Multiple Spin-echo Sequence Research Articles

Separate MRI quantification of dispersed (ferritin‐like) and aggregated (hemosiderin‐like) storage iron

A new MRI method is proposed for separately quantifying the two principal forms of tissue storage (nonheme) iron: ferritin iron, a dispersed, soluble fraction that can be rapidly mobilized, and hemosiderin iron, an aggregated, insoluble fraction that serves as a long-term reserve. The method utilizes multiple spin echo sequences, exploiting the fact that aggregated iron can induce nonmonoexponential signal decay for multiple spin echo sequences. The method is validated in vitro for agarose phantoms, simulating dispersed iron with manganese chloride, and aggregated iron with iron oxide microspheres. To demonstrate feasibility for human studies, preliminary in vivo data from two healthy controls and six patients with transfusional iron overload are presented. For both phantoms and human subjects, conventional R(2) and R(2)* relaxation rates are also measured in order to contrast the proposed method with established MRI iron quantification techniques. Quantification of dispersed (ferritin-like) iron may provide a new means of monitoring the risk of iron-induced toxicity in patients with iron overload and, together with quantification of aggregated (hemosiderin-like) iron, improve the accuracy of estimates for total storage iron.

3D 23Na MRI of human skeletal muscle at 7 Tesla: initial experience

To evaluate healthy skeletal muscle pre- and post-exercise via 7 T (23)Na MRI and muscle proton T(2) mapping, and to evaluate diabetic muscle pre- and post-exercise via 7 T (23)Na MRI. The calves of seven healthy subjects underwent imaging pre- and post-exercise via 7 T (23)Na MRI (3D fast low angle shot, TR/TE = 80 ms/0.160 ms, 4 mm x 4 mm x 4 mm) and 1 week later by (1)H MRI (multiple spin-echo sequence, TR/TE = 3,000 ms/15-90 ms). Four type 2 diabetics also participated in the (23)Na MRI protocol. Pre- and post-exercise sodium signal intensity (SI) and proton T(2) relaxation values were measured/calculated for soleus (S), gastrocnemius (G), and a control, tibialis anterior (TA). Two-tailed t tests were performed. In S/G in healthy subjects post-exercise, sodium SI increased 8-13% (p < 0.03), then decreased (t(1/2) = 22 min), and (1)H T(2) values increased 12-17% (p < 0.03), then decreased (t(1/2 )= 12-15 min). In TA, no significant changes in sodium SI or (1)H T(2) values were seen (-2.4 to 1%, p > 0.17). In S/G in diabetics, sodium SI increased 10-11% (p < 0.04), then decreased (t(1/2) = 27-37 min) without significant change in the TA SI (-3.6%, p = 0.066). It is feasible to evaluate skeletal muscle via 3D (23)Na MRI at 7 T. Post-exercise muscle (1)H T(2) values return to baseline more rapidly than sodium SI. Diabetics may demonstrate delayed muscle sodium SI recovery compared with healthy subjects.

Optimized, unequal pulse spacing in multiple echo sequences improves refocusing in magnetic resonance.

A recent quantum computing paper (G. S. Uhrig, Phys. Rev. Lett. 98, 100504 (2007)) analytically derived optimal pulse spacings for a multiple spin echo sequence designed to remove decoherence in a two-level system coupled to a bath. The spacings in what has been called a "Uhrig dynamic decoupling (UDD) sequence" differ dramatically from the conventional, equal pulse spacing of a Carr-Purcell-Meiboom-Gill (CPMG) multiple spin echo sequence. The UDD sequence was derived for a model that is unrelated to magnetic resonance, but was recently shown theoretically to be more general. Here we show that the UDD sequence has theoretical advantages for magnetic resonance imaging of structured materials such as tissue, where diffusion in compartmentalized and microstructured environments leads to fluctuating fields on a range of different time scales. We also show experimentally, both in excised tissue and in a live mouse tumor model, that optimal UDD sequences produce different T(2)-weighted contrast than do CPMG sequences with the same number of pulses and total delay, with substantial enhancements in most regions. This permits improved characterization of low-frequency spectral density functions in a wide range of applications.

