Biomedical subjects

J P Lefebvre

Publications and source records attributed to J P Lefebvre.

8 recordsLinked to original sources

PubMed2002

Contrast and velocity ultrasonic tomography of long bones.

Our objective is to derive quantitative sound speed images of cortical bone using ultrasonic transmission tomography. Cortical bone is a highly refracting medium, i.e., the sound velocity changes abruptly across the interface between soft tissue and bone. It results in a loss of data compared to classical tomography in soft tissues. In order to correct for degradation by refraction effects, the classical acquisition procedure of projection data is modified: the transducers are oriented according to Snell's law of refraction with the aim of optimizing the sound propagation as parallel longitudinal rays inside the bone. This strategy allows the subsequent application of straight-ray reconstruction by the backprojection technique, which is a classical procedure in x-ray tomography. The method is validated with Plexiglas solid cylinders and tubes immersed in water. Improved sound velocity images are then derived using conventional Radon transform of the experimental time-of-flight data. The method is then extended to in vitro human femur immersed in water. The geometry of the bone cross-section is reconstructed from measurements using ultrasonic reflection tomography. The result is then introduced in the calculation of the position and orientation of the transducers, which are associated with the parallel acoustical paths in bone in the transmission measurements. The procedure leads to significant restoration enhancement over the non corrected image. The mean value of the velocity of 3,200 ms(-1) in the cortical shell is consistent with the values known from literature. These preliminary quantitative images using combined reflected and transmission ultrasound show promise for bone imaging.

Elasticity↗

PubMed2002

Circular antenna for breast ultrasonic diffraction tomography.

Compared to echography, which exploits only the reflected field, ultrasonic diffraction tomography improves image resolution by combining the total diffracted field. For breast cancer imaging, this improvement reinforces contrast between various breast tissues and structures by eliminating some interference phenomena such as speckle and then allowing parameterization of the images. Our work concerns the development of an experimental set-up for fast acquisition of the diffracted field and construction of two-dimensional tomographic images. For this purpose, we developed a multichannel ultrasound circular antenna with eight focused transducers.

Algorithms↗

PubMed2001

Cancellous and cortical bone imaging by reflected tomography.

This paper deals with the inverse scattering problem observed when ultrasonic waves are used to analyze biological media. The objective is to image cancellous and cortical bone by ultrasonic reflected tomography (URT). Because strong contrast and high absorbance bodies such as bones cannot be imaged at usual ultrasonic high frequencies (> 1 MHz), we adapted for low-frequency URT (< 1 MHz) our tomographic set-up and reconstruction and acquisition tools, previously developed for weakly scattered media. Indeed, when the frequency of the transducer decreases, the penetration length of the wave increases, which unfortunately makes resolution poor, inappropriate for bone imagery. To improve resolution, we extend the generalized inversion in the complementary bandwidth of the electro-acoustic set-up (Papoulis deconvolution). This resolution enhancement for human porous vertebrae and human and animal femur showed that high-resolution images can be obtained with low-frequency URT.

Animals↗

PubMed2000

Ghosts in the Born images of a layer probed by acoustic waves.

The Born inversion of time records of acoustic pulses reflected from a layer object gives rise to the images with ghost-like artifacts that make it difficult to pick out the exact thickness and composition of the object. The origin of these ghosts is explained herein and a means is proposed for their suppression.

Artifacts↗

PubMed1997

High resolution low frequency ultrasonic tomography.

Ultrasonic reflection tomography results from a linearization of the inverse acoustic scattering problem, named the inverse Born approximation. The goal of ultrasonic reflection tomography is to obtain reflectivity images from backscattered measurements. This is a Fourier synthesis problem and the first step is to correctly cover the frequency space of the object. For this inverse problem, we use the classical algorithm of tomographic reconstruction by summation of filtered backprojections. In practice, only a limited number of views are available with our mechanical rig, typically 180, and the frequency bandwidth of the pulses is very limited, typically one octave. The resolving power of the system is them limited by the bandwidth of the pulse. Low and high frequencies can be restored by use of a deconvolution algorithm that enhances resolution. We used a deconvolution technique based on the Papoulis method. The advantage of this technique is conservation of the overall frequency information content of the signals. The enhancement procedure was tested by imaging a square aluminium rod with a cross-section less than the wavelength. In this application, the central frequency of the transducer was 250 kHz so that the central wavelength was 6 mm whereas the cross-section of the rod was 4 mm. Although the Born approximation was not theoretically valid in this case (high contrast), a good reconstruction was obtained.

