Tierärztliche Hochschule Hannover
Einsatz von Fiber tracking in der Darstellung des kaninen Rückenmarks
INAUGURAL – DISSERTATION
zur Erlangung des Grades eines Doktors der Veterinärmedizin – Doctor medicinae veterinariae –
(Dr. med. vet.)
vorgelegt von Marc Karlheinz Hobert
Bad Hersfeld
Hannover 2012
Wissenschaftliche Betreuung: 1. Prof. Dr. med. vet. Andrea Tipold
2. PD Dr. med. vet. Veronika M. Stein
Klinik für Kleintiere
1. Gutachterin: Prof. Dr. med. vet. Andrea Tipold
2. Gutachter: Prof. Dr. med. vet. Wolfgang Baumgärtner
Tag der mündlichen Prüfung: 10.05.2012
Diese Dissertation wurde im Rahmen der Forschergruppe (FOR 1103) „Neurodegeneration und -regeneration bei ZNS-Erkrankungen des Hundes“ finanziell durch die Deutsche Forschungsgemeinschaft (DFG) (FOR TI 309/4-1) gefördert.
Man merkt nie, was schon getan wurde, man sieht immer nur, was noch zu tun bleibt.
Marie Curie (1867 – 1934)
Polnisch-französische Chemikerin, Physikerin
und zweimalige Nobelpreisträgerin, Physik und Chemie
Teile der Ergebnisse der vorliegenden Dissertation wurden auf folgenden Tagungen und Fachkongressen präsentiert:
Hobert, M.K., Dziallas, P., Stein, V.M., Tipold, A.:
Darstellung von Rückenmarksfasern bei Hunden mittels Fibertracking 19. Jahrestagung der DVG-Fachgruppe Innere Medizin und Klinische Laboratoriumsdiagnostik, Leipzig, 04.02.-05.02.2011,
Tierärztliche Praxis (K), 1: A18-19 (ISSN: 1434-1239), S. 21, 2011 Auszeichnung mit dem 3. Posterpreis der Fachgruppe InnLab
Hobert, M.K., Stein, V.M., Dziallas, P., Wolf, D., Tipold, A.:
Visualisation of spinal cord tracts by fibertracking in dogs First International Workshop of Veterinary Neuroscience, DFG Research Unit (FOR) 1103, p. 27
Hannover, 31.03.-02.04.2011
Hobert, M.K., Stein, V.M., Dziallas, P., Wolf, D., Tipold, A.:
Fiber tracking of the spinal cord in dogs – from trial stage to clinical application
24th Annual Symposium “Genetic Neurological Diseases”, no. 29 Trier, Germany; 22.-24.09.2011
Hobert, M.K., Stein, V.M., Wolf, D., Dziallas, P., Tipold, A.:
Fiber tracking zur Visualisierung von Rückenmarksschäden beim Hund 20. Jahrestagung der Fachgruppe Innere Medizin und Klinische
Laboratoriumsdiagnostik, Göttingen, 03.02.-04.02.2012, Tierärztliche Praxis (K), 1/2012: V35 (ISSN: 1434-1239), 2012
Inhaltsverzeichnis
I. Einleitung……… 7
II. Publikationen………. 11
A. Evaluation of healthy spinal cord by diffusion tensor imaging, fiber tracking, fractional anisotropy, and apparent diffusion coefficient measurement in 13 dogs……. Abstract………. Introduction……… Materials and methods……….…. Results………. Discussion……….… Conclusions……….. Conflict of interest statement………. Acknowledgements……… References………... 11 12 13 14 18 21 23 23 23 24 B. Evaluation of fiber tracking as a prognostic marker for long-term motor functional recovery in dogs with intervertebral disk disease………. Abstract……….. Introduction……….… Material and methods……….…. Results……….. Discussion……….. Acknowledgements……….… Author disclosure statement………... References……….… 27 28 29 30 36 44 48 48 48 III. Zusammenfassende Darstellung der Ergebnisse beider Studien ……… 51
IV. Übergreifende Diskussion……… 60
V. Zusammenfassung (deutsch)………. 66
VI. Zusammenfassung (englisch)………. 68
VII. Schrifttumsverzeichnis………. 70
VIII. Anhang……… 74
IX. Danksagung……… 129
I. Einleitung
Seit seiner Einführung im Jahre 1994 ist das diffusion tensor imaging (DTI) eine in der Humanmedizin häufig eingesetzte Technik. Sie wird vor allem für die Erforschung der Integrität von Gehirn- und Rückenmarksbahnen bei verschiedenen neurologischen Erkrankungen genutzt (Basser und Mattiello et al. 1994). Fundamentale Bedeutung erlangte diese Technik für die Planung und Durchführung chirurgischer Resektionen von Gehirntumoren. Mittels des sogenannten (sog.) Fiber Tracking (FT), einer weiterführenden Technik des DTI und nicht-invasiven bildgebenden Diagnostik, konnten erstmals funktionell bedeutsame Zentren des Gehirns überprüft und somit intraoperativ verschont werden.
Dadurch eröffnete sich die Möglichkeit, auch nach der Entfernung des Tumors, zum Beispiel das Sprechen durch Schonung des Sprachzentrums zu gewährleisten. Durch diese Eigenschaft zählt die DTI-Technik zur sog. funktionellen Magnetresonanztomographie (fMRT) (Stippich 2010).
Der Mechanismus, den sich das DTI zunutze macht, ist das Identifizieren und anschließende Berechnen der Hauptdiffusionsrichtung von Wasser-Protonen in unterschiedlichen Geweben (Moseley, Cohen et al. 1990; Basser und Pierpaoli 1996). Den verschiedenen Richtungen der Wasserdiffusion werden dabei sog. Farbcodierungen zugeordnet, sodass ein Bild des Gewebes mit bis zu drei Hauptfarben und deren farblichen Abstufungen entsteht. Die Codierung der drei Farben für das Fiber tracking erfolgt einheitlich: die Farbe blau codiert für die sog. head-feet- (HF) oder auch kraniokaudale Hauptdiffusionsrichtung, rot für die Diffusion mit hauptsächlicher rechts-links-Ausrichtung (RL) und grün für die anterior- posterior (AP) bzw. dorsoventrale Hauptdiffusionsrichtung. Für die Hauptdiffusions- richtungen bei der Farbcodierung sind unterschiedliche Bezeichnungen vorhanden, da die ursprüngliche Terminologie aus der Humanmedizin stammt und diese in der Tiermedizin nur bedingt anwendbar ist. Zur Berechnung der Hauptdiffusionsrichtung der Wassermoleküle müssen durch das DTI mindestens sechs Richtungen der Diffusion gemessen werden.
