HEART
's radio-nuclear image of the heart is based sullarivelazione traciante the signals emitted by a radioactive that is injected, is taken up by the myocardium.
It allows to assess, using different radiopharmaceuticals and different modes of stove, various morphological and functional aspects of the heart:
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substances that mark the mass of blood -> studies of ventricular function, shunts, regurgitation;
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substances that are located on muscle infarction in proportion to the flow and cellular integrity, indicating the disease areas as areas "cold" (negative indicator) -> Diagnosis of ischemia, injury, vitality;
- metabolic substances that are not picked up by the cells when their metabolism is reduced or inadequate perfuzione substances that accumulate in cells that use an anaerobic metabolism (18FDG, 13NH3). -> studies of the myocardium, hibernating, stunned.
Sintigrafia myocardial perfusion or miocardioscintigrafia
myocardial perfusion scintigraphy is widely used in daily assessment of patients with suspected or known coronary artery disease.
Combined effort has proven to increase diagnostic sensitivity and specificity 's ECG.
Pincipi methodological
Most radiolabelled used for this method are similar monovalent cation potassium. The first to be used around 1973, to demonstrate perfusion defects induced by stress, were the 43K and 81Rb. dek limits of 43K and 81Rb were the high energy of the first, the second emission tomography and its short half-life of both. They were then abandoned and completely replaced by 201Tl too similar monovalent cation potassium.
The 201Tl injected once, is picked up by myocardium in proportion to the flow, alal cell integrity and the regional pool of potassium (2% delal dose at rest and during exercise 5-6%). The entry of monovalent cations in cardiac miocellule attivom essentially is a complex phenomenon that requires the intervention of the exchange system sodium-potassium ATPase dependent.
A reduction of flow in one area or anatomical or functional alteration of membrane transport system, a lack of adequate energy-producing or failure to use the same cellular energy lead to a reduced caotazione the radiocompound.
The 201Tl over other radioisotopes had other previous the following advantages:
And the following disadvantages:
Immediately after the injection radiotallio cien picked up by the myocardium in proportion to reionale blood flow and extraction fraction, which is defined as the amount that is extracted from the blood proveiente the first pass from the heart. It varies in a range between 85% and 90%. Acidosis and hypoxia are able to reduce the ejection fraction of thallium.
linear The reaction between thallium uptake and flow is maintained under conditions of increased oxygen demand and dostribuzione radiocompound of the ventricle is homogenous stress in patients with normal coronary although for high levels of flow, as a maximum effort, the extraction fraction undergoes a decrease in proportion. If there are blockages in the coronary artery for coronary flow reserve, ie, the ability to increase the flow downstream of the stenosis is impaired, shows an uneven distribution of thallium, which reflects the disproportion between the increase of flow induced stress in healthy areas and dallla the failure to increase the areas of distribution of coronary obstruction.
Once Thallium has been picked up by the myocardium of a characteristic phenomenon occurs CEH is said to redistribution and that is, within a few minutes dll'iniezione (4-5 ') the radiotalliocomincia to move through the different compartments (blood-myocardium - other districts) tend to rebalance concentrations. In this way, portions of myocardial tissue that received the least share of Thallium to give blood and tissue that it has received at least until reaching an equilibrium concentration. Within 3-4 hours this phenomenon of redistribution generally is completed so that the scintigraphic image, obtained at this time can be regarded as the baseline.
In terms of exercise hyperemia concentration of thallium in the myocardium is greater than in normally perfused blood and this gradient forms the basis for a migration of compoeto blood from the heart. This cleaning mechanism where there is reduced flow reduction, as the areas of hypoperfused myocardium do not receive high flow nor the concentration of thallium, and so the gradient between Thallium miocadio hypoperfused and blood is lower than in the rest of the myocardium. For this assumption the ischemic areas, which have reduced uptake in early images after standardization effort have ridistribuzioneo Itard images, while areas of necrosis, which are not enlarge the uptake of Thallium neanceh belatedly, have also ipocaptanti or absent uptake both early and late after that effort.
The Isonitrili of 99mTc are complex cations are distributed as thallium, are less expensive, can be used in the kits always disponibiliattingendo Mo-99m Tc generator, and for their more energy, suffer less attenuation of the soft parts (breast, etc. ..).
99mTc MIBI or sestamibi is the prototype of these radiolabelled. it enters the cell cytosol by passive diffusion under a concentration gradient dependent flow, which does not require the intervention of the mechanism of transportation of membrane ATPase employee.
