UC Berkeley

The magnetic gradient fields used in magnetic resonance imaging (MRI) have a component which is parallel to the uniform field B₀ = B₀z, as well as a component perpendicular to B₀. The component parallel to B₀ is used in spatial encoding. The component perpendicular to B₀, called the ``concomitant gradient," causes image distortions (by altering the magnitude and direction of the total field) if its magnitude approaches B₀ at any point in the field of view (FOV). In a conventional imaging sequence, the presence of the concomitant gradients limits the maximum gradient that can be used with a given B₀ field or, conversely, limits the minimum B₀ field that can be used with a given gradient field.

This thesis describes an implementation of the so-called ``zero-field MRI" (ZFMRI) pulse sequence, which allows for imaging in an arbitrarily low B₀ field. The ZFMRI sequence created an effective unidirectional gradient field by using a train of π pulses to average out the concomitant gradient components during encoding. The signals were acquired using a low-transition temperature dc Superconducting QUantum Interference Device (low-T_c dc SQUID) coupled to a first-order axial gradiometer. The experiments were carried out in a liquid helium dewar which was magnetically shielded with a single-layer mu-metal can around the outside and a superconducting Pb can contained within the helium space. We increased the filling factor of the custom-made, double-walled Pyrex insert by placing the liquid alcohol sample, at a temperature of approximately -50°C, at the center of one loop of the superconducting gradiometer, which was immersed in the helium bath.

Using the aforementioned sequence and apparatus, images were acquired in the limit of zero static field, using gradients of up to 100 μT/m over a 23 mm FOV. The change in field magnitude over the FOV due to gradients was up to 10 times larger than the magnitude of any static field present in the dewar (static fields arose from residual magnetic fields and were 1 μT or less). These images were free of concomitant gradient distortions. Images encoded using a

conventional imaging sequence under similar conditions were also acquired; the conventional images were irreparably distorted.

The limitations of the present ZFMRI sequence implementation are considered, as well as how the procedure could be made more practical with regard to imaging time. The extension of the technique to unshielded operation in a uniform ambient field is discussed, as are other methods of mitigating or eliminating concomitant gradient distortions.