Which of the following delivers the alrgest thyroid dose?
Exposure dose to patients can be expressed as entrance skin exposure (ESE), sometimes referred to as skin entrance exposure (SEE). Exposure can also be expressed in terms of organ dose. Organ doses to the gonads, bone marrow, breast, thyroid, lens, and lung can be determined. Patient position and beam restriction often make a significant difference in patient dose. Examinations performed PA, rather than AP, often decrease exposure to sensitive organs. This is so because the lower energy x-ray photons will be absorbed by the anatomic structures closer to the x-ray source, and the higher energy photons will penetrate and exit the part (penetrating the sensitive part rather than being absorbed by it). PA abdomen radiographs deliver less quantity dose to the reproductive organs than do AP abdomen radiographs. An AP skull projection (80 kVp) delivers about 90 mrad to the thyroid, whereas a PA skull (80 kVp) radiograph delivers 8 mrad; the PA esophagus (110 kVp) radiograph delivers 9 mrad, and the PA chest (120 kVp) radiograph delivers about 1 mrad. (Bontrager and Lampignano, 6th ed., p. 67)