In this blog post, we’ll explore the principles and characteristics of CT and MRI, and examine the concepts of computer graphics used to represent the inside of the human body, as well as how sweep and mesh-based modeling techniques are applied to medical imaging.
What exactly do CT and MRI do? They are leading medical imaging technologies that allow us to visualize the interior of the human body—which is otherwise invisible to the naked eye. To briefly explain their respective principles: Magnetic Resonance Imaging (MRI) involves placing the human body inside a magnet that generates a strong magnetic field, then using radiofrequency waves to alter the signals from hydrogen nuclei within the body. The signals, which vary depending on the tissue type, are measured and reconstructed by a computer to produce an image. MRI does not use ionizing radiation like X-rays and is particularly effective at providing detailed images of soft tissues such as the brain, spinal cord, nerves, muscles, ligaments, and tendons.
In contrast, computed tomography (CT) uses X-rays to capture images of the body from multiple angles; a computer then reconstructs this data to produce cross-sectional images. In a CT scan, the patient enters a machine with a circular opening, and an X-ray source and detector rotate around the patient to collect data from multiple angles. When a computer reconstructs this data, it creates cross-sectional images of the body’s interior. Compared to plain X-rays, CT scans reduce the overlap of structures, allowing for a more detailed examination of bones, organs, soft tissues, and blood vessels.
Why did I suddenly mention CT and MRI? The reason is simply to explain the importance of computer graphics by relating it to their respective strengths and weaknesses. Even today, CT and MRI play a crucial role in examining the interior of the human body, diagnosing diseases, and developing treatment plans. However, since medical imaging is fundamentally based on data obtained from the actual human body, it focuses on representing the anatomical structures and conditions at the time of imaging. Furthermore, a single image may have limitations when it comes to intuitively grasping complex three-dimensional structures, the spatial relationships between organs, or their movements. One of the technologies used to overcome these limitations is computer graphics and 3D medical image processing.
Graphics refers to the technology of generating and processing images and videos on a computer using graphic display devices and graphic input/output devices. Simply put, graphics encompasses various techniques for generating, transforming, and rendering images that we can see in two- or three-dimensional space using a computer. In the medical field, graphics technology can be utilized in various ways, such as reconstructing the internal structures of the body in 3D based on data obtained from CT or MRI scans, or highlighting specific tissues for better visualization. In fact, techniques such as surface rendering and volume rendering are used for 3D reconstruction of medical images, and these technologies are employed to visualize 2D medical images in 3D form.
One of the key advantages of these graphics techniques is the freedom of expression they offer. This means that structures difficult to observe directly in the human body can be represented in various ways on a computer. Leveraging this advantage, graphics are used to depict the interior of the human body or to present medical images in three dimensions. Here, we will examine sweep-based modeling and mesh-based modeling as two representative methods for representing human anatomy using computer graphics. Sweep technology involves creating curved surfaces or three-dimensional objects by moving a curve or cross-section along a fixed path; it can be used efficiently when modeling structures that are easy to express mathematically. This process can utilize mathematical representations such as Bézier curves or Bézier surfaces. In contrast, mesh technology involves connecting multiple polygons, such as triangles, to form the desired shape; it is useful for representing complex and irregular forms rather than regularly repeating structures.
So, how are these two representative technologies used to depict the interior of the human body? First, the sweep method can be effectively used to depict structures that extend in a long, continuous line and take on tube- or cylinder-like shapes, such as human arteries, veins, airways, esophagi, and capillaries. It can also be used to represent structures that have a consistent curved surface but vary in cross-sectional size or shape depending on their location, such as the small intestine, large intestine, pancreas, and stomach. In particular, if the direction of the curves and the changes in the cross-sections can be defined relatively regularly, mathematical models can be used to efficiently represent the structure. The G1 continuity referred to here means the property where the direction of the tangent is continuous at the point where two curves or surfaces meet. By utilizing this continuity, it is possible to represent a shape where the structure flows naturally without sudden breaks.
