WO2014158080A1

WO2014158080A1 - Method for accurate positioning of the detector in a mobile imaging system

Info

Publication number: WO2014158080A1
Application number: PCT/SE2014/050344
Authority: WO
Inventors: Dianna BONE
Original assignee: Adolesco Ab
Priority date: 2013-03-27
Filing date: 2014-03-20
Publication date: 2014-10-02
Also published as: SE537029C2; SE1350388A1

Abstract

The invention relates to amethod for adjusting the position of the detector in a mobile imaging system. The system comprises a rotatable detector (14a) with a collimator (15) having collimator holes arranged at a slant angle (σ) and control electronics (12'). The method comprises obtaining, by using the detector, at least two, preferably three projections of the object to be imaged from different directions, by rotating the detector through an angle (α) between each projection acquisition. Then the projection coordinates for the centre of gravity of the object in the respective projectionare identified. The actual position of the imaged object using the coordinates in said projectionsis calculated. Also, the displacement of the detector from the centre of gravity of the system field of view iscalculated, and the detector position is adjusted by said displacement.

Description

METHOD FOR ACCURATE POSITIONING OF THE DETECTOR IN A MOBILE

IMAGING SYSTEM

The present invention relates to myocardial scintigraphy in general, specifically to a mobile system for inter alia enabling imaging in acute situations without the need of moving a patient. Primarily it relates to a method and means for positioning the imaging system accurately.

Background of the Invention

Several studies have shown that myocardial scintigraphy is the most efficient method for the diagnosis and prognosis of ischemic heart disease. As an example this technique has exhibited a sensitivity of 96% for coronary stenosis compared to a 35% sensitivity using rest EKG in contrast to using biomarkers such as troponin I, myocardial scintigraphy can detect or exclude ischemic regions and infarctions in an earlier stage of the disease progression. This allows a considerably faster decision as to which intervention is to be carried out/ performed.

Despite the advantages with myocardial scintigraphy the technique is used relatively infrequently in the diagnosing of acute ischemic heart disease, e.g. in an emergency care ward. One of the main reasons for this is thought to be the fact that an installed Single-photon emission computed tomography (SPECT) system is stationary and it is not always possible to move a patient to the location where such a system is placed.

In the 1980s it was demonstrated that it is possible to generate tomographic images from projections taken at an oblique angle. This new technique was referred to as ectomography. It differs from conventional SPECT imaging where the gamma camera, which is used as the detector, rotates at least 180° around the patient, by the provision of a collimator having slanted holes rotating around its own axis. The imaged layer will thus differ between the two techniques, resulting in SPECT being more useful to image organs located deeper inside the body, whereas ectomography is more useful for the imaging of smaller organs located closer to the body surface. Summary of the Invention In view of the drawback of the prior art SPECT systems, i.e. the stationary character thereof requiring patients to be moved to the location of the SPECT apparatus for investigation, the present inventors have devised a novel imaging apparatus, based on the ectomography principle. The novel apparatus is mobile, making it particularly useful for acute and emergency situations, and is optimized for three-dimensional heart investigations.

The object of the present invention is to provide a method for accurately positioning of the detector in the novel imaging system.

The novel method is defined in claim 1 , and comprises obtaining, by using the detector, at least two, preferably three projections of the object to be imaged from different directions, by rotating the detector through an angle (a) between each image recording; identifying the coordinates for the centre of gravity of the object in the respective projection; calculating the actual position of the imaged object using the coordinates of said projections; calculating the displacement of the detector from the centre of gravity of the system field of view; and adjusting the detector position by said displacement. The positioning method and system comprises several features that renders it advantageous, and these features are the subject of dependent claims.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus not to be considered limiting on the present invention, and wherein

Fig. 1 shows a mobile imaging system; Fig. 2 is a view of the mechanical positioning device;

Fig. 3a shows the positioning device in inoperative state;

Fig. 3b shows the positioning device in operative state; Figs 4a-e illustrates the method of determining the necessary adjustment of the detector;

Fig. 5 illustrates means for operating the wheels of the system;

Fig. 6 is an example of a slant hole collimator; and

Fig. 7 is a schematic illustration of the method in a set-up with a patient. Detailed Description of Preferred Embodiments

In order to provide optimal imaging it is required that the heart (or other imaged object) is located within the system field of view. However, it is difficult to position the detector at an exact and correct position and readjustment of the apparatus some small distance such as one or a few centimetres is mostly necessary.

