Patent Publication Number: US-10311586-B2

Title: Systems and methods for anatomy-based registration of medical images acquired with different imaging modalities

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application represents the national stage entry of PCT International Application PCT/IB2014/001878, filed Sep. 18, 2014. The contents of this application are hereby incorporated by reference as set forth in their entirety herein. 
     BACKGROUND OF THE INVENTION 
     The field of the invention is systems and methods for medical imaging. More particularly, the invention relates to systems and methods for co-registering medical images acquired with different imaging modalities. 
     Different imaging modalities have different strengths and weaknesses for visualizing different structures. For example, x-ray computed tomography (“CT”) is particularly good at imaging bony structures and magnetic resonance imaging (“MRI”) is particularly good at obtaining soft tissue contrast. 
     For surgical planning and intra-operative guidance, it can be desirable to simultaneously display information from multiple imaging modalities to leverage the benefits of each modality. For example, in orthopedic surgery the visualization of bones, ligaments, and tendons is important to the surgical intervention. The ability to register images obtained from different modalities can be challenging because images obtained with different modalities may depict different anatomical and physiological structures, such as bone versus soft tissue. 
     Thus, there remains a need to provide systems and methods that are capable of reliably combining image data from multiple modalities, such as CT and MRI, and displaying the results in an effective manner during an intra-operative procedure. For instance, the registration of multiple three-dimensional imaging volumes usually relies on the identification of common anatomic, or other, landmarks in each of the imaging volumes. When the imaging volumes have significantly different relative contrasts in these key structures (e.g., bone versus tissue), as is often the case with CT and MRI, the registration task becomes very challenging. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned drawbacks by providing a system and method for registering a first medical image with a second medical image using a computer system. A first medical image acquired with a first imaging modality is provided to the computer system. The first imaging modality is at least one of x-ray imaging or magnetic resonance imaging (“MRI”). A second medical image acquired with an MRI system is also provided to the computer system. A first set of anatomical features is identified in the first medical image, and a second set of anatomical features, which has a known spatial relationship with the first set of anatomical features, is identified in the second medical image. The first set of anatomical features includes at least one anatomical landmark on a bony structure, and the second set of anatomical features includes at least one of an anatomical landmark on a bony structure, or a portion of a fibrous connective tissue. The computer system is then directed to determine registration parameters based on the first set of anatomical features, the second set of anatomical features, and the known spatial relationship between the first and second sets of anatomical features. The computer systems is then directed to co-register the first medical image and the second medical image using the determined registration parameters. 
     The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart setting forth the steps of an example method for co-registering medical images acquired with different imaging modalities based on anatomical features identified in the images and on known spatial relationships between the anatomical features; 
         FIG. 2  is an example of images acquired with a computed tomography (“CT”) imaging system and of a digitally reconstructed radiograph (“DRR”); 
         FIG. 3  is an example of images acquired with a magnetic resonance imaging (“MRI”) system using an ultrashort echo time (“UTE”) pulse sequence, and of a DRR; 
         FIGS. 4A-4D  depict examples of bony structures and soft tissue, including muscles and fibrous connective tissue, and the co-registration of these anatomical features based on the known spatial relationships between the bony structure and the fibrous connective tissue; 
         FIG. 5  is a block diagram of an example of an MRI system; 
         FIG. 6A  is an example of a CT imaging system; 
         FIG. 6B  is a block diagram of an example of a CT imaging system; and 
         FIG. 7  is a block diagram of an example computer system that can be configured to implement the methods described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Described here are systems and methods for co-registering medical images obtained with different imaging modalities. For instance, images obtained with x-ray imaging, such as x-ray computed tomography (“CT”), can be co-registered with images obtained with magnetic resonance imaging (“MRI”). Because different imaging modalities generate images that have different visualization characteristics for tissues, it is often difficult to directly register images obtained with different imaging modalities. 
     The systems and methods of the present invention overcome the limitations of co-registering images obtained with different imaging modalities by identifying anatomical features in the images obtained with the different imaging modalities and utilizing known spatial relationships between the anatomical features to align the images. As one example, if one image provides a good visualization of bony structures, and another image provides good visualization of soft tissues, including fibrous connective tissue, then the two images can be co-registered based on anatomical features in the respective images and the known spatial relationships between those features. For instance, regions on the bony structure where fibrous connective tissues originate or insert can be identified and used to align the bony structure image with the soft tissue image based on the known anatomical relationship between the bony structure and fibrous connective tissues. 
