Quantitative magnetic resonance imaging of subcutaneous adipose tissue.

BACKGROUND/PURPOSE: Quantitative transverse relaxation time (T(2)) magnetic resonance (MR) imaging has been used with the aim to characterize subcutaneous adipose tissue. Protons in adipose tissue have a fast exchange behavior giving bi-component transverse relaxation processes with short and long relaxation time values depending on the tissue properties. METHODS: MR images were acquired on a 1 T Siemens MR scan using a multi-spin-echo sequence. A high sensitive surface coil, enabling low noise MR images with voxel size of 10 mm(3), was used for performing accurate quantitative T(2) imaging. These acquisition parameters were determined by a preliminary study performed on an oil phantom known to be a valuable model for mimicking in vivo adipose tissue. In vivo study of the thigh adipose tissue was carried out on 30 volunteers. 20 women with various clinically diagnosed cellulite grades and 10 males, among them five showed overweight. Tissue characterization was finally performed through the analysis of the T(2) distributions. RESULTS: Phantom study showed that improvements in the precision in T(2) measurement are obtained at the expense of the spatial resolution. Uncertainties in T(2) measurements are three times lower by considering a region of interest of 3 x 3 pixels compared with a pixel by pixel analysis. The in vivo study showed that women groups present higher mean short T(2S) component values than men. Histogram of T(2) distribution showed that the maximum amplitude is observed at a lower value for the overweight men group. In addition, larger values around the septae were visualized on the long relaxation time images. CONCLUSIONS: This study shows that precise T(2) map of adipose tissue can be computed. The balance between precision and spatial resolution is examined. Preliminary results relative to tissue organization and to difference between clinical groups proves the potential of the quantitative MRI.

In vivo visualization of hyaluronic acid injection by high spatial resolution T2 parametric magnetic resonance images.

BACKGROUND/PURPOSE: In recent years, increasing use of injectable resorbable fillings has been reported for facial wrinkle treatment. However, the physiological processes involved such as the localization and subsequent diffusion of the injected product in skin tissues are poorly documented. This may be noninvasively achieved using quantitative magnetic resonance imaging (MRI), which is duly presented in this pilot study. METHODS: Hyaluronic acid (HA) was injected intradermally in the forearm of a young male volunteer. High-resolution MRI scans using a surface antenna were performed just after injection, and after 2, 4 and 9 months. Morphological images were compared with transverse relaxation time (T(2)) images computed from a pixel-by-pixel analysis. RESULTS: On high-resolution morphological MR images the HA injection is barely visible, but with quantitative MRI the zone of injection is clearly seen. This is due to HA having a distinctly different transverse relaxation time, T(2) approximately 600 ms, compared with dermal and hypodermal tissues, 35 and 80 ms, respectively. CONCLUSION: These preliminary results demonstrate the ability of the T(2) images for in vivo visualization of the filler agent and also for characterization of tissue modifications. In addition, the diffusion and progressive degradation of the filler agent can be monitored by T(2) measurements over time.

High spatial resolution quantitative MR images: an experimental study of dedicated surface coils.

Measuring spin-spin relaxation times (T2) by quantitative MR imaging represents a potentially efficient tool to evaluate the physicochemical properties of various media. However, noise in MR images is responsible for uncertainties in the determination of T2 relaxation times, which limits the accuracy of parametric tissue analysis. The required signal-to-noise ratio (SNR) depends on the T2 relaxation behaviour specific to each tissue. Thus, we have previously shown that keeping the uncertainty in T2 measurements within a limit of 10% implies that SNR values be greater than 100 and 300 for mono- and biexponential T2 relaxation behaviours, respectively. Noise reduction can be obtained either by increasing the voxel size (i.e., at the expense of spatial resolution) or by using high sensitivity dedicated surface coils (which allows us to increase SNR without deteriorating spatial resolution in an excessive manner). However, surface coil sensitivity is heterogeneous, i.e., it--and hence SNR--decreases with increasing depth, and the more so as the coil radius is smaller. The use of surface coils is therefore limited to the analysis of superficial structure such as the hypodermic tissue analysed here. The aim of this work was to determine the maximum limits of spatial resolution and depth compatible with reliable in vivo T2 quantitative MR images using dedicated surface coils available on various clinical MR scanners. The average thickness of adipose tissue is around 15 mm, and the results obtained have shown that obtaining reliable biexponential relaxation analysis requires a minimum achievable voxel size of 13 mm3 for a conventional volume birdcage coil and only of 1.7 mm3 for the smallest available surface coil (23 mm in diameter). Further improvement in spatial resolution allowing us to detect low details in MR images without deteriorating parametric T2 images can be obtained by image filtering. By using the non-linear selective blurring filter described in a previous work, the voxel size was reduced to 0.8 mm3, allowing us to detect microstructures such as fibrous septae while preserving precision in T2 measurements. This paper provides practical information allowing us to perform reliable T2 quantitative MR micro images. High resolution imaging with dedicated surface coils, which is only well-suited to near surface organs, might lead to highly valuable results in this context, especially to analyse the hypodermis involved in the lipodystrophy seen in patients with human immuno-deficiency virus (HIV).

A post-processing method for multiexponential spin-spin relaxation analysis of MRI signals.

Quantitative MR imaging is a potential tool for tissue characterization; in particular, proton density and proton relaxation times can be derived from MR signal analysis. However, MR image noise affects the accuracy of measurements and the number of tissue parameters that can be reliably estimated. Filtering can be used to limit image noise; however this reduces spatial resolution. In this work we studied, using both simulations and experiments, a filter called a 'selective blurring filter'. Compared to other classical filters, this filter achieves the best compromise between spatial resolution and noise reduction. The filter was specifically used to reliably determine the bi-component transverse relaxation of protons in adipose tissue. Long and short relaxation times and the relative proton fraction of each component were obtained with a degree of uncertainty of less than 10% and an accuracy of 95%.

