Electromagnetic and Thermal Aspects of Radiofrequency Field Propagation in Ultra-High Field MRI

In MRI, a radiofrequency (RF) pulse is used to generate a signal from the spins that are polarized by a strong magnetic field. For higher magnetic field strengths, a higher frequency of the RF pulse is required in order to match the Larmor frequency. A higher frequency, in turn, leads to a shorter wavelength. This results in undesirable spatial fluctuations of the RF magnetic field. Those fluctuations are caused by interference and lead to reduced image quality. Not only the magnetic field, but also the electric field is inhomogeneous. Part of the electric field is absorbed which results in tissue heating. Due to interference, hotspots are expected in the energy absorption patterns, as well as the tissue temperature distribution. Tissue heating can be harmful in case the reached temperature is too high. In the first part of the thesis, the energy absorption and resulting tissue temperature in the head during 7T MRI was investigated using simulations. It was shown that the current safety guidelines regarding the maximal allowable energy absorption closely match the maximum allowable temperature. Therefore patient-to-patient variations in anatomy and physiology, can lead to a higher temperature than expected, thus lead to a risk for the patients. Therefore, the remainder of the work was focused on mapping of one of the input variables: the dielectric properties. It was shown that the dielectric properties can be mapped with MRI using Electrical Properties Tomography (EPT). Paradoxically, the inhomogeneous RF field is in this case very useful; the local dielectric properties are encoded in the propagation pattern of the RF field, as they affect the local wave behavior. Therefore, it seems beneficial to measure with a high RF frequency, thus at a high static magnetic field strength. This was evaluated using a comparison study at 1.5, 3 and 7T of EPT of the brain. It was found that measuring at a higher static magnetic field strength indeed leads to a higher sensitivity of EPT. However, certain assumptions which are needed for EPT, are less valid at higher field strengths. Therefore it was concluded that for the investigated case, 3T was the best field strength for conductivity mapping. For permittivity mapping, however, it was found that the sensitivity at 3T was much lower, and therefore, 7T was the optimal field strength for this case. Mapping of the dielectric properties is not only useful for RF safety assessment of MRI, but can also be used to image tumors. In the thesis it was shown that in brain tumors the electrical conductivity is elevated compared to the healthy surrounding tissue. This finding was consistent with earlier work on conductivity measurements, however, in that case invasive measurement techniques were used. One remaining challenge for EPT, is to reduce the errors which originate from two presently used assumptions, such that the accuracy of the reconstruction improves and the maps can be used as input for simulations. Clinically, more insight should be gained on the physiological changes that are the basis for the elevated conductivity

Imaging strategies in the management of oesophageal cancer: what's the role of MRI?

To outline the current role and future potential of magnetic resonance imaging (MRI) in the management of oesophageal cancer regarding T-staging, N-staging, tumour delineation for radiotherapy (RT) and treatment response assessment.PubMed, Embase and the Cochrane library were searched identifying all articles related to the use of MRI in oesophageal cancer. Data regarding the value of MRI in the areas of interest were extracted in order to calculate sensitivity, specificity, predictive values and accuracy for group-related outcome measures.Although historically poor, recent improvements in MRI protocols and techniques have resulted in better imaging quality and the valuable addition of functional information. In recent studies, similar or even better results have been achieved using optimised MRI compared with other imaging strategies for T- and N-staging. No studies clearly report on the role of MRI in oesophageal tumour delineation and real-time guidance for RT so far. Recent pilot studies showed that functional MRI might be capable of predicting pathological response to treatment and patient prognosis.In the near future MRI has the potential to bring improvement in staging, tumour delineation and real-time guidance for RT and assessment of treatment response, thereby complementing the limitations of currently used imaging strategies.aEuro cent MRI's role in oesophageal cancer has been somewhat limited to date.aEuro cent However MRI's ability to depict oesophageal cancer is continuously improving.aEuro cent Optimising TN-staging, radiotherapy planning and response assessment ultimately improves individualised cancer care.aEuro cent MRI potentially complements the limitations of other imaging strategies regarding these points.Biological, physical and clinical aspects of cancer treatment with ionising radiatio

B1-based SAR reconstruction using contrast source inversion-electric properties tomography (CSI-EPT)

