Stripe Sensor Tomography and application to microcoil Magnetic Resonance Imaging

摘要

Summary form only given. Magnetic resonance imaging (MRI) has undergone a multitude of innovations in the past decade. In conventional MRI, the major contribution to image resolution is the strength of the gradient magnetic fields used for frequency and phase encoding. A novel imaging technique, stripe sensor tomography (SST), replaces conventional 2-D radiation required for radiation tomography by a linear array sensor and angular scanning. SST can be applied to MRI where the array replaces the receiver antenna and 2D electromagnetic radiation (magnetic field gradients in MRI) are no longer required to acquire a MR image. This prospect of stripe sensor tomography in the application of MRI is demonstrated in the submicron scale. Conducted studies show that angular scanning during nuclear magnetic resonance (NMR) signal acquisition with a special stripe sensor array acting as the NMR receiver antenna will also yield an MRI. NMR and MRI are often confused to be equivalent. To clarify, we note that MRI is an application of NMR in which an extra set of magnetic (gradient) fields are superimposed on the NMR signal to obtain spatial discrimination that is detected and deconvoluted by Fourier transform analysis; this extra step in MRI accounting for the image formation. Stripe sensor tomography can therefore circumvent the requirement of gradient magnetic fields and will hopefully be of prime importance to the medical arena both for convenience and long-term safety measures. The experimental MRI set-up was carried out with a 1.35 Tesla Hallbach cylinder magnet as applications relevant to medical MRI usually range from 15-170 MHz (.3-4 Tesla) and are as of today strictly limited to 8 Tesla due to undetermined effects of electromagnetic (EM) radiation on living systems. For the technique to be applicable, the dimensions of the microcoils comprising the stripe sensor array must be less than the dimension of the sample to be imaged. This inadvertently causes the coil width (ga-nuge) to reach electrical skin depth at the operating frequency since the skin depth is naturally smaller in the low radiofrequency (RF) regime relevant to medical MRI. As a major task of post-experimental analysis, this study investigated the optimal parameters for the stripe sensor image resolution. SST image resolution is primarily determined by the inter-loop spacing of the linear array sensor. In the SST submicron scale, signal to noise ratio (SNR) follows distinct criteria due to the proximity of wire size to electrical skin depth. The resistances due to neighboring wires affects SNR and in turn the resolution. After completing the proof of principle and setting up the MRI system where the new sensor is to be configured, MRI resolution for SST was studied and compared to conventional MRI resolution and a crude mathematical model for the expected resolution was derived. In summary, it was found that SST offers an imaging methodology less dependent on other parameters of the MRI experiment and circumvents many necessities required for imaging with radiation ranging from the selectivity of transmitter excitation pulses to power requirements. It may well also present a better alternative for microcoil MRI imaging in the future. As this work investigates applications to submicron scale (MRI), it is hoped that one day MRI will reach atomic scale resolution. Other than lifting the requirement for potentially harmful gradient field radiation, SST-MRI can mean considerably more compact MRI machines that can image more definite features of the human body and perhaps at the subcellular level.