Biomedical Engineering Reference
In-Depth Information
MRI
Optical
PET
Source of
signal
Nuclei with magnetic moment,
i.e., odd number of protons
and/or neutrons
Fluorescent compounds
bioluminescent compounds
Metastable positron
emitting radionuclide
B
0
S
1
Physical
principle
γ
γ
rf
Absorption
Emission
h
ν
l
M
0
e
-
h
ν
ex
e
+
M
xy
γ
γ
S
0
Nuclear magnets align along
magnetic ield
B
0
. Application
of radiofrequency (rf ) pulse
generates
transverse
magnetization
M
xy
,
the
MRI signal
Absorption of photon generates
excited state
S
1
, from which
system relaxes to the ground-
state
S
0
by
emission of a photon
of lower energy (luorescence)
Positron (e
+
) emitted by radio
nucleid is anihiliated through
interaction with electron (e
-
),
generating
two γ-photons
traveling in opposite direction.
Inluence of
environment
on signal
Relaxation times:
T
1
,
T
2
,
T
2
,
T
1p
water difusion: ADC
water exchange rates:
k
ex
Fluorescence quantum yield
luorescence lifetime
absorption scattering
Scattering (Compton)
Spatial
information
Frequency encoding of spatial
information
Requires solution of inverse
problem of electrodynamics:
difuse photon propagation
Electronic collimation/
coincidence detection:
anihiliation event has occured
on line of response (LoR)
Spatial
resolution
100 μm
1-2 mm
1-2 mm
FIGURE 7.2
Features of imaging modalities MRI, l uorescence imaging, and PET. MR images represent
the weighted distribution of tissue protons, predominantly those of water and adipose tissue. The signal is
weighted by parameters such as the relaxation times, diffusion properties, or proton exchange rate, which
depend on the local environment. Spatial encoding is achieved by applying magnetic i eld gradients: as the
resonance frequency depends on the local magnetic i eld, the spatial information is directly encoded in fre-
quency information. Spatial resolution in animal imaging is of the order of 100 μm. Fluorescence imaging
measures the distribution of l uorescent molecules (dyes, quantum dots, or l uorescent proteins). As light is
heavily scattered by tissue, photon propagates as a diffusion wave. In order to determine the localization
and intensity of a l uorescent source the inverse problem has to be solved by iterative procedures. Spatial
resolution is of the order of 1 mm. PET measures the distribution of radionuclides that decay by emitting a
positron. As antimatter particles, positrons are captured by electrons after traveling through tissue for a short
distance (positron range), the annihilation process generating two γ-photons traveling in opposite direction.
The detector system consists of a ring of scintillation crystals. Coincidence detection of the two photons
allows determining a line of response (LOR), on which the annihilation process (not the radionuclide decay)
has occurred. Measuring a sufi cient number of LORs allows reconstructing the PET image.
7.2.1 C
ONVENTIONAL
I
MAGING
Classical imaging yields morphological and physiological information. Intrinsic contrast in images
is governed by the interaction of radiation with tissue and hence on the biophysical properties of
tissue. For x-ray-based techniques such as computer tomography (CT) the incident x-ray beam is
attenuated upon passage through matter due to scattering at electrons. MRI maps the distribution
of protons (water) in tissue. The signal intensity is governed by a multitude of parameters: proton
density, spin relaxation properties that describe interaction of the protons interrogated with their
environment, macro- and microscopic motion in a magnetic i eld and chemical exchange reac-
tions that alter the local environment of protons. This multiparameter dependence explains the
high soft-tissue contrast provided by MRI methods. In ultrasound, the pressure wave is rel ected
by tissue interfaces and the contrast arises from differences in tissue elasticity, compressibility and
backscattering.
