Echo projection imaging-a method to obtain NMR images undistorted by magnetic field inhomogeneities

An image correction protocol to reduce distortion for 3-T stereotactic MRI

BACKGROUND

Image distortion limits application of direct 3-T magnetic resonance imaging for stereotactic functional neurosurgery.

OBJECTIVE

To test the application of a method to correct and curtail image distortion of 3-T magnetic resonance images.

METHODS

We used a phantom head model mounted on a platform with the dimensions and features of a stereotactic frame. The phantom was scanned within the head coil of a Philips Achieva 3T X series (Philips Medical Systems, Eindhoven, the Netherlands). For each scan, 2 images were obtained-the normal and the reversed images. We applied the inverted gradient correction protocol to produce a corrected x, y, and z coordinates. We applied the Cronbach test or coefficient of reliability to assess the internal consistency of the data.

RESULTS

For all analyzed data, the P value was >.05, indicating that the differences among the observers were not statistically significant. Moreover, the data rectification proved to be effective, as the average distortion after correction was 1.05 mm. The distortion varied between 0.7 mm and 3.7 mm, depending on the target location.

CONCLUSION

This study examined a rectifying technique for correcting geometric distortion encountered in magnetic resonance images related to static field inhomogeneities (resonance offsets), and the technique proved to be highly successful in producing consistently accurate stereotactic target registration. The technique is applicable to all routinely used spin-echo MRI.

On the utility of spectroscopic imaging as a tool for generating geometrically accurate MR images and parameter maps in the presence of field inhomogeneities and chemical shift effects

Lack of spatial accuracy is a recognized problem in magnetic resonance imaging (MRI) which severely detracts from its value as a stand-alone modality for applications that put high demands on geometric fidelity, such as radiotherapy treatment planning and stereotactic neurosurgery. In this paper, we illustrate the potential and discuss the limitations of spectroscopic imaging as a tool for generating purely phase-encoded MR images and parameter maps that preserve the geometry of an object and allow localization of object features in world coordinates. Experiments were done on a clinical system with standard facilities for imaging and spectroscopy. Images were acquired with a regular spin echo sequence and a corresponding spectroscopic imaging sequence. In the latter, successive samples of the acquired echo were used for the reconstruction of a series of evenly spaced images in the time and frequency domain. Experiments were done with a spatial linearity phantom and a series of test objects representing a wide range of susceptibility- and chemical-shift-induced off-resonance conditions. In contrast to regular spin echo imaging, spectroscopic imaging was shown to be immune to off-resonance effects, such as those caused by field inhomogeneity, susceptibility, chemical shift, f(0) offset and field drift, and to yield geometrically accurate images and parameter maps that allowed object structures to be localized in world coordinates. From these illustrative examples and a discussion of the limitations of purely phase-encoded imaging techniques, it is concluded that spectroscopic imaging offers a fundamental solution to the geometric deficiencies of MRI which may evolve toward a practical solution when full advantage will be taken of current developments with regard to scan time reduction. This perspective is backed up by a demonstration of the significant scan time reduction that may be achieved by the use of compressed sensing for a simple phantom.

Numerical determination of the susceptibility caused geometric distortions in magnetic resonance imaging

The goal of this work is the design of highly accurate surgical navigation methods purely based on magnetic resonance imaging. In this context we numerically examine the geometrical distortions which occur in magnetic resonance imaging. We extend an existing method for computing magnitude and direction of distortions for any internal point. In particular, a multi-grid approach for a fast and efficient calculation of the static magnetic field throughout the imaging volume is presented and compared to the analytical solution for simple geometries. We found that shifts in the range of up to 2.5 mm occur in MRI of femur bones with 1.5 Tesla. Our new method was implemented and has been found capable of accurately correcting for geometrical distortions within reasonable computing times. In particular, we show that the registration accuracy for mutual information (MI) based MR-CT fusion can be much improved. Thus the value of the optimization functional in MI registration for MR-CT substantially increases after our distortion correction.

Magnetic resonance spectroscopic imaging for visualization and correction of distortions in MRI: high precision applications in neurosurgery

We present a method for the quantification and correction of geometrical/intensity distortions of magnetic resonance images predominantly caused by bulk magnetic susceptibility shifts due to susceptibility heterogeneities of measured biologic tissues and shape of the object under investigation. The method includes precise and fast measurements of the static magnetic-field distribution inside the measured object and automated data processing. Magnetic-field deviations in the range (-2.4; 2.6) ppm were found in the human brain at B0 = 1.5 T. For routinely used imaging parameters, with a read gradient strength of about approximately 1 mT/m, the magnetic-field perturbations in the human brain can cause geometrical distortions up to +/-4 mm and intensity changes up to +/-50%. MR images corrected by the described method are suitable for planning high precision applications in neurosurgery.