     The systems and methods of the present invention thus identify anatomical or functional landmarks in a first imaging volume acquired with a first imaging modality and then relate those landmarks to other anatomical or functional landmarks in other imaging modalities to assist in the registration between those imaging volumes. The use of these anatomical or functional landmarks can improve registration accuracy and speed or can simplify the registration complexity and may therefore assist in performing these kinds of registrations in an intra-operative environment. 
     Referring now to  FIG. 1 , a flowchart is illustrated as setting forth the steps of an example method for co-registering medical images acquired with different imaging modalities based on different anatomical features identified in the images. 
     The method includes providing a first medical image acquired with a first imaging modality, as indicated at step  102 . As one example, the first imaging modality can be x-ray imaging and the first medical image can be an image acquired with a CT system. As such, the first medical image provides visualization of bony structures in the subject being image. 
       FIG. 2  illustrates examples of typical CT images of a skull in three orthogonal planes in addition to a digitally reconstructed radiograph (“DRR”) in the upper right panel. These images are representative of images acquired with x-ray imaging that provide good visualization of bony structures. 
     Referring again to  FIG. 1 , the method also includes providing a second medical image acquired with a second imaging modality, as indicated at step  104 . As one example, the second imaging modality can be MRI and the second medical image can be an image acquired with an MRI system. In some embodiments, the second medical image can be acquired using diffusion-weighted MRI techniques, including diffusion tensor imaging (“DTI”), to visualize organized, fibrous structures such as fibrous connective tissue. The known relationships between fibrous connective tissues and their associated bones can then be used for the registration, as described below. In some other embodiments, the second medical image can be acquired using ultrashort echo time (“UTE”) MRI methods, which can be used to directly image bony structures. 
       FIG. 3  illustrates example of typical magnetic resonance images acquired using a UTE sequence, in addition to a DRR in the upper right panel. These images are representative of images acquired with an MRI system that provide good visualization of bony structures and, therefore, can be used to identify anatomical features based on those bony structures similar to the anatomical features that can be identified in x-ray images. 
     Referring again to  FIG. 1 , a first set of anatomical features is identified in the first medical image, as indicated at step  106 . In some embodiments, these anatomical features can be manually identified by a user. In some other embodiments, these anatomical features can be identified automatically or semi-automatically, such as by using atlas-based methods. For instance, an image-based atlas that indicates the visualization of anatomical structures in the first imaging modality can be used to automatically or semi-automatically identify the first set of anatomical features in the first medical image. 
     By way of example, the first set of anatomical features can include bony structures or portions thereof. For instance, the first set of anatomical features can be anatomical landmarks or other locations on a bony structure. In some embodiments, the first set of anatomical features includes one or more insertion points or origin points on a bony structure. An insertion point can include those points on a bony structure where a muscle inserts via a tendon, or where other fibrous connective tissue, such as a ligament or fascia, inserts on the bony structure. Likewise, an origin point can include those points on a bony structure where a muscle originates via a tendon, or where other fibrous connective tissue, such as a ligament or fascia, originates on the bony structure. Because the anatomical locations of these origin and insertion points are well known, they can be used as anatomical landmarks for co-registering images in which bony structures may not be visible. 
     A second set of anatomical features is identified in the second medical image, as indicated at step  108 . In some embodiments, these anatomical features can be manually identified by a user. In some other embodiments, these anatomical features can be identified automatically or semi-automatically, such as by using atlas-based methods. In general, the second set of anatomical features is different from, but related to, the first set of anatomical features. For instance, an image-based atlas that indicates the visualization of anatomical structures in the second imaging modality can be used to automatically or semi-automatically identify the second set of anatomical features in the second medical image. 
     By way of example, because bony structures are difficult to image with MRI, the other anatomic structures that have known spatial relationships with bony structures can be identified and used as the basis for registering a magnetic resonance imaging volume to a bony-structure based volume, such as an x-ray imaging or CT imaging volume. 
     As one example, the second set of anatomical features can include fibrous connective tissue or portions thereof. For instance, the second set of anatomical features can include the beginning or end point of a tendon or ligament. In general, these terminal points of a tendon or ligament correspond to an origin or insertion point on a bony structure. As such, the terminal points of the tendon or ligament have a unique spatial relationship with the related origin or insertion point on the bony structure. The different anatomical feature sets thus provide different, but complementary information that can be used to improve the co-registration of the first and second medical images. As will be described below, this spatial relationship can be used as the basis for calculating registration parameters between the first medical image and the second medical image. 