Technisch können jedoch bis zu 33 Diffusionsrichtungen detektiert und zur Auswertung herangezogen werden (Wheeler-Kingshott 2002; Facon und Ozanne et al. 2005; van de Looij, Mauconduit et al. 2011).
Auf Basis der DTI-Messungen an drei definierten sog. Regions of Interest (ROIs) lassen sich zwei weitere Parameter zur Evaluierung der Hauptdiffusion der Wassermoleküle berechnen.
Dies sind die Fraktionale Anisotropie (FA) und der sog. Apparent Diffusion Coefficient (ADC).
Die FA gibt Informationen über die Richtung der Wasserdiffusion in jedem einzelnen Voxel und somit über die Faserintegrität der weißen Substanz (Basser und Pierpaoli 1996; Schöne- Bake 2010). Die FA wird durch einen sog. Skalar beschrieben. Dies ist ein Wert ohne Einheit (Basser und Pierpaoli 1996; Ducreux, Lepeintre et al. 2006; DeBoy, Zhang et al. 2007). Der zweite Parameter, der ADC, beschreibt die Stärke der Wasserdiffusion und setzt sich aus den drei Diffusionsrichtungen (x-, y- und z-Achse) zusammen. Im Gegensatz zum FA besitzt der ADC eine Einheit und wird in x10-3mm2/s (Basser und Pierpaoli 1996; Levine and Fosgate et al. 2009; Schöne-Bake 2010) angegeben.
In der Tiermedizin gibt es bisher nur sehr wenige Studien über den Einsatz der Technik des Fiber tracking. Diese wenigen Studien belegen jedoch, dass FT das Rückenmark (RM) von Säugern mit seinen axonalen Bündeln, auch als Rückenmarksbahnen bezeichnet, in seiner plastischen Erscheinung darstellen kann und somit eine Bewertung seiner Integrität ermöglicht (Conturo, Lori et al. 1999; Mori and van Zijl 2002; Wheeler-Kingshott CA 2002;
Facon und Ozanne et al. 2005; Johansen-Berg and Behrens 2006; Murakami, Morimoto et al.
2008; Danielian, Iwata et al. 2010; Stippich 2010; Pease and Miller 2011).
Bandscheibenvorfälle (BSV) sind eine der häufigsten neurologischen Erkrankungen beim Hund. Sie treten mit einer Häufigkeit von bis zu 2% in der Gesamthundepopulation auf (Bray and Burbidge 1998). Heutzutage wird die Magnetresonanztomographie (MRT) als die am besten geeignete Technik der Bildgebung zur Diagnosestellung von BSV beim Hund angesehen (Sether, Yu et al. 1990). Vergleichende Studien haben gezeigt, dass das MRT ausgezeichnet die intraoperativen Befunde in Bezug auf Lokalisation und Ausdehnung des vorgefallenen/komprimierenden Bandscheibenmaterials widerspiegelt (Besalti, Ozak et al.
2005; Naude, Lambrechts et al. 2008). Das konventionelle MRT kann neben dem vorgefallenen Bandscheibengewebe pathologische Veränderungen des Rückenmarks selbst detektieren, die sich häufig als Hyperintensität in der T2-gewichteten Sequenz darstellen.
Eine Hyperintensität entspricht den pathologischen Befunden Blutung, Ödem, Nekrose und Myelomalazie (Kulkarni, McArdle et al. 1987; Bondurant, Cotler et al. 1990; Flanders, Spettell
et al. 1996; Ito, Matsunaga et al. 2005; Boekhoff, Flieshardt et al. 2011), die zum Teil durch Evaluierung weiterer MRT-Sequenzen eingegrenzt werden können. Häufig gestaltet es sich als äußerst schwierig, anhand dieser Befunde eine Prognose für die funktionelle Wiederherstellung des Patienten zu treffen. Meist werden zusätzlich die Befunde aus der klinisch-neurologischen Untersuchung zur Prognosestellung herangezogen. Die Ausdehnung der Hyperintensität des Myelons in der T2-gewichteten MRT-Sequenz in Relation zur Länge des 6. Hals- bzw. 2. Lendenwirbelkörpers kann zur Einschätzung angewendet werden (Laitinen and Puerto 2005; Ensinger EM 2010; Boekhoff, Flieshardt et al. 2011). Dennoch stellen diese MR-Parameter lediglich eine makroskopische Befundung des Rückenmarks dar.
Neuere Techniken, wie das DTI, enthalten Informationen, die auf die Integrität auf zellulärer Ebene schließen lassen. Somit ermöglichen neuere Techniken, wie das DTI und Fiber tracking, subtile pathologische Veränderungen der weißen Substanz in einem frühen Stadium zu detektieren und einen Schaden der Architektur der Rückenmarksfasern genauer zu beurteilen (Kale, Gupta et al. 2006; Chahboune, Mishra et al. 2009).
Die vorliegende Studie verfolgte zweierlei Ziele:
Zum Ersten sollte versucht werden, die Technik des DTI auf den Hund zu übertragen und somit Möglichkeiten der Durchführung von Traktographien bzw. Fiber tracking am kaninen Rückenmark zu etablieren. Zu diesem Zweck wurden 13 Hunde mit gesundem Rückenmark untersucht und in eine erste Studie eingeschlossen. Diese Hunde dienten zudem der Erstellung von Referenzwerten für das Fiber tracking des physiologischen kaninen Rückenmarks.
Zum Zweiten sollte die Technik des DTI an Hunden mit Bandscheibenvorfällen angewendet werden, um die Möglichkeit der Darstellung pathologischer Veränderungen des Rückenmarks nach Rückenmarkstrauma zu evaluieren. Zudem sollte die Hypothese bestätigt werden, dass Fiber tracking mit seinen objektiven FA- und ADC-Werten eine genauere Definition der Prognose ermöglicht, als dies bisher der Fall ist. Dazu wurden in einer zweiten Studie an 20 Hunden mit Bandscheibenvorfällen Fiber tracking-Daten erhoben und diese mit Referenzwerten verglichen, sowie in einem Follow-up mit dem funktionellen Outcome der Patienten korreliert.
