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Recently, several surveies on diffusion tensor imagination of the average nervus have been published. However, assorted imagination and Reconstruction parametric quantities were used. The intent of this survey was to consistently measure the optimum b-value for DTI and fiber tractography of the average nervus at 3.0T every bit good as optimum Reconstruction parametric quantities for fibre tractography.

1.2. Materials and methods

This is a prospective survey carried out with local ethical board blessing and written informed consent from all survey topics. 45 healthy voluntaries ( 15 work forces, 30 adult females ; average age, 41A±3.4 old ages ) underwent diffusion tensor imagination of the right carpus at 3.0T ( Achieva X-series, Phillips, Best, the Netherlands ) utilizing an 8-channel carpus spiral ( Achieva Sense, Philips, Best, the Netherlands ) . A single-shot echo-planar-imaging sequence ( TR/TE 10123/40ms ) was acquired from each topic at four different b-values ( 800, 1000, 1200, and 1400 s/mm2 ) .

Post processing and Fiber-tractography was performed by two independent readers utilizing dedicated Software ( Philips Achieva Version.X.X, Best, the Netherlands ) . FA, ADC and color-coded diffusion maps were calculated and fiber piece of lands were generated utilizing four different fibre tractography algorithms incorporating different FA thresholds and different Reconstruction angles.

Fiber tractography was so evaluated quantitatively and qualitatively.

1.3. Consequences

Fiber tractography generated significantly longer fibres in DTI acquisitions with higher b-values ( 1200 and 1400 s/mm2 ) compared to acquisitions with b-values of 800 and 1000 s/mm2 ( p & lt ; 0.001 ) . The overall quality of fibre tractography ( fiber length, homogeneousness, denseness and conformity with anatomy ) was best at a b-value of 1200 s/mm2 ( p & lt ; 0.001 ) . The tracking algorithm utilizing a minimal FA threshold of 0.2 and a maximal angulation of 10A° was significantly better than all other Reconstruction algorithms.

1.4. Decision

At 3.0T, the optimum b-value for DTI is 1200 s/mm2 and the optimum Reconstruction parametric quantities for fibre tractography are a minimal FA threshold of 0.2 and a maximal Reconstruction angle of 10A° .

2. Introduction

Recently, several pilot surveies sing the application of diffusion tensor imagination ( DTI ) and fiber tractography to peripheral nervousnesss ( e.g. , the median, radial, and ulnar nervus in the carpus, the peroneal and tibial nervus in the knee/calf/ mortise joint, every bit good as the sciatic nervus ) have been published. Possible clinical applications include the rating of peripheral nervousnesss in compressive neuropathies such as the carpal tunnel syndrome ( CTS ) or tracking of peripheral nervousnesss in the presence of malignant tumor. ( 1-7 )

DTI is based on magnetic resonance imagination ( MRI ) and reveals micro structural features of biological tissues by detecting the random motion of H2O molecules ( Brownian gesture ) . DTI is particularly advantageous for tissues incorporating extremely organized microstructures ( e.g. cell membranes, vascular constructions or axon cylinders ) , given that in these tissues H2O can non freely spread in all waies. This inhomogeneity of diffusion is called anisotropy and can be quantified with DTI. ( 8-11 )

To obtain diffusion-weighted images, a brace of strong gradient pulsations is added to the pulse sequence. The first pulsation dephases the spins, and the 2nd pulsation rephases the spins. If the spins move between the gradient pulsations, rephasing will be uncomplete and the image will demo a signal-attenuation proportional to the sum of H2O diffusion in the specific orientation of the applied magnetic field gradients in 3-dimensional infinite. ( 1, 12-14 )

The diffusion-weighted images obtained with at least six different diffusion gradient orientations can be used to cipher a tensor ( e.g. a 3 ten 3 matrix ) . The chief diffusion way will be indicated by the tensor ‘s chief eigenvector. Quantitative diffusion prosodies such as maps for the evident diffusion coefficient ( ADC ) , fractional anisotropy ( FA ) , and color-coded diffusion maps, every bit good as fibre tractography can so be computed.