Next, meshes are effective for depicting objects with complex surfaces—such as internal organs—that are difficult to implement using only mathematically simple equations. A prime example is the heart. Structures with complex and irregular shapes, like the heart, are relatively easy to represent using a mesh method, which constructs the overall shape by connecting numerous small triangles. Strictly speaking, it is not impossible in principle to represent all shapes that can be expressed as meshes using mathematical surfaces. However, representing complex shapes using a single mathematical equation or a complex surface can make calculations and modeling extremely difficult. In contrast, meshes are widely used in practical computer graphics because they can approximate complex shapes by connecting many relatively simple polygons. They are particularly useful when modeling structures with irregular or complex curvature. As such, sweeps and meshes are each suited for different types of structures, and in practical graphics applications, either method can be selected appropriately or both can be used together depending on the purpose.
Using graphics in this way greatly increases the variety of images we can depict and allows us to represent the same medical data from various perspectives. In particular, reconstructing data obtained from CT or MRI into three dimensions can help users intuitively understand the spatial relationships of complex anatomical structures better than when viewing only two-dimensional images. Three-dimensional models of medical images are used for purposes such as preoperative planning or verifying specific anatomical structures, and actual medical software incorporates features such as two- and three-dimensional visualization and multi-plane reconstruction using CT and MRI data.
Rather than simply stating that the costs associated with using computer graphics are uniformly lower than those of CT or MRI equipment, it is more accurate to understand them in terms of the software and computing resources required to process and visualize existing medical image data. CT and MRI require specialized equipment, facilities, maintenance, and trained personnel for imaging; in particular, since CT uses X-rays, radiation doses must be properly managed when necessary. In contrast, graphics-based 3D modeling offers the advantage of processing acquired data on a computer to generate new forms of visual information. Therefore, it is more accurate to view graphics not as a technology that replaces CT or MRI, but as a complementary technology that enables the interpretation and representation of information obtained from these images in a wider variety of ways.
Furthermore, by utilizing graphics and medical image processing, it is possible to go beyond simply displaying actual human anatomical structures and create models based on various assumptions for use in education, surgical planning, and simulation. However, models created using computer graphics cannot automatically predict a patient’s future condition or the onset of a disease. Predicting diseases or forecasting treatment outcomes requires a variety of data—including clinical information, imaging data, biometric data, and statistical and computational models—as well as a rigorous validation process. Therefore, the strength of computer graphics lies not in accurately predicting the future, but in visually recreating various scenarios based on actual data, thereby helping medical professionals and researchers more easily understand complex structures and information.
With current graphics technology, it is possible to create personalized 3D models that reconstruct the internal anatomy of a specific patient using computer graphics, provided sufficient medical imaging data is available. Based on imaging data such as CT and MRI scans, a patient’s anatomical structure can be reconstructed, and specific areas can be isolated for visualization or examined from various angles. Furthermore, if data showing changes over time is available, dynamic visualization depicting the movement of organs or blood vessels is also possible. These technologies can assist medical professionals in understanding a patient’s anatomical structure and planning treatments or surgeries, and they can also be utilized in research and medical education. In fact, medical imaging software already includes features that utilize CT and MRI data to visualize 3D anatomical structures and support preoperative planning.
As such, the human body and computer graphics are becoming increasingly inseparable. While CT and MRI provide imaging data of the body’s interior—information that cannot be seen directly with the naked eye—computer graphics reconstruct and represent that data as various forms of two-dimensional and three-dimensional images, helping us understand it more intuitively. In particular, modeling techniques such as sweeps and meshes can be used to represent a wide range of human anatomical structures on a computer, from relatively regular structures like blood vessels and airways to complex organs such as the heart. As medical imaging and computer graphics technologies continue to advance, we will see an increasing variety of applications that more precisely reproduce the anatomical structures of actual patients and utilize them in medical education, surgical planning, simulation, and research.
