Abstract:
Disclosed herein are cerebrospinal diffusion phantoms which include a housing having a shape and size configured for insertion into a magnetic resonance coil in one or more preselected poses. A scaffold support structure is mounted on an interior of said housing and a plurality of elongated diffusion mimicking members supported on the support array. The elongated diffusion mimicking members are affixed to the scaffold support structure such that elongated diffusion mimicking members extend in directions needed to substantially emulate a 3 dimensional arrangement of cerebrospinal diffusion fiber tracts in a living organism; as well as modules for elimination of resolution based-bias, angular accuracy evaluation, diffusion rate calibration, and quality assurance image referencing. Each elongated diffusion mimicking member includes an aqueous component which can undergo diffusion along the elongated diffusion mimicking member. The phantom includes a cerebrospinal tissue mimic matrix material contained in the housing enveloping the array of elongated diffusion mimicking members. The housing is made of a material whose magnetic susceptibility substantially matches that of the cerebrospinal mimic matrix material.

Description:
FIELD 
     The present disclosure relates to medical phantom configured to be used for baseline calibration of MRI devices. 
     BACKGROUND 
     In the field of medicine phantoms or simulators have significant utility. Phantoms allow for the evaluation, analysis, and performance optimization of various imaging devices including magnetic resonance imaging (MRI) devices. They are more readily available and provide better consistency than use of a living specimen or cadaver. 
     In some biological tissues the diffusion of water is dependent on its interactions with the surrounding environment. Macromolecular structures, fibers, membranes and the dimension of the volumes containing the diffusing water reveal different information regarding the tissue in which the diffusion occurs. This information may provide insight into the architecture of anatomical and sub-anatomical structures; for example axonal fibers in the central nervous system. 
     Diffusion tensor imaging (DTI) is an MRI based imaging method that measures the anisotropic rate of water diffusion. The high degree of organization of white matter in the brain leads water to diffuse more rapidly in directions along white matter tracts because physical barriers such as myelinated axonal walls restrict water movement in other directions. 
     This unique ability of DTI to closely examine the fine structural changes of biological tissue by measuring anisotropic diffusion of water is very useful in determining the fine structure of white matter, delineating the boundaries of necrotic or damaged tissue, detection and confirmation of neurodegenerative diseases and brain disorders not found out by general medical imagining. The importance of DTI is underscored by the fact that it is the only way of studying white matter structure in vivo. This is a key element in being able to understand how these connections in the brain are affected during the progression of various diseases, and how cognitive and behavioral systems are linked to these changes. 
     The ability of DTI to describe connectivity in the brain has been clinically relevant for the study of neurological disorders as it can reveal abnormalities in white matter fiber structure and provide models of brain functionality. 
     This connectivity relies on the fact that functioning white matter consists of multiple axons contained within myelin sheaths with multiple axons arranged collinearly to form fascicles or bundles of nerve cells. The extra-cellular water contained in the spaces between myelin sheaths experiences anisotropic (directionally dependent) diffusion along the direction of the fascicle as the space is heavily restricted in perpendicular directions. 
     Presently, MR diffusion protocols may be used to measure both the rate and anisotropic property of diffusion. Differences in the rate of diffusion are associated with differences in cellular density which may vary between adjacent sub-anatomical structures—for example between ventricles and white matter, or, between healthy and some diseased tissues. For example cystic tumours would be characterized by regions of low (hypo-) cellularity and dense tumours such as a fibrous metastatic tumour would be characterized by high (hyper-) cellularity. The directionality (anisotropy) of diffusion is associated with tissue organization. Greater diffusional anisotropy indicates a more strongly directed diffusion, or highly structured tissue, such as white matter fiber bundles. 
     The practical applicability of DTI is limited by variation in the diffusion indices in different MR scanners; its inability to resolve multiple fiber populations; as well as by variations caused by the use of different imaging parameters (e.g. those used in longitudinal or multicenter trials). Therefore, the development of a standard DTI phantom to serve as a baseline for calibrated measurement and validated imaging would find utility. 
     Presently diffusion phantoms typically focus on either mimicking the rate or anisotropic nature of diffusion but not both simultaneously. 
     SUMMARY 
     The present disclosure describes an apparatus, approach, and methodology to produce cerebrospinal diffusion phantoms. The phantoms include a housing having a shape and size configured for insertion into a magnetic resonance coil in one or more preselected poses. A modular scaffold support structure is mounted on an interior of the housing and a plurality of elongated diffusion mimicking members supported on the modular support array. The elongated diffusion mimicking members are affixed to the adjustable scaffold support structure such that elongated diffusion mimicking members extend in directions needed to substantially emulate a 3 dimensional arrangement representative of cerebrospinal diffusion fiber tracts in a living organism. Each elongated diffusion mimicking member contains an aqueous component which can undergo diffusion along the elongated diffusion mimicking member. The phantom includes a cerebrospinal tissue mimic matrix material contained in the housing enveloping the array of elongated diffusion mimicking members. The housing is made of a material whose magnetic susceptibility reasonably matches that of the cerebrospinal mimic matrix material. 