     In some embodiments, however, the second set of anatomical features can be the same as the first set of anatomical features. For instance, the anatomical features can include bony structures or portions thereof, as described above. In such embodiments, the first imaging modality may be x-ray imaging and the second imaging modality may be MRI, in which an imaging technique that is capable of providing visualization of bony structures is utilized. For example, UTE MRI methods can be used to directly image bony structures. The co-registration of the magnetic resonance images and x-ray images can then be based on direct bony structure landmark registration between the UTE and x-ray imaging volumes. 
     These embodiments are particularly useful for radiation treatment planning and guidance, where it is desirable to reduce the unnecessary radiation dose imparted to the patient. Accordingly, the radiation treatment can be guided using magnetic resonance images that are acquired during the treatment procedure and co-registered with a CT imaging volume that was acquired during the treatment planning stage. With this approach, the patient does not need to be exposed to additional radiation from an x-ray imaging source during treatment. 
     After the first and second anatomical feature sets have been identified in the first and second medical images, respectively, registration parameters are calculated based on the known spatial relationship between the first and second sets of anatomical features, as indicated at step  110 . For example, the first and second anatomical feature sets can be related by the known origin and insertion of tendons, ligaments, aponeuroses, fasciae, or other fibrous connective tissue on bony structures. Based on these known spatial relationships, a transformation between the two anatomical feature sets can be calculated and used to co-register the first and second medical images. 
     In some embodiments, additional information from non-anatomical landmark sources can be provided to guide the registration process, as indicated at step  112 . For example, external, common reference points can be used to further guide the registration. As one example, in orthopedic surgery various positioning devices may be fixed to the patient. When such devices are in use, landmark points can be defined as being located on the device, or as being related to the known attachment points of the device. These additional landmark points, which can be visualized in the first medical image, the second medical image, or both, can be used as additional landmark points for registration purposes. 
     The calculated registration parameters are thus used to co-register the first and second medical images, as indicated at step  114 . The aforementioned processes can be repeated for any number of images in an image series or image volume such that the image series or image volume can be co-registered. 
     An illustrative example of the foregoing registration method can be described with respect to  FIGS. 4A-4D . By way of example,  FIG. 4A  represents a first medical image of a subject acquired with a first imaging modality and  FIG. 4B  represents a second medical image acquired with a second imaging modality. For instance,  FIG. 4A  represents an image acquired with x-ray imaging, which has good visualization characteristics for bone, and  FIG. 4B  represents an image acquired with MRI, which has good visualization characteristics for soft tissues, but poor visualization characteristics for bony structures. 
       FIG. 4A  represents an image of a bony structure  10  in a subject. Because the anatomy of the bony structure is known, the origin and insertion points for different fibrous connective tissues that attach to the bony structure  10  can be identified in the image. These anatomical features can include the origins and insertions of muscles via tendons, or the origins and insertions of different ligaments or fascia. In the example illustrated in  FIG. 4A , the insertion points  12  for different muscles that attach to the bony structure  10  are known and can be identified in the image. 
       FIG. 4B  represents an image of soft tissues, including muscles  14  that surround the bony structure  10 . The image represented in  FIG. 4B  depicts the muscles  14  well, but does not provide significant differentiation of tendons and ligaments. Thus, in some embodiments, the soft tissue image can be acquired using an imaging technique that improves the visualization of fibrous connective tissue, including tendons, ligaments, fasciae, and aponeuroses. 
     An example of a soft tissue image acquired using an imaging technique that improves the visualization of fibrous connective tissue is illustrated in  FIG. 4C , in which tendons  16  are depicted in addition to the muscles  14 . By way of example, the image technique that improves the visualization of fibrous connective tissues can include diffusion-weighted imaging. More particularly, the soft tissue image can include a tractography map that is produced using information from diffusion tensors that are computed from diffusion-weighted magnetic resonance images. This tractography map depicts the microstructure of the fibrous connective tissue and, thus, provides an accurate visualization of the terminal points of the fibrous connective tissue. 
     By spatially registering the end points of the tendons  16  in the soft tissue image and the insertion points  12  on the bony structure  10 , registration parameters can be calculated and used to co-register the first and second medical images. As such, the respective image volumes can be aligned to show the soft tissue in the appropriate spatial orientation and scaling relative to the bony structure, as illustrated in  FIG. 4D . 
