6-心脏MR对比剂

• Classification of MR contrast media is based on a. Distribution in tissues b. Changes in tissue contrast
c. Specificity for special cells or tissue
• The effects of different agents on the relaxation rate (R1) of normal and reperfused infarcted myocardium have been compared in rat hearts. • The data indicate the following: a) Gd-based contrast medium increased R1 of infarcted myocardium significantly more compared with normal myocardium at 5 minutes, followed by a gradual, but parallel, decline in R1 of both regions; b) Gd-DTPA-albumin produced a small increase in R1 of normal and infarcted myocardium at an early phase, followed by a sharp increase in R1 of infarcted, but not normal, myocardium; c) Mn+2 based contrast medium caused an identical increase in R1 of both regions in the first 5 minutes because of the nonspecific distribution of the contrast medium, followed by a rapid decrease in R1 of infarcted, but not normal, myocardium. Overall, provided the largest R1 difference between normal and infarcted myocardium at a later time than Gd-based media.
MRI CONTRAST MEDIA (CARDIAC)
2018-02-21
OVERVIEW
• MR contrast media, paramagnetic and super-paramagnetic metal complexes, are made from metals containing unpaired electrons. These agents are used to enhance the capability of MRI and MRA. • The inherent contrast of the blood and myocardium on MR depends largely on proton concentration and longitudinal (T1) and transverse (T2) relaxation times. MR contrast media and pulse sequences can manipulate tissue contrast. The effects of MR contrast agents on signal intensity are described in terms of T1 and T2 relaxivities (R1 and R2). For any contrast medium, the R2/R1 ratio can be used in determining whether the agent is causing predominantly T2 shortening (decrease in SI) or T1 shortening (increase in SI).
• In coronary artery occlusion and stenosis models remote myocardium appears bright in seconds, and at this time the area at risk appears relatively dark because of delayed arrival of the contrast agent.
CHARACTERIZATION AREA AT RISK
• Accurate sizing of the area at risk using noninvasive method provides valuable information for guiding therapeutic interventions aimed at enhancing collateral blood flow(angiogenesis), preserving peri-infarction zones, and preventing LV remodelling. • First-pass MR perfusion demonstrates the wash-in of the contrast medium in normal and ischemic myocardium.
• Functional and first-pass perfusion MRI were also used in a canine model of coronary artery stenosis for the quantification of functional and perfusion deficits in the area at risk before and after dipyridamole administration. it was found that the extent of functional defect was smaller than the perfusion defect during vasodilation. Other studies have shown that the region demarcated by extracellular MR contrast media underestimates the size of area at risk by at least 10% because of the rapid diffusion of these agents into the territory through collateral vessels. • The spatial extent of the area at risk has been recently measured using intracellular manganesebased MR contrast media. • The size of area at risk on MRI was identical to that measured at post-mortem. • The definition of the area at risk on MRI was based on the reduce uptake of Mn+2 and weak T1 changes compared with normal myocardium.
• Recent animal studies indicated that some of the bold-pool contrast media(Gadomer, Gd-DTPA-24-cascade-polymer, and MS-325) provide longer delineation of area at risk than extracellular media. • Another study used first-pass perfusion MRI to detect and quantify the spatial extent of the area at risk during brief coronary artery occlusion and reperfusion relative to the true size of the area at risk as defined in ex vivo using histochemical stain. Investigators found that there is a linear relationship between the regional contrast enhancement of the area at risk and the blood flow during postocclusive hyperemia, but that the size of the area at risk was significantly smaller than at postmortem. This difference in size may reflect an influence of coronary collateral circulation.
CHARACTERIZATION OF OCCLUSIVE INFARCTION
• A number of clinical studies have indicated that the site of acute infarction can be visualized as a bright region of T2-weighted spin echo and T2-weighted triple inversion recovery sequence. • Although T2-weighted images can detect the presence of myocardial damage, a portion of the area that exhibits increased DI is probably related to the presence of myocardial edema, rather than the myocardial infarction. Especially in the acute phase the demarcated region likely encompasses both viable and nonviable tissue. • Furthermore, the difficulty in measuring the extent of myocardial infarction from T2-weighted images is in defining the boundaries of infarction and the lack of differential enhancement in chronic infarction. • The mechanism by which extracellular MR contrast media concentrate in infarcted myocardium is most likely related to the increase infractional distribution volume because of the loss of cell membrane integrity in acute infarctions and increase in extracelluar matrix structure in chronic(dense collagenous scar tissue) infarctions. Despite the marked structural difference between acute and chronic infarction, hyper-enhancement is observed for both with the use of extracellular agents. • A recent study showed that bold-pool contrast media can discriminate acute from chronic infarction. It showed that the blood-pool agent Vistarem enhances acute myocardial infarction, but not chronic (scar) infarction. The lack of enhancement of scar tissue can be attributed to poor vascularization and intact residual vessels.