INTRODUCTION - Fundamentals of Three-Dimensional Digital Image Processing - page 9

X-ray attenuation coecient at specific points within the object is measured,
and this along with spatial changes show the internal structure of the sub-
ject, allowing use of the data as 3D shape information. Currently, recording
similar resolutions both between and within slices is somewhat dicult, with
spacing between slices tending to be to some extent larger. Developments in
the technology are extremely rapid, however, and so such problems will likely
be improved soon.

Remark 1.4. More specifically, this is referred to by names such as positron
emission tomography (PET) and single photon emission CT (SPECT). These
technologies are attracting attention as ways to measure brain functioning. As
with X-ray CT, the measurement of the integral of attenuation along the path
of an X-ray is required. Recent devices allow for imaging while simultaneously
performing X-ray CT.

(b) Magnetic resonance imaging

Magnetic resonance imaging (MRI) is a technique in which the subject is
placed within a magnetic field that is altered at a specific frequency, causing
the subject to emit signals at the spin resonance frequencies of the nuclei of
its component atoms, and the strength of those emissions are measured. By
placing the subject in a precisely controlled magnetic field and controlling in-
formation from signal detection positions, the strength of the resonance signal
at any arbitrary point and surrounding micro scale area can be measured, even
from within the subject [Bankman00]. The strength of the resonance signal
gives the number (ratio) of atoms corresponding to the signal in that area.
At present the technique is used mainly on human subjects as a way to view
hydrogen ratios, making it an effective tool in cancer and tumor diagnosis, as
well as acquiring information about brain, heart, muscle, and other soft tis-
sue form and composition. Isotropic data are more easily obtained than with
theX-rayCTmethoddescribedabove,thoughsomedistortioniscausedby
empty cavities. Types of information that can be obtained using this method
are still increasing rapidly.

(c) Tissue sample microscopy

In medicine, examination of pathological samples is performed by hardening
an actual sample, shaving off slices in thicknesses measured in microns, and
then observing those slices under an optical microscope. By taking and digital-
izing sequential photographs of the image in the microscope (or by connecting
a CCD camera to directly acquire images from the microscope), a 3D image
can be obtained. When using this technique, however, it is dicult to keep
the distancing within the slice and between slices the same, and aligning the
position of adjacent slices is problematic. In theory large-scale 3D images of
up to 1000 × 1000 × 500 elements can be obtained, but digitalization requires