Motion artifact correction in MRI using generalized projections

An algorithm that suppresses translational motion artifacts in magnetic resonance imaging (MRI) by using post processing on a standard spin-warp image is presented. It is shown that translational motion causes an additional phase factor in the detected signal and that this phase error can be removed using an iterative algorithm of generalized projections. The method has been tested using computer simulations and it successfully removed most of the artifact. The algorithm converges even in the presence of severe noise.

An improved algorithm for 2-D translational motion artifact correction

The quality of magnetic resonance imaging systems has improved to the point that motion is a major limitation in many examinations. Translational motion in the imaging plane causes the phase of the data to be corrupted. An algorithm using computer post-processing is proposed to correct the phase of the data, and hence remove the artifact. This algorithm has superior convergence properties to an earlier algorithm, which is achieved by incorporating additional prior information specific to the situation. The algorithm is verified using a Shepp and Logan phantom with simulated motion in the imaging plane. It is shown that the algorithm can correct both periodic and random motion, and that the algorithm is not significantly degraded when noise is present.

Noise reduction in MR echo planar image reconstruction

It is shown that the noise level in EPI (echo planar imaging) can increase very significantly if the final image is deconvolved to remove the ghost artifacts. This noise amplification is caused by divisions of small numbers, which scale the noise variance up. The scaling factor is a function of the frequency variable in the uniform sampling direction, and is small for low-frequency components and large for high-frequency components. A window function to provide an inverse scaling of data so that the noise scaling factor is canceled exactly is proposed. Since the signal energy is concentrated in the low-frequency range, it is not reduced significantly when windowing is applied. The window function is compared to the commonly used Hamming window and is shown to have good frequency response. The algorithm has been tested with computer simulations and has been verified to be able to raise the signal-to-noise ratio by 50% in the final image.

An algorithm for the suppression of translational motion artifacts in MRI

The quality of Magnetic Resonance imaging systems has improved to the point that the motion is the most important limitation in many examinations. Gross movements of the patient cause a ghost artifact in the image which can interfere with the diagnosis. In this paper a new method is presented to suppress the translational motion artifact caused by gross movements. The method uses postprocessing on a standard two-dimensional Fourier transform image. It is shown that translational motion causes an additional phase factor in the detected signal and that this phase error can be removed using an iterative algorithm of generalised projections. The method has been tested using computer simulations and successfully removed most of the artifact, even in the presence of noise.

The relation of low frequency restoration methods to the Gerchberg-Papoulis algorithm

In magnetic resonance imaging, low frequency components can be allowed to saturate the analog to digital converter to reduce the quantization noise. These components can be estimated using least squares error estimation based low frequency restoration methods or the iterative Gerchberg-Papoulis algorithm. In this paper, we show the relationship between the closed form estimation methods and the iterative algorithm, propose a method for improving the speed of iteration, and discuss the advantages and disadvantages of two types of methods.

Purification, characterization, and localization of linamarase in cassava

We have purified cassava (Manihot esculenta) linamarase to apparent homogeneity using a simplified extraction procedure using low pH phosphate buffer. Three isozymes of cassava linamarase were identified in leaves based on differences in isoelectric point. The enzyme is capable of hydrolyzing a number of beta-glycosides in addition to linamarin. The enzyme is unusually stable and has a temperature optimum of 55 degrees C. Immunogold labeling studies indicate that linamarase is localized in the cell walls of cassava leaf tissue. Since linamarin must cross the cell wall following synthesis in the leaf for transport to the root, it is likely that linamarin must cross the cell wall in a nonhydrolyzable form, possibly as the diglucoside, linustatin. In addition, we have quantified the levels of linamarin and linamarase activity in leaves of cassava varieties which differ in the linamarin content of their roots. We observed no substantial differences in the steady state linamarin content or linamarase activity of leaves from high or low (root) cyanogenic varieties. These results indicate that the steady state levels of linamarin and linamarase in leaves of high and low cyanogenic varieties are not correlated with the varietal differences in the steady state levels of linamarin in roots.

