Patient sensing and indicating arrangement for a computed tomography system

An arrangement is provided for sensing and indicating if a cross section of a patient extends beyond an image reconstruction circle during examination. The arrangement is positioned within a generally vertical gantry having a generally cylindrical opening for receiving the patient. The arrangement includes a plurality of light emitting sources disposed within the gantry and which are closely adjacent the reconstruction circle. Each light emitting source projects a light beam along a line which is parallel with the plane of the reconstruction circle and tangent to a cylinder having a diameter and central axis in agreement with the reconstruction circle. The light beams extend substantially across the opening in the gantry and generally inscribe the reconstruction circle. A plurality of photodetectors are disposed within the gantry and aligned to receive each of the light beams. The photodetectors are interconnected to the system by means for indicating an interruption of any of the light beams between any of the sources and detectors.

BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
This invention relates generally to a system for performing computed x-ray 
tomography and more particularly to an arrangement for precisely sensing 
and indicating that a patient extends beyond an image reconstruction 
circle of the system. 
2. DESCRIPTION OF THE PRIOR ART 
In one method of computed tomography a patient is supported for being 
translated along a longitudinal axis which is usually horizontally 
disposed. The axis coincides with the center of rotation of a rotatable 
gantry which has an x-ray source on one side of the center of rotation and 
a multiple array of x-ray detectors on the other side. A fan shaped x-ray 
beam that is thin in the longitudinal direction is projected through the 
patient as the gantry rotates so that the detectors may develop signals 
indicative of x-ray transmission characteristics along a plurality of 
paths through the patient undergoing examination. Analog signals 
representative of x-ray attenuation by all of the volume elements in a 
layer of the patient at various rotational angles are then converted to 
digital signals which are used by a computer to produce signals which may 
thereafter be used to produce a reconstructed image of the layer. The 
image is reconstructed by analyzing all of the signals within a central 
cross-sectional area of the system known as the "reconstruction circle." 
The analog signals are analyzed and converted based upon all of the x-ray 
attenuation taking place within the reconstruction circle. If the cross 
section of the patient is either too large or improperly positioned so 
that a portion of the patient extends outside of the reconstruction 
circle, the x-ray attenuation of that portion will be erroneously 
interpreted. The resulting reconstructed image might not be clear or 
accurate and could require reexamination, or could even provide 
misinformation. It is therefore extremely important to know, before the 
scanning process, whether the patient is properly positioned within the 
reconstruction circle to alleviate the necessity of re-examination. 
One method of sensing an oversized or improperly positioned patient 
involves the application of mechanical templates and sizing devices 
physically over and around the patient while the patient is in the staged 
position. Such devices are awkward to handle, time consuming and are 
typically removed prior to transaltion of the patient into the system. The 
patient-oversize checks by this method are not always performed closely 
adjacent to the scan plane. There is time for the patient to shift out of 
position, between the time of checking and the time of scanning. 
A problem associated with the prior art is that the oversized patient check 
is not done automatically and can be overlooked. Another problem is that 
there is no opportunity to monitor the patient once in the gantry and 
between successive scans to insure that the patient remains within the 
reconstruction circle. 
There are situations where the scans will be desired by the physician or 
radiology technician even though the patient may extend beyond the 
reconstruction circle. It is important however that these situations 
always be acknowledged and recorded for proper caution in interpretation 
of the scans. 
Accordingly, one object of this invention is to provide an arrangement to 
automatically sense and indicate that a patient extends beyond an image 
reconstruction circle of a computed tomography system. 
Another object is to monitor the size and position of the patient over the 
area to be examined immediately prior to initiating the scanning 
operation. 
Another object is to monitor the size and position of a patient by an 
arrangement which does not interfere with the patient or the scanning 
operation and is immediately adjacent to the scan plane of the system. 
Another object is to indicate an oversized patient condition on a control 
panel of the system. 
Still another object of this invention is to provide an arrangement to 
indicate the presence of an oversized patient on an operator's console 
which requires an operator response, such as, to either abort the scan or 
override and continue with the examination. 
