C. J. G. Bakker

Multi-exponential relaxation analysis with MR imaging and NMR spectroscopy using fat-water systems.

This study was undertaken to evaluate the feasibility of multiexponential relaxation data analysis to MR imaging techniques. The first part of this study contains accurate relaxation time measurements performed on a conventional spectrometer. In the second part, essentially the same measuring techniques were applied but now on standard whole body MR imaging equipment. T2 relaxation was measured using multi-echo techniques, T1 relaxation using multiple inversion recovery measurements. Manganese chloride solutions were used for verification of the single exponential model. Water and fat mixtures were considered for multi-exponentiality. Pure fat showed an intrinsic two-exponentiality in T1 and T2 relaxation. Mixtures of fat and water were analyzed and could at best be characterized by two exponentials, although at least three exponentials were known to be present. From the two-exponential fit the relative amounts of fat and water were calculated and compared with the mixture composition. Statistical criteria are discussed to discriminate between single and double exponential behavior in relaxation curves. It is concluded that the time consuming IR measurements for the determination of multiple T1 relaxation are not applicable in a clinical environment. Multiple T2 relaxation can be determined in a reasonable amount of time using multiple echo measurements in one image acquisition. It is shown that the observed values of T1 and T2 from tissues with intrinsic multiexponential relaxation behavior, measured with MR imaging or MR relaxation techniques on a whole-body imager or a conventional spectrometer, depend strongly on the way the experiments are set up and on the model accepted for data analysis.

Derivation of quantitative information in NMR imaging: a phantom study

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. 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.

Restoration of Signal Polarity in a Set of Inversion Recovery NMR Images

A method is described which enables unambiguous retrieval of sign information in a set of magnetic resonance magnitude images of the inversion recovery type. The proposed method starts from the observation that the inversion recovery curve S is a monotonically increasing function of the inversion time TI, and comes down to finding the zero-crossing time TI0 of this curve for each pixel within the image. ¿S¿ and S are then related by S(TI) = -¿S(TI)¿ for TI ¿ TI0 and S(TI) = + ¿S(TI)¿ for TI ¿ TI0. The method, which does not require additional knowledge with respect to any of the NMR parameters involved, is shown to be effective when at least four inversion recovery images with different inversion times of a particular object slice are available. The efficacy of sign retrieval is demonstrated by imaging experiments on phantoms and human subjects. The validity of the polarity restoration method is established by viewing its results against the results of conventional methods, i.e., NMR spectroscopy.

Lanthanide bearing microparticulate systems for multi-modality imaging and targeted therapy of cancer.

The rapid developments of high-resolution imaging techniques are offering unique possibilities for the guidance and follow up of recently developed sophisticated anticancer therapies. Advanced biodegradable drug delivery systems, e.g. based on liposomes and polymeric nanoparticles or microparticles, are very effective tools to carry these anticancer agents to their site of action. Elements from the group of lanthanides have very interesting physical characteristics for imaging applications and are the ideal candidates to be co-loaded either in their non-radioactive or radioactive form into these advanced drug delivery systems because of the following reasons: Firstly, they can be used both as magnetic resonance imaging (MRI) and computed tomography (CT) contrast agents and for single photon emission computed tomography (SPECT). Secondly, they can be used for radionuclide therapies which, importantly, can be monitored with SPECT, CT, and MRI. Thirdly, they have a relatively low toxicity, especially when they are complexed to ligands. This review gives a survey of the currently developed lanthanide-loaded microparticulate systems that are under investigation for cancer imaging and/or cancer therapy.

Precision in calculated ϱ, T1 and T2 images as a function of data analysis method

In NMR imaging rho, T1 and T2 images are usually calculated from a set of partial saturation, saturation recovery or inversion recovery experiments with multiple echoes and multiple repetition times. Several methods can be envisaged to extract parameter images from such a set of source images. These methods to a greater or lesser extent take advantage of the fact that a multiple echo/multiple repetition time experiment provides a set of largely independent T1 and T2 measurements. In this study several data analysis methods, including weighted and non-weighted averaging of results of independent T1 and T2 measurements, weighted and non-weighted averaging of source images prior to data reduction and simultaneous three-parameter fitting, were compared against another in terms of precision, computational efficiency and robustness. The predicted performance of the examined methods was verified by stochastic simulation experiments.

Calculation of Zero-Crossing and Spin-Lattice Relaxation Time Pictures in Inversion Recovery NMR Imaging

A method is described which permits accurate pixel-by-pixel estimation of zero-crossing time t10 and spin-lattice relaxation time T1 images from a set of inversion recovery images with different inversion times. The method is effective when at least three inversion recovery images of a particular object sike are available, and does not require starting values or additional knowledge with respect to any of the NMR parameters involved. In the proposed method T1 is calculated from the zero-crossing time t10 of the inversion recovery curve for each point in the image, in a way analogous to the null-point determination of T1 in a conventional NMR experiment.

Concerning the Derivation and Representation of Parameter Images in NMR Imaging

In this work several aspects of the derivation and representation of parameter images in NMR imaging are taken into consideration. First we discuss the use of statistical parameter estimation techniques to determine the minimum variance bound (MVB) on the precision of any parameter determined from an NMR experiment and give some practical examples. Then we explore the effect of experiment design, i.e. the choice of sequence type and timings, on the precision of the estimated parameters achievable per unit experiment time. In the third section we compare some practical methods to extract p, T1 and T2 images from a set of partial saturation, saturation recovery or inversion recovery experiments which to a greater or less extent take into account the fact that a multiple echo/multiple repetition time experiment provides a set of largely independent T1 and T2 measurements. In the final section a provisional attempt is made to develop a non-linear gray scale for the display of ρ, T1 and T2 images which takes into account the estimated precision of the displayed parameter values.

Some Aspects of Mr Image Processing and Display: Simulation Studies, Multiresolution Segmentation, and Adaptive Histogram Equalization

In the years 1982–1984 we have designed and built a software system for the quantitative processing, analysis and display of magnetic resonance 2D and 3D images and timeseries of images. This system supports two research projects in our university, “NMR in Oncology”, and “MRI Processing and Simulation Studies”.

DERIVATION OF QUANTITATIVE INFORMATION IN NMR IMAGING: A PHANTOM STUDY

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.