     The phantom may include a module that enables the tuning of partial voxel volumes to decrease resolution based biases in DTI imaging. DTI imaging may be successfully applied for the quantification of fiber integrity and nerve fiber bundles consisting of a single fiber tract. To resolve fiber crossings, more elaborate diffusion imaging techniques like Q-Ball Imaging (QBI) may be used. 
     The phantom may include a module that enables the resolution of fiber crossings for evaluation of angular accuracy. In order to make accurate measurements of the relative difference between anisotropic and isotropic conditions within in vivo scans, operators need baseline or ‘ground-truth’ value for both of these diffusion states relative to one another. The phantom includes a module to enable such isotropic diffusion signal measurement. 
     In an embodiment there is disclosed a method for generating biomimetic micro-lumen structure containing bundles of flexible micro-rods from multi-material elements, comprising: 
     providing a spool of bicomponent micro fiber strand comprised of micro-rods of polypropylene embedded in a matrix of polyvinyl alcohol, unwinding the biocomponent micro fiber strand from said spool and winding it onto a generally square shaped spindle to generate a rod bundle with a set number of aligned bicomponent micro fiber strands, and said rod bundle having opposed ends; 
     immersing the rod bundle in water to dissolve the matrix of polyvinyl alcohol and thereafter placing the rod bundle in an ultrasonication bath, and repeating until all the polyvinyl alcohol has been removed and water is entrapped between adjacent micro-rods of polypropylene; 
     applying tension to the rod bundle by pulling the opposed ends of said rod bundle in opposite directions to align the micro-rods with each other in the bundle; and 
     fastening said rod bundle on an interior of a cerebrospinal phantom to mimic diffusion tracts in cerebrospinal tissue. 
     A further understanding of the functional and advantageous aspects of the invention can be realized by reference to the following detailed description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments disclosed herein will be more fully understood from the following detailed description thereof taken in connection with the accompanying drawings, which form a part of this application, and in which: 
         FIG. 1  is perspective view of an embodiment of a cerebrospinal diffusion phantom. 
         FIG. 2  is an elevation view of the diffusion phantom of  FIG. 1 . 
         FIG. 3  is an elevation view of the diffusion phantom similar to  FIG. 2  but rotated 90 degrees. 
         FIG. 4  is a cut out view of the elevation view of  FIG. 1  showing the internal structure of the phantom. 
         FIG. 5  is an exploded view of the phantom shown in  FIGS. 1 to 4 . 
         FIG. 6  is an extended exploded view of all components of the phantom of  FIGS. 1 to 4 . 
         FIG. 7  is a cross sectional view of a nineteen (19) hole micro-rod which may form part of the present phantom having different sizes of hole diameters. 
         FIG. 8  is a cross sectional view of a nineteen (19) hole micro-rod which may form part of the present phantom having one size of hole diameters. 
         FIG. 9  is a cross sectional view of an embodiment of a flexible micro used to mimic cerebrospinal diffusion fiber tracts in the present diffusion phantom. 
         FIG. 10  is a cross sectional view of an alternate embodiment of a flexible micro-rod used to mimic cerebrospinal diffusion fiber tracts in the present diffusion phantom. 
         FIG. 11  shows a cross section of an embodiment of a pulled micro rod forming part of the present disclosure. 
         FIG. 12  shows a cross section of an alternate embodiment of a pulled micro-rod forming part of the present disclosure. 
         FIG. 13  shows a cross section of yet another alternate embodiment of a pulled micro-rod forming part of the present disclosure. 
         FIG. 14  shows the bi-material flexible micro-rod (Material B within Material A) before processing. 
         FIG. 15  shows the bi-material flexible micro-rod after processing (Material B only) with generated micro-lumen structure. 
         FIG. 16  shows the multi-material micro-rods before separation process. 
         FIG. 17  illustrates how the strands are spun onto a spindle, heated and ultrasonicated in a water bath. 
         FIG. 18A  illustrates the formed rod bundles fastened on the phantom. 
         FIG. 18B  illustrates a processed DTI image of micro-rod bundles supported within the phantom. 
         FIG. 19A  shows and exploded view of the diffusion phantom. 
         FIG. 19B  shows an assembled view of diffusion phantom seen in  FIG. 19A . 
         FIG. 20  illustrates an exemplary orthopedic phantom. 
         FIG. 21  is a perspective view showing a diffusion phantom constructed in accordance with the present invention being inserted into an MRI machine. 
