The present invention relates generally to magnetic resonance imaging (MRI), and more particularly, to a method and apparatus that employs a higher-order gradient field to achieve spatial encoding of a moving object.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, spatially linear magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Typically, these linear magnetic field gradients are used to achieve spatial encoding in MR imaging. For example, a conventional linear Z gradient can be used as a phase encoding gradient in which spatial encoding is achieved by stepping the phase encoding gradient through all the required k-space values while the object remains stationary. However, such systems are limited to the field of view (FOV) of the magnet, or if a moving table is employed, steps must be taken to offset the effects of movement. It would be desirable to image the longitudinal axis of a patient without the need of an oversized magnet. Such large Z-direction FOVs may be acquired by moving the patient while scanning, then combining data either during or after image reconstruction.
Prior art methods used to acquire longitudinally extended FOV images typically use the conventional linear magnetic field gradients Gx, Gy, and Gz which are modified to offset the effects of patient movement. That is, such prior art methods that rely on encoding using the linear gradients, must either piece together raw data to reconstruct an image or piece together multiple reconstructed “sub-images.”
It would therefore be desirable to have a method and apparatus capable of scanning an object continuously while it moves through the magnet without the need to patch data together in k-space or in image space.