Beam hardening effects of lead shielding during bone scintigraphy in the horse

Abstract

Lameness in the horse is one of the most common clinical entities encountered in equine veterinary practice. Obscure lamenesses, equivocal radiographic findings or non–definable lameness that does not respond to regional anaesthesia, often warrant the use of further, more advanced diagnostic imaging modalities such as nuclear scintigraphy. Bone scintigraphy in the horse is a useful diagnostic imaging modality to help identify sites of active bone metabolism, identified as increased radiopharmaceutical uptake (IRU) in the affected area on a scintigram. The radioactive nuclide, 99mtechnetium (99mTc), is bound to freezedried and specially prepared pharmaceutical compounds such as methylene-diphosphonate (MDP) or disodium-oxydronate (HDP) and methylene-hydroxy-diphosphonate (MHDP), resulting in radiopharmaceuticals which are readily absorbed in areas of increased osteogenic activity. Usually, 99mTc–MDP is injected into a horse intravenously and the bone phase initiated approximately 2.5 to 3 hours later. Depending on the presenting complaint and clinicians’ requests, the procedure may involve extensive parts of the horse’s anatomy and thus personnel may be subjected to at least 1.5 to 2 hours of scanning time. 99mTechnetium is a metastable compound which decays by means of gamma ray emission, the majority of which are at an energy peak of 140.5keV. Once injected into the horse, there is extensive interaction within various tissues of the patient, but especially within large masses of muscle and bone. The resulting emitted polychromatic beam emanating from the horse has a vast spectrum of radiation energies. These energies emitted from the surface of the patient reach bystanding personnel with resultant radiation exposure. It is thus of paramount importance to establish the amount of radiation to which personnel are subjected, and whether conventional lead shielding as used in radiography, decreases the exposure during scintigraphic examinations. Due to the fact that there is interaction of the emitted radiation energies within the lead apron itself, resultant characteristic radiation produced by this interaction may theoretically be more harmful than the polychromatic spectrum emitted from the horse. The removal of lower energy radiation and thereby increasing the average energy of a spectrum is known as beam hardening. Five average sized horses were scanned without lead shields and using combinations of lead shields of varying lead thicknesses on the horse and lead aprons as would be worn by personnel. All resultant energy spectra were measured with a spectrometer and recorded. The energies emitted from horses injected with 99mTc–MDP differed from the energy spectrum of the pure, non–injected radiopharmaceutical. A large component of lower energies was emitted as a result of patient physical matter interaction. These energies averaged at 88–90keV. Once lead shielding was applied, two energy peaks were seen, one at 83–88keV and another at the typical gamma peak of 140.5keV for 99mTc. Depending on the thickness of the lead shielding, the heights of the two peaks varied. Generally, the thicker the lead coats, or combined lead coats (on the patient and on personnel), the smaller the 140.5keV peak and the higher the 83– 88keV peak. This finding was attributed to characteristic x-rays emitted from the lead shielding through the interaction with the 140.5keV of 99mTc. Surprisingly, there was no evidence of the expected beam hardening, the average energy of the spectrum before lead shielding was higher (up to 94.1keV) than the average energy of the spectrum recorded behind lead shielding (up to 88keV). Instead, lead shielding resulted in slight “softening” of the typical 99mTc gamma spectrum. The 140.5keV peak from technetium is theoretically biologically safer than the 83– 88keV peak emitted by characteristic radiation of lead coats. Personnel were exposed to lower energy scatter emitted from the horse at any rate, regardless of any application of lead shielding. The overall intensity of radiation exposure behind lead shielding, however, was reduced by 90%. Therefore, despite altering the gamma spectrum of 99mTc into a biologically potentially more harmful lower peak of 83keV, the wearing of lead shielding during bone scintigraphy is strongly recommended, as it not only reduced the intensity of radiation considerably, but also removes the harmful lower energy scatter emitted from the patient that would otherwise reach bystanding personnel. Further studies are needed to assess the ability of non-leaded shields to effectively shield the polychromatic energy spectrum emitted from horses during bone scintigraphy, and analyse the characteristic energy spectra emitted by these shields.