Modeling of radio-frequency induced currents on lead wires during MR imaging using a modified transmission line method

Cataloged from PDF version of article.Purpose: Metallic implants may cause serious tissue heating during magnetic resonance (MR) imaging. This heating occurs due to the induced currents caused by the radio-frequency (RF) field. Much work has been done to date to understand the relationship between the RF field and the induced currents. Most of these studies, however, were based purely on experimental or numerical methods. This study has three main purposes: (1) to define the RF heating properties of an implant lead using two parameters; (2) to develop an analytical formulation that directly explains the relationship between RF fields and induced currents; and (3) to form a basis for analysis of complex cases. Methods: In this study, a lumped element model of the transmission line was modified to model leads of implants inside the body. Using this model, leads are defined using two parameters: impedance per unit length, Z, and effective wavenumber along the lead, k t. These two parameters were obtained by using methods that are similar to the transmission line theory. As long as these parameters are known for a lead, currents induced in the lead can be obtained no matter how complex the lead geometry is. The currents induced in bare wire, lossy wire, and insulated wire were calculated using this new method which is called the modified transmission line method or MoTLiM. First, the calculated induced currents under uniform electric field distribution were solved and compared with method-of-moments (MoM) calculations. In addition, MoTLiM results were compared with those of phantom experiments. For experimental verification, the flip angle distortion due to the induced currents was used. The flip angle distribution around a wire was both measured by using flip angle imaging methods and calculated using current distribution obtained from the MoTLiM. Finally, these results were compared and an error analysis was carried out. Results: Bare perfect electric, bare lossy, and insulated perfect electric conductor wires under uniform and linearly varying electric field exposure were solved, both for 1.5 T and 3 T scanners, using both the MoTLiM and MoM. The results are in agreement within 10 mean-square error. The flip angle distribution that was obtained from experiments was compared along the azimuthal paths with different distances from the wire. The highest mean-square error was 20 among compared cases. Conclusions: A novel method was developed to define the RF heating properties of implant leads with two parameters and analyze the induced currents on implant leads that are exposed to electromagnetic fields in a lossy medium during a magnetic resonance imaging (MRI) scan. Some simple cases are examined to explain the MoTLiM and a basis is formed for the analysis of complex cases. The method presented shows the direct relationship between the incident RF field and the induced currents. In addition, the MoTLiM reveals the RF heating properties of the implant leads in terms of the physical features of the lead and electrical properties of the medium. © 2011 American Association of Physicists in Medicine

Intravascular magnetic resonance imaging (MRI)

This chapter discusses the advantages of using MRI in intravascular operations. Beginning with a brief explanation of some sample intravascular MR operations, the chapter goes on to outline the visualization problem of catheters under MRI. Some of the fundamental catheter coil designs are shown and a range of imaging techniques for catheter tracking are discussed. Finally, safety issues related both specifically to these catheter coil designs and to MRI more generally are explored. © 2014 Woodhead Publishing Limited All rights reserved

Quadrichambered ventricles

Double chambered is a term that has been used to describe the subdivision of a ventricle as a result of anomalous septum or muscle bundle. Subdivision of the left ventricular cavity is a rare cardiac anomaly compared to subdivision of the right ventricle. This case features a double chambered right ventricle and a rare double chambered left ventricle at the same time. (Cardiol J 2010; 17, 3: 303–305

Modeling of radio-frequency induced currents on lead wires during MR imaging using a modified transmission line method

