A method for simulation of pulsed pressure fields from arbitrarily shaped, apodized and excited ultrasound transducers is suggested. It relies on the Tupholme-Stepanishen method for calculating pulsed pressure fields, and can also handle the continuous wave and pulse-echo case. The field is calculated by dividing the surface into small rectangles and then Summing their response. A fast calculation is obtained by using the far-field approximation. Examples of the accuracy of the approach and actual calculation times are given. >

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Calculation of pressure fields from arbitrarily shaped, apodized, and excited ultrasound transducers

American College of Cardiology Clinical Expert Consensus Document on Standards for Acquisition, Measurement and Reporting of Intravascular Ultrasound Studies (IVUS). A report of the American College of Cardiology Task Force on Clinical Expert Consensus Documents.

By using a multiple receiving coil composed of receiving coils, an imaging portion of a subject is subjected to a first pulse sequence to create n sensitivity images (701 to 703) fewer than the examination images. When these sensitivity images are created, an NMR signal is measured for only the low-frequency region of the k space. A second pulse sequence from which a phase encode step is removed is conducted to create m (m>n) examination images (704, 705) of the subject by using the receiving coils. When sensitivity distributions (707, 708) of the receiving coils are determined for the sensitivity images (701 to 703), and if there are no sensitivity distributions corresponding to the slice positions of the examination images (704, 705), they are determined by slice interpolation using the sensitivity distributions (701 to 703), and the aliasing artifacts of the examination images (704, 705) are removed by matrix operation by using the sensitivity distributions (707, 708).

Magnetic resonance imaging device

In oncology, the term 'hyperthermia' refers to the treatment of malignant diseases by administering heat in various ways. Hyperthermia is usually applied as an adjunct to an already established treatment modality (especially radiotherapy and chemotherapy), where tumor temperatures in the range of 40-43 degrees C are aspired. In several clinical phase-III trials, an improvement of both local control and survival rates have been demonstrated by adding local/regional hyperthermia to radiotherapy in patients with locally advanced or recurrent superficial and pelvic tumors. In addition, interstitial hyperthermia, hyperthermic chemoperfusion, and whole-body hyperthermia (WBH) are under clinical investigation, and some positive comparative trials have already been completed. In parallel to clinical research, several aspects of heat action have been examined in numerous pre-clinical studies since the 1970s. However, an unequivocal identification of the mechanisms leading to favorable clinical results of hyperthermia have not yet been identified for various reasons. This manuscript deals with discussions concerning the direct cytotoxic effect of heat, heat-induced alterations of the tumor microenvironment, synergism of heat in conjunction with radiation and drugs, as well as, the presumed cellular effects of hyperthermia including the expression of heat-shock proteins (HSP), induction and regulation of apoptosis, signal transduction, and modulation of drug resistance by hyperthermia.

The cellular and molecular basis of hyperthermia.

Time reversal of ultrasonic fields represents a way to focus through an inhomogeneous medium. This may be accomplished by a time-reversal mirror (TRM) made from an array of transmit-receive transducers that respond linearly and allow the incident acoustic pressure to be sampled. The pressure field is then time-reversed and re-emitted. This process can be used to focus through inhomogeneous media on a reflective target that behaves as an acoustic source after being insonified. The time-reversal approach is introduced in a discussion of the classical techniques used for focusing pulsed waves through inhomogeneous media (adaptive time-delay techniques). Pulsed wave time-reversal focusing is shown using reciprocity valid in inhomogeneous medium to be optimal in the sense that it realizes the spatial-temporal matched filter to the inhomogeneous propagation transfer function between the array and the target. The research on time-reversed wave fields has also led to the development of new concepts that are described: time-reversal cavity that extends the concept of the TRM, and iterative time-reversal processing for automatic sorting of targets according to their reflectivity and resonating of extended targets. >

Time reversal of ultrasonic fields. I. Basic principles

The transducer is probably the single most important component of any ultrasonic imaging system. A basic introduction to the problems and paradoxes of transducer design is given. After introducing the piezoelectric equations and discussing important transducer material such as lead zirconate titanate and polyvinylidene difluoride, the techniques for modeling the electromechanical impulse response are reviewed. Quarter-wave matching and short pulse techniques are discussed. The prediction of the ultrasound field of plane, spherical, and conical transducers is reviewed with emphasis on the spatio-temporal impulse response technique. Finally, the use of the above approaches is illustrated in a very practical fashion for three interesting transducer geometries: 1) a split aperture device with two focal lengths, 2) a five-element annular array, and 3) a 37.5 degree conical/annular array hybrid transducer.

Ultrasound Transducers for Pulse-Echo Medical Imaging

A new non-invasive method for monitoring apoptosis has been developed using high frequency (40 MHz) ultrasound imaging. Conventional ultrasound backscatter imaging techniques were used to observe apoptosis occurring in response to anticancer agents in cells in vitro, in tissues ex vivo and in live animals. The mechanism behind this ultrasonic detection was identified experimentally to be the subcellular nuclear changes, condensation followed by fragmentation, that cells undergo during apoptosis. These changes dramatically increase the high frequency ultrasound scattering efficiency of apoptotic cells over normal cells (25- to 50-fold change in intensity). The result is that areas of tissue undergoing apoptosis become much brighter in comparison to surrounding viable tissues. The results provide a framework for the possibility of using high frequency ultrasound imaging in the future to non-invasively monitor the effects of chemotherapeutic agents and other anticancer treatments in experimental animal systems and in patients.

