289
Magnetic Resonance Imaging
Since each element has its own Larmar frequency, the element that responds to
the alternating field is the one whose Larmar frequency corresponds to the frequency
of the field. This means that the resulting magnetic resonance happens only in the
element with the Larmar frequency corresponding to the frequency of the alternative field. In many MRI systems, the frequency of the alternating field is tuned to
detect the Larmar frequency of hydrogen. By scanning a range of frequencies and
examining (detecting) the frequency spectrum of the emitted NMR signal or FID,
one theoretically can perform a comprehensive chemical identification of the tissue.
Spectral imaging will be discussed in detail later in this chapter.
To summarize the resonance process described earlier, the imaging process can
be described as the following steps:
Step 1: The tissue is exposed to a homogeneous static magnetic field.
Step 2: After all protons are aligned, the imaging system exposed the sample to
a train of RF pulses at the respective Larmor frequency through the tissue.
This causes the molecules having protons excited by the RF pulses generate
and emit FID signals.
Step 3: The emitted FID signals are collected by the magnetic detectors.
The FID signal collected in Step 3 is used to determine the total number of free
protons. While the exact number of free protons in a biological medium is not very
informative by itself, the relative volumetric distribution of free protons in the tissue
identified by the previously described process is a very informative characteristic.
Specifically, since the presence and density of protons in hydrogen and other elements can identify the distribution of molecules in the tissue. This relative volumetric distribution is therefore used to form an image of the tissue. The detection of
proton density is performed using the measurement of some time intervals that are
described next.
As shown in Figure 15.6, due to the external RF signal, the magnetization M
changes the alignment and will no longer be parallel to the external field. As can
be seen in Figure 15.6, the magnetization vector, instead of aligning with the static
magnetic field (in blue), rotate around the z-axis with a frequency corresponding
to the frequency of the external alternating field. The magnetization is split into a
90° radio frequency pulse
m xy
T 2 Time
FID
FIGURE 15.6 Disorienting RF pulse that strikes all protons out of alignment from the
external magnetic field and the resulting FID RF pulse. (Courtesy of Siemens AG, Medical
Solutions, Magnetic resonance; brochure: Magnets, flows and artifacts.)
Magnetic Resonance Imaging
Since each element has its own Larmar frequency, the element that responds to
the alternating field is the one whose Larmar frequency corresponds to the frequency
of the field. This means that the resulting magnetic resonance happens only in the
element with the Larmar frequency corresponding to the frequency of the alternative field. In many MRI systems, the frequency of the alternating field is tuned to
detect the Larmar frequency of hydrogen. By scanning a range of frequencies and
examining (detecting) the frequency spectrum of the emitted NMR signal or FID,
one theoretically can perform a comprehensive chemical identification of the tissue.
Spectral imaging will be discussed in detail later in this chapter.
To summarize the resonance process described earlier, the imaging process can
be described as the following steps:
Step 1: The tissue is exposed to a homogeneous static magnetic field.
Step 2: After all protons are aligned, the imaging system exposed the sample to
a train of RF pulses at the respective Larmor frequency through the tissue.
This causes the molecules having protons excited by the RF pulses generate
and emit FID signals.
Step 3: The emitted FID signals are collected by the magnetic detectors.
The FID signal collected in Step 3 is used to determine the total number of free
protons. While the exact number of free protons in a biological medium is not very
informative by itself, the relative volumetric distribution of free protons in the tissue
identified by the previously described process is a very informative characteristic.
Specifically, since the presence and density of protons in hydrogen and other elements can identify the distribution of molecules in the tissue. This relative volumetric distribution is therefore used to form an image of the tissue. The detection of
proton density is performed using the measurement of some time intervals that are
described next.
As shown in Figure 15.6, due to the external RF signal, the magnetization M
changes the alignment and will no longer be parallel to the external field. As can
be seen in Figure 15.6, the magnetization vector, instead of aligning with the static
magnetic field (in blue), rotate around the z-axis with a frequency corresponding
to the frequency of the external alternating field. The magnetization is split into a
90° radio frequency pulse
m xy
T 2 Time
FID
FIGURE 15.6 Disorienting RF pulse that strikes all protons out of alignment from the
external magnetic field and the resulting FID RF pulse. (Courtesy of Siemens AG, Medical
Solutions, Magnetic resonance; brochure: Magnets, flows and artifacts.)