Evaluation of proton density by magnetic resonance imaging: phantom experiments and analysis of multiple component proton transverse relaxation

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Abstract
The accuracy of proton density measurements using a standard whole-body MR imager operating at 1.5 T was estimated. First, phantom experiments were performed to examine the possibility of an intensity correction. For the test phantom the systematical errors in the computed proton densities were reduced from 5 to 1% after correction. Secondly, proton transverse relaxation curves of biological tissues were measured in vitro on an MR spectrometer. A multi-exponential analysis of the data shows that for spin-echo times TE)10 ms in total between 10 and 30% of the protons of the tissue do not contribute to the image signal. In all tissues a proton component with a free induction decay (FID) time T2* (32 mu s was observed. In the time range TE(10 ms two proton components can be distinguished in muscle and fatty tissue. It is concluded that the hydrogen density of biological tissues can be evaluated at best with an overall error of 10% from MR images for TE)10 ms. This accuracy is insufficient for a pixel-orientated neutron therapy planning.