the multimodal source of the invention can be provided in a wide variety of configurations . fig1 shows a cross sectional first embodiment of the source 10 of the invention comprising a point source in the form of a spherical source with both radioactive and ct / mri / optical target materials . the point source can be conveniently sized at approximately 5 mm in diameter or smaller . in this embodiment , a core 12 of the source can be formed of a matrix ( such as a polymeric resin , a cement , a silicone , a ceramic , a polymer gel , etc .) with one or more radionuclides such as ag - 110m , am - 241 , au - 195 , ba - 133 , c - 14 , cd - 109 , ce - 139 , co - 57 , co - 60 , cs - 137 , eu - 152 , gd - 151 , gd - 153 , ge - 68 , hg - 203 , ir - 192 , i - 125 , i - 129 , i - 131 , lu - 173 , lu - 177m , mn - 54 , na - 22 , ra - 226 , rh - 101 , ru - 103 , ru - 106 , sb - 125 , se - 75 , sn - 113 , sr - 90 , ta - 182 , te - 123m , tl - 204 , th - 228 , th - 229 , th - 230 , y - 88 , zn - 65 , and zr - 95 , ( with ba - 133 , co - 57 , ge - 68 , na - 22 , gd - 153 , cs - 137 and se - 75 being particularly good nuclides ) mixed in , and a coating of barium or iodine salts , bone - equivalent density material such as alumina ceramic , as well as other known compounds such as dense plastics , polymers or glass that are ct target material mixed in an outer coating layer 14 that surrounds core 12 to provide ct targets . for a source that is usable with pet or spect scanners and mri scanners , the coating 14 can , for example , comprise gadolinium salts and other known mri target materials in a matrix , such as a polymeric resin base . the ordering of materials making up of core 12 and outer layer 14 can be reversed if desired , and it is likewise possible to provide a point source with mri and ct and optical target substances mixed together and included as a single coating 14 . it is also possible to provide the ct , mri and / or optical target substances mixed together with the radionuclide in core 12 and forego the outer coating having mri , ct and / or optical target substances incorporated therein . if desired , additional coatings can be provided over the point source . referring to fig2 , it is also possible to provide a single point source 20 which incorporates mri , ct and / or optical target substances in either of two separate coatings 24 and 26 over a core 22 . fig3 and 4 show an exploded and a partially exposed side view , respectively , of an exemplary point source 30 with a radioactive element 32 surrounded by a ring of ct / mri / optical target material 34 . this point source can be made by providing a ring of ct / mri / optical target material 34 , sized , for example , at about 250 microns to about 3 cm with a well 36 located therein , and with a cylindrical , spherical or cubical shaped radioactive portion 32 sized with a diameter of about 100 microns to 5 mm . a glue or a polymeric resin or other material or covering such as plastic film can be deposited or used to cover over radioactive portion 32 to seal it in place to ring of ct / mri / optical target material 34 . other dimensions and configurations can be provided . fig5 is a side view of an exemplary line source 40 having both radioactive and ct / mri / optical target material incorporated uniformly through an active core area 44 of source 40 . a protective outer layer 42 ( such as stainless steel , a plastic tubing , etc .) envelopes active core area 44 . line source may be rigid or flexible , and , for example , can have a diameter of 100 microns to 2 cm and a length of 70 cm or smaller . fig6 is a side view of an exemplary embodiment of a line source 50 having alternating segments of radioactive material 52 and ct / mri / optical target material 54 . a protective outer layer 56 ( such as stainless steel , a plastic tubing , etc .) envelopes active core area comprising alternating sections 52 and 54 . line source 50 may be rigid or flexible , and , for example , can have a diameter of 100 microns to 2 cm and a length of 70 cm or smaller . fig7 is a partially exposed top plan view of an exemplary embodiment of a plane source 60 with both radioactive material and ct / mri / optical target material incorporated uniformly throughout the active area 62 of the source surrounded by a protective envelope 64 . the active area can be sized as desired , such as 75 cm × 65 cm or smaller such 1 cm × 2 cm . active areas can be provided in any number of desired shapes , such as circular ( not shown ) or rectangular areas , with both the radioactive material and ct / mri / optical target material being dispersed uniformly throughout the active area of the source . fig8 is a side view of an exemplary embodiment of a cylinder source 70 with radioactive and ct / mri / optical target material distributed throughout an active volume 72 in a container 74 . cylinder source 70 can , for example , have an active dimension of around 30 cm diameter × 30 cm height . other sizes can be provided as desired . as shown in fig9 , in an alternate embodiment of a cylinder source 80 , radioactive material and ct / mri / optical target material are distributed in their own discrete regions 82 and 84 within a container 86 . although not shown , the cylindrical containers of the embodiments of fig8 and 9 can be replaced with a box shaped container if desired . anthropomorphic shapes such as a heart , breast , torso , or thyroid mimic , with radioactivity and ct / mri / optical target material distributed throughout the active volume or in discrete regions within the volume of the source can also be provided . fig1 is a partially exposed side view of an anthropomorphic breast phantom 90 , with radioactive and ct / mri / optical target material distributed even throughout the active volumes of material 92 ( such as polymeric resins , cements , silicones , polymer gels , ceramics , etc . ), with active volumes 92 being encased with walls 94 . fig1 is another embodiment of a partially exposed side view of an anthropomorphic breast phantom 100 , wherein radioactive material is distributed in one region of the volumes 102 a and 102 b and ct / mri / optical target material are distributed throughout other active areas 104 a - g , with the structure being surrounded by walls 106 . in this embodiment , as well as the other embodiments described above and below , the discrete regions of material 104 a - g can be constructed to provide different visibility densities for a single modality and / or multiple modalities . for example , the active areas can have differing levels of radioactivity ( from no level to a high level ) and / or can comprise radioactive materials with increasing amounts of ct , mri and / or optical target materials . turning to fig1 and 13 , there are shown a perspective view and a cross - sectional view , respectively , of an exemplary small animal phantom 110 that includes objects that mimic , for example , animal structures , such as a spine 112 and ribs 114 , and rodlike structures 116 and 118 ( e . g ., of different diameter ) that can be used to calibrate a scanner and / or measure the scanner &# 39 ; s resolution . the mimicking objects are contained in a background of radioactive material 120 , which in turn are contained in a container 122 . in all embodiments of the multimodal sources , the amount of the nuclide can be varied as required by the specific application . by way of example , the amount can range from about 10 nanocuries ( mci ) ( 370 becquerels ( bq )) or less to about 10 millicuries ( mci ) ( 370 megabecquerels ( mbq )) or more . moreover , combinations of different radionuclides can be provided in a single source . it is desirable for the material matrix to be a solid , semi - solid , or cross - linked gel type of material ( such as silicones ) so that the positioning of any objects in the matrix remains constant . any of these embodiments may also include internal structures such as points , cylinders , rods , planes , etc ., that provide varying levels of visibility for each modality . as noted above , for ct scanning , a “ target material ” or “ target substance ” has a mass density greater than about 1 g / cc so as to be visible against a water or water equivalent background but having an upper mass density that is specific to the x - ray energy of the ct scanner so as not to leave artifacts . artifacts interfere with image quality and it is therefore desirable to eliminate them in images . for example , for a scanner using 50 kvp x - rays , the upper limit of the mass density greater than about 2 g / cc would leave artifacts in the image . for scanners using up to 120 kvp x - rays , the upper limit of the mass density is about 3 to 3 . 5 g / cc . for mr scanners , the “ target material ” or “ target substance ” is a paramagnetic material . for optical scanners , a “ target material ” or “ target substance ” is scintillation material along with a material to activate the scintillation material . having thus described exemplary embodiments of the present invention , it should be understood by those skilled in the art that the above disclosures are exemplary only and that various other alternatives , adaptations and modifications may be made within the scope of the present invention . the presently disclosed embodiments are to be considered in all respects as illustrative and not restrictive . the scope of the invention being indicated by the claims , rather than the foregoing description , and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein .