Quantitative computed tomography is a form of medical imaging in which data from a series of X-ray images creates a two- or three-dimensional model of a part of the body. In general, computed tomography (CT) refers to the practice of using this type of X-ray image to create a more complete image. Frequently abbreviated QCT, the term "quantitative computed tomography" distinguishes the type of analysis used in this method. Quantitative computed tomography is most common in the field of bone densitometry, the measurement of bone mineral density (BMD), but it also has other applications.
In quantitative computed tomography, the patient or subject places the body part to be scanned within a CT scanner. Most QCT involves the spine or an extremity such as the forearm. In spinal scans, the patient or subject lies down within the scanner. In peripheral quantitative computed tomography (pQCT), the patient or subject places the forearm in a smaller scanner while seated or standing.
The CT scanner contains an X-ray tube and sensor, which rotate around the body part in a circular or spiral pattern. The X-ray device takes a series of pictures of the body part, then transmits them to a computer. Special QCT software analyzes the images, creating a model of the scanned area. This image can be three-dimensional or two-dimensional, depending on the scanner and the goal of the scan.
The primary difference between quantitative computed tomography and other forms of computed tomography is in the analysis performed by the computer. In most computed tomography, the software produces a composite visual image for the doctor or researcher to examine. The purpose of this type of visual examination is to detect fractures, lesions or other symptoms in the scanned bone or soft tissue. QCT takes the data provided by the scanner and uses it to generate numerical values for the volume, mass and density of bone.
Quantitative computed tomography possesses a number of advantages over other forms of measuring bone density. One of the most important is its ability to distinguish between cortical bone, which lines the outside of the bones, and trabecular bone, the softer tissue which makes up the center of the bone. Trabecular bone is much more metabolically active than cortical bone, meaning that the two types of bone are replaced at different rates. As a result, the two types of bone will show different rates of change in bone mineral density.