Therefore, the main object of the present invention is to provide accurate positioning of the imaging apparatus by fine-tuning/ adjusting the position of the detector in order to optimize the imaging procedure. To meet this object there are provided three features that each separately or together are usable to optimize the positioning.

The first feature is a visual interface comprising at least two projections of the heart taken at different angles, and a control unit/ calculation unit which enables the determination of the optimal detector position and subsequent accurate positioning of the detector. In order to enable to make a final accurate position adjustment of the detector with respect to the heart there is provided a method and device which on the basis of at least two, preferably three, different projections of the heart acquired at different collimator rotation angles, enables very fine adjustment of the detector position to a correct position. This is herein referred to as a "sighting system" and "sighting method" and will be described with reference to Figs. 4a-e.

Another feature is a mechanical position adjustment unit designed as a means to substantially reduce the force required to move the rather heavy equipment in small increments for the adjustment of the position of the detector. This mechanical unit is disclosed in Figs. 1-2, and will be described in detail with reference to these Figs. A further feature, which is optional, is the provision of an automatic positioning of the detector when the optimal position has been determined by the control unit.

Fig. 1 shows schematically an embodiment of an entire mobile imaging system generally designated 10. It comprises a chassis 11a and an essentially vertically arranged frame l ib, a main cabinet 12 housing control electronics 12', a display 13, a detector 14a, suitably a so called gamma camera (or scintillation camera also referred to as an Anger camera) mounted on an essentially horizontal beam 14b mounted to the frame 1 lb, a rotatable collimator 15 having a plurality of holes arranged at a slant angle o of typically 30 degrees (an example of a slant hole collimator is shown in Fig. 6), a height adjustment unit 16 and a mechanical position adjustment unit 17 for fine tuning of the detector position. The system also has a front wheel pair FW and a rear wheel pair RW. The detector in Fig. 6 comprises a processor P, an amplifier AMP, photomultipliers PM, a scintillator crystal SC, a Slant Hole Collimator SHC made of lead, and schematically an organ is shown into which a substance emitting gamma radiation has been introduced. For the purpose of this application "Y direction" refers to the longitudinal direction of the system, and "X direction" is a transverse direction with respect to the system, whereas "Z direction" is a direction perpendicular to the detector surface.

The system is generally used as follows. Since it is provided with wheels i.e. mobile and not too big, it is easily and quickly moved to a location where a patient is, instead of moving the patient. When the mobile system arrives at the patient the front of the chassis 11 is run in under the bed on which the patient lies. The detector 14a is provided at a nominal height such that it always goes clear of the patient in this operation. When it has been roughly positioned, the position adjustment system is activated. The detector 14a is then properly positioned within just a few minutes and images can be acquired.

Now the mechanical positioning unit 17 will be described in detail with reference to the schematic illustrations in Figs. 2 and 3a-3b. As can be seen in Fig. 1 the mechanical positioning unit 17 is preferably located on the chassis 1 la at the underside thereof, in the vicinity of the front part of the entire apparatus, i.e. at a position essentially vertically beneath the detector 14a and collimator 15. In operation the mechanical positioning unit 17, which functions like a jack, is activated such that a support member will be lowered using e.g. hydraulics to be brought in contact with the ground or floor. Continued activation will provide an upward force, thereby lifting the front part of the system. Thus the front wheels will no longer rest against the ground. This is illustrated in Figs 3a and 3b. In Fig. 2 the details of the positioning unit 17 are shown.

The positioning unit 17 is essentially an "X/Y" table, which is a well known

mechanism per se. Here the unit comprises a pair of members referred to herein as slide blocks, a bottom slide block 19 and a top slide block 20. Each slide block comprises two parts. The bottom slide block 19 has a lower part 19' resting on the floor in operative position, i.e. when the unit 17 has been lowered so as to lift the chassis 11, and an upper part 19" slideably coupled to the lower part. In the same manner the top slide block 20 comprises a lower part 20' and an upper part 20" slideably coupled to each other. In one embodiment the coupling is a rail-like structure. However, any arrangement providing low friction slideabililty is applicable. Thereby the respective upper and lower parts of each slide block exhibit a very low friction between them, which renders them easily movable without applying much force.