         FIG. 22  high level diagram of a diffusion phantom system. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments and aspects of the disclosure will be described with reference to details discussed below. The following description and drawings are illustrative of the disclosure and are not to be construed as limiting the disclosure. Numerous specific details are described to provide a thorough understanding of various embodiments of the present disclosure. However, in certain instances, well-known or conventional details are not described in order to provide a concise discussion of embodiments of the present disclosure. 
     As used herein, the terms “comprises” and “comprising” are to be construed as being inclusive and open ended, and not exclusive. Specifically, when used in the specification and claims, the terms “comprises” and “comprising” and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components. 
     As used herein, the term “exemplary” means “serving as an example, instance, or illustration,” and should not be construed as preferred or advantageous over other configurations disclosed herein. 
     As used herein, the terms “about” and “approximately” are meant to cover variations that may exist in the upper and lower limits of the ranges of values, such as variations in properties, parameters, and dimensions. 
     The present specification discloses numerous example embodiments. The scope of the present patent application is not limited to the disclosed embodiments, but also encompasses combinations of the disclosed embodiments, as well as modifications to the disclosed embodiments. 
     References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     Furthermore, it should be understood that spatial descriptions (e.g., “above,” “below,” “up,” “left,” “right,” “down,” “top,” “bottom,” “vertical,” “horizontal,” etc.) used herein are for purposes of illustration only, and that practical implementations of the structures described herein can be spatially arranged in any orientation or manner. 
     Numerous exemplary embodiments are described as follows. It is noted that any section/subsection headings provided herein are not intended to be limiting. Embodiments are described throughout this document, and any type of embodiment may be included under any section/subsection. Furthermore, disclosed embodiments may be combined with each other in any manner. 
     As used herein, the term “patient” is not limited to human patients and may mean any organism to be treated using the diffusion phantoms disclosed herein. 
     As used herein, “hydrogels” refer to materials that are formed by crosslinking polymer chains, through physical, ionic or covalent interactions and are known for their ability to absorb water. An example of a physical interaction that can give rise to a hydrogel is by thermal treatment of the liquid hydrogel precursor which, prior to being subjected to a freeze thaw cycle is a liquid or near liquid. The process of freezing the liquid precursor acts to freeze the water contained in the polymer/water mixture and ice particles causes the polymer strands to be topologically restricted in molecular motion by other chains thus giving rise to the “entanglement’ cross linking to produce the hydrogel. Hydrogels that have been produced by a freeze thaw cycle are sometimes referred to as “cryogels”. 
     Hydrogels characterized by cross linking that are produced through ionic or covalent interactions typically require a cross linking (XL) agent and/or an initiator and activation by methods such as heat or radiation. 
     Description of Inner and Outer Housings 
     Referring to  FIGS. 1, 2, 3, 4 and 5 , a cerebrospinal diffusion phantom constructed in accordance with the present invention is shown generally at  10 . Diffusion phantom  10  comprises a base section  12 , an exterior housing  14  adapted to couple with base section  12 , and a handle  16  located in housing  14  adapted to be gripped by a user moving the phantom  10  around. 
       FIG. 2  also shows a scanning location reference marker  11  which provides a reference for locating the phantom  10  in a MRI machine. Phantom  10  in  FIG. 2  also provides registration targets  13  where a navigation pointer tool can select these registration targets  13  to register phantom  10  with a medical navigation system can be mounted. 
       FIGS. 5 and 6  shows an exploded view of phantom  10  in which it can be seen that base section  12  mounts to a seal plate  30 , inner housing  32 , and outer shell  18 . When the phantom  10  is assembled, as can be seen from  FIG. 5  one (1) o-ring  15  seals the inner housing  32  and the two (2) o-rings  13  and  14  seal the outer housing  18  and inner housing  32 . 
     In reference to the embodiment shown in  FIG. 4 , a central pillar  46  mounts to the base  12  and acts as support for the circular micro-rod bundle mounts  47 , the resolution module  48  and the q-ball module  49 . A micro-rod bundle  40  are shown associated with the circular bundle mounts  47 . A plug  50  is used to seal the seal plate  30  after topping up matrix fluid during assembly. 
     The circular bundle mounts  47  act as a configurable support structure to the micro-rod bundles  40  and serve an additional purpose as fluid buffers to prevent fluid motion during a scanning protocol should the matrix material be fluid. This feature is to improve image clarity. 
       FIG. 6  is an extended exploded view of all components of the phantom of  FIGS. 1 to 4 . Micro-rod bundles  40  can be seen in  FIGS. 5 and 6  where they are mounted in various orientations. 
     Referring to  FIG. 5 , once base  12  and internal housing  32  are sealed together, they are locked together using five (5) bolts  20  which are passed through holes in base  12  and corresponding holes in the peripheral shoulder in internal housing  32 , indicated by the dashed lines and the bolts are threaded into their corresponding nuts as shown. 