SUMMARY OF THE INVENTION 
The invention is directed to an arrangement for sensing and indicating if a 
cross section of a patient extends beyond an image reconstruction circle 
during examination by a computed tomography system. The tomography system 
includes a source of an x-ray beam and a detector disposed on opposite 
sides of a generally vertical gantry. The terms x-ray and x-ray source are 
used herein for the sake of brevity and convenience, but these terms 
should be construed as embracing gamma radiation and the gamma sources and 
other penetrating radiation and sources as well. The gantry has a 
generally cylindrical opening for receiving the patient. The patient is 
supported on a generally horizontal table which is translatable along the 
longitudinal axis generally centered within the opening of the gantry. The 
table is displacable for the purpose of disposing longitudinal successive 
layers of the patient into the path of the x-ray beam. 
The sensing and indicating arrangement is generally described as at least 
one light beam which is generally tangent to the reconstruction circle and 
which is directed to a corresponding number of photo sensors which signal 
an interruption of any of the light beams. The arrangement comprises at 
least one light emitting source disposed within the gantry and which is 
closely adjacent the reconstruction circle. Each light emitting source 
projects a light beam along a line which is generally parallel with the 
plane of the reconstruction circle and generally tangent to a cylinder 
having a diameter and central axis in agreement with the reconstruction 
circle. A corresponding number of photo detectors are each disposed within 
the gantry and aligned to receive each of the light beams. The 
photosensors are interconnected to the system by means for indicating an 
interruption of any of the light beams between any of the sources and the 
detectors.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIGS. 1 and 2 there is shown a portion of a computed 
tomography system 10. A table for supporting a patient undergoing 
tomography examination is generally designated by referenced numeral 12. 
The table has a top 14 on which a patient may be supported in a recumbent 
position. Top 14 is translatable in the longitudinal direction so that it 
extends in cantilever fashion from table 12 for the purpose of disposing 
successive longitudinal layers of a patient's body in the path of a 
scanning x-ray beam. The mechanism for advancing and retracting the 
patient longitudinally is within the housing of table 12 and is not shown 
in detail since it forms no part of the present invention. 
X-ray scanning and obtaining x-ray attenuation data on a multitude of small 
volume elements in the patient is carried out with components of a gantry 
which is generally designated by the referenced numeral 16. Gantry 16 is 
generally vertical having a cylindrical opening 18 for receiving the 
patient for examination. The gantry 16 includes a base 20 about which the 
components of the gantry can be tilted about a transverse axis and rotated 
about a central longitudinal axis 22. 
Basic elements of the gantry 16 are shown in FIGS. 3 and 4 attached to 
rotatable structure 24. Rotatable structure 24 includes bearing structure 
25 for rotation thereon about longitudinal axis 22. In general terms, 
rotatable structure 24 has a frame comprised of side members 26 and 28, a 
top cross member 30 and a bottom cross member 32. An x-ray source is 
mounted to cross member 30 by means of a bracket assembly 36 and 
positioned so that the central ray of the x-ray beam eminating from the 
x-ray source passes through the longitudinal axis 22. Mounted to the 
output side of x-ray source 34 is a collimator device 38 which in this 
particular design collimates the x-ray beam into a thin fan-shaped beam 
which originates from a substantially point source on the target of an 
x-ray tube in source 34. In a commercial embodiment, by way of example, 
the longitudinal thickness of the x-ray beam is about 1 centimeter and a 
slice or layer of corresponding thickness in the patient is penetrated by 
the beam for each scan. An x-ray detector and data acquisition assembly 40 
is mounted to bottom cross member 32 and is aligned with collimator 38. 
The detector portion has a curved x-ray input window behind which there is 
an array of adjacent detector cells, which are not visible. Each cell 
produces output signals corresponding with the intensity of the plurality 
of rays that radiate from the x-ray source and penetrate continuous series 
of volume elements of a patient layer that is disposed along the 
longitudinal axis during x-ray examination of the patient. A suitable 
detector is shown in U.S. Pat. No. 4,031,396 by Whetten et al and assigned 
to the assignee of this application. Other detector forms are known to 
those skilled in the art. The descreet plurality of analog signals 
representative of x-ray attenuation by small volume elements in the 
patient are processed in a data acquisition system after which the analog 
signals are converted to digital signals which are used by a computer, not 
shown, to execute the image reconstruction algorithm. The signals and 
reconstruction algorithm are computed based upon all of the x-ray 
attenuation taking place within a central area shown as a dashed line and 
known as the reconstruction circle 42. Reconstruction circle 42 is 
concentric with the central axis of rotation of gantry 16 which is also 
longitudinal axis 22. The reconstruction circle of this particular 
embodiment has a diameter of 42 centimeters. The above described computed 
tomography system is generally known in the art. 
