Interventional MR Elastography for MRI-Guided Percutaneous Procedures

International audiencePURPOSE : MRI-guided thermal ablations require reliable monitoring methods to ensure complete destruction of the diseased tissue while avoiding damage to the surrounding healthy tissue. Based on the fact that thermal ablations result in substantial changes in biomechanical properties, interventional MR elastography (MRE) dedicated to the monitoring of MR-guided thermal therapies is proposed here. METHODS : Interventional MRE consists of a needle MRE driver, a fast and interactive gradient echo pulse sequence with motion encoding, and an inverse problem solver in real-time. This complete protocol was tested in vivo on swine and the ability to monitor elasticity changes in real-time was assessed in phantom. RESULTS : Thanks to a short repetition time, a reduction of the number of phase-offsets and the use of a sliding window, one refreshed elastogram was provided every 2.56 s for an excitation frequency of 100 Hz. In vivo elastograms of swine liver were successfully provided in real-time during one breath-hold. Changes of elasticity were successfully monitored in a phantom during its gelation with the same elastogram frame rate. CONCLUSION : This study demonstrates the ability of detecting elasticity changes in real-time and providing elastograms in vivo with interventional MRE that could be used for the monitoring of thermal ablations

Kolmiulotteinen lämpötilamittaus protoniresonanssin avulla

Proton resonance frequency (PRF), by which it precesses in the magnetic field, alters due to change in temperature, which can be detected with magnetic resonance imaging (MRI). MRI scanner uses protons’ nuclear magnetic resonance phenomenon. The target is first excited with a radio frequency pulse, then its relaxation to initial stage is observed. Parts with different temperatures can be mapped according to the characteristics of the signal they emit during relaxation. PRF thermometry is recognized as the best method to study in vivo temperature distribution with MRI scanner. PRF thermometry is favored due to a good large scale linearity and tissue independence. When tissue containing water is heated, the hydrogen bonds between water molecules are soften as a result of increased Brownian motion. When hydrogen bonds are weaker, the magnetic shielding from electron cloud around proton is stronger. Now that the magnetic shielding is stronger, the local magnetic field of that proton is weakened. Lower magnetic field leads to lower proton nuclear magnetic resonance. Change in nuclear magnetic resonance can be detected with phase difference mapping as a phase shift in phase images with MRI scanner. Noninvasiveness is universally justified in clinical medicine. Diseases and tumors in living tissues can be noninvasively treated with hyper- or hypothermia. Abnormal situations can be detected by observing the temperature changes in the body. MRI scanner can be used to examine tissue temperatures during temperature treatments. Temperature mapping can also be used to monitor unwanted tissue heating related to MRI examinations. The purpose of this thesis is to produce volumetric thermometry data with proton resonance, and to optimize the imaging parameters in order to achieve the best signal-to-noise ratio for magnetic resonance thermometry.Protonin resonanssitaajuus (PRF), jolla se prekessoi magneettikentässä, muuttuu lämpötilan muutoksen johdosta, joka voidaan nähdä magneettikuvauslaitteen avulla. Magneettikuvauslaite käyttää hyväkseen protonien ydinmagneettista ilmiötä. Kohdetta viritetään ensin radiotaajuuspulssilla, jonka jälkeen seurataan sen palautumista alkutilaan. Eri lämpöiset alueet kohteessa voidaan kartoittaa niiden lähettämän eri taajuisen signaalin avulla palautumisen aikana. PRF muutos on tunnustettu parhaaksi menetelmäksi seurattaessa elävien kudosten sisäisiä lämpötilaeroja magneettikuvauslaitteen avulla. Etuna PRF menetelmässä toisiin magneettikuvauksen avulla tehtäviin lämpömittausmenetelmiin on sen hyvä lineaarisuus laajalla mittausalueella, ja riippumattomuus kudostyypistä. Kun vettä sisältävä kudos lämpenee, siinä olevien vesimolekyylien väliset vetysidokset heikkenevät lisääntyvän lämpöliikkeen vuoksi. Kun vetysidokset heikkenevät, kasvaa veden protonien ympärillä olevien elektronipilvien magneettinen suojaus. Kun magneettinen suojaus kasvaa, kokee protoni magneettikuvauslaitteen magneettikentän paikallisesti heikompana. Kun paikallisesti koettu magneettikenttä on heikompi, on myös protonin resonanssitaajuus pienempi. Tämä havaitaan magneettikuvauslaitteen vaihekuvissa vaihe-erona. Vaihemuutoskartan avulla voidaan kartoittaa kohteen lämpötilaeroja. Minimaalinen kajoamattomuus on yleisesti perusteltua lääketieteellisissä hoidoissa. Kehon lämpötilamuutosten avulla saadaan tietoa kehon anomaalisista tiloista. Elävissä kudoksissa olevia tauteja ja kasvaimia voidaan hoitaa kajoamattomasti lämpö- ja kylmäterapialla. Magneettikuvauslaitteella voidaan seurata kudosten lämpötilaa hoitojen aikana, sekä kartoittaa kehon poikkeavia lämpötilaeroja. Vaihekartoituksen avulla voidaan seurata myös magneettikuvaukseen liittyvää kudosten lämpenemistä. Tämän työn tavoitteena on muodostaa lämpötilatietoa kuvaava tilavuuskartta protonin resonanssitaajuuteen perustuvalla menetelmällä, ja optimoida kuvausparametrejä parhaan signaalikohinasuhteen saavuttamiseksi lämpötilamittauksen osalta

