Passage The Radiopharmaceutical 2-Deoxy-2-(¹⁸F)fluoro-D-Glucose (Abbreviated as ¹⁸F-FDG) Is a Modified Form

Passage
The radiopharmaceutical 2-deoxy-2-(¹⁸F) fluoro-D-glucose (abbreviated as ¹⁸F-FDG) is a modified form of D-glucose in which the hydroxyl group on the 2-carbon of the molecule has been replaced with a positron-emitting ¹⁸F radionuclide.  Synthesis of ¹⁸F-FDG is accomplished in several steps involving both nuclear and chemical reactions, as shown in Figure 1.
Figure 1  Synthetic scheme for 2-deoxy-2-(¹⁸F) fluoro-D-glucoseInitially, a target of ¹⁸O-enriched water (¹⁸OH₂) is bombarded by protons using a cyclotron to achieve a (p-n) nuclear reaction in which a proton hitting the ¹⁸O nucleus is absorbed by the nucleus, causing the ejection of a neutron and forming an ¹⁸F nucleus.  Following separation of the resulting ¹⁸F⁻ anions from the residual ¹⁸OH₂ via an ion exchange column, ¹⁸F⁻ is used in a nucleophilic substitution reaction with an acetylated mannose triflate substrate.  Subsequent hydrolysis to remove the acetyl protecting groups yields ¹⁸F-FDG.Because ¹⁸F-FDG is an analog of D-glucose, it is readily taken into cells by the same metabolic processes as D-glucose.  Once inside the cells, the positron emissions of the ¹⁸F radionuclide can be detected by positron emission tomography (PET) scans to produce diagnostic medical images of objects such as tumors.  Emissions from the ¹⁸F radionuclide subsequently diminish over time via radioactive decay (Figure 2) .
Figure 2  Nuclear decay of the ¹⁸F radionuclideBecause of limitations in the spatial resolution of PET scanners, when an object (such as a tumor) is smaller than 30 mm in diameter, the measured specific uptake value (SUV) of ¹⁸F-FDG accumulated within the object is lower than the actual SUV.  The recovery coefficient (the ratio of the measured SUV to the actual SUV) shows that the measured SUV decreases as the size of the object decreases below the 30-mm threshold (Figure 3) .
Figure 3  Recovery coefficient as a function of object size in a PET scan with a spatial resolution of 7 mm using ordered subsets expectation maximization (OSEM) image processing
Adapted from: S. Yu. "Review of F-FDG Synthesis and Quality Control." Biomed Imaging Interv J. ©2006 Department of Biomedical Imaging, Faculty of Medicine, University of Malaysa; and P. E. Kinahan, et al., "Positron emission tomography-computed tomography standardized uptake values in clinical practice and assessing response to therapy." Semin Ultrasound CT MR. ©2010 Elsevier.
-Compared to the effective nuclear charge of ¹⁸O, the effective nuclear charge of ¹⁸F is:

A) higher, because ¹⁸F and ¹⁸O have the same number of core electrons but ¹⁸F has fewer neutrons.
B) lower, because ¹⁸F has more valence electrons than ¹⁸O.
C) higher, because ¹⁸F and ¹⁸O have the same number of core electrons but ¹⁸F has more protons.
D) lower, because ¹⁸F has an equal number of protons and electrons.

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