Between the upper part 19" of the lower slide block 19 and the lower part 20' of the top slide block there is provided a bearing arrangement 21 (rotary bearing) enabling the two slide blocks to rotate with respect to each other. This bearing arrangement is essential to the function, since it will accommodate the angular displacement of the two slide blocks when the detector unit is moved laterally (X-direction) . In this way it allows easy X-Y movement of the entire system such that it enables accurate

positioning of the detector and collimator unit 14a, 15. When a correct position has been arrived at the system is locked in that position during image recording. In operative position the upper part 20" of the top slide block 20 is rigidly connected to the chassis 11. The lower part 19 Of the bottom slide block 19 is resting against the ground or floor, and is thus stationary during position adjustment. Preferably the slideblocks 19 and 20, respectively, are arranged perpendicularly with respect to each other if they have non-square geometry.

Thus, when the adjustment device 17 is in operative position, as shown in Fig. 3b, the low friction rotary bearing arrangement 21 will enable small movements with very little force applied, and the displacement of the entire system with respect to the ground will be accommodated by the slidable coupling between the parts of each slide block 19, 20.

There is also provided a mechanism for lowering the adjustment unit 17. This mechanism comprises in one embodiment a linear actuator 23, which can comprise a hydraulic, pneumatic or electric actuator coupled to an actuating bar 24 that is also coupled to the upper part 20" of the top slide block 20. The upper part 20" of the top slide block 20 is also coupled to the chassis 11 via link units 25 in the form of yokes pivotally connected to the upper part 20" and to the chassis 11, as can be clearly seen in Figs. 3a and 3b.

Thus, in non-operative position (Fig. 3a) the entire position adjustment unit 17 is retracted such that it rests essentially against the bottom of the chassis, or at least such that it goes clear of the ground or floor when the mobile imaging system is transported. When the linear actuator 23 is energized it will pull on the upper part 20" of the top slide block 20, whereby the entire unit 17 will swing downwards by virtue of the linking yokes 25 being pivotally connected as shown.

When the lower part 19' of the bottom slide block 19 hits the ground, the continued pulling exerted by the linear actuator 23 will cause the front of the entire mobile imaging system to raise such that the wheels goes clear of the ground with about 5 mm (Fig. 3b), very much like the function of a common jack.

As already mentioned, in this elevated position it is very easy to adjust the detector using very little force. Now the sighting method and system will be described in detail with reference to Figs. 4a-e.

Thus, the mobile imaging system that comprises a chassis 11a having a front end and a rear end, the front end being configured to be insertable under a bed on which a patient is located, a rotatable detector 14a with a collimator 15, attached to the chassis such that it can be positioned over the patient in the bed configured to register images of an internal organ such as the heart of the patient, also comprises control electronics 12', a device for accurate positioning of the detector, comprising a presentation unit 13 for displaying at least two, preferably three projections of the object to be imaged viewed from different directions, said projections obtained with the detector 14a; one screen marker or cursor is displayed on the presentation unit for each displayed projection, movable to a position essentially centred over the object in the corresponding projection; means for positioning at least two of said cursors essentially centred over the object in the corresponding projection; means for calculating the projected coordinates of the object centre i.e. the point, where the projected lines from two markers cross, the lines having the same directions as the corresponding collimator holes; means for calculating the displacement required for moving the detector to a position where the projected object centre essentially coincides with the centre of the system field of view and means for adjusting the position of the detector by moving the detector the calculated respective displacements in X and Y directions.

Preferably, the means for calculating displacement is configured to calculate the distance between the detector surface and the object centre as Z=r*tan(o) where r is the distance between one of the markers and the projected object centre and o the angle between the detector surface and the corresponding collimator holes, whereby the required displacement in the Z direction is dZ=R*tan(sigma)*0.5-Z where R is the detector radius, and wherein the means for adjusting the position of the detector also is configured to enable movement of the detector in said Z direction.

