Motion artifact reduction techniques in magnetic resonance imaging

Ankara : The Department of Electrical and Electronics Engineering and the Institute of Engineering and Science of Bilkent Univ., 1991.Thesis (Ph.D.) -- Bilkent University, 1991.Includes bibliographical references.It is shown that the expansion/shrinkage and rotational motions of the body cause phase and amplitude distortions and non-rectangular sampling over the A:-domain. If these distortions are not compensated then the reconstructed image will suffer from ''the motion artifact'. The mathematical relation between motion and motion artifact is given. If the motion of the body is known, it is possible to obtain motion artifact free images. The motion is estimated either by using the information in the acquired data or by direct measurement. These estimates and the relation between motion and artifact are used to compensate the phase and amplitude distortions. Using the non-rectangular samples over the ¿-domain the rectangular samples are obtain by the aid of the singular value decomposition method. And finally, the inverse Fourier transform of these calculated samples gives the motion artifact free image. The proposed method is tested by simulations. For the estimation of the motion, two methods are proposed and tested. The first method is an iterative image reconstruction method. The second one uses the navigator echoes to obtain the amount of motion.Atalar, ErginPh.D

In vivo MRI with Concurrent Excitation and Acquisition using Automated Active Analog Cancellation

Magnetic resonance imaging (MRI) provides excellent cross-sectional images of the soft tissues in patients. Unfortunately, MRI is intrinsically slow, it exposes patients to severe acoustic noise levels, and is limited in the visualization of certain tissues such as bone. These limitations are partly caused by the timing structure of the MRI exam which first generates the MR signal by a strong radio-frequency excitation and later acquires the weak MRI signal. Concurrent excitation and acquisition (CEA) can overcome these limitations, but is extremely challenging due to the huge intensity difference between transmit and receive signal (up to 100 dB). To suppress the strong transmit signals during signal reception, a fully automated analog cancellation unit was designed. On a 3 Tesla clinical MRI system we achieved an on-resonance analog isolation of 90 dB between the transmit and receive path, so that CEA images of the head and the extremities could be acquired with an acquisition efficiency of higher than 90% at sound pressure levels close to background noise. CEA with analog cancellation might provide new opportunities for MRI in tissues with very short T2 relaxation times, and it offers a silent and time-efficient MRI acquisition

Specific absorption rate reduction using nonlinear gradient fields

The specific absorption rate is used as one of the main safety parameters in magnetic resonance imaging. The performance of imaging sequences is frequently hampered by the limitations imposed on the specific absorption rate that increase in severity at higher field strengths. The most well-known approach to reducing the specific absorption rate is presumably the variable rate selective excitation technique, which modifies the gradient waveforms in time. In this article, an alternative approach is introduced that uses gradient fields with nonlinear variations in space to reduce the specific absorption rate. The effect of such gradient fields on the relationship between the desired excitation profile and the corresponding radiofrequency pulse is shown. The feasibility of the method is demonstrated using three examples of radiofrequency pulse design. Finally, the proposed method is compared with and combined with the variable rate selective excitation technique

Tumoral calcinosis: Clinical and biochemical aspects of a patient treated with vinpocetine

We describe a tumoral calcinosis (TC) patient with a calcified mass around the hip joint. Once diagnosed, she was followed up with vinpocetine [14-ethoxycarbonyl-(3 alpha,16 alpha-ethyl)/14,15-eburnamenine] treatment, which was reported to be effective in eliminating TC in a previous study. We think that vinpocetine contributed, at least in part, to the process of TC elimination. Although there are many reports about the surgical excision of calcified masses in TC, local recurrences are not uncommon. Our success with conservative treatment has encouraged us to refrain from surgical procedures in patients with TC. (C) 2006 European Federation of Internal Medicine. Published by Elsevier B.V All rights reserved