Mohammed T. Abou-Saleh

Principles and Practice of Geriatric Psychiatry.Editors: Professor John R. M. Copeland, Dr Mohammed T. Abou-Saleh and Professor Dan G. BlazerCopyright & 2002 John Wiley & Sons LtdPrint ISBN 0-471-98197-4 Online ISBN 0-470-84641-064Magnetic Resonance ImagingK. Ranga R. KrishnanDuke University Medical Center, Durham, NC, USAThe property that is the basis for magnetic resonance imaging isthe interaction between hydrogen and a magnetic field. Ahydrogen atom is a nuclear magnet which, when placed on themagnetic field, aligns itself in the direction of the field. In theprocess of aligning it develops a spin at a frequency which is calledthe Larmor frequency. If the magnetic field is varied, then thefrequency of this oscillation also varies. Besides hydrogen, when anucleus contains an odd number of either protons or neutrons, itis magnetic. As indicated earlier, the frequency of oscillation ofhydrogen is proportional to the strength of the magnetic field.Resonance occurs when the applied frequency (magneticfrequency) is the same as that of the object. Magnetic nuclei ina magnetic field can be stimulated by other magnetic fields. Thesefields can usually be created by radio-frequency waves. When theradio-frequency waves are tuned to the Larmor frequency of theatomic nuclei, namely hydrogen, then the direction of oscillationof the atom moves towards the direction of the newly appliedmagnetic field. When the field is turned off, the nuclei relax, emitabsorbed energy and return to the state prior to the radiofrequency stimulation. This process can be repeated over and overagain, provided that enough time is allowed for the relaxation ofthe atom. When consideration is given to an entire objectcomposed of billions of atoms, the relaxation of these atomswill depend upon local effects, i.e. what molecule the atom isattached to, etc., the most common attachment being water forhydrogen. The signal detected when the hydrogen atom relaxesafter the radio frequency and pulse has stopped can be detected byusing a radio frequency antenna tuned to the Larmor frequency.The decay of this signal over time is called free induction decay.The time to recover two-thirds of the magnetization after stoppingthe radio-frequency stimulus is called the T1 relaxation time, orspin lattice relaxation time. This relaxation time is influenced bythe environment of the hydrogen atom and can vary between graymatter, white matter, and fluid. T1 primarily refers to thelongitudinal vector of relaxation, which is along the direction ofthe magnetic field. Another vector at right angles to the field,called the transverse vector, can also be measured, and itsrelaxation is called T2 relaxation. By convention, this refers to thetime when it disappears, rather than the two-thirds used formeasuring T1. T2 is always shorter than T1. Based on theproperties of the longitudinal and transverse vector, one acquiresimages that are predominately T1-weighted, or those which arepredominately T2-weighted, a third type that appear intermittentin appearance. In a T1-weighted image, fluid appears dark. In aT2-weighted image, fluid appears bright and white. Scanningsequences can be optimized by emphasizing one contrast vs.another. This allows a better distinction of both pathology andnormal tissue.MAGNETIC RESONANCE MORPHOMETRYMagnetic resonance morphometry basically utilizes the acquiredimages to identify objects and then to quantitate their volumes.Volumes can be estimated in either two dimensions or three. Themost common method of estimation is to use two-dimensionalslices through a given object. A number of studies have indicatedthat systematic sampling is better than the randomized samplingof the particular object of interest. Segmentation of the object canbe accomplished manually or by semi-automated means, andmore recently in an automated fashion. Factors that effect thesensitivity and accuracy of the estimation of the volume includethe number of slices that go through the object, the orientation ofthe slices, contrast, and any inhomogeneities in the magnetic field.Magnetic resonance morphometry methods have greatlyimproved over the last decade and are now widely utilized forstudying a variety of neuropsychiatric disorders. The techniques,which were initially primarily manual outlining of an object, havevastly improved and now include semi-automated and automatedtechniques to estimate the volume of the object.GERIATRIC PSYCHIATRYThe application of MRI in geriatric psychiatry has been extensive.There are two broad areas to which MR techniques have beenapplied. One is to evaluate the presence of gross pathology andthe other is to evaluate morphemetric changes in a variety ofdisorders in the elderly. MRI is often utilized to look for tumors,infarcts, etc. when one suspects disease in a patient. MRI candetect space-occupying lesions and, given that images can beacquired in three dimensions, it can often provide betterresolution than computed tomography (CT) (it must be kept inmind that newer forms of CT provide sufficiently high resolution).MRI is not useful in identifying calcifying objects, and in thisparticular case CT is far better. The use of contrast agents, such asgadolinium, can distinguish any breaking of the blood–brainbarrier and often these agents are utilized to produce additionalcontrast of pathological tissue. Full-blown infarcts can be easilyidentified on MRI. In addition, MRI can be utilized to measureblood flow through large vessels, a technique known as calledmagnetic resonance angiography.BRAIN ATROPHYThe second aspect as it relates to geropsychiatry is a measurementof brain atrophy and dementia. Patients with frontal temporalPrinciples and Practice of Geriatric Psychiatry, 2nd edn. Edited by J. R. M. Copeland, M. T. Abou-Saleh and D. G. Blazer&2002 John Wiley & Sons, Ltd