ABSTRACT
OBJECTIVE: The present work introduces an alternative to the conventional B0-gradient spatial phase encoding technique. By applying far off-resonant radiofrequency (RF) pulses, a spatially dependent phase shift is introduced to the on-resonant transverse magnetization. This so-called Bloch-Siegert (BS) phase shift has been recently used for B1(+)-mapping. The current work presents the theoretical background for the BS spatial encoding technique (BS-SET) using RF-gradients. MATERIALS AND METHODS: Since the BS-gradient leads to nonlinear encoding, an adapted reconstruction method was developed to obtain undistorted images. To replace conventional phase encoding gradients, BS-SET was implemented in a two-dimensional (2D) spin echo sequence on a 0.5 T portable MR scanner. RESULTS: A 2D spin echo (SE) measurement imaged along a single dimension using the BS-SET was compared to a conventional SE 2D measurement. The proposed reconstruction method yielded undistorted images. CONCLUSIONS: BS-gradients were demonstrated as a feasible option for spatial phase encoding. Furthermore, undistorted BS-SET images could be obtained using the proposed reconstruction method.
Subject(s)
Algorithms , Image Enhancement/methods , Magnetic Resonance Imaging/methods , Equipment Design , Image Processing, Computer-Assisted/methods , Magnetic Resonance Imaging/instrumentation , Models, Theoretical , Phantoms, Imaging , Plant Stems , Radio WavesABSTRACT
OBJECT: Eddy current compensation by dynamic reference phase modulation (eDREAM) is a compensation method for eddy current fields induced by B 0 field-cycling which occur in delta relaxation enhanced MR (dreMR) imaging. The presented method is based on a dynamic frequency adjustment and prevents eddy current related artifacts. It is easy to implement and can be completely realized in software for any imaging sequence. MATERIALS AND METHODS: In this paper, the theory of eDREAM is derived and two applications are demonstrated. The theory describes how to model the behavior of the eddy currents and how to implement the compensation. Phantom and in vivo measurements are carried out and demonstrate the benefits of eDREAM. RESULTS: A comparison of images acquired with and without eDREAM shows a significant improvement in dreMR image quality. Images without eDREAM suffer from severe artifacts and do not allow proper interpretation while images with eDREAM are artifact free. In vivo experiments demonstrate that dreMR imaging without eDREAM is not feasible as artifacts completely change the image contrast. CONCLUSION: eDREAM is a flexible eddy current compensation for dreMR. It is capable of completely removing the influence of eddy currents such that the dreMR images do not suffer from artifacts.
Subject(s)
Magnetic Resonance Imaging/methods , Animals , Artifacts , Humans , Image Interpretation, Computer-Assisted , Magnetic Phenomena , Magnetic Resonance Imaging/statistics & numerical data , Mice , Mice, Nude , Models, Theoretical , Neoplasms, Experimental/pathology , Phantoms, Imaging , Signal-To-Noise RatioABSTRACT
OBJECT: Delta relaxation enhanced magnetic resonance (dreMR) is a new imaging technique based on the idea of cycling the magnetic field B (0) during an imaging sequence. The method determines the field dependency of the relaxation rate (relaxation dispersion dR (1)/dB). This quantity is of particular interest in contrast agent imaging because the parameter can be used to determine contrast agent concentrations and increases the ability to localize the contrast agent. MATERIALS AND METHODS: In this paper dreMR imaging was implemented on a clinical 1.5 T MR scanner combining conventional MR imaging with fast field-cycling. Two improvements to dreMR theory are presented describing the quantification of contrast agent concentrations from dreMR data and a correction for field-cycling with finite ramp times. RESULTS: Experiments demonstrate the use of the extended theory and show the measurement of contrast agent concentrations with the dreMR method. A second experiment performs localization of a contrast agent with a significant improvement in comparison to conventional imaging. CONCLUSION: dreMR imaging has been extended by a method to quantify contrast agent concentrations and improved for field-cycling with finite ramp times. Robust localization of contrast agents using dreMR imaging has been performed in a sample where conventional imaging delivers inconclusive results.