Die im Rahmen dieser Studie erhobenen Daten wurden in zwei Publikationen zusammengefasst, von denen eine am 29.03.2012 bei der Zeitschrift „The Veterinary Journal“ eingereicht wurde und die zweite zur Einreichung bei der Zeitschrift „Veterinary Radiology and Ultrasound“ vorbereitet ist.
II. Publikationen
Die folgende Publikation wurde am 29.03.2012 bei der Zeitschrift “The Veterinary Journal“
eingereicht.
A. Evaluation of healthy spinal cord by diffusion tensor imaging, fiber tracking, fractional anisotropy, and apparent diffusion coefficient measurement in 13 dogs
M. K. Hobert*, V. M. Stein, P. Dziallas, D. Wolf, and A. Tipold
Department of Small Animal Medicine and Surgery, University of Veterinary Medicine Hannover, Buenteweg 9, 30559 Hannover, Germany
*Corresponding author: Tel. +49 511 953 6297.
E-mail address: marchobert@web.de (M.K. Hobert).
Abstract
Functional magnetic resonance (fMR) imaging offers plenty of new opportunities in the diagnosis of central nervous system diseases. Diffusion tensor imaging (DTI) is a technique sensitive to the random motion of water providing information about tissue architecture. We applied DTI to healthy spinal cords of 13 dogs of different breeds and body weights in a 3.0T magnetic resonance (MR) scanner. The aim was to study fiber tracking (FT) patterns by tractographies and the variations of the fractional anisotropy (FA) and the apparent diffusion coefficient (ADC) observed in the spinal cords of dogs with different sizes and at different locations (cervical and thoracolumbar). For that reason we added a DTI sequence to the standard clinical MR protocol. The values of FA and ADC were calculated by means of three so-called regions of interest defined on the cervical or the thoracolumbar spinal cord (ROI 1, 2, and 3). The shape of the spinal cord fiber tracts was well illustrated following tractography and the exiting nerve roots could be differentiated from the spinal cord fiber tracts. Routine MR scanning times were extended for 8 to 12 minutes, depending on the size of the field of view (FOV), the slice thickness, and the size of the interslice gaps. In small breed dogs (< 15 kg body weight) the fibers could be tracked over a length of approximately 10 vertebral bodies with scanning times of about 8 minutes, whereas in large breed dogs (> 25 kg body weight) the traceable fiber length was about 5 vertebral bodies which took 10 to 12 minutes scanning time.
FA and ADC values showed mean values of 0.447 (FA), and 0.560x10-3mm2/s (ADC), respectively without any differences with regard to different dog sizes and spinal cord segments examined. In conclusion, FT is suitable for the visualization of the canine spinal cord and the exiting nerve roots. The FA and ADC values offer an objective measure for evaluation of the spinal cord fiber integrity in dogs.
Keywords
canine; tractography; DTI; FA; ADC
Introduction
Currently, conventional magnetic resonance imaging (MRI) is the most widely used technique to examine the central nervous system (CNS). This technique enables the visualization of structures such as the brain, the spinal cord, and the surrounding soft tissues.(McConnell, Garosi et al. 2005; Garosi, McConnell et al. 2006;
Shanmuganathan, Gullapalli et al. 2008) Furthermore, pathological findings such as hemorrhages, edema, and physical injuries can be visualized.(Kulkarni, McArdle et al. 1987; Bondurant, Cotler et al. 1990; Flanders, Spettell et al. 1996) Pathological MRI changes of the myelon caused by spinal cord compression appear as hyperintense lesions in T2 weighted images in conventional MRI.(Boekhoff, Flieshardt et al. 2011) These lesions might represent edema, hemorrhage but also necrosis or myelomalacia.(Ito, Matsunaga et al. 2005; Boekhoff, Flieshardt et al.
2011) The histopathological correlate cannot be exactly determined in vivo.
Therefore, further imaging techniques are required to substantiate the prognosis in compressive spinal cord diseases. Diffusion-weighted imaging (DWI) is a special technique of functional MR (fMR) imaging that has the capability to assess changes in random motion of water protons in vivo.(Moseley, Cohen et al. 1990; Basser and Pierpaoli 1996) Diffusion tensor imaging (DTI) is an advanced technique of DWI that measures at least six diffusion directions and offers the possibility to track and visualize axonal fiber bundles by so-called tractographies.(Wheeler-Kingshott CA 2002; Facon and Ozanne et al. 2005; van de Looij, Mauconduit et al. 2011) Evaluation of the translation of extracellular water molecule diffusion within white matter fibers is possible due to DTI utilization of anisotropic water diffusion in organized tissues such as the brain and spinal cord.(Moseley, Cohen et al. 1990;
Basser and Pierpaoli 1996; van de Looij, Mauconduit et al. 2011) Originally, DTI was used presurgically in human medicine to plan function-preserving surgery by saving motor and speech reliable areas of the brain.(Stippich 2010) Today, technical advancement enables reconstruction of white matter tracts in 3D images not only of the brain but also of the spinal cord. For this purpose specialized fiber tracking (FT) algorithms and a tensor map are used to utilize tractographies that make the spinal cord educible in a plastic shape in mammals. (Conturo, Lori et al. 1999; Mori and van Zijl 2002; Wheeler-Kingshott CA 2002; Facon and Ozanne et al. 2005; Johansen- Berg and Behrens 2006; Murakami, Morimoto et al. 2008; Danielian, Iwata et al.
2010; Stippich 2010; Pease and Miller 2011)
To prove the hypothesis that DTI with subsequent fiber tracking is suitable for visualization of the canine spinal cord, DTI had to be applicable within a reasonable time frame additionally to a standard clinical MR protocol to define reference values for the healthy canine spinal cord. DTI and subsequent fiber tracking were performed in the spinal cord of 13 dogs of different breeds to assess the integrity of spinal cord
fibers. Additionally, fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values were calculated for the three regions of interest (ROI 1, 2, and 3) on the ADC map at an Extended MR Workspace (Version 2.6.3.2 HF 3 2010, Philips Medical Systems Nederland). The FA value gives information about the directionality of the diffusion at each voxel and subsequently about the fiber integrity.(Basser and Pierpaoli 1996; Schöne-Bake 2010) FA values represent so-called scalar values.(Basser and Pierpaoli 1996; Ducreux, Lepeintre et al. 2006; DeBoy, Zhang et al. 2007) The ADC value describes the strength of the water diffusion and is composed of the values of all three diffusion directions (x-, y-, and z-axis). ADC values are described with the unit x10-3mm2/s.(Basser and Pierpaoli 1996; Levine and Fosgate et al. 2009; Schöne-Bake 2010)
Fiber tracking (FT) patterns and variations of fractional anisotropy (FA) and apparent diffusion coefficient (ADC) should be evaluated in different healthy spinal cord regions of dogs with different sizes using a 3.0T MR scanner. These values will serve as reference values for comparison of fiber tract integrity in compressive spinal cord diseases to generate a potential additional prognostic tool.
