During the last decade, three-dimensional (3-D) imaging has rapidly become an essential part of the visualization tools used in a variety of diagnostic procedures in radiology. With the latest generation of multi-detector computed tomography (CT) and magnetic resonance imaging (MRI) scanners it is possible to acquire high-resolution dynamic images of the heart in a single breath-hold. These studies provide isotropic data that have identical resolution in the three dimensions, providing exquisite detailed anatomical information. With 3-D volume rendering techniques it is possible to generate high-quality images offering a realistic anatomical view of the heart and vessels allowing the physicians and care providers to better visualize anatomical structures and morphological anomalies. Due to the inherent function of the heart it is necessary to be able to display the images in a dynamic mode adding a temporal dimension to the images. This dimension, often referred to as the fourth dimension, is obtained by acquiring images of the heart at different phases of the cardiac cycle allowing a better assessment of cardiac motion and function.
Another rapidly growing domain of medical imaging is the evolution toward functional and molecular imaging for the assessment of biological and metabolic pathways. Positron emission tomography (PET) using radio-labeled tracers represents the most common molecular imaging modality, but other techniques based on molecular markers are nowadays emerging in other modalities, such as MRI, offering new perspectives of functional imaging and a great hope to increasing the specificity and sensibility of diagnostics and detection of varieties of diseases. These modalities are complementary to classic conventional and anatomical imaging like CT and MRI. The ability to combine the functional or molecular data with the anatomical images adds a new dimension to the images. This functional or molecular component is often referred to as the fifth dimension.
Today's physicians need new tools and new ways to explore these multidimensional data generated by modern imaging techniques. New software platforms need to be simple and intuitive and provide extremely fast and powerful processing capabilities allowing the users to easily navigate and explore complex anatomical and functional features of these examinations.
This is the main rationale behind the development of a new generation of image navigation tools that is more suitable for multidimensional datasets. The OSIRIX software developed by collaboration between University of California, Los Angeles and the University of Geneva is one of such software platforms that allow users to efficiently and conveniently navigate through large sets of multidimensional data without the need for high-end expensive hardware. Most importantly, the system was also developed under the new open source paradigm based on the integration of existing open source software libraries and allowing other institutions and developers to contribute to this project.
The New Vision of the Heart
The ability to acquire 3-D dynamic images of the heart has revolutionized the perception of the heart and great vessels. Cardiac MRI has rapidly exceeded the capabilities of more traditional techniques such as echocardiography by providing images of superior quality and unlimited access to hidden anatomical structures that are more difficult to access by ultrasound. Dynamic cine-MRI sequences allow better assessment of cardiac function and detection of regional wall motion anomalies and hemodynamic alterations. The introduction of pharmacological stress procedures allows further detection of alterations of heart function during stress-induced ischemia. Quantitative analysis of ventricular wall motion and cardiac function provides very valuable clinical information for enhanced diagnostic capabilities of these imaging techniques. Three-dimensional rendering technique further enhances the ability to visualize complex anatomical structures such as congenital heart anomalies, as well as assessment of anatomical changes following surgical interventions.
More recently, multi-detector CT scanners have emerged as an extremely promising technique for evaluation of the heart and great vessels, particularly for their ability to visualize coronary arteries and non-invasive detection of coronary artery disease (CAD).The ability to acquire ECG-gated high-resolution of images of the heart at extremely high speed allows dynamic 3-D data sets of the heart to be acquired in a single breath-hold. These data, also referred to as 4-D data, provide the temporal component that allows for evaluation of cardiac function and wall motion. Furthermore, the ability to process the data and create 2-D or 3-D views of the coronary arteries offers a higher flexibility for analysis and visualization of individual coronary artery vessels. Special processing and rendering tools are necessary for generating volume-rendered images as well as reformatting 2-D image maps along each vessel for better assessment of segmental narrowing or occlusions.
Multimodality scanners combining PET and CT imaging modalities have emerged on the market and are widely adopted for oncology investigations. With the advent on the market of ultrafast multi-detector CT scanners combined with PET scanners the path is open to a new generation of clinical investigations combining anatomical and metabolic information. Hybrid scanners allow images from the two modalities to be acquired simultaneously, resulting in perfectly registered sets of images.
The visualization of the two sets of images requires graphic tools allowing progressive blending of the two for better localization of metabolic alterations depicted by PET over the corresponding anatomical structures obtained from the CT images. In the particular case of the heart the dynamic property of the heart requires additional temporal discrimination of the cardiac motion. This results in an additional dimension to the data where the linear correlation from CT to PET represents a fifth dimension.
The graphical representation and the interactive manipulation of these multidimensional data represent a technical challenge, but also a challenge in designing the proper users' interface to allow easy and intuitive navigation through the data. To further facilitate and improve the navigation and image manipulation functions in five dimensions, innovative pointing devices and multidimensional navigation devices that can be used in conjunction with the standard mouse and keyboard are necessary. In a recent study it was found that the device that is most adapted to such functions is a standard programmable video-editing jog-wheel device that is widely used by professional video editors. This device allows rapid navigation in multiple data sets and multiple dimensions with a single hand. The addition of this low-cost pointing device increases the ability of the user to navigate rapidly and to switch among different display functions in realtime.
