fig1 shows a typical ct scanner having a gantry assembly 10 pivotally mounted on a u - shaped support structure 12 to allow tilting of the gantry assembly plus or minus 20 degrees from the vertical . the gantry assembly 10 carries the x - ray beam and detection equipment as well as the translational and rotational apparatus for manipulating the beam in the plane of the scan circle in the round gantry opening 14 , approximately 20 inches in diameter . the servo motor controllers and associated apparatus which drive the gantry equipment are located in the scan power module 16 immediately behind the gantry 10 . also part of the typical ct scanner , although not shown in fig1 are a display terminal , image processor , heat exchanger , x - ray power supply , scanner console and x - ray controls . these features form part of a standard &# 34 ; delta scan &# 34 ;™ system , currently marketed by the assignee of this application . fig1 and 3 illustrate the physical features of the patient support system for abdominal scanning . the system for brain scans is shown in fig4 and 5 and 6 while a portion of the electronics for automatic table indexing , common to both brain and abdominal scans , is represented in fig7 and 8 . in fig1 and 3 the patient support system for abdominal scans includes three separate elements : a wheeled front table 18 , a smaller , wheeled rear table 20 and a thin , plastic and wood patient pallet 22 . the front table is connected to the gantry support 12 by a detachable , pivotally mounted rigid link 24 , in a position such that the projecting end of the front table barely extends into the gantry opening 14 without intersecting the scan circle plane 26 ( fig2 ). similarly , the rear table is fastened to the gantry support 12 by a rigid detachable ( nonpivoting ) link 28 in substantial alignment with the front table , such that the cantilevered end of the rear table 20 proximate to the end of the front table 20 , barely extends into the gantry opening 14 without intersecting the scan circle 26 . the patient pallet 22 rests on top of and across the stainless steel tops of the front end and rear tables 18 and 20 such that its narrow - waisted section 22a spans the gap between the front and rear tables 18 and 20 . thus the section 22a is the only portion of the patient support system which intersects the scan circle . consequently , besides air , the x - ray beams will encounter only the patient and a small volume of low density material in the patient pallet 22 . the front table 18 comprises a table top 30 with conveyer belt 32 , cantilevered on a box - shaped support 34 housing a dc belt motor and reduction gearing ( not shown ), mounted on a wheeled carriage 36 . the conveyer belt 32 is a single continuous loop with a hinged metal seam ( not shown ) driven by a drive drum 38 and tensioned by belt take up idler rollers 41 . the drive drum is driven by the dc motor which is velocity controlled by a tachometer on the motor shaft and the motor control circuit , all commercially available . the table top 30 supports the conveyer belt at all points along the top path of the belt . the cantilevered end of the table top 30 adjacent to the gantry 10 includes a protruding stainless steel nose portion 39 over which the belt 32 rides . at the other end of the table top 30 , the belt disappears through a slot 40 . conventional side guard rails are used on the front table as well as a failsafe bar 42 at the outermost end of the table top 30 . the failsafe bar 42 actuates a microswitch which shuts down the system to prevent further excursion of the patient along the belt . to accommodate tissue sections of different girth , the front and rear table 18 and 20 have separate elevational controls which allow the table tops to move up and down at a 30 degree angle to insure clearance from the gantry structure 10 when tilted a maximum of 20 degrees forward . precise coplanarity of the tops of the front and rear tables 18 and 20 is not essential because of the flexibility of the pallet 22 . the front table 18 must roll away from the gantry when the table 18 is being lowered to keep the front table from hitting the gantry when the top of the gantry is tilted toward the rear . this is accomplished automatically by pivoting the rigid link 24 at the bottom of the support structure 12 . the top of the rear table 20 supported by linear bearings ( not shown ) is elevated on a 30 ° diagonal toward the gantry 10 by means of a linear drive system 41 with a ball screw and ac motor , while the entire front table 18 including the carriage 36 is raised by ac motor - driven wheel links 43 which pivot the wheels oppositely about respective axes 43a . for abdominal scans , the patient pallet 22 is laid on top of the front table 18 . the patient is positioned on the pallet 22 so that the section of the patient &# 39 ; s body that is to be scanned is over the narrow - waisted part 22a of the pallet 22 . the pallet is then driven by the conveyor belt 32 until the low friction skids 44 engage the stainless steel top of the rear table 20 . the pallet 22 continues to be driven by the belt , with the pallet sliding over the rear table , until the patient is positioned for the first scan . the front and rear tables may be raised or lowered vertically under operator control in order to keep the patient centered in the gantry opening . the patient &# 39 ; s weight on the pallet 22 causes sufficient friction between the high friction surface of the pallet 22 and conveyor belt 32 to allow the palleted patient to be moved back and forth through the scan circle 26 . for brain or head scans , the rear table 20 and patient pallet 22 are not used . instead , as shown in fig4 and 6 , a detachable unitary , restraint assembly 46 is used in conjunction with the front table 18 alone since all of the support for the patient &# 39 ; s head can be accomplished from one side of the gantry 10 . the head restraint assembly 46 includes a rectangular restraint body pad 48 with a safety belt 50 attached to a metal base plate 52 carrying a hinged , thin , plastic , contoured headrest 54 held in place by a ratchet hinge mechanism 56 and raised or lowered by hand . a pair of cheek pad restraint assemblies 58 are mounted on either side of the plate 52 . each restraint assembly 58 includes a vertical pole 60 with an articulated boom - like arm 62 having a pivoted cushion pad support 64 on one end . knurled knobs on a holder 66 allow precise positioning of the cushion pads 68 against the patient &# 39 ; s face as shown in fig5 . for a brain scan , the head restraint assembly 46 is placed on the front table conveyor belt 32 instead of the patient pallet 22 . the patient is placed on a restraint body pad 48 with his head in the headrest 54 . the head is positioned at the prescribed angle by adjusting the headrest 54 . next , the cheek pads 64 are located on the patient &# 39 ; s face and secured in position by means of the holder 66 . the operator then energizes the conveyor belt drive motor , and the upper portion of the belt 32 moves toward the gantry opening 14 until the patient &# 39 ; s head is located within the scan circle 26 . the belt 32 may be moved in or out slowly for more precise positioning by manual controls as discussed below . the front table can also be raised or lowered separately to center the head tissue section in the scan circle 26 . for either brain or abdominal scains , the conveyor belt 32 provides axial positioning of the patient . the functions of the belt drive system are to advance the patient further into or retract the patient from the gantry opening , to recognize an extreme limit of travel in either direction and , in the automatic mode , to index the patient further out of the gantry opening by preestablished intervals , for example , 26 millimeters , after each one of a series of complete scans by the ct scanner . the latter function is the only one which requires a control signal from the image processor ( not shown ) of the ct scanner . this signal , designated for explanation herein as &# 34 ; scan over ,&# 34 ; is a signal internally produced by the image processor indicating that the procedure for one scan of the scan circle has been completed , the scanner is idle and the patient may be moved to another location for the next scan . the other functions reside solely in the table itself . in fig7 the table belt drive consists of a dc motor 70 driving a gear reducer ( not shown ) connected to the belt via a chain driven drum 38 ( fig2 ). the dc motor 70 is bidirectional and is driven by positive and negative high and low dc voltages to produce slow or fast forward or reverse rotation , i . e ., inward , toward the gantry opening , or outward , from the gantry . the belt motor 70 is operated by a conventional belt motor controller 72 such as that manufactured by cleveland machine control , inc ., or graham , inc . typical voltages are + 0 . 7 volts dc and + 10 volts dc for slow and fast outward belt motion , respectively , and - 0 . 7 volts and - 10 volts dc for slow and fast speed inward belt motion . manual belt control 74 can be employed by the operator ( see fig2 ) to energize the belt motor 70 via a selectable analog control voltage source 76 to which the motor controller 72 is responsive . the metal seam joining the ends of the continuous belt 32 is designed to be located below the top of the table 30 in fig2 at all times to insure patient comfort and to utilize the seam in a limit switching function . metal proximity sensors 78 and 80 are located at either end of the table top 30 to sense the passage of the metal seam . the seam travels between the sensors 78 and 80 on the underside of a loop and are never indexed to the top of the table . accordingly , the seam sensor 78 indicates the outer limit of travel of the belt along the top of the table away from the gantry opening 14 . likewise , the seam sensor 80 establishes the inner limit of travel of the belt on the top of the table toward the gantry opening 14 . in either case , the sensing of the seam causes respective one shot ( monovibrator ) circuits 82 and 84 in fig7 to actuate respective magnetic latching relays 86 and 88 . the relays 86 and 88 remain latched even if power is turned off to insure that the machine will remember that it is at the limit when it is turned on the next time . if the belt is at its outer limit , the logic output of the latching relay 86 produces an input to the selectable analog control voltage circuit 76 to disable the production of the belt out signal at either the fast or slow speed . however , operating the belt manually in the in direction automatically resets the relay 86 . similarly , if the belt has reached the inner limit , the corresponding relay 88 disables the belt in signal , and operation of the belt in the outward direction automatically resets the relay 88 . the position of the belt 32 is indicated to the operator by a belt travel indicator system 90 . a table position incremental rotary encoder 92 ( fig2 ) dirven by the belt near the drive drum 38 on the underside of the table top 30 provides distance information as to how far the belt has moved . the encoder circuit 92a ( fig7 ) interprets belt travel by creating phased encoder a and encoder b signals as shown in fig8 . these signals are directionally encoded to insure that a pulse for every millimeter of belt travel will occur at the same absolute position of the encoder ( belt ). according to the direction of rotation , the encoder b output enables a trailing edge pulse on the a output to trigger the count up one - shot ( included in enocder circuit 92a ) and the b output enables a leading edge pulse to trigger the count down singal . the count up and count down signals are labeled tb up and tb dn . in fig7 the tb up and tb dn signals are sent to the increment and decrement inputs respectively of the digital up / down counter 94 . the output of the counter 94 is passed via a decoder / driver circuit 96 to a numerical display 98 located on the table top 30 alongside the belt 32 as shown in fig1 . the indicator 98 shows , preferably in millimeters , how far the belt has moved from wherever the operator depressed the zero reset button for the first scan . this allows the operator to position a patient for the first scan a precise distance from a convenient or customary anatomical reference , e . g ., the o - m line on the head , and then by operating the manual table controls to move the patient a measured distance to a second scanning position . this procedure is used for either brain or abdominal work . regardless of the reading on the table display , if the seam sensor 78 indicates that the outer limit has been reached , the corresponding output of the latching relay 86 will advise the decoder / driver 96 to display an &# 34 ; h &# 34 ; corresponding to the &# 34 ; home &# 34 ; position . likewise , the output of the latching relay 88 is used to indicate that the inner limit has been reached by displaying an &# 34 ; i .