Purpose: Metallic implants may cause serious tissue heating during magnetic resonance (MR) imaging. This heating occurs due to the induced currents caused by the radio-frequency (RF) field. Much work has been done to date to understand the relationship between the RF field and the induced currents. Most of these studies, however, were based purely on experimental or numerical methods. This study has three main purposes: (1) to define the RF heating properties of an implant lead using two parameters; (2) to develop an analytical formulation that directly explains the relationship between RF fields and induced currents; and (3) to form a basis for analysis of complex cases. Methods: In this study, a lumped element model of the transmission line was modified to model leads of implants inside the body. Using this model, leads are defined using two parameters: impedance per unit length, Z, and effective wavenumber along the lead, k t. These two parameters were obtained by using methods that are similar to the transmission line theory. As long as these parameters are known for a lead, currents induced in the lead can be obtained no matter how complex the lead geometry is. The currents induced in bare wire, lossy wire, and insulated wire were calculated using this new method which is called the modified transmission line method or MoTLiM. First, the calculated induced currents under uniform electric field distribution were solved and compared with method-of-moments (MoM) calculations. In addition, MoTLiM results were compared with those of phantom experiments. For experimental verification, the flip angle distortion due to the induced currents was used. The flip angle distribution around a wire was both measured by using flip angle imaging methods and calculated using current distribution obtained from the MoTLiM. Finally, these results were compared and an error analysis was carried out. Results: Bare perfect electric, bare lossy, and insulated perfect electric conductor wires under uniform and linearly varying electric field exposure were solved, both for 1.5 T and 3 T scanners, using both the MoTLiM and MoM. The results are in agreement within 10 mean-square error. The flip angle distribution that was obtained from experiments was compared along the azimuthal paths with different distances from the wire. The highest mean-square error was 20 among compared cases. Conclusions: A novel method was developed to define the RF heating properties of implant leads with two parameters and analyze the induced currents on implant leads that are exposed to electromagnetic fields in a lossy medium during a magnetic resonance imaging (MRI) scan. Some simple cases are examined to explain the MoTLiM and a basis is formed for the analysis of complex cases. The method presented shows the direct relationship between the incident RF field and the induced currents. In addition, the MoTLiM reveals the RF heating properties of the implant leads in terms of the physical features of the lead and electrical properties of the medium. © 2011 American Association of Physicists in Medicine

Reliability and validity of the Turkish translation of the beliefs about medicines questionnaire (BMQ-T) in patients with Behçet’s disease

OBJECTIVES: The aim of this study was to evaluate the reliability and validity of the Turkish translation of the Beliefs about Medicines Questionnaire (BMQ-T, ©Prof. Rob Horne) for patients with Behçet's disease. METHODS: This methodological study enrolled a sample of 125 patients. The scale was adapted to Turkish through a process including translation, comparison with versions in other languages, back translation, and pretesting. Construct validity was evaluated by factor analysis. Medication adherence evaluated as poor, moderate and good according to the Morisky Medication Adherence Scale (MMAS). BMQ-T scores compared along medication adherence status groups. RESULTS: In our study, as in the original scale, the factor analysis confirmed that the BMQ-T had a four-factor structure explaining 54.73% of the total variance. The BMQ-T had acceptable internal consistency (Cronbach's alpha coefficient: Specific Necessity=.812; Specific Concerns=.672; General Harm=.677; General Overuse=.656), adequate test-retest reliability (intraclass correlation coefficients: Specific Necessity=.715; Specific Concerns=.680; General Harm=.678; General Overuse=.327). Specific Necessity and Specific Concerns scores were significantly different between medication adherence status groups. CONCLUSIONS: The psychometric properties of the BMQ-T were consistent with those reported in the original study. The BMQ-T was found to be a valid and reliable tool for evaluating beliefs about medicines in patients with Behçet's disease

Modeling of electrodes and implantable pulse generator cases for the analysis of implant tip heating under MR imaging