Magnetic resonance imaging follow-up of liver growth of neuroendocrine tumors in an experimental mouse model

Liver metastases in patients with gastroenteropancreatic (GEP) endocrine tumors represent the main factor of adverse prognosis in this tumor type and thus have a strong effect on the therapeutic strategies. Currently, magnetic resonance imaging (MRI) is considered the modality of choice for the noninvasive, in vivo detection of liver metastases. Dedicated MRI protocols suitable for following liver lesion evolution on an experimental model of endocrine tumors could be valuable. An experimental animal model mimicking the clinical situation of intrahepatic dissemination has been designed. The goal of this study was to characterize liver lesions in this athymic nude mouse model and assess the detection sensitivity of MRI using a physiological gating strategy optimized for high magnetic fields. The experiments were performed at 7 T using a dual cardiac-respiratory-triggered multiple spin-echo sequence. This protocol was used to carry out a longitudinal follow-up of hepatic lesions in a group of eight nude mice at different stages: Day 7 (D7), Day 12 (D12), Day 17 (D17) and Day 24 (D24). The hepatic lesion volume fraction (HLVF) was quantified using an adaptive segmentation procedure based on a dual-reference limit. Mean transverse relaxation time T(2) values were quantified from multiple spin-echo images. The first lesions were detected at stage D12 on images with 20-ms TE. From D12, the HLVF increased significantly with stage. The mean T(2) values also increased significantly at D17 and D24. In conclusion, the level of detection and characterization of liver lesions were performed using a devoted protocol with a dedicated high-field MRI synchronization strategy. In future studies, MRI could be used to monitor the effects of targeted therapies on liver endocrine metastases in preclinical animal models.

The application of T1 and T2 relaxation time and magnetization transfer ratios to the early diagnosis of patellar cartilage osteoarthritis

We compare the T1 and T2 relaxation times and magnetization transfer ratios (MTRs) of normal subjects and patients with osteoarthritis (OA) to evaluate the ability of these techniques to aid in the early diagnosis and treatment of OA. The knee joints in 11 normal volunteers and 40 patients with OA were prospectively evaluated using T1 relaxation times as measured using delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), T2 relaxation times (multiple spin-echo sequence, T2 mapping), and MTRs. The OA patients were further categorized into mild, moderate, and severe OA. The mean T1 relaxation times of the four groups (normal, mild OA, moderate OA, and severe OA) were: 487.3 +/- 27.7, 458.0 +/- 55.9, 405.9 +/- 57.3, and 357.9 +/- 36.7 respectively (p <0.001). The mean T2 relaxation times of the four groups were: 37.8 +/- 3.3, 44.0 +/- 8.5, 50.9 +/- 9.5, and 57.4 +/- 4.8 respectively (p < 0.001). T1 relaxation time decreased and T2 relaxation time increased with worsening degeneration of patellar cartilage. The result of the covariance analysis showed that the covariate age had a significant influence on T2 relaxation time (p < 0.001). No significant differences between the normal and OA groups using MTR were noted. T1 and T2 relaxation times are relatively sensitive to early degenerative changes in the patellar cartilage, whereas the MTR may have some limitations with regard to early detection of OA. In addition, The T1 and T2 relaxation times negatively correlate with each other, which is a novel finding.

Evaluation of dose delivery accuracy of gamma knife using MRI polymer gel dosimeter in an inhomogeneous phantom

Polymer gel dosimetry is still the only dosimetry method for directly measuring three-dimensional dose distributions. MRI Polymer gel dosimeters are tissue equivalent and can act as a phantom material. Because of high dose response sensitivity, the MRI was chosen as readout device.In this study dose profiles calculated with treatment-planning software (LGP) and measurements with the MR polymer gel dosimeter for single-shot irradiations were compared.A custom-built 16 cm diameter spherical plexiglas head phantom was used in this study.Inside the phantom, there is a cubic cutout for insertion of gel phantoms and another cutout for inserting the inhomogeneities. The phantoms were scanned with a 1.5T MRI (Siemens syngo MR 2004A 4VA25A) scanner. The multiple spin-echo sequence with 32 echoes was used for the MRI scans. Calibration relations between the spin-spin relaxation rate and the absorbed dose were obtained by using small cylindrical vials, which were filled with the PAGAT polymer gel from the same batch as for the spherical phantom.1D and 2D data obtained using gel dosimeter for homogeneous and inhomogeneous phantoms were compared with dose obtained using LGP calculation.The distance between relative isodose curves obtained for homogeneous phantom and heterogeneous phantoms exceed the accepted total positioning error (>±2mm).The findings of this study indicate that dose measurement using PAGAT gel dosimeter can be used for verifying dose delivering accuracy in GK unit in presence of inhomogeneities.