Materials and methods Animals
Thirteen dogs of different breeds were included in the current study with the consent of the dog-owners. To fulfill the inclusion criteria, dogs had no clinical signs of an underlying disorder of the spinal cord and had physiologically appearing spinal cords as assessed by standard MR protocol. The 13 dogs underwent routine clinical MRI protocols under general anesthesia for diagnostic work-up of their suspected underlying disease (Table 1). Dogs were excluded if there was any suspicion of a neurological disorder affecting the spinal cord, history of diseases or previous surgery of the spinal cord. The study was conducted according to the ethical rules of the University of Veterinary Medicine Hannover and approved by the promotion Committee and the appointee for animal protection.
Breed, body weight, age, diagnosis, and results of FA and ADC measurements of 13 dogs with healthy spinal cords Table 1.gender: m; male, f; female, mc; male castrated, fn; female neutered, FT; fiber tracking, FA; fractional anisotropy, ADC; apparent diffusion coefficient, nd; not done (technical error)
doggender breedbody weight (kg)
age (months) diagnosisFA (ROI 1) FA (ROI 2) FA (ROI 3) ADC (ROI 1) (10-3mm2/s) ADC (ROI 2) (10-3mm2/s) ADC (ROI 3) (10-3mm2/s) FT in the cervical spinal cord 1f Golden Retriever 3093osteosarcoma of the temporomandibular joint 0.448 nd nd0.413nd nd 2mAmerican Staffordshire terrier 2671cerebellar abiotrophy0.5310.4370.6310.4530.8050.498 3mcEnglish bulldog1627head bobbing0.3380.3810.464nd0.5580.561 4mGerman wirehaired pointer 3079presumed neoplastic lesion in the right lobus occipitalis0.2910.3370.316ndndnd 5f Mixed breed1466idiopathic epilepsy0.5560.4910.4510.4620.4900.532 FT in the thoracolumbar spinal cord 6mGerman wirehaired pointer 3281presumed neoplastic lesion in the right lobus occipitalis0.5120.4090.5270.8570.7820.811 7fnGriffon Bruxellois675Caudal occipital malformation syndrome0.4210.3830.4460.5720.6640.496 8mcMiniature poodle721lumbosacral stenosis0.4690.4240.4030.5110.5790.626 9mMiniature poodle10145gastrointestinal foreign body0.5370.4160.4440.5330.4900.445 10mAiredale terrier 3399idiopathic epilepsy0.5500.5020.4930.5260.5410.521 11f Westhighland white terrier 711idiopathic epilepsy0.3620.4160.4880.4890.5100.488 12fnMixed breed1484lymphoma in the spleen0.5940.460.3870.217nd0.542 13mMalinois2746orthopedic disease0.4090.400.4010.6540.620.624 Mean 19.3869.07- 0.4630.4210.4540.5170.6040.559 Median 75.00- 0.4690.4160.4490.5110.5690.532 Range6-3311-145- 0.291-0.5940.337-0.5020.316-0.6310.217-0.8570.49-0.8050.445-0.811
Breed, body weight, age, diagnosis, and results of FA and ADC measurements of 13 dogs with healthy spinal cords
MRI
We used a 3.0T Philips Achieva MRI scanner (Philips Medical Systems Nederland, PC Best, The Netherlands) with one or a combination of two of the following different phased array coils: SENSE (sensitivity encoding)-spine-coil with 15 channels, SENSE-knee-coil with 8 channels, SENSE-neurovascular-coil TO with 8 channels, a combination in terms of dual-coil-imaging with the SENSE-neurovascular-coil and the SENSE-spine coil in large breed dogs.
The following parameters were used for acquisition of the T2-weighted sequences:
TSE sequence, TR = 3000-6000 milliseconds, TE = 80-120 milliseconds, slice thickness 2.2-3.0 mm, with a 0.2-0.3 mm interslice gap, ACQ matrix size 256x204 in the protocol used for head scans with a reconstruction matrix of 512. In the protocol for thorax/abdomen scans an ACQ matrix of 448x333 with a reconstruction matrix of 1024 and 880, respectively was used. Twenty images were obtained so that the FOV varied from 180 to 273 mm according to the dog’s size. Voxel-size ranged from 0.7x0.88 to 0.91x2.2 and 0.91x3.0 mm, respectively.
DTI/FT
A so-called Reg DTI-high isoSENSE sequence was used for DTI (measurement method: DWI SE, repetition time: 12,000 milliseconds; excitation time: 70 milliseconds), with measurement of 33 diffusion directions (with a motion-probing gradient in 33 orientations). The FOV varied from 180 mm to 273 mm, according to the dog’s size. Number of b-values: 2; voxel-size: 2.0x2.0x2.0 mm. The ACQ matrix had a size of 122x111 (MxP) with a reconstruction matrix of 128. A total of 60-95 slices (thickness: 2-3 mm) with no interslice gap were obtained. Dynamic stabilization was used to improve image consistency across dynamics. The DTI sequence had a length of 8 to 12 minutes for each dog and was performed subsequently to the standard clinical MRI protocol. This protocol comprised T2-weighted (T2-w) sequences in sagittal and transversal planes, a transversal HEMO sequence and a fluid saturation sequence (Fluid Attenuated Inversion Recovery, FLAIR). Additionally, a dorsal plane was superimposed on the other planes when the head was scanned.