99mTc MIBI does not present the phenomenon delal redistribution. Once injected it is outsourced in the myocardium where it remains fixed freezing the image, which does not change until the disappearance of the tracer. For this reason, to compare images with those of baseline stress, you need to inject another dose of 99mTc MIBI. recently because of the short half-life of 99mTc permits (6 hours), was introduced a protocol that provides the baseline control throughout the day to avoid discomfort to the patient to return for a second time (single day).
SPET In assessing the heart based on the short axis, the axis and along the horizontal axis.
The pathological
consist of fixing defects in myocardial radiocompound of which may be reversible or irreversible and location in the three standards and projections related coronary concerned alal . Uptake defects in the septum and anterior wall delal ogeneralmente are due to obstruction of the anterior interventricular artery (IVA), defects of the posterior wall of the right coronary or lower alal (C.dx) or circumflex (CFX), posterior wall defects circonflessaCfx the side. The isolated defects of the apex although that is sprayed from multiple vessels, usually indicate an involvement of the CFX. A lack of uptake of the free wall of the RV was descrittocome segnno prossimaledelal right coronary artery stenosis.
myocardium
It is shown that there are areas of myocardium severely and chronically ipopperfuse ( hibernating myocardium) and other long hypoperfused after vasoconstrictor stimuli such as stress (myocardial stunned), which are able to develop only a minimal metabolic activity, just enough for their survival but inadequate to ensure the movement of contractile or membrane-active phenomena. In both cases it is viable myocardium. In fact stunned myocardium and hibernating are two conditions which have different pathophysiology as a basis of viable myocardium and representing, respectively, acute and chronic adaptations to myocardial ischemic conditions. my cardio stunned (stunned ) is a mechanical dysfunction that persists for hours or days after brief ischemia, spontaneously reversible, but serious, caused dallariperfusione post ischemia. The hibernating myocardium ( iberned ) is a condition of silent hypoperfusion blood, of indefinite duration, which is determined during ischemia, a kind of adaptation to the reduced availability of oxygen Reduced labor ocontrattile, reversible with the restoration of Article dek blood flow with bypass, PTCA or medication.
Metabolic Tracer
the existence of common areas of viable myocardium, but strongly ischemic, Enlarged to maintain only a minimal metabolic activity (hibernating myocardium), but akinetic and incapable of active functions of the membrane, requiring the use of specific methods for their detection and to demonstrate, by now acquired the myocardium of this type is able to recover contractility and metabolic activity, if successfully revascularized. Inadequate myocardial perfusion of a district, is called effort to hear discrepancy between demand and supply of O2, is due to a reduction in primary flow for vasospasm, causing a string of metabolic effects of nature, mechanical, clinical.
using metabolic tracers day you can evaluate myocardial metabolic changes that occur in the tissues downstream of the blocked coronary artery. For these studies can be used to detect gamma-camera scintigraphy with SPET investigations or performing with PET with positron-labeled metabolic substrates.
PET methodological principles
The myocardium in normal conditions of oxygenation used preferentially as substrate metabolism, free fatty acids, you fall in hypoxia, glucose becomes the main metabolic source. If there is low intake of O2 jams meccanismoossidativo the free fatty acids, which accumulate within cells as triglycerides, while the concentration of glucose available, resulting in part from the increased cleavage of glicogenoed in part by increased uptake exogenous glycogen. Although in this way has increased the amount of glicolisianaerobia the accumulation of hydrogen ions to inhibit the ends of key stages regulation of enzyme activity resulting in a lower glucose utilization and decreased production of metabolic energy. If the supply of O2 is restored, as happens in the next step transient ischemia, before they produce a necrotic damage, glucose utilization remains high for a variable period and the oxidation of free fatty acids decreases. This phenomenon, which in areas of severely hypoperfused myocardium may persist for a long time, even at rest and in complete absence of symptoms, it can be demonstrated by PET for the storage of choice in such a place, similar to the labeled glucose (18FDG ).
icon corresponds to the PET therefore an ideal tool for measuring the regional uptake of deoxyglucose glucosioi by its analogue in the areas of infarction who were forced to change their metabolism in the glycolytic ischemia. To make more precise localization of metabolic phenomena, can be used with the PET tracer positronic also suitable to measure blood flow in order to assess the topographic correspondence between ischemic areas and their metabolic comportamneto. To this were used flow tracers such as 82Rb or 13N-ammonia after intravenous injection. Afetti authors studied patients with unstable angina without evidence of previous myocardial in some of these, while nesun perfusion defect at rest were indications, however, could be demonstrated areas of increased glucose utilization even at a distance from ischemic episodes. The data in these cases indicate the existence of chronic metabolic alterations in loading zones, which undergo repeated episodes of ischemia, or otherwise served by highly stenotic coronary arteries downstream of which is always present ischemia and tissue hypoxia.
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