Quantitative computed tomography or QCT

Quantitative computed tomography is a procedure which uses CT scan for quantitative assessment of the bones or bone mineral density. It is mainly used in osteoporotic patients.  By Osteoporosis is a skeletal disease, characterized by low bone mass and micro-architectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture.

There are various methods to estimate bone mineral density in osteoporosis.  The goal is the measurement of bone strength, to predict fractures, and to differentially assess the effects of aging and treatment.

Plain x-ray was the earliest tool used. Cortical morphometry involved measurements of cortices of the second metacarpal of the non-dominant hand (metacarpal index).

Single and dual-energy photon absorptiometry utilized radionuclide sources.

Dual-energy X-ray absorptiometry or DXA was based on low dose x-ray radiations has become the widely used procedure. It is fast and can be applied to central and peripheral skeletal sites.

In recent years, better CT technology and the advantages of CT over DXA in the differentiation of cortical and trabecular bone mineral density and others,  the use of quantitative computed tomography in musculoskeletal research studies is increasing.

Quantitative computed tomography is used to quantify bone mineral density in the spine, proximal femur, forearm, and tibia.

It offers many advantages over DXA

• Separation of cortical and trabecular bone
• Determination of trabecular volumes of interest independent of degenerative changes in the spine
• 3D geometric parameters can be determined
• Measures true density in gm/cm­³ centimeter in contrast to dual-energy X-ray absorptiometry, which determines density measured in g/cm².
• Quantification of bone mineral density with QCT is independent of bone size, which is not the case for DXA.

The only constraint is the radiation exposure.

pQCT denotes peripheral quantitative computed tomography which is done on the appendicular skeleton, such as the arms or legs.

Dedicated peripheral CT scanners to measure BMD and bone morphology in the radius and tibia are smaller, more mobile and less expensive that whole-body CT scanners.

Basis of Quantitative Computed Tomography

Computed tomography utilizes X-rays and provides an image which is based on the linear X-ray absorption coefficients of the tissues through which it passes.

The Hounsfield scale, also known as CT numbers is a quantitative scale for describing radiodensity.

The scale is obtained from a linear transformation of the measured attenuation coefficients. Attenuation coefficient quantifies how much the beam is “attenuated”, i.e., weakened by the material it is passing through. It is a measure of how easily a material can be penetrated by the beam.

Thus, a change of one Hounsfield unit represents a change of 0.1% of the attenuation coefficient of water since the attenuation coefficient of air is nearly zero.

Calibration of CT  is based on the arbitrary definitions of air and water

• Radiodensity of distilled water at standard temperature and pressure (STP) = 0 HU
• Radiodensity of air at STP = -1000 HU
• Bone density varies from +700 (cancellous bone) to +3000 (cortical bone)

Bone, absorb more X-rays, have a high HU number and appear white on the image.

Image Generation by CT Scan

It is a two-step process

• Scan data acquisition
• Tomographic reconstruction by a calculating the image from the acquired data

Phantoms

Imaging phantoms, or simply “phantoms”, are specially designed that when scanned respond in a similar manner to human tissues and organs would.

To transform HU into bone mineral equivalents an appropriate bone mineral phantom is included in the scan field.

Phantoms are kind of cross calibrators and help to avoid the use of live controls/cadaveric controls and issues associated with them.

For quantitative computed tomography,  calibration phantoms contain various concentrations of material with similar X-ray attenuation characteristics to bone

Central Quantitative Computed Tomography

Spine

In spine, quantitative computed tomography gauges mineral density is gauged from analyzing the vertebral bones.

For 2 D CT, 8–10 mm slice through the middle of each vertebra [generally L1–L3] is obtained.

Fractured vertebrae should not be analyzed as inclusion of the endplate will cause overestimation of bone mineral.

An oval region of interest (or PacMan shape) is chosen to include as much of the vertebral trabecular bone as possible

The measurements obtained are the trabecular BMD in mg/cm³ of individual vertebrae scanned and a mean BMD, from which are calculated the T score and Z score.

On spiral multidetector CT, a volume of tissue is scanned. To limit radiation, two vertebrae are scanned, generally L1 and l2.

Proximal Femur

3D QCT of the hip is a new procedure and not yet clinically established. QCT of the proximal femur is carried out on clinical whole-body CT scanners.

The scan region typically starts 1-2 cm above the femoral head and extends a few centimeters below the lesser trochanter.

Peripheral Quantitative Computed Tomography

Standard pQCT scanners work in step and scan mode.

These use distal radius distal sites containing for trabecular bone and shaft location for cortical bone in nondominant limb. Currently, only a few types of pQCT scanners are in use.

Peripheral sites can be scanned on MDCT scanners more quickly than on dedicated pQCT.

Use of  QCT in Clinical Situations

Diagnosing  osteoporosis

By WHO definition, osteoporosis is a T score at or below 2.5 calculated by DXA of the lumbar spine, femoral neck, total hip, and distal 33% radius.

Definition of osteoporosis does not apply to any other densitometric method than DXA. If we apply WHO criteria of a T score to spinal QCT would lead to an earlier age of diagnosis of osteoporosis and overclassification.

But bone mineral density as measured by quantitative computed tomography has at least the same ability to predict vertebral fractures as spine DXA.

Though diagnosis cannot be made on QCT, in absence of DXA measurement due to present criteria, treatment can be started if there is fracture probability in conjunction with clinical risk factors.

But values of spinal QCT could be used to initiate treatment if DXA values are not available.

 Fracture Prediction

pQCT of the distal radius can be used for predicting hip fragility fractures in the spine in postmenopausal women but in men, it is equivocal as of now.

Data has shown that structural parameters from 3D hip QCT were independently related to hip fracture risk.

Application in children

Spinal QCT and pQCT can be used in children. pQCT, has been used more widely than central QCT due to radiation issues.

Comparison of DXA and Quantitative Computed Tomography

DXA measures a two-dimensional ‘areal’ bone mineral density which should be designated as BMD_a.

It is bone size dependent.

QCT measures true volumetric bone mineral density designated as BMD, and is not size dependent.

DXA measures integral (cortical and trabecular) BMD_a whereas QCT allows separate measures of BMD of the trabecular and cortical bone compartments. DXA evaluates a projected area of the investigated volume but not the volume itself. As a consequence, only a so-called ‘areal’ bone mineral density BMD typically measured in g/cm², can be determined.

However, in literature, the distinction between BMD and BMDa is rarely made.

With trabecular bone being eight times more metabolically active than cortical bone, QCT can be more sensitive to change than is DXA-BMD_a.

Trabecular bone mineral density of the lumbar spine on QCT can be used to monitor age, disease and treatment-related BMD changes.

The differentiation of cortical and trabecular bone is one of the unique advantages of QCT compared to DXA.

With QCT, the assessment of biomechanical properties can be done by

• 3-point bending for the shaft of radius
• Axial compression for the distal metaphysic and vertebra
• Fall simulation on the outstretched hand
• Single leg stance and fall simulations for the hip

Several studies have determined the association between whole bone strength, QCT-derived bone mineral density, and bone geometry.

With QCT, a finite element analysis is done by integration of density and geometry and better accounts for regional variations than the separation of a bone into sub-volumes. FEA has been reported to be a strong predictor of whole bone strength.

Conclusion

Quantitative computed tomography is currently predominantly a research tool which has some important advantages over DXA in studies of the skeleton and is being increasingly utilized.

Inaccuracies in quantitative computed tomography are contributed to by marrow fat, partial volume averaging and the threshold levels used and lies variably between 5% and 15%.