ABC | Volume 114, Nº2, February 2020

Update Update of the Brazilian Guideline on Nuclear Cardiology – 2020 Arq Bras Cardiol. 2020; 114(2):325-429 Table 28 – Suggestions for dose limitation, adjusted to body mass index (BMI), using conventional gamma camera technology 24 One-day protocol BMI Dose 1 (mCi) Dose 2 (mCi) < 25 8 24 25 to 30 9 27 30 to 35 10 30* > 35 12 36* Two-day protocol BMI Dose 1 (mCi) Dose 2 (mCi) < 25 8 8 25 to 30 9 9 30 to 35 10 10 > 35 12 12 *Give preference to the two-day protocol. Table 29 – Suggestions for dose limitation, adjusted to body mass index (BMI), using gamma cameras with cadmium-zinc-telluride technology 24 One-day protocol BMI Dose 1 (mCi) Dose 2 (mCi) < 25 4 12 25 to 30 4.5 13.5 30 to 35 5 15* > 35 6 18* Two-day protocol BMI Dose 1 (mCi) Dose 2 (mCi) < 25 4 4 25 to 30 4.5 4.5 30 to 35 5 5 > 35 6 6 *Give preference to the two-day protocol. is directly converted into an electric pulse when it comes into contact with CZT detectors, increasing energy resolution and dispensing with photomultipliers, which makes the detectors much finer, lighter , and more sensible to photon detection . Protocols have suggested that it is possible to combine faster exams with lower dosimetry. Owing to high costs, they are still not widely used in Brazil, where there are few more than a dozen gamma cameras with this new technology , which has already begun to contribute relevant publications to the international scenario. 321 One of the limitations, known as the “Achilles heel” of myocardial perfusion image interpretation is the attenuation which gamma rays undergo when they pass through tissues before reaching the detector. The problem lies in the fact that attenuation may simulate myocardial perfusion defects, and their recognition as cases of “false-positive” results greatly depends on the observer’s experience. These defects occur more frequently in the inferior wall in men (especially in patients with abdominal obesity), and they are described as diaphragmatic attenuation. In women, they are most commonly found in the anterior wall, due to attenuation caused by breast tissue. These imaging defects are more common in obese patients, for which reason they require higher injected doses. When the defects found are tenuous, when they occur in a similar way during resting and stress, and when they are accompanied by thickness in the walls of LV without abnormalities, the myocardial perfusion study may frequently be interpreted as normal, thus sparing the patient other investigations, both those that involve radiation and those that do not, and minimizing costs. Another way to lower the attenuation artifacts is to apply specific machines with other sources of radiation, such as an x-ray emitting tomograph, which calculates tissue attenuation factors and applies attenuation correction to the photons emitted by the radioactive tracer, reducing the effects of attenuation by means of software. This equipment, however, adds costs to the exam, and it is difficult in terms of financial viability. Even in the USA, which is the country with the highest volume of myocardial scintigraphy studies, it is estimated that only 20% of medical services routinely apply this method. Finally, another method, which is widely applied in practice to resolve apparent defects in uptake generated by attenuation of gamma photons when they penetrate tissue, is to acquire images in the prone and supine positions , especially during the stress phase, which significantly reduces the artifacts described. Some services have routinely implemented this practice, even considering the increased gamma camera utilization time with a new image acquisition series. With respect to performing stress exam alone in order to reduce radiation by avoiding the resting dose, the following should be considered: 1. It is possible for the observing doctor to succeed in interpreting a study as “absence of ischemia,” based only on stress imaging, thus avoiding the resting injection. In order to do this, the observer must be confident that the image is perfectly normal and free of any perfusion defects, including “attenuation artifacts,” which are generally recognized by comparing stress and resting images. The alternative is described above, routinely applying attenuation correction, but this would involve increased costs. 2. Another situation, which is not frequent, but which may occur, is when the patient has homogenous tracer distribution in the LV and apparently normal perfusion, but with an observed transient increase or dilation in the same cavity during the stress phase, when compared to resting images (transient ischemic dilation). The following may, additionally, be observed: a) drop in LVEF following stress, compared to resting; and/or b) increased uptake in the RV (generally not visualized) during the phase corresponding to stress, in comparison with resting; these are markers of poor prognosis, even in the presence of perfusion which is apparently “normal” owing to homogenous distribution. In these situations, the patient is exceptionally identified as high-risk based on other findings which are not solely 397