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QCT, The Most Accurate Method of Measuring Bone Mineral Density?

1997; Oxford University Press; Volume: 12; Issue: 11 Linguagem: Inglês

10.1359/jbmr.1997.12.11.1954

1523-4681

Jean Weigert,

Medical Imaging and Analysis

To the Editor: The recent article by Grampp et al.1 comes to the conclusion that quantitative computer tomography (QCT) is the most accurate method of measuring bone mineral density. QCT had the strongest correlation to age and menopause related bone loss and the highest sensitivity for distinguishing between normal and osteoporotic bone. Many studies have shown similar results.2-8 QCT is fast, convenient, accurate, and precise. However, many fallacies are presented by nonimagers who do not understand this modality. It is not high in radiation. A low-dose scanning technique can be utilized to minimize radiation dosage (EDE) to 60 μSv/examination, a dose similar to that for a posteroanterior (PA) chest X-ray, three times less than a mammogram, and considerably less than background radiation of 2500 μSv per anum.9 It is interesting to note that some of the newer technology for dual X-ray absorptiometry (DXA) has almost as much radiation dosage as QCT.10 The only area radiated in a CT scan is in a direct line of the beam. When there are four slices through four vertebral bodies, those are the only regions being radiated. In addition, often several areas are imaged on DEXA: PA spine, lateral spine, hip, and on occasion a plain film radiograph of the spine as well. This combination is certainly higher in radiation dosage than any QCT. As to the precision of QCT, adequate quality control is important in all modalities. Newer QCT programs than that utilized by Dr. Grampp's group allow even better precision and reproducibility.9-11 Three-dimensional (3D) QCT (QCT PRO Mindways Corp., South San Francisco, CA, U.S.A.) utilizes 3-mm contiguous slices of two vertebral bodies to isolate a cube of pure trabecular bone determined by a reconstructed image in the sagittal, coronal, and axial planes. Reproducibility is improved because the actual position of the patient on the CT scanner is no longer in the hands of the technologist. The computer program isolates a specific portion of bone to be measured. In addition, this new 3D technology allows for reconstruction of the hip, enabling isolation of trabecular and cortical bone in various regions of the proximal femur.12 Quality control is strictly maintained: various parameters are evaluated, including machine variation within the CT scanner, phantom accuracy, and the computer program itself. These superior quality controls ensure that when Mrs. Jones has her QCT in San Francisco, California it can be accurately compared with one in West Hartford, Connecticut. This may not be the case with all DXA technology. Availability of QCT is an interesting issue. The market has been flooded with a variety of planer imaging modalities that can be set up free-style in clinician offices. QCT requires the utilization of a state-of-the-art CT scanner which can only be found in imaging departments or imaging centers. Radiologists are becoming more aware that they can provide the best means of measuring bone mineral density. We have a superb technology at our fingertips and we must not be afraid to use it. Educating the referring physician to the superiority of this modality will be key. Dr. Grampp's article provides additional validation of this fact.