Purpose: The authors purpose is to model the case of an implantable pulse generator (IPG) and the electrode of an active implantable medical device using lumped circuit elements in order to analyze their effect on radio frequency induced tissue heating problem during a magnetic resonance imaging (MRI) examination. Methods: In this study, IPG case and electrode are modeled with a voltage source and impedance. Values of these parameters are found using the modified transmission line method (MoTLiM) and the method of moments (MoM) simulations. Once the parameter values of an electrode/IPG case model are determined, they can be connected to any lead, and tip heating can be analyzed. To validate these models, both MoM simulations and MR experiments were used. The induced currents on the leads with the IPG case or electrode connections were solved using the proposed models and the MoTLiM. These results were compared with the MoM simulations. In addition, an electrode was connected to a lead via an inductor. The dissipated power on the electrode was calculated using the MoTLiM by changing the inductance and the results were compared with the specific absorption rate results that were obtained using MoM. Then, MRI experiments were conducted to test the IPG case and the electrode models. To test the IPG case, a bare lead was connected to the case and placed inside a uniform phantom. During a MRI scan, the temperature rise at the lead was measured by changing the lead length. The power at the lead tip for the same scenario was also calculated using the IPG case model and MoTLiM. Then, an electrode was connected to a lead via an inductor and placed inside a uniform phantom. During a MRI scan, the temperature rise at the electrode was measured by changing the inductance and compared with the dissipated power on the electrode resistance. Results: The induced currents on leads with the IPG case or electrode connection were solved for using the combination of the MoTLiM and the proposed lumped circuit models. These results were compared with those from the MoM simulations. The mean square error was less than 9%. During the MRI experiments, when the IPG case was introduced, the resonance lengths were calculated to have an error less than 13%. Also the change in tip temperature rise at resonance lengths was predicted with less than 4% error. For the electrode experiments, the value of the matching impedance was predicted with an error less than 1%. Conclusions: Electrical models for the IPG case and electrode are suggested, and the method is proposed to determine the parameter values. The concept of matching of the electrode to the lead is clarified using the defined electrode impedance and the lead Thevenin impedance. The effect of the IPG case and electrode on tip heating can be predicted using the proposed theory. With these models, understanding the tissue heating due to the implants becomes easier. Also, these models are beneficial for implant safety testers and designers. Using these models, worst case conditions can be determined and the corresponding implant test experiments can be planned. © 2015 American Association of Physicists in Medicine

The diagnosis and clinical importance of tumour neovascularisation from coronary artery to right atrial myxoma

Myxomas are the commonest cardiac tumours and are usually localised in the atria. Neovascularisation in cardiac myxomas has been shown in previous case reports. However, the clinical importance of neovascularisation in cardiac myxomas is not well understood. In our case report, we present a right atrial myxoma in a 46 year-old woman admitted to our hospital with exertional angina and dyspnea. Coronary angiography revealed the presence of tumour neovascularisation from the right coronary artery, with no evidence of coronary artery stenosis. We thus speculate that neovascularisation of myxoma may cause typical anginal symptoms as a result of coronary steal phenomenon. Coronary angiography might help in the evaluation of the neovascularisation process and also in indicating surgery.Myxomas are the commonest cardiac tumours and are usually localised in the atria. Neovascularisation in cardiac myxomas has been shown in previous case reports. However, the clinical importance of neovascularisation in cardiac myxomas is not well understood. In our case report, we present a right atrial myxoma in a 46 year-old woman admitted to our hospital with exertional angina and dyspnea. Coronary angiography revealed the presence of tumour neovascularisation from the right coronary artery, with no evidence of coronary artery stenosis. We thus speculate that neovascularisation of myxoma may cause typical anginal symptoms as a result of coronary steal phenomenon. Coronary angiography might help in the evaluation of the neovascularisation process and also in indicating surgery

A Case of Linear Porokeratosis Superimposed on Disseminated Superficial Actinic Porokeratosis

We present a female patient with linear porokeratosis of her right arm since childhood. At the age of 67 years she additionally developed disseminated superficial actinic porokeratosis (DSAP) involving both lower legs. This uncommon coexistence of two different types of porokeratosis fulfils the clinical criteria of a type 2 segmental manifestation of an autosomal dominant skin disorder, being superimposed on the ordinary nonsegmental lesions and reflecting loss of heterozygosity that occurred at an early developmental stage. In DSAP molecular evidence of this concept is so far lacking, but such proof has already been provided in several other autosomal dominant skin disorders. Molecular analysis of cases of type 2 segmental involvement may help elucidate the genetic defect causing DSAP