Quantitative MCE – offline image processing
Quantification of MCE images obtained at rest and at stress allows for the calculation of myocardial blood volume and the speed at which erythrocytes travel. The product of these two gives the myocardial blood flow (MBF). The ratio of MBF at stress compared to MBF at rest then yields the coronary flow reserve (CFR). There are numerous ways to quantify MCE images, but probably the most commonly used is assessment of images following microbubble destruction using a packet of high MI ultrasound pulses (i.e. destruction-replenishment imaging).
As detailed in chapter 3, scientific experiments performed in the 1990s revealed the following:
- Once a steady-state of contrast has been achieved in the microcirculation (usually with contrast infusion), the peak signal intensity represents the capillary or myocardial blood volume
- The rate at which contrast replenishment occurs after bubble destruction (i.e. microbubble velocity) represents red blood cell velocity (since contrast agents are essentially RBC tracers)
- If a graph is drawn plotting contrast video intensity (VI – y axis) against time (x axis), an exponential curve is obtained.
- Mathematical analysis reveals that the plateau represents the peak MBV (denoted A) and the initial slope of the curve is the microbubble velocity (denoted β). A x β then gives MBF
How to perform MCE Quantification: A Step-by-Step Guide :
- Acquire frames following microbubble destruction
- Launch MCE quantification software package
- Find flash frames and start with first frame after flash stops (myocardium should be black, implying successful bubble destruction)
- Place regions of interest (ROI) in myocardial segments of interest (usually one per segment) and re-position through the cycle, to ensure in each frame ROIs are positioned on the myocardium. ROIs usually placed on the end-systolic frame, as myocardium is thicker so greater chance of including only myocardium and not LV cavity or epicardium
- Analyse – time curves for each region will be obtained
- Software will usually automatically provide values for A and β
- Usually one needs to use the “background subtraction” button (or similar) to remove any ‘noise’ from tissue harmonic(s)
- Analysis of the ROIs produces a time replenishment curve for each ROI and usually will give an automatically calculated value for A and β.
Figure 7: Example of a patient who sustained an anterior MI. Apical two-chamber view during MCE with absence of contrast opacification (solid arrows) at the apex and anterior wall, which were akinetic. The unaffected segments in the inferior wall (outlined arrows) show normal contrast intensity (A). The replenishment curves in the akinetic segment (yellow) demonstrates very low peak contrast intensity (A), microbubble velocity(β) and MBF, in comparison with remote normal segment (red).
Despite a wealth of data from research studies attesting to the value of MCE quantification, uptake in the clinical arena has been noticeably slower than anticipated. Possible reasons for low clinical use of MCE quantification techniques include:
- Time required for post-processing often not available in busy echocardiography departments
- Inability to perform quantification on machines from all vendors
- Uncertainty whether the quantitative values obtained from different software packages are equivalent However, quantification of coronary flow reserve (CFR) is reliably calculated as a ratio so the absolute values for A and β are not essential.
Contrast echocardiography is a versatile, safe and practical technique with a variety of potential applications, ranging from enhancement of cardiac structures in patients with sub-optimal image quality through to calculation of the coronary flow reserve.
One should be aware of the different methods used during contrast imaging and be aware of the artefacts to look out for and how to correct them, where possible. MCE can be performed with a variety of stress techniques and can be interpreted qualitatively, semi-quantitatively or quantitatively using off-line software.