GB2452064A

GB2452064A - Apparatus And Method For Scanning A Patient And Detecting a Mismatch Between Scans

Info

Publication number: GB2452064A
Application number: GB0716438A
Authority: GB
Inventors: David Schottlander; Timor Kadir; Julian Bernard Hayball
Original assignee: Siemens Medical Solutions USA Inc
Current assignee: Siemens Medical Solutions USA Inc
Priority date: 2007-08-23
Filing date: 2007-08-23
Publication date: 2009-02-25
Also published as: GB0716438D0

Abstract

A patient is scanned using a CT and then a PET scanner (PET-CT). The PET scan is registered with the CT scan as each field of view image is processed. The PET image is conformed to the CT scan using an image manipulation subsystem and the degree of manipulation required compared with a threshold. In the event that the threshold is crossed then an alarm is issued as movement will have occurred which may later hamper the diagnosis of the scans by a clinician. This ensures that errors are eliminated at an early stage in the process improving efficiency of the process. A patient scanning apparatus is claimed with means to compare at least part of the scan with other parts of the scan or with another scan to determine a mismatch in image features and means to compare the measure of the mismatch with a predetermined mismatch and to provide an alarm depending on the comparison.

Description

Araratus And Method For Scanning A Patient This invention relates to apparatus and method for scanning a patient, in particular, using a Positron Emission Tomography (PET) and another scanning technique such as Magnetic Resonance imagery (MRI) or Computer Tomograpny (CT) scans.

PET-CT is a hybrid imaging technology combining a PET and a CT scan in one machine. The CT scan provides anatomical information and the PET scan functional information. The CT scan is also used for the correction of the attenuation undergone by the PET emission photons. Consequently, it is necessary for the scans to be accurately registered when combined to give an accurate basis for subsequent diagnosis.

The time required for producing a PET scan is considerably longer than that required for producing a CT scan. A CT scan may be carried out in a few minutes whereas a PET scan can take over thirty minutes. Movement of the patient during the scanning process can cause difficulties when the two scans are later registered. It may be difficult or even impossible to achieve accurate registration. As a result of this a rescan may be necessary causing significant disruption to the processing of patients. Furthermore, a CT rescan increases the patient's exposure to radiation which is undesirable. A subsequent PET rescan may be of poorer quality due to the decay in the radiopharmaceutical used.

Usually the CT is the first scan to be made in the protocol for scanning a patient. Then the PET scan is performed. PET scanners only have a limited field of view and it is therefore necessary to move the patient through a number of so called "bed positions" and for a scan to be carried out at each position. A patient will therefore have to remain in a particular position for many minutes.

The data from the scans is then processed and "reconstructed" with the PET scan and the CT scan displayed side by side to a technician where the images are checked for quality. The images are then sent to a physician for consideration. The time elapsed from the scan to the physician viewing the output may be thirty or forty minutes. If errors or quality problems are observed at this stage and they require a rescan then the disruption to the process for the patient and other patients may be considerable. In particular, for the PET scan, the radiopharmaceutical may have to be re-administered and the patient required to wait for an hour or so to allow it to be evenly absorbed.

Delays in the throughput of the scanning process thus produce significant problems for the patient and the clinicians The present invention arose in an effort to alleviate these problems.

In its broadest aspect the invention provides patient scanning apparatus comprising a scanner for carrying out a scan to produce data representative thereof, means to compare at least part of the scan with other parts of the scan or with another scan to determine a mismatch in image features, means to compare the measure of the mismatch with a predetermined mismatch and to issue an alarm in response to the comparison.

The mismatch may be of boundary features detected by a boundary detection method or system or it may be a measure of the lack of registration between a first scan and a second scan. The comparison is preferably made on the basis of parts of the scan but may be carried out for the whole scan. The predetermined mismatch may be a pattern but in the the preferred embodiment the movement is compared with a threshold. A pattern may permit movement in certain areas to be less significant in triggering an alarm.

According to the invention in a more detailed aspect there is provided a patient scanning apparatus comprising a scanner for carrying out a first scan and a second scan to produce data representative thereof, which second scan being captured from multiple fields of view, means to register data as each field of view is captured by the second scan with data from the first scan wherein the means to register includes means to manipulate the data of the field of view to achieve registration and to provide a measure of the manipulation required, means responsive to the measure of manipulation to generate an alarm when a predetermined manipulation is exceeded.

Preferably, a threshold for the manipulation is used to determine an alarm condition when the threshold is crossed.

