A preliminary report on medical radiation exposures to the US population based on publicly available sources of data estimated that the collective dose received from medical uses of radiation has increased by Ͼ700% between 1980 and 2006. 1 Computed tomography (CT) has had an annual growth rate of Ͼ10% per year and accounted for Ϸ50% of the collective dose in 2006. Approximately 65% of the collective CT dose is from studies of chest, abdomen, and pelvis. In 2006, cardiac CT accounted for 1.5% of the collective CT dose; however, utilization of cardiac CT is expected to rise, with the potential to further increase exposure to the population. 1 Nuclear medicine studies in the United States have increased by 5% annually to 20 million in 2006 and accounted for Ϸ25% of the 2006 collective medical radiation dose. Among nuclear medicine studies, cardiac imaging represented 57% of the number of studies and Ϸ85% of the radiation dose. 1 A number of publications on imaging with CT, fluoroscopy, or radioisotopes have emphasized the risks that may be associated with exposure to ionizing radiation. [2][3][4] To make informed decisions concerning the use of medical radiation in imaging procedures, the following are important components: (1) A working knowledge of the principles and uncertainties of the estimation of patient dose and biological risk; (2) a comparison of the risks of radiation exposure with the risks of activities in daily life; and (3) recognition of the potential risk of failing to make important diagnoses or treatment decisions if imaging is not performed because of safety concerns.There is no federal regulation of patient radiation dose, with the exception of mammography. Most federal and state regulations are aimed at equipment performance or the handling of nuclear materials. Therefore, appropriate utilization of the equipment or nuclear material in cardiac imaging, to maintain the dose as low as reasonably achievable, is the responsibility of the imaging physician and facility. The purpose of this Science Advisory is to provide a conceptual framework and make general recommendations for the safe use of cardiac imaging that relies on ionizing radiation. Parameters of Dosimetry CT and FluoroscopyThe parameters by which ionizing radiation is quantified differ among imaging modalities. 4 The amount of radiation produced by an imaging device can be described using exposure, expressed in International System of Units (SI) units of coulombs per kilogram (C/kg), or air kerma, expressed in SI units of milligrays (mGy). This document will use the term exposure, which refers to the amount of ionization produced in air by photon irradiation. Exposure can be measured for CT and fluoroscopy with ionization chambers within test objects (phantoms) or at body surfaces with minimal difficulty. Measurable or easily derived parameters, such as entrance skin exposure in radiography and fluoroscopy and the weighted CT dose index (CTDI w ) in CT, are useful to establish diagnostic reference levels for radioThe American Heart Ass...

The shielding of positron emission tomography (PET) and PET/CT (computed tomography) facilities presents special challenges. The 0.511 MeV annihilation photons associated with positron decay are much higher energy than other diagnostic radiations. As a result, barrier shielding may be required in floors and ceilings as well as adjacent walls. Since the patient becomes the radioactive source after the radiopharmaceutical has been administered, one has to consider the entire time that the subject remains in the clinic. In this report we present methods for estimating the shielding requirements for PET and PET/CT facilities. Information about the physical properties of the most commonly used clinical PET radionuclides is summarized, although the report primarily refers to fluorine-18. Typical PET imaging protocols are reviewed and exposure rates from patients are estimated including self-attenuation by body tissues and physical decay of the radionuclide. Examples of barrier calculations are presented for controlled and noncontrolled areas. Shielding for adjacent rooms with scintillation cameras is also discussed. Tables and graphs of estimated transmission factors for lead, steel, and concrete at 0.511 MeV are also included. Meeting the regulatory limits for uncontrolled areas can be an expensive proposition. Careful planning with the equipment vendor, facility architect, and a qualified medical physicist is necessary to produce a cost effective design while maintaining radiation safety standards.

A comprehensive performance testing program is an essential ingredient of high-quality single-photon emission computed tomography (SPECT). Many of the procedures previously published are complicated, time consuming, or require a special testing environment. This Task Group developed a protocol for evaluating SPECT imaging systems that was simple, practical, required minimal test equipment, and could be performed in a few hours using processing software available on all nuclear medicine computers. It was designed to test rotational stability of uniformity and sensitivity, tomographic spatial resolution, uniformity and contrast, and the accuracy of attenuation correction. It can be performed in less than three hours and requires only a Co-57 flood source, a line source, and a tomographic cylindrical phantom. The protocol was used 51 times on 42 different cameras (seven vendors) by four different individuals. The results were used to establish acceptable ranges for the measured parameters. The variation between vendors was relatively small and appeared to reflect slight differences in basic camera performance, collimation, and reconstruction software. Individuals can use the tabulated values to evaluate the performance of individual systems.

Performance of a prototype dual-energy digital chest radiography unit in detecting calcified and noncalcified simulated pulmonary nodules was compared with that of a highly optimized, conventional system. Nodules ranging in size (0.5, 1.0, and 1.6 cm), in number (five to 11), and in calcium content (0-25 mg) were superimposed over the lungs of a frozen, unembalmed, human chest phantom. For each technique, six observers examined 50 posteroanterior projections with different randomized nodule locations. Detection consisted of locating and assigning a level of confidence to each perceived nodular opacity. The resulting plots of the true-positive fraction versus the mean number of false-positive calls per projection indicate that for both calcified and noncalcified nodules, the digital unit performed significantly better (P less than .01).

A modified receiver operating characteristic (ROC) study was performed in which five readers were asked to locate multiple nodules on images of an anthropomorphic phantom obtained with a prototype digital radiographic chest unit and with a conventional chest unit. Results indicate that when nodules were projected over the lungs, a significantly greater number (significant at the 5% level) were identified on conventional radiographs, whereas for nodules projected over the mediastinum, the digital images were notably superior (difference significant at the 2% level). An error analysis of the multiple nodule problem and pseudo-ROC curves are presented. The modified ROC study does not suffer from the positional ambiguity inherent in most ROC studies and is efficient in acquiring data.