a-stable-phantom-material-for-optical-and-acoustic-imaging

A Stable Phantom Material for Optical and Acoustic Imaging

Establishing tissue-mimicking biophotonic phantom materials that provide long-term stability are imperative to enable the comparison of biomedical imaging ...

National Physical Laboratory

The aim of this work was to establish the optimum acoustic characterisation approach and insonation transmit beam parameters for subharmonic signal generation with 'native' and 'altered' populations of a commonly-used microbubble contrast agent. Dynamic contrast-enhanced (DCE) ultrasound is a non-invasive method of imaging the microvasculature, typically implemented using harmonic imaging. Subharmonic imaging, in which echoes at half the fundamental frequency are detected, detects signals which are generated by the ultrasound contrast agents (UCAs) but not by tissue. However, optimal transmission parameters and furthermore, the optimum acoustic characterisation method have not been established. The subharmonic response of 'native' and 'altered' UCA, altered through decantation, was investigated at transmit centre frequencies 1.8-5 MHz and pulse lengths 1-8 cycles. The 'altered' UCA had reduced polydispersity (1-4 µm: 82% bubble volume), compared to 'native' (4-10 µm: 57% bubble volume). A custom-built narrow-band acoustic characterisation system was found to be more appropriate for acoustic characterisation compared to the commonly used broadband pulse-echo approach. Both UCA generated the highest subharmonic signal at pulse length of 3-cycles. The maximum 'native' subharmonic signal was generated at a transmit centre frequency of 1.9 MHz, corresponding to a subharmonic at 0.95 MHz. This optimal frequency increased in the 'altered' population to 2.3-2.5 MHz, bringing the subharmonic above 1 MHz and hence into a range amenable to clinical abdominal imaging transducers. The use of subharmonic signal detection coupled with a modified UCA size distribution has potential to significantly improve the quantification sensitivity and accuracy of DCE ultrasound imaging.

Optimisation of the transmit beam parameters for generation of subharmonic signals in native and altered populations of a commercial microbubble contrast agent SonoVue®.

Tissue mimicking materials (TMMs), typically contained within phantoms, have been used for many decades in both imaging and therapeutic applications. This review investigates the specifications that are typically being used in development of the latest TMMs. The imaging modalities that have been investigated focus around CT, mammography, SPECT, PET, MRI and ultrasound. Therapeutic applications discussed within the review include radiotherapy, thermal therapy and surgical applications. A number of modalities were not reviewed including optical spectroscopy, optical imaging and planar x-rays. The emergence of image guided interventions and multimodality imaging have placed an increasing demand on the number of specifications on the latest TMMs. Material specification standards are available in some imaging areas such as ultrasound. It is recommended that this should be replicated for other imaging and therapeutic modalities. Materials used within phantoms have been reviewed for a series of imaging and therapeutic applications with the potential to become a testbed for cross-fertilization of materials across modalities. Deformation, texture, multimodality imaging and perfusion are common themes that are currently under development.

Tissue mimicking materials for imaging and therapy phantoms: a review.

This article reports the experimental validation of a method for correcting underestimates of peak compressional pressure ( p) , peak rarefactional pressure ( p) , and pulse intensity integral (pii) due to hydrophone spatial averaging effects that occur during output measurement of clinical linear and phased arrays. Pressure parameters ( p , p , and pii), which are used to compute acoustic exposure safety indexes, such as mechanical index (MI) and thermal index (TI), are often not corrected for spatial averaging because a standardized method for doing so does not exist for linear and phased arrays. In a companion article (Part I), a novel, analytic, inverse-filter method was derived to correct for spatial averaging for linear or nonlinear pressure waves from linear and phased arrays. In the present article (Part II), the inverse filter is validated on measurements of acoustic radiation force impulse (ARFI) and pulsed Doppler waveforms. Empirical formulas are provided to enable researchers to predict and correct hydrophone spatial averaging errors for membrane-hydrophone-based acoustic output measurements. For example, for a 400- [Formula: see text] membrane hydrophone, inverse filtering reduced errors (means ± standard errors for 15 linear array/hydrophone pairs) from about 34% ( p) , 22% ( p) , and 45% (pii) down to within 5% for all three parameters. Inverse filtering for spatial averaging effects significantly improves the accuracy of estimates of acoustic pressure parameters for ARFI and pulsed Doppler signals.

Hydrophone Spatial Averaging Correction for Acoustic Exposure Measurements From Arrays-Part II: Validation for ARFI and Pulsed Doppler Waveforms.

This paper reports underestimation of peak compressional pressure ( ), peak rarefactional pressure (  ), and pulse intensity integral () due to hydrophone spatial averaging of acoustic radiation force impulse (ARFI) beams generated by clinical linear and phased arrays. Although a method exists for correcting for hydrophone spatial averaging for circularly-symmetric beams, there is presently no analogous method for rectangularly-symmetric beams generated by linear and phased arrays. Consequently, pressure parameters ( ,   , and ) from clinical arrays are often not corrected for spatial averaging. This can lead to errors in Mechanical Index (MI) and Thermal Index (TI), which are derived from pressure measurements and are displayed in real-time during clinical ultrasound scans. ARFI beams were generated using three clinical linear array transducers. Output pressure waveforms for all three transducers were measured using five hydrophones with geometrical sensitive element sizes (d) ranging from 85 to 1000 μm. Spatial averaging errors were found to increase with hydrophone sensitive element size. For example, if d = 500 μm (typical membrane hydrophone), frequency = 2.25 MHz and F/# = 1.5, then average errors are approximately -20% (p), -10% (p), and -25% (pii). Therefore, due to hydrophone spatial averaging, typical membrane hydrophones can exhibit significant underestimation of ARFI pressure measurements, which likely compromises exposure safety indexes.

Aoife M. Ivory

Ultrasound and Underwater Acoustics Group,
Department of Medical,
Marine and Nuclear Physics,
Ultrasound and Underwater Acoustics Group, Department of Medical, Marine and Nuclear Physics

National Physical Laboratory

A Stable Phantom Material for Optical and Acoustic Imaging

Aoife M. Ivory

Ultrasound and Underwater Acoustics Group,Department of Medical,Marine and Nuclear Physics,Ultrasound and Underwater Acoustics Group, Department of Medical, Marine and Nuclear Physics

National Physical Laboratory

A Stable Phantom Material for Optical and Acoustic Imaging

Ultrasound and Underwater Acoustics Group,
Department of Medical,
Marine and Nuclear Physics,
Ultrasound and Underwater Acoustics Group, Department of Medical, Marine and Nuclear Physics