Muscular Contraction
With increasing energy demands, skeletal muscle relies more on glycolysis of glucose to meet those demands. Active contraction of skeletal muscle, particularly during the first half-hour following FDG injection, will be responsible for increased uptake. Heavy muscular activity (weight-lifting, heavy exercise, etc.) within 24 hours prior to injection may also demonstrate increased FDG uptake, in an attempt to replenish glycogen stores. If a particular set of muscles were used during an activity, the increased uptake may be limited only to those. For example increased uptake in the pectoral muscles following heavy bench pressing, or in respiratory muscles (intercostals, diaphragm crura, neck strap muscles) in a patient with pulmonary disease and labored respiration. Altered muscle-use following surgery or injury can also produce unusual asymmetric patterns of muscular uptake.

Common sites of skeletal muscular uptake:

• Ocular muscles
• Muscles of mastication
• Neck strap muscles
• Tongue - especially the base
• Larynx
• Forearms
• Hands
• Large muscles of the legs

Effect of Insulin on skeletal muscle
More diffuse and generalized muscle and fat uptake may be seen as a result of a hyperinsulnemia following meals and insulin injections in diabetics.

Possible limitations:
FDG can be 'stolen' by muscles, due to increased demand or hyperinsulinemia, reducing the sensitivity of disease detection thereby increasing the number of false negatives.

Preventive measures:

• Avoid heavy activity at least 24 hrs prior to scan.
• Avoid any activity immediately following injection prior to imaging. Preferably have patient rest in a quiet dark room alone (no talking, no chewing gum) during the uptake period.
• Avoid insulin injections a few hours prior to scan. Have diabetic patient follow normal morning routine and then fast prior to scanning in the afternoon.