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To present a new method, S0 estimation of the free induction decay combined with a single spin echo measurement (SOFIDSE), that enables simultaneous measurements of R2*, R2, and R2' in order to quantify the local concentration of holmium microspheres (Ho-MS) for radioembolization.SOFIDSE estimates R2* and the signal magnitude at time point 0, S0, from a multigradient echo readout of the free induction decay and subsequently estimates R2 using S0 and a single spin echo, from which R2' is deducted. The method was evaluated by comparing SOFIDSE R2 values with values obtained from shifted spin echo (SSE) measurements on a phantom setup containing Ho-MS and from dual spin echo measurements on a healthy volunteer.On average, SOFIDSE showed a small overestimation of R2 values compared with SSE independent of the microsphere concentration. R2' values determined by subtraction of either SOFIDSE R2 or SSE R2 from R2* showed excellent agreement (correlation coefficient = 1; P = 9 · 10(-11)). The Ho-MS-induced R2' values obtained by SOFIDSE were insensitive to the R2 value of the tissue in which they resided.SOFIDSE enables quantification of Ho-MS, in media with spatially or temporally varying R2 values, in a single acquisition.

Accurate localization of interventional devices, for example, needles and brachytherapy seeds, is desired for interventional procedures. MRI is usually considered unsuitable for this purpose, as the induced signal voids and signal pile-ups do not necessarily represent the exact location of the devices. Center-out radial sampling with off-resonance reception (co-RASOR) has been shown to solve this problem by repositioning the signal pile-up into the geometrical center of the interventional devices. However, the multiple acquisitions required for co-RASOR resulted in a low efficiency and unsuitability for near real-time interventional purposes. Herein, we aim to increase the efficiency of co-RASOR by relying on multiple off-resonance reconstructions of a single acquisition rather than on multiple acquisitions. The soundness of this approach is shown by demonstrating the equivalence of acquisition co-RASOR and reconstruction co-RASOR, both theoretically and experimentally. An algorithm is proposed and evaluated to obtain the geometric centers of the devices, while suppressing the background. This procedure is shown to be effective, in vitro as well as ex vivo, and to yield signal intensity increases in the order of 150-400% of the average signal, in the geometric center of a brachytherapy seed and a needle, respectively. The geometric accuracy of the resultant images is confirmed by computed tomography.

Proton resonance frequency shift (PRFS)‐based MR thermometry is hampered by temporal field changes. Temporal changes in the susceptibility distribution lead to field changes and are therefore a possible source of errors. The susceptibility of fat, χfat, is temperature dependent, in the same order of magnitude as the temperature dependence of the chemical shift of water (0.01 ppm/° C). PRFS‐based temperature measurements may therefore be corrupted by non‐local field effects due to temperature induced susceptibility changes in fatty tissue. We performed simulations to quantify the influence of dχfat/dT on PRFS‐based MR temperature maps during thermal interventions in the breast. Susceptibility distributions were calculated for a 3D breast model without and with a stationary Gaussian temperature distribution and a maximum temperature of 57° C. Subsequently, the magnetic field was calculated in Fourier‐domain from these susceptibility distributions. Changes in the magnetic field distribution were quantified b...

In this review we describe current applications and future perspectives of MR angiography, MR flow quantification, and interventional MRI in hemodialysis access management. Each section starts with a brief overview of the main techniques that are currently available or under development. This is followed by a survey of the pertinent literature. Each section concludes with a discussion of the reported findings and an indication of research opportunities.

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.

The visualization of the devices is the first step in making MR a useful tool for monitoring and guiding endovascular interventions. Basically there are three approaches to achieving this aim: active tracking, passive tracking, and the technique of locally induced field inhomogeneities (see Chaps. 7 and 8). In active tracking, a small receiver coil is built into the tip of the catheter or guidewire so as to actively identify its spatial position (Ackermann et al. 1986; Dumoulin et al. 1993; Wildermuth et al. 1997). Active tracking allows localization of the tip of a device in tens of milliseconds. The three-dimensional (3D) coordinates can be used to display the tip of the device as a white or colored dot on any previously acquired MR image. Also, the 3D coordinates can be used to steer the image acquisition.