By manipulating the data to achieve registration, making a measure of the required manipulation and comparing this with a threshold, and doing this on a field of view basis, errors caused by patient movement wiU be detected much earlier than if the second scan is carried out and the data combined with the data of the first scan at a later stage by visual inspection by a clinician.

The threshold may be one for all views or may be set to be particular thresholds for particular areas. For example, if the clinician is most interested in the head then the threshold for that field of view will be set to ensure that low manipulation is required whilst other parts of the body may have a threshold which permits relatively greater manipulation. An input device may be provided to allow the thresholds to be input or to select pre-stored thresholds. The thresholds may be stored as sets to be applied when a particular type of scan is input. For example, a brain, chest or lower abdomen scan type may be selected using a suitable user interface.

The invention also provides a method of scanning a patient.

A specific embodiment of the invention will now be described, by way of example only, with reference to the drawings in which: Figure 1 shows a scanning arrangement in accordance with a first embodiment of the invention; Figure 2 shows a method of operating the apparatus in accordance with the invention; Figure 3 shows in block diagram form a second embodiment of the invention; Figure 4 shows the method for operating the second embodiment of the invention; and Figure 5 shows two scans and associated memory tables recording radio activity levels at selected regions of interest.

A scanning arrangement 1 includes a scanning ring or doughnut 2 along the axis 3 of which is mounted a patient carrying platen or bed 4. The bed 4 is movable by motors under the control of a position controller 5 relative to the ring 2 in the directions indicated by labelled arrow 6 to enable the patient to be scanned along the length of the patient's body.

The ring 2 includes sensors (not shown) for use in a first scan and a second scan. The first scan is a CT scan and the output is passed to a CT scanner subsystem 7. The second scan is a PET scan and the sensor outputs are passed to a PET scanner subsystem 8.

The CT scan covers the length of the patient's body as indicated by labelled arrow 9.

The PET scan covers the head and the torso of the patient as indicated by labelled arrow 10. The PET scan is provided by a number of fields of view taken at different bed positions as indicated by the subdivisions 10 a to g. Each field of view is fifteen degrees for this particular PET scanner.

A manipulation subsystem 10 is provided which receives images from the CT and the PET subsystems. The purpose of this subsystem will be described later.

Both the CT and the PET subsystems provide images to a further viewing system 11 for displaying to a clinician.

The CT scanner subsystem includes an image reconstructor 70 which processes the data to reconstruct an image of volumes in the patient. The images are stored in local memory 71 and output to the viewing system 11 for manipulation and display. The local memory is in the form of a table which may be accessed by the image manipulation subsystem 10 via a link 12.

The PET scanner subsystem 8 includes an image reconstructor 80 which processes the incoming sensor data for a field of view into an image of that part of the patient's body.

The images are held in a memory table 81 and the images may be accessed from there by the viewing system 11 and also the image manipulation system 10 along link 13.

The image manipulation system 10 comprises a microprocessor based manipulation engine 100 and associated memory 101 holding a program, an image correlator 102 operably coupled to the manipulation engine 100, a threshold memory 103 for holding movement threshold's input by a user interface device 104 and an alarm unit 105 which displays an alert to a user on a screen.

The scanning arrangement operates in the following manner as will be explained with reference to figure 2. In a first step 200, the patient is prepared for the scans. This involves the administration of a suitable radio-tracer to the patient by injection and a waiting period to allow for even absorption and loading of the patient onto the bed 4. A user enters a scan type into the manipulation system 10 in step 201. For example, a head and a torso scan is selected which results in the appropriate thresholds for movement to be selected in step 202 by the manipulation system 10. In step 203, a CT scan is performed with the patient being moved through the scanning distance 9. The scan data is passed to the CT subsystem for reconstruction and storage of the images in scan memory 71. In the next step, 204 the PET scan is performed by movement of the patient through the bed positions 10 a to g. The data is passed to the PET subsystem where it is reconstructed into images which are stored in the field of view memory 81.

In step 205, the CT image is downloaded by the manipulation system 10, this step is carried out by the manipulation system at the same time as the PET scan for the first field of view lOa. When the PET scan of the first field of view is available in memory 81, it is downloaded to the manipulation subsystem 10 in step 206. That field of view is manipulated by the manipulation engine 100 and correlator 102 with the pertinent part of the CT image in step 207. A measure of the manipulation required to achieve a satisfactory matching or registration is made and compared with the threshold held in memory 103 in step 208. If the threshold has not been crossed then the next field of view is scanned by a return to step 204 and the process of capture, downloading and manipulation, repeated. If the threshold has been crossed then the alarm is triggered and remedial activity carried out in step 209. The remedial activity may be for example a request to the patient to remain still before the PET scan is repeated by a return to step 204.