Open Source Navigation Tools
The need for new image display and navigation software exceeds the capacity of commercial software manufacturers to rapidly adapt and provide new tools that are needed by clinical and academic community. Typically, tools required to rapidly manipulate large sets of 3-D and 4-D data sets are hard to find and are only available on very high-end expensive workstations. Besides, they usually lack some specialized sets of tools that are specific for a certain number of applications and diagnostic or therapeutic tasks. Open-source software has gained momentum in recent years because it allows more dynamic and rapid development by a large number of developers from different institutions. Not only does open source software tend to be more robust than traditional commercial software because it is being developed and debugged by a large community of users and developers, but it also often provides features that are more adapted to the users' needs because it is being developed by a community of users. Until recently, open source developments were relatively uncommon in medical applications and in particular in medical imaging. One recent open source development is the OSIRIX software. This open source program allows the community of users to share new developments that could emerge from different institutions using the OSIRIX program.
The author believes that OSIRIX can rapidly evolve into a new tool that will assist radiologists, cardiologists, and other physicians in interpreting new multidimensional examinations. It has the potential to adapt more rapidly to the emerging needs of the medical community than commercial solutions that are traditionally less flexible and driven by market rules that force vendors to be more reluctant to adopt multiple innovations, some of which may not have a sufficient commercial value when serving only a minority of users. It is also an ideal platform for the development of research tools and innovative image analysis and processing tools. In its current version, OSIRIX provides all the necessary tools for 3-D display and manipulation of dynamic cardiac data sets from CT,MRI, ultrasound, angiograms, and PET-CT modalities. It provides intuitive user interface to navigate through 4-D and 5-D cardiac imaging data. It also provides a flexible expandable architecture that allows plug-in components to be added to the program for quantitative analysis and data measurements. Generic measurement tools of functional parameters such as ventricular volumes and ejection fraction are simple examples of quantitative tools that can be added to the program and applied to different imaging modalities for quantitative analysis of cardiac function.
Conclusions
Radiology imaging modalities are evolving from conventional sets of 2-D tomographic slices to 3-D volumetric image sets extending to a fourth or fifth dimension with temporal and functional data that can be acquired with ultrafast CT and MR scanners and with combined PET/CT scanners. To allow radiologists and clinicians to conveniently and efficiently interpret these large exam sets, traditional image viewers commonly available on Picture Archiving and Communication System (PACS) workstations have to be re-designed and tailored to a new paradigm of multidimensional image navigation, visualization, and manipulation. This is why a completely new image navigation environment was developed, and by making it available for free under open source it is expected that participation will be encouraged from different institutions that are willing to invest time and resources in the development and exploration of new means for complex image interpretation and diagnostic tasks. The OSIRIX software also provides an attractive and cost-effective alternative to radiologists and healthcare providers that have increasing needs for multidimensional image processing and manipulation and cannot afford the extra cost of high-end workstations currently available on the market. The availability of OSIRIX certainly facilitates the communication between radiologists and referring physicians by allowing them to share the same convenient platform for image display and navigation.
The author believes that such tools are critical for patient care and patient management with the increasing number of image-based therapeutic and surgical procedures. In these innovative procedures it is critical for adequate patient care that the performing physician has full access to the same tools and image data that are available to the interpreting radiologists. Furthermore, the author also believes that a wider availability of 3-D visualization tools and multi-dimensional navigation tools will greatly enhance the quality of communication between physicians and patients. Physicians will have the ability to use these tools to explain and educate the patients about their illness and better explain the effect of the different treatments or therapeutic procedures.
OSIRIX also provides a completely new perspective on medical imaging in general by providing the necessary tools for exploring the fourth dimension, the temporal dimension, the fifth dimension, and the functional or molecular dimension of studies like Cardiac-CT or PET-CT. With the rapid evolution of imaging modalities and the shift toward molecular imaging procedures, this new paradigm in multidimensional image navigation may become even more prevalent in general diagnostic tasks where interpreting physicians will have to integrate information from different functional and molecular components into the diagnostic process. It was also shown that applying image fusion and multidimensional visualization tools to existing imaging procedure, such as MRI, by combining T1 and T2 or functional data can also improve and facilitate the review and interpretation of these images. It is also demonstrated that 4-D studies of dynamic cardiac sequences and combination of flow-sensitive, and dynamic sequences are becoming part of the requirements for adequate interpretation of cardiac and cardiovascular imaging procedures.
With the development of OSIRIX, a new image navigation platform and its distribution as an open source software allowed the demonstration of the potential value of 4-D viewers and image fusion with functional or molecular data. The author believes that such tools will become will become an essential part of next-generation Digital Images and Communications in Medicine (DICOM) viewers and PACS workstations, in the same way that 3-D rendering or myocardial perfusion reserve (MPR) image reformatting have made their way into the mainstream and become standard in modern radiology.