Doctor of Philosophy

dissertationMinimally invasive thermal therapy under Magnetic Resonance Imaging (MRI) guidance is becoming popular with several applications in the process of getting FDA approval. The ability to determine in near real-time the temperature map of a tumor and its surrounding tissue makes MR thermometry very attractive and well suited for thermal treatment. The proton resonance frequency shift (PRF) is currently the gold standard method for temperature monitoring using MRI. However, its incapacity to measure temperature in fatty tissue limits the scope of its applicability. The spin lattice relaxation time T1, on the other hand, has shown good temperature sensitivity and works well in all types of tissues. In this dissertation, we have addressed a number of challenges currently affecting MRI thermometry. A non-CPMG Turbo Spin Echo (TSE) sequence has been implemented to monitor the temperature rise due to the high RF power deposition inherent to this sequence at high field (3T and higher). This new implementation allows TSE sequences to be used safely without altering their high contrast properties which make them appealing in clinical settings. Tissue damage assessment during thermal therapy is critical for the safety of the patient. We have developed a new hybrid PRF-T1 sequence that has the capability to provide simultaneously in near real-time the temperature map and T1 information, which is a good indication of the state of the tissue. The simplicity and the real-time capability of the newly developed sequence make it an ideal tool for tissue damage assessment. Temperature monitoring during thermal therapy in organs with large fat content have been hindered by the lack of an MRI thermometry method that can provide simultaneous temperature in fat and aqueous tissue. A new sequence and acquisition scheme have been developed to address this issue. In sum, this dissertation proposed several pulse sequence implementation techniques and an acquisition scheme to overcome some of the limitations of MR thermometry

Monitoring temperature distribution during laser-tissue interaction for cancer treatment using magnetic resonance thermometry.