The manipulation subsystem 10 may operate in accordance with a number of known image manipulation methods. For example, one approach may be to use a multi-modal deformable registration algorithm such as those described in "A survey of medical image registration" by JB. Antoine Maintz, M. Viergever, Medical Image Analysis 2(1):1-36, 1998 Typically such methods attempt to optimise a similarity function chosen for the application under some transformation model using a particular type of optimiser. For registration of PET and CT Mutual Information is a widely used similarity function. The transformation model can be based on Cubic B-Splines, Radial Basis Functions or similar methods. A gradient descent based optimiser is typically used. The registration algorithm matches the two images by adjusting the transformation such that the similarity function is maximised.

The measure of the degree of local deformation is found by the registration algorithm.

After registration each point in the image has associated with it a deformation vector -that is a vector that indicates the magnitude and direction of the required deformation.

The total amount of deformation can be calculated by simply summing up the magnitude of the deformation vectors. Alternatively, one could measure the degree of local stretching, that is the local change in volume. This can be done by calculating the determinant of the Jacobian: dEx dEx dFx ci5 dv dz -dFv dFv dFv /i (IV dz dFz dFz dFz dx dv dz where Fx, Fy and Fz represent the components of the deformation grid in the x y and z directions and the "d" operator represent partial derivatives.

By utilising the invention any problems with the PET scan registering with the CT scan are identified at a time when the correction is best made. This minimises disruption to the throughput of patients and maximises the utilisation of staff and equipment.

in alternative embodiments of the invention, patient parameters such as age, weight, height, gender and conditions or combinations of these may be input to the apparatus.

The threshold or set of thresholds may be selected based on these parameters. In essence a selection of a pre-stored patient model may be made. For example, persons suffering from a chest complaint would reasonably be expected to move over the upper torso area to a greater extent than a patient without such a condition. The selected model would then permit a higher threshold for manipulation over the chest region to prevent unnecessary alarms. A patient's weight may have a similar bearing. A high body mass index may indicate a level of body fat which may also have an effect on the ability to achieve a good registration and hence, a need to increase the threshold for the manipulation may be required.

In a second embodiment of the invention, shown in Figure 3, images from successive fields of view of a scan are processed to detect boundaries. The boundaries are then compared and, in the event that there is a significant mismatch, an alarm is triggered since the mismatch will indicate significant movement has occurred between the scans covering the field of view. In this case the boundaries of the PET scan are detected but other scans may be used.

In this embodiment the PET scanner provides data which is reconstructed by the image reconstructor as before and each field of view image is input to a image manipulation subsystem 300. The images are stored into memory 301 by a boundary detection processor 302. When two images are stored, then the boundary detection processor 302 determines from the images the boundary features of each image. These are passed to a boundary feature comparator 303. The boundary feature comparator 303 compares the boundary features and determines a mismatch. This is compared with a threshold held in memory 304. If the threshold is exceeded then the boundary comparator 303 sends an alarm signal to the alarm system 305. The alarm may be an audible or visual one as before. The process for this embodiment is shown in figure 4.

In a first step 400, the patient is prepared for the scanning process. This involves for a PET scan the administration of a radio tracing chemical and a waiting period for the chemical to be evenly absorbed into the body. The patient enters the scanning room and lies on the bed. A scanning technician enters a threshold for movement into the apparatus in step 401. The threshold may take into account the factors referred to in the description of the firt embodiment or indeed may be pre-programmed and selected by

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selection of a scan type. The scan is then started in step 402. A first field of view is scanned and the data sent to the image reconstructor where the image is reconstructed in step 403 If there is another scan to be carried out the bed is moved to the next field of view position and the scan repeated, decision step 404 returning to step 402.

The reconstructed image for the first field of view is in step 404 input to the manipulation sub-system 300 and stored in memory 301. The next scan for the next field of view is stored in the same way in step 405. For each pair of stored scans the boundary is detected in step 406. The boundary mismatch is determined in step 407 as earlier described. The mismatch is determined and compared with the threshold in decision step 408. If the threshold is not exceeded then the next field of view is compared by a return to step 406 using the second field of view with a third field of view. If the threshold is exceeded, then an alarm is indicated and remedial action taken in step 409. The remedial action may be to perform a re-scan for the fields of view affected or a complete rescan or other remedial action such as a warning to the patient to remain still or an annotation of the scan data to the effect that movement occurred over that part of the scan. The latter may be particularly useful where it is deemed that part of the scan is not essential for the diagnosis.

In a further embodiment of the invention, regions of interest in the scans are defined.

