Quantitative Computed Tomography (QCT) and Osteoporosis
Quantitative computed tomography (QCT) is a medical technique that measures bone mineral density (BMD) using a standard X-ray Computed Tomography (CT) scanner with a calibration standard to convert Hounsfield Units (HU) of the CT image to bone mineral density values. Quantitative CT scans are primarily used to evaluate bone mineral density at the lumbar spine and hip.
Figure 1 (above): A QCT of spine showing five fixed reference densities in the calibration device under the patient’s back that help adjusting CT machine and calculating accurate bone mineral density (BMD).
Bone densitometry is one of the most advance branches of orthopedics that discusses the evaluation of osteoporosis through the measurement of bone density. While a number of imaging techniques are prevailing today to successfully identify the bone mass density (BMD), quantitative computed tomography (QCT) is the most accurate and advance radiological imaging technique available. Another widely recommended radiological imaging technology for bone density measurement is the dual-energy X-ray absorptiometry (DXA). DXA is sometimes inefficient for the proper evaluation of osteoporosis. Because, changes in bone density in trabecular (soft bone) and cortical (hard bone) bones are different, and the 2-D or “areal” view of a DXA image cannot identify them separately. QCT facilitates the 3-D volumetric measurement of bone and separately represents interior trabecular or spongy bone tissue from the dense cortical bone wall. As a result, QCT reveals the true bone density and is the most successful in detecting changes in trabecular bones, which is extremely useful for early recognition of osteoporosis and during monitoring the therapeutic effects of osteoporosis treatments. This article discusses the fundamental of bone densitometry, the use of DXA and QCT in bone densitometry, how to perform a high quality QCT, and some fundamental differences among QCT and DXA.
Bone Mineral Density (BMD)
Bone mineral density is measured by identifying the amount of bone mineral per unit area or volume of bone tissue. The commonly known BMD measurement techniques are conventional X-ray radiography, Dual-energy X-ray Absorptiometry (DXA), Quantitative Computed Tomography (QCT), and Quantitative Ultrasonography. Among them, DXA and QCT are becoming frequently used methods for this purpose.
Though osteoporosis is a systemic disease, effects of the disease is more prominent on spine, hip, distal radius, and proximal femur. The standard procedure of BMD measurement involves the use of X-ray radiation. While the X-ray is passing through the patient’s body, some radiation is absorbed by the bone tissue, and a fraction passes through the body. This escaped X-ray beam is captured on the radio sensitive plate, and the radiolucency of the beam is measured. The amount of X-ray that passes through the body directly correlates with the bone mineral density (BMD). The result of BMD is expressed in terms of mineral per area (i.e., g/cm2) or mineral per volume (i.e., mg/cc). This primary result is then converted into T score and Z score to identify the osteoporosis. A T score is the standard deviation of a patient’s bone mass density (BMD) from that of a referenced score of a young, healthy population with similar sex and ethnicity. A T score is a statistical measure that directly correlates with the fracture risk. T scores can be obtained from both DXA and QCT scanning. Fracture risks become doubled or tripled for each point deviation from the average T score of similar age and sex group. A deviation between -1 to -2.5 is recognized as osteopenia while a deviation of -2.5 or lower is recognized as osteoporosis. A Z score is the standard deviation of a patient’s BMD from the average BMD of same age and gender. The Z score helps patients to evaluate their disease relative to their own age group. Z score is helpful in that a patient may be osteoporotic by his or her T score but the disease condition can be much better compared to their own age group. The risk of osteoporosis for an individual becomes doubled for each point below the standard Z score. Both the T score and Z score can be derived from DXA and QCT.
Dual-energy x-ray absorptiometry (DXA):
The most widely used method of bone mineral density (BMD) measurement for osteoporosis evaluation is called dual-energy x-ray absorptiometry (DXA). DXA uses a low dose of radiation and is relatively easy to operate. It uses two X-ray beams with separate intensities.
Using two separate beams overcomes the limitations of radiation absorption by the soft tissue components, hence, appropriate condition of the bone mass is revealed. The amount of X-ray absorbed by the bone is proportional to the bone mass present. The information is converted into T score and Z score to compare them with the bone mass density of healthy populations. DXA is used for both the diagnosis of osteoporosis and the evaluation of osteoporosis risk in older people, normally over 50 years for women and 60 years for men. Usually lumber spine, proximal femur, and distal radius are scanned for this purpose, and the results are expressed in mineral per area or g/cm2.
