Chemistry Reference
In-Depth Information
Gadoteric acid
O
O
-
O
N
N
O
-
Gd
O
-
N
N
O
O
-
O
FIgure 1.14
Dotarem: A typical contrast agent used in MrI.
1.5
WhAt Is MAgnetIc resonAnce IMAgIng (MrI)?
Magnetic resonance imaging (MrI) is an imaging technique based on the principles of nuclear magnetic resonance (nMr),
which provides microscopic chemical and physical information about molecules. Instead of obtaining information about
chemical shifts and coupling constants, MrI gives spatial distribution of the intensity of water proton signals in the body [24].
MrI measures the relaxation of free hydrogen nuclei when they realign to their original state in the direction of the magnetic
field after having been excited by a radio frequency (rf). Different image contrasts can be achieved by using different pulse
sequences or by changing the imaging parameters such as longitudinal relaxation time (T
1
) and transverse relaxation time (T
2
).
contrast agents are also used to improve the quality of the image (Figure 1.14) [25].
1.5.1
Basic Principles
Approximately 63% of the human body is primarily fat and water, which are comprised of many hydrogen atoms. Thus, MrI
focuses mostly on nMr signals from hydrogen nuclei. nuclei are charged particles that have characteristic motion or
precession that produces a small magnetic moment. In the presence of a magnetic field, the nuclei would move about it in a
phenomenon known as the Larmor precession. The frequency of Larmor precession is proportional to the applied magnetic
field strength as defined by the Larmor frequency ω
0
, ⇒ ω
0
= γB
0
, where γ is the gyromagnetic ratio and B
0
is the strength
of the applied magnetic field. The gyromagnetic ratio is a nuclei-specific constant, for hydrogen, γ = 42.6 MHz/Tesla. A
strong uniform magnetic field of 1.5 or 3 Tesla is generally used in a typical human scanner [24-26]. MrI measures the
relaxation of free hydrogen nuclei after they have been excited by a radio frequency. The electric field of the radio frequency
creates a new magnetic field, B
1
, which induces protons away from the original field, B
0
. The nuclei spins acquire enough
energy to tilt/flip and precess. This 'flip' is time- and power-dependent on the B
1
field and hence the rF pulse. When the
rF field is off, the protons are able to realign to their original state in the direction of the magnetic field, B
0
, by T
1
and T
2
relaxation. In a strong magnetic field, the hydrogen nuclei spin is aligned in a direction parallel to the field. This process is
illustrated in Figure 1.15.
The signal recorded in MrI is the energy given or lost after relaxation of the nuclei from the rF excitation. This signal is
the “spin echo,” which is composed of multiple frequencies, reflecting different positions along the magnetic field gradient.
Fourier transform is used to process these frequencies where the magnitude of the signal at each frequency is proportional
to the hydrogen density at that location, thus allowing images to be constructed. Hence, spatial information in MrI is
encoded in the frequency of the signal, which is dependent on the local value of the magnetic field. The generation of MrI
images requires the combination of both spatial and intensity information. The signal intensity is mostly affected by the T
1
and T
2
relaxation parameters. although in general, the overall quality of the image is strongly dependent on hardware design
such as the transmitting and receiving coil design.
In order to understand how the contrasts of the images are generated, it is important to note that each proton has a
unique T
1
and T
2
, which are parameters that can be easily altered [27-29]. Proton relaxation is a process of realigning