Suitably said cursor is a ring, the size of which is preferably adjustable, or at least large enough to enclose the projection of the heart. Thus, in order to enable to make a final accurate position adjustment of the detector with respect to the heart there is provided a method and device which on the basis of at least two, preferably three, different projections of the heart acquired at different collimator rotation angles, enables very fine adjustment of the detector position to a correct position. By using only two or three projections instead of all the projections necessary for a complete image, the imaging during the position adjustment can be performed much more rapidly than the acquisition of a complete image thus simplifying and speeding up detector alignment. For proper image acquisition it is important that the object to be imaged is within the system field of view. In limited view tomography the field of view is a cone, the frustum of a cone, a double cone or the frustum of a double cone with its axis coincident with the collimator axis of rotation. The position of an object relative to the detector is uniquely determined by its positions in two different projections acquired with different collimator rotation angles. If the object is visible in and contained within three different projections acquired with three different collimator rotation angles it will, due to the circular symmetry of the field of view, be within the system field of view.

The preferred embodiment of the sighting system presents three different projections of the object in question, herein exemplified by the heart muscle, acquired with the three collimator rotation angles al, a2 and a3, typically 0, 120 and 240 degrees, on a monitor screen. The display on the monitor M for a sequence of operations is shown in Figs. 4a-e.

Furthermore three markers, here represented as rings 41, 42, 43, which is a preferred embodiment, are presented on the monitor screen. Two of these markers can be moved like cursors on the monitor screen using an input device such as a computer "mouse". In particular they can be moved so as to be centred over the projection of the heart muscle shown on the monitor. It is not strictly necessary to use rings, any marker that can be positioned so as to be centred over the object in question, e.g. a heart, on the screen would do. The position of the third marker is automatically computed in a manner described below. Thus, each marker is associated with one of the projections of the heart, the projections being designated PI, P2 and P3, respectively in the following. In Fig. 4a three projections PI, P2, P3 are shown on the monitor M. Here the projected heart is ideally positioned and fully within the field of view. One can see that the centres of the three projected hearths form an equilateral triangle (indicated with a broken line) with its centre of gravity at the centre of the detector.

In Fig. 4b a typical situation before positional adjustment has been achieved is illustrated. Two of the projections do not fit fully inside the field of view.

In Fig. 4c two markers 41, 42 have been placed on the projections PI and P2, respectively. In 4d a third marker 43 has been automatically placed on the third projection P3, and finally in Fig. 4e the situation in which the detector is correctly positioned is shown, i.e. the centre of gravity of the triangle formed by the projections PI, P2, P3 coincide with the centre of gravity of the field of view CGFV.

Thus, the operator centres two of the rings over the projections of the imaged object with the aid of a mouse or other device. Two of the three rings can be positioned in this manner. Using the coordinates of the two rings manually positioned the system calculates the position of the third marker, and thus the third ring is automatically placed on the third projection. The diameter of the rings can be adjustable so as to fit the object in question, or in the alternative be at least so large so that they can fully enclose the projection of the object to be imaged. When the camera has been properly positioned and new projections have been obtained and two of the markers properly placed each ring will be centered over the corresponding projection of the object and each ring will be located in the center of the projection and contained within the corresponding projection. It should be noted that in the above description three projections are foreseen. It is, however, sufficient with two. The third projection, which may be omitted, is used for increased operator confidence and convenience.

With PI having the coordinates (xl, yl) and P2 having the coordinates (x2,y2), o the slant angle of the collimator holes and R the radius of the detector the position (x,y,z) of the imaged object relative to the detector will be (the scale is here for simplicity assumed to be 1: 1): I - V2 - ta{« 21x2

x = - ———

tg(s 2) - tg<s I)

^ _ tg(« 2>yi - t Cs I—y2 ÷ tgf _:a l ; t—f(s2 _: (3E2 - si)

tg(s ) - tg(s 1)

These x, y values represent the position in a plane, whereas, as indicated previously, also the height above the patient may be required to be adjusted. This height is represented by the z value: z = (x3 - ^-tg!f j Furthermore, the centre coordinates of P3 will be:

The necessary adjustments of the detector position will be: dx= -¾ dy = -

7

These values for the X-, Y- and Z-movements of the detector, respectively, will be displayed on the monitor. Thereby it is an easy matter to adjust the position.