SPARE: A robust method for magnetic resonance imaging in inhomogeneous fields

An imaging sequence based on a spin-echo train has been developed which is free from geometric distortions in the imaging plane due to main field inhomogeneity. Such inhomogeneities, and chemical shifts, cause only a displacement in the selected slice, which is minimized by the use of high gradient strengths and short radiofrequency pulses. Additionally, variations in the radiofrequency field strength cause variations in the image amplitude but cause no other artifacts. This allows the use of low-flip-angle refocusing pulses, reducing the power deposition to levels which are safe in vivo at high field strengths. The sequence was implemented on a Bruker whole-body 3T system. Example images from a perfluorocarbon phantom and a human head are presented.

Line-narrowing approaches to solid state NMR imaging: pulsed gradients and second averaging

There are questions in materials sciences which can be addressed by NMR imaging. However, not all materials exhibit sufficient molecular motion to reduce dipolar couplings to the level at which the familiar techniques of medical NMR imaging are appropriate. For the more general case of these less mobile materials, line-narrowing methods, specialized to imaging, are appropriate. We have earlier demonstrated that such imaging sequences adequately overcome the fundamental dipolar broadening in engineering polymers; however, these sequences are subject to certain practical limitations which we address. We present a class of homonuclear line-narrowing imaging methods which intercalate short gradient pulses ( ca. 5 ps) into the RF pulse sequence. Such a strategy improves spatial resolution and signal sensitivity by reducing extraneous broadening from off-resonance effects. In a further implementation, the chemical shift and susceptibility terms in the spin hamiltonian are suppressed by ‘second averaging’ about a particular axis, while the hamiltonian for the gradient pulses is aligned along that axis: the result is a further improvement in spatial resolution. Prospects for the future of solid state NMR imaging are considered.

Effect of geometrical distortion correction in MR on image registration accuracy

In this article we investigate the effect of geometrical distortion correction in MR images on the accuracy of the registration of X-ray CT and MR head images for both a fiducial marker (extrinsic point) method and a surface-matching technique. We use CT and T2-weighted MR image volumes acquired from seven patients who underwent craniotomies in a stereotactic neurosurgical clinical trial. Each patient had four external markers attached to transcutaneous posts screwed into the outer table of the skull. The MR images are corrected for static field inhomogeneity by using an image rectification technique and corrected for scale distortion (gradient magnitude uncertainty) by using an attached stereotactic frame as an object of known shape and size. We define target registration error (TRE) as the distance between corresponding marker positions after registration and transformation. The accuracy of the fiducial marker method is determined by using each combination of three markers to estimate the transformation and the remaining marker to calculate registration error. Surface-based registration is accomplished by fitting MR contours corresponding to the CSF-dura interface to CT contours derived from the inner surface of the skull. The mean point-based TRE using three noncollinear fiducials improved 34%-from 1.15 to 0.76 mm-after correcting for both static field inhomogeneity and scale distortion. The mean surface-based TRE improved 46%-from 2.20 to 1.19 mm. Correction of geometrical distortion in MR images can significantly improve the accuracy of point-based and surface-based registration of CT and MR head images. Distortion correction can be important in clinical situations such as stereotactic and functional neurosurgery where 1 to 2 mm accuracy is required.

H magnetic resonance imaging of rigid polymeric solids

Implementation of the 1H magnetic resonance single-point imaging technique has provided new opportunities for the direct imaging of rigid solids, particularly semicrystalline polymers with spin-spin relaxation times, T2*, of the order of 10 microseconds and greater. Potential applications of industrial relevance in the following areas are briefly discussed and illustrated: fabricated/processed parts, photochemical degradation, oxidation, and blend characterization.

Phase constrained encoding (PACE): a technique for MRI in large static field inhomogeneities

In spin echo imaging, magnetization is assigned to a location defined by its frequency of rotation. In the presence of a static magnetic field inhomogeneity, however, this location does not correspond to the true location of the magnetization. This paper describes a magnetic resonance imaging technique called phase constrained encoding (PACE) that assigns magnetization to its true location through the use of a spin echo train and alternating readout gradients. Small artifactual side-bands occur in the point spread function but can be minimized or eliminated using higher gradient strengths, more echoes, and/or additional acquisitions. Implementation of a simple version of this technique confirms simulations.

