The discovery of x rays over 100 years ago heralded a new way of visualizing the human body. The use of x rays produces a radiographic shadow in a two-dimensional image of the three-dimensional (3D) structures within the body. Although this imaging approach is extremely useful and is still in use today, all three-dimensional information is lost to the physician. Many attempts have been made to develop imaging techniques in which three-dimensional information within the body was preserved in a recorded image. In the early 1970s, the introduction of CT revolutionized diagnostic radiology. For the first time, three-dimensional information was presented to the physician as a series of tomographic, two-dimensional (2D) image slices of the body. In addition, for the first time in radiology, computers became central in the processing and display of the images. The availability of true 3D anatomical information stimulated the field of 3D visualization for a variety of applications in diagnostic radiology [1-3].
The history of ultrasound imaging is much more recent than x-ray imaging. Following the pioneering work of Wild and Reid  in the 50's, the medical use of ultrasound progressed slowly. A-mode systems, producing oscilloscope traces of acoustic reflections, preceded systems producing B-mode grey-scale images of the anatomy, which was followed by systems producing real-time tomographic images of the anatomy and blood flow. The image quality of medical ultrasound has advanced from low-resolution, bi-stable images to images with much greater detail, making ultrasound an important and often indispensable imaging modality in disease diagnosis and obstetrics. Because it is not invasive and because of the improved image quality, and the addition of blood flow and perfusion information by means of the Doppler effect, ultrasonography is progressively achieving a greater role in radiology, cardiology, and in image-guided surgery and therapy. The major advantages of ultrasound imaging are:
- The ultrasound transducer is small and easily manipulated, allowing the generation of real-time tomographic images at orientations and positions controlled by the user.
- The ultrasound image has sufficient resolution (0.2 mm to 2.0 mm) to display details of many structures within the body.
- The ultrasound imaging system is inexpensive, compact, and mobile.
- Ultrasound imaging can provide real-time images of blood velocity and flow, allowing the physician to map vascular structures ranging in size from arteries to angiogenic tumour vessels.
In spite of these very important advantages, ultrasound imaging still suffers from several limitations, which academic investigators and imaging companies are addressing.
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