Microstructural analysis of foam by use of NMR R 2 dispersion

The spin–spin relaxation rate R 2 (=1/T 2) in hydrogel foams measured by use of a multiple spin echo sequence is found to be dependent on the echo time spacing. This property, referred to as R 2-dispersion, originates to a large extent from molecular self-diffusion of water within internal field gradients that result from magnetic susceptibility differences between the gel and air phase. Another contribution to the R 2 relaxation rate is surface relaxation. Numerical simulations are performed to investigate the relation between the foam microstructure (the mean air bubble radius and standard deviation of the air bubble radius) and foam composition properties (such as magnetic susceptibilities, diffusion coefficient and surface relaxivity) at one hand and the R 2-dispersion at the other hand. The simulated R 2-dispersions of gel foam are in agreement with the measured R 2-dispersions. By correlating the R 2-dispersion parameters and simulated microstructure properties a semi-empirical relationship is obtained that enables the mean air bubble size to be derived from measured R 2-dispersion curves. The R 2-derived mean air bubble size of a hydrogel foam is in agreement with the bubble size measured with X-ray micro-CT. This illustrates the feasibility of using 1H R 2-dispersion measurements to determine the size of air bubbles in hydrogel foams and of alveoli in lung tissue.

Quantitative T2 Mapping of Matrix-Associated Autologous Chondrocyte Transplantation at 3 Tesla

To evaluate magnetic resonance (MR) T2 mapping for characterization of cartilage repair tissue following matrix-associated autologous cartilage transplantation (MACT). Fifteen patients were evaluated following MACT using a 3T MR scanner. Patients were categorized into 2 postoperative intervals: I: 3-13 months, II: 19-42 months. Mean T2 relaxation times calculated from multiple spin-echo sequence were determined in regions of interest (MACT and normal hyaline cartilage) and T2 line profiles through the repair tissue and control sites were acquired. Mean global T2 values of repair tissue in group I were significantly higher than at control sites (P < 0.05). Repair tissue in group II showed no significant difference to control sites. Repair tissue T2 line profiles normalized over time toward the control sites. T2 mapping allows visualization of cartilage repair tissue maturation. Global T2 repair tissue values approach that of control sites after more than 1.5 years, similar behavior is seen in the zonal organization.

SU-FF-T-138: Designing a Multiple Spin-Echo Pulse Sequence Optimized for Polymer Gel Dosimetry

Purpose: To formulate a systematic method to establish an optimized MRI pulse sequence using an experimental MRI scanner as an example for polymergeldosimetry (PGD). Methods and Materials: 3D dosimetry for clinical applications needs 2×2×2mm3spatial resolution with dose uncertainty lower than 3%. Fundamentals of MRI physics lead to optimized imaging parameters for R2 measurements. We chose a multiple spin‐echo technique of CMPG with variable crusher pulses. The 2D imaging technique was supplemented with interleaved slice acquisition for higher efficiency. The optimized parameters were implemented in a 1.5T MR system (Magnex Scientific, Oxfordshire, UK) using the Apollo pulse programming/data acquisition system (Tecmag Inc., Houston, TX). Measurements were carried out for a phantom containing varying R2 values. Results and Discussion: The signal‐to‐noise ratio (SNR) of baseline signals of our scanner was low; thereby, acquisitions had to be repeated for 25 times to raise SNR to a value greater than 50. The spin echo time was determined to be 22 ms based on the available gradient field strength. Since TR should be longer than 3 times of the longest T1, TR was set to 3 sec. The number of spin echoes (NSE) has to be sufficiently large since the uncertainty of the estimated R2 value decreases with increasing NSE. The number is limited because the signal noise departs from Gaussian as the signal decays. For SNR=50, NSE should be between 15 and 20 for R2 ranging from 2 to 10 s−1. For these imaging parameters the total scan takes 21.3 hours to obtain 48 2‐mm slices. We showed that increasing SNR broadens the measurable range of the R2 value and reduces NSE. Conclusion:A key requirement of high resolution and high precision MRI for PGD is to achieve SNR larger than 50.