     The flat bottom section of base  18  (i.e., exterior end) allows it to be securely placed on a bench-top or other flat surface. The handle provided by  34  in this section provides a grip for when a user is placing it into (and removing it from) an MRI head-coil. In addition, the flat surfaces  51  on the upper housing section  18  allows the user to steady the phantom  10  before transporting. 
     It should be noted that in some embodiments the phantom  10  is produced from a material capable of withstanding a freeze-thaw cycle if using a cryogel as matrix material. In addition the phantom  10  may include a marker (not shown) for landmarking and/or correct orientation in the MR coil. 
     In an alternate embodiment, the housing may be constructed and function as follows. Using various ties, the fiber modules are attached at their ends and along their lengths to three circular elements that are designed to enable the maximum number of fastening locations. These circular elements are attached to a center column that is mounted to an inner housing and they can slide, and be fastened to various locations on the column. These elements are unique in that they enable multiple configurations for fiber bundles to be positioned in x,y,z directions, ‘kissing’, diagonally, curved and interweaving. This center column also enables attachment of modules for various modules described in further detail below. 
     Description of Micro-Rods 
     Water can diffuse a radial distance of approximately 6-10 μm between the time of excitation and signal acquisition for a standard DTI protocol. This means that water within this distance from a micro-lumen channel wall or micro-rod wall will demonstrate restricted diffusion. Water that is not within this distance of a wall or barrier, will maintain free diffusion. 
     To increase the level of anisotropic diffusion within a micro-rod channel or against a micro-rod, it is useful to restrict the diffusion more in the radial direction. With increasing radial restriction (i.e. reduced radial dimension), this decreases water in the voxel which may decrease the received anisotropic signal. To promote ideal signal, the cross-sectional surface area needs to be increased to get more water within the voxel. 
     Two possible embodiments are disclosed in  FIGS. 5 and 6 , with respect to the fiber emulating micro-rod bundles  40  being mounted on the circular bundle mounts  47  in phantom  10  or in additional phantoms. In one embodiment, a flexible micro-lumen rod containing at least one (1) and as many as nineteen (19) micro-lumen channels of a diameter that allows an extreme aspect ratio may be used which facilitates the detection of diffusion in a liquid when the micro-lumen channels are filled with the liquid. The micro-lumen rod is comprised of a flexible material that allows it to be bent to span multiple directions and to isolate the liquid interior from the matrix material and it is cut to give desired lengths without cracking or otherwise deteriorating the inner structure. The micro-lumen channels can be filled by a number of methods including vacuum backfilling. 
       FIG. 7  shows a cross sectional view of one such embodiment of a flexible micro-rods  52 ,  54  and  56  having nineteen (19) channels of three different sizes. A single central channel  56  has the largest diameter, six (6) channels  54  of a slightly smaller diameter surround the central micro-lumen channel  56 , and twelve (12) smaller diameter channels  52  surround channels  54 .  FIG. 8  shows a cross sectional view of another embodiment of a flexible micro-rod  70  which also contains nineteen (19) micro-channels  72  all having the same diameter. 
     The micro-rod embodiments in  FIGS. 7 and 8  are non-limiting example embodiments of flexible micro-rods with multiple micro-lumen channels. It may be conceived that other micro-rod permutations with any number of micro-lumen channels and/or diameter sizes may be used. As an example, the micro-lumen channels as seen in  FIGS. 7 and 8  may have a size in the range of 0.5 micrometers to 10 micrometers. 
     In an alternate approach, a flexible plastic micro-rod that is threaded with thread sizes proportional to the size of voxels may be used. This approach to axon fiber mimicry accounts for the fact that diffusion is restricted in the volume closest to the surface. The shapes disclosed herein are designed to balance the trade-off between increasing signal and restricting radial diffusion. 
       FIGS. 9 to 13  illustrates the cross-sectional views of several non-limiting embodiments of flexible pulled micro-rods used to mimic cerebrospinal diffusion fiber tracts in a diffusion phantom. The cross-section can be consistent along the entire length of the micro-rod and should be designed to maximize the wettable surface area of the micro-rod. For example, in the micro-rod structure  80 . In  FIG. 9 , indentations with circular cross-sections  84  would maximize the wettable surface area. 
       FIGS. 10 to 13  illustrates cross sections of alternate embodiments of pulled micro-rods forming part of the present disclosure.  FIG. 10  is a cross sectional view of an alternate embodiment of a flexible micro-rod used to mimic cerebrospinal diffusion fiber tracts in the present diffusion phantom. In this embodiment in  FIG. 10 , a shape such as a rhodonea curve (where k=4) is representative of an idealized cross section where the surface area is maximized in each voxel. In a further embodiment, a fractal pattern (not shown) can also be used to maximize the wettable surface area and result in greater diffusion restriction in each voxel. 