In dogs with suspected intracranial diseases, the cervical spinal cord was scanned (dogs 1, 2, 3, 4, and 5). In eight dogs the thoracolumbar region (dogs 6, 7, 8, 9, 10, 11, 12, and 13) of the spinal cord was examined which is the region mostly affected by intervertebral disk disease in dogs.(Levine and Fosgate et al. 2009) T2-w images
were used for orientation of the fiber tracking reconstruction to ensure the anatomical allocation of the reconstructed fibers. These T2-w images were imported into the DTI map. The subsequent tracking of the fibers, so-called tractography, was performed on an Extended MR workspace (Version 2.6.3.2 HF 3 2010, Philips Medical Systems Nederland) with the special application tool FiberTrak.
a b
Figure 1a. Coloring of the directional fiber tracking according to the course of water diffusion. The data were color coded with blue indicating head-to-feet or craniocaudal direction, red denoting right-to-left (R-L) direction, and green indicating anterior-to-posterior (A-P) or dorsoventral orientation.
Figure 1b. DTI raw data image with water proton diffusion coloring as described above. In the center (white circle) a relatively large blue colored area is visible which identifies the spinal cord.
The DTI map depicts the spinal cord by use of the color-coded water diffusion direction and the anatomical orientation by implementing the related T2-w sequence into the DTI map (Figure 1a and 1b). The color-coded mapping assigns a specific color to the according direction of water. The water molecules within the axonal bundles of the spinal cord white matter prefer a craniocaudal or so-called head-feet diffusion direction. This direction is coded with blue color. Therefore, the spinal cord is consequently depicted in blue (Fig. 1b). The right-left direction is assigned to red color, and green is the color coding for the dorsoventral or posterior-anterior diffusion of the protons in water molecules (Fig. 1a).
Tracking of the spinal cord fibers, so called tractographies, allowed the visualization of the spinal cord fibers and enables the determination of freely definable regions of interest (ROIs).Three ROIs, namely ROI 1, ROI 2, and ROI 3 were set on the spinal cord itself (grey and white matter) with a distance of once to twice the length of a
vertebral body in between the particular ROIs. By these selected anatomical regions the reproducibility of the measured values in the healthy spinal cord tissue of dogs should be proven. With special angle adjustments it was possible to visualize not only the axonal bundles of the spinal cord itself but also the outgoing dorsal and ventral nerve roots. The corresponding algorithm included minimal fractional anisotropy of 0.15, implying that the direction of diffusion anisotropy was followed until tracking was terminated when it reached a voxel with a FA of < 0.15.
Furthermore, it included a maximal angle variation from 27.0° to 45° and a minimal length of the fibers of 10 mm. Directional fiber coloring and detailed illustration of the fibers was selected to achieve a more detailed visualization of the axonal bundles.
For each single ROI the corresponding FA- and ADC-values were calculated.
Statistics
Testing for normal distribution of the data obtained was performed with the Kolmogorov-Smirnov and the Shapiro-Wilk normality tests. Differences between results of different ROIs were assessed by ANOVA with a Dunnett post-hoc test. An error probability of 0.05 (P) was used as significance level for all statistical tests.
Results
Thirteen dogs of different breeds and ages with healthy spinal cords were imaged in a 3.0T MRI. FT studies allowed the visualization of the axonal bundles of the cervical (n = 5) and thoracolumbar (n = 8) spinal cord by tractographies in all 13 dogs. The shape of the bundles was restiform and well differentiable from the surrounding tissues. Due to the main limiting factors namely scanning time and length of the coils the spinal cord could be scanned over a length of about 10 vertebral bodies in small breed dogs (< 15 kg body weight) and 5 vertebral bodies in large breed dogs (> 25 kg body weight), respectively. Accordingly, the FT scanning protocol took 8 to a maximum of 12 minutes in addition to the standard clinical MR protocol.
The spinal cord DTI data generated were transferred to the Extended MR Workspace and displayed after 3D reconstruction. The main diffusion direction of water molecules within tissues define the color coding in fiber tracking. Fibers are depicted in blue, red or green for craniocaudal, right-left or dorsoventral orientation, respectively (Fig. 1a). As water molecules within the spinal cord prefer a
craniocaudal diffusion direction along white matter fibers they are illustrated mainly in blue color (Fig. 1b and Fig. 2).
Figure 2. Fiber tracking study of the lumbar spinal cord of a dog (no. 13) with the T2-w sequence included (lateral view, left is cranial and right is caudal). The axonal bundles of the spinal cord are restiform and appear mainly in blue color due to the mainly craniocaudal direction of water diffusion. Exiting dorsal nerve roots are colored green. Axonal bundles could be tracked over a length of approximately five vertebral bodies in this large breed dog.
Interestingly, the exiting nerve roots were not depictable in all dogs with the standard adjustments (maximum angle variation of 27.0°). Hence, the maximum angle variation had to be modulated. Increasing the maximum angle from 27.0° to 45.0°
allowed visualization of both, the ventral and the dorsal nerve roots extending the vertebral canal through the intervertebral foramen in all 13 dogs. The ventral nerve roots were colored green due to the mainly dorsoventral or anterior-posterior diffusion direction, whereas the dorsal nerve roots were coded in red because of their mainly right-left course (Fig. 3).
Figure 3. Blue color-coded axonal bundles of the L1 to L4 spinal cord segment with exiting dorsal (green) and ventral (red) nerve roots. Maximum angle variation 45.0°. The exciting dorsal and ventral nerve roots are depicted in green and red, respectively, according to the main direction of water diffusion (white arrows).
Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were assessed for 13 healthy dogs of different breed and at different spinal cord segments. The mean FA value in the healthy spinal cords was 0.447 with a range of 0.291 to 0.631 and a standard error of 0.0163 (Table 1). The median of the FA was 0.448 (Fig. 4a).
The median of the ADC value in healthy spinal cords was 0.532 x10-3mm2/s and the mean ADC value was 0.560 x10-3mm2/s with a range from 0.217 to 0.857 x10-3mm2/s and a standard error of 0.0126 (Fig. 4b, Table 1). No statistically significant difference could be demonstrated between different sites of the spinal cords measured by the three ROIs (P (FA) = 0.8045 and P (ADC) = 0.9688). Evaluation of FA and ADC did also not reveal a statistically significant difference between small and large breed dogs (P (FA) = 0.787 and P (ADC) = 0.143).
