Versatile coil design and positioning of transverse-field RF surface coils for clinical 1.5-T MRI applications.

Clinical MRI/MRS applications require radio frequency (RF) surface coils positioned at an arbitrary angle alpha with respect to B(0). In these experimental conditions the standard circular loop (CL) coil, producing an axial RF field, shows a large signal loss in the central region of interest (ROI). We demonstrate that transverse-field figure-of-eight (FO8) RF surface coils design are not subject to the same amount of signal loss in the central ROI as loop coils when their orientations are changed. The 1.5-T CL and FO8 prototypes (diameter = 10 cm) were built on Plexiglas using copper strips (width = 4 mm, thickness = 100 mum). The two linear elements of the FO8 coil were 1 cm apart. Axial spoiled gradient echo (SPGR) images of a phantom containing doped water were acquired with the coil plane at alpha=0 degrees , 45 degrees , and 90 degrees . As alpha increases, the CL images show, in the central ROI, a signal that decreases from a maximum value to zero. Whereas the FO8 images show, in the same ROI, a signal that varies little from the maximum value (20%). Optimized FO8 coils can be oriented with the coil plane positioned along any direction with respect to B(0) without significant signal loss. Transverse RF coil design should be useful for clinical MRS studies and also for parallel imaging techniques where versatile RF coils disposed along arbitrary directions are required.

EPR imaging from projections: errors due to misalignment of projection centres and their rectification by a novel acquisition modality.

Continuous wave and pulsed wave electron paramagnetic resonance imaging (EPRI) makes use of classical methods of acquisition of projections. Acquisition/reconstruction techniques, such as spin-echo, gradient-echo, etc, cannot be applied to EPRI because they would require very short switching times for the gradient coils. Due to the use of the polar acquisition technique, it is necessary to define a centre of rotation about which the measured projections are rotated during the reconstruction process. This centre represents the point at which the field gradient coils must produce zero magnetic field. Due to the presence of a magnetic field control system that serves to compensate for field variations, principally due to heating, some interference can occur in the control system between the main magnetic field and the magnetic field produced by the gradient coils. The effect changes as the orientation changes. This results in a shift of the centres of the projections as a function of the variation of magnetic field produced by the gradient coils on the control Hall probe. If this condition is present, some artefacts can appear on the reconstructed image. This effect is irrelevant when EPR is used for imaging of paramagnetic probes whose linewidths are of the order of 10(-4) T, while it can be significant in the case of linewidths of the order of 10(-5) T or lower or when EPR is used in microimaging applications (i.e. for high values of magnetic field gradient). We describe the effects that misalignments of the projections have on the reconstructed images. We present a useful method for estimating the real position of the centre and correcting the measured projections before the application of the reconstruction algorithm. Moreover, we demonstrate the functioning of our technique by presenting some examples of EPR reconstruction collected by an X-band EPR imaging apparatus.