Theoretical studies of the effect of the dipolar field in multiple spin-echo sequences with refocusing pulses of finite duration

It has been observed recently that the finite duration of refocusing rf pulses in a multiecho acquisition of the signal formed under the influence of the dipolar field leads to significant signal attenuation [S. Kennedy, Z. Chen, C.K. Wong, E.W.-C. Kwok, J. Zhong, Investigation of multiple-echo spin-echo signal acquisition under distant dipole–dipole interactions, Proc. Int. Soc. Magn. Reson. Med. 13 (2005) 2288]. Hereto, we quantify the phenomenon by evaluating analytically the influences of both the distant dipolar field (DDF) and transverse relaxation T 2 on the magnetization in a multiecho pulse sequence based on correlation spectroscopy revamped by asymmetric z-gradient echo detection (CRAZED). Analytic expressions for the magnetization were obtained, which demonstrate explicitly the origin of rephased signal in the presence of the finite π pulses in the multiecho train. The expressions also explain the effects of the DDF and T 2 during the refocusing pulses on the signal strength, and show the substantial signal dependence on the phase of the rf pulses. We show that when the DDF effect during the pulse is canceled, the signal rises primarily during the free evolution time in the acquisition period. This elucidates the signal attenuation when the rf pulses cover a significant proportion of time in the sequence. In addition, we performed an optimization on the number of refocusing pulses that maximizes the total acquired signal using parameters for water, brain white matter, and muscle. We found that maximal signal-to-noise ratio is obtained when the pulse duration approximately equals the free evolution time in the samples with a wide range of T 2.

3D polymer gel dosimetry using a 3D (DESS) and a 2D MultiEcho SE (MESE) sequence

The utilization of 3D techniques in Magnetic Resonance Imaging data aquisition and post-processing analysis is a prerequisite especially when modern radiotherapy techniques (conformal RT, IMRT, Stereotactic RT) are to be used. The aim of this work is to compare a 3D Double Echo Steady State (DESS) and a 2D Multiple Echo Spin Echo (MESE) sequence in 3D MRI radiation dosimetry using two different MRI scanners and utilising N-VInylPyrrolidone (VIPAR) based polymer gels.

Contrast‐driven approach to intracranial segmentation using a combination of T2‐ and T1‐weighted 3D MRI data sets

To develop a strategy for structural brain imaging when using FSL software for segmentation and subsequent volumetry. Three-dimensional (3D) structural MRI of 1-mm isotropic resolution was performed on a 3-Tesla clinical imaging system. Prescribed signal evolution of a multiple spin-echo (SE) sequence with variable refocusing flip angle for T2 weighting, and a modified driven equilibrium Fourier transform (MDEFT) sequence were used for T1 weighting. Postprocessing included rigid-body coregistration, brain extraction, and segmentation using the tools of the FSL 3.2 software package. T2 weighting provided reliable delineation of the subarachnoidal space, while T1 weighting provided better segmentation of gray matter (GM) and white matter (WM). The combination of T1-weighted (T1-w) and T2-w data allowed the identification of a T2-hypointense class of "nonbrain" (NB) representing larger vessels and structures of connective tissue, as well as partial volume of bone and air-filled cavities. Brain extraction on T2-w data and subsequent segmentation of the combined T1- and T2-w intensity distribution into four classes are recommended.

Basic investigations on the performance of a normoxic polymer gel with tetrakis‐hydroxy‐methyl‐phosphonium chloride as an oxygen scavenger: Reproducibility, accuracy, stability, and dose rate dependence

