Chapter 11

3D Medical Imaging

Philip G. Batchelor, P.J. “Eddie” Edwards, and Andrew P. King

Abstract This chapter overviews three-dimensional (3D) medical imaging and the associated analysis techniques. The methods described here aim to reconstruct the inside of the human body in three dimensions. This is in contrast to optical methods that try to reconstruct the surface of viewed objects, although there are similarities in some of the geometries and techniques used. Due to the wide scope of medical imaging it is unrealistic to attempt an exhaustive or detailed description of techniques. Rather, the aim is to provide some illustrations and directions for further study for the interested reader. The first section gives an overview of the physics of data acquisition, where images come from and why they look the way they do. The next section illustrates how this raw data is processed into surface and volume data for viewing and analysis. This is followed by a description of how to put images in a common coordinate frame and a more specific case study illustrating higher dimensional data manipulation. Finally, we describe some clinical applications to show how these methods can be used to provide effective treatment of patients.

11.1 Introduction

Medical imaging dates back to the discovery of X-rays by Wilhelm Röntgen in November 1895. His discovery was published in early 1896 and rapidly led to the proliferation of 2D X-Ray imaging equipment. The clinical implications of the fact that X-rays pass through the soft tissue and can image bone was quickly realized

and X-rays were used to image fractures and diagnose the presence and location of foreign bodies such as gunshot wounds.

Much of the 3D imaging described in the earlier chapters of this book is based on one or more optical cameras imaging a visible surface. In contrast, the aim of 3D medical imaging is to model structures beneath the surface of the skin. The techniques of stereo and multi-view reconstruction, such as those described in Chap. 2, can also be applied to X-rays, which have the same projective geometry as camera images. For example, the 3D shape of vessels can be reconstructed from biplanar 2D X-rays [32]. However, such applications have been largely part of research and, for over 70 years since its inception, the field of medical imaging remained largely 2D.

In the 1970s, however, research began on imaging modalities that produce a fully 3D representation of the patient. The first of these was computed tomography (CT), a 3D reconstruction using X-rays. The second was magnetic resonance imaging (MRI) which utilizes the nuclear magnetic resonance effect. These developments were in part only possible due to the increasing power of computing occurring at the same time. Both of these modalities produce volumetric images. These can be considered as a series of aligned slices but really form a continuous 3D block of data. The individual elements are generally referred to as voxels rather than pixels. In a sense, the surface data described in much of this book is not truly 3D, since it represents a 2D surface, albeit a surface embedded in 3D space. In this chapter, we will concentrate on fully volumetric 3D medical imaging modalities, but also look at how surfaces can be extracted from or registered to such images.

Despite being three dimensional imaging modalities, the most common way for a radiologist to view these images is as a series of 2D slices. Until remarkably recently, these would actually be viewed as printed films on a light box. However, with the widespread adoption of standardized networking and reviewing of images via picture archiving and communication systems (PACS), viewing on a computer screen is now the norm. Viewing as 2D slices does enable the full image data to be explored, but viewing in 3D can provide improved perception and the majority of radiologists are now used to navigating and viewing datasets in 3D.

Chapter Outline In Sect. 11.2, we will summarize the principles behind the leading 3D anatomical medical imaging modalities, namely CT and MRI, as well as briefly touching on positron emission tomography (PET). CT and MRI are sometimes described as anatomical modalities, whereas PET is considered functional, in the sense that it shows where metabolism is occurring. There are also functional forms of MRI, however, but discussion of this is beyond the scope of this chapter. In Sect. 11.3, we present methods for surface extraction and volumetric visualization. The following section deals with volumetric image registration, while Sect. 11.5 presents segmentation methods. Section 11.6 considers higher dimensional imaging with the example of diffusion tensor MRI and the final main section describes some clinical applications of 3D imaging; in particular, surgical guidance.