     Referring to  FIG. 11 , micro-rod  94  includes a flexible pulled micro-rod material  96  showing six (6) indents or channels along the length of micro-rod  94 . The embodiment of a micro-rod  100  in  FIG. 12  shows a flexible pulled micro-rod material  102  having eight (8) indents or channels  104  running along the length of the rod  100 . In a further embodiment shown in  FIG. 13  at micro-rod  100 , a pulled flexible micro-rod material  112  is shown having eight (8) channels  114 . 
     The indents or channels in the micro-rod surface seen in  FIGS. 11 to 13  are chosen to be of a size to be narrow in comparison to the distance water can diffuse on the timescale of a DTI protocol, thereby restricting the possible diffusion in all directions except for along the direction of the fiber. It will be appreciated that the embodiments of  FIGS. 9 to 13  are only exemplary in nature. 
     Description of Bicomponent Micro-Rods 
     The embodiment shown in  FIG. 14  incorporates the use of multi-rod bundles wherein the multi-rod bundles contain a multitude of bicomponent rods before the separation process (i.e., pre-processing stage). At this stage, there are a multitude of rods of Material B  122  embedded in Material A  120 . These bicomponent rods may be extruded from the same spinneret (small, thimble-shaped, metal nozzle having fine holes through which a spinning solution is forced to form a micro-rod), resulting in both polymers contained within the same micro-rod. 
       FIG. 14  further depicts the cross-section of a bicomponent rod. In this embodiment, there are 61 micro-rods of Material B  122  made out of polypropylene (PP) that is surrounded by a water-soluble material Material A  120 . Material A  120  in this embodiment may be polyvinyl alcohol (PVA). The micro-rods  122  made of Material B have substantially uniform diameter and are embedded through Material A  120  in substantially uniform spacing. Those skilled in the art would be able to determine appropriate substitutes for these materials. The ‘sea’ material can be removed by placing the fibers in warm water for a few hours, or using a combination of warm water and ultra-sonication as an example. 
       FIG. 15  shows the bi-material flexible micro-rod after removal of the Material A constituent, as well as, the unique micro-lumen regions generated by this process. It is these regions that function to provide anisotropic restriction of diffusion motion. The scale of the micro-lumen structure is dependent on the tightness of the packing of the micro-rod bundle, the diameter of the Material B micro-rods  122 , and the sectional geometry of the rods themselves (i.e., they may contain the internal lumen structures as alluded to in  FIGS. 7 to 13 ). Further, each micro-rod  122  of Material B is of substantially uniform spacing &amp; uniform diameter based on manufacture tolerance requirements and/or limitations. 
     Description of Method of Forming Diffusion Fibers 
       FIG. 16  shows the bicomponent rods before the separation process. At this stage, there are a multitude of micro-rods  122  of material B (polypropylene (PP) embedded in the matrix  120  made of material A (polyvinyl alcohol (PVA)).  FIG. 16  also illustrates bicomponent rod windings of fiber strands  126  on a production bobbin  124 . In  FIG. 16 , the bicomponent rods  130  are wound on a bobbin  124  with  144  bicomponent rods per strand. Each strand  126  contains approximately 8800 polypropylene (PP) micro-rods (material B). The fiber strands  126  are wound onto a square shaped spindle  128  (see  FIG. 17 ) using a motorized spinner to generate a rod bundle with a set number of aligned bicomponent rods  130 . The revolutions are counted to determine the total number of micro-rods within the rod bundle. For example, 200 revolutions equates to 400 strands segments per bundle, resulting in a total of 3.5 million micro-rods per bundle. 
     To remove matrix  120  made of material A and introduce water between the micro-rods  122  (material B), the U-bolt containing the rod bundle is placed into a water bath for dissolving material A. Thereafter the material is then placed in an ultrasonication bath. The warm water and ultrasonication is then repeated one or more times to ensure complete removal of the PVA (material A). 
     In this embodiment, the strands  126  in  FIG. 16  are initially brown in colour before removal of the PVA, and become white after the dissolving and ultrasonication process. Sonication also breaks up the micro-rods  122  (material B) and allows water to become entrapped between them. 
     The flexible micro-rod bundles  122  are then secured at each end to maintain alignment of the fibers using thread or zipties and are removed from the U-bolt. The flexible micro-rod bundles  122  can then be wrapped or manipulated to maintain a tight flexible micro-rod bundle and then fastened in various orientations to the interior of inner housing  32  suitable for MR imaging. In this example, the flexible micro-rod bundles  122  are tightly bundled using various techniques which may include, but are not limited to sewing thread, heat shrink tubing collars, ziptie collars, twisted fiber, or no manipulation. The zipties at the ends help keep tension on the flexible micro-rod bundles  122  to reduce motion during scanning. 