&# 34 ; it is often desirable to make numerous adjacent scans . iterative incrementing of the axial position of the patient can be carried out automatically . since ct scanning systems require software capabilities for processing images , the computer which processes the images can be borrowed to serve as a programmable controller for the automatic table out function since it requires minimum storage . thus , the programmed computer in the image processor would interrogate the console switches and at the appropriate time in the scan procedure , signal the belt drive to start moving in the out direction . when the digital distance information from the encoder on the belt matches the programmed distance in the computer , the computer would signal the belt drive to stop . this operation is represented in fig7 in an operative hardware form for the sake of explanation of the functional requirements . the counting , comparing and switching functions , of course , can be carried out by software , if desired . the system of fig7 causes the belt to move in the outward direction at slow speed for a predetermined distance after each successive scan has been completed . the pulses which increment the counter 100 are the tb dn ( belt out ) pulses form the table position encoder circuit 92a . when the image processor determines that the scan procedure at a given location is complete , the processor issues a scan over signal . this logic signal resets the counter 100 to zero . the scan over signal is also connected to the set input of an r - s flip - flop 104 . changing of the scan over line from low to high sets the flip - flop 104 input labeled auto table out which is passed to the selectable analog control voltage circuit 76 which causes the belt to begin to move slowly in the outward direction under the control of the belt motor controller 72 . while the belt is moving , tb dn pulses are being produced by the encoder circuit 92a . each one of these pulses increments the counter 100 one unit . the output of the counter 100 is fed to a comparator 102 provided with a reference count by the latch 103 . the belt continues to move and the counter 100 continues to count up until its count matches the reference count and the comparator 102 issues a stop signal . the stop signal is passed via an or gate 106 to reset input of the flip - flop 104 , resetting the flip - flop and removing the auto table out signal to stop the belt motor 70 . the stop signal from the comparator 102 is also fed back to the image processor to indicate to the ct scanner that the patient is ready for the next scan . as a precaution , a timer 107 started by the auto / out signal will reset the flip - flop 104 after a maximum allowable interval . without any other type of control function , this incrementing of the patient &# 39 ; s axial position and the taking of successive scans could continue until the seam sensor 78 senses the outer limit . at that point , the output of the latching relay 86 through the or gate 106 would reset the flip - flop 104 to remove the auto table out signal even if the comparator 102 had not yet issued its stop signal . in addition , the outward motion of the belt 32 would be disabled directly by the disable belt out signal to the analog control voltage circuit 76 . as a further precaution because of the use of an automatic mode for patient transport , a failsafe bar switch 42a can be tripped by the patient &# 39 ; s feet at the end of the table to remove the auto table out signal and to disable belt motion in the outward direction . instead of allowing the circuitry to increment axial position indefinitely up to the limit , the control system can lock after a predetermined number of increments , for example , by counting stop signals . this implementation would allow the operator to determine a central plane , for example at 520 millimeters , and then back up six increments and set the number of increments at 13 in order to cover an equal volume of tissue in front of and behind the central plane . the invention may be embodied in other specific forms without departing from its spirit or essential characteristics . the present embodiments are , therefore , to be considered in all respects as illustrative and not restrictive , the scope of the invention being indicated by the claims rather than by the foregoing description , and all changes which come within the meaning and range of the equivalents of the claims are , therefore , intended to be embraced therein .
a versatile patient table system for transverse axial scanners employs front and rear tables with a spanning patient pallet for abdominal scans . a motorized conveyer belt on the front table increments the axial position of the patient . for brain scans the rear table and patient pallet are replaced by a head restraint assembly with a hinged headrest , check pad restraint members and a body pad which rests on the conveyer belt on the front table . in the automatic mode of operation , the belt moves either the body pad or the patient pallet out from the scanner in controlled increments triggered by the image processor . the hinged metal seam of the belt is used for indexing the extreme outward and inward positions .