Figure 4. Results of fractional anisotropy (FA) and apparent diffusion coefficient (ADC) measurements in spinal cords of 13 dogs.
a. FA values were assessed for three different regions of interest (ROI) with ROI 1 (dotted box), ROI 2 (pale grey box), and ROI 3 (dark grey box) representing the values of the FAs at the different ROIs. No statistically significant difference occurred between ROIs (P = 0.8045). The box on the right (white) comprises the FA values of all 13 dogs together.
b. ADC values were evaluated for the three defined ROIs, ROI 1 (dotted box), ROI 2 (pale grey box), and ROI 3 (dark grey box). No statistically significant difference between ROIs could be detected (P = 0.9688). The boxes contain the middle 50% of the sample values, Tukey boxplots display minimum and maximum, lower and upper quartiles, and median, and contain 95% of the values. FA = fractional anisotropy, ADC = apparent diffusion coefficient, ROI = region of interest, all = values of all 13 dogs and cervical and thoracolumbar spinal cord comprised
Discussion
So far, DTI was applied presurgically in human medicine to plan function-preserving brain surgery by saving special areas of motor and speech function.(Stippich 2010) Nowadays the application of this technique is also possible for white matter tract reconstruction in 3D images for the spinal cord.(Wheeler-Kingshott CA 2002; Facon and Ozanne et al. 2005) Specialized fiber tracking (FT) algorithms and a tensor map are used to utilize tractographies that allow reproduction of the spinal cord in a plastic shape (Conturo, Lori et al. 1999; Stippich 2010).
This study shows that the technique of diffusion tensor imaging (DTI) with subsequent tractography is applicable to dogs of different breeds and size/body weight and allows the visualization of canine spinal cord in a remarkably detailed explicitness on a cellular level, respectively the water diffusion along the axonal
bundles. The scans were performed with a 3.0 Tesla (T) MR scanner. In the current study alive dogs were included although Fujiyoshi et al. (2007) determined that the data of dead and alive animals are highly similar.(Fujiyoshi, Yamada et al. 2007) In a previous study by Pease and Miller (2011) a 1.5T scanner was used and beagles were used as controls. The authors could show that the technique of fiber tracking is feasible in dogs. In the current study DTI was performed with a 3.0T scanner, which offers more detailed precision. A short scanning time as offered by the 3.0T scanner is necessary to introduce the technique into the daily clinical practice. Furthermore, reference values were established for a different scanner type and for dogs of different breeds and sizes. A special sequence, termed Fac Reko Reg DTI-high iso SENSE, with a duration of 8 to 12 minutes was added to the routine clinical protocol.
The scanning time of this additional sequence did not excessively extend the overall scanning time and can therefore be added to the clinical standard protocol to serve as a prognostic factor. For analysis of the DTI data a specialized software is required.
This software was part of the corresponding MR work space. The field of view (FOV) for the DTI-scan was defined on the sagittal section plane. This was sufficient because the post-processing on the work station assembled a three dimensional (3D) reconstruction data set. Hence, the tractography can be rotated in all directions in this 3D-reconstruction.
The FA values of all 13 scanned dogs with healthy spinal cords showed a range of 0.291 to 0.631 independent of the measured spinal cord segment (ROI 1, 2, and 3).
The results of the measurements were reproducible and the values did not differ significantly between the ROIs. Breed and body weight of the dogs had no influence on the distribution of measured values. The FA mean value determined in the current study (0.447) is lower compared to the mean value described by Pease and Miller (2011; 0.72043). (Pease and Miller 2011) However, in the study by Pease and Miller FA values were recorded in only one medium-sized dog breed and only in one spinal region, the cervical spinal cord. Values of the current study are comparable with data established for healthy cats ranging from 0.36 to 0.62. (Cohen-Adad, Benali et al.
2007) The ADC values of the current study showed a wider range than the FA values, namely from 0.217 to 0.857 x10-3mm2/s. Pease and Miller (2011) detected comparably higher ADC values with a mean of 1.0253 x10-3mm2/s for their control group consisting of eleven normal Beagle dogs scanned in the cervical region of the spinal cord.(Pease and Miller 2011) Differences might be explained by the scanner
used. Therefore, reference values should be established for each scanner and software used for the measurements. In the current study single ROI measurements were not evaluable in four dogs (see Table 1). A technical error such as a measurement mistake in context with an inaccurate setting of the ROIs on the spinal cord or calculation error of the extended workspace is suspected. Another source of error might be the inclusion of the surrounding cerebrospinal fluid and fat in the ROIs as this leads to partial volume artefact and results in significantly lower FA values.
(Renoux, Facon et al. 2006)
Conclusions
DTI data not only offer the possibility to visualize axonal bundles of the spinal cord and the extending nerve roots in a plastic way but also to assess the axonal bundles by definite values, such as FA and ADC. These objective values provide further important information about the directionality of the diffusion (FA) and the strength of the diffusion itself (ADC) within the neuronal structures of the CNS. The evaluation of the spinal cord integrity is no longer depending on the viewers experience and the presence or absence of indirect means such as a T2 hyperintensity visible in the myelon in sagittal plans after spinal cord compression. Objective values to characterize the spinal cord are comprehensible using fMRI.
These additional data can be used therefore for further studies to evaluate spinal cord injuries in dogs, since data are collected on a cellular level and may reveal abnormalities that are occult on conventional MRI scans.
Conflict of interest statement
None of the authors of this paper has a financial a personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.
Acknowledgements
This study was partly funded by the German Research Foundation (DFG, FOR 1103/TI 309/4-1). V.M. Stein was supported by the Frauchiger Stiftung, Berne, Switzerland.
References
Bareyre, F. M. and M. E. Schwab (2003). "Inflammation, degeneration and regeneration in the injured spinal cord: insights from DNA microarrays."
Trends Neurosci 26(10): 555-63.
Basser, P. J., J. Mattiello, et al. (1994). "Estimation of the effective self-diffusion tensor from the NMR spin echo." J Magn Reson B 103(3): 247-54.
Basser, P. J. and C. Pierpaoli (1996). "Microstructural and physiological features of tissues elucidated by quantitative-diffusion-tensor MRI." J Magn Reson B 111(3): 209-19.
Beaulieu, C. (2002). "The basis of anisotropic water diffusion in the nervous system - a technical review." NMR Biomed 15(7-8): 435-55.
Besalti, O., A. Ozak, et al. (2005). "The role of extruded disk material in thoracolumbar intervertebral disk disease: a retrospective study in 40 dogs."
Can Vet J 46(9): 814-20.
Boekhoff, T. M., C. Flieshardt, et al. (2011). "Quantitative Magnetic Resonance Imaging Characteristics : Evaluation of Prognostic Value in the Dog as a Translational Model for Spinal Cord Injury." J Spinal Disord Tech.