Specific absorption rate (SAR) assessment is essential for safety purposes during MR acquisition. Online SAR assessment is not trivial and requires, in addition, knowledge of the electric tissue properties and the electric fields in the human anatomy. In this study, the potential of the recently developed CSI-EPT method to reconstruct SAR distributions is investigated. This method is based on integral representations for the electromagnetic field and attempts to reconstruct the tissue parameters and the electric field strength based on [Formula: see text] field data only. Full three-dimensional FDTD simulations using a female pelvis model are used to validate two-dimensional CSI reconstruction results in the central transverse plane of a 3T body coil. Numerical experiments demonstrate that the reconstructed SAR distributions are in good agreement with the SAR distributions as determined via 3D FDTD simulations and show that these distributions can be computed very efficiently in the central transverse plane of a body coil with the two-dimensional approach of CSI-EPT

Imaging of oesophageal cancer with FDG-PET/CT and MRI

Integrated 2-[F-18]-fluoro-2-deoxy-D-glucose (FOG) PET/CT and magnetic resonance imaging (MRI) with functional features of diffusion-weighted imaging (DWI) are advancing imaging technologies that have current and future potential to overcome important limitations of conventional staging methods in the management of patients with oesophageal cancer. PET/CT has emerged as an important part of the standard work-up of patients with oesophageal cancer. Besides its important ability to detect unsuspected metastatic disease, PET/CT may be useful in the assessment of treatment response, radiation treatment planning, and detection of recurrent disease. In addition, high-resolution T2-weighted MRI and DWI have potential complementary roles. Recent improvements in MRI protocols and techniques have resulted in better imaging quality with the potential to bring improvement in staging, radiation treatment planning, and the assessment of treatment response. Optimal use and understanding of PET/CT and MRI in oesophageal cancer will contribute to the impact of these advancing technologies in tailoring treatment to the individual patient and achieving best possible outcomes. In this article, we graphically outline the current and potential future roles of PET/CT and MRI in the multidisciplinary management of oesophageal cancer. (C) 2014 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved

Hyperthermia treatment planning for cervical cancer patients based on electrical conductivity tissue properties acquired in vivo with EPT at 3 T MRI

Introduction The reliability of hyperthermia treatment planning (HTP) is strongly dependent on the accuracy of the electric properties of each tissue. The values currently used are mostly based on ex vivo measurements. In this study, in vivo conductivity of human muscle, bladder content and cervical tumours, acquired with magnetic resonance-based electric properties tomography (MR-EPT), are exploited to investigate the effect on HTP for cervical cancer patients. Methods Temperature-based optimisation of five different patients was performed using literature-based conductivity values yielding certain antenna settings, which are then used to compute the temperature distribution of the patient models with EPT-based conductivity values. Furthermore, the effects of altered bladder and muscle conductivity were studied separately. Finally, the temperature-based optimisation was performed with patient models based on EPT conductivity values. Results The tumour temperatures for all EPT-based dielectric patient models were lower compared to the optimal tumour temperatures based on literature values. The largest deviation was observed for patient 1 with ΔT90 = -1.37 °C. A negative impact was also observed when the treatment was optimised based on the EPT values. For four patients ΔT90 was less than 0.6 °C; for one patient it was 1.5 °C. Conclusions Electric conductivity values acquired by EPT are higher than commonly used from literature. This difference has a substantial impact on cervical tumour temperatures achieved during hyperthermia. A higher conductivity in the bladder and in the muscle tissue surrounding the tumour leads to higher power dissipation in the bladder and muscle, and therefore to lower tumour temperatures

Dynamic contrast-enhanced MRI for treatment response assessment in patients with oesophageal cancer receiving neoadjuvant chemoradiotherapy

