The advancement of endoscopic transnasal skull base surgery requires high-resolution, precise, real-time visualization. Three-dimensional endoscopy represents an important potential development to aid visualization. When assessed directly against standard 2D endoscopic visualization, 3D was the preferred method for surgeons, both practicing and in-training. “Adding binocular vision through novel 3D imaging and rendering technology to endoscopic approaches has the potential to reduce mistakes in movement, provide more visual anatomic cues by more clearly illuminating depth relationships, and reduce learning curves for novice surgeons”.
3 D in Neuroimaging
The neuronavigation systems have become standard tools for planning and guidance of surgery of both spine and brain. These systems make use of MRI or CT multislice images and produce 3 D models for this purpose. In this chapter, we will discuss about the basics of MRI (magnetic Resonance Imaging) and CT (Computed Tomography) and then production of 3 D images. A 3D data set can be formed either by stacking slices (multi-slice acquisition) or by acquiring the data in the 3D Fourier domain.
Basics of MRI
The MRI machine consists of:
Fig: MRI system
The MRI signal arises from protons in the body, primarily water, but also lipid. The patient is placed inside a strong magnetic field, which produces a static magnetic field typically more than 10,000 times stronger than the earth’s magnetic field. Each proton, being a charged particle with angular momentum, can be considered as acting as a small magnet. The protons align in two configurations, with their internal magnetic field, aligned either parallel or antiparallel to the direction of the large static magnetic field, with slightly more in the parallel state. The protons precess around the direction of the magnetic field and the frequency of precession is proportional to the strength of the static magnetic field. Application of a weak radiofrequency (RF) field causes the protons to precess coherently, and the sum of all the protons of precessing is detected as an induced voltage in the tuned detector coil.
Magnetism of the body
Most frequently, the MR (Magnetic resonance) signal is derived from hydrogen nuclei (meaning the atomic nuclei in the hydrogen atoms). Most of the body’s hydrogen is found in the water molecules. Few other nuclei are used for MR. Hydrogen nuclei (also called protons) behave as small compass needles that align themselves parallel to the field. This is caused by an intrinsic property called nuclear spin (the nuclei each rotate as shown in Fig 4.1). By the “direction of the nuclear spins” we mean the axis of rotation. The protons are randomly aligned and there is no net magnetization.
Fig 4.1: Alignment of protons in human body
When a strong magnetic field B0 (1.5 T) is applied in the z- direction, the protons align themselves in parallel and anti-parallel direction and precess around the magnetic field with a particular frequency called Lamor frequency. The parallel protons are more in number when compared to anti-parallel protons because of their lesser energy of magnetization. Thus there will be a net magnetization proportional to the difference in the number of parallel and anti-parallel protons. The net magnetization will be in the z-direction which is the direction of applied magnetic field as shown in Fig 4.2.