Instrumentation for rapid MR image synthesis

Noise removal from multiple MRI images

We introduce a novel technique for magnetic resonance image (MRI) restoration, using a physical model (spin equation). We determine a set of three basis images (proton density and nuclear relaxation times) from the MRI data using a nonlinear optimization method, and use those images to obtain restorations of the original image. MRIs depend nonlinearly on proton density, two nuclear relaxation times, T1 and T2, and two control parameters, echo time (TE) and relaxation time (TR). We model images as Markov random fields and introduce a maximum a posteriori restoration method, based on nonlinear optimization, which reduces noise while preserving resolution.

MR fluoroscopy: technical feasibility

A method of magnetic resonance image acquisition and reconstruction is described in which high imaging rates and fast reconstruction times are allowed. The acquisition is a modification of the basic FLASH sequence but with a restricted number N of phase encodings. The encodings are applied sequentially, periodically, and continuously. Images are formed by sliding a window of width N encodings along the acquired data and reconstructing an image for each position of the window. In general the acquisition time per image exceeds the time between successive images, and the method thus has a temporal lag. Experimental studies were performed with a dynamic phantom using 48 phase encodings and a TR of 20 ms, for an image acquisition time of about 1 s. The image display rate in the reconstructed sequence was 12.5 images/s, and the image sequence portrayed the motion of the phantom. Additional studies were done with 24 encodings. It is shown how the sliding window technique lends itself to high-speed reconstruction, with each newly acquired echo used to quickly update the image on display. The combination of the acquisition technique described and a hardware implementation of the reconstruction algorithm can result in realtime MR image acquisition and reconstruction.

Magnetic resonance image synthesis from an interleaved saturation recovery/inversion recovery sequence

An interleaved saturation recovery/inversion recovery (SR/IR) pulse sequence allows the sign of the IR signal to be retained even if images are formed from the magnitude of the two-dimensional complex Fourier transforms of the raw data. Synthesis of images by a linear combination of SR and signed IR pixel values has proved a valuable method of relaxation time (T1) contrast manipulation without the need to re-image the slice. Linear combination has also allowed IR images with a wide range of time-to-inversion (TI) values to be synthesized from an SR/IR pair obtained with TI = 200 ms and time-to-recovery (TR) = 1000 ms, thus avoiding the need for computationally intensive calculations based on complicated relaxation equations.