Mohammed T. Abou-Saleh

26 PRINCIPLES AND PRACTICE OF GERIATRIC PSYCHIATRYinvasive procedure. As a result, the ‘normality’ of the sample instudies using PEG can be easily criticized.With advances in computer technology, two new brain imagingmethods have been recently introduced, computed X-ray tomography(CT) and magnetic resonance imaging (MRI). The majoradvantage common to both techniques is the opportunity to studyboth the CSF spaces and the brain parenchyma in great detail.Computed X-ray tomography is a relatively simple and inexpensivetechnique, with the advantage of short study acquisitiontimes. The method is limited, however, by low spatial and densityresolution, partial volume artifacts (the effect of the coexistence ofmultiple tissue and fluid components in the same volume unit) andbone-hardening artifacts (false elevation of brain CT numbersadjacent to the skull) 5,8 . These technical problems have limited theability of CT to quantitate brain changes with aging.Magnetic resonance imaging takes advantage of the magneticcharacteristics of tissue protons to study the brain. The mainadvantages of this technique are the excellent resolution of graymatter, white matter and CSF and the high sensitivity to diseaseprocesses, due to the fact that multiple factors (proton density,proton environment, etc.) contribute to the signal characteristicsof each structure. The images are free of bone artifacts and thebrain can be examined across all three major planes (i.e. axial,coronal and sagittal). Another advantage of MRI technology isthat it does not utilize ionizing radiation. Disadvantages of MRIare the partial volume artifacts, the relatively longer studyacquisition times and the fact that claustrophobic subjects cannottolerate the examination due to the physical characteristics of theequipment 9,10 .Method of Image AnalysisAnatomical studies of the aging brain have used either qualitativeor quantitative measures of brain morphology. In qualitativestudies, raters employ various scales to examine the parameters ofinterest including, for example, the degree of cortical atrophy andventricular enlargement. Qualitative ratings are relatively easy touse, are clinically relevant 11 , do not depend on advancedinstrumentation and sophisticated computations, and may displaysignificant correlations with more quantitative methods 12 . On theother hand, the accuracy of these ratings depends on the skill ofthe raters and, as such, it is difficult to compare the results ofstudies from different authors using different scales. In addition,the sensitivity and resolution of the scales are limited by thenumber of rating categories (usually three to five) 13 .The quantitative measurements can be categorized as volumetric,planimetric (area measurement of regions of interest) orlinear (distance measurement between points of interest)(Table 7.1). Volumetric methods are very accurate (according tovalidation studies on phantoms) and are very sensitive to changesin brain size 8,14,15 . The accuracy of the volume measurements isincreased by obtaining relatively thin imaging slices (