ADC is a step of diffusion magnitude, depicting average diffusivity. FA is a scalar parametric quantity, depicting the grade of anisotropy. FA is scaled to obtain values between 0 and 1 ; 0 stand foring random isotropic diffusion and 1 stand foring complete anisotropy. In color-coded maps the directional constituents of the chief eigenvector are assigned to different colourss ( typically red, green, and blue ) . The resulting image is weighted with the FA map to except tissues with isotropous diffusion. Fiber tractography is a line extension method used to visualise DTI informations. Anisotropic structures such as nervousnesss or nervus packages can be tracked and displayed on color-coded 3-dimensional images. The most normally used fibre tracking method is, to propagate a line from a seed point by following the local vector orientation. For fiber tractography assorted Reconstruction parametric quantities have been used in the past literature. ( 1, 8, 14 )

DTI is a good established technique in the cardinal nervous system, yet its application to the peripheral nervous system is disputing presumptively because in most other tissues H2O proton denseness is lower than in the cardinal nervous system. The primary parametric quantity finding the sensitiveness in a diffusion-weighted sequence is described by the b-value, a user defined parameter relative to the amplitude and continuance of the diffusion-sensitizing gradients. Increasing b-values reflect increasing diffusion weighting of a DTI acquisition but do besides take to a lower signal/noise ratio ratio ( SNR ) . ( 1, 3-6, 12, 15 )

To the best of our cognition, so far there has merely been one study, sing the systematical rating of the optimum b-value for DTI of peripheral nervousnesss. ( 1 ) This survey nevertheless was performed utilizing a 1.5T MR unit. At present, informations for higher field strengths ( i.e. 3.0T ) are still non available in the literature. Yet the optimum b-value for 3.0T is expected to be different than for 1.5T, since 3.0T MR units normally offer stronger gradient systems in add-on to a much higher SNR.

Therefore, the intent of this survey was to consistently measure the optimum b-value for DTI and fiber tractography of the average nervus at 3.0T every bit good as optimum Reconstruction parametric quantities for fibre tractography.

Please note: For farther information on MR imaging natural philosophies, T1- and T2-weighted and diffusion weighted MR imagination every bit good as tractography, delight refer to the Appendix.

3. Materials and methods

3.1. Study topics

This is a prospective cross-sectional survey with anterior blessing by the institutional reappraisal board ( IRB blessing figure ( KEK-ZH-Nr. ) , 2009-0133/5 ) . This survey was conducted harmonizing to the Helsinki declaration with written informed consent obtained from all survey topics.

Between April and June 2010, 45 healthy voluntaries were included in this survey ( 15 work forces, 30 adult females ; average age, 39 old ages ; average age, 41A±3.4 old ages ; age scope, 22-66 old ages ) . Inclusion standard was age & gt ; 18. Exclusion standards were general contraindications for MRI ( e.g. pacesetter ) , gestation, history of anterior surgery and cardiovascular, pneumonic, endocrinal, metabolic, neurological, neuromuscular, or musculoskeletal upsets. All 45 topics underwent MRI of the right carpus. All topics were right-handed.

3.2. MR imaging

All MR images were acquired with a 3.0T MRI scanner ( Achieva X-series, Philips, Best, the Netherlands ) , utilizing an eight channel wrist spiral ( Achieva Sense, Philips, Best, the Netherlands ) . This MRI system allows field gradient amplitudes up to 80 mT/m, or batch rates up to 200 mT/m/s.

The carpus spiral was positioned in the centre of the magnet dullard, since old surveies achieved a significantly better quality of the echo-planar images, compared to a sidelong place, normally used for anatomical imagination. ( 5 ) All imagination was performed with topics placed in the scanner in prone place, with their right manus extended over the caput ( “ demigod ” place ) .

The survey protocol included a standard T1 weighted turbo spin reverberation ( TSE ) MR sequence ( repeat clip ( TR ) / echo clip ( TE ) , 636/21 MS ; matrix size, 400 ten 264 millimeter ; field of position ( FOV ) , 120 ten 80 millimeter ; acquisition voxel size 0,3 ten 0,3 ten 4 millimeter ; reconstructed voxel size 0,15 ten 0,15 ten 4 millimeter ; figure of pieces, 25 ; TSE factor 3 ; figure of signal acquisitions ( NSA ) 1 ; sense factor 2 ; acquisition clip, 4:38 min ) that was used as anatomical mention. In add-on, four spin-echo-based single-shot echo-planar imagination ( EPI ) MR sequences ( TR/TE, 10123/40 MS ; matrix size 100 ten 82 millimeter ; FOV 120 x 99 ten 100 millimeter ; acquisition voxel size 1,2 ten 1,2 ten 4 millimeter ; reconstruced voxel size 0,54 ten 0,54 ten 4 millimeter ; figure of pieces 25 ; NSA 2, fat suppression SPAIR ; EPI factor 45 ; sense factor 2, acquisition clip 6:06 min ) with diffusion sensitising gradients were performed in each topic utilizing four different b-values 800, 1000, 1200 and 1400 s/mm2. Each acquisition included 15 different diffusion gradient orientations, distributed equally to the surface of the unit sphere. Prior to the DTI acquisition, high-order shimming with a XX-cm FOV was applied to cut down inhomogeneities of the chief magnetic field in the imaging country. Slice location and all other image parametric quantities were unbroken indistinguishable in all acquisitions.

