Fluorescent Thyroid Scanning: A New Method of Imaging the Thyroid

Abstract

A method has been developed for imaging the thyroid gland through the use of K-shell fluorescence. This work was stimulated by the pioneer efforts of Jacobson (4) and Roy et al. (7), whose studies demonstrated the feasibility of radiologic detection of in vivo thyroidal iodine. Although it is well known that the thyroid gland selectively traps and incorporates significant amounts of iodine, it is not generally appreciated that the iodine content of the average thyroid gland is only 0.04 per cent by weight (6). Nevertheless, this quantity is sufficient to act as the target in the system to be described. The technic for imaging the thyroid gland which is presented in this communication incorporates the basic scheme of an x-ray fluorescent spectrometer. The equipment is a modification of the TMC Model 331 photon spectrometer used in conjunction with a dysprosium-159 radiation source. The dysprosium is placed in the flanges of a collimator, and a lithium-drifted silicon crystal (30 mm2 with 3 mm depleted region) is located behind the collimator in a low temperature vacuum chamber. A human thyroid gland, embedded in a plastic neck phantom, serves as the target in this study and is placed in front of the source detector system (Figs. 1 and 2). Dysprosium 159 is used as the irradiation source because it produces a reasonably monochromatic 44.5 keV x ray. This energy is above the K-shell edge of iodine, which is 33.2 keV. It is also high enough above the characteristic radiation of iodine not to interfere as background. When the 44.5 keV source x ray interacts with a K-shell electron in the iodine atom, a photoelectron is produced and a vacancy is created in the K shell. An electron falls from one of the outer shells to replace it, and a characteristic x ray is produced. The probability that this will occur is 87 per cent. When the electron is from the L shell, the 28.5 keV Kα characteristic x ray is generated, and when the x ray is from the M shell, the 32.3 Kβ characteristic x ray is produced. Production of Kα x rays predominates over the production of Kβ x rays in a ratio of 7.5/1 (1). It is the Kα x ray with a half-value layer in soft tissue of ≈2 cm that is used to produce the thyroid image in this system. The detector is a 30 mm2 lithium-drifted silicon crystal with a depleted layer of 3 mm. The energy resolution of this crystal in the energy range of 30 keV is better than 500 eV full width at one-half maximum at 78° K. The efficiency of collection is ≈66 per cent. The target phantoms in this study consisted of normal thyroid glands (obtained at autopsy) fixed in formalin and embedded in a plastic (Lucite) cylinder 11 cm in diameter. At no point was the gland closer than 3 mm to the surface of the phantom, and the average thyroid mass had 1 cm of tissue equivalent over it. The neck phantom was then placed against the source-detector system, and an image was obtained.