The visceral and subcutaneous abdominal adipose tissue (AT) area and the subcutaneous hip AT area were assessed by magnetic resonance imaging (MRI) in 12 growth hormone-deficient adults before and after 6 mo of replacement with recombinant human growth hormone (rhGH) and in 12 healthy control subjects. The data obtained by MRI were compared with circumference measurements of waist and hip. Growth hormone-deficient patients compared with control subjects had a higher visceral AT area (P = 0.003) and subcutaneous AT area (P = 0.013); there was no significant difference in subcutaneous hip AT area. Six months of rhGH replacement reduced the subcutaneous hip AT area (19.8%), the subcutaneous abdominal AT area (15.6%), and particularly the visceral AT area (38.2%), resulting in fat areas that were not different from those of control subjects. Furthermore, this study shows that in contrast with control subjects, circumference measurements are not useful to predict AT areas in growth hormone-deficient patients and cannot be used to assess changes in AT areas during rhGH replacement.

Magnetic resonance imaging was used to study the effect of weight loss on three fat depots: the visceral and subcutaneous abdominal depots and the subcutaneous depot at trochanter level. Changes in fat depots were compared with changes in circumference measures and the waist-hip ratio (WHR) in obese men (n = 38) and women (n = 40). Mean weight loss was (mean +/- SD) 12.9 +/- 3.5 kg (P < 0.001). The proportional reduction of fat was largest in the visceral depot (men 40%, women 33%). Less fat was lost subcutaneously, especially at trochanter level (men 29%, women 26%). WHR decreased significantly in both sexes (P < 0.001). Change in WHR was not significantly related to the absolute reduction in visceral fat. Total body-fat loss showed a stronger association with subcutaneous fat loss than with visceral fat loss. The findings suggest that fat distribution may change with weight loss, particularly by the loss of visceral fat, but changes in WHR are not appropriate for evaluating changes in this fat depot.

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.

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.

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. Absolute Value(S)and S are then related by S(TI) = -Absolute Value(S(TI)) for TI =/TI(0) and S(TI) = + Absolute Value(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.

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”.

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.

This in vitro study was undertaken to examine whether water proton spin-lattice relaxation in biological tissues is adequately described by a single time constant T1, to define under what circumstances a multi-exponential approach is indicated, and to study the implications of multi- exponentiality for quantitative NMR imaging. Water proton relaxation curves were measured with the 180-tau-90 method at 60 MHz. Uni- and bi-exponential curves were fitted to the empirical curves using chi 2 as a criterion for the goodness of fit. An F-test was applied to test the validity of each exponential term as it was added to the fitting function. Taking into account experimental accuracy, the uni-exponential model appeared to be an adequate description of the relaxation data for necrotic tissue. Eyelens and fat showed distinct bi- exponentiality , while liver, spleen, salivary gland, tumour, and muscle presented intermediate cases. The bi-exponential analysis generally yields a minor component with a fast relaxation time, T11 less than 20 ms, and a slow relaxation major component with T12 greater than 300 ms. A simplified bi-exponential model is proposed for implementation in quantitative NMR imaging. The results seem to be consistent with current views about water proton spin-lattice relaxation in biological tissues.

Proton spin-lattice relaxation times were measured with pulsed NMR spectroscopy at 60 MHz for a range of tissues from healthy and tumour-bearing mice. T1 appeared to be significantly longer in the tumour than in any other tissue. Clearly distinct values of T1 were found for the vital and necrotic parts of the tumour. Non-involved tissues of tumour-bearing mice, viz. spleen, muscle, and salivary gland tissue, showed an increase in T1 relative to normal values. Irradiation of healthy mice with 6 MeV X-rays to a total-body dose of 15 Gy induced a shortening of T1 in the spleen. Similar experiments on tumour-bearing mice showed a decrease of T1 in the tumour, the spleen, the kidneys, and the liver. Attempts were made to interpret the results in terms of tissue hydration.

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.

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.