Bondurant, F. J., H. B. Cotler, et al. (1990). "Acute spinal cord injury. A study using physical examination and magnetic resonance imaging." Spine (Phila Pa 1976) 15(3): 161-8.
Bray, J. P. and H. M. Burbidge (1998). "The canine intervertebral disk: part one:
structure and function." J Am Anim Hosp Assoc 34(1): 55-63.
Chahboune, H., A. M. Mishra, et al. (2009). "DTI abnormalities in anterior corpus callosum of rats with spike-wave epilepsy." Neuroimage 47(2): 459-66.
Cohen-Adad, J., H. Benali, et al. (2007). "Methodology for MR diffusion tensor imaging of the cat spinal cord." Conf Proc IEEE Eng Med Biol Soc 2007: 323- 6.
Conturo, T. E., N. F. Lori, et al. (1999). "Tracking neuronal fiber pathways in the living human brain." Proc Natl Acad Sci U S A 96(18): 10422-7.
Danielian, L. E., N. K. Iwata, et al. (2010). "Reliability of fiber tracking measurements in diffusion tensor imaging for longitudinal study." Neuroimage 49(2): 1572-80.
DeBoy, C. A., J. Zhang, et al. (2007). "High resolution diffusion tensor imaging of axonal damage in focal inflammatory and demyelinating lesions in rat spinal cord." Brain 130(Pt 8): 2199-210.
Ducreux, D., J. F. Lepeintre, et al. (2006). "MR diffusion tensor imaging and fiber tracking in 5 spinal cord astrocytomas." AJNR Am J Neuroradiol 27(1): 214-6.
Ensinger EM, F. C., Boekhoff TM, Fork M, Kramer S, Tipold A (2010).
"Magnetresonanztomographische Befunde des Myelons bei 34 Hunden mit zervikalem Bandscheibenvorfall: Gibt es Hinweise auf die Prognose? ."
Kleintierpraxis 55(10): 537-546.
Facon, D., A. Ozanne, et al. (2005). "MR diffusion tensor imaging and fiber tracking in spinal cord compression." AJNR Am J Neuroradiol 26(6): 1587-94.
Flanders, A. E., C. M. Spettell, et al. (1996). "Forecasting motor recovery after cervical spinal cord injury: value of MR imaging." Radiology 201(3): 649-55.
Fujiyoshi, K., M. Yamada, et al. (2007). "In vivo tracing of neural tracts in the intact and injured spinal cord of marmosets by diffusion tensor tractography." J Neurosci 27(44): 11991-8.
Garosi, L., J. F. McConnell, et al. (2006). "Clinical and topographic magnetic resonance characteristics of suspected brain infarction in 40 dogs." J Vet Intern Med 20(2): 311-21.
Ito, D., S. Matsunaga, et al. (2005). "Prognostic value of magnetic resonance imaging in dogs with paraplegia caused by thoracolumbar intervertebral disk extrusion:
77 cases (2000-2003)." J Am Vet Med Assoc 227(9): 1454-60.
Johansen-Berg, H. and T. E. Behrens (2006). "Just pretty pictures? What diffusion tractography can add in clinical neuroscience." Curr Opin Neurol 19(4): 379- 85.
Kale, R. A., R. K. Gupta, et al. (2006). "Demonstration of interstitial cerebral edema with diffusion tensor MR imaging in type C hepatic encephalopathy."
Hepatology 43(4): 698-706.
Kulkarni, M. V., C. B. McArdle, et al. (1987). "Acute spinal cord injury: MR imaging at 1.5 T." Radiology 164(3): 837-43.
Laitinen, O. M. and D. A. Puerto (2005). "Surgical decompression in dogs with thoracolumbar intervertebral disc disease and loss of deep pain perception: A retrospective study of 46 cases." Acta Vet Scand 46(1-2): 79-85.
Lasjaunias, P., A. Berenstein, et al. (2001). "Spinal and spinal cord arteries and veins." Surgical neuroangiography: 146-164.
Le Bihan, D. (1995). "[Diffusion, perfusion and functional magnetic resonance imaging]." J Mal Vasc 20(3): 203-14.
Levine, J. M., G. T. Fosgate, et al. (2009). "Magnetic resonance imaging in dogs with neurologic impairment due to acute thoracic and lumbar intervertebral disk herniation." J Vet Intern Med 23(6): 1220-6.
McConnell, J. F., L. Garosi, et al. (2005). "Magnetic resonance imaging findings of presumed cerebellar cerebrovascular accident in twelve dogs." Vet Radiol Ultrasound 46(1): 1-10.
Mori, S. and P. C. van Zijl (2002). "Fiber tracking: principles and strategies - a technical review." NMR Biomed 15(7-8): 468-80.
Moseley, M. E., Y. Cohen, et al. (1990). "Diffusion-weighted MR imaging of anisotropic water diffusion in cat central nervous system." Radiology 176(2):
439-45.
Murakami, A., M. Morimoto, et al. (2008). "Fiber-tracking techniques can predict the degree of neurologic impairment for periventricular leukomalacia." Pediatrics 122(3): 500-6.
Naude, S. H., N. E. Lambrechts, et al. (2008). "Association of preoperative magnetic resonance imaging findings with surgical features in Dachshunds with thoracolumbar intervertebral disk extrusion." J Am Vet Med Assoc 232(5):
702-8.
Olby, N., J. Levine, et al. (2003). "Long-term functional outcome of dogs with severe injuries of the thoracolumbar spinal cord: 87 cases (1996-2001)." J Am Vet Med Assoc 222(6): 762-9.
Pease, A. and R. Miller (2011). "The use of diffusion tensor imaging to evaluate the spinal cord in normal and abnormal dogs." Vet Radiol Ultrasound 52(5): 492-7.
Renoux, J., D. Facon, et al. (2006). "MR diffusion tensor imaging and fiber tracking in inflammatory diseases of the spinal cord." AJNR Am J Neuroradiol 27(9):
1947-51.
Schöne-Bake, J.-C. (2010). Darstellung von Veränderungen der Fraktionellen Anisotropie großer Fasertrakte bei Temporallappenepilepsie mit Hilfe von DTI und TBSS. Life & Brain Forschungszentrum Bonn, Bereich NeuroCognition -
Imaging. Bonn, Klinik und Poliklinik für Epileptologie der Universität Bonn: 22- 23.
