The thyroid shield is a protective collar, typically made of lead or a lead-equivalent material, worn around the neck during certain medical imaging procedures. Its purpose is to reduce unnecessary ionizing radiation exposure to the thyroid gland. The thyroid, a small, butterfly-shaped gland, is sensitive to radiation-induced damage, especially in children and young adults. Using this barrier is a direct application of the “As Low As Reasonably Achievable” (ALARA) principle, minimizing potential radiation dose without affecting image quality.
Diagnostic Imaging Where Shielding is Standard
Thyroid shielding is routinely provided during projection radiography procedures where the thyroid gland is near the primary X-ray beam, but not the target of the image. The shield works by blocking scattered radiation, which is X-ray energy deflected from the primary beam as it passes through the patient’s body. These scattered rays are the main source of unintended radiation dose to surrounding tissues.
Dental X-rays, particularly those involving the upper jaw or panoramic views, are a common example where a thyroid collar is standard protocol. The thyroid is located directly below the chin, placing it in the path of scattered radiation from the dental beam. Studies show that using a thyroid shield during these procedures can reduce the absorbed radiation dose to the gland significantly.
In general radiography, such as chest X-rays or X-rays of the spine or extremities, the thyroid shield is also useful. Although the primary beam is aimed away from the neck, the shield acts as a barrier against scattered radiation reaching the thyroid from the imaged area. A properly fitted shield minimizes this secondary exposure.
Why Shields Are Excluded in Certain Procedures
Despite the benefits, thyroid shields are not appropriate for all medical imaging and can sometimes be detrimental. The primary reason for exclusion is the potential to create image artifacts, which appear as bright or obscured areas on the scan. These artifacts are caused by the high density of the lead material, which interferes with the X-ray detection process and can render the image unusable.
Computed Tomography (CT)
In CT scans, the use of external shielding is often discouraged, especially when the thyroid is in the direct path of the X-ray tube rotation. The shield can cause severe streaking artifacts, potentially obscuring the anatomy the physician needs to see. Furthermore, the dense shield can trick automated exposure control (AEC) systems into increasing the X-ray output, inadvertently raising the radiation dose to the patient’s entire body.
Nuclear Medicine and Fluoroscopy
Shields are also typically excluded from Nuclear Medicine scans, which involve injecting a radioactive tracer into the patient. These procedures rely on the thyroid gland actively taking up the tracer to create the image. Placing a dense external shield would block the signals the imaging equipment is trying to detect, making the scan impossible to interpret. In complex fluoroscopy procedures, a shield may also be omitted if it would obscure the necessary field of view.
How Thyroid Shields Offer Protection
A thyroid shield functions by attenuation, reducing the intensity of radiation as it passes through the material. The shield is constructed with a core of high-density material, such as lead, which effectively absorbs or scatters X-ray photons. A standard thyroid collar typically offers a lead equivalency of 0.5 millimeters, sufficient to attenuate a large percentage of scattered X-rays.
Patient Advocacy and Proper Shield Placement
Patients should ask the technologist about the use of a thyroid shield before any X-ray procedure, especially where its use is routine or beneficial. For the shield to be effective, its placement is extremely important. The collar must fit snugly and completely cover the entire thyroid gland area at the base of the neck. Patients should confirm that the shield is positioned correctly and does not extend into the area being imaged, which could cause artifacts.