Foundations of Medical Imaging and Signal Recording - Biomedical Signal Analysis: Contemporary Methods and Applications - page 16

attenuation term span the entire body because of the coincidence detec-
tion of paired γ -rays, the so-called annihilation photons. The imaging
equation is given as

ϕ ( l, θ )= K R
−R

A ( x ( s ) ,y ( s )) ds (1.9)

where K represents a constant that includes the constant factors, such
as detector area and eciency, that influence ϕ . The image quality in
both SPECT and PET is limited by resolution, scatter, and noise. PET
has its main clinical application in oncology, neurology, and psychiatry.
An important area is neurological disorders, such as early detection of
Alzheimers disease, dementia, and epilepsy.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is a non-invasive imaging method
used to render images of the inside of the body. Since the late 1970s, it
has become one of the key bioimaging modalities in medicine. It reveals
pathological and physiological changes in bod tissues as nuclear medicine
does, in addition to structural details of organs as CT does.
The MRI signal stems from the nuclear magnetism of hydrogen
atoms located in the fat and water of the human body, and is based
on the physical principle of nuclear magnetic resonance (NMR). NMR is
concerned with the charge and angular momentum possessed by certain
nuclei. Nuclei have positive charge and, in the case of an odd atomic
number or mass number, an angular momentum Φ. By having spin,
these nuclei are NMR-active. Each nucleus that has a spin also has a
microscopic magnetic field. When an external electric field is applied,
the spins tend to align with that field. This property is called nuclear
magnetism. Thus, the spin systems become macroscopically magnetized.
In MR imaging, we look at the macroscopic magnetization by con-
sidering a specific spin system (hydrogen atoms) within a sample. The
“sample” represents a small volume of tissue (i.e., a voxel). Applying
a static magnetic field B 0 causes the spin system to become magne-
tized, and it can be modeled by a bulk magnetization vector M .Inthe
undisturbed state, M will reach an equilibrium value M 0 parallel to the
direction of B 0 , see figure 1.10(a).
It's very important to note that M ( r ,t ) is a function of time and