in the x - ray apparatus of fig1 an x - ray tube x directs a beam of x - rays along a radiation axis a1 to the x - ray responsive face f1 of an x - ray image intensifier ii . the radiation axis passes through a zone z occupied by a patient under examination . the x - ray tube x and image intensifier ii are supported on a frame consisting of a standard 1 which may be floor or ceiling mounted , a rotor 2 journalled in the standard for rotation on a horizontal axis a2 , two transverse members 3 and 4 centrally pivoted on the rotor 2 at 6 and 7 , and upper and lower horizontal arm members 8 and 9 pivotally connected to the transverse members at points 10 , 11 , 12 and 13 so as to form an adjustable parallelogram as fully described in u . s . pat . no . 3 , 892 , 967 . the horizontal members 8 and 9 respectively have extensions 8a and 9a beyond the parallelogram , the x - ray tube x being pivotally supported on a skew axis ax through the lower arm extension 9a . as will be explained in detail the image intensifier ii and a film changer fc are mounted on a frame plate 14 which turns about a pivot 16 so as to interchange the positions of the intensifier and changer to and from the radiation axis a1 as indicated by the arcuate , double - headed arrow 15 in fig1 . the interchange pivot 16 comprises a rotary bearing 17 mounting the assembly of frame plate , changer fc and intensifier ii on a carriage 18 , rotating about a skew axis as through the extension 8a of the upper arm 8 ( fig4 ). links 21 and 22 between one transverse member and the carriage 18 and x - ray tube x maintain the optical axes of the x - ray tube and intensifier or film changer aligned on the radiation axis a1 through the isocenter c as the parallelogram of the transverse and horizontal members is skewed by rotation about the pivot points 10 , 11 , 12 , 13 . the composite angulation by skewing of the radiation axis and rotation about the rotor axis a2 permits examination of a patient from spherical loci equidistant from the isocenter c within a patient . while the prior x - ray apparatus as so far described operates satisfactorily in general it has two disadvantages . firstly , if the optical axis af of the film changer is mounted for simple rotation into coincidence with the radiation axis a1 , its radially extreme edge f3 will intrude into the patient zone z to the hazard or disturbance of the patient . as shown in fig1 this hazard does not exist because the film changer is displaced vertically to a position from which its extreme edge will not swing into the patient zone . however , when in this displaced position as shown in fig1 and 2 , either the film changer optical axis af will not swing into alignment with the radiation axis a1 because the interchange pivot 16 is not coincident with the film changer axis af ( and the optical axis of the intensifier ), or , if these axes and pivot are coincident , the pivot is offset from the combined center of gravity of the film changer and intensifier so that swinging about the pivot shifts the center of gravity of the upper arm 8a and unbalances the system about the rotor axis a2 . according to the present invention the above disadvantages are avoided by mounting both x - ray receptors , the image intensifier ii and film changers , for sliding movement relative to the interchange pivot 16 , as indicated by vertical double - headed arrows in fig1 . in fig2 and 3 are shown linear bearings 26 and 27 on which the film changer fc and image intensifier ii respectively slide . fig4 and 5 illustrate a drive mechanism for coordinating joint rotational and concomitant sliding movement of the film changer and image intensifier . a motor m mounted on the carriage 18 turns a sprocket wheel 28 coupled by a chain 29 to a pulley or sheave 31 on a shaft set in the carriage 18 on the interchange pivot axis 16 such that rotation of the motor swings the frame plate , changer and intensifier assembly about the interchange axis . a stop 32 set in the carriage 18 slides in a slot 33 concentric with the pivot axis 16 limiting movement of the assembly to ninety degrees , thus swinging the image face f1 of the intensifier out of coincidence with an image plane perpendicular to the radiation axis a1 and swinging the image face f2 of the film changer into the same , vacated image plane but not into a position aligned with the radiation axis . the pulley carries on its periphery a belt 41 anchored at one end 42 to the frame plate and at the other end to a bracket 43 connected by a compression spring 44 pivoted to an anchor 46 on the film changer fc . rotation of the pulley 31 simultaneously with swinging the frame plate , film changer , intensifier assembly presses the bracket 43 against the spring which resists compression until the stop 32 arrests swinging movement . further drive from the pulley belt then requires the spring to yield and advances the film changer lengthwise on its linear bearings in the direction of the arrow 47 until its optical axis af aligns with the radiation axis a1 . as shown in fig4 the film changer fc and image intensifier ii are coupled by a bell crank 36a , 36b , pivoted on a stud 37 anchored on the frame plate 14 . links 38 and 39 connect the ends of the bell crank respectively to the film changer fc and image intensifier ii . thus , as the film changer advances lengthwise after the end of its swing , the image intensifier retracts on its linear bearings 27 parallel to the advance of the film changer . the two arms 36a and 36b of the bell crank have lengths inversely proportional to the masses of the film changer and image intensifier respectively . a typical intensifier weighs 160 pounds as against a film changer weight of 130 pounds and the crank arms are proportionally unequal . fig6 shows the film changer initial position fc before interchange in dashed outline , its position fc * after rotation in phantom , and in solid line its final position fc ** after sliding movement from rotated position . the image intensifier has a corresponding initial dashed line position ii , a rotated position ii *, and a final position ii **. as the image intensifier and film changer rotate about the interchange pivot 16 their combined center of gravity does not shift and upset the balance with the x - ray tube because the interchange pivot is coincident with the combined center of gravity . however , when the film changer has swung to its rotated position fc * its optical axis af is offset from the radiation axis a1 . this offset however prevents the extremity f3 from swinging into the patient zone as indicated by the arrows 15 *. the offset of the film changer axis from the radiation axis is thus corrected by sliding the film changer lengthwise to its final interchange position ii *, and then slides to its final interchange position ii ** maintaining the center of gravity in its initial locus . during rotation the spring 44 holds the extremity f3 of the film changer at a radius from the interchange pivot 16 less than the distance from the interchange pivot to the zone z so as to prevent intrusion of the film changer into the patient zone . thus , whereas prior apparatus for interchange of a film changer and an image intensifier or like x - ray receptor destroyed the balance between the x - ray tube and receptor arms and intruded into the patient zone , such hazards are avoided by the present system of concomitant rotation and sliding which also prevents intrusion into the patient zone and effects alignment of the image face of either receptor in the same image plane on the radiation axis , all while maintaining a fixed locus for the center of gravity of the changer and intensifier , and balance of the system about a rotational axis . it should be understood that the foregoing description is for the purpose of illustration only and this invention includes all modifications and equivalents which fall within the scope of the appended claims .
in medical x - ray apparatus including a stand mounting an x - ray tube and two x - ray receptors such as an image intensifier and a film exchanger , the two x - ray receptors are mounted on a common frame pivoted to present either receptor at the image plane of the x - ray tube . additionally the two receptors slide on the frame and are linked by a bell crank for coordinated rotation and sliding movement which restores their common center of gravity to its initial locus as one receptor is interchanged for the other . balance of the receptors with the x - ray tube on the frame is thereby maintained .