3.3. Post-processing and fibre tractography

For computations, measurings and fiber tractography, all images were transferred to an independent workstation and post-processed by two independent readers ( P.E. , D.M. ; both readers were trained in post-processing DTI information ) utilizing dedicated Software ( Philips Achieva Version.X.X, Best, the Netherlands ) .

First, ADC maps, FA maps, and color-coded diffusion maps, were calculated. Then, FA and ADC values were measured within a specific part of involvement ( ROI ) incorporating the nervus fibres of involvement ( ROI technique ) . Measurements were done at three different transaxial degrees defined by the undermentioned anatomical constructions: distal radioulnar articulation, pisiform bone and unciform bone bone. The standard T1-weighted TSE sequence was used as an anatomical mention to guarantee the proper arrangement of the ROIs on the FA and ADC maps. Size, form and location of the ROIs were unbroken indistinguishable for all measurings in the four different b-value acquisitions of each survey topic.

Fiber-tractography was performed by taking three initial seed ROIs through which the fibres were tracked. A freehand ROI was positioned in the FA and color-coded maps at the same three transaxial degrees as mentioned above. The seed ROIs for fiber-tractography were somewhat larger than the existent cross-sectional country of the average nervus, in order to include all nervus fibres. Care was taken non to include any relevant environing anatomical constructions ( e.g. , vass or sinews ) . Based on old literature, four different fibre Reconstruction algorithms incorporating two different FA threshold values ( 0.2 / 0.3 ) and two different angulation tolerances ( 10A° / 20A° ) were chosen as Reconstruction parametric quantities. Fiber trailing was terminated if FA values were below the selected threshold or if fiber angulation exceeded the selected tolerance angle.

Overall, a sum of 720 fiber piece of land images were generated ( 4 b-value acquisitions x 4 fiber Reconstruction algorithms x 45 survey topics = 720 fiber piece of land images ) . All fiber tract images were electronically stored on the workstation for a subsequent qualitative rating.

3.4. Quantitative and qualitative analysis

The quantitative and qualitative analysis was performed by the same two independent writers who post-processed the DTI datasets.

Standards for qualitative rating included fiber piece of land homogeneousness, fiber tract denseness, fiber length and the fibre tracts conformity to anatomy. The stored fibre piece of land images were presented in random order. Both readers were blinded to the corresponding b-values and the personal information of the topic. Both readers ranked the image quality in consensus utilizing a four point rank graduated table ( from 4 = best to 1 = worst ) .

3.5. Statistical analysis

All computations were performed by two writers ( P.E. , R.G. ) utilizing ExcelA® ( release 14.0, Microsoft, Redmond, WA, USA ) and SPSSA® ( release 18.0, SPSS Inc. , Chicago, IL, USA )

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4. Consequences

Fiber tractography generated significantly longer fibres in DTI acquisitions with higher b-values ( 1200 and 1400 s/mm2 ) compared to acquisitions with b-values of 800 and 1000 s/mm2 ( p & lt ; 0.001 ) . The overall quality of fibre tractography ( fiber length, homogeneousness, denseness and conformity with anatomy ) was best at a b-value of 1200 s/mm2 ( p & lt ; 0.001 ) . The tracking algorithm utilizing a minimal FA threshold of 0.2 and a maximal angulation of 10A° was significantly better than all other Reconstruction algorithms.

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5. Discussion

Study Subjects ( Male Female )

At 3.0T, the optimum b-value for DTI is 1200 s/mm2 and the optimum Reconstruction parametric quantities for fibre tractography are a minimal FA threshold of 0.2 and a maximal Reconstruction angle of 10A° .