       FIG. 17  illustrates how the strands are spun onto a spindle  128 , heated and ultrasonicated in a water bath  129 . In this embodiment, the spindle  128  is attached to a controlling motor which rotates the strands off of the bobbin  124  as seen in  FIG. 16 . 
     One advantage to using bicomponent micro-rod materials is that the alignment of the inner material  122  (material B) within a bicomponent rod  130  remains unaffected during the winding process and can only shift during the removal process of matrix  122  (material A). During this process, all micro-rods made of material B are under tension which should allow the material B to remain in an aligned configuration. This provides more uniform packing of the material B micro-rods  122  once the material A material is removed, in turn providing more uniform micro-lumen avenues between the material B micro-rods  122  where the anisotropic diffusion of water occurs. 
     Description of Micro-Rod Bundles Mounted on Module 
       FIG. 18A  illustrates the formed micro-rod bundles, shown from the process illustrated in  FIGS. 7 to 17 . In  FIG. 18A , the phantom is supported by the internal scaffold support structure also referred to as circular bundle mounts  47  so that the phantom simulates brain fibers travelling in all three orthogonal directions. 
     Using various ties, the micro-rod bundle modules are attached at their ends and along their lengths to three circular elements that are design to enable the maximum number of fastening locations. These circular elements are attached to a center column that is mounted to the main housing and they can slide, and be fastened to various locations on the column. These elements are unique in that they enable near infinite configurations for fiber bundles to be positioned in x,y,z directions, ‘kissing’, diagonally, curved and interweaving. 
       FIG. 18B  illustrates a processed DTI image of micro-rod bundles supported within the phantom. The top image of  FIG. 188  shows the DTI image of the micro-rod bundles. The bottom image of  FIG. 18B  provides a close-up magnified view of two strand of the micro-rod bundles. 
       FIGS. 18A and 18B  are illustrative of a head phantom support structure for scanning the head region, however, the micro-rod bundles  122  and circular bundle mounts  47  may be incorporated into additional phantoms for scanning of other anatomical body parts (i.e., a diffusion phantom for a leg, spine, hip, abdominal regions, etc.) where anisotropic diffusion of water may be present in tissue and nerve images. 
     Tuning Matrix for T1 and T2 Values 
     As seen in  FIG. 5 , when assembled, internal housing  32  is sealed against base section  12  using o-ring  15 , creating a liquid tight seal that encloses the matrix material. The micro-rod bundles  40  as shown in  FIG. 18  are submerged in this matrix material. 
     Anisotropic diffusion is a function of the aspect ratio of the lumen micro-structure generated by the flexible micro-rod elements. By having an extreme length (i.e., infinitely long on the time scale of the MR acquisition) and a small width and height, this acts to restrict the direction that diffusion can take place to the direction of the micro-rod elements. Thus the liquid can be water or an aqueous based solution of a material to tune the MR visibility (e.g. copper sulfate solution). 
     To improve the MR visibility of the matrix material in the phantom, one can tune the MR properties of the matrix material to increase the relative signal within a typical MR diffusion measurement. The MR relaxation properties which control the relative amount of signal generated within an imaging sequence are the T1 and T2 relaxation times. The T1 relaxation rate determines how quickly the MR signal recovers in between repeated data acquisitions, thus to maximize signal in a DTI acquisition the T1 relaxation time should be short compared to the MR imaging repetition rate (TR). Similarly the T2 relaxation rate determines how quickly the MR signal decays away when trying to measure it so the T2 relaxation rate should be long relative to the time before data is acquired (commonly referred to as the time of echo, TE). As the liquid in the micro-lumen structure is aqueous, one can add soluble materials such as copper, nickel, and/or iron salts to change and optimize the T1 and T2 responses. 
     Description of Matrix Material 
     The diffusion phantom  10  disclosed herein may be filed with a matrix material which is chosen to be magnetic resonance (MR) compatible and give MR signals including signals in the range of human tissue. These materials could include but are not limited to polyvinyl alcohol (PVA) cryogel, PVA solution, cross-linked polyacrylate polymer gel, water, mineral oil or a solution of salt such as copper sulfate or similar materials. Exemplary formulations are disclosed in international publication WO/2015/003271, which is incorporated herein by reference in its entirety. 
     The matrix material is also interchangeable as the micro-rod bundles are modular and separable from the matrix. In other words the matrix material may be removed leaving the micro-rod bundles intact in its preselected configuration and replaced with a different matrix material if that is desired. 