/pdf/ultrasound-imaging-of-apoptosis-high-resolution-non-invasive-88ixajwg7u.pdf

Ultrasound imaging of apoptosis: high-resolution non-invasive monitoring of programmed cell death in vitro, in situ and in vivo.

Large blood vessels can produce steep temperature gradients in heated tissues leading to inadequate tissue temperatures during hyperthermia This paper utilizes a finite difference scheme to solve the basic equations of heat transfer and fluid flow to model blood vessel cooling Unlike previous formulations, heat transfer coefficients were not used to calculate heat transfer to large blood vessels Instead, the conservation form of the finite difference equations implicitly modelled this process Temperature profiles of heated tissues near thermally significant vessels were calculated Microvascular heat transfer was modelled either as an effective conductivity or a heat sink An increase in perfusion in both microvascular models results in a reduction of the cooling effects of large vessels For equivalent perfusion values, the effective conductivity model predicted more effective heating of the blood and adjacent tissue Furthermore, it was found that optimal vessel heating strategies depend on the microvascular heat transfer model adopted; localized deposition of heat near vessels could produce higher temperature profiles when microvascular heat transfer was modelled according to the bioheat transfer equation (BHTE) but not the effective thermal conductivity equation (ETCE) Reduction of the blood flow through thermally significant vessels was found to be the most effective way of reducing localized cooling

https://iopscience.iop.org/article/10.1088/0031-9155/40/4/001/pdf

Large blood vessel cooling in heated tissues: a numerical study.

Ultrasound (US) spectral analysis methods are used to analyze the radiofrequency (RF) data collected from cell pellets exposed to chemotherapeutics that induce apoptosis and other chemicals that induce nuclear transformations. Calibrated backscatter spectra from regions-of-interest (ROI) were analyzed using linear regression techniques to calculate the spectral slope and midband fit. Two f/2 transducers, with operating frequencies of 30 and 34 MHz (relative bandwidths of 93% and 78%, respectively) were used with a custom-made imaging system that enabled the collection of the raw RF data. For apoptotic cells, the spectral slope increased from 0.37 dB/MHz before drug exposure to 0.57 dB/MHz 24 h after, corresponding to a change in effective scatterer radius from 8.7 to 3.2 m. The midband fit increased in a time-dependent fashion, peaking at 13dB 24 h after exposure. The statistical deviation of the spectral parameters was in close agreement with theoretical predictions. The results provide a framework for using spectral parameter methods to monitor apoptosis in in vitro and in in vivo systems and are being used to guide the design of system and signal analysis parameters. (E-mail: mkolios@ryerson.ca) © 2002 World Federation for Ultrasound in Medicine & Biology.

Ultrasonic spectral parameter characterization of apoptosis.

A 50 MHz ultrasound backscatter microscope has been built to measure the acoustic properties of vascular tissues and blood over the frequency range from 35-65 MHz. High resolution (45 microns) ultrasound backscatter microscope images of nine femoral and eight common carotid human artery samples were made and compared with corresponding histological sections. Individual tissue layers were selected using these images for quantitative measurement of the frequency dependent backscatter. Backscatter measurements were made in each layer of an artery at two different angles of incidence: along the axis of the artery (axial direction) and at 90 degrees to this measurement radially out from the center of the artery (radial direction). Scattering was found to be higher in elastic arteries (carotid) than in the muscular arteries (femoral). The largest difference was found in the media where the average scatter (measured in the radial direction at 50 MHz) increased from 0.002 sr-1 mm-1 in muscular arteries to 0.4 sr-1 mm-1 in elastic arteries. Large differences in scattering between measurements made in the axial and radial direction were also found. Again, the largest differences were found in the media where scattering (at 50 MHz) in carotid arteries increased from 0.003 sr-1 mm-1 measured in the axial direction to 0.4 sr-1 mm-1 measured in the radial direction. The speed of sound and attenuation in the artery wall of each sample were measured. Speed of sound measurements were found to range between 1579-1628 ms-1. The average attenuation in the artery wall increased from 4 dB mm-1 at 30 MHz to 10 dB mm-1 at 60 mHz. This is higher than the attenuation measured in blood which increased from 1.6 dB mm-1 to 5 dB mm-1 over the same frequency range. The backscatter coefficient for flowing blood was measured for flow velocities up to 36 cms-1. At flow velocities below 18 cms-1 a level of scattering of 0.0005 sr-1 mm-1 (at 50 MHz) was found. An increase in scattering of 1.6 times was measured when the flow velocity was increased to 36 cms-1. All measurements were made at 37 degrees C. The relevance of these results to clinical imaging and image interpretation is discussed.

John W. Hunt

Papers

Ultrasound Transducers for Pulse-Echo Medical Imaging

Ultrasound imaging of apoptosis: high-resolution non-invasive monitoring of programmed cell death in vitro, in situ and in vivo.

Large blood vessel cooling in heated tissues: a numerical study.

Ultrasonic spectral parameter characterization of apoptosis.

Measurement of the ultrasonic properties of vascular tissues and blood from 35-65 MHz.