The displacement in X and Y directions can be performed manually or by motors coupled top the rear wheels. The displacement in Z direction is performed suitably by moving the horizontal beam 14b, either manually but preferably using a motor. Although it is convenient for many reasons to have operator control as described above, it is also within the inventive concept to have this process performed

automatically. This can be achieved by using image recognition software to identify the position of the centre of gravity of the projections of the imaged object. In such case there is no need for operator intervention and strictly speaking not even a monitor screen for displaying the projections is required. However, for control purposes, it is nevertheless a preferred feature to display the projections for visual verification of correctness in position. The sequence of operation when performing the method with operator intervention is as follows.

Fig. 4b shows the display on the monitor in an initial position before adjustment. Only one ring 42 is in correct position with respect to the heart. Now one of the other rings 43 is moved manually on the monitor screen such that it covers the heart, see Fig. 4c. The centre of gravity 45 of the triangle formed by the three rings is automatically moved to a new position. Then a third ring 44 is moved automatically to cover a third projection of the heart, Fig. 4d. Again, the centre of gravity 45 is changed, and now the deviation between current and optimal position is calculated. This deviation is displayed on the monitor and the adjustment can easily be performed, either manually or automatically as described below, and the display will than look like in Fig. 4e.

After the adjustment, optionally a new set of projections can be acquired so as to verify the correctness of the position.

In an automated mode, the control electronic suitably running an image recognition software would process the data of each image and provide the identification of the centre of gravity of the object in each projection, and calculate the triangle thus formed, i.e. spanned by the centres of gravity. Then the deviation between current and optimal position is calculated as above, and the adjustment performed like before.

Preferably the adjustment of the detector position in accordance with the calculated required displacement is effected automatically. This can be implemented, as shown in Fig. 5, by the provision of electric motors 51 , 52 which are provided to drive the rear wheels RW of the mobile apparatus. Thus, when it is desired to move the detector in the Y-direction the motors will drive each wheel in the same direction, whereas when a movement in the X-direction is required the motors will drive the wheels in opposite directions. In order to record the actual displacement during the adjustment operation there is in one embodiment provided a means for the detection of the position with respect to the ground or floor. The information regarding the position is synchronized with the data from the mechanical position adjustment unit 17, and this enables a determination of when the detector has reached an optimal position.

The position detection can be implemented by using the technology on which a so called optical mouse is based, see Fig. 3a. An image recorder (camera) C continuously records the ground or floor and by digital image processing changes in the image are recorded by sequentially comparing the recorded images, and hence the speed and direction of the displacement can be determined. Preferably a light source such as a LED is provided to enhance the image.

Of course any other type of position detection could be used as long as speed and direction of the displacement can be recorded and fed back to the system.

The method is illustrated in some further detail in Fig. 7. It comprises recording successive projections of e.g. a heart at different, preferably three different angles using the detector 60. The signals from the detector 60 representing each projection are stored in a memory 62 as pixels representing images that can be displayed on a display screen or monitor 64.

In one embodiment the control electronics 66 is programmed to execute an image recognition software. Thus, the data, i.e. the image pixels are retrieved form memory, and the control electronics, by executing the image recognition software, identifies the imaged objects and their respective centres of gravity and thus the coordinates of said centers of gravity.

The centre of gravity of each of the projections having been registered at different angles by means of the detector, each represent a corner of an equilateral triangle, which in its turn will have a centre of gravity directly given by the coordinates of the center of gravity of the projections, by simple geometric consideration.

Thus, since the field of view of the system is known in terms of coordinates in the reference frame of the apparatus itself, the deviation of the centre of gravity of the geometric figure spanned by said projections is calculated by the control electronics, and the deviation is presented as a required displacement of the detector in the X-, Y- and optionally Z-directions, in order that the detector be positioned properly for the investigation.

In one embodiment the data representing the displacements are fed as control signals to a couple of motors 68 individually driving the rear wheels 69 whereby an automatic adjustment of the detector is possible.

