Relaxation Time T₁, T₂ and Proton Density Images in NMR Imaging

Abstract

Pure T₁, T₂ and proton density (ρ) images can be computed from three or more different NMR images. Computed images can be useful for several reasons: a) they are objective, since they are independent of pulse sequence and scan parameters. b) arbitrary composite images can be synthesized from computed images. c) biochemical information can be obtained from relaxation times, so quantitative diagnosis is possible using T₁ and T₂ images. For these reasons, several methods of producing computed images have been tried.
However, with these methods, there are several practical problems such as large systematic error and long total scan time. This paper describes how several sets of NMR pulse sequences and scan parameters were investigated, keeping total scan time constant, to find which of them gave computed images with best resolution and minimum systematic error for a given scan time.
Pulse sequences and scan parameters were optimized to yield minimum variance of computed images, using the law of error propagation, for a given range of T₁, T₂ and ρ. We found that theoretically the combination Inversion Recovery 3 Spin Echo and Saturation Recovery 4 Spin Echo pulse sequence gave the best compromise between scan time and resolution.
The effect of slice profile and errors in RF pulses-causes of systematic error-were analyzed in order to find ways to remove or reduce them.
Using this method computed T₁, T₂ and ρ images were obtained for the human head and for various phantoms. Computed values agreed closely with values measured using analytical methods. We conclude from these results that the combination Inversion Recovery 3 Spin Echo and Saturation Recovery 4 Spin Echo pulse sequence gives the best compromise between scan time, resolution and error.