Measurements of extracellular volume fraction and capillary permeability in tissues using dynamic spin-lattice relaxometry: studies in rabbit muscles.

Dynamic MR longitudinal R(1) relaxometry after administration of a gadolinium contrast bolus (Gd-DTPA) has been used for in vivo measurements of the extracellular volume fraction (v) and the capillary permeability (k min(-1)) in rabbit muscles to distinguish between red slow- and white fast-twitch muscle fiber types. For this purpose a protocol imaging sequence has been used which allows fast R(1) measurements during the contrast agent uptake. Physiological tissue parameters, k and v, were obtained by computing procedures assuming a simplified monoexponential plasma model. These were shown to be about twice as large in the slow-twitch semimembranosous proprius muscle (SP), containing 100% oxidative type-I fiber, that in the fast-twitch rectus femorus muscle (RF), containing only 6% type-I fiber type. The capillary permeability has been found to be 0.25 +/- 0.02 min(-1) for the (SP) and 0.10 +/- 0.01 min(-1) for the (RF). Similarly, the extracellular volume fractions were 0.189 +/- 0.015 and 0.082 +/- 0.006 respectively, in close agreement with literature data and experimental results obtained by invasive radionuclide measurements. For the pool of the 10 studied animals, no significant variation among animals was observed in the extracellular volume fraction and the capillary permeability for the different muscle fiber types. The dynamic relaxometry method used is easy to implement on conventional MR imagers and has potential applications in muscle diseases. The method has also potential applications for tissue characterization based on extracellular volume and capillary permeability quantification. In particular, the method can be used for the evaluation of tumors and their responses to therapies.

Rapid relaxation times measurements by MRI: an in vivo application to contrast agent modeling for muscle fiber types characterization.

This paper is a description of a simulation method to evaluate the contrast in NMR imaging and its aim is to help to optimize the use of contrast media in clinical imaging. Indeed, there is a need to define objective criteria in order to choose among several contrast media the ones that are the most effective and to define their optimal conditions of use, such as: the dose to be injected, the required time after injection to obtain the best enhancement and the optimal imaging sequence parameter values. The method is based on NMR signal simulation in the presence of contrast media and requires the fast measurement of the T1 and T2 relaxation times to obtain the dynamic relaxometry variation of tissues after contrast injection. In this work the fast imaging techniques that are to be described enable the measurement of T1 and T2 with a 30sec temporal resolution on 128*256 matrix images. The accuracy of the method was assessed in rabbit muscles after the injection of two gadolinium chelates (Gd-DTPA and Gd-DOTA) with the aim of improving the in vivo characterization of fast-twitch and slow-twitch muscle fiber types. The simulation results were in close agreement with contrast image analysis and showed, for relevant clinical doses, a small efficacy for both chelates. The interest of the proposed simulation method lies in the fact that it enables to objectively compare the efficacy of different contrast agents, to forecast the efficacy of a given contrast reagent and to define the optimal dose and the optimal imaging sequence parameters that give the best contrast. This simulation method obviates numerous prior experiments to evaluate the benefit expected from different contrast media. The method, which has been evaluated here for muscle investigations is applicable to any tissue analysis and can help to guide the best condition of use of contrast agents in MR imaging.

In vivo tissue extracellular volume fraction measurement by dynamic spin-lattice MRI relaxometry: application to the characterization of muscle fiber types.

RATIONALE AND OBJECTIVES: The extracellular volume fraction (v) was estimated in leg rabbit muscles by MRI dynamic longitudinal relaxation rate (R1) relaxometry to distinguish between slow- and fast-twitch muscle fiber types. METHOD: The extracellular volume fraction was calculated from the dynamic increase of the longitudinal relaxation rate after intravenous administration of a gadolinium (Gd-DTPA) contrast bolus, assuming a biexponential plasma concentration model. RESULTS: It has been shown that the extracellular volume fraction increases with the slow fiber content (oxidative type I); the maximal value (v = 0.186+/-0,018) was obtained in pure slow-twitch muscle fiber (100% type I). CONCLUSION: NMR extracellular volume estimates closely agree with those obtained using the more classic invasive isotopic method (99mTc-DTPA) carried out on the same rabbit strain and with data reported in the literature. The method has potential applications to characterize the pathophysiologic status of tissues. It is also applicable to a wide range of tissues and pathologies, in particular for the characterization of malignant tissues and their response to therapies.

MRI dosimetry: a fast quantitative MRI method to determine 3D absorbed dose distributions.

RATIONALE AND OBJECTIVES: Magnetic resonance imaging (MRI) techniques seem to be very promising for 3D dosimetry studies, but long imaging acquisition time limits their use. A new fast T1 mapping protocol, easy to implement on a conventional MR imager, has been used to determine dose distributions on Fricke gels. METHODS: The method has been tested on manganese chloride (MnCl2) doped ferrous gelatin gels. The T1 measuring times range from 1 minute 40 seconds to 3 minutes 30 seconds for a 256x256 matrix image. RESULTS: The two- and three-dimensional profiles agree with those obtained with conventional dosimetry techniques (ion chambers). The precision and the spatial resolution principally depend on the signal-to-noise ratio of the used imaging RF coil. For example, for a surface coil, the accuracy is about 2.5% with a 1.56 mm spatial resolution. CONCLUSION: These preliminary results support the feasibility of the proposed technique for accurate MRI dosimetry studies and also have potential for various clinical quantitative MRI applications.