Claims

CLAIMS:

1 . A method for adjusting the position of the detector in a mobile gamma camera system for the imaging of internal organs, the system comprising a gamma camera ( 14a) with a rotatable collimator ( 15) having collimator holes arranged at a slant angle (o); control electronics ( 12); the method comprising: obtaining, by using the gamma camera, at least two, preferably three projections of the an internal organ to be imaged, from different directions, by rotating the collimator between each projection acquisition; identifying the projection coordinates for the centre of gravity of the internal organ in the respective projection; calculating the actual position of the internal organ using the coordinates in said projections; calculating the required displacement of the gamma camera from the centre of gravity of the system field of view, the system field of view being a cone, the frustum of a cone, a double cone or the frustum of a double cone with its axis coincident with the collimator axis of rotation; and adjusting the gamma camera position by said calculated displacement.

2. The method according to claim 1 , wherein the identification of the

coordinates for the centre of gravity of each object in the respective projection is made by running a program comprising algorithms for image recognition in the control electronics.

3. The method according to claim 1 or 2, wherein the identification of the coordinates for the centre of gravity of each object in the respective projection comprises presenting the projections on a presentation unit, such as a graphics display; providing one movable cursor for each projection on said presentation unit; and positioning said cursors at the centre of gravity of each projection; and storing the coordinates of said centres of gravity for further calculations.

4. The method according to claim 1 , wherein the detector generates signals representing pixels of images and said pixels belonging to each projection are stored in a memory.

5. The method according to claim 4, wherein the step of identifying projection coordinates comprises retrieving data from said memory and displaying the images represented by said data on a display subdivided in three segments, equally angularly spaced on the display, whereby the centre of the display represents the centre of gravity of the field of view of the system, such that one projection is displayed in each display segment; generating a cursor/ marker on said display for each projection, the position of each cursor/ marker being operator controllable; and positioning the cursor centred on each projection.

6. The method according to claim 3, comprising registering the coordinates of said cursors and feeding the coordinates to the control electronics, which is programmed to calculate the coordinates of the centre of gravity of the geometric figure spanned by said projections, e.g. a triangle, and calculating the deviation of said coordinates of the geometric figure from the centre of gravity of the system field of view.

7. The method according to claim 1 , wherein the step of identifying projection coordinates comprises retrieving data from said memory and processing the data using image recognition software executed by the control electronics to identify the centre of gravity of each projection.

8. The method according to any preceding claim, comprising calculating the centre of gravity of the geometric figure spanned by said projections, i.e. a triangle, spanned by the three projections and calculating the deviation of said coordinates of the geometric figure from the centre of gravity of the system field of view.

9. The method according to any preceding claim, wherein the adjustment is made by manually moving the detector the required distances in X- and Y- directions, respectively.

10. The method according to any preceding claim 1-8, wherein the adjustment is made by feeding signals corresponding to the required displacement of the detector to the control electronics, which generates actuation signals for motors individually driving a respective rear wheel on said mobile imaging system.

1 1. The method according to any preceding claim, wherein the calculation of the position of the imaged object is performed according to the following equations: yl - γ2 - 4- ig{«2}x2

s = ^: —— ^:—

*g(«2) - tg{« 1}

_ _ tg(« 2>■_y: i - tg(g I—v2 tgf _:a l _:—t—f(s2 _: (3E2 - si )

t¾{s 2> - tg(S 1) wherein xl and yl , and x2 and y2 represent the coordinates of two projections, and wherein al , a2 represent rotation of the detector through a predetermined angle, and x and y represent the true coordinates of the object in a plane.

12. The method according to claim 4, further comprising calculation of the the height of the detector above the patient, this height being represented by a third coordinate:

13. The method according to claim 5, wherein the required adjustment of the detector position is given by

wherein dx, dy and dz are the distances it is necessary to move the detector in each direction x, y, and z, respectively.

14. The method according to claim 1 , wherein the positioning said cursors at the centre of gravity of each projection is performed manually.

15. A mobile imaging gamma camera system comprising a chassis (11a) having a front end and a rear end, the front end being configured to be insertable under a bed on which a patient is located, a gamma camera (14a) with a rotatable collimator (15) having collimator holes arranged at a slant angle (o); control electronics {12); a presentation unit; wherein the control electronics is programmed to perform the method according any of claims 1-14.