Signal to Noise Ratio

Best trailing parametric quantities

Potential clinical usage

Restrictions: deformation artefacts, SNR, Software dependence

6. Appendix

6.1. Physicss of MR Imaging

An MR image represents the comparative response of biological tissues to absorbed wireless frequence energy. Normally MR imaging observes the karyon of H atoms because of the comparative copiousness of H2O in the human organic structure. Their distribution and characteristic belongingss vary depending on their physical and chemical environment. Depending on the ascertained parametric quantities assorted types of contrast can be generated. This makes MR imaging a really various technique. ( 9, 16, 17 )

A H atom consists of a nucleus incorporating a individual proton with a positive charge and of a individual negatron with a negative charge revolving the karyon. Any karyon with an uneven atomic mass or uneven atomic figure possesses the intrinsic quantum belongings of spin, harmonizing to the Pauli Exclusion Principle. As a charged atom with spin, the karyon induces a corresponding magnetic field, doing it a magnetic dipole. Normally, at room temperature without the presence of an external magnetic field, the magnetic orientation of a aggregation of H karyon ( protons ) will be indiscriminately distributed harmonizing to the rules of Brownian gesture. ( 18-20 ) However when an outer magnetic field B0 is applied, the protons will be given to presume magnetic orientations either parallel or anti-parallel to the outer magnetic field, somewhat prefering the parallel orientation, tantamount to the low energy province. Therefore, the amount of their magnetic dipole minutes consequences in a little net magnetisation vector analogue to the outer magnetic field. Incompatibilities in the alliance of the protons magnetic orientation with the outer magnetic field let the spin vectors of the protons experience a torsion, doing their precession around the outer magnetic field ‘s longitudinal axis with a specific frequence proportional to the strength of the outer magnetic field. This frequence is called the Larmor frequence and is described by the Larmor equation:

I‰ ( frequence ) = I? ( gyro-magnetic ratio invariable ) B0 ( outer magnetic field )

The amount of the precession-equivalents of all H2O protons is the signal ( electric current in a receiving system spiral ) measured in MR imagination. Larmor precession is a resonance phenomenon, leting a transportation of energy to the precessing protons by a radiofrequency ( RF ) pulsation of the same frequence. Prior to an RF pulsation, the net magnetisation vector is aligned parallel to the chief magnetic field B0 and the longitudinal axis. As energy is absorbed from an applied RF pulsation, the net magnetisation vector rotates off from the longitudinal way. The sum of rotary motion ( called the somersault angle ) depends on the strength and continuance of the RF pulsation.

A 90A° RF pulsation rotates the net magnetisation into the cross plane. A 180A° RF pulsation rotates the net magnetisation by 180A° into the opposite way. ( 16, 21 )

6.2. T1- and T2-weighted images

T1 relaxation is defined as the rate at which net magnetisation realigns with the longitudinal way after a 90A° RF pulsation rotates is applied. The definition of T1 is the clip elapsed until the longitudinal magnetisation to make 63 % of its concluding value, presuming a 90A° RF pulsation. Different tissues have different rates of T1 relaxation. If an image is acquired while T1 relaxation curves are widely separated, T1-weighted contrast will be maximized.

T2 relaxation occurs when spins in the high and low energy province exchange energy but do non free energy to the environing lattice.

T2* is characterized by B0 inhomogeneity and loss of cross magnetisation at a rate greater than T2.

T2 relaxation can be explained by a loss of coherency or synchronism of the protons. During an RF pulsation, the protons precess in stage. After a 90A° RF pulsation, the stages of the protons desynchronize because each proton precesses at a somewhat different velocity due to assorted effects such as spin-spin interactions or local inhomogeneities of the magnetic field. What observed is, is the vector amount of all stages ; a loss of synchronism of the stages leads to the loss of signal in MR imagination. T2 relaxation is defined as the clip elapsed until the cross magnetisation decays to 37 % of its original value. Different tissues have different rates of T2 relaxation. T2 weighting is obtained by infixing a weighting period between the 90A° RF pulsation and information acquisition. This clip period is called echo clip ( TE ) . If TE is increased, T2-weighted contrast will be maximized.

The exact mechanism taking to longer or shorter T2 relaxation is non wholly understood. T2 relaxation times seem to be prolonged in environments where H2O protons can freely topple ( e.g. , less viscousness or fewer supermolecules with which to interact ) . Therefore fluids have the longest T2s ( 700-1200 MS ) , and H2O based tissues are in the 40-200 MS scope, while fat based tissues are in the 10-100 MS scope.

T2 weighted images in MRI are frequently thought of as “ pathology scans ” because aggregations of unnatural fluid appear brighter against the darker normal tissue. A typical illustration is the formation of hydrops, taking to a significantly prolonged T2-weighted signal.

