Browsing by Author "Vesanen, Panu"
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- 3D geometric correction in 2D multi-slice acquired MR images used in radiation therapy planning
Perustieteiden korkeakoulu | Master's thesis(2020-03-16) Haukipuro, Eeva-SofiaThe role of MRI in the external radiation therapy treatment has increased in the past few decades significantly due to its superior soft-tissue contrast and absence of ionizing radiation. The two challenges concerning the lack of electron density values and limited geometric accuracy to use MR images in RT have been overcome. MR images can either be aligned with CT images or converted to pseudo-CT images for radiation dose planning, and the required geometric accuracy can be achieved by optimizing sequences and utilizing geometric correction algorithms. The purpose of this thesis was to improve the geometric accuracy of MR images by implementing 3D geometric correction to 2D multi-slice acquired images. Until now, this feature has not been available in Philips MR scanners. The advantages of 2DMS acquired images over 3D images are the reduced sensitivity to motion artifacts and the contrast that is preferred by radiologists. The geometric correction algorithm itself was not modified, since the existing algorithm for 3D images could be reused. The performance and the geometric accuracy achievable with the algorithm were evaluated with phantom imaging and the effect of the algorithm on human anatomy was evaluated with volunteer imaging. Altogether 21 subjects were imaged with Ingenia 1.5 T, Ingenia 3 T and 1.5 T MR scanner of Elekta Unity. The results reveal that the geometric distortion is significantly reduced when through-plane correction is applied alongside in-plane correction. The greatest effects are shown on the edges of the field-of-view where the effect of gradient non-linearities are the largest. It was also proven that the algorithm performs the geometric correction almost as well in 2DMS as in 3D images. - Combined ultra-low-field MRI and MEG: instrumentation and applications
School of Science | Doctoral dissertation (article-based)(2013) Vesanen, PanuMagnetic resonance imaging (MRI) is a noninvasive method that allows the study of the interior structure of matter. Today, MRI is widely used in medical diagnosis and research, thanks to its versatile contrast and the lack of ionizing radiation. Conventionally, the signal-to-noise ratio of an MRI measurement scales with the strength of the applied magnetic field. This has driven the development of MRI scanners towards fields of 3 T and above. Ultra-low-field (ULF) MRI is an emerging technology that uses microtesla-range magnetic fields for image formation. The low signal-to-noise ratio is partly compensated for by prepolarizing the sample in a field of 1 – 200 mT and using superconducting quantum interference devices (SQUIDs) for signal detection. Advantages of ULF MRI include unique low-field contrast mechanisms, flexibility in the sequence design, and the possibility to construct a silent scanner with an open geometry. ULF MRI is also compatible with magnetoencephalography (MEG), which uses SQUIDs to record the magnetic field produced by neuronal activity. With a hybrid scanner combining MEG and MRI, both the structure and function of the human brain can be studied with a single device. In this Thesis, a hybrid MEG-MRI device was designed, constructed, and tested. The system is based on a commercial whole-head MEG device that was modified to accommodate ULF-MRI functionality. In particular, the effects of the various magnetic fields applied inside a magnetically shielded room were studied. To prevent the harmful effects of the eddy currents caused by changing magnetic fields, a self-shielded polarizing coil was designed and constructed. Moreover, the conventional SQUID design was modified in order to develop sensor modules that tolerate the relatively strong polarizing field. Finally, the device was used to measure MEG data and ULF-MR images of the human brain. In addition to the instrumentation development, several applications of ULF MRI were investigated. A method for imaging electric current density was presented. The technique takes advantage of the flexibility of ULF MRI by encoding the signal in zero magnetic field. Furthermore, the temperature dependence of the MRI relaxation times was studied. Drastic variations were found as a function of the field strength. The results were used to reconstruct temperature maps using ULF MRI. The results presented in this Thesis demonstrate that upgrading MRI functionality into an existing commercial MEG device is a feasible concept. Such a device has the potential to enable new methods and paradigms for neuroscientific research. The possibility of taking advantage of the unique low-field contrast is an interesting subject for further research. - Compressed sensing in parallel magnetic resonance imaging
Helsinki University of Technology | Master's thesis(2009) Vesanen, PanuMagnetic resonance imaging (MRI) is a non-invasive method that allows the study of interior structures of matter. MRI is based on magnetizing a sample, manipulating the magnetization, and detecting the magnetic field that the sample produces. Today, MRI is widely used in medical imaging. Reductions in the MRI measurement time are important because they correspond to, e.g. higher throughput of MRI scanners or enhanced spatial resolution of images. It is well-known that natural images, such as MR images, are compressible. Compressed sensing (CS) is a method that exploits the compressibility of signals in order to measure and reconstruct them efficiently. Moreover, in parallel MRI (pMRI), the magnetic fields produced by the magnetized sample are measured with multiple coils simultaneously. The differing sensitivity profiles of the coils allow a faster rate of information flow compared to single-coil receiving. In this work, CS and pMRI are combined to achieve shortened measurement times. Theory of CS, pMRI, and their combination are reviewed. Simulations were conducted to demonstrate the performance of the method in idealistic conditions. In addition, experimental measurements were performed to validate the technique in practice. Simulation results indicate that the combination of CS with pMRI reduces the measurement time by 30-50 %. Experimentally, reductions of 10-20 % were obtained. - pH-arvon vaikutus veden T1-relaksaatioon matalassa magneettikentässä
Perustieteiden korkeakoulu | Bachelor's thesis(2012-09-06) Mäntylä, Janne