     Description of Use Diffusion Phantom Utility 
     One use for diffusion phantoms disclosed herein is for calibration and support of diffusion weighted magnetic resonance imaging (DW-MRI). A gold standard for the quantitative validation of DW-MRI is crucial for clinical purposes but is still not available. For the determination of the accuracy and precision and the evaluation of artifacts in a DW-MRI experiment, a phantom is required which has a well-known structure and diffusion behaviour similar to that in brain white matter. The use of phantoms with a well-known connectivity and anisotropy would also be useful for testing fiber tracking algorithms. Moreover, the origin of the DW-MRI signal in brain white matter is not completely understood. Several models exist, based on specific assumptions about the diffusion in the complex geometry of brain white matter. Validation of those models is also necessary. 
     Advantages of Micro-Rod Use for Lumen Generation 
     The diffusion phantom disclosed herein has several advantageous features. It can be configured to produce a diffusion signal along tracts in well-defined paths. The diffusion is produced using flexible micro-rods to generate multiple lumen microstructures, filled with water, or other useful liquids such as aqueous solutions containing contrast agents or salts that can help minimize magnetic susceptibility differences between fluid and micro-rods. These micro-rods can include preexisting lumen structures in their aspect ratio to increase the number of lumen within the flexible micro-rod bundle, as shown in  FIGS. 7 to 13 . This increases the diffusion signal since in this manner a greater volume of water will experience restricted diffusion at the lumen walls. It is noted that several separate lumen side by side are more effective than one large lumen, depending on the size. For example, if one large lumen is small enough that it will restrict the radial diffusion to the point of measurement then this is advantageous (if the wall is thin enough). It is preferred to maximize the amount of water in the voxel while also restricting diffusion so that there is enough non-water micro-lumen tube material to adequately restrict the diffusion. 
     As an alternative to using micro-rods with enclosed lumen, diffusion can be created with micro-rods with sectional profiles that are optimized to increase the perimeter area (i.e., outer surface). The diffusion signal can be increased in a scalable and predictable way by increasing the number of micro-rods passing through the same voxel. In this embodiment, it is preferable to increase the number of lumen to the point where the restricted radial diffusion is such that it can be measured in the DTI protocol. Any further increase in number of lumen would be unfavourable since there will be less water per voxel. This limit in the number of lumen needed will change based on the b-value of the diffusion sequence. For higher b-values there would be required more lumen, and conversely, for lower b-values, less lumen. This is because the b-value determines what diffusion length the system is sensitive to A lower b-value typically refers to a larger diffusion lengths and a higher b-value typically refers to a shorter diffusion lengths. 
     Description of Micro-Rod Bundle Support Structures 
     As discussed above, the micro-rod bundles  40 ,  122  are placed in a scaffold support structure comprised of circular holders  47  that allows for predictable, repeatable and stable mechanical positioning of the micro-rod bundles. When mounted on this scaffold structure, the micro-rod orientations may be chosen to demonstrate the ability to distinguish diffusion in orthogonal directions, along diagonal paths and in curved paths that change direction. Thus, the micro-rod bundles  40 ,  122  may be configured to provide a curved path and a U-shaped path to give some non-limiting exemplary configurations. 
     In one embodiment, the micro-rod bundle positioning can be configured to provide simulation of tractography of brain white matter fiber tracts wherein the simulated brain fiber tractography can display brain fiber tracts that touches, crosses or interweaves. 
     Thus, the set of micro-rod bundles described here is idealized simulation of nerve fibers, in that all orthogonal directions and curved paths are covered. In this embodiment, simulated nerve fibers that cross each other in the same voxel can be distinguished, and simulated nerve fibers that run together and then separate, can also be distinguished. 
     Description of Overall System 
       FIG. 21  is a perspective view showing a diffusion phantom  10  resting on a table  130  being inserted into an MRI device  132 . The phantom  10  is placed on a flat surface  24  on table  130  as it is positioned inside the MRI machine. 
       FIG. 22  high level system diffusion phantom system. The diffusion phantom  10  (or phantom calibration body) is placed onto into a MRI device  132 , where the received signal can be acquired, processed and shared. The MRI device  132  may be connected to a computer processor  134 , database  136  and computer readable media  138 . 
     Description of Additional Modules 
     Partial volume effects (when a voxel contains two or more types of material) can be problematic in post-processing. By housing multiple micro-rod bundle thicknesses, an included resolution module  48 , as seen in  FIGS. 4 and 6 , can be used to develop scanning methods that decrease resolution based biases. The resolution module  48  includes bicomponent rod bundles of varying diameters that can create DTI signals corresponding to the varying diameters. Resolution module  48  enables the MR technical to tune the MR machine protocols to obtain the desired DTI resolution. 
     In a further embodiment, the QBI (Q-ball imaging) module  49 , as seen in  FIGS. 4 and 6 , can be used to validate more elaborate diffusion imaging techniques like Q-Ball Imaging by enabling the resolution of fiber crossings for evaluation of angular accuracy. The module is comprised of a column-mountable fixture which supports three intersecting and crossing rings of bicomponent rod bundles of different diameter. QBI module  49  is mounted to the center pillar  46  as seen in  FIGS. 4 and 6 . For supporting Q-Ball imaging, a sound measurement tool such as QBI module  49  is indispensable. 