As previously discussed, it is important that the patient be centered 
within the reconstruction circle during scanning by the system. A suitable 
arrangement for facilitating central alignment of the patient in a staged 
position is described in U.S. Pat. No. 4,117,337, entitled "PATIENT 
POSITIONING INDICATION ARRANGEMENT FOR A COMPUTED TOMOGRAPHY SYSTEM" and 
filed concurrently herewith in the name of P. Staats and assigned to the 
same assignee as this application. However, even though a patient may be 
centrally aligned with the reconstruction circle 42 at a staged position, 
the patient may shift out of position or be too large and extend beyond 
the reconstruction circle. In order to precisely indicate if a portion of 
a patient extends beyond the reconstruction circle a sensing arrangement 
is provided and is a principle feature of this invention. 
The arrangement is characterized by a set of sources and sensors, such as a 
photo cell system as described below, inscribing the reconstruction circle 
42. Any energy source which can be directed and sensed, such as 
electromagnetic, infra red and other portions of the energy spectrum could 
be provided to inscribe the desired area, and the energy beam sources are 
not limited to light sources. The arrangement of this preferred embodiment 
comprises light emitting sources 44, 46 and 48 projecting light beams 50, 
52 and 54. The light sources are mounted on supporting arms 56 and 58 so 
that each light beam is projected along a line which is generally parallel 
to the plane of reconstruction circle 42 and generally tangent to a 
cylinder in space having a diameter and central axis 22 in agreement with 
the reconstruction circle. In this preferred embodiment, light source 48 
is positioned by the supporting arm 58 so as to project light beam 54 
generally horizontally directed substantially across opening 18 and 
tangent to reconstruction circle 42. Light sources 44 and 46 are 
positioned by support arms 56 and 58 so as to project light beams 50 and 
52 generally vertically directed substantially across opening 18 but which 
are 2.5 centimeters behind the plane of reconstruction circle 42 so as not 
to interfere with the x-ray beam eminating from collimator 38. A bottom 
horizontal beam is not necessary since the patient is confined by 
supporting table top 14 and thereby the lower sector is always within the 
reconstruction circle. The three light beams 50, 52 and 54 generally 
define three sides of a square having sides of 42 centimeters and inscribe 
the 42 centimeter reconstruction circle. When rotatable structure 24 is 
rotated with gantry 16, the light beams define a 42 centimeter circle. A 
suitable light emitting source is known as the Skan-a-matic L 130 which is 
a #12 bi-pin 6.3 volt and 0.18 amp, lamp. 
Corresponding detectors 60, 62 and 64 are also mounted to support arms 56 
and 58 and are aligned to receive each of said light beams at the opposite 
side of opening 18. A suitable photodetector is commercially available as 
the Skan-a-matic P 130 having a rise and fall time of 3 microseconds and a 
forward current of 20 milliamps. The light emitting sources 44, 46 and 48 
and photodetectors 60, 62 and 64 are electrically wired in parallel so 
that an interruption of any of light beams 50, 52 or 54 would produce a 
signal indication. 
The three mutually perpendicular light beams 50, 52 and 54 inscribing the 
reconstruction circle 42 and substantially in the same plane as the 
reconstruction circle is the preferred embodiment of the arrangement. The 
location of the light beams within the gantry also serve to automatically 
reduce the sensing height correspondingly with the reduced height of the 
reconstruction circle when the gantry is in the tilted position. However, 
a variety of positions and quantity of sources and detectors could 
alternatively be arranged to inscribe the reconstruction circle. A single 
light beam (as 52), which is tangent to or closely adjacent to one point 
of the reconstruction circle 42 could be rotated to make a perimeter check 
of the entire reconstruction circle. 
