An exact form for the magnetic field density of states for a dipole

We present an analytical form for the density of states for a magnetic dipo le in the center of a spherical voxel. This analytic form is then used to e valuate the signal decay as a function of echo time for different volume fr actions and susceptibilities. The decay can be considered exponential only in a limited interval of time. Otherwise, it has a quadratic dependence on time for short echo times and an oscillatory decaying behavior for long ech o times. (C) 2001 Elsevier Science Inc. All rights reserved.