Purpose To explore and evaluate the potential value of dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) for the prediction of pathologic response to neoadjuvant chemoradiotherapy (nCRT) in oesophageal cancer. Material and methods Twenty-six patients underwent DCE-MRI before, during (week 2–3) and after nCRT, but before surgery (pre/per/post, respectively). Histopathologic tumour regression grade (TRG) was assessed after oesophagectomy. Tumour area-under-the-concentration time curve (AUC), time-to-peak (TTP) and slope were calculated. The ability of these DCE-parameters to distinguish good responders (GR, TRG 1–2) from poor responders (noGR, TRG ⩾ 3), and pathologic complete responders (pCR) from no-pCR was assessed. Results Twelve patients (48%) showed GR of which 8 patients (32%) pCR. Analysis of AUC change throughout treatment, AUCper-pre, was most predictive for GR, at a threshold of 22.7% resulting in a sensitivity of 92%, specificity of 77%, PPV of 79%, and a NPV of 91%. AUCpost-pre was most predictive for pCR, at a threshold of −24.6% resulting in a sensitivity of 83%, specificity of 88%, PPV of 71%, and a NPV of 93%. TTP and slope were not associated with pathologic response. Conclusions This study demonstrates that changes in AUC throughout treatment are promising for prediction of histopathologic response to nCRT for oesophageal cancer

Dynamic contrast-enhanced MRI for treatment response assessment in patients with oesophageal cancer receiving neoadjuvant chemoradiotherapy

Purpose To explore and evaluate the potential value of dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) for the prediction of pathologic response to neoadjuvant chemoradiotherapy (nCRT) in oesophageal cancer. Material and methods Twenty-six patients underwent DCE-MRI before, during (week 2–3) and after nCRT, but before surgery (pre/per/post, respectively). Histopathologic tumour regression grade (TRG) was assessed after oesophagectomy. Tumour area-under-the-concentration time curve (AUC), time-to-peak (TTP) and slope were calculated. The ability of these DCE-parameters to distinguish good responders (GR, TRG 1–2) from poor responders (noGR, TRG ⩾ 3), and pathologic complete responders (pCR) from no-pCR was assessed. Results Twelve patients (48%) showed GR of which 8 patients (32%) pCR. Analysis of AUC change throughout treatment, AUCper-pre, was most predictive for GR, at a threshold of 22.7% resulting in a sensitivity of 92%, specificity of 77%, PPV of 79%, and a NPV of 91%. AUCpost-pre was most predictive for pCR, at a threshold of −24.6% resulting in a sensitivity of 83%, specificity of 88%, PPV of 71%, and a NPV of 93%. TTP and slope were not associated with pathologic response. Conclusions This study demonstrates that changes in AUC throughout treatment are promising for prediction of histopathologic response to nCRT for oesophageal cancer

The peri-esophageal connective tissue layers and related compartments : visualization by histology and magnetic resonance imaging

An organized layer of connective tissue coursing from aorta to esophagus was recently discovered in the mediastinum. The relations with other peri-esophageal fascias have not been described and it is unclear whether this layer can be visualized by non-invasive imaging. This study aimed to provide a comprehensive description of the peri-esophageal fascias and determine whether the connective tissue layer between aorta and esophagus can be visualized by magnetic resonance imaging (MRI). First, T2-weighted MRI scanning of the thoracic region of a human cadaver was performed, followed by histological examination of transverse sections of the peri-esophageal tissue between the thyroid gland and the diaphragm. Secondly, pretreatment motion-triggered MRI scans were prospectively obtained from 34 patients with esophageal cancer and independently assessed by two radiologists for the presence and location of the connective tissue layer coursing from aorta to esophagus. A layer of connective tissue coursing from the anterior aspect of the descending aorta to the left lateral aspect of the esophagus, with a thin extension coursing to the right pleural reflection, was visualized ex vivo in the cadaver on MR images, macroscopic tissue sections, and after histologic staining, as well as on in vivo MR images. The layer connecting esophagus and aorta was named 'aorto-esophageal ligament' and the layer connecting aorta to the right pleural reflection 'aorto-pleural ligament'. These connective tissue layers divides the posterior mediastinum in an anterior compartment containing the esophagus, (carinal) lymph nodes and vagus nerve, and a posterior compartment, containing the azygos vein, thoracic duct and occasionally lymph nodes. The anterior compartment was named 'peri-esophageal compartment' and the posterior compartment 'para-aortic compartment'. The connective tissue layers superior to the aortic arch and at the diaphragm corresponded with the currently available anatomic descriptions. This study confirms the existence of the previously described connective tissue layer coursing from aorta to esophagus, challenging the long-standing paradigm that no such structure exists. A comprehensive, detailed description of the peri-esophageal fascias is provided and, furthermore, it is shown that the connective tissue layer coursing from aorta to esophagus can be visualized in vivo by MRI