[Preparation of monoclonal antibodies against Trichinella spiralis and localization of antigens]

BALB/c mice were infected per os with the infective larvae (L1) of Trichinella spiralis, and challenged by injecting 0.2ml soluble L1 antigen 3 days before cell fusion. SP2/0 myeloma and immune spleen cells were fused at the ratio of 1:5 in the presence 30%PEG (MW 4,000). The fusion rate and antibody positive rate were 92.1% and 16.6% respectively using ELISA. Among these the chromosomes of 4 strains were 2n greater than 100 and that of the SP2/0 myeloma cells 2n less than or equal to 70. The titer of McAb in the ascites was found to be 1/640 or 1/1,280. Eight strains were of IgG1, 1 IgG2a and 3 IgM in an agar system. Three strains of McAbs, which could recognize specifically the L1 antigen fractions by immuno-blotting technique, were used as probes to localize the target antigens in sections of T. spiralis L1. Staining was performed using an indirect technique consisting of goat anti-mouse IgG-conjugated horseradish peroxidase on de-paraffin sections, and substrate DAB. The results revealed that the antigens reacted most strongly with the specific McAbs were located in the cuticle and stichosome, followed by the lining of gut. Among the 3 strains of McAbs used, 2E5F11A5 reacted most strongly with the antigen, followed by 2E5E9E4. 2E5E9G5 was the weakest.

Slice profile improvement for a clinical MRI system

Optimal control methods have been recently introduced to improve the design of selective radio frequency pulses and several optimized selective pulses that can produce excellent slice profiles have been reported. These pulses usually require high peak rf amplitudes to implement and thus can not be widely utilized because of the limitations of the specific absorption rate and the rf power amplifier of a clinical system. We have a Siemens 1.5 T MRI clinical system. Several pulse files which consist of the bandwidth matched 90 degrees and 180 degrees selective pulses are provided. Some of these can produce excellent slice profiles. However, they can only be used in the pulse sequences with the pulse length of 5.12 msec. The purpose of this paper is to improve the slice profiles produced by the pulse file in the pulse sequences with the shorter 2.56 msec pulse length. A pulse file optimized by the conjugate gradient method is proposed to substitute the 2.56 msec Siemens pulse file. Our experimental results confirm that the slice profiles and images are improved by the optimized pulse file with a lower peak voltage. The proposed pulse file can also be applied in other clinical MRI systems.

An efficient algorithm for MR image reconstruction without low spatial frequencies

It is demonstrated that if the image to be reconstructed is known to have some zero-valued pixels, the dynamic ranges can be better used by disregarding the largest signals and using signal restoration methods. Low-frequency and high-frequency signals are related, using the knowledge that some pixels are zero, by a set of linear equations in which the number of equations is equal to the number of zero pixels, and the number of unknowns is equal to the number of low-frequency signal samples rejected. An improved Fourier transform (FT), magnetic resonance (MR) imaging method based on a least-square-error (LSE) technique, and an efficient algorithm for signal restoration when the low-frequency components are discarded are presented. In this method, the regions of support in both the image domain and the frequency domain can have arbitrary shapes, and all zero pixels in the image domain can be taken into account. The algorithm has been tested on simulated and experimental data with acceptable results.

Hypersomatotropism in pregnant women, as measured by a human liver radioreceptor assay

Radioreceptor assays (RRAs) provide useful information about the bioactivity of peptide hormones. We have developed a RRA for human GH using membranes prepared from human liver. The assay has 1% cross-reactivity with human PRL, 0.11% cross-reactivity with human chorionic somatomammotropin, and negligible cross-reactivity with bovine and rat GH. The assay has distinct advantages over the IM-9 lymphocyte RRA in having reduced nonspecific interference from serum proteins and providing a uniform source of receptors which can be stored for long periods at -70 C. The coefficient of variation of the assay is 13%. In studies of 55 sera from short children with normal or elevated serum GH concentrations the mean ratio of RRA GH to immunoradiometric assayed GH was 1.28. This assay provided important information about the total GH activity in pregnancy. There was a progressive rise in RRA GH during pregnancy, reaching 64.7 +/- 2.5 (+/- SE) micrograms/L at term. This level is 2-3 times higher than suggested by earlier estimates made with monoclonal antibody RIAs for pituitary GH. At term the contributions to total GH activity of serum are less than 3% from the pituitary, 12% from human chorionic somatomammotropin reacting with the GH receptor, and 85% from placental GH. Despite this great increase in receptor-reactive GH, somatomedin activity by bioassay is reduced, and IGF-I, determined by RIA after acid-gel filtration, is minimally elevated.