Functional analysis with DTI and diffusion-neurography of cranial nerves

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1 Functional analysis with DTI and diffusion-neurography of cranial nerves Poster No.: C-1942 Congress: ECR 2013 Type: Educational Exhibit Authors: J. P. Martínez Barbero, T. Martín Noguerol, A. Luna Alcalá; Jaén/ ES Keywords: Speech disorders, Diagnostic procedure, MR-Functional imaging, MR-Diffusion/Perfusion, Neuroradiology brain DOI: /ecr2013/C-1942 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 30

2 Learning objectives Define an appropriate technical design of brain DTI sequence to evaluate cranial nerves. Learn how to obtain high-quality tractographic reconstructions of normal cranial nerves according to their different segments Highlight the clinical role of DTI in the evaluation of cranial nerves pathology Page 2 of 30

3 Background Diffusion Tensor Imaging (DTI) techniques are based on free diffusion of water molecules due to the Brownian movement, which defines that any molecules in liquid or gaseous compounds tend to move in a random and probabilistic way in any space direction. DTI takes advantage of this quality of water molecules to reflect brain structure. In the CNS, movement of water molecules is constrained in its freedom by the organized neuronal axons. Their motion is no longer random as it is conditioned by myelinated axon fat walls, which prevents their free passage into the extracellular space. Fig. 1 References: Neurorradiología, Clínica las Nieves - Jaén/ES Page 3 of 30

4 Fig. 2 References: Neurorradiología, Clínica las Nieves - Jaén/ES The DTI acquisition is performed by multiple directional vectors of B, which let us define for each voxel included in the study the tendency (a probabilistic calculation) of the water molecules contained therein to move in a certain direction (anisotropy). Thus, the higher our values of B in the acquisition, and the greater the number of addresses that we acquire, the more information we will have to post-process. Page 4 of 30

5 Fig. 3 References: Neurorradiología, Clínica las Nieves - Jaén/ES Therefore, for each voxel of white matter we will get a fractional anisotropy (FA) value which determines the level of restriction of free water diffusion, providing information about nerve fibers direction, permitting us to define the fiber tracts. Page 5 of 30

6 Fig. 4 References: Neurorradiología, Clínica las Nieves - Jaén/ES Linking pixel by pixel where the higher diffusion directions are aligned it is possible to define fibers or maximum diffusivity paths. Stopping criteria: - Maximum angulation between diffusion direction in two consecutive pixels: Angle between the principal diffusion direction higher than a certain value. - Region where the are none principal diffusion direction: Fractional Anisotropy lower than a certain value. Page 6 of 30

7 Fig. 5 References: Neurorradiología, Clínica las Nieves - Jaén/ES Page 7 of 30

8 Fig. 6 References: Neurorradiología, Clínica las Nieves - Jaén/ES Page 8 of 30

9 Imaging findings OR Procedure details DWI: BASIC SEQUENCEDESIGN Scanning parameters of the DWI sequence must be optimized in order to increase SNR and CNR. DWI is prone to motion and magnetic susceptibility artifacts since the majority of DWI are based on EPI sequences. As a general rule, conventional DWI has a limited spatial resolution. In order to increase the DWI sequence quality several rules should be followed: - use fat supression sequences: the use of fat suppression sequences allows us to increase the dynamic range of the DWI reducing the chemical shift-induced ghosting artifacts. Although inversion-recovery approaches such as STIR are useful for imaging large areas, the use chemical fat selective saturation is more appropriate (e.g: SPAIR; CHESS...) for smaller areas of interest due to their better SNR. Therefore, selective fat suppression is better for soft tissue studies - minimize T1 saturation: TR should be long enough to avoid T1 saturation effects, which can result in falsely low ADC values. - use short TE: which can be done increasing the bandwidth (up to a maximum of 1500 MHz), using parallel imaging (SENSE factor up to 2) and by increasing the gradient intensity in the gradient lobes - increase the number of adquisitions (NEX), although it is time consuming. Because the noise is disruptive and the signal is additive - decrease the field of view (FOV) to a minimum in the phase encoding direction. - do not increase the resolution in plane to levels where the noise increases significantly or image quality decreases severely because it will decrease the quality of ADC maps. To enlarge the FOV may have a similar result. - trace approach/gradient overplus : The use of three orthogonal motion-probing gradients to produce a single diffusion direction allows us to improve the gradient strength by square root of three. Therefore, this approach reduces the effective TE, increases the SNR and minimizes susceptibility, EPI, or motion artifacts. - SNR may be increased by using higher field strength (3T magnets), reducing TE, applying higher gradient power, using a short EPI train, and using phase-array coils with more number of elements Our scanning protocol for cranial nerves DTI: Page 9 of 30

