Quantitative Computed Tomography (QCT) is a medical imaging technique that uses a standard X-ray computed tomography (CT) scanner to generate detailed three-dimensional (3D) images of bones. QCT assesses bone density and structure, playing a role in diagnosing and monitoring conditions like osteoporosis, a disease characterized by reduced bone mass and increased fracture risk. It provides insights into areas such as the lumbar spine and hip.
How QCT Measures Bone Density
QCT uses computed tomography technology to create 3D images of bones, allowing for a volumetric measurement of bone mineral density (BMD). This differs from other methods that provide a two-dimensional, or “areal,” view of bone density. During a QCT scan, a calibration phantom is placed under the patient to ensure accurate density measurements. These phantoms typically contain materials like calcium hydroxyapatite or potassium phosphate, representing different bone mineral densities, which help convert the CT image’s Hounsfield units (HU) into BMD values.
The QCT system can differentiate between cortical and trabecular bone, which are the dense outer layer and the spongy inner network of bone, respectively. This capability is beneficial in areas like the spine and hip, where both types of bone are present. For instance, QCT spine measurements primarily characterize trabecular bone, while hip measurements mainly characterize cortical bone. QCT data acquisition can involve volumetric CT scanning of the spine, often with a calibration device.
The Unique Value of QCT
QCT provides volumetric bone density measurements, which can be more sensitive to subtle changes in bone health compared to other methods, particularly in the spine. QCT can isolate and measure the metabolically active trabecular bone, which often shows early signs of osteoporosis. This allows for earlier detection of low bone mass, potentially enabling earlier interventions.
The technique is useful in complex cases where other bone density assessment methods might be less accurate. For example, patients with spinal arthritis, vertebral compression fractures, scoliosis, or aortic calcification can receive more reliable BMD measurements. These conditions can affect the accuracy of two-dimensional scans due to calcification or structural changes. QCT is also helpful for individuals with extreme obesity or very low body mass index.
Understanding Your QCT Results
QCT results are typically reported in milligrams per cubic centimeter (mg/cm³). A spine bone density value in the range of 80-120 mg/cm³ indicates osteopenia, a condition of low bone mass. Bone density values below 80 mg/cm³ are classified as osteoporosis. The risk for fracture increases as bone density decreases.
Your QCT report will include T-scores and Z-scores. A T-score compares your bone density to that of a healthy young adult population. For postmenopausal women and men aged 50 and older, a T-score of -1.0 or above indicates normal bone density, while a score between -1.0 and -2.5 suggests osteopenia. A T-score of -2.5 or below is diagnostic of osteoporosis.
A Z-score compares your bone density to that of individuals of the same age and gender. These results should always be interpreted by a healthcare professional, considering your overall health and individual risk factors.
Preparing for Your QCT Scan
Preparing for a QCT scan involves a few steps. Inform your doctor about any recent medical procedures that involved contrast agents, such as barium or iodine, as these can interfere with the scan. Remove all metal objects, including jewelry, piercings, and removable dentures, before the scan to prevent image artifacts. Wearing comfortable, loose-fitting clothing is advisable.
A QCT scan lasts only a few minutes per scanned area. While QCT uses X-rays, the radiation exposure is low and comparable to background radiation over a short period. Low-dose protocols are continuously being developed to further reduce radiation exposure. The procedure is non-invasive, and you will lie still on a padded table during the scan.