Sether, L. A., S. Yu, et al. (1990). "Intervertebral disk: normal age-related changes in MR signal intensity." Radiology 177(2): 385-8.
Shanmuganathan, K., R. P. Gullapalli, et al. (2008). "Diffusion tensor MR imaging in cervical spine trauma." AJNR Am J Neuroradiol 29(4): 655-9.
Stippich, C. (2010). Presurgical functional magnetic resonance imaging. Radiologe, Abteilung Diagnostische und Interventionelle Neuroradiologie, Universitätsspital Basel. 2: 110-22.
van de Looij, Y., F. Mauconduit, et al. (2011). "Diffusion tensor imaging of diffuse axonal injury in a rat brain trauma model." NMR Biomed.
Wheeler-Kingshott CA, H. S., Parker GJ, Ciccarelli O, Symms MR, Miller DH, Barker GJ (2002). "Investigating cervical spinal cord structure using axial diffusion tensor imaging." Neuroimage 16(1): 93-102.
B. Titel Paper 2
Das folgende Manuskript ist zur Einreichung bei der Zeitschrift “Veterinary Radiology and Ultrasound“ vorbereitet.
B. Evaluation of fiber tracking as a prognostic marker for long-term motor functional recovery in dogs with intervertebral disk disease
M. K. Hobert1*, P. Dziallas1, D. Wolf1, K. Rohn2, A. Tipold1, and V. M. Stein1
1Department of Small Animal Medicine and Surgery, University of Veterinary Medicine Hannover, Buenteweg 9, 30559 Hannover, Germany, 2Institute of Biometry, Epidemiology, and Information Processing, University of Veterinary Medicine Hannover, Bünteweg 2, D-30559 Hannover, Germany
*Corresponding author: Marc Hobert
Department of Small Animal Medicine and Surgery University of Veterinary Medicine Hannover
Buenteweg 9 30559 Hannover Germany
marchobert@web.de tel. +49 511 953 6297 fax +49 511 953 6204
Abstract
Intervertebral disk disease is a common neurologic disorder in dogs resulting in different degrees of clinical signs ranging from spinal pain to paresis or plegia with loss of nociception. Prognostic statements for functional recovery are challenging and rely on assessment of the severity of clinical signs supported by presence or absence and extent of hyperintensity in T2-weighted MR imaging. However, this prognostic assessment may not be congruent with the effective outcome in many dogs. Diffusion tensor imaging (DTI) measures random motion of water protons in tissue such as spinal cord white matter. The computed tractographies enable the visualization of the spinal cord axonal bundles and allows the evaluation of fiber tract integrity and might therefore aid in predicting neurologic outcome. We hypothesized that DTI detects differences in dogs suffering from IVDD compared to healthy controls that correlate with injury severity and can therefore serve as a prognostic indicator for functional motor recovery. Twenty dogs of different breeds suffering of IVDD were scanned using a standard clinical MR protocol with subsequent DTI sequence. Three defined regions of interest (ROIs) were placed on the spinal cord, at the IVDD epicenter as well as in the segment cranial and caudal to it for which values of fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were calculated. FA values of the pooled ROIs showed higher values in the dogs with IVDD ( = 0.475, ROI 1, 2, and 3) compared to the control group ( = 0.447). In contrast, the mean ADC values showed lower values in dogs with IVDD ( = 0.504 x 10-3mm2/s) compared to the control group. ( = 0.560 x 10-3mm2/s). However, in the spinal cord segment caudal to the IVDD (p = 0.0004) a significant decrease of the FA was detected in dogs with IVDD. Evaluation of axonal bundle architecture correlated with the outcome as dogs with fiber ruptures did not show neurological improvement and were euthanized. Fiber tracking enables the reconstruction of spinal cord axonal bundles and exclusively offers acquisition of quantitative data to assess spinal cord integrity. It therefore has the potential to aid as a prognostic indicator for functional motor recovery.
Key words: IVDD, MRI, DTI, canine, prognosis, fiber tracking, fractional anisotropy, apparent diffusion coefficient, disk prolapse, axonal bundles
Introduction
Intervertebral disk disease (IVDD) is one of the most common neurological diseases in dogs with a prevalence of up to 2% in the canine population.1 Currently the most sensitive diagnostic tool to validate IVDD is magnetic resonance imaging (MRI). 2 MRI findings excellently reflect site and side of the lesion as confirmed by surgery or necropsy.3, 4 Changes detected with the standard clinical MR protocol, namely T2- weighted (T2-w) , Short-Tau Inversion Recovery (STIR), and T1-weighted (T1-w) sequences, merely indicate pathological changes such as hemorrhages, edema, necrosis, and myelomalacia of the spinal cord or the surrounding soft tissue.5-9 Determination of a prognosis in dogs with IVDD is challenging. Commonly, a combination of clinical findings such as severity of neurological deficits, duration of loss of deep pain perception, presence and extent of spinal cord hyperintensities in T2-w MRI sequences and cerebrospinal fluid biomarkers are used as markers for a successful outcome.8, 10-12 Newer non-invasive techniques such as diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI) collect data on the tissue microstructure including axons and myelin and therefore reflect functional axonal integrity. DTI allows the detection of subtle white matter changes in the early pathologic state.13, 14 This growth in information could enable the clinician to establish a more accurate prognosis for recovery in compressive spinal cord diseases in dogs.
DTI has the ability to gather alterations in random motion of water molecules in tissue in vivo.15, 16 Data collection for DTI can assess from at least 6 up to 33 orientations of water diffusion. Subsequently to the data acquisition DTI offers tracking and reconstruction of spinal cord axonal fiber bundles by so-called tractographies.16-18 The molecular basis of DTI is the utilization of anisotropic water diffusion in organized tissues such as white matter in the brain or spinal cord.18-20
In human medicine DTI was originally used presurgically to plan function-preserving surgery by saving motor and speech reliable areas of the brain.15 To date reconstruction of white matter tracts in 3D images are not only possible in the brain but also in the spinal cord. Specialized fiber tracking (FT) algorithms and a so-called tensor map are used to generate tractographies that allow the visualization of the spinal cord in a plastic shape in various mammalian species.15-17, 21-26
Definition of so called regions of interest (ROIs) on the axonal fiber bundles of the spinal cord offers the possibility to assess two additional quantitative parameters, the fractional anisotropy (FA) and the apparent diffusion coefficient (ADC). The ADC
