Derivation of quantitative information in NMR imaging: a phantom study

Abstract

The use of NMR imaging as a quantitative research tool requires insight into the relationship between various imaging techniques and their resultant images. Work was undertaken to elucidate this relationship by using the following procedure. First, a theoretical model of NMR imaging under various pulse sequences was elaborated. Subsequently, a series of inversion recovery and saturation recovery images of a particular object slice was generated by varying the sequence parameters. Finally, pure rho, T1 and T2 images of that slice were obtained by solving the corresponding model equations. This procedure was applied to a test phantom containing tubes with suitable reference substances, including aqueous solutions of agar, manganese chloride and deuterium, and water-fat mixtures. The concentration of various samples was chosen such as to yield rho, T1 and T2 values usually encountered in clinical NMR imaging. Experiments were carried out with a prototype resistive NMR imager with a static magnetic field of 0.14 T, corresponding to a hydrogen proton resonance frequency of 5.9 MHz. For most samples a weighted non-linear regression analysis showed the theoretical model to produce an adequate parametrisation of the data at the 5% significance level, given the number of data points and the experimental accuracy. The quantitative information extracted from the NMR imaging experiments, i.e. rho, T1 and T2, appeared to be in good agreement with the results of conventional methods, including NMR spectroscopy. The clinical efficacy of the proposed methods is currently being investigated.

Proton relaxation times and interstitial fluid pressure in human melanoma xenografts

“…shortening (Jakobsen et al, 1995 (Schneiders et al, 1983;Bakker et al, 1984). A relatively simple model was used to calculate T, and T2 in the present work, ignoring the possibility that the relaxation process might be bi-or multiexponential.…”

T 1 : The Longitudinal Relaxation Time

“…during contrast agent uptake studies). The steady-state approach uses the same Equations (5.3) and (5.4) to predict the signal in the sequence, but makes use of the fact that magnetization is in equilibrium with the applied RF (Bakker et al, 1984a). After about five repeats the sequence in Figure 5.4 would reach steady state.…”

“…If the transverse magnetization is not crushed throughout the sequence, it will lead to the formation of spin echoes and stimulated echoes. In theory these additional signals can be taken account of (Bakker et al, 1984a;Vold et al, 1973). In practice, as their amplitudes will depend to varying degrees on T 2 , T 2 * and T 1 , in most T 1 measurement sequences it is better to crush the transverse magnetization throughout, except just before data acquisition.…”

“…The latter approach is better as it overcomes the problem of scanner drift between acquisitions. The method of Bakker et al (1984a) can be used to calculate the expected ratios of signals for the different pulse sequences (Kurland, 1985). Kurland (1985) compared various two point sequences and found that the IR/SE was superior to the IR+SE, SE+SE and SE/SE sequences in terms of signalto-noise ratio in the T 1 measurement per unit measurement time.…”

Optimization of signal‐to‐noise ratio in calculated T1 images derived from two spin‐echo images

“…For example, only a few workers (9)(10)(11)(12)(13) have taken into account the distributions of tip angles through an imaging slice caused by the interaction of magnetic field gradients and the rf pulse envelope.…”