spar material that would be encountered by the x rays would lie along the plane designated x in fig8 that thickness may be represented as a b where a is the thickness of third wall 282 taken along plane x and b is the thickness of the second wall 280 in its preferred form the spar has a maximum thickness a b of less than 0 625 inches for purposes of contrast a cross section of a prior art table spar is shown in fig1 the prior art spar is comprised of a rectangle having first walls 284 and second walls 286 as can be seen x rays directed towards the prior art spar would be required to pass through a maximum thickness equal to the length d of one of the second walls 286 as will be appreciated in light of the preceding discussion alternative spar cross sections such as the one shown in fig9 may also be used to minimize x ray shadowing the alternative cross section of fig9 is comprised of six sides 287 of equal length as can be seen in fig7 a sleeve 72 is disposed around the table spar 66 near the proximal end 70 of the spar the sleeve supports a rectangular column 74 which extends normally of the table spar 66 and which has a square cross section the column has a.