A contrast medium is a substance that enhances the information contained in an image produced by medical diagnostic equipments used in fields like traditional and digital Radiology, Magnetic Resonance, Nuclear Medicine and Ultrasounds.
Contrast media can either be swallowed or injected intravenously to enhance the contrast existing between anatomical structures. Ultrasound contrast media are available as solutions containing air or gas micro-bubbles that can efficiently reflect the ultrasound beams used during the investigation.
The first problems that had to be solved in order to have effective contrast media were those related to the stability and the diameter of micro-bubbles themselves. The solution to the stability problem was key to obtain a contrast medium that could dwell in the blood stream long enough to allow the completion of an ultrasound investigation with contrast enhancement. The solution to the diameter problem was necessary to obtain a contrast medium that could pass through the pulmonary circulation and that, once into the systemic circulation, could act as a contrast-enhancing medium in all organs.
At present two systems are commonly used to collect information from the micro-bubbles injected into the blood stream. The first requires the use of programmes that can generate pulses with a mechanical index above 1.2 (the mechanical index is the ratio between negative pressure peak and frequency. Such index measures the power of an ultasound beam and it provides useful indications about the capability the beam has to destroy the micro-bubbles contained in the tissues found within the scanning planes. The higher the mechanical index, the higher the number of micro-bubbles destroyed and consequently of harmonic waves generated.) The second detection system uses a low mechanical index to make micro-bubbles resonate on the second harmonic wave without causing the destruction of micro-bubbles themselves.
The contrast media currently available are mainly made of free or encapsulated micro-bubbles containing air or other gasses. Destruction detection systems are currently associated to static modes (pulse inversion, flash echo, C3).
New detection software products have recently been introduced: they use a high mechanical index and can detect in real time the information coming from destroyed micro-bubbles through Imaging, Energy or Power Doppler. When using these systems, a time interval between observations should be used so as to see reperfusion phenomena after the destruction of micro-bubbles. Detection systems with a low mechanical index mainly provide real time observations of the vascular behaviour of organs and lesions.
The imaging modality of these systems is mainly based on the detection of
the second harmonic waves coming from micro-bubbles perfusing an organ or
tissue. The micro bubble destruction technique can also be used in association
to these systems to assess the reperfusion process in subsequent phases. This
section of the website will outline the pros and cons and the possible indications
to the use of the different contrast media.