ABSTRACT
The word tomography consists of two Greek words tomos (slice) and graphein (draw). Historically, the term tomography has referred to a technique of Xray photography in which only one plane of the internal structure inside the object is photographed in shape focus. Linear and transaxial tomography are two examples. In the linear tomography as shown in Figure 2.1, an X-ray source and a
photographic plate (which is parallel to the cross section of an object) are placed on two sides of the object. By moving the X-ray source at a fixed speed parallel to the cross section in one direction, and moving the plate at an appropriate speed in the opposite direction, a point in the cross section (denoted by •) is always projected onto the same point in the plate, but the point above or below the plate is projected onto different points in the plate. Thus, on the photographic plate the cross section stands out while the remainder of the object is blurred. In transaxial tomography as shown in Figure 2.2, an object sits in a turn-
table in an upright position. The photographic plate is on a horizontal table next to the object. X-rays are directed obliquely through the object and projected onto the photographic plate. Both the object and the photographic plate are rotating in the same direction and at the same speed. Only those points actually on the focal section of the object remain in sharp focus throughout a rotation. Points that are above or below the focal section are blurred. Nowadays, tomography refers to the cross-sectional imaging of an object
from either transmission or reflection data collected by illuminating the object from many different directions. Computed tomography (abbreviated as CT) is different from conventional
tomography in that the images of the cross sections in CT are not influenced by the objects outside those sections. In X-ray transmission CT (abbreviated as X-ray CT), a single X-ray source
is contained in a tube and the detector device consists of an array of X-ray detectors. The X-ray tube and the collimator are on one side and the detector device and the data acquisition unit are on the other side of the object. Both
✲
✛
❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏❏
❫ ✂ ✂ ✂ ✂ ✂ ✂ ✂ ✂ ✂✂
✌
x-ray source
photographic plate
cross section
t1 t2
t1 t2
•
⋆ ⋆
FIGURE 2.1
Linear tomography: t1 and t2 denote the time instants and −→ represents the moving direction.