Magnetic resonance (MR)-based polymer gel dosimetry using normoxic polymer gels, represents a new dosimetric method specially suited for high-resolution three-dimensional dosimetric problems. The aim of this study was to investigate the dose response with regard to stability, accuracy, reproducibility, and the dose rate dependence. Tetrakis-hydroxy-methyl-phosphonium chloride (THPC) is used as an oxygen scavenger, and methacrylic acid as a monomer. Accuracy, reproducibility, and dose resolution were determined for MR protocols at low spatial resolution (typical for clinical scanners), medium, and microimaging-resolution protocols at three different dose levels. The dose-response stability and preirradiation-induced variations in R2, related to the time interval between preparation and irradiation of the polymer gel, were investigated. Also postirradiation stability of the polymer gel was considered. These experiments were performed using a 60Co beam (E = 1.2 MV) in a water phantom. Moreover, we investigated the dose rate dependence in the low, medium, and saturation dose region of the normoxic polymer gel using a linear accelerator at photon energy of 25 MV. MR scanning was performed on a 3 T whole body scanner (MEDSPEC 30/80, BRUKER BIOSPIN, Ettlingen, Germany) using several coils and different gradient systems adapted to the acquired spatial resolution investigated. For T2-parameter selective imaging and determination of the relaxation rate R2 = 1/T2, a multiple spin echo sequence with 20 equidistant echoes was used. With regard to preirradiation induced variations R2 increases significantly with the increasing time interval between the polymer gel preparation and irradiation. Only a slight increase in R2 can be observed for varying the postirradiation-time solely. The dose reproducibility at voxel volumes of about 1.4 x 1.4 x 2 mm3 is better than 2%. The accuracy strongly depends on the calibration curve. THPC represents a very effective oxygen scavenger in methacrylic acid and gelatin. Polymer gels containing THPC offer high sensitivity to dose but their dose response also strongly depends on dose rate in the medium and high dose region.

Robust prescan calibration for multiple spin-echo sequences: application to FSE and b-SSFP

The collection of fast imaging techniques that use multiple spin-echo (MSE) sequences relies on a precise phase relationship between spin echoes and stimulated echoes that form along the radiofrequency refocusing pulse train. Failure to achieve this condition produces dark banding artifacts that result from destructive interference between signal coherence pathways. Satisfying this condition on the microsecond timescale required is technically challenging for conditions involving strong diffusion-weighted gradients, for arbitrary orientation acquisitions and at large field strengths with high-resolution acquisitions. Two clinically significant MSE sequences, fast spin echo (FSE) and balanced steady-state free precession (b-SSFP), are investigated in this work using a 4-T whole-body scanner. We developed a readout-projection-based prescan technique that ensures coherent signal formation by utilizing banding artifacts to automatically adjust gradient balance. Subsequent image acquisition using the results of this prescan permits the formation of coherent-echo images, which are robust under challenging imaging conditions. The robustness of this approach is demonstrated for FSE and b-SSFP images obtained from the knees of human volunteers. We believe that the use of this prescan calibration technique for the alignment of signal pools in MSE sequences is critical at high fields and will facilitate the implementation of high-quality clinically significant sequences such as FSE and b-SSFP.

Multiple spin-echo spectroscopic imaging for rapid quantitative assessment of N-acetylaspartate and lactate in acute stroke.

Monitoring the signal levels of lactate (Lac) and N-acetylaspartate (NAA) by chemical shift imaging can provide additional knowledge about tissue damage in acute stroke. Despite the need for this metabolic information, spectroscopic imaging (SI) has not been used routinely for acute stroke patients, mainly due to the long acquisition time required. The presented data demonstrate that the application of a fast multiple spin-echo (MSE) SI sequence can reduce the measurement time to 6 min (four spin echoes per echo train, 32 x 32 matrix). Quantification of Lac and NAA in terms of absolute concentrations (i.e., mmol/l) can be achieved by means of the phantom replacement approach, with correction terms for the longitudinal and transversal relaxation adapted to the multiple spin-echo sequence. In this pilot study of 10 stroke patients (symptom onset < 24 hr), metabolite concentrations obtained from MSE-SI add important information regarding tissue viability that is not provided by other sequences (e.g., diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI)). Metabolic changes extended beyond the borders of the apparent diffusion coefficient (ADC) lesion in nine of the 10 patients, showing a rise in Lac concentrations up to 18 mmol/l, while NAA levels sometimes dropped below the detection level. Considerable differences among the patients in terms of the Lac concentrations and the size of the SI-ADC mismatch were observed.

