The technical problem addressed in this report is the passive detection of noise-like signals in the non-stationary noise and interference typical of the ocean environment. Applications could include the counting of ships entering or leaving a harbor, or transiting by some point of interest perhaps in harbor traffic control situations. This class of remote sensing application might require the deployment of many compact, self contained systems that are infrequently interrogated or that relay the transit data to other platforms at infrequent intervals. Thus, the detection problem could be compounded by system level considerations such as cost, environmental integrity, volume and operational lifetime. It is in this context that the above technical problem is addressed.

Transit detection in the ocean: Some methods and criteria

Motivation In the case of the hyperpolarized gas imaging of the lungs, a cost effective vest-coil (to be described in detail in the Hyperpolarized Gas section) is used. Normally this coil geometry gives nearly ideal coil sensitivity. However, not all patients are of similar sizes, and therefore when imaging larger patients, the vest coil does not completely circumscribe the patient. This leads to severe non-uniformities in the sensitivity profile near the open section of the vest coil, following the 1/r 2 principle of coil sensitivity. This, in turn, leads to significant attenuation of the MR signal from the affected portion of the lung volume. The goal of this project is to correct the inhomogeneities using only the acquired scan. This would allow complete backwards compatibility with all previous scans, and allow the process to work without requiring any additional use of the rather expensive 3 He gas. It would be a success if the correction preserved the ventilation defects, as described in the Background section, and the ADC map, also described in the Background section, while simultaneously correcting for the coil inhomogeneities to some extent. The theory and methods sections, later in the paper, describe this process in far greater detail. Background Basics of Magnetic Resonance Imaging Magnetic Resonance Imaging (MRI) is a type of medical imaging system which uses strong magnetic fields and RadioFrequency (RF) pulses and coils to produce images. The basic requirements for MRI are that the object being imaged can tolerate the high magnetic fields, does not produce a major magnetic field itself, and that it is or contains non-zero spin molecules as defined by quantum mechanics. The basic principle behind the imaging is to excite the atoms of the object using an RF pulse, and then to 'listen' to the RF output as they decay back to a steady state. Using that basic principle, research and clinically relevant images can be acquired using non-ionizing radiation. The RF energy used is at a frequency of γB where γ is the gyromagnetic ratio of a specific atom, and B is the magnetic field strength. The typical imaging of water uses a frequency of ~64MHz because γ is ~43MHz/T and B is typically 1.5T where T is the magnetic field strength in Tesla [9]. The MRI hardware has to be tuned very precisely to that frequency in order to maximize the signal and Signal to Noise Ratio (SNR).

Single Coil Sensitivity Mapping and Correction using Spatial Harmonics

The problem of detecting target signals in an ocean environment using active sonar is complicated by the non-stationary background which usually consists of both ambient ocean noise and reverberation. In the present paper, a signal processing system capable of detecting a signal in nonstationary noise is introduced. This system makes use of the mean and variance time functions of the nonstationary noise background in order to design estimation filters which will cope with the nonstationarity. Appropriate statistics of the noise and signal (tone burst) plus noise have been obtained and are used to determine the probabilities of false alarm and detection and the receiver operating characteristics.

D. Gupta

Papers

An adaptive thresholding system for functioning in nonstationary noise backgrounds