Quantitative Computed Tomography (QCT) is a specialized medical imaging test used to assess bone health and strength. It helps identify and monitor osteoporosis, a disease that weakens bones. QCT provides insights into bone mineral density (BMD), helping healthcare providers understand a person’s bone health status.
What is a QCT Scan?
QCT, or Quantitative Computed Tomography, is a type of CT scan designed to measure bone mineral density and evaluate bone strength. It creates a three-dimensional image of the bone, allowing for volumetric measurement of bone density, particularly in the lumbar spine and hip. These are common sites for osteoporosis-related fractures, making QCT valuable for assessment.
The scan uses a standard CT scanner in conjunction with a calibration standard, typically a solid phantom, placed under the patient. The scanner measures X-ray attenuation, converting these measurements into bone mineral density values. This process provides a precise and detailed picture of the bone’s internal structure and density.
How QCT Compares to Other Bone Density Tests
QCT offers distinct advantages over other bone density tests, particularly Dual-energy X-ray Absorptiometry (DXA). DXA provides a two-dimensional measurement of bone density, combining the dense outer layer (cortical bone) and the spongy inner part (trabecular bone). QCT provides a three-dimensional, volumetric measurement, allowing separate analysis of trabecular and cortical bone.
This ability to isolate trabecular bone is a significant benefit because it is often affected earlier and to a greater extent by bone loss conditions like osteoporosis. QCT can detect early bone loss more sensitively than DXA. QCT measurements are also less affected by conditions that can artificially inflate DXA results, such as spinal degenerative diseases, disc space narrowing, aortic calcification, osteophytes, and obesity.
While QCT generally involves a slightly higher radiation dose than DXA, the dose is still considered low and within safe limits for diagnostic imaging. The benefits of more accurate and detailed bone density assessment often outweigh the minimal increase in radiation exposure.
The QCT Scan Process
Before a QCT scan, patients may receive specific instructions, such as avoiding calcium supplements for at least 24 hours prior to the test. Patients are also advised to wear loose, comfortable clothing and remove any metal objects, such as zippers, buttons, or jewelry, which could interfere with the imaging. It is also important to inform the healthcare provider about any recent medical exams involving contrast materials, like barium or iodine, as these substances can affect scan results.
During the QCT scan, the patient lies on a cushioned table that slides into the CT scanner. The procedure is non-invasive and generally quick, typically taking between 10 to 30 minutes. The patient must remain still throughout the scan, sometimes holding their breath for a few seconds. A trained radiographer will be present to guide the patient and address any concerns, operating the scanner from an adjacent control room while maintaining communication.
Interpreting QCT Scan Results
QCT scan results provide detailed information about bone mineral density, which is used to assess fracture risk and guide osteoporosis management. For the lumbar spine, QCT results are typically reported as volumetric BMD measurements in milligrams per cubic centimeter (mg/cm³). The American College of Radiology (ACR) provides specific guidelines for interpreting these values: a BMD above 120 mg/cm³ is considered normal, a BMD between 80 mg/cm³ and 120 mg/cm³ indicates osteopenia, and a BMD below 80 mg/cm³ suggests osteoporosis.
For the hip, QCT can produce areal BMD measurements and T-scores comparable to DXA scans. A T-score compares an individual’s bone density to that of a healthy young adult of the same sex and ethnicity. A T-score between -1.0 and -2.5 suggests osteopenia, while -2.5 or lower indicates osteoporosis. Z-scores may also be reported, comparing a patient’s BMD to the average BMD of individuals of the same age and sex, providing additional context. A radiologist or specialist interprets these results, and a healthcare provider then discusses them with the patient to determine the appropriate course of action.