Contrast Detection

See Contrast Imaging for introduction to contrast agents and imaging of these.

Current methods for contrast imaging rely on driving the contrast bubbles into strong nonlinear oscillations by the imaging pulse. The transmit frequency is then forced to be around or often below the resonance frequency of the contrast bubbles. Contrast bubbles that are available and practical for clinical use generally have resonance frequencies in the range 1-3 MHz, given by their size and shell stiffness. This implies that with current contrast agent detection techniques the transmit frequency is typically below 3 MHz, thus limiting the spatial image resolution for contrast agent imaging of for example the breast, the prostate, the thyroid, the carotid arteries, in gynecology, and for some gastrointestinal applications.

SURF Imaging is a way to overcome the limitations described above allowing detection at clinically-high frequencies by using a dual-band method in which two ultrasound pulses are superimposed in time. A low-frequency pulse is used (e.g 0.5-2 MHz) to manipulate the contrast agent around its resonance frequency, changing its scattering properties. A high-frequency pulse (e.g. 3-14 MHz) is used for high-resolution detection of the changes in scattering caused by the low-frequency manipulation pulse.

The imaging pulse is used for high-resolution contrast agent detection. By transmitting several SURF-pulse complexes in the same direction, changing the phase of the imaging pulse relative to the manipulation pulse (see Figure 1), it is possible to generate a contrast agent detection signal and strongly suppress the tissue signal.

 

 

The most basic of these pulsing schemes is a two-pulse setting, where the imaging pulse is first placed in a compression phase of the manipulation pulse and then in a rarefaction phase as illustrated in Figure 2. In this situation a single microbubble is first imaged in a compressed state and then in an expanded state with the difference providing a detection signal.

Figure 2: Manipulation and imaging of a contrast bubble in a compressed and expanded state.

 

A SURF Imaging contrast detection scheme can then be set up as follows:

• Two SURF pulses are transmitted in the same direction; the second with an inverted manipulation pulse (as seen in Figure 1).

• The low-frequency manipulation pulse is filtered out on receive.

• The two received high-frequency echoes are processed to obtain the contrast signal.

Using this scheme, the high-frequency tissue signal may then be suppressed, providing a contrast agent detection signal.

 

The use of a dual-band method decouples the contrast agent detection frequency from the resonance frequency of the microbubbles, allowing high-frequency contrast agent imaging. The frequency of the manipulation pulse can be tailored to suit different types of ultrasound contrast agents, by tuning it with respect to the resonance characteristics of the agent.