Axial CT in the diagnosis of osteoporosis
QCT is a X-ray absorptiometric technique such as SXA, and DXA (Single and Dual X-ray Absorptiometry), but it is different from these methods of measurement because it provides separate estimates of trabecular and cortical bone BMD as a true volumetric mineral density in mg/cm3. It measures high-turnover trabecular bone in consecutive vertebrae of the spine (usually two to four vertebrae out of T12 to L4), using commercial CT scanners and a bone mineral reference standard to calibrate each scan. Beginning from an initial lateral localized image and using a low-dose technique with the gantry angled parallel to the vertebral end plate, single 8 to 10 mm-thick sections are obtained through the midplane of each of these vertebrae (Fig. 1). A region of interest (ROI) is manually positioned in the anterior portion of trabecular bone of the vertebral body for analysis. It is possible to automate the sagittal location of midvertebral slices and the axial placement of ROIs to improve precision and reduce acquisition and analysis time. A software automatically locates the vertebral body, maps its outer edges, and employs anatomic landmarks, such as the spinous process and spinal canal and it calculates in this way size and location of the ROIs. Either trabecular, cortical, or integral (cortical and trabecular) bone ROIs are defined by these systems. The basivertebral vein and sclerotic foci such as bone islands have to be excluded. Hounsfield units (HU) (also known as CT number) are used to measure the CT density of the selected area of interest within a slice through a vertebral body. Then, comparing the CT number of the trabecular bone to that of the compartments of the calibration standard, it is possible to achieve a conversion to mg/cm3. The calculated densities for the vertebrae are averaged and compared to those of a normal population. Normative data are gender-and race-specific.
Women have a different bone density curve over age than men. This difference is based on an accelerated bone loss in women soon after the onset of menopause, superimposing the normal physiologic bone loss that occurs in both men and women in aging. Moreover, absolute normal bone density values are race-dependent. Some studies show that black-race has higher bone density values than white-race (both in men and women). At present solid hydroxyapatite or calcium carbonate are used as calibration materials (Fig. 2). They are quite different from the first used liquid calibration reference phantoms. Infact, these last contained varying concentrations of bipotassium hydrogen phosphate (K2HPO4), with the drawback of limited long- term stability of the solutions. In this way scanning was difficult and inaccurate: air bubbles developed in the solution because of the transpiration of fluid from the solution into the plexiglas shell of the phantom.
Figure 1 – Lateral scout view of the lumbar spine used to determine vertebral levels for axial scans.
A study of the spine with QCT takes about 30 minutes. The skin radiation dose is generally 100 to 300 mrem. Actually, only a small portion of marrow is irradiated during a QCT study of the spine, so that the effective dose or whole-body equivalent dose is generally in the range of only 3 mrem (30 |jSv). The local- izer scan that precedes the actual QCT study will add an additional 3 mrem to the effective dose. These values are quite acceptable compared to a natural background radiation of approximately 20 mrem per month. Make your pharmacy dollar go further and buy asthma inhalers online
Figure 2 – Axial slice, 10 mm thick, through L2 vertebral body, showing the bone equivalent calibration phantom positioned under the patient and an elliptical ROI in the vertebral trabecular bone. The solid reference phantom is based on calcium carbonate, which enables any whole body CT scanner to perform accurate and reliable bone mineral density measurements.
Category: Main
Tags: (QCT), Bone Mineral Density (BMD), volumetric QCT