Sammendrag
A dual-band method for ultrasound contrast agent detection is demonstrated in vivo in patients with prostate cancer and thyroid nodules. The method is named Second-Order UltRasound Field Imaging, with the acronym SURF Imaging. It relies on simultaneously transmitting two ultrasound pulses with a large separation in frequency. Here, a low-frequency pulse of 0.9 MHz is combined with a high-frequency pulse of 7.5 MHz. The low-frequency pulse is used to manipulate the properties of the contrast agent, and the high-frequency pulse is used for high-resolution contrast imaging. An annular array capable of transmitting the low- and high-frequency pulses simultaneously was constructed and fitted to a mechanically scanned probe used in a GE Vingmed System 5 ultrasound scanner. The scanner was modified and adapted for the dual-band transmit technique. In-house software was written for post-processing of recorded IQ-data, thus real-time imaging was not possible.
Generally, the method decouples the contrast agent detection frequency from the resonance frequency of the microbubbles, allowing imaging at clinically-high frequencies. The low-frequency pulse is used (e.g. 0.5-2 MHz) to manipulate the properties of the contrast agent around its resonance frequency, changing the back-scattering from the high-frequency pulse (e.g. 3-14 MHz) which is used for high-resolution detection. The general principle of insonifying ultrasound contrast agents with the above described pulse complex is also known as Radial Modulation Imaging in the literature. Here, the name SURF Imaging is used to recognize the fact that transmitting such a pulse complex also infers changes in the forward generated tissue nonlinearities which needs to be accounted for. The low-frequency manipulation pulse generates a local change in the speed of sound experienced by the high-frequency pulse. In the data presented here, this effect is estimated and compensated in the obtained images.
The results present contrast-processed B-mode images from patients enrolled in two ongoing pilot studies, aimed at imaging prostate cancer and thyroid nodules respectively. The studies are approved by the local ethics committee and the patients provide written informed consent. The obtained images show contrast agent detection in both cases, demonstrating that SURF imaging works well in a clinical setting. For the studied thyroid nodules, contrast-to-tissue ratios ranging from 15-20 dB was obtained, and wash-in curves of contrast agents within a region-of-interest are presented. For the prostate cases, due to scanner limitations in IQ-data storage, wash-in curves were not possible to obtain. Images displaying Maximum Intensity Projections are shown for both categories of patients.
The results demonstrate the potential of SURF Imaging as an ultrasound contrast detection technique in a clinical setting for high ultrasound frequencies.
Generally, the method decouples the contrast agent detection frequency from the resonance frequency of the microbubbles, allowing imaging at clinically-high frequencies. The low-frequency pulse is used (e.g. 0.5-2 MHz) to manipulate the properties of the contrast agent around its resonance frequency, changing the back-scattering from the high-frequency pulse (e.g. 3-14 MHz) which is used for high-resolution detection. The general principle of insonifying ultrasound contrast agents with the above described pulse complex is also known as Radial Modulation Imaging in the literature. Here, the name SURF Imaging is used to recognize the fact that transmitting such a pulse complex also infers changes in the forward generated tissue nonlinearities which needs to be accounted for. The low-frequency manipulation pulse generates a local change in the speed of sound experienced by the high-frequency pulse. In the data presented here, this effect is estimated and compensated in the obtained images.
The results present contrast-processed B-mode images from patients enrolled in two ongoing pilot studies, aimed at imaging prostate cancer and thyroid nodules respectively. The studies are approved by the local ethics committee and the patients provide written informed consent. The obtained images show contrast agent detection in both cases, demonstrating that SURF imaging works well in a clinical setting. For the studied thyroid nodules, contrast-to-tissue ratios ranging from 15-20 dB was obtained, and wash-in curves of contrast agents within a region-of-interest are presented. For the prostate cases, due to scanner limitations in IQ-data storage, wash-in curves were not possible to obtain. Images displaying Maximum Intensity Projections are shown for both categories of patients.
The results demonstrate the potential of SURF Imaging as an ultrasound contrast detection technique in a clinical setting for high ultrasound frequencies.