90° radio frequency pulse
FID
T 2
T 2
Time
*
290
Biomedical Signal and Image Processing
FIGURE 15.7 FID pulse with time constant T 2 * and the decay of the transverse magnetization with time constant T 2 . (Courtesy of Siemens AG, Medical Solutions, Magnetic resonance;
brochure: Magnets, flows and artifacts.)
transverse component, M xy , and a longitudinal component, M z , for signal analysis.
The resulting angle between the magnetization vector and the z-axis is called the RF
flip angle, or the flip angle for short.
As the RF pulse excitation is applied, the transverse component M xy starts rotating around z-axis at a flip angle and an angular frequency corresponding to the
Larmor frequency. As the excitation pulse dies, the deviation angle of the traverse
component from the z-axis decreases exponentially with in time. The relaxation time
of M xy vector, i.e., the time it takes for this component to die away, is defined as the
transverse relaxation time T 2 , or spin–spin relaxation time. This relaxation time is
shown in Figure 15.7. The decaying of the traverse magnetic vector induces an oscillating but decaying voltage that is detected by a receiver coil located around the
tissue. By registering this voltage, the relaxation time T 2 is registered (Table 15.1).
The longitudinal component M z also changes as the RF pulse reaches to the
zero state. This component, as illustrated in Figure 15.8, becomes larger and
larger until it reaches its maximum when the complete realignment with the
static field is achieved. The realignment rate of the longitudinal component M z
is described by a longitudinal relaxation time T 1 , which is called the spin-lattice
relaxation time. The voltages detected by the detector coils can also identify the
relaxation time T 1 .
TABLE 15.1
Longitudinal and Transverse Spin
Relaxation Times for Various Tissues
Tissue
T 1 (s)
T 2 (s)
Brain
0.5–1
0.06–0.1
Fat
0.2–0.7
0.05–0.09
Muscle
1–1.8
0.02–0.07
Note: T 1 and T 2 values for tissues at 1.5 T Magnetic
Field Strength.
FID
T 2
T 2
Time
*
290
Biomedical Signal and Image Processing
FIGURE 15.7 FID pulse with time constant T 2 * and the decay of the transverse magnetization with time constant T 2 . (Courtesy of Siemens AG, Medical Solutions, Magnetic resonance;
brochure: Magnets, flows and artifacts.)
transverse component, M xy , and a longitudinal component, M z , for signal analysis.
The resulting angle between the magnetization vector and the z-axis is called the RF
flip angle, or the flip angle for short.
As the RF pulse excitation is applied, the transverse component M xy starts rotating around z-axis at a flip angle and an angular frequency corresponding to the
Larmor frequency. As the excitation pulse dies, the deviation angle of the traverse
component from the z-axis decreases exponentially with in time. The relaxation time
of M xy vector, i.e., the time it takes for this component to die away, is defined as the
transverse relaxation time T 2 , or spin–spin relaxation time. This relaxation time is
shown in Figure 15.7. The decaying of the traverse magnetic vector induces an oscillating but decaying voltage that is detected by a receiver coil located around the
tissue. By registering this voltage, the relaxation time T 2 is registered (Table 15.1).
The longitudinal component M z also changes as the RF pulse reaches to the
zero state. This component, as illustrated in Figure 15.8, becomes larger and
larger until it reaches its maximum when the complete realignment with the
static field is achieved. The realignment rate of the longitudinal component M z
is described by a longitudinal relaxation time T 1 , which is called the spin-lattice
relaxation time. The voltages detected by the detector coils can also identify the
relaxation time T 1 .
TABLE 15.1
Longitudinal and Transverse Spin
Relaxation Times for Various Tissues
Tissue
T 1 (s)
T 2 (s)
Brain
0.5–1
0.06–0.1
Fat
0.2–0.7
0.05–0.09
Muscle
1–1.8
0.02–0.07
Note: T 1 and T 2 values for tissues at 1.5 T Magnetic
Field Strength.