Abstract:
An enclosure or burrow restrains an awake animal during an imaging procedure. A tubular body, made from a radiolucent material that does not attenuate x-rays or gamma rays, accepts an awake animal. A proximal end of the body includes an attachment surface that corresponds to an attachment surface of an optically transparent and optically uniform window. An anti-reflective coating may be applied to an inner surface, an outer surface, or both surfaces of the window. Since the window is a separate element of the enclosure and it is not integrally formed as part of the body, it can be made with optically uniform thickness properties for improved motion tracking of markers on the animal with a camera during the imaging procedure. The motion tracking information is then used to compensate for animal movement in the image.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application relates to U.S. Pat. No. 8,170,302 “System and Method for Generating Motion Corrected Tomographic Images”, and United States Patent Application Publication Number 2008/0317313 “System and Method for Tracking Motion for Generating Motion Corrected Tomographic Images”, with each being incorporated herein by reference as if included at length. 
     THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to medical imaging of small animals and more specifically to an enclosure for accommodating small animals while being imaged in an awake state. 
     2. Description of the Related Art 
     Motion of a living subject during medical imaging procedures can cause image artifacts. The sources of motion can vary and may include restlessness, scratching, respiration and heart beating, to small movements due to pressure changes over the cardiac cycle. In some cases, motion artifacts degrade the diagnostic value of an image, wasting significant time and money. 
     Efficient methods for testing new drugs are very important to the pharmaceutical industry. The ability to screen test subjects for effects of a particular drug is an essential element in the process of product development. Small animals are essential for pharmaceutical testing, and mice, in particular, are useful for modeling human diseases. Efforts to scale down clinical medical imaging systems for smaller subjects have allowed medical researchers to obtain high-resolution computed tomography (CT) images of small animals for disease studies. Noninvasive imaging techniques, such as X-ray, CT, and positron emission tomography (PET), have been developed for small animal medical imaging applications. For example, small animal imaging is currently being used in cancer research to monitor tumor growth and regression in mice. 
     While anatomical models are useful for studying drug effectiveness, it is very often desirable to screen test subjects for physiological effects of a drug. PET and single photon emission computed tomography (SPECT) are among current techniques used for functional medical imaging. Because animal test subjects must be kept awake during the screening process in order to monitor functional processes, either the animal must remain motionless for the duration of the scan or its movements must be tracked, measured and recorded with a high degree of precision and accuracy. Unrestrained awake animals tend to move around rapidly and present imaging challenges. Although sedation and physical restraint can be used to impede animal motion for this type of medical scan, both methods have the potential to alter the neurological and physiological processes that are being studied. 
     U.S. Pat. No. 8,170,302 “System and Method for Generating Motion Corrected Tomographic Images”, and United States Patent Application Publication Number 2008/0317313 “System and Method for Tracking Motion for Generating Motion Corrected Tomographic Images” both discuss the use of an enclosure or burrow to restrain an awake animal during imaging. Optical and reflective markers are adhered to the restrained but awake animal during imaging. Two or more cameras track the relative motion of the markers so that the animal&#39;s motion can later be accommodated for, and corrected, during image processing. The enclosure is optically transparent to the illumination wavelength and optically uniform so that the external images of the animal can be made without significant image distortion. A one-piece enclosure with optically uniform properties in the location of the optical and reflective markers can be challenging to manufacture. 
     Despite the teachings provided in the above publications, improvements to animal enclosures are needed in order to provide distortion-free images for motion tracking and further advance the art of small animal imaging for medical research. 
     BRIEF SUMMARY OF THE INVENTION 
     Disclosed are several examples of enclosures for restraining an animal during a medical imaging procedure. The enclosures include a tubular body made of a radiolucent material and a separate window made of an optically clear material that is affixed to the body. The window provides an undistorted view of the restrained animal during the medical imaging procedure. Because the window is a separate element of the enclosure, it can be made with optically uniform properties in the location of the reflective markers for improved motion tracking. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The apparatus may be better understood with reference to the following drawings and detailed description. Non-limiting and non-exhaustive descriptions are described with reference to the following drawings. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating principles. In the drawings, like referenced numerals may refer to like parts throughout the different figures unless otherwise specified. 
         FIG. 1  illustrates an exemplary burrow with a restrained and animal; 
         FIG. 2  illustrates an exemplary body portion of the burrow illustrated in  FIG. 1 ; 
         FIG. 3  illustrates exemplary window portions of the burrow illustrated in  FIG. 1 ; 
         FIG. 4  illustrates an exemplary insert for use inside the body portion of  FIG. 2 ; and 
         FIG. 5  illustrates an exemplary air supply means and sensing means for use with the burrow of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to  FIGS. 1-3 , an enclosure  100  restrains an awake animal (A), such as a rat or a mouse, during a medical imaging procedure. In order to correct the images for the 3D motion of the awake animal (A), two cameras are used to capture the positions of the artificial markers (M) affixed to the animal (A), which are illuminated by infrared light emitting diodes (LEDs) as described in the incorporated references. The imaging procedure and motion correction equipment and methods are fully described in the incorporated references and a full description will not be included here for brevity. In the example of  FIG. 1 , the enclosure  100  includes a body  102  and a viewing window  104  that is a separate and detachable element from the body  102 . 
