In vivo multiple spin echoes imaging of trabecular bone on a clinical 1.5 T MR scanner

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Abstract

In vivo multiple spin echoes (MSE) images of bone marrow in trabecular bone were obtained for the first time on a clinical 1.5 T scanner. Despite of a reduced sensitivity of the MSE trabecular bone images with respect to the cerebral matter ones, it is possible to observe some features in the MSE trabecular bone images that may be useful in the diagnosis of osteopenic states. Two different CRAZED-type MSE imaging sequences based on spin-echo and EPI imaging modalities were applied in phantom and in vivo. Preliminary experimental results indicate that EPI imaging readout seems to conceal the MSE contrast correlated with pore dimension in porous media. However it is still possible to detect anisotropy effects related to the bone structure in MSE-EPI images. Some strategies are suggested to optimize the quality of MSE trabecular bone images.

Introduction

Recently, there has been much interest in MRI based on the contrast arising from multiple spin echoes (MSE) [1] or intermolecular multi-quantum coherences (iMQCs) [2], a manifestation of the dipolar field generated by residual intermolecular dipolar couplings in liquids. This occurs in the presence of a correlation magnetic field gradient with amplitude Gc that breaks the magnetic isotropy of the sample so that intrinsic long-range dipolar couplings give rise to the refocusing of a signal. Hence the correlation gradient generates a magnetization helix resulting in spatial modulation of the sample magnetization. In heterogeneous systems, the amplitude of the MSE signal depends on sample heterogeneity over a correlation distance dc = π/(γGct) [2] (where t is the gradient pulse duration [1], [3]) which is half a cycle of the magnetization helix, thus providing a novel contrast mechanism that can be tuned to a specific length scale.

Various MR techniques based on MSE effects, such as the CRAZED sequence [2], have been applied for characterizing tissues and heterogeneous systems [3], [4], [5], [6], [7], [8], [9], [10]. Furthermore functional magnetic resonance imaging (fMRI) and self-diffusion weighted imaging have been demonstrated using iMQCs [11], [12]. Despite of a lower signal-to-noise ratio (SNR) with respect to the single-quantum images, the iMQCs images have revealed features that were undetectable with conventional MRI. IMQCs images are characterized by a contrast based on the dipolar interaction correlation distance as well as by a higher sensitivity to magnetic susceptibility distributions due to the dipolar demagnetizing field.

For morphologic studies of trabecular bone, multiple spin-echoes (MSE) offers an alternative contrast mechanism that depends on the porosity of the bone, as recently demonstrated ex vivo at 7 T [8], [9], [10].

In particular, two competing contrast mechanisms are active in porous media. One is directly related to the mean number of coupled spins at the correlation distance dc since only the magnetization arising from spins at a distance dc is detected. The other mechanism depends upon the susceptibility differences between coupled spins at this distance. In particular, susceptibility effects in porous media produce a drop of the signal intensity when the correlation distance dc corresponds to the pore diameter. In previous studies, we have shown a distinct relation between the porous structure and the correlation distance dc, by using bovine trabecular bone of the calf [8], [9], [10]. In particular, by varying the gradient strength and/or the duration of the gradient pulse, the MSE image contrast could be varied and unique information about the trabecular bone microstructure could be obtained. Especially, by using this approach, the pore size of the trabecular bone could be measured quantitatively and the data obtained from the MSE images were consistent with those obtained by scanning electron microscopy (SEM).

Thus, these previous high-field studies [8], [9], [10] demonstrated the feasibility of such an approach, which however could become impractical at lower field strengths used in clinical routine. Particularly, the successful implementation of MSE imaging at 1.5 T, has the potential to provide useful information in bone disease. In the contest of trabecular bone imaging, an important issues regards what kind of image encoding that is to be used. The imaging version of the CRAZED-type sequence is obtained by adding a single-shot echo planar (EPI) [13], [14] read-out [4], [6] or a conventional sequential line-scan (SE) technique [3], [7], [8], [9], [15]. The advantage of the EPI read-out is based on its speed that compensates for the high number of signal averages necessary to obtain a sufficient SNR in the MSE image. However since k-space coverage with EPI is obtained by a series of image gradient reversals the resulting image will be weighted in T2* and signal drop-outs and distortions due to susceptibility artifacts at tissue interfaces may hamper an accurate interpretation of the image and its contrast. This effect may be particularly bothersome in tissues with rapid T2* decay, such as trabecular bone.

In this paper, we outline some strategies that we have adopted to optimize MSE imaging of bone marrow in the trabecular bone at 1.5 T. A feasibility analysis of the MSE-imaging method is presented and CRAZED-type sequences with different image encoding are discussed and applied on a phantom and in vivo on a clinical 1.5 T MR scanner. In trabecular bone and porous media phantom, we found that the T2* weighted image obtained by using EPI read-out seems to conceal the novel MSE contrast linked to the porous dimension in the CRAZED-type sequence.

Section snippets

Methods and material

Two CRAZED-type imaging sequences, both based on a conventional sequential line-scan (MSE-SE) and EPI (MSE-EPI) read-out, were implemented on a Siemens Vision MR-scanner operating at 1.5 T (Siemens Medical System, Erlangen, Germany). The images were obtained by using a circular polarized volume head-coil for RF transmission and reception.

The first sequence (Fig. 1a) consisted of two slice-selective RF pulses (90° and 120°) spaced by a delay time τ combined with a pair of magnetic gradient

Results

To demonstrate the sensitivity of MSE imaging to microstructure, MSE images of a spherical phantom characterized by pore structures with a diameter of about 0.5 mm were obtained at 1.5 T. As recently demonstrated at 7 T [9], the MSE signal intensity of that phantom should exhibit a signal intensity drop when the correlation distance dc, related to the duration and amplitude of the applied correlation gradient, matches the interstitial pore diameter. To investigate the MSE signal variation,

Discussion and conclusion

In the present work, CRAZED-type sequences have been used to obtain, for the first time, in vivo trabecular bone MSE images at 1.5 T. The intrinsic features of the MSE contrast, that enables the detection of the pore size of trabecular bone, may prove useful in the diagnosis of the osteopenic diseases.

To start with, we verified the feasibility of detecting the relation between the correlation distance dc and the pore dimension in a porous system, at a magnetic field strength of 1.5 T, as

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