Allan J. Lightman
Rapid prototyping is having an impact in several areas related to health care. Surgical planning and the fabrication of prostheses have received the greatest prominence, in part due to the dramatic nature of the application. RP systems are key to developing new modalities in other areas of use, such as specialized drug delivery carriers. Surgical applications represent the majority of the activities in Europe and Japan, although there are other applications being pursued.
In the early 1970s, a new mode of X-ray imaging was developed based upon tomographic scanning (computerized tomography, or CT). This modality differs from traditional X-ray shadowgraphy in key aspects, including the fact that linear images are collected along one plane of the object at a time. These images are taken from a variety of angles. The plane of interrogation is then shifted and the process repeated. The information collected for each plane is numerically analyzed to derive the spatial distribution of the X-ray densities within the plane. The information from each plane can then be put together to provide a volumetric image of the structure. Standard CT scanners achieve a resolution of 512 x 512 elements within a layer (1,024 x 1,024 capability is now available in more advanced systems). To scan organs or other regions of interest, the patient is stepped through the measurement plane with a typical pitch of 2-3 mm. Finer scanning of selected regions can be accomplished so long as the total X-ray exposure is kept within safe limits. The numerically reconstructed X-ray density spatial distribution from each "slice" is then printed on X-ray film so that the data presented to the radiologist and/or surgeon is in traditional format (Fig. 12.1).
At about the same time as CT was demonstrated, application of nuclear magnetic resonance (NMR) as an interrogation probe was also demonstrated. The name evolved over the years to magnetic resonance interferometry (MRI) and then finally to MR scanning. MR differs from CT in at least two key aspects: (1) the MR system measures the density of a specific nucleus, and (2) the measurement system is volumetric (interrogation of the entire body, within the measurement volume, is done all at one time). Typically, the MR system is tuned to hydrogen, a common constituent in most soft-tissue cellular matter. It is assumed that this measurement will define the spatial locations of organs by differentiating them according to the densities of hydrogen within their tissues. The system can be tuned to other nuclear species, so long as the nucleus has a magnetic moment (that is, an odd number of protons and/or neutrons). As a result of the diagnostician's familiarity with interpreting CT scan data, MR scan data is also computed and presented in a layer-by-layer format.
These two systems, CT and MR, present the finest resolution capability available in diagnostic systems, achieving volumetric resolutions of about 2 mm in each direction. More recently, spiral-scan CT has been developed. In this modality, X-ray data is collected circumferentially as the patient is continuously moved through the scanner. This spiral data collection path provides partial data at every location along the scan. Using interpolation algorithms, a complete scan can be put together at any desired plane, yielding reconstructed slices at considerably finer spacing than stepping systems achieve. The numerical analysis is thought to be more robust, due to availability of the partial data at every plane. Validation tests using phantoms are still underway. Currently, users of spiral-scan CT are claiming isotropic resolution of 0.5 mm or better. Data from other scanning systems (PET, SPECT, etc.) are considerably more coarse. Their output is also presented in a layer-by-layer format.
The information from CT scanner systems provides a host of potential medical applications. Detailed information can be electronically shared among practitioners, thus permitting distributed consultation. This form of telemedicine is providing expert assistance in remote locations. Another application gaining widespread use is the creation of virtual images of the constituents mapped in the images. Full three-dimensional geometry can be assembled from the data. These images can then be formed into stereoscopic presentations viewed from the perspective of a designated platform, providing the equivalent of a "fly-through." These same images, in a static format, can be presented to surgeons during operations, using "heads-up" displays, to guide them (that is, computer-assisted surgery, or CAS). The images are oriented by the use of registration fiducials, located on the patient and visible in the image, and a tracker determining the surgeon's location and view angle relative to the patient. In addition, this layer data format presents a ready path to control current RP systems, which also function on a layer-by-layer basis. This potential transformation was recognized early in RP development, and accurate anatomical RP models were fabricated. This physical realization of CT data has been termed "real virtuality." These technologies may be able to significantly impact the cost of medical care. Budget demands are placing considerable emphasis on field evaluation. The focus in the United States is on telemedicine and CAS. It is thought that these technologies will have a significant monetary impact. The use of telemedicine is especially important, due to the large physical size of the United States and the difficulty of providing highly trained physicians and surgeons at remote sites. Europe is smaller in size and Japan is much smaller, so issues of distance are less significant there. Telemedicine is still receiving attention, in part due to the application of the same technologies to electronic archiving of patient files. CAS and the application of RP models are under strong development in both regions. There is also a significant effort in Australia in applying RP models for surgical planning. CAS and RP are viewed as complementary, and the RP models offer the advantage of providing stereotactic feedback to the surgeon. Also, the RP models provide a medium for practice efforts; the results can be taken into the surgical theater and used as templates. Furthermore, in complex reconstructions, models of the desired results are used to provide feedback for the surgeon to gauge how close the actual surgical results approach the planned results.