     The body  102  is a tubular structure, defining an interior volume for accepting the animal (A), and having a cross sectional shape that is preferably circular (shown). Shapes such as square, rectangular, triangular, elliptical, or some other shape may be used. The shape of the body  102  must position and restrain the animal A to allow viewing of one or more optical markers (M), adhered to the animal (A), through the window  104  while the imaging procedure is being performed. A distal end  106  of the body  102  provides ingress and egress for the animal (A), while a proximal end  108  includes an attachment surface  110  for mating with a corresponding attachment surface  110 ′ of the window  104 . 
     Since the body  102  is disposed between the animal (A) and an imaging source (not shown), it must be made from a material that permits the passage of radiant energy with relatively little attenuation by absorption. Any radiolucent material that does not attenuate x-rays or gamma rays is suitable if it can be formed into the appropriate shape. Although optically clear PYREX brand glass tubes made by Corning Incorporated have been used successfully for the body  102 , visible wavelength (optical) transparency is not required for this component. Glass can be formed into round tubular shapes with relative ease, but other shapes are possible as well. 
     Other radiolucent materials, such as, carbon fiber reinforced plastics, glass fiber reinforced plastics or Polyetheretherketon (PEEK) thermoplastic materials will allow radiant energy to pass through them with little attenuation. Since the body  102  does not require strong mechanical strength, a body  102  made from a plastic material is acceptable. A body  102  made from a plastic material is also relatively easy to form into numerous shapes and is inexpensive when compared with other materials. Depending on the material, injection molding, blow molding, spin molding, or other manufacturing processes may be used to form the body  102  into the required shape. 
     A diffuse or non-reflecting interior surface  112  of the body  102  is preferred in order to prevent specular reflections from being imaged by the optical motion cameras (not shown) through the window  104 . The non-reflecting interior surface  112  may be a condition of the material selected, a surface treatment (e.g., sanded, sand blasted, etc. . . . ), or may be achieved with a coating as will be discussed in greater detail later. An opposite, exterior surface  114  faces away from the animal (A) and its reflectivity will not affect the optical motion cameras. 
     In some examples, the body  102  defines one or more apertures  116  for providing fresh air, food, medicine, or other items to the animal (A) during the imaging procedure. The apertures  116  may be circular, rectangular, slotted, or otherwise shaped and can be molded directly into the body  102  during forming, or can be machined into the body  102  after forming. The apertures  116  may be placed anywhere along the body  102 , but placing one or more apertures  116  in close proximity to the animal (A) is preferred. The apertures also allow any water vapor to exit the enclosure  100  without forming condensation on the window  104 . 
     The optically transparent and optically uniform window  104  is affixed to the proximal end  108  of the body  102  at cooperating attachment surfaces  110 ,  110 ′ with an attachment means  118 . The window  104  may be hemispherical dome shaped or may be flat disc shaped as shown in the examples of  FIG. 3 , or may be otherwise shaped. By manufacturing the window  104  as a separate component from the body  102 , the optical clarity and uniformity of the thickness can be more closely controlled. A more uniform window  104  thickness improves the optical transparency, produces a very low optical distortion, and allows for more accurate motion tracking of the animal (A). 
     Optical glass such as BK7 glass available from ESCO Products provides excellent transmittance throughout the visible and near infrared spectra and down to 350 nm in the ultraviolet spectrum. A window thickness of up to 2 mm (0.079 inch) is preferred to avoid the need for optical correction, although windows of greater than 2 mm (0.079 inch) thickness may be used if the optical distortion is corrected. A thickness tolerance of +/−0.1 mm (0.004 inch) is generally sufficient to reduce optical distortion. Optics components are available from commercial vendors to meet the requirements for thickness, optical quality and thickness tolerance. Optical glass windows of 1 mm (0.039 inch) thickness have been used successfully for imaging of small animals (A). 
     The window  104  has both an interior surface  120  that faces the animal (A) and an opposite-facing exterior surface  122 . In some examples, at least one of the interior  120  and exterior  122  surfaces is coated with an anti-reflective material  124 . A coating of anti-reflective material  124  is optimized to allow the wavelength of light used to illuminate the animal (A) during the imaging procedure to pass through the window  104 . This ensures that the infrared light emitting diodes don&#39;t produce reflections on the window  104  that would interfere with the 3D motion tracking. 
