David M. Kramer

T2 restoration and noise suppression of hybrid MR images using Wiener and linear prediction techniques

The authors address the problem of enhancing hybrid magnetic resonance (MR) images degraded by T2 effects and additive measurement noise. To reduce imaging time, MR signals are acquired using hybrid imaging (HI) sequences such as rapid acquisition relaxation-enhanced (RARE) and fast spin-echo (FSE). With these techniques, T2 effects act as a distortion filter. This T2 filter affects the signal and results in image spatial resolution and/or contrast loss. Furthermore, the amplitude and phase discontinuities in the T2 filter frequency response function may generate serious ringing artifacts. These distortions will damage image quality and affect object detectability. The authors use the Wiener filter and linear prediction (LP) technique to process HI MR signals in the spatial frequency domain (K-space) and the hybrid domain, respectively. Based on the average amplitude symmetry constraint of the spin echo signal, the amplitude frequency response function of the T2 distortion filter can be estimated and used in the Wiener filter for a global T2 amplitude restoration. Then, the linear prediction technique is utilized to obtain the local signal amplitude and phase estimates around the discontinuities of the frequency response function of the T2 filter. These estimates are used to make local amplitude and phase corrections. The effectiveness of this combined technique in correcting T2 distortion and reducing the measurement noise is analyzed and demonstrated using experiments on both phantoms and human studies.

Applications of voxel shifting in magnetic resonance imaging.

Due to the nature of three-dimensional Fourier transform (3-DFT) data acquisition in magnetic resonance imaging (MRI), the spatial relation between a resolved volume element (a voxel) and the object can be manipulated easily. Those manipulations have practical consequences in terms of registering slice positions with respect to features of interest in producing oblique reformatted images where volume resolution is preserved, and in generating reformatted images that project the viewing plane onto a surface through the object that has an arbitrary shape.

MRI findings in subjects with breast implants.

One-hundred and nineteen implants were imaged in an open-sided low-field magnetic resonance imaging system. Thirty-four of these implants were removed, and postoperative reports were made available. We found a variety of signs associated with the implants, including internal structures, loss of saline in double-lumen implants, intracapsular and extracapsular fluid accumulations, signal dropout regions most likely associated with calcifications, and debris and fluid in the silicone, as well as degradation of the silicone, disruptions of the capsule and bulging, and extracapsular silicone. Conclusions of intact versus ruptured were made on the basis of these findings and found to be confirmed in 32 of 34 cases, with 1 false-positive and 1 false-negative result. Based on MRI criteria for a broad sample of the population, 27 percent of the implants were considered ruptured. (Plast. Reconstr. Surg. 96: 852, 1995.)

Characteristics of magnetic resonance sequences used for imaging silicone gel, saline, and gel-saline implants at low field strengths.

RATIONALE AND OBJECTIVES.Low-field magnetic resonance imaging (MRI) evaluation of breast implants is described, with emphasis on the capabilities provided by different imaging sequences. METHODS.Open architecture MR images using a .064-T permanent magnet and three-dimensional Fourier transform and inversion recovery sequences were obtained. A breast coil was designed and built for this project. Symptomatic and asymptomatic patients with silicone, saline-silicone and saline implants, and phantoms were part of this study. Phase images were used to differentiate protons in silicone, water, and fat. RESULTS AND CONCLUSIONS.Low-field MRI permitted differentiation of silicone, water, and fat. Implant anatomy and surrounding pathology could be imaged and identified.

Magnetic resonance imaging characteristics of multiple sclerosis plaques imaged with two-dimensional and three-dimensional Fourier transform techniques at low and mid field strengths.

A total of 58 multiple sclerosis lesions from 16 patients were used to characterize the performance of a low field system for the detection of edematous (e.g., water‐elevating) brain lesions. Contrast, signal‐to‐noise, and their product (signal difference to noise) were measured for two‐dimensional and three‐dimensional Fourier transform techniques at low field strength (640 G) and compared to two‐dimensional sequences at mid field strength (3,500 G). The results showed numerically that low field‐strength magnetic resonance imaging can reliably detect multiple sclerosis lesions, and, by extension, other water‐elevating lesions, although with lower confidence levels.

Ghost imaging in MRI

Needle biopsies and other interventions done under MR Fluoroscopy sometimes do not show the target well, either because the rapid sequence does not have adequate contrast or because a contrast agent may have washed out of the target. In these cases, an image that shows the target can be saved and scaled to match the spatial parameters of the fluoroscopic sequence, and used as a virtual or ghost field upon which the fluoroscopic images are superimposed, thus providing a view of the target, useful for needle pre-localization and for monitoring its progress as it is inserted.

Two-gradient-echo, two-dimensional, Fourier transform multisection imaging: comparison with spin-echo imaging.

A multisection, two‐dimensional, Fourier transform, doublegradient‐ echo magnetic resonance imaging sequence with partial flip‐angle excitation and section doubling by radiofrequency encoding approximately doubles the signal‐to‐noise ratio obtainable from a conventional spin‐echo sequence at low field strength, while maintaining essentially equivalent contrast.

Surface definition technique for clinical imaging

Surface descriptions are difficult to specify. Though image processing techniques are well established to generate nearly any planar or three-dimensionally curved surface, methods to describe such complex shapes are often disorienting. Even the best intentioned interface for surface description can confuse a seasoned user. This paper introduces a surface definition technique that is simple, accurate and intuitive for the needs of routine medical image analysis. We describe a procedure to define a curved surface based on surface intersection points in a series of parallel images. In this medical context, points selected describe a surface that contains pathology of diagnostic interest. Using this technique diagnostic views are generated that conform to natural anatomic shape, physicians are no longer restricted to orthogonal or even single curve surfaces. This user interface provides analytic descriptions to produce surface views that use a Fourier-shift technique for reconstruction. Surfaces through a volume are produced with resolution equal to that of the original data set. Example images are illustrated.