Image Processing Reference
In-Depth Information
images, or even four-dimensional images representing spatial-spectral or spatial-
temporal (i.e., in cardiac imaging) distributions. In addition, no mechanical adjust-
ments to the imaging machinery are involved in generating these images.
Another peculiarity of the MRI technique is that MR signals used for image
formation come directly from the object itself. In this sense, MRI is a form of
emission tomography similar to position emission tomography (PET) and single-
photon computed tomography (SPECT). But unlike PET or SPECT, no injection
of radioactive isotopes into the object is needed for signal generation in MRI.
There are other forms of tomography in use, including transmission tomography
and diffraction tomography. X-ray computer-assisted tomography (CT) belongs
to the first category, while most acoustic tomography is of the diffraction type.
In both cases, an external signal source is used to “probe” the object being imaged.
MRI operates in the radio frequency (RF) range, as shown in Figure 1.1. There-
fore, the imaging process does not involve the use of ionizing radiation and does
not have the associated potential harmful effects. However, because of the unique
imaging scheme used, the resulting spatial resolution of MRI is not limited by the
probing (or working) frequency range as in other remote-sensing technologies.
Wavelength
(m)
Frequency
(Hz)
Energy
(eV)
10
−
16
10
10
10
24
Gamma rays
10
−
14
10
8
10
22
X-rays
10
−
12
10
6
10
20
10
−
10
10
4
10
18
10
2
10
−
8
Ultraviolet
10
16
Visible light
Infrared
10
−
6
1
10
14
10
−
4
10
−
2
10
12
Microwaves
10
−
2
10
−
4
MR
imaging
10
10
10
−
6
1
10
8
10
2
10
−
8
10
6
Radiowaves
10
4
10
−
10
10
4
10
6
10
−
12
10
2
FIGURE 1.1
Electromagnetic wave range for MR and MRI.
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