Biomedical subjects

Nico de Jong

Publications and source records attributed to Nico de Jong.

7 recordsLinked to original sources

PubMed2003

Nonlinear coded excitation method for ultrasound contrast imaging.

Coded excitation with compression on receive is used in medical ultrasound (US) imaging to increase signal-to-noise ratio (SNR) and penetration depth. We performed a computer simulation study to investigate if chirped pulse excitation can be applied in US contrast agent imaging to increase SNR and contrast-to-tissue ratio (CTR) and, thus, reduce contrast agent destruction and tissue harmonics. A new nonlinear compression technique is proposed that selectively compresses the second harmonic component of the response. We compared a chirp of 9.4-micros duration, 2-MHz centre frequency, 45% relative bandwidth to a Gaussian pulse with equal centre frequency and bandwidth. For peak pressures between 50 and 300 kPa, we found for resonant bubbles an increase in response between 10 and 13 dB. Moreover, the axial resolution after compression was comparable to axial resolution of conventional imaging. This effect was relatively insensitive to peak excitation pressure and was largest for bubbles having resonance frequency around the centre frequency of the excitation.

Capsules↗

PubMed2003

Visualization of myocardial perfusion after percutaneous myocardial septal ablation for hypertrophic cardiomyopathy using superharmonic imaging.

Harmonic imaging is used for detection of ultrasound contrast agents in myocardial perfusion studies. However, harmonic imaging has limitations because of the presence of tissue harmonics, which results in less specificity and sensitivity, thus, lower contrast-to-tissue ratio. We describe a clinical example using superharmonic imaging. This technique detects the third, fourth, and fifth harmonics. These harmonics are not created in tissue, resulting, hence, in a high contrast-to-tissue ratio. After myocardial alcohol ablation for hypertrophic cardiomyopathy areas of nontreated and treated myocardium, normal and low flow could be visualized with superharmonic imaging.

Cardiomyopathy, Hypertrophic↗

PubMed2003

Harmonic ultrasonic field of medical phased arrays: simulations and measurements.

With the introduction of harmonic imaging, the design of new array transducers for tissue and contrast imaging became indispensable. Hence, prior knowledge of harmonic beams is essential to attain optimal harmonic performances. For that purpose, a new numerical algorithm that solves the parabolic nonlinear wave equation is developed. The algorithm is based on finite differences and performs exclusively in time domain. Pulsed harmonic fields emitted by a medical transducer were measured and computed at different mechanical indices. Simulations and measurements showed very good agreement for all the harmonic components.

Algorithms↗

PubMed2002

Contrast harmonic imaging.

The behavior of ultrasound contrast agents depends highly on the acoustic pressure of the insonified ultrasound wave. For low pressure the expansion and compression is linear to the pressure, for medium acoustic pressure nonlinear behavior starts to occur and for high pressures, but still in the diagnostic range transient scattering can be noticed, resulting in an enhanced scattering followed by a disappearance of the bubble. The nonlinear and transient regime can be utilized for imaging of the contrast agent in or nearby tissue. The magnitude of the nonlinear signal from the contrast has to compete with the nonlinear component of the ultrasound wave, which is generated during propagation. It is shown that contrast is superior to tissue when using low frequencies and imaging the third or fourth harmonic of the transmitted frequency.

Journal Article↗

PubMed2002

Improved contrast to tissue ratio at higher harmonics.

The challenge in ultrasound contrast imaging is a better discrimination between the perfused tissue and the contrast bubbles, which is usually expressed by contrast to tissue ratio (CTR). Imaging based on the second harmonic frequency showed a higher CTR than imaging at the fundamental frequency. However, because of nonlinear propagation of ultrasound waves, harmonic frequencies are generated. These harmonic frequencies will be linearly reflected by the tissue and therefore limit the CTR at the second harmonic frequency. In order to reduce the scattering of tissue at harmonic frequencies and by that increase the CTR, nonlinear distortion has to be reduced. We demonstrate in this study that the CTR increases with the harmonic number. The increase is substantial when transmitting at lower frequencies. To take advantage of the higher harmonics (third, fourth, fifth and the ultraharmonics and termed here super harmonics), we have developed a new phased array transducer with a wide frequency band. In-vitro measurements using the new probe show an increase of 40 dB of the CTR for super harmonic components over the conventional second harmonic system. The increase in CTR is in agreement with the calculations using existing models for the response of encapsulated bubbles and known theory of nonlinear propagation.

Journal Article↗

PubMed2002

Super harmonic imaging: a new imaging technique for improved contrast detection.

For ultrasound contrast agents (UCA), nonlinear imaging now has become fundamental. All of the current contrast-imaging methods are dominantly based on the nonlinear response of UCA bubbles. The discrimination between the perfused tissue and the UCA is the challenge in the field of UCA-imaging. This differentiation is usually associated or expressed by the ratio of the scattered power from the contrast agent to the scattered power from the tissue and is termed "contrast-to-tissue ratio" (CTR). Second harmonic imaging showed a better discrimination between tissue and UCA than fundamental imaging because of a higher CTR. We demonstrate, in this study, that the CTR increases as a function of the order of the harmonic frequency. Currently, due to the limited bandwidth of the transducers, only the second harmonic is selectively imaged, resulting in images with a superior quality to fundamental images, but still degraded and not optimal because of the harmonic generation in the underlying tissue (due to nonlinear propagation) and hence giving a limited CTR. To increase the CTR and to take advantage of the higher harmonics (third, fourth, fifth and the ultraharmonics and termed here super harmonics), we have developed a new phased array transducer. The array transducer contains two different types of elements arranged in an interleaved pattern (odd and even elements). The total number of elements is 96. The elements can operate separately and at a distinct frequency, enabling separate transmission and reception modes. The odd elements (48) operate at typically 2.8 MHz center frequency and 80% bandwidth. The even elements (48) have a center frequency of 900 kHz with a bandwidth of 50%. In vitro measurements using the dual frequency probe show an increase of 40 dB in the CTR for super harmonic components over the conventional second harmonic system. The increase in CTR is in agreement with the calculations using existing models for the response of encapsulated bubbles and known theory of nonlinear propagation. Animal experiments have demonstrated the feasibility of this approach using commercially available UCA and showed a similar increase of the CTR.

Contrast Media↗

PubMed2002

Basic acoustic properties of microbubbles.

Small (encapsulated) gas bubbles in a contrast medium react to an external oscillating pressure field with volume pulsations. Depending on the magnitude of the ultrasound wave, the vibrations will be related either linearly or nonlinearly to the applied acoustic pressure. For low acoustic pressures, the instantaneous radius oscillates linearly in relation to the amplitude of the applied external pressure field. The oscillation of the bubble is governed by parameters such as resonance frequency, damping coefficients, and shell properties. For higher amplitudes of the external field, the pulsation of the bubbles becomes nonlinear. The spectrum of the scattered ultrasound wave also contains higher harmonics of the emitted frequency in addition to the fundamental frequency. The emitted frequency, bubble size, and nonlinear propagation effects have significant influence on the harmonic generation. For encapsulated bubbles exposed to even higher acoustic amplitudes, their scattering effectiveness increases dramatically and becomes transient. The scattered frequency spectrum broadens, containing higher harmonics. This consequence is due to rupture, disappearance, change of gas content, etc. Using these specific characteristics of the contrast bubbles will open new perspectives in imaging and analysis for medical diagnosis.

Acoustics↗