One of the major challenges in the field of cancer treatment is to induce a prolonged systemic immune response. Laser immunotherapy has shown its potential as a minimally invasive alternative for many cancer patients due to its ability to target and destroy cancerous cells while leaving minimal damage to surrounding tissue. Temperature distribution is a crucial factor during laser-tissue interaction in the treatment of tumors. In order to optimize laser immunotherapy, it is important to monitor the temperature distribution in target tissue during laser irradiation. Among the currently available temperature measurement techniques, Magnetic Resonance Thermometry (MRT) is the only feasible method for non-invasive, real-time, three-dimensional temperature mapping. In this study, we investigated the temperature distribution in gel phantom during laser irradiation using MRT. Spherical gel phantoms, as targets containing a light-absorbing agent, indocyanine green (ICG), of different concentrations were embedded inside normal phantom gels or in chicken breast tissue and were irradiated by an 805 nm diode laser. A 7.1 Tesla magnetic resonance imaging (MRI) system and a specially designed algorithm were used for ex vivo temperature mappings. The MRI signals were recorded using a fast low-angle shot (FLASH) sequence during laser irradiation with different values of MRI parameters such as TE (echo time), TR (repetition time), and NEX (number of excitation). The relative position of the proton, as determined by its phase, was obtained from the MRI signal. The phase data were reconstructed to obtain temperature distributions. Temperature elevation of target gels containing 0.056% ICG irradiated with laser of 2.0 watt/cm2 power density showed better selectivity compared to that of 0.08% and 0.10% ICG. However, phantom gels in chicken breast tissue with 0.08% ICG treated with laser of 1.0 watt/cm2 power density showed a considerable temperature elevation in the range of 10-35oC. Our results suggest that the effectiveness of the photothermal treatment depends on various parameters such as laser power, dye concentration, time of laser irradiation, and time of relaxation. This study provided important information for optimizing laser immunotherapy. Key Words: Cancer treatment, laser immunotherapy, magnetic resonance thermometry (MRT), magnetic resonance imaging (MRI), indocyanine green (ICG), gel phantoms, fast low-angle shot (FLASH), TE (echo time), TR (repetition time), NEX (number of excitation), temperature mapping

K-space data processing for Magnetic Resonance Elastography (MRE)

International audienceObject: Magnetic Resonance Elastography (MRE) requires substantial data processing based on phase image reconstruction, wave enhancement and inverse problem solving. The objective of this study is to propose a new, fast MRE method based on MR raw data processing, particularly adapted to applications requiring fast MRE measurement or high elastogram update rate.Material and Methods: The proposed method allows measuring tissue elasticity directly from raw data without prior phase image reconstruction and without phase unwrapping. Experimental feasibility is assessed both in a gelatin phantom and in the liver of a porcine model in vivo. Elastograms are reconstructed with the raw MRE method and compared to those obtained using conventional MRE. In a third experiment, changes in elasticity are monitored in real-time in a gelatin phantom during its solidification by using both conventional MRE and raw MRE.Results: The raw MRE method shows promising results by providing similar elasticity values to the ones obtained with conventional MRE methods while decreasing the number of processing steps and circumventing the delicate step of phase unwrapping. Limitations of the proposed method are the influence of the magnitude on the elastogram and the requirement for a minimum number of phase offsets.Conclusion: This study demonstrates the feasibility of directly reconstructing elastograms from raw data

Tissue magnetic susceptibility mapping as a marker of tau pathology in Alzheimer's disease.

Alzheimer's disease is connected to a number of other neurodegenerative conditions, known collectively as 'tauopathies', by the presence of aggregated tau protein in the brain. Neuroinflammation and oxidative stress in AD are associated with tau pathology and both the breakdown of axonal sheaths in white matter tracts and excess iron accumulation grey matter brain regions. Despite the identification of myelin and iron concentration as major sources of contrast in quantitative susceptibility maps of the brain, the sensitivity of this technique to tau pathology has yet to be explored. In this study, we perform Quantitative Susceptibility Mapping (QSM) and T2* mapping in the rTg4510, a mouse model of tauopathy, both in vivo and ex vivo. Significant correlations were observed between histological measures of myelin content and both mean regional magnetic susceptibility and T2* values. These results suggest that magnetic susceptibility is sensitive to tissue myelin concentrations across different regions of the brain. Differences in magnetic susceptibility were detected in the corpus callosum, striatum, hippocampus and thalamus of the rTg4510 mice relative to wild type controls. The concentration of neurofibrillary tangles was found to be low to intermediate in these brain regions indicating that QSM may be a useful biomarker for early stage detection of tau pathology in neurodegenerative diseases