Biomedical Engineering Reference
In-Depth Information
distinct as with FDG. However, for many patients there is also tracer uptake in
the skin, providing an important landmark which can be exploited during the
registration process. Despite the limitations outlined above, techniques are
available to cope with difficulties in correlating brain images from PET with
those from MRI, as will be discussed.
For PET images from nonbrain regions (neck, thorax, heart, etc.) the task is
even more complicated in regard to identification of anatomical structures.
The most widely used tracer is FDG, since in oncological studies where tumors
or metastases are searched for, this tracer yields the most valuable results. It has
been shown, however, that reliability in tumor detection can be improved by
image registration with CT or MRI.
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The reason lies in the discriminative
power between normal and pathological uptake, with clues coming from
functional imaging with PET and anatomical confirmation from MRI that
high PET tracer uptake in a particular structure should be interpreted as nor-
mal or pathologic. Color Figure 9.3* shows an example of a PET study with
some focal uptake in the thorax. The precise localization cannot be deduced
without anatomical correlation. There are further limitations in registration of
PET and MR images from regions other than the brain, because the thorax or
the abdomen cannot be regarded as rigid bodies. Special acquisition protocols
that might help in these cases and other related issues are discussed below.
9.3
Properties of Magnetic Resonance Images
Magnetic Resonance Imaging (MRI) is a primary diagnostic tool for generat-
ing structural images of the living human body. In contrast to PET, it does not
involve ionizing radiation but instead applies electromagnetic radiation with
wavelength of approximately 0.3 m, hence with much lower energy than that
used with x-ray computed tomography (CT) or emission tomography like
PET. For obtaining tomograms, the nuclear magnetic resonance of the hydro-
gen nuclei (i.e., protons) is mainly used because of its intrinsic sensitivity
combined with the fact that the human body is primarily composed of H
O.
Together these ensure a sufficiently strong signal. The key to MR imaging is
the design of pulse sequences, which are applied in order to obtain images
with desired contrast. A long list of contributions from many researchers
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with
ever more refined pulse sequences and detection techniques has led to faster
image acquisition and reconstruction, providing high-resolution images of
the brain or other parts of the body, with a wide variety of tissue contrasts.
An example of brain images of the same anatomy showing different con-
trast is given in Figure 9.4. Here the selection of appropriate echo time (TE)
and repetition time (TR) changes the sequence's sensitivity to tissue properties
such as T1 and T2 relaxation times, producing a differentiation among cere-
bral gray matter, white matter, and cerebral spinal fluid (CSF), respectively.
* Color Figures follow page 22.