     In a further embodiment (not shown), an isotropic diffusion module can be mounted to center pillar  46 , similar to one seen in  FIGS. 4 and 6 . The isotropic module can enable calibration to a series of diffusion rates. The isotropic diffusion module is comprised of a column-mountable fixture which supports a multitude of vials containing a water-soluble polymer (i.e., povidone) in an aqueous solutions of various concentrations. 
     In an alternate embodiment, the phantom can accommodate a quality assurance module (QA module). One such example is the American College of Radiology (ACR) accreditation module. The QA module may be a separate module from the DTI module. 
       FIGS. 19A and 19B  illustrates an exemplary diffusion phantom with a QA module and an anisotropic DTI module.  FIG. 19A  shows and exploded view of the diffusion phantom with both a DTI module  140  and QA module  142 .  FIG. 19B  shows an assembled view of diffusion phantom seen in  FIG. 19A . 
     The QA module  142 , as seen in  FIG. 19A  includes necessary elements required for quality assurance verification and validation for ACR MRI accreditation. QA module  142  enables measurements of geometric accuracy, high-contrast spatial resolution, slice thickness accuracy, slice position accuracy, image intensity uniformity, percent-signal ghosting and low-contrast object detectability. 
     In further embodiments (not shown), the diffusion phantom as seen in  FIGS. 19A and 19B  may further comprise of a column-mountable plate. In an alternate embodiment, the center pillar  46  as seen in  FIGS. 4 and 6  is secured at both ends by a vibration dampening element (not shown). A vibrational damping element will act to reduce vibration of the micro-rod bundle modules during the scanning process. This feature is to improve the image clarity. 
     In further alternate embodiments, microelectromechanical systems (MEMS) such as accelerometers and drop sensors can be attached to phantom  10  to monitor excess vibration. Further sensors such as thermometers can also be attached to phantom  10  to monitor temperature fluctuations. 
     Different bicomponent rods may be able to represent differentially myelinated nervous tissue by varying the spaces between the close packed structures of the difference radii of the micro-rod elements. This may be illustrated in  FIG. 16  where by changing the relative spacing between micro-rod  122  within water-soluble Material A  120  would generate the DTI imaging characteristic of differentially myelinated nervous tissue. This gives us the ability to approximate the diffusion properties of a variety of structures. 
     A person skilled in the art using the aforementioned method of creating differentially myelinated nervous tissue would be able to create simulated version of different types (e.g. various types of tissues, such as tendons, ligaments, spinal cord, different fiber groups such as, corticospinal, SLF, IFO, corpus collosum; various nerve tissue models such as pediatric, natal, neonatal, in-utero; and different disease and injury states such as multiple sclerosis, edema, traumatic brain injury. 
     In a further embodiment, different processing conditions may be used to partially process the bicomponent rods to remove the water soluable matrix  120 , (Material A) in  FIG. 16 , which could show partial diffusion and blocked channels along sections of the simulated axon fiber. 
       FIG. 20  illustrates an exemplary orthopedic phantom. The orthopedic phantom as seen in  FIG. 20  may be used to mimic the structure of a knee, hip, spine or other orthopedic structures where it can be imaged in a MRI. One objective of the orthopedic phantom is to provide a calibration of these anatomical structures before an actual procedure. 
     The orthopedic phantom displays the diffusion tensor image (DTI) generation process and shows the flexibility of the micro-rod bundle manufacturing process to mimic diffusion signals corresponding to different tissues or different tissue states in the body. 
     The orthopedic phantom of  FIG. 20 , consist of a number of components to simulate bony structure, soft tissue, tendons and ligaments and fluids. The orthopedic phantom as seen in  FIG. 20  illustrates a sectional view of a knee having bone structure  150 , bone marrow  152  and simulated muscle tissue  154 . Embedded within the simulated muscle tissue  154  are micro-rod bundles. In a further embodiment, the orthopedic phantom as seen in  FIG. 20  may also include resolution and spatial modules to mimic staples or bone screws to test scanning abilities where an orthopedic implant is present. 
     While the applicant&#39;s teachings described herein are in conjunction with various embodiments for illustrative purposes, it is not intended that the applicant&#39;s teachings be limited to such embodiments. On the contrary, the applicant&#39;s teachings described and illustrated herein encompass various alternatives, modifications, and equivalents, without departing from the embodiments, the general scope of which is defined in the appended claims. 
     Except to the extent necessary or inherent in the processes themselves, no particular order to steps or stages of methods or processes described in this disclosure is intended or implied. In many cases the order of process steps may be varied without changing the purpose, effect, or import of the methods described.