Determination of background gradients with diffusion MR imaging

A new magnetic resonance (MR) imaging technique, opposite-polarity pulsed-field-gradient technique, with which the effects of background magnetic field gradients can be separated from the effects of diffusion, is described. It is based on the processing of two sets of diffusion-weighted images, the acquisition parameters of which differ only in the polarity of the applied diffusion pulses. The two effects can be separated because the cross term (bc) of the gradient factor function is antisymmetric with respect to reversal of the sign of the applied diffusion pulses. The technique permits simultaneous measurement of the spatial distribution of both the diffusion constants and background magnetic field gradients, with the same spatial resolution as the parent images from which they were derived. The technique has been validated with a phantom in which the spatial distribution of susceptibility-induced background gradients is known, the results showing excellent agreement with theory. The technique was applied to two systems in which the spatial distribution of the background gradient is unknown. Sources of error in the measurement of background gradients and (unrestricted) diffusion constants are analyzed, including the effects of voxel size, partial volumes, and interactions between background and imaging gradients.

Echo planar imaging of perfluorocarbons

Emulsions of perfluorotributylamine (FTBA) and perflubron were evaluated for their utility in 19F echo planar imaging. Fluorine images of the emulsions were obtained in a phantom and two mice that had been predosed. Both agents, but particularly perflubron, show potential for fluorine echo planar studies because of the long spin-spin relaxation times of the CF3 resonances. High resolution thin slice images obtained in as little as 26.6 ms are presented.

The use of adaptive algorithms for obtaining optimal electrical shimming in magnetic resonance imaging (MRI)

A method of determining the dc coil current values to electrically shim the static magnetic fields used in magnetic resonance imaging (MRI) using modified steepest descent adaptive algorithm is described. Using a 32 cm diameter by a 40 cm long water phantom as the test volume, the algorithm achieved field homogeneities of 0.2 parts per million (ppm) peak-to-peak within a 20 cm diameter spherical imaging volume, and 1.3 ppm peak-to-peak within the entire phantom. The algorithm achieved an inhomogeneity variance of 0.18 ppm2. The shim system was successfully modeled as a sum of adaptive linear combiners. The model contains 13 parameters that can be varied, 12 shim coil currents, and the receiver mixer frequency. The model was then used to predict key adaptive algorithm parameters. Experimental verification of these parameters lends support to the accuracy of the model.

Spin-Inversion Imaging: A Technique for NMR Imaging under Magnetic Fields with High Field Nonuniformities

An NMR imaging method, called spin invesion (SI) imaging, is described for imaging under a magnetic field with high nonuniformity. 180 degrees flips are applied between sampling times of the FID to remove the effects of field nonuniformities on the FID. The only requirement for SI imaging is to have high enough RF power. This requirement may be satisfied safely for small volume imaging, for example, head scanning. The advantages of SI imaging are that the images produced are almost independent of the effects of main-field nonuniformities and chemical shifts, and that there is no stringent requirement on the gradient pulse shapes. In this paper, the technique is described, gradient and sampling period requirements are derived, methods for reducing the RF peak power needed are developed, and a computer simulation to demonstrate the technique is presented.

Echo projection imaging-a method to obtain NMR images undistorted by magnetic field inhomogeneities

It is proposed to use a modified form of Mansfield's echo planar imaging to obtain NMR images which are entirely undistorted by background inhomogeneities of the magnetic field. In the proposed method, a train of 180 degrees pulses is applied in the presence of a periodically switched or sinusoidally modulated linear field gradient. The time-domain signal is sampled at half the distance between the 180 degrees pulses. At these points the magnetization will be modulated by the gradient, but will be independent of any mechanisms of inhomogeneous broadening, such as static field inhomogeneities, local susceptibility effects, or chemical shifts. A Fourier transform of the function comprising these points will therefore yield a faithful projection of the spin density, although the magnitude of the superimposed gradient need not be large compared to the inhomogeneous broadenings. This paper demonstrates the application of the proposed pulse sequence to a small-scale one-dimensional phantom. The major problem in upscaling this technique to human-scale dimensions lies in the limited available and allowed RF power, which in turn limits the maximal tolerable field inhomogeneities as well as the maximal practical field strength. An analysis of the tolerance of the proposed technique to these factors is presented, based on numerical simulation of its performance, using the Bloch equations. It is concluded that its use may be feasible on low-field systems, providing the advantages of increased signal-to-noise, lower required gradient strength, and drastically reduced sensitivity to the homogeneity and stability of the magnetic field, at the expense of larger RF power.