Synopsis

Diffusion tensor imaging (DTI) is promising for evaluation of lumbar nerve root compression in the extraforaminal area. A quantitative assessment using T₂ relaxometry is also promising to evaluate nerve injury. Hence, nerve root quantification using both DTI and T₂ relaxation properties may improve the diagnosis of nerve roots in patients with lower back pain, but it requires a long scan time. In this study, we developed a new sequence to simultaneously obtain both diffusion parameters and T₂ value in one single scan (Diffusion-Relaxation Matrix: DRM). DRM simultaneously provides diffusion tensor imaging and T₂ map without prolongation of acquisition time. This quantitative combination may be helpful to further assess the lumbar nerve root pathology.

PURPOSE

Lumbar nerve root entrapment caused by lumbar foraminal stenosis and disc herniation, commonly occur in the extraforaminal area¹, can be a cause of low back pain; therefore, it is important to evaluate pathologic lesions in the extraforaminal area. Diffusion tensor imaging (DTI) based on a single-shot diffusion-weighted echo planar imaging (DW-EPI) is promising for evaluation of lumbar nerve root compression in the extraforaminal area. Several studies have shown that DTI and its tractography can visualize lumbar nerve disorders and quantitatively evaluate by using fractional anisotropy (FA)^2-4. FA values is decreased at the nerve compression injuries due to nerve degeneration^5.6. Besides, quantitative assessment using T₂ relaxometry is also promising to evaluate nerve injury^7-9. T₂ values correlated with nerve injury and its functional recovery⁷. Hence, nerve root quantification using both DTI and T₂ relaxation properties may improve the identification of the specific site of any degenerative and inflammatory changes along the nerve roots of patients with lower back pain¹⁰. However, in fact, a separate acquisition of DTI and T₂ relaxometry requires long scan duration and it is hardly feasible in routine clinical examination.

In this study, we developed a new sequence to simultaneously obtain both diffusion related parameters and T₂ value in one single scan (Diffusion-Relaxation Matrix: DRM). The purpose of this study was to evaluate the feasibility of DRM sequence.

METHODS

Theory and pulse sequence:

DRM is based on single-shot dual-echo DW-EPI sequence [Fig.1]. This sequence achieves dual echo-time (TE) by adding an extra refocusing pulse and EPI readout. Since motion-probing gradients (MPG) are applied around first refocusing pulse only, the effective diffusion time of both TEs are not changed. Accordingly, DRM reconstruction can simultaneously provide apparent diffusion coefficient (ADC) maps and FA maps of both short and long TEs (54ms and 120ms in this study), and T₂ maps of both b=0s/mm² and high b-value (b=800s/mm² in this study) images in one single scan [Fig.2].

Experiments:

A total of five volunteers were examined with 1.5T whole-body clinical system (Ingenia, Philips Healthcare). The study was approved by the local IRB, and written informed consent was obtained from all subjects.

The T₂ maps obtained by DRM sequence were assessed for accuracy by comparing with a conventional 2D multi-echo spin-echo (SE) T₂ mapping sequence with use of dedicated phantom with various T2 values (filled with stepwise-diluted indigestible dextrin and soluble calcium) and human nerve roots. We measured the T₂ value of the phantoms and nerve (average value of both the left and right dorsal root ganglions of L5 and S1). Spearman’s correlation coefficient in T₂ values of phantoms were assessed. The T₂ values were assessed by one-way repeated measures ANOVA and the post-hoc Tukey test. We also measured the ADC maps and FA maps with respective TEs and compared with the values reported in literature^10-14.

Imaging parameters for DRM dual-echo DTI were: voxel size=3.33*2.21*3.50mm, FOV=320*253mm, 50 slices, b-value=0 and 800s/mm², MPG=15 directions, TR=10900ms, TE=54 and 120ms, and total acquisition time=6m00s. Imaging parameters for multi-echo SE were: voxel size=1.5*1.8*3.5mm, FOV=320*253mm, 20 slices, TR=2600ms, TE=12ms*8, ETL=8, and scan duration=4min51s.

RESULTS and DISCUSSION

Fig.3(a) shows the comparison of T₂ values among multi-echo SE and DRM-T2 with respective b-values. The T₂ values of nerve in DRM-T2 with both b-values indicated slightly shorter time compared to those of conventional T₂ mapping. The following points can be given as reasons. First, DRM calculated the T₂ value by only two TEs. This may potentially cause unstable T₂ assessments. Besides, partial volume effect, due to larger voxel size and image distortion of EPI, might also affect the measured T₂ value. It is possible that T₂ value changes due to the contamination from adjacent tissues. Nevertheless, the T₂ values of dedicated phantoms measured in DRM-T2 well correlated with the one measured in multi-echo SE T₂ mapping (R²=0.9986, P<0.01) [Fig.3(b)]. Thus, the T₂ values measured in DRM-T2 may have same trends with one measured in conventional T₂ mapping.

Fig.4 shows the FA and ADC values with respective TEs. Both the FA and ADC values with short TE indicated close to the values reported in literature^10-14. These results suggest that the dual-echo acquisition does not adversely affect for assessment of these diffusion values. Furthermore, FA values were slightly decreased and ADC values are slightly increased by prolonging the TE [Fig.4]. Additionally, T₂ values were slightly decreased due to the diffusion gradients [Fig.3(a)]. Although further clinical investigation is needed, these changes might be clinically useful in addition to FA, ADC and T₂ values themselves.

CONCLUSION

Acknowledgements

References

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