The amplitude of the net magnetization is related to several parameters, as previously described in Chapter 3. It is reasonable to expect that one of the parameters that affects the amplitude of the magnetic resonance (MR) signal might be the number of hydrogen nuclei within the volume of the sample. If no hydrogen nuclei are present, no signal should be expected. Conversely, if the sample is rich in hydrogen, a strong MR signal may be expected. Voxels with high hydrogen concentration or proton density (PD) appear bright. This reasoning holds true only to a certain extent.

MRI signal amplitude depends on the presence or absence of hydrogen nuclei and is also sensitive to the environment of the hydrogen nuclei. How hydrogen is bound within a molecule also influences the amplitude of the MR signal.

The signal from tightly bound hydrogen dephases and disappears very quickly. Therefore the typical MR signal received originates from loosely bound hydrogen nuclei, which are called mobile hydrogen. Hydrogen nuclei found in liquids are mobile.

An example of this effect is bone. Cortical bone appears black on an MR image because its MR signal is totally dephased before the receiver is turned on. This is not due to the absence of hydrogen; rather, the hydrogen nuclei are tightly bound to the molecule. As a result, bone typically looks like air, which has a low hydrogen concentration. However, medullary bone is visible because of the fat located in the spaces between the trabeculae and in the marrow cavities.

PD is the measure of the concentration of mobile hydrogen nuclei available to produce an MR signal. The higher the concentration of mobile hydrogen nuclei, the stronger the net magnetization at equilibrium (M₀) and the more intense the MR signal. A strong MR signal results in a better MR image.

Figure 6-1 shows three pixels highlighted in the cross section of the patient. Each represents a voxel containing different tissue with different concentrations of mobile hydrogen nuclei. The tissue with the highest proton density (PD) also has the largest value of M₀. Table 6-1 shows averages of relative values of PD for several tissues.

When a patient is positioned in the magnetic field of an MRI system, the net magnetization of the patient does not change instantly. In fact, the M_Z changes rapidly at first and then relatively slowly (Figure 6-2; see also Appendix A). The longitudinal magnetization, M_Z, reaches equilibrium after approximately five T1 relaxation times.

Once equilibrium is reached and M_Z equals M₀, this condition of net magnetization continues indefinitely unless the spins are disturbed. If the magnet is turned off or the patient is removed from the magnetic field, the individual nuclear magnetic moments reposition randomly. This causes the net magnetization along the Z-axis to disappear, so M_Z returns to zero (Figure 6-3).

The time constant that describes the rate at which M_Z returns to M₀ is the T1 relaxation time. For tap water, T1 is about 2500 ms. For tissue in vivo, T1 can be as short as 100 ms for fatty tissues and as long as 2000 ms for bodily fluids such as cerebrospinal fluid (Table 6-2).

An equation describing how M_Z relaxes toward equilibrium has an exponential form. The further M_Z is from M₀, the faster it approaches M₀. As the ensemble of nuclear spins gets closer to M₀, M_Z approaches more slowly.

The ensemble of nuclear spins relaxes to 0.63 M₀ in one T1. As a result, it takes approximately five T1 for a spin ensemble to return to M₀

Related posts:

Magnetic Resonance Imaging Hardware Partially Parallel Magnetic Resonance Imaging Chemical Shift and Magnetization Transfer Fourier Transforms in Magnetic Resonance Imaging Nuclear Magnetism Steady State Gradient Echo Imaging

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