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Magnetic resonance imaging (MRI) interprets the nuclear relaxation or relaxivity of hydrogen nuclei in resident water molecules. Hydrogen nuclei in water have two magnetic spin states: alpha and beta. In the presence of an applied magnetic field, these spins will align with the field in an "excited" state and then relax back to the "ground" state, producing a signal with intensity proportional to relaxivity. Frequently, an MRI contrast agent is used to increase the relaxation time in coordinated molecules, resulting in a more intense signal.Commonly used contrast agents include gadolinium-based agents. Depending on its environment, gadolinium (Gd) can have eight or nine sites in its coordination sphere, allowing Gd³⁺ to interact with eight water molecules as in Gd(H₂O) ₈³⁺. The electronic configuration of gadolinium is shown in Figure 1.
Figure 1 Electronic configuration of gadoliniumGd³⁺ alone is toxic to cells because its ionic radius is similar to that of Ca²⁺, allowing Gd³⁺ to displace Ca²⁺ in biologically important settings. Therefore, Gd³⁺ must be coordinated to an organic ligand such as 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) to allow it to pass through the body safely. When chelated, DOTA displaces the water around gadolinium and leaves only one coordination site for a water molecule (Figure 2) .
Figure 2 Gd-DOTA with a water coordinated to Gd³⁺Zinc ions (Zn²⁺) are required to store insulin and are co-released when insulin is secreted by the pancreas. To detect zinc ions in the body, researchers have designed a variation of Gd-DOTA called Gd-daa3 with two diaminoacetate (daa) arms that preferentially bind to Zn²⁺ ions (Figure 3) . When Zn²⁺ is absent, these arms coordinate to Gd³⁺ and create a nine-coordinate complex that prohibits water from binding to Gd³⁺. Binding of diaminoacetate arms to Zn²⁺ frees Gd³⁺ to coordinate with water molecules. This configuration results in increased signal intensity in the parts of the cell where Zn²⁺ is located.
Figure 3 Gd-daa3 when Zn²⁺ is present
Adapted from Louie A. MRI biosensors: a short primer. J Magn Reson Imaging. 2013;38(3) :530-9.
-Gadolinium becomes ionized to Gd³⁺ when it loses electrons from which orbital(s) ?

Question

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Magnetic resonance imaging (MRI) interprets the nuclear relaxation or relaxivity of hydrogen nuclei in resident water molecules. Hydrogen nuclei in water have two magnetic spin states: alpha and beta. In the presence of an applied magnetic field, these spins will align with the field in an "excited" state and then relax back to the "ground" state, producing a signal with intensity proportional to relaxivity. Frequently, an MRI contrast agent is used to increase the relaxation time in coordinated molecules, resulting in a more intense signal.Commonly used contrast agents include gadolinium-based agents. Depending on its environment, gadolinium (Gd) can have eight or nine sites in its coordination sphere, allowing Gd³⁺ to interact with eight water molecules as in Gd(H₂O) ₈³⁺. The electronic configuration of gadolinium is shown in Figure 1.

Figure 1 Electronic configuration of gadoliniumGd³⁺ alone is toxic to cells because its ionic radius is similar to that of Ca²⁺, allowing Gd³⁺ to displace Ca²⁺ in biologically important settings. Therefore, Gd³⁺ must be coordinated to an organic ligand such as 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) to allow it to pass through the body safely. When chelated, DOTA displaces the water around gadolinium and leaves only one coordination site for a water molecule (Figure 2) .

Figure 2 Gd-DOTA with a water coordinated to Gd³⁺Zinc ions (Zn²⁺) are required to store insulin and are co-released when insulin is secreted by the pancreas. To detect zinc ions in the body, researchers have designed a variation of Gd-DOTA called Gd-daa3 with two diaminoacetate (daa) arms that preferentially bind to Zn²⁺ ions (Figure 3) . When Zn²⁺ is absent, these arms coordinate to Gd³⁺ and create a nine-coordinate complex that prohibits water from binding to Gd³⁺. Binding of diaminoacetate arms to Zn²⁺ frees Gd³⁺ to coordinate with water molecules. This configuration results in increased signal intensity in the parts of the cell where Zn²⁺ is located.

Figure 3 Gd-daa3 when Zn²⁺ is present
Adapted from Louie A. MRI biosensors: a short primer. J Magn Reson Imaging. 2013;38(3) :530-9.
-Gadolinium becomes ionized to Gd³⁺ when it loses electrons from which orbital(s) ?

Quizplus · Accepted Answer

11ecba2d_118c_b023_a3bf_93453036c89b_MD0008_00 The electron configuration of an atom describes the placement of electrons within the atom and can be used to determine order of electron removal during ionization. In standard notation, the electron shell is denoted by the principal quantum number (n), and subshells are typically denoted by a letter (eg, s, p, d, or f). The principal quantum number is related to the distance of an orbital from the nucleus, whereas the subshells are related to orbital shape. The full electron configuration of an atom lists all orbitals and the number of electrons occupying each. However, these lists can become cumbersome for elements with many electrons, so a shorthand version is often used in which the preceding noble gas on the periodic table is listed followed by the remainder of the configuration. For instance, the electron configuration for gadolinium can be reduced from the long form: to the much simpler [Xe]6s²4f⁷5d¹. Note here that Gd shows an exception to the usual orbital filling order because the energy difference between the 4f and 5d orbitals is small and promoting an electron to the 5d orbital stabilizes the electron by minimizing electronic repulsion. During ionization, electrons are removed from the valence electron shell with the greatest principal quantum number first because these electrons are farthest from the nucleus and are therefore the least tightly bound. Once all electrons from that shell are removed, additional electrons will be removed from the shell with the next greatest principal quantum number. Therefore, gadolinium will lose its first two electrons from the 6s subshell followed by one electron from the 5d subshell to produce Gd³⁺, which has the configuration [Xe]4f⁷. (Choices A, B, and C) Gadolinium does not lose any electrons from the 4f orbital. Atoms tend to lose the outermost valence electrons first because these electrons are farthest from the nucleus. Educational objective: The electron configuration of an atom describes the placement of all the electrons within the atom. During ionization, electrons are removed from the orbital with the greatest principal quantum number.

Passage Magnetic Resonance Imaging (MRI) Interprets the Nuclear Relaxation or Relaxivity