure; and DXA has ongoing maintenance costs, unlike QCT systems that consist only of a software application and solid-state phantoms. In Australia, Medicare reimbursement is identical for DXA and QCT. Total examination time including report generation can be completed in less than 10 minutes.

Originally developed in the early 80’s, QCT has several advantages over Dual-energy X-ray Absorptiometry (DXA) for the measurement of BMD  in patients with degenerative disease and arthritis but was superseded by DXA early on, primarily due to the high cost and limited availability of early CT scanners. In addition, these early CT scanners where slow and so a QCT bone density test might take 30 minutes or so which made the exam expensive. During the 1990’s DXA became the most prevalent method of bone density testing and is now recommended as the “gold standard” method despite some well recognized deficiencies in this approach. Importantly, the proportion of fractures attributable to osteoporosis based on a standard definition of using a DXA BMD T-score of less than −2.5 is very modest, ranging from less than 10% to 44%.

While the x-ray doses from standard abdominal and pelvic CT scans can be relatively high, QCT bone density testing uses lower quality, noisier images and thicker scan slices than normal which reduces the x-ray dose considerably. Combined with the dose reduction algorithms on modern CT scanners, these low-dose CT scan protocols mean that the amount of radiation required is only around 500-800μSv which is comparable to a set of mammograms or around 1/4 of annual background radiation (depending on where you live).

QCT measurements at the hip got FDA clearance for clinical use in 2001. The QCT measurement of BMD at the hip is made in a way that produces a measurement and T-Score equivalent to DXA. A pelvic scan from the top of the femoral head to just below the lesser trochanter is used. The orientation of the proximal femur may be adjusted on screen by the operator for reproducible results. An automatic 2D ROI is then located at the femoral neck and used to compute a DXA-equivalent areal BMD measure. An areal BMD T-score at the femoral neck is used by the WHO for the definition of osteoporosis and so measurement at the hip is becoming standard of care. This T-Score may also be used for fracture risk probability calculation in the WHO FRAX® tool.

This may have been a problem back in the 1990’s when CT scanners where slow and 2D QCT spine bone density measurements relied on gantry tilt to orient a single scan slice through the middle of a vertebral body. Nowadays, QCT exams use volumetric scan data so that any spine or hip orientation correction can be dealt with in software. Modern multi-slice helical CT scanners can acquire volumetric QCT hip and spine scan data in 1-10 seconds and so the whole examination takes around 5-10 minutes, about the same as a DXA exam. Because of their fast scan times, these CT machines frequently have spare time available, especially in a rural community hospital setting, where the CT scanner is installed for emergency cases.

This may have been a problem back in the 1990’s when 2D QCT spine bone density measurements relied on gantry tilt to orient a single scan slice through the middle of a vertebral body. That process was very operator-dependent and less to reproducibility issues. Nowadays, QCT exams use volumetric scan data so that any spine or hip orientation correction can be dealt with in software. Published short-term precision estimates of BMD measurement by volumetric QCT have been calculated as CV% for BMD at the lumbar spine (0.8%) and areal BMD at the total hip (0.82%) and femoral neck (0.69%). This compares favorably with DXA areal BMD measurements, for which published precision estimates are similar for the lumbar spine (1.1%), total hip (0.65%) and femoral neck (1.66%).

The accuracy of BMD is how close the measurement by densitometry is to actual calcium content of the bone (the ash weight of bone). Because a BMD measurement in trabecular bone incorporates both bone and marrow, the accuracy of BMD measurement by QCT is sometimes questioned with reference to the relative proportions of red and fatty marrow content of the bone. Whereas the conversion from red to fatty marrow tends to finish by the mid-20s in the hip and peripheral skeleton, the vertebrae show a gradual age-related increase in the proportion of fat in the bone marrow, which starts in youth and continues through old age. An increase in fatty marrow is indistinguishable from a decrease in bone. The inclusion of fatty marrow in the vertebral QCT BMD measurement results in accuracy errors ranging from 5% to 15% depending on the age group. In comparison, the BMD measurement accuracy of DXA lies in the region of 10-15%; the error in accuracy being related to the same issue regarding marrow fat content of the bone and to the assumptions DXA makes in taking soft tissue as a reference. DXA makes assumptions about body composition and soft tissues that means that inaccuracies may occur with inhomogeneities in soft tissue composition and with large changes in weight between scans. Exactly how this may corrected for in adults in uncertain. In the cases of both DXA and QCT BMD measurements, comparing this measurement to a normal reference population makes systematic accuracy errors (as opposed to precision errors) of questionable importance in most situations.

QCT measurements at the hip got FDA clearance for clinical use in 2001. The QCT measurement of BMD at the hip is made in a way that produces a measurement and T-Score equivalent to DXA. A pelvic scan from the top of the femoral head to just below the lesser trochanter is used. The orientation of the proximal femur may be adjusted on screen by the operator for reproducible results. An automatic 2D ROI is then located at the femoral neck and used to compute a DXA-equivalent areal BMD measure. An areal BMD T-score at the femoral neck is used by the WHO for the definition of osteoporosis and so measurement at the hip is becoming standard of care. This T-Score may also be used for fracture risk probability calculation in the WHO FRAX® tool.

QCT has a very important place in academic and clinical research: in a clinical research mode, QCT is often used to help understand how disease affects the skeleton and how innovative therapies impact the density and geometry of the skeleton.  In pharmaceutical clinical trials, QCT provides substantially more information than the traditional bone densitometry with DXA. However, QCT is also being used clinically in hundreds of medical imaging centers around the world, both for routine BMD screening and for patients for whom DXA cannot produce an adequate BMD measurement due to scoliosis, spinal instrumentation or else degenerative diseases such as arthritis. There are now 4 or 5 commercially available QCT systems in existence and somewhere in the region of 5000 QCT systems have been installed globally in the past 15 years.