The principle underlying QCT Densitometry and other bone mass measurements (SPA, DPA, DXA/QDR) is that calcified tissue will absorb more x-rays than surrounding tissue so that the CT density measurement can be used to measure total bone mass within a sample of tissue. The method used by Mindways is the same one developed and used at the University of California, San Francisco, since 1978, using the same calibration phantom and scanning procedures. With proper technique, precision for the conventional (2D) method is 2-3%, and about 1% for 3D QCT, so monitoring patients at yearly intervals yields clinically useful results.

How is QCT different from other methods?
QCT and DXA both measure spinal bone mass, a significant advantage over methods, e.g. ultrasound, which measures bone in the peripheral skeleton where bone density is slow to change in response to disease therapy. Spinal bone is a mixture of high-turnover trabecular (spongy) bone and slowly-changing cortical (compact) bone. DXA/QDR measures the sum of these two compartments but also includes aortic calcification and osteophytes in the calculation of bone mineral in the spine. Only QCT isolates the metabolically-active trabecular bone for analysis. Lateral DXA, a newer approach, has recently been shown to have a sensitivity intermediate between the high sensitivity of QCT and the somewhat lower one of conventional DXA for detection of osteoporosis, but it uses 4-10 times the radiation exposure, is less precise, and the study time is increased compared to conventional DXA/QDR. New ultrasound methods for the knee or the heel are being used for osteoporosis screening, but unlike QCT or DXA they do not measure bone mass, and cannot be used to measure the spine where most osteoporotic fractures occur.

How is the QCT examination performed?
The QCT examination is performed on on any modern CT scanner and takes approximately 10 minutes:

• A lateral scout scan of the lower thoracic and lumbar spine is taken.

• 10 mm thick scans are obtained through the mid-plane of 4 vertebral bodies between T11 and L4. The average density (CT value) of 4 cm3 of trabecular bone is measured in each vertebra and expressed as a bone mineral equivalent by comparison with a K2HPO4 bone standard scanned with the patient.

• The scout and axial scans are evaluated quantitatively and any related or incidental findings (vertebral compressions, kidney stones, etc.) are noted in the report. The average bone density of the patient is then:

What about radiation exposure?
All bone density measurements excepting ultrasound use small amounts of radiation to determine the amount of bone present. For a conventional QCT study, the exposure is restricted to four 10 mm-thick slices of the abdomen and a lateral digital radiograph. There is no measurable gonadal exposure for the low-dose methods normally used for QCT. The relative radiation exposure to the bone marrow is about the same as 5 millirem to the whole body. For comparison, a chest x-ray would be about 3 millirem whole-body-equivalent, a cross-country plane flight is about 2 millirem, and natural background is about 365 millirem per year.

Reimbursement
Insurance companies and Medicare may reimburse for QCT exams with proper documentation. The CPT code for QCT bone densitometry is:

76070, CT Bone Density study

Insurance typically ranges from $125 to $150, while Medicare is typically about $110.

Four clinical indications for QCT, defined by the National Osteoporosis Foundation
1. To assess bone density of peri-menopausal women for initiation of estrogen replacement therapy.
2. To establish a diagnosis of osteoporosis or assess its severity in the context of general clinical care.
3. To monitor bone density in patients receiving glucocorticoid therapy.
4. To diagnose low bone density in patients with metabolic disorders such as mild primary hyper-parathyroidism.