6- 8 Mean brain radiodensity is also abnormal in multiple sclerosis, 9 and mean brain radiodensity shows a statistically significant decline as a function of aging. It is well-established that mean brain radiodensity is abnormal in hypoxic-ischemic encephalopathy, for example, and quantitative assessment appears to have diagnostic and prognostic implications. Because most brain imaging research has been conducted using MR imaging, there is little information on the diagnostic value of the brain radiodensity measure. 5 CT has the additional capability of voxel-based assessment of radiodensity, expressed in terms of Hounsfield units, a tissue property measure that is unique to this technique. 3, 4 The volumetrics of CT imaging data have been less extensively researched, though a recent study of automated segmentation of clinical head CT data shows that the brain volume is statistically smaller in patients with Alzheimer disease. In neuroimaging, brain volumetrics can have diagnostic value, as illustrated by prospective MR imaging studies of neurodegenerative diseases such as age-related dementia, 1 amyotrophic lateral sclerosis, 2 and multiple sclerosis.
Such databases facilitate research into quantitative CT by helping define the normal statistical variance, identify outliers, and correlate measurements with pathology. Recent improvements in image quality and the availability of postprocessing software enable the generation of large reference databases from the existing clinical archives. Concerns over radiation exposure have limited prospective human subject research in CT and consequently have limited the investigation into the value of quantitative CT. Hounsfield, Nobel Lecture, 1979ĬT is a highly calibrated and scaled imaging technique capable of volumetric and radiodensity assessment, though this capability is rarely used in the clinical setting. “… it became apparent that the conventional methods were not making full use of all the information the X-rays could give.” G. ABBREVIATION: BPF brain parenchymal fraction
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