     Referring particularly now to  FIG. 5 , an example of a magnetic resonance imaging (“MRI”) system  500  is illustrated. The MRI system  500  includes an operator workstation  502 , which will typically include a display  504 ; one or more input devices  506 , such as a keyboard and mouse; and a processor  508 . The processor  508  may include a commercially available programmable machine running a commercially available operating system. The operator workstation  502  provides the operator interface that enables scan prescriptions to be entered into the MRI system  500 . In general, the operator workstation  502  may be coupled to four servers: a pulse sequence server  510 ; a data acquisition server  512 ; a data processing server  514 ; and a data store server  516 . The operator workstation  502  and each server  510 ,  512 ,  514 , and  516  are connected to communicate with each other. For example, the servers  510 ,  512 ,  514 , and  516  may be connected via a communication system  540 , which may include any suitable network connection, whether wired, wireless, or a combination of both. As an example, the communication system  540  may include both proprietary or dedicated networks, as well as open networks, such as the internet. 
     The pulse sequence server  510  functions in response to instructions downloaded from the operator workstation  502  to operate a gradient system  518  and a radiofrequency (“RF”) system  520 . Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system  518 , which excites gradient coils in an assembly  522  to produce the magnetic field gradients G x , G y , and G z  used for position encoding magnetic resonance signals. The gradient coil assembly  522  forms part of a magnet assembly  524  that includes a polarizing magnet  526  and a whole-body RF coil  528 . 
     RF waveforms are applied by the RF system  520  to the RF coil  528 , or a separate local coil (not shown in  FIG. 5 ), in order to perform the prescribed magnetic resonance pulse sequence. Responsive magnetic resonance signals detected by the RF coil  528 , or a separate local coil (not shown in  FIG. 5 ), are received by the RF system  520 , where they are amplified, demodulated, filtered, and digitized under direction of commands produced by the pulse sequence server  510 . The RF system  520  includes an RF transmitter for producing a wide variety of RF pulses used in MRI pulse sequences. The RF transmitter is responsive to the scan prescription and direction from the pulse sequence server  510  to produce RF pulses of the desired frequency, phase, and pulse amplitude waveform. The generated RF pulses may be applied to the whole-body RF coil  528  or to one or more local coils or coil arrays (not shown in  FIG. 5 ). 
     The RF system  520  also includes one or more RF receiver channels. Each RF receiver channel includes an RF preamplifier that amplifies the magnetic resonance signal received by the coil  528  to which it is connected, and a detector that detects and digitizes the I and Q quadrature components of the received magnetic resonance signal. The magnitude of the received magnetic resonance signal may, therefore, be determined at any sampled point by the square root of the sum of the squares of the I and Q components:
 
 M =√{square root over ( I   2   +Q   2 )}  (1);
 
     and the phase of the received magnetic resonance signal may also be determined according to the following relationship: 
     
       
         
           
             
               
                 
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     The pulse sequence server  510  also optionally receives patient data from a physiological acquisition controller  530 . By way of example, the physiological acquisition controller  530  may receive signals from a number of different sensors connected to the patient, such as electrocardiograph (“ECG”) signals from electrodes, or respiratory signals from a respiratory bellows or other respiratory monitoring device. Such signals are typically used by the pulse sequence server  510  to synchronize, or “gate,” the performance of the scan with the subject&#39;s heart beat or respiration. 
     The pulse sequence server  510  also connects to a scan room interface circuit  532  that receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  532  that a patient positioning system  534  receives commands to move the patient to desired positions during the scan. 
     The digitized magnetic resonance signal samples produced by the RF system  520  are received by the data acquisition server  512 . The data acquisition server  512  operates in response to instructions downloaded from the operator workstation  502  to receive the real-time magnetic resonance data and provide buffer storage, such that no data is lost by data overrun. In some scans, the data acquisition server  512  does little more than pass the acquired magnetic resonance data to the data processor server  514 . However, in scans that require information derived from acquired magnetic resonance data to control the further performance of the scan, the data acquisition server  512  is programmed to produce such information and convey it to the pulse sequence server  510 . For example, during prescans, magnetic resonance data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server  510 . As another example, navigator signals may be acquired and used to adjust the operating parameters of the RF system  520  or the gradient system  518 , or to control the view order in which k-space is sampled. In still another example, the data acquisition server  512  may also be employed to process magnetic resonance signals used to detect the arrival of a contrast agent in a magnetic resonance angiography (“MRA”) scan. By way of example, the data acquisition server  512  acquires magnetic resonance data and processes it in real-time to produce information that is used to control the scan. 