T1 and T2 relaxation processes occur at the same time After a 90A° RF pulsation, dephasing of the transverse magnetisation occurs while the longitudinal magnetisation is restituted but T2 decay occurs 5 to 10 times more quickly than T1 recovery. After a few seconds, most of the cross magnetisation will be dephased and most of the longitudinal magnetisation will be restituted. ( 9 )

6.3. Diffusion MR Imaging

In an isotropic environment, such as cerebrospinal fluid, H2O molecules diffuse freely at equal rates of about 0.1 mm/sec in all waies. In tissues incorporating extremely organized microstructures ( e.g. cell membranes, vascular constructions or axon cylinders ) H2O molecules can non freely spread in all waies, but sooner diffuse in the way aligned with the internal construction. This inhomogeneity of diffusion is called anisotropy. ( 22 )

It has been long, but non widely known that MR imagination is capable of quantifying diffusional motion of molecules utilizing diffusion-weighted imagination ( DWI ) techniques, as described by Stejskal and Tannner in1965. ( 23 ) In 1985 LeBihan integrated the diffusion burdening technique developed by Stejskal and Tanner into MR Imaging. The first of import clinical application of DWI was the sensing of shot in its acute stage. As anisotropic H2O diffusion in extremely ordered tissues, such as the encephalon, had been observed, Basser, Mattliello and LeBihan established DTI in the 1990s, which became a widely used technique ( 8-11 )

In DTI, each voxel is defined by a rate of diffusion and a preferable way of diffusion in three dimensional infinite. To obtain diffusion-weighted images, a brace of strong gradient pulsations is added to the pulse sequence. After the first gradient pulsation is applied, protons start to precess at different rates, depending on their location in the gradient field. In analogy to T2 relaxation, these differences in the precession rate lead to scattering of the stage and signal loss. However, if an indistinguishable gradient pulsation of opposite magnitude is later applied, protons can be refocused or rephrased. The refocusing will non be perfect for protons that have moved between the two gradient applications. Therefore, the mensural signal is reduced. This manner an acquisition is sensitized to motional procedures such as flow or diffusion in a specific way. The primary parametric quantity finding sensitiveness in a diffusion-weighted sequence is described by the b-value. This parametric quantity is relative to the incline and continuance of the diffusion-sensitizing gradients.

In order to mensurate a tissue ‘s complete diffusion profile, repeated scans with different waies of the diffusion sensitising gradients are required. For DWI three gradient-directions are usually sufficient to gauge the mean diffusivity. Clinically, so called trace-weighted images are used to name vascular shots in the encephalon, by early sensing of hypoxic hydrops. For DTI nevertheless, the belongingss of each voxel are normally calculated by vector or tensor math from six or more different diffusion weighted acquisitions, significantly increasing acquisition clip. The chief diffusion way will be indicated by the tensor ‘s chief eigenvector. In color-coded maps the directional constituents are assigned to different colourss ( typically red, green, and blue ) . The resulting image is weighted with the FA map to except tissues with isotropous diffusion. FA is a scalar parametric quantity, depicting the grade of anisotropy. FA is scaled to obtain values between 0 and 1 ; 0 stand foring random isotropic diffusion and 1 stand foring complete anisotropy. ADC is a step of diffusion magnitude, depicting average diffusivity.DTI is a comparatively simple theoretical account of the diffusion procedure, presuming homogeneousness and one-dimensionality of diffusion within each image voxel, ensuing in a individual tensor-ellipsoid per voxel.

Recently, more advanced theoretical accounts of the diffusion procedure have been proposed that purpose to get the better of the failings of the diffusion tensor theoretical account ( e.g. crossings of nervousnesss ) Amongst others, these include q-space imagination and generalised diffusion tensor imagination. ( 1, 8, 12-14, 20, 21, 24-28 )

6.4. Tractography

The directional information of a DTI acquisition can be exploited to execute tractography in anisotropic tissues. Hempen constructions such as nervousnesss or nervus packages can be tracked and displayed on color-coded 3-dimensional images. The most normally used fibre tracking method is, to propagate a line from a seed point by following the local vector orientation.

Moseley presented an abstract with the first tractogram in 1992. Further progresss in the development of tractography can be accredited to Mori, Pierpaoli, Lazar, Conturo and many others. ( 1, 8, 14 )

Echo planar imaging

In single-shot echo-planar imagination, all spatial-encoding informations of an image can be obtained after a individual radio-frequency excitement. Echo-planar imaging offers major advantages over conventional MR imagination, such as decreased imagination clip, decreased gesture artefacts, and the possibility to image rapid physiologic procedures in the human organic structure.

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