Quantitative Computed Tomography (QCT)
QCT is another widely used radiological procedure to perform bone densitometry using a CT scanner. QCT facilitates both the assessment of low bone mass and the clinical evaluation of bone mass therapy. During bone mass assessment, both the spine and hip are scanned. Scanning different areas of axial skeleton and heap allows radiologists to assess the differences in bone density in those areas, as some parts of the bones are more deteriorated to the osteoporotic effects. QCT can be utilized for early diagnosis of osteoporosis by identifying trabecular or soft bone tissue that becomes more susceptible to osteoporosis effects than the cortical or hard bones.
QCT uses X-ray beam from multiple angles and then combines the data to construct a 3-D view of the scanned object. QCT involves a comparatively lower dose of radiation than that of a standard CT scan but significantly higher level of radiation than DXA. The X-ray attenuation is expressed in terms of Hounsfield Units (HU). The Hounsfield Units are then transformed into milligrams hydroxyapatite per cubic centimeter (mg/cc) representing the BMD.
QCT is used to distinguish existing fractures, the risk of vertebral fracture, and age-related bone loss. Sometimes high resolution CT is performed to analyze the trabecular structure with very high level of precision that cannot be achieved in other means. BMD of a lumbar spine can be obtained through standard CT scanners equipped with special software for the measurement purpose. Sometimes, the peripheral measurement of BMD is done by small-bore CT scanners specifically designed for this purpose.
Normal CT scanners have some limitations in obtaining the precision level during the assessment of longitudinal bones. A peripheral QCT (pQCT) device can overcome the limitations of axial QCT and can separately assess trabecular and cortical bones. pQCT can produce information on both mass and distribution of bone materials as well as bone geometry at important appendicular sites. This information can be integrated to distinguish the bone’s strength and its ability to withstand bending and torsional loads.
Despite DXA is widely recommended as a densitometry tool, the use of QCT is increasing in recent years due to some advantages of QCT over DXA, such as i) QCT allows separate assessment of cortical and trabecular bones; ii) results obtained from QCT represent true volumetric density in mg/cc; iii) unlike DXA, the results are free of errors deriving from the degenerative disk disease and aortic calcification. However, there are some disadvantages with QCT, such as the amount of radiation exposure is higher than that of DXA. QCT also costs significantly higher than DXA.
How to obtain appropriate results from QCT
A radiologic technologist should carefully investigate the physician’s request and other documentations or medical records for QCT examination, such as clinical demonstration or any specific information necessary for the test. Identifying specific reasons for examination will help the radiologic technologist to perform a scan and interpret the result accurately. The result of a non-IV contrasted abdominal or pelvic scan can be utilized for the osteoporotic evaluation. So a previous history of CT scan can save the patient from additional exposure to radiation.
Before performing a QCT, technologists should demonstrate the procedure to the patients clearly and make them comfortable. As the CT table moves the patient through the scanner’s round bore, some patients may become claustrophobic, so this needs to be addressed earlier to overcome fear. Technologists have to ensure proper positioning of the patients during scanning. For example, a patient’s hands should be over the head, and the scan table should be free of pillows and other comfort materials used for positioning. Any metal object within the scan area must be removed.
Quality assurance is an important requirement for QCT. A radiologic technologist should perform routine quality assurance procedures in all steps of operations. The QCT scan protocols need to be consistent with a patient’s previous scans for best reproducibility, which is an essential requirement during follow-up scans. Standard radiological guidelines as well as the specific manufacturer’s guidelines should be followed to reduce the radiation dose and to obtain a greater quality image. Calibration of the CT scanner should be performed carefully to avoid accidental drift of the CT numbers. The selection of appropriate vertebrae is the most important requirement of QCT scanning. Previous reports must be reviewed properly in case of follow-up examinations, and same vertebrae should be selected. A QCT spine examination should include a lateral view of lumber spine. Appropriate levels of ROI (position and size) must be maintained, particularly during follow-up scans, for good reproducibility. Vertebrae containing fractures, deformities, metastases, etc. should be avoided from the scan. Previous surgery areas should be excluded; however, if necessary, they can be included after the proper review of a source image confirming the absence of a lytic or blastic process.