     The data processing server  514  receives magnetic resonance data from the data acquisition server  512  and processes it in accordance with instructions downloaded from the operator workstation  502 . Such processing may, for example, include one or more of the following: reconstructing two-dimensional or three-dimensional images by performing a Fourier transformation of raw k-space data; performing other image reconstruction algorithms, such as iterative or backprojection reconstruction algorithms; applying filters to raw k-space data or to reconstructed images; generating functional magnetic resonance images; calculating motion or flow images; and so on. 
     Images reconstructed by the data processing server  514  are conveyed back to the operator workstation  502  where they are stored. Real-time images are stored in a data base memory cache (not shown in  FIG. 5 ), from which they may be output to operator display  512  or a display  536  that is located near the magnet assembly  524  for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage  538 . When such images have been reconstructed and transferred to storage, the data processing server  514  notifies the data store server  516  on the operator workstation  502 . The operator workstation  502  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
     The MRI system  500  may also include one or more networked workstations  542 . By way of example, a networked workstation  542  may include a display  544 ; one or more input devices  546 , such as a keyboard and mouse; and a processor  548 . The networked workstation  542  may be located within the same facility as the operator workstation  502 , or in a different facility, such as a different healthcare institution or clinic. 
     The networked workstation  542 , whether within the same facility or in a different facility as the operator workstation  502 , may gain remote access to the data processing server  514  or data store server  516  via the communication system  540 . Accordingly, multiple networked workstations  542  may have access to the data processing server  514  and the data store server  516 . In this manner, magnetic resonance data, reconstructed images, or other data may be exchanged between the data processing server  514  or the data store server  516  and the networked workstations  542 , such that the data or images may be remotely processed by a networked workstation  542 . This data may be exchanged in any suitable format, such as in accordance with the transmission control protocol (“TCP”), the Internet protocol (“IP”), or other known or suitable protocols. 
     Referring particularly now to  FIGS. 6A and 6B , an example of an x-ray computed tomography (“CT”) imaging system  600  is illustrated. The CT system  600  includes a gantry  602 , to which at least one x-ray source  604  is coupled. The x-ray source  604  projects an x-ray beam  606 , which may be a fan-beam or cone-beam of x-rays, towards a detector array  608  on the opposite side of the gantry  602 . The detector array  608  includes a number of x-ray detector elements  610 . Together, the x-ray detector elements  610  sense the projected x-rays  606  that pass through a subject  612 , such as a medical patient or an object undergoing examination, that is positioned in the CT system  600 . As one example, each x-ray detector element  610  produces an electrical signal that may represent the intensity of an impinging x-ray beam and, hence, the attenuation of the beam as it passes through the subject  612 . In some configurations, each x-ray detector  610  is capable of counting the number of x-ray photons that impinge upon the detector  610 . During a scan to acquire x-ray projection data, the gantry  602  and the components mounted thereon rotate about a center of rotation  614  located within the CT system  600 . 
     The CT system  600  also includes an operator workstation  616 , which typically includes a display  618 ; one or more input devices  620 , such as a keyboard and mouse; and a computer processor  622 . The computer processor  622  may include a commercially available programmable machine running a commercially available operating system. The operator workstation  616  provides the operator interface that enables scanning control parameters to be entered into the CT system  600 . In general, the operator workstation  616  is in communication with a data store server  624  and an image reconstruction system  626 . By way of example, the operator workstation  616 , data store sever  624 , and image reconstruction system  626  may be connected via a communication system  628 , which may include any suitable network connection, whether wired, wireless, or a combination of both. As an example, the communication system  628  may include both proprietary or dedicated networks, as well as open networks, such as the internet. 
     The operator workstation  616  is also in communication with a control system  630  that controls operation of the CT system  600 . The control system  630  generally includes an x-ray controller  632 , a table controller  634 , a gantry controller  636 , and a data acquisition system  638 . The x-ray controller  632  provides power and timing signals to the x-ray source  604  and the gantry controller  636  controls the rotational speed and position of the gantry  602 . The table controller  634  controls a table  640  to position the subject  612  in the gantry  602  of the CT system  600 . 
