Relationship among SAR, RF frequency and tissue

SAR is one of the most important parameters and as a guideline for MRI safety. The relationships among SAR, different tissues and different resonant frequencies have not been discussed quantitatively. In this abstract, we present some comparisons and results about this important issue.published_or_final_versio

Optimization of a multi-channel transmit, Quadrature Receive Birdcage Coil

A multi-channel transmit, quadrature receive birdcage coil are optimized by using singular value decomposition (SVD) method. Compared with conventional transceiver quadrature birdcage, this coil has dramatically improved B1 + filed homogeneity and relative SNR, with slight penalty of increased average SAR.published_or_final_versio

Functional morphology of shortleaf pine (Pinus echinata Mill) fire-adapted traits

Shortleaf pine is one of the most important trees in the southeastern United States for lumber, plywood, and pulpwood production. Throughout its range, shortleaf pine is subjected to frequent fires that can be detrimental to seedlings, often leading to death of the stem (top-kill). Frequent fire occurrence has led to the development of fire-adapted traits in shortleaf pine, specifically thick bark and sprouting from the basal crook after top-kill. The thick bark of shortleaf pine acts as a protective barrier for the cambium layer during a fire; while the basal crook is insulated from fire in the soil, protecting auxiliary buds that can initiate sprouts after top-kill. Our study aims to determine what fire adapted traits are responsible for protecting shortleaf pine throughout its life, from seedling to mature tree. The objectives of our study were to characterize shortleaf pine sprouting after top-kill and the development of bark thickness in relation to stem size. To address our objectives, we top-killed shortleaf pines in the Clemson Experimental Forest, SC and the Ouachita National Forest, AR, and measured bark thickness and counted sprouts from the basal crook after a growing season. We found defense mechanism varied by tree size: smaller trees relied on sprouting after top-kill, while larger trees did not sprout as readily. We hypothesize that the larger trees did not sprout because the thicker bark would have protected the cambium layer to decrease the probability of mortality, ultimately demonstrating a shift in fire-defense mechanisms throughout the life of the tree

Minimizing power of RF pulse by genetic algorithm

In excitation phase of MRI, there is only a very small portion of the RF pulse energy used for rotating the net magnetization, most of them are deposited into the imaging object as heat. So, reducing the RF pulse power can effectively decrease the heat deposited. In this work, genetic algorithm is introduced to minimize the power of RF pulse in the pulse design procedure, while keeping the excited profile almost unchanged. Simulation result showed that the power of RF pulse can be reduced by about 17% using GA minimization, while the excitation profile homogeneity almost has no change.published_or_final_versio

Optimization of HTS Tape Coil Design

HTS tapes are promising materials for RF coil due to their advantages of easier fabrication, easier frequency adjustment and lower cost over HTS films. A theoretical model was developed to investigate the relationship of Q with coil size, solder joint and resonant frequency. The model showed that Q increased with size and frequency. The model would be helpful in coil design to increase Q and hence increase SNR. Bi (2223) tape coils with different sizes and resonant frequencies were fabricated to verify the theoretical model.published_or_final_versio

Multi-Resolution Successive Iteration (MRSI) for Non-Cartesian SENSE Reconstruction

An improved reconstruction strategy ¡ª the multi-resolution successive iteration (MRSI) method is proposed for non-Cartesian SENSE. The conjugate gradient (CG) reconstruction is performed in several successive phases with different level of resolution. In such a fashion, the iteration approaches the desired image with increasing resolution. In vivo results show that the total reconstruction time can be significantly reduced with this novel method.published_or_final_versio

Theoretical and Experimental Investigation of the Relationship among SAR, Tissues and Resonant Frequencies in MRI

The SAR of muscle, brain and bone are investigated theoretically and experimentally. The average SAR(s) of these three tissues at 0.5T, 1.5T and 3T are simulated by finite difference time domain (FDTD) method. The SAR of muscle, brain and bone increase 7.49 folds, 10.87 folds and 12.92 folds respectively when the field increases from 0.5T to 3T. The SAR of muscle is 1.72-fold over brain and 8.74-fold over bone at 1.5T. Experiments have been carried out to obtain SAR using phantoms to simulate human tissues at 1.5T. The experiment results agree with the simulation well and within difference of 5%.published_or_final_versio

Comparison of Three Applied Volume Coils at 7T

The B1 + field homogeneity, SAR and SNR of shielded birdcage, TEM and microstrip volume coils are compared at 7T in this work by finite difference time domain (FDTD) method. It seems that the birdcage coil has the best unload B1 + homogeneity but the TEM has the best loaded B1 + homogeneity. On the other hand, the microstrip coil seems better in terms of SAR and average SNR than other two coils at ultra high field MRI.published_or_final_versio

Comparison of B1 Field Homogeneity for Shielded Birdcage, TEM and Microstrip Volume Coils at 300MHz

In this work the B1 fields of shielded birdcage, TEM and microstrip volume coils were simulated by the finite difference time domain (FDTD) method at 300MHz under the same geometry conditions. The simulation results indicate that the shielded birdcage coil has better B1 field homogeneity compared with the TEM and microstrip volume coils, but might have more radiation loss than the TEM and Microstrip volume coils.published_or_final_versio

A new array design using tunable loop microstrip (TLM) coil

A new array, tunable loop microstrip (TLM) array for MRI has been developed. The theoretical analysis of decoupling of a 2-element TLM array is based on microstrip theory. The experiments of comparison between the TLM array and a conventional loop array show that TLM array has great decoupling performance. This effectiveness is further demonstrated by 1.5T cardiac images. Thus, without overlap of array elements TLM array will be particularly useful for parallel imaging applications.published_or_final_versio