10 For the specific study of cranial nerves, acquisition differs from that used In assessment of white matter, and in our study we optimized values. We have used a DWI SE sequence with TR of 11,395 and TE 70ms, Flip Angle 90, Voxel Size 1.5 x 1.5 x 1.2 mm, with 16 Directions and 2 B values (0 and 1000), in a Philips Intera 1.5T Magnet. During the postprocessing of tracts, we adapted values with respect to conventional tractography, using a minimum value of fractional anisotropy of 0.10 (compared to conventional 0.15) with a maximum angle of direction change of the fibers of 50º degrees (vs. 27 ), and minimum fiber length of 5 mm. We use both T1 or T2 3D sequences for anatomical location, depending on the suspected pathology, scanning protocol (eg, in MS studies, for the evaluation of optic neuritis, we use a 3D GE T1-weighted sequence, which is already included in our scanning protocols). In the other hand, for the evaluation of the olfactory nerve and cranial nerves of the posterior fossa, we prefer T2-weighted 3D sequences (CISS, Balance, DRIVE ) due to their better contrast to define the small cranial nerves In order to obtain the best possible images of cranial nerves in our institution we have three different protocols for acquisition, depending on their anatomical location Olfactory nerve: coronal acquisition in anterior cranial fossa (green box) Optic n and III, IV, V and IV cranial nerves : axial acquisition (red box) Lower cranial nerves: VII + VII and IX-XII : axial acquisition(blue box) The use of small FOV in DTI acquisition decrease time of adquisition, improve S/N ratio and reduce motion artifacts. Page 10 of 30

11 Fig. 7 References: Neurorradiología, Clínica las Nieves - Jaén/ES Page 11 of 30

12 Images for this section: Fig. 8 Page 12 of 30

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29 Conclusion - High-resolution DTI-based tractography allows to evaluating normal anatomy of most cranial nerves - DTI and tractography are useful in the evaluation of various diseases of the cranial nerves along with other morphological and functional sequences Page 29 of 30

30 References S.A. Smith, Z.R. Williams, J.N. Ratchford, S.D. Newsome et al. Diffusion Tensor Imaging of the Optic Nerve in Multiple Sclerosis: Association with Retinal Damage and Visual Disability. AJNR Am J Neuroradiol Oct 2011, 32: M.-Y. Wang, P.-H. Qi D.-P. Shi. Diffusion Tensor Imaging of the Optic Nerve in Subacute Anterior Ischemic Optic Neuropathy at 3T AJNR Am J Neuroradiol Aug : Mikael Skorpil, Tyler Rolheiser, Harold Robertson, Anders Sundin, Per Svenningsson. Diffusion tensor fiber tractography of the olfactory tract. Magnetic Resonance Imaging 29 (2011) Alexandra Borges, Jan Casselman. Imaging the trigeminal nerve. European Journal of Radiology 74 (2010) Avneesh Chhabra, Gustav Andreisek, Theodoros Soldatos, Kenneth C. Wang et al. MR Neurography: Past, Present, and Future. AJR 2011; 197: Page 30 of 30