In this paper we present how a miniature fiber optic pressure sensor based on micro-optical mechanical systems (MOMS) could solve most of the problems associated with fluidic pressure transduction presently used for triggering purposes in IABP therapy. The small size of the MOMS (0 550 μm) allows a positioning of the sensor directly at the tip of the intra-aortic catheter, exactly where the pressure should be monitored. With outstanding performances in terms of resolution and frequency fidelity, this absolute pressure sensor can precisely detect small pressure variations such as the dicrotic notch in the intra-aortic pressure waveform, which is used as a trigger point in IABP therapy. Such technology could probably help in the development of a less invasive therapy with reduced catheter size associated with reduction of vascular complications such as ischemia.
The presented optical fiber sensor has intrinsic immunity to electromagnetic fields and noise perturbations. Furthermore, the patented white-light cross-correlation technology of the signal conditioner makes it immune to optical fiber binding and highly tolerant to optical losses. Such solution is extremely well adapted for in situ pressure monitoring in many medical applications.
In a series of papers we have developed various mathematical models for the solid angle centered at a scintillation point and subtended by a circular photomultiplier tube (PMT) in a gamma camera. In this third paper, the scintillation crystal, where the gamma ray is converted to visible light, shows a very diffusive plane in the PMT direction. The media on each side of the boundary are homogeneous. We develop a full mathematical model for the PMT response. This model depends on several physical parameters. They can be set up according to the kind of diffusion occurring at the interface. We also show that the direct and refracted solid angles are particular cases of the diffusive model. Then, for computational purposes, we approximate the model to bring its four-level integral representation to a two-level one. Numerical results show the effectiveness of our mathematical model of the solid angle function transmitted through a diffusive plane.
Light distribution in a standard scintillation camera is a complex process. The photons come across many different optical materials and many types of specular and rough optical surfaces. Complexity is further added to the model when the spatial and angular sensitivities of the detection components--the photomultipliers--are considered. To be able to correctly predict the PSF of a gamma camera, we developed a Monte-Carlo ray-tracing model which was subsequently compared to data measured on an existing gamma camera head (PRISM 3000 from Picker International Inc.). The experimental configuration was first replicated: geometry, optical properties of the crystal, light guide, photomultiplier tube window and photocathode, index matching fluid and gamma ray energy. Several other parameters, such as back mirror reflectivity and border reflectivity, were the optimized. Finally an a posteriori modelization of the scattered refracted and reflected fields at the rough interface between the crystal and the light guide was obtained by fitting simulation results to experimental data.
Gamma camera technology has evolved during the past two or three decades and is now a mature product. This paper will show that important gains can still be made at the detection level by modifying some optical components and by considering a new description of the physical phenomena. The first design modification to the detector would be to match the indices of all optical materials, from the crystal to the photomultiplier tube's window. The second and equally important point where improvement is possible is in the elimination of the spatial/spectral distortions. We will show that a complete description of the scintillation process is only possible when taking into account the depth-of-interaction (DOI) of the gamma in the crystal. Finally, the spectral contamination caused by gamma rays undergoing Compton interaction either in the object or in the detector itself is addressed by the Holospectral imaging technique. In this approach, events from the whole spectrum are accepted (as opposed to the energy windowing presently in use) and formatted into a series of energy frames. Statistical analysis is then performed on these multidimensional data to segregate object-related variance and contamination.
Previously, we developed a classical solid angle function that is valid only when the light is traveling within a homogeneous medium. As soon as the light path contains a refractive interface, the direct solid angle formula is invalid. A different approach must be used if one is to include refraction effects in the solid angle formulation. The variables of integration are given more of a physical interpretation than a geometrical one: by using the emitting point instead of the detection aperture as the basis for the coordinates system, we are able to use the symmetry of the light distribution to simplify the bounds of integration. With carefully chosen coordinate changes, we are thus able to obtain an expression for the solid angle subtended by a circular aperture from a point source situated in a different optical medium. The final refracted solid angle formula also includes the expression of Fresnel's transmission coefficient.
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