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 24 – Main characteristics of perfusion myocardial radiotracers labeled with positron emitters Rubidium-82 ( 82 Rb) Ammonia labeled with Nitrogen-13 ( 13 NH 3 ) Water labeled with Oxygen-15 ( 15 O-H 2 O) 18 F-Flurpiridaz Physical half-life 1.27 min 9.97 min 2.04 min 110 min Extraction fraction (flow) 40% to 70% 94% to 98% 95% to 100% > 90% Means of production 82-strontium/rubidium ( 82 Sr/Rb) generator Cyclotron Cyclotron Cyclotron Advantages • Commercially available in the form of a generator • Capable of evaluating flow quantitatively • Short half-life for quick tests • Low radiation • High contrast resolution • Capable of evaluating flow quantitatively • Potential for use with exercise • Ideal for quantification of flow • Short half-life for quick tests • Low radiation • High contrast resolution • Capable of evaluating flow quantitatively • Potential for use with exercise • May be distributed by central production Disadvantages • Short half-life does not allow for exercise • Lower resolution • Requires a local cyclotron • Heterogeneity of distribution • Requires a local cyclotron • Short half-life does not allow for exercise • Not commercially available to its methodological complexity, high operational costs, and low availability of devices and tracers. Recent technological advances have reduced the costs, and its increasingly frequent use in oncology has resulted in increased equipment availability. Nowadays, non-invasive estimation of absolute coronary flow and flow reserve with this technique has become possible and has been validated. 197 The scenario is contrary in Brazil, however. In spite of a small amount of experience using research protocols, this method is not available in clinical practice. Even though PET cameras are distributed throughout the country, there is a lack of other radiotracers for the modality as well as a lack of economic viability. 8.3. Radioactive Tracers for Use in Positron Emission Tomography Different available radiotracers make it possible to identify vasoactive, metabolic, or neurological processes that are present in diverse cardiomyopathies and atherosclerosis, on the molecular level. The images acquired allow for evaluation of the cardiovascular system on different levels, including: perfusion, 201-203 metabolism, 204-207 sympathetic innervation, 208,209 and inflammation; 210-212 depending on the tracer utilized. Myocardial perfusion studies using PETmay be performed using different tracers, each of which possesses specific characteristics, advantages, and disadvantages (Table 24). Rubidium-82 ( 82 Rb) and ammonia labeled with nitrogen-13 ( 13 NH 3 ) have been approved for clinical use by the Food and Drug Administration (FDA), and they are the most commonly used in the USA. On the other hand, water labeled with oxygen-15 ( 15 O-H 2 O) has been used mainly for research in the USA, as it diffuses freely between blood and the myocardium, which makes it ideal for quantitative flow measures. In South America and Brazil, associated costs, especially with tracers, have limited their use. Initial experience with myocardial perfusion using PET and 82 Rb have been conducted at the Heart Institute of São Paulo (InCor, acronym in Portuguese). 213 As 82 Rb is the only one of these tracers produced in a generator system, from strontium-82, it has an advantage in relation to the others whose production depends on a cyclotron. These generators may be transported, and, specifically in case of Brazil, they may be imported especially for this purpose. Their short physical half-life of 76 seconds constitutes another favorable aspect, given that it implies very low dosimetry for the patient, with estimated exposure lower than 2 milliSieverts (mSv), in a stress and resting protocol, including tomography for attenuation correction. On the other hand, stress with exercise becomes unviable, considering the elevated costs, making it possible only in high-volume centers that perform around 40 exams weekly. 214 Payment tables for medical procedures in Brazil, to date, restrict the payment of PET for oncological indications, which complicates its use for cardiology. The use of 13 NH 3 requires a cyclotron, the installation of which has extremely high costs, but it provides high contrast images, due to its high first-pass extraction fraction. It offers good accuracy for absolute measure of myocardial blood flow (MBF), and its relatively long half-life of approximately 10 minutes, makes physical exercise viable. 215 18 F-flurpiridaz is a new tracer. Although its production requires a cyclotron, its labeling with fluoride-18 makes it possible to utilize the production and distribution systems that are already widely available for oncological use of PET. Due to its relatively long half-life of 110 minutes, it is appropriate for associated use with an exercise testing, and its high first-pass extraction fraction also makes it ideal for flow quantification. Its use is currently being evaluated in a phase-III study. 216-220 8.4. Use of PET for Assessment of Myocardial Ischemia PET has advantages over conventional SPECT, including higher spatial resolution and contrast rate, higher sensitivity than the tracers classically used in MPS (thallium-201 and technetium-99m-labeled radiopharmaceuticals), and higher specificity, considering the attenuation correction system based on coupled CT, which results in better capability to differentiate true perfusion defects from attenuation artifacts. 201,221-223 These advantages are notably applied to some special populations, such as obese patients and women with voluminous breasts, in whom gamma ray attenuation in soft tissue may be a factor of greater importance to the final quality of cardiac images. 358