The most likely locations for a patient to extend beyond the reconstruction 
circle is at either side. The recumbent body tends to be horizontally 
oval-shaped in cross section and also the arms can sometimes be extended 
to the side and out of the circle. As an alternative arrangement, two 
vertically tangent beams (such as 50 and 52) and which would not even need 
to rotate, would provide a sensing arrangement with some degree of 
reliability. 
Another set of light beams could alternatively be arranged forward of 
gantry 16 and in the path of the translatable tabletop 14 along 
longitudinal axis 22. The forwardly arranged beams would be tangent to a 
cylinder having a diameter and central axis in agreement with the 
reconstruction circle 42 and thereby sense if the patient extended beyond 
the circle as the patient was translated adjacently beside the beams and 
into the gantry. 
Referring to FIG. 5 a block diagram describes how the arrangement is 
incorporated into the tomography system. The patient sensing and 
indicating arrangement of this invention is designed to inform the 
computer and the operator if the patient's anatomy under examination is 
not within the reconstruction circle. The arrangement is incorporated 
electrically into the control panel, and into the software of the computer 
of the system. The lamp and detector arrangement is activated prior to 
advancing the patient into the scanning position. The system in this 
preferred embodiment is designed so that the patient is translated into 
the deepest scan, through the sensing arrangement, and then successively 
sequenced out of the gantry for the additional examining scans. The above 
sequence allows the entire area of interest to be automatically sensed, 
and confirmed to be within the reconstruction circle, prior to initiating 
the first scan and thereby generally precludes interruption of the 
examination. The arrangement remains activated throughout the examination. 
The block designated "lamps and detectors" 66, represents the light beam 
arrangement wired in parallel so that an interruption of any light beam 
will produce a signal to "logic" circuitry 68. The logic circuitry 68 then 
initiates and "indicator signal" 70 on the control panel, initiates a 
signal to the "computer interface board" 72 and initiates a signal to 
inhibit the "control of x-ray scan" 74. The tomography scanning operation 
is thereby discontinued at this point and computer interface circuit board 
72 prints a "patient oversize" statement at the computer terminal 76. The 
operator now has two choices, one is to go ahead with the scan and include 
the patient oversize information for that particular scan file, and two is 
to abort the current scan and try scanning again. The system will not 
continue unless an operator responds to the statement with a command such 
as "override" at computer terminal 76. If the override command is given, 
circuit board 72 will transmit this statement through the logic to 
continue operation of the system even though the patient may be extended 
beyond the reconstruction circle. 
Alternatively, the system could be arranged to automatically abort the 
scanning operation at any time after the detectors sense that a portion of 
the patient extends beyond the reconstruction circle. An alternative 
system could also incorporate a simple override or reset switch on the 
control panel which could reactivate the system and continue with the 
scanning examination. 
As an additional safety feature, once the detectors signal to the logic 
that a portion of the patient extends beyond the reconstruction circle the 
logic also initiates a signal to the elevation drives (not shown) of the 
table and the tilt drive (not shown) for the gantry to prohibit any 
movement of the table or the gantry as long as the oversize condition 
exists. This prevents movement of the table or the gantry which might 
bring the patient in contact with the surface of the cylindrical opening 
(16) of the gantry. 
The arrangement of this invention automatically senses and indicates if a 
patient extends beyond an image reconstruction circle of a computed 
tomography system. The arrangement does not interfere with the patient or 
the scanning operation and is immediately adjacent the scan plane of the 
system to monitor the size and position over the area of the patient to be 
examined, immediately prior to initiating the scanning operation. If a 
portion of the patient is found to extend beyond the reconstruction circle 
the arrangement cooperates with the software of the system to provide an 
oversized indication lamp on the control panel and to initiate a statement 
to the computer terminal which requires an operator response in order to 
continue the examination. 
While specific embodiments of the present invention have been illustrated 
and described herein, it is realized that modifications and changes will 
occur to those skilled in the art. It is therefore to be understood that 
the appended claims are intended to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.