     Anti-reflective materials  124  reduce reflections due to illumination by means of optical interference. This reduction in reflections also increases the light transmission through the window  104 , thus improving the contrast of the resulting image. Anti-reflective materials  124  are applied to the interior surfaces  120 , the exterior surfaces  122 , or both surfaces to achieve the bandwidth requirements and reflectance reduction. These include broadband and narrow bandpass coatings with single and multi-layers. Acceptable techniques for applying these coatings include evaporative and ion assisted deposition, advanced plasma reactive sputtering, and physical vapor deposition for example. 
     Both near-infrared and visible LED&#39;s are used for illumination of the markers (M) with wavelength ranges from 640 nm to 850 nm although wider wavelength ranges may be used as can other types of illumination. For these wavelengths, both broadband and narrowband anti-reflective coatings  124  can be used, although narrowband coatings will usually provide better anti-reflection results. Typically, maximum reflectivities of less than 1% are achieved with broadband coatings and less than 0.2% with narrowband coatings. Actual requirements for reflectivity depend on the specific application and the need to reduce artifacts (e.g., reflections) for image analysis. Parameters include wavelength, wavelength bandwidth, and angle of incidence with respect to reflection reduction. Anti-reflective materials such as Calcium Fluoride (CaF2) and various other metal oxides could also be used. An example of a low cost, anti-reflective material  124  is Magnesium Fluoride (MgF2) for broadband visible and near infrared applications. This material  124  can be applied using the physical vapor deposition process for example. 
     The equations that describe the total reflectance of a multi-layer film structure are given by: 
               [         B           C         ]     =       {       ∏     p   =   1     q     ⁢     [           cos   ⁢           ⁢     δ   p             i   ⁢           ⁢   sin   ⁢           ⁢       δ   p     /     η   p                   i   ⁢           ⁢     η   p     ⁢   sin   ⁢           ⁢     δ   p             cos   ⁢           ⁢     δ   p             ]       }     ⁡     [         1             η   η           ]                     δ   p     =       2   ⁢           ⁢   π   ⁢           ⁢     N   p     ⁢     d   p     ⁢   cos   ⁢           ⁢     θ   p       λ                 Y   =     c   B                 R   =       (         η   0     -   Y         η   0     +   Y       )     ⁢     (         η   0     -   Y         η   0     +   Y       )             
Where,
 
     q: number of layers 
     δ: phase term 
     η: optical admittance of the layers 
     Np: Complex Refractive Index 
     tp: physical thickness of the layer 
     λ: wavelength 
     θp: angle of incidence 
     Y: the optical admittance of the stack 
     R: Reflectance of the stack 
     The window  104  is affixed to the attachment surface  110  of the body  102  at a corresponding attachment surface  110 ′ via attachment means  118 . The corresponding attachment surfaces  110 ,  110 ′ may have the same or different shapes and surface areas. In some examples, a silicone adhesive of the type offered by MasterBond of Hackensack, N.J., USA is used as the attachment means  118 . The silicon adhesive remains pliant and allows the window  104  to be periodically removed from the body  102  for cleaning, coating, polishing, or replacement. In other examples, a snap fit, a threaded joint, hook and loop fasteners, magnets, or other means such as screws, clips, engaged pins or springs may be used as the attachment means  118 . 
     In order to accommodate smaller sized animals (A) (e.g., juvenile mice) an insert  126  may be disposed inside the body  102  in some examples. The insert  126  decreases the volume of the body  102  to more adequately restrain the smaller sized animal (A) during an imaging procedure. The insert  126  includes a reducer portion  128  that defines a reduced volume for accepting the small animal (A). The reducer portion  128  may also define one or more apertures  116  as describe above with respect to the body  102 . Two or more radially extending spacers  130  may extend from the reducer portion  128  and are appropriately sized to support and center the reducer portion  128  within the body  102 . The reducer portion  128  preferably has a similar cross sectional shape as the body  102 , but may have a different shape as well. The insert  126  is also made of a radiolucent material to allow imaging of the small animal (A) as described above with respect to the body  102 . 
     In order to further prevent condensation from forming on the interior surface  120  of the window  104 , an air supply means  132 , such as a tube or a duct, may be disposed inside the body  102 . A fan, pump, compressor, tank, or other air supply means (not shown) provides temperature and/or humidity controlled air to the animal (A) via the air supply means  132 . Sensing means  134  may also be included within the body  102 . The sensing means  134  may include sensors for monitoring one or more vital signs of the animal (A) such as heart rate, blood pressure, temperature for example. 
     While this disclosure describes and enables several examples of an enclosure for restraining awake animals, other examples and applications are contemplated. Accordingly, the invention is intended to embrace those alternatives, modifications, equivalents, and variations as fall within the broad scope of the appended claims. The technology disclosed and claimed herein may be available for licensing in specific fields of use by the assignee of record.