The radioactivity of the regions of interest are determined and then the level of radioactivity is monitored over time. Significant changes in the detected level of radioactivity will indicate that the patient has moved since for any given region of interest the radioactivity should be at a relatively steady state after absorption is complete. Thus, a detected change will not be due to a change in the radioactivity of the region in the patient's body but rather a movement of the region of interest in the field of view of the scan. This will constitute an image mismatch.

In figure 5a, a view of a simplified patient image is shown. It is a "slice" through a patient's torso. The slice 500 includes three regions of interest 501, 502 and 503. This slice is held in memory and the regions of interest 501, 502 and 503 identified. For each, region of interest the level of radio-activity determined and held in memory. This is represented in the figure as a memory table in which the region of interest has associated with it the detected level of radio-activity. Thus, region of interest 501 is detected as having a level of 20, region of interest 502 a level of 60 and region of interest 503 a level of 51.

In a later scan shown in figure 5a, it Will be apparent that movement has occurred because the torso boundary has changed. The system detects this by again measuring the radio-activity detected over the regions of interest 501 to 503. The memory table now shows that the level of activity for the regions of interest have changed to 15, 65 and 60 for the regions of interest 501, 502 and 503 respectively. Comparison of these changes with a predetermined threshold will indicate that significant movement has occurred and that there is therefore a mismatch in the image features. An alarm state will then be entered.

It will be appreciated that the embodiments may be provided in one apparatus for selection of the scanning method or indeed the methods may be combined. Claim

Claims

1. Patient scanning apparatus comprising a scanner for carrying out a scan to produce data representative thereof, means to compare at least part of the scan with other parts of the scan or with another scan to determine a mismatch in image features, means to compare the measure of the mismatch with a predetermined mismatch and to provide an alarm output dependent upon the comparison.

2. Apparatus as claimed in claim 1 wherein the predetermined mismatch is a threshold value and the alarm is issued if the threshold is crossed.

3. Apparatus as claimed in claim 1 or 2 wherein the scanner is a PET scanner.

4. Apparatus as claimed in claim 3 wherein the PET scanner captures data representing multiple fields of view and comprising means to determine from the data from at least some of the fields of view boundary features, means to compare the boundary features in the views to determine the measure of mismatch.

5. Apparatus as claimed in claim I, 2 or 3 comprising a scanner for carrying out a first scan and a scanner for carrying out a second scan to produce data representative thereof, at least which second scan being captured from multiple fields of view, wherein the means to determine the mismatch comprises means to register features between scans as data for each field of view is captured by the second scan and means to manipulate the data of the field of view to achieve registration and to provide a measure of the manipulation required and hence the measure of mismatch.

6 Apparatus as claimed in claim 5 wherein the mean to manipulate the data of the field of view applies a deformation field to the data to achieve registration and the Jacobian of the deformation field compared with the manipulation threshold.

7. Apparatus as claimed in any claim 5 or 6 wherein the second scanner is a CT scanner.

8. Apparatus as claimed in claim 1, 2 or 3 wherein the means to determine the mismatch determines the level of radio-activity at regions of interest in the field of view and compares the levels over time to determine the mismatch between scans and hence to determine that patient movement has occurred.

9. A patient scanning method comprising: performing a scan of at least part of the patient; comparing at least part of the scan with another part of the scan or another scan; determining from the comparison a mismatch of features; deriving a measure of the mismatch; comparing the measure with a predetermined mismatch; and dependent on the result of the comparison providing an alarm signal.

10. A method as claimed in claim 9 wherein the predetermined mismatch is a threshold and the alarm signal is provided in response to the crossing of the threshold.

11. A method as claimed in claim 9 or 10 wherein the features are boundary features.

12. A method as claimed in claim 11 wherein features of a first scan are compared with features of a second scan.

13. A method as claimed in claim 12 wherein fields of view of the second scan are compared with at least part of the second scan.

14. A method as claimed in claim 13 wherein fields of view of the second scan are registered with corresponding parts of the first scan by manipulation of the field of view and a measure of the manipulation required made and compared to a threshold, and in the event of the threshold being crossed producing an alarm signal.

15. A method as claimed in claim 14 wherein a deformation field is applied to the data to achieve registration and the Jacobian of the deformation field compared with the manipulation threshold

16. A method as claimed in any preceding claim wherein the second scan is a PET scan.

17. A method as claimed in claim 16 wherein the first scan is a CT scan.

18. A method as claimed in claim 9 wherein the radioactivity of at least one region of interest in a field of view in a first scan is compared with the radioactivity of the region of interest in the field of view in a further scan to determine the image mismatch.