Quantitative computed tomography

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. 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. In general, solid phantoms placed in a pad under the patient during CT image acquisition are used for calibration. These phantoms contain materials that represent a number of different equivalent bone mineral densities. Usually either calcium hydroxyapatite (CaHAP) or potassium phosphate (K2HPO4) are used as the reference standard. QCT was invented at University of California San Francisco (UCSF) during the 1970s. Douglas Boyd, PhD and Harry Genant, MD used a CT head scanner to do some of the seminal work on QCT.At the same time, CT imaging technology progressed rapidly and Genant and Boyd worked with one of EMI’s first whole body CT systems in the late 1970s and early 1980s to apply the quantitative CT method to the spine, coining the term 'QCT.' Genant later published several articles on spinal QCT in the early 1980s with Christopher E. Cann, PhD. Today, QCT is being used in hundreds of medical imaging centers around the world, both clinically and as a powerful research tool. Originally, conventional 2D QCT used individual, thick CT slice images through each of multiple vertebrae which involved tilting the CT scanner gantry to align the slice with each vertebra. Today, modern 3D QCT uses the ability of CT scanners to rapidly acquire multiple slices to construct three-dimensional images of the human body. Using 3D imaging substantially reduced image acquisition time, improved reproducibility and enabled QCT bone density analysis of the hip. QCT exams are typically used in the diagnosis and monitoring of osteoporosis. At the spine, QCT is used to measure the bone mineral density of only the spongy interior bone separately from the dense cortical bone that forms the exterior walls of the vertebrae. The trabecular bone has much higher metabolic activity than the cortical bone and so is affected by age, disease and therapy-related changes earlier and to a greater degree than cortical bone. This means that QCT of the spine has an advantage compared to other bone density tests because earlier changes in bone mineral density may be detected . Clinically, QCT is used at the hip to produce areal BMD measurements and T-Scores that are equivalent to DXA measurements. The exam can be done without particular attention to the positioning of the patient’s limbs because the software allows the hip anatomy to be manipulated after the image is captured, allowing the exam to be performed on patients with arthritic hips who may find traditional exams uncomfortable.