The effect of water molecular self-diffusion on quantitative high-resolution MRI polymer gel dosimetry

In polymer gel dosimetry, magnetic resonance imaging (MRI) is used to determine the spin–spin relaxation rate (R2) which in turn can be correlated with absorbed dose to provide a map of the spatial distribution of the absorbed dose in the irradiated dosimeter. High accuracy, precision and reproducibility of these dose maps are essential. Moreover, for dose verification around brachytherapy sources used for intravascular brachytherapy, a high spatial resolution is required (typically 0.01–0.1 mm). To achieve these microscopic resolutions, strong imaging gradients are applied. The Brownian motion of water molecules in the presence of these strong magnetic field gradients causes an attenuation of the MR signal. When using a multiple spin–echo sequence, this may result in a significant deviation in the measured R2. The diffusion-related change in R2 at high resolutions was investigated experimentally and correlated with predictions that were obtained numerically and algebraically. Diffusion weighting is determined by the self-diffusion coefficient D, and imaging parameters, quantified by the b-factor. The b-factor was calculated for a multiple spin–echo sequence for different gradient strengths and gradient pulse durations. The variations in R2 that were observed when changing the matrix size and slice thickness are explained. It is shown that a linear correlation between the matrix size and the variation in R2 is based on the diffusion weighting caused by the read-out gradients and slice selective gradients. In conclusion, the essence of taking into account molecular self-diffusion to quantify variations in the measured dose–R2 response when using high-resolution MRI in polymer gel dosimetry is emphasized.

Pathway selection by pulsed field gradients.

A method for pathway selection in a multiple spin-echo pulse sequence applying crusher gradients before and after each pi pulse, to dephase unwanted pathways, is described. This method selects the only pathway that would contribute to the measurable signal if the pi pulses were perfect (1,-1,1,-1, em leader ). Good pathway selection is essential in pulse programming, especially when the CPMG condition is not met. An interactive applet was also developed to deal with these calculations.

Optimization of multiple spin–echo sequences for 3D polymer gel dosimetry

The overall performance of polymer gel dosimeters for three-dimensional radiation dosimetry is determined by the temporal and spatial stability of the gels, dose sensitivity and image quality with respect to both systematic and stochastic deviations. The dose resolution (DpΔ) is determined by the dose sensitivity and the signal-to-noise ratio (SNR) in the dose images. The dose sensitivity can be altered by changing the chemical composition of the polymer gel. The SNR is determined by the scanner and the imaging sequence.In the dose verification of conformal radiotherapy treatments the chosen number of slices may reach a number of 10–20. For these experiments, to obtain a sufficient SNR within a reasonable measurement time using a certain MR scanner, the imaging sequence should be optimized. A few other studies have emphasized the importance of optimizing the imaging sequence with respect to dose resolution (DpΔ) or SNR but do not give quantitative values for the optimal sequence parameters for scanning a polymer gel dosimeter in three dimensions.In this paper, it is proved that a multiple spin–echo sequence is preferable to a single spin–echo sequence. It is also shown that when using a multiple spin–echo sequence it is not the inter-echo time that should be optimized but the number of echoes. An algebraical expression is derived for the dose resolution in terms of sequence parameters. A mathematical formalism and look-up tables are provided that can be used to optimize both a single and a slice-selective multiple spin–echo sequence to acquire a set of dose images at various locations. The use of the optimization protocol is illustrated by some examples. The optimization protocol enables the user to derive the optimal sequence parameters to acquire a set of dose maps obtained by quantitative T2 imaging for each polymer gel dosimeter within the shortest time possible.

The return of the frequency sweep: designing adiabatic pulses for contemporary NMR.

Frequency-modulated (FM) pulses that function according to adiabatic principles are becoming increasingly popular in many areas of NMR. Often adiabatic pulses can extend experimental capabilities and minimize annoying experimental imperfections. Here, adiabatic principles and some of the current methods used to create these pulses are considered. The classical adiabatic rapid passage, which is a fundamental element upon which all adiabatic pulses and sequences are based, is analyzed using vector models in different rotating frames of reference. Two methods to optimize adiabaticity are described, and ways to tailor modulation functions to best satisfy specific experimental needs are demonstrated. Finally, adiabatic plane rotation pulses and frequency-selective multiple spin-echo sequences are considered.

Characterization of trabecular bone by dipolar demagnetizing field MRI.

A multiple spin-echo (MSE) sequence has been applied for the first time to study trabecular bone ex vivo. The second echo generated by the demagnetizing field presents discrete drops in signal intensity for certain values of the pitch of the magnetization helix created by the correlation gradient. These dips may reflect characteristic pore sizes in the trabecular bone specimens. This hypothesis is supported by similar experiments performed on a phantom with uniform pore size distribution. Trabecular bone images weighted in the MSE contrast mechanism are reported.