Computer Simulation↗

PubMed1995

Nebulizer performance: AFLM study. Association Française de Lutte contre la Mucoviscidose.

This study was conducted by the AFLM order to determine the performance characteristics of 12 commercially available nebulizers (6 ultrasonic and 6 jet) used in the treatment of cystic fibrosis (CF). The nebulizers were connected to a circuit which simulated the ventilation of a CF child and CF adult, and were tested using three drug solutions: tobramycin (T), colistin (C), and amiloride (A). Nebulizer performance was evaluated according to the volume of drug solution delivered in 10 min during the simulated inspiratory phase (VI), drug granulometry (G%), drug concentration modification in the nebulizer reservoir (delta C), and percentage of efficiently aerosolized drug EA%). The ultrasonic devices delivered a significantly higher VI than the jet nebulizers (p < 0.0001) for all three study drug. Ventilation rate did not influence VI. Regarding granulometry, higher percentages of T and A were found to be contained in droplets ranging from 0.5 to 5.0 micron following ultrasonic nebulization. Drug concentration modifications were independent of the nebulizer used but were influenced by drug type; overconcentrations of T and A were observed (delta C = +10.5 +/- 18.6 and +13.4 +/- 8.9%, respectively). On average, the ultrasonic devices achieved a higher EA% than the jet nebulizers (17.3 +/- 6.7 and 9.7 +/- 9.6%, respectively). This study highlights the significant variability in performance of different nebulizer types and empahsizes the importance of accurately testing nebulizers prior to clinical use so that the most efficacious nebulizer/drug combinations may be prescribed.

Administration, Inhalation↗

PubMed1993

Laser tomography of heterogeneous scattering media using spatial and temporal resolution.

Time-resolved tomography is performed in transillumination by using 527 nm picosecond pulses from a passively mode-locked doubled Nd/glass laser and a streak camera to select photons according to their flight time. This work reports on the increase in contrast of a time-resolved profile of a 2 mm radius opaque object embedded in a scattering medium, constituted of diluted milk in a 30 mm thick cell. For spatial analysis, the emerging photons are detected through a 6 mm slit at the outlet face of the cell. Transmission profiles obtained as a function of time show that the contrast is enhanced for the shortest flight times, while the 'shadow' of the object is no longer detected after about 100 ps. Moreover, improvements in contrast are studied for different configurations of the model, to analyse separately the role of collimated and scattered photons. It is expected that such a tomographic method based on time-resolved absorption could be applied to imaging for more complex biological structures in the red and near-infra-red range.

Humans↗

PubMed1989

In vivo transillumination of the hand using near infrared laser pulses and differential spectroscopy.

In vivo dual wavelength differential spectrography was performed on the hand of an adult male, using a collimated transillumination device. A pulsed laser with sufficiently high peak power and sufficiently low energy was employed so that transillumination could be realized without thermal damage. Spectrochemical analysis based on the absorbance of oxygen transporting molecules (OTM), i.e. haemoglobin in blood vessels and myoglobin in muscles, was performed along a 70 mm scanning line within the near-red infrared range. The two wavelengths used (675 and 800 nm) were chosen on the basis of the absorption spectrum of haemoglobin. The profiles computed with differential spectrography data related to the oxygen saturation of OTM are closely correlated with X-ray densitometry of the most vascularized tissues in the hand along the scanning line. In addition, at 675 nm, the profiles are rapidly modified as a function of the oxygen supply to the hand. Considering the accuracy obtained for the spatial localization of the OTM redox state, it is expected that these results could be applied to imaging.

Adult↗