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Imaging
Figure 10: High-mechanical-index Figure 11: High-mechanical-index Figure 12: High-mechanical-index
Intermittent Power Doppler Images at Intermittent Power Doppler Images Intermittent Power Doppler Images at
Rest and During Stress at Rest and During Stress Rest and During Stress
Rest Stress Rest Stress Rest Stress
Images obtained at rest and during stress showing normal
High-mechanical-index (MI) intermittent power Doppler High-mechanical-index (MI) intermittent power Doppler images
myocardial perfusion.
images obtained at rest and during stress showing a obtained at rest and during stress showing a fixed defect (arrow).
reversible defect (arrow).
the blood volume fraction of that pool. For example, during normal is obtained. Patients with normal perfusion and function have excellent
conditions, for every 100 microbubbles within a sample volume in the outcome, while those in whom both are abnormal have the worst
LV cavity, there will be eight microbubbles within a similar-sized outcome. Intermediate outcome is noted in those with normal perfusion
sample volume in the myocardium. When normalized to that from the despite abnormal function. These patients include those with spontaneous
LV cavity, the acoustic intensity measured from the myocardium after reperfusion (about one-sixth of the AMI patients) and those with non-
background subtraction (to eliminate native backscatter from ischemic cardiomyopathies.
myocardial tissue) provides a measure of myocardial blood volume
fraction (since LV cavity is 100% blood).
45
As in end-systole, 90% of the Figure 8 illustrates a large perfusion defect that helped make the
myocardial blood volume fraction is composed of capillary blood, and diagnosis of AMI in a patient with chest pain and normal
a single end-systolic CE image provides an assessment of capillary electrocardiogram (ECG) (see left panel). This patient had total
density in the different myocardial regions. occlusion of a dominant left circumflex coronary artery that was
opened successfully. A repeat study showed excellent myocardial
At steady state, the microbubbles within the myocardium are reperfusion except in a small apical region that showed no reflow (see
destroyed with high-energy ultrasound pulse(s) so that microbubbles right panel). A month later there was normal wall motion in all
are no longer seen in the myocardium. Imaging is then performed to myocardial segments except the apex, which continued to show
measure the rate of microbubble reappearance, which reflects akinesia. Figure 9 illustrates a case of Tako-Tsobu syndrome where
erythrocyte velocity (see top panel of Figure 7). Time versus acoustic apical ballooning (indicated by the arrows) is seen in the B-mode end-
intensity (AI) curves can be generated from different myocardial systolic image (see panel A), but myocardial perfusion is normal (see
regions (see bottom panel of Figure 7) and fitted to an exponential panel B).
91
Based on this study in the emergency department, the
function: y = A(1-e-bt), where y is AI at a pulsing interval t, A is the patient was not taken to the catheterization laboratory and the
plateau AI b representing myocardial blood volume, and b is the rate regional dysfunction resolved spontaneously. In the absence of prior
constant that represents the rate of rise of AI (and thus mean infarction, the detection of CAD on myocardial perfusion imaging is
microbubble velocity). Since blood flow is a volume of blood moving based on the occurrence of reversible perfusion defects during
at a mean velocity, the product of A and b reflects tissue blood flow).
44
pharmacological or exercise stress. CE can be used to detect
coronary stenosis and to quantify the degree of MBF mismatch during
This approach can be used at rest and during stress. At rest it is usually pharmacological stress. Figure 10 demonstrates normal perfusion. At
used to assess risk area for the diagnosis of acute myocardial infarction rest, microbubble replenishment should occur in four to five seconds
(AMI) and other acute coronary syndromes,
46–60
define the extent of if blood flow is normal (see Figure 7). Therefore, the rest image (see
collateral blood flow,
61–65
determine the infarct size after reperfusion,
51–53
left panel) is captured at the fourth heartbeat after bubble
and assess myocardial viability.
46–53,66–71
During stress it is used to make the destruction. If blood flow reserve is normal, the myocardium should
diagnosis of CAD.
72–87
Non-stress detection of CAD has also been reported replenish within one second at stress, hence the stress image (right
using CE where the technique utilizes the increase in arteriolar blood panel) is captured at the first heartbeat after bubble destruction. In
volume to detect coronary stenosis.
88,89
Several studies have reported on the normal setting, these two images (rest and stress) should look
the use of CE for the diagnosis of AMI in the emergency department. In similar. If there is a significant stenosis in the absence of prior
these studies, adding regional function assessment by CE increased the infarction, the rest image should look normal (see left panel in Figure
prognostic information of the clinical variables significantly. When 11), while the stress image should show a defect (indicated by the
myocardial perfusion assessment is added, further additional information arrow in Figure 11).
14 US CARDIOLOGY
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