     The DAS  638  samples data from the detector elements  610  and converts the data to digital signals for subsequent processing. For instance, digitized x-ray data is communicated from the DAS  638  to the data store server  624 . The image reconstruction system  626  then retrieves the x-ray data from the data store server  624  and reconstructs an image therefrom. The image reconstruction system  626  may include a commercially available computer processor, or may be a highly parallel computer architecture, such as a system that includes multiple-core processors and massively parallel, high-density computing devices. Optionally, image reconstruction can also be performed on the processor  622  in the operator workstation  616 . Reconstructed images can then be communicated back to the data store server  624  for storage or to the operator workstation  616  to be displayed to the operator or clinician. 
     The CT system  600  may also include one or more networked workstations  642 . By way of example, a networked workstation  642  may include a display  644 ; one or more input devices  646 , such as a keyboard and mouse; and a processor  648 . The networked workstation  642  may be located within the same facility as the operator workstation  616 , or in a different facility, such as a different healthcare institution or clinic. 
     The networked workstation  642 , whether within the same facility or in a different facility as the operator workstation  616 , may gain remote access to the data store server  624  and/or the image reconstruction system  626  via the communication system  628 . Accordingly, multiple networked workstations  642  may have access to the data store server  624  and/or image reconstruction system  626 . In this manner, x-ray data, reconstructed images, or other data may be exchanged between the data store server  624 , the image reconstruction system  626 , and the networked workstations  642 , such that the data or images may be remotely processed by a networked workstation  642 . This data may be exchanged in any suitable format, such as in accordance with the transmission control protocol (“TCP”), the internet protocol (“IP”), or other known or suitable protocols. 
     Referring now to  FIG. 7 , a block diagram of an example computer system  700  that can be configured to co-register medical images acquired with different imaging modalities, as described above, is illustrated. The medical images to be co-registered can be provided to the computer system  700  from the respective medical imaging systems, such as an MRI system and a CT system, or from a data storage device, and are received in a processing unit  702 . 
     In some embodiments, the processing unit  702  can include one or more processors. As an example, the processing unit  702  may include one or more of a digital signal processor (“DSP”)  704 , a microprocessor unit (“MPU”)  706 , and a graphics processing unit (“GPU”)  708 . The processing unit  702  can also include a data acquisition unit  710  that is configured to electronically receive data to be processed, which may include first and second medical images, image series, or image volumes. The DSP  704 , MPU  706 , GPU  708 , and data acquisition unit  710  are all coupled to a communication bus  712 . As an example, the communication bus  712  can be a group of wires, or a hardwire used for switching data between the peripherals or between any component in the processing unit  702 . 
     The DSP  704  can be configured to receive and processes the first and second medical images. The MPU  706  and GPU  708  can also be configured to process the first and second medical images in conjunction with the DSP  704 . As an example, the MPU  706  can be configured to control the operation of components in the processing unit  702  and can include instructions to perform processing of the first and second medical images on the DSP  704 . Also as an example, the GPU  708  can process image graphics. 
     In some embodiments, the DSP  704  can be configured to process the first and second medical images received by the processing unit  702  in accordance with the algorithms described above. Thus, the DSP  704  can be configured to identify anatomical features in the images, to calculate registration parameters based on the identified anatomical features and known spatial relationships therebetween, and to co-register the images using the registration parameters. 
     The processing unit  702  preferably includes a communication port  714  in electronic communication with other devices, which may include a storage device  716 , a display  718 , and one or more input devices  720 . Examples of an input device  720  include, but are not limited to, a keyboard, a mouse, and a touch screen through which a user can provide an input. 
     The storage device  716  is configured to store images, whether provided to or processed by the processing unit  702 . The display  718  is used to display images, such as images that may be stored in the storage device  716 , and other information. Thus, in some embodiments, the storage device  716  and the display  718  can be used for displaying the images before and after registration and for outputting other information, such as data plots or other reports based on the registration process. 
     The processing unit  702  can also be in electronic communication with a network  722  to transmit and receive data, including CT images, MR images, and other information. The communication port  714  can also be coupled to the processing unit  702  through a switched central resource, for example the communication bus  712 . 
     The processing unit  702  can also include a temporary storage  724  and a display controller  726 . As an example, the temporary storage  724  can store temporary information. For instance, the temporary storage  724  can be a random access memory. 
     The present invention has been described in terms of one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.