Quantitative Computed Tomography (QCT) is an advanced medical imaging procedure used to accurately measure bone mineral density (BMD). This technique utilizes a specialized CT scan to create a three-dimensional (3D) assessment of bone structure. QCT provides a detailed, volumetric analysis of the bone, primarily focusing on the hip and the lumbar spine, making it a specialized tool for assessing bone health.
The Technology Behind QCT
QCT uses a standard CT scanner, incorporating specialized software and a calibration standard to quantify bone density. The process converts the gray-scale values (Hounsfield units) from the CT images into precise bone mineral density values. This conversion relies on a phantom, a material of known density, placed beneath the patient during the scan to ensure accurate calibration.
The core technical distinction of QCT is its ability to perform a volumetric analysis, providing a density measurement in milligrams per cubic centimeter (mg/cm³). This three-dimensional measurement allows for the separation and analysis of different bone tissue types. For example, QCT can isolate the metabolically active trabecular (spongy) bone within the vertebral body from the dense outer cortical bone.
The trabecular bone is often affected earlier and more significantly by bone loss than the cortical shell. By focusing only on this inner structure, QCT provides a purer measure of bone health that is less influenced by surrounding calcifications or structural changes. The scan creates a contiguous volume of images, typically using a low-dose protocol to minimize radiation exposure while capturing the necessary detailed data.
QCT Versus Standard Bone Density Testing
The most common method for measuring bone density is Dual-Energy X-ray Absorptiometry (DXA), which provides an areal (two-dimensional) projection of bone density in grams per square centimeter (g/cm²). Because DXA measures all mineral in the path of the X-ray beam, its results can be artificially elevated by degenerative changes in the spine. Conditions like spinal arthritis, osteophytes (bone spurs), and aortic calcification can falsely increase the measured density on a DXA scan, potentially masking true bone loss.
QCT circumvents these limitations by utilizing its three-dimensional capability to focus exclusively on the trabecular bone, which is less affected by these external factors. This makes QCT particularly useful for patients with severe spinal degenerative disease or significant obesity, where positioning and accurate measurement with DXA may be compromised. The ability to separately analyze the trabecular bone also offers superior sensitivity for detecting subtle changes, which can be useful for monitoring the effectiveness of certain osteoporosis treatments.
The volumetric measurement provided by QCT is inherently more precise because it accounts for the actual size and volume of the bone tissue, unlike the areal density of DXA, which projects a 3D structure onto a 2D plane. This difference means that QCT T-scores for the spine are often lower than those obtained from DXA for the same patient, leading to different diagnostic thresholds.
Preparing for and Undergoing the Scan
The QCT procedure is non-invasive and typically takes between 10 and 30 minutes. Preparation is straightforward, primarily involving the removal of all metal objects, such as jewelry, zippers, or buttons, from the area to be scanned to prevent image artifacts. Patients may also be asked to wear a gown.
Patients lie flat on a cushioned table that slides into the CT scanner, which resembles a large ring. During the scan, remaining completely still is important to ensure image clarity and accuracy. Although QCT uses a low-dose protocol, the radiation exposure is generally higher than a DXA scan, but it is comparable to other common diagnostic imaging procedures.
To ensure the most accurate results, patients may be advised to temporarily stop taking calcium supplements for at least 24 hours before the test. It is also important to inform the healthcare provider of any recent imaging tests involving contrast materials, such as barium or iodine dyes, as these substances can interfere with the bone density calculations.
Interpreting the Bone Density Results
QCT results are typically reported using a volumetric density value, expressed in milligrams of mineral per cubic centimeter (mg/cm³). This measurement is then used to determine a patient’s T-score and Z-score, which are statistical comparisons to a reference population. The T-score compares the patient’s bone density to that of a healthy young adult of the same sex, while the Z-score compares it to an age-matched and sex-matched peer group.
For QCT of the spine, the interpretation relies on specific density thresholds because the standard World Health Organization (WHO) T-score criteria, derived from DXA, are not appropriate for the volumetric measurement. The American College of Radiology (ACR) recommends that a trabecular BMD value between 80 and 120 mg/cm³ indicates low bone mass, or osteopenia. A value below 80 mg/cm³ is considered diagnostic of osteoporosis.
QCT performed at the hip often uses software to generate an areal BMD measurement and T-score equivalent to a DXA result, allowing the use of the standard WHO diagnostic classification. These results provide a comprehensive picture of fracture risk, offering distinct measurements for the hip and the spine. The volumetric data helps clinicians track bone deterioration and monitor a patient’s response to therapy.