Dosimetry in Head and Neck Computed Tomography Scans: The Use of Bismuth Shielding of Sensitive Organs

Abstract

Computed Tomography (CT) scans usually promote a higher radiation exposure than a conventional radiography examination. Thus, they contribute to the increase of the patient absorbed dose and the collective dose of the population, being a worry for the public health. The head and neck CT protocols are among those that deposit the highest dose on patient and increase the risk of developing cancers, especially in children and teenagers. In this work, absorbed dose values are evaluated in organs in the head and neck CT scans using anthropomorphic phantoms. Aiming to reduce thyroid and lens doses in head and neck scans, specific bismuth shields were used for these organs and the values of absorbed dose in organs were evaluated. For head scan were proposed a patient's head tilt technique. The absorbed doses were recorded by radiochromic film strips placed in radiosensitive organs such as lenses and thyroid, as well as nearby organs close to the irradiation field, such as the nasopharynx, salivary glands, parotid glands, hypophysis, spinal cord and breasts. CT scans were performed on three different imaging center equipment’s. This study describes in detail the methodology applied for the evaluation of the dose reduction in the lenses and thyroid without and with the use of bismuth shielding, as well as the use of the technique of tilting the patient's head. The data obtained allowed to observe the dose variation in the organs studied. In the head CT scan the use of the bismuth shielding for eyes reduced the absorbed dose in the lenses up to 46% and the preposition of the patient's head tilting in the head CT scan obtained an absorbed dose reduction in the lenses of more than 80%. The cervical CT scan with the use of bismuth shielding for thyroid reduced more than 50% the absorbed dose in this organ. The results showed the efficacy of the use of bismuth shielding and the positioning of the patient with the head tilted, without compromising the image quality of the examination.