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Machine settings (influence of the mechanical index)

An introduction to contrast

Non-Invasive Imaging

In this section, we will be describing in greater detail the interaction between mechanical index and microbubble behaviour and the impact of dynamic range on microbubble concentration.


 

When exposed to ultrasound waves, microbubbles can respond in one of three ways:

1- Linear oscillation

2- Non-linear oscillation
Thus, microbubbles are not just passive reflectors but active generators of sound energy. This is a key feature of contrast imaging. The manner in which the bubbles behave is related to the degree that they are stimulated (i.e. the acoustic energy they are exposed to) – this is defined by the Mechanical Index (MI) and, as a legal requirement, is always displayed on all ultrasound machines. This is outlined in figure 3 below and can also be seen in the links to the accompanying video clips above.

3- Destruction (oblique implosion)
The mechanical index is an estimate of the maximum amplitude of the pressure pulse in tissue and is an indicator of the likelihood of adverse mechanical (i.e. non-thermal) bio-effects (streaming and cavitation). The mechanical index of the ultrasound beam is the amount of negative acoustic pressure within a ultrasonic field and is used in contrast echocardiography to modulate the output signature of UCAs to incite different microbubble responses.

At low acoustic power (MI < 0.2), the acoustic response is considered as linear, as the microbubbles undergo oscillation with compression and rarefaction that are equal in amplitude and thus no special contrast enhanced signal is generated. Microbubbles act as strong scattering objects due to the difference in impedance between air and liquid, and the acoustic response is optimized at the resonant frequency of a microbubble.

At intermediate acoustic power (MI between 0.2–0.5), non-linear oscillation occurs (bubbles undergo rarefaction that is greater than compression). Ultrasound waves are created at harmonics of the delivered frequency. The harmonic response frequencies are different from that of the incident wave (fundamental frequency). These contrast enhanced ultrasound signals are microbubble-specific.

At high acoustic power (MI > 0.5), microbubble destruction begins with subsequent transient emission of high intensity signals that are very rich in non-linear components. Intermittent imaging becomes necessary to allow the capillaries to refill with further microbubbles. It should be noted that microbubble destruction occurs to some degree at all mechanical indices.

Figure 3 : The impact of varying mechanical index on microbubble behaviour (Lindner J et al. Curr Probl Cardiol (2002); 11; 454-519)


The mechanical index is an estimate of the maximum amplitude of the pressure pulse in tissue and is an indicator of the likelihood of adverse mechanical (i.e. non-thermal) bio-effects (streaming and cavitation). The mechanical index of the ultrasound beam is the amount of negative acoustic pressure within a ultrasonic field and is used in contrast echocardiography to modulate the output signature of UCAs to incite different microbubble responses.

At low acoustic power (MI < 0.2), the acoustic response is considered as linear, as the microbubbles undergo oscillation with compression and rarefaction that are equal in amplitude and thus no special contrast enhanced signal is generated. Microbubbles act as strong scattering objects due to the difference in impedance between air and liquid, and the acoustic response is optimized at the resonant frequency of a microbubble.

At intermediate acoustic power (MI between 0.2–0.5), non-linear oscillation occurs (bubbles undergo rarefaction that is greater than compression). Ultrasound waves are created at harmonics of the delivered frequency. The harmonic response frequencies are different from that of the incident wave (fundamental frequency). These contrast enhanced ultrasound signals are microbubble-specific.

At high acoustic power (MI > 0.5), microbubble destruction begins with subsequent transient emission of high intensity signals that are very rich in non-linear components. Intermittent imaging becomes necessary to allow the capillaries to refill with further microbubbles. It should be noted that microbubble destruction occurs to some degree at all mechanical indices.

Dynamic range

The dynamic range represents the extent or spectrum (range) of ultrasound signals which can be processed. It is expressed in Db. It is simply the ratio of the highest signal intensity, amplitude or voltage before the signal is saturated to the lowest perceived signal intensity, amplitude or voltage just before it is eliminated from the system.

Contrast Echo Box: Introduction

The graph above demonstrates concentration of microbubbles (y axis) versus time (x axis). Following immediate injection, the peak concentration often results in over-saturation (A) and thus one has to wait until the concentration falls back inside the dynamic range of the system, indicated here by the ‘imaging window’ (B - black rectangle). As time progresses, the concentration of microbubbles decreases to the point where it will then fall below the ‘detection threshold’ (C) and, at this stage, further contrast-enhanced imaging is not feasible without further contrast administration.

In the following chapter, we discuss the issue of whether to use a bolus injection or continuous infusion of contrast agent – this graph helps to explain one of the advantages of an infusion, namely that the rate of administration can be tailored (individualized) to each patient so that the contrast concentration remains within the dynamic range for significantly longer time period, thus facilitating imaging.


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