Fat and water differentiation by nuclear magnetic resonance imaging

Subtraction of in-phase and opposed-phase images in dynamic MR mammography

PURPOSE

To develop and to evaluate an advanced image acquisition and analysis method for collecting T(1)-weighted dynamic 3D MR mammography data sets by using a combined in-phase (IP) and opposed-phase (OP) imaging procedure.

MATERIALS AND METHODS

3D MR mammography data sets were acquired by applying an interleaved gradient-echo OP and IP imaging sequence during administration of contrast agent. A phantom data set, two volunteer breast data sets, and six patient breast data sets were recorded. Subtraction of dynamic in-phase magnitude images was performed for clinical assessment. In addition, the magnitude subtraction (SIPOP) as well as the complex subtraction (cSIPOP) of the IP and OP magnitude and phase images were considered.

RESULTS

The detection of small lesions, lesion boundaries, and tumor offshoots in fatty tissue was improved by the subtraction of IP and OP images without the risk of signal cancellation due to partial volume effects.

CONCLUSION

Dynamic MR mammography acquisition of IP and OP images in combination with appropriate data processing yields important supplementary information that can support routinely applied diagnostics of breast lesions that are fully embedded in fatty tissue by only marginally increasing acquisition time.

In-vivo quantitative hydrolipidic map of perirenal adipose tissue by chemical shift imaging at 4.7 Tesla

OBJECTIVE

In this work, chemical shift magnetic resonance imaging is used for in vivo quantitative evaluation of fat and water content in the perirenal white adipose tissue.

DESIGN AND MEASUREMENTS

Experiments were carried out on female Sprague-Dawley rats with a 4.7 T magnet. Fat and water fractions were computed pixel-by-pixel from the chemical shift selective images with an algorithm of reconstruction that allowed parametric maps (called hydrolipidic maps) to be produced with a pixel size of 625 x 625 microm.

RESULTS

Our findings indicate that, in the perirenal adipose tissue, the water content ranges between 15% and 20%, with slight differences between the ventral and dorsal portions, and between the left and right deposits. The mesenteric adipose tissue, observed for comparison, has a mean water content of 30%.

CONCLUSION

The present work demonstrates that methods based on magnetic resonance imaging can be useful tools for non-invasive in vivo quantitative mapping of the hydrolipidic content of adipose tissues.

Chemical shift imaging at 4.7 tesla of thymus in young and old mice

We propose an experimental protocol, based on chemical shift magnetic resonance imaging (CSI) that improves the methods presently available for the in vivo study of the thymus in small animals. Male Balb/c mice were examined in an imager-spectrometer equipped with a 4.7 T magnet. Three groups of animals with different ages were used: the first group consisted of 3-month-old mice (n = 5), the second group of 19-month-old mice (n = 5), and the third group of 26-month-old mice (n = 4). The identification of thymic parenchyma was obtained by two (T1-weighted spin-echo and CSI water-selective) images. The T1-weighted spin-echo image provided a detailed anatomical description of the organs located in the thorax. The CSI water-selective image provided a detailed description of thymic location, shape, and dimensions. The cross-sectional area of the thymus, measured from CSI images, showed a decreasing trend with increasing age. The values of the thymus-muscle contrast-to-noise ratio were measured in both spin echo and CSI images. While the contrast between thymus and muscle was greatly improved in the young and presenescent group, the difference was not statistically significant in the senescent group. In conclusion, the proposed method allows the study of thymic modification during the passage from young to pre-senescent age and from presenescent to old age. This method could be useful in studies in which experimental manipulation or drug treatments produce changes in the dimension and fat content of this organ. The proposed protocol, based on CSI, appears to be an improved methodology for study of the thymus.

Quantification of oil and water in preserved reservoir rock by NMR spectroscopy and imaging

Reservoir rock analysis by proton NMR requires separation of the response into brine and crude oil components. Tests on preserved core from a North Sea chalk reservoir show that spin-lattice relaxation time distributions can be used to distinguish the two fluids. NMR estimates of oil and water saturations for 1.5" diameter core examined in a 10 MHz Bruker Minispec spectrometer closely match fluid contents determined by distillation. The spin-lattice relaxation contrast mechanism developed for core samples can be applied in the quantitative analysis of NMR images. The relaxation data are compared with data from chemical shift imaging on the same core sample. The results indicate that it will be possible to monitor changes in fluid distributions, in this and similar systems, under dynamic conditions such as in a waterflood.

Coupled-spin fast spin-echo MR imaging

Distinguishing between lipid and water-containing tissues is clinically important. Current techniques rely on the chemical shift difference between fat and water resonances or differences in relaxation times of the tissues, or a combination of both. A method is presented for separating the signals of lipid protons from those of water protons by using fast spin-echo magnetic resonance imaging based on the principle that lipid protons behave differently from water protons in multi-echo sequences. Two images are acquired with different echo train lengths and echo spacing but with identical TEs, and then subtracted to exploit differences in the behavior of lipid and water protons in multi-echo sequences. The method is insensitive to B0 inhomogeneities or susceptibility effects and provides separate lipid and water images with a high signal-to-noise ratio. The advantages of the method are demonstrated with phantom studies and clinical examples.

A chemical shift selective inversion recovery sequence for fat-suppressed MRI: theory and experimental validation

Fat-suppression techniques are used extensively in routine proton nuclear magnetic resonance imaging to produce images free from chemical shift artifacts and dynamic range problems. A hybrid fat-suppression sequence is studied which combines the principle of short time inversion recovery with chemical shift selective imaging. The aim of this study is to provide a theoretical understanding of the role of the sequence parameters, as well as to compare this hybrid sequence with its most closely related conventional fat-suppression techniques, namely selective pre-saturation and short time inversion recovery (STIR) imaging. The hybrid technique is shown to be robust in normal use, and more tolerant than the conventional methods to mis-settings of parameters such as inversion time, as well as tip angle and frequency bandwidth of the fat selective pulse.