Earl Zastrow

Development of Anatomically Realistic Numerical Breast Phantoms With Accurate Dielectric Properties for Modeling Microwave Interactions With the Human Breast

Computational electromagnetics models of microwave interactions with the human breast serve as an invaluable tool for exploring the feasibility of new technologies and improving design concepts related to microwave breast cancer detection and treatment. In this paper, we report the development of a collection of anatomically realistic 3-D numerical breast phantoms of varying shape, size, and radiographic density which can readily be used in finite-difference time-domain computational electromagnetics models. The phantoms are derived from T1-weighted MRIs of prone patients. Each MRI is transformed into a uniform grid of dielectric properties using several steps. First, the structure of each phantom is identified by applying image processing techniques to the MRI. Next, the voxel intensities of the MRI are converted to frequency-dependent and tissue-dependent dielectric properties of normal breast tissues via a piecewise-linear map. The dielectric properties of normal breast tissue are taken from the recently completed large-scale experimental study of normal breast tissue dielectric properties conducted by the Universities of Wisconsin and Calgary. The comprehensive collection of numerical phantoms is made available to the scientific community through an online repository.

3D computational study of non-invasive patient-specific microwave hyperthermia treatment of breast cancer

Non-invasive microwave hyperthermia treatment of breast cancer is investigated using three-dimensional (3D) numerical breast phantoms with anatomical and dielectric-properties realism. 3D electromagnetic and thermal finite-difference time-domain simulations are used to evaluate the focusing and selective heating efficacy in four numerical breast phantoms with different breast tissue densities. Beamforming is used to design and focus the signals transmitted by an antenna array into the breast. We investigate the use of propagation models of varying fidelity and complexity in the design of the transmitted signals. An ideal propagation model that is exactly matched to the actual patients breast is used to establish a best-performance baseline. Simpler patient-specific propagation models based on a homogeneous breast interior are also explored to evaluate the robustness of beamforming in practical clinical settings in which an ideal propagation model is not available. We also investigate the performance of the beamformer as a function of operating frequency and compare single-frequency and multiple-frequency focusing strategies. Our study suggests that beamforming is a robust method of non-invasively focusing microwave energy at a tumor site in breasts of varying volume and breast tissue density.

A Computational Study of Time Reversal Techniques for Ultra-Wideband Microwave Hyperthermia Treatment of Breast Cancer

We present a computational study of the application of time reversal (TR) principles to microwave hyperthermia treatment of breast cancer. A wideband source is excited at the tumor location (the desired focus) in an electromagnetic (EM) simulation based on the finite-difference time-domain (FDTD) method and the transmitted wave is recorded at multiple antenna locations. The FDTD-computed signals are time reversed for transmission into the breast. The same set of FDTD-computed signals is also used in a comparative investigation of a space-time beamforming technique, which has been previously studied for microwave hyperthermia. We discuss the relation between these two approaches, and compare the focusing efficacy and heating selectivity of the TR and beamforming approaches using FDTD EM and thermal simulations with anatomically realistic numerical breast phantoms. Promising results from both methods are obtained.

Safety assessment of breast cancer detection via ultrawideband microwave radar operating in swept-frequency mode

Ultrawideband (UWB) microwave radar is a promising modality for detecting early-stage breast cancer. In our current prototype, we synthesize an antenna array by scanning a single antenna to each array position and sequentially transmitting a swept-frequency signal and receiving the backscatter. Our UWB radar operates in the 1-11 GHz frequency band. While it has been assumed that low-power UWB microwave breast cancer detection technology poses no health risk, it is important to formally evaluate the absorption of electromagnetic energy in the breast to establish the safety of such exposure prior to clinical implementation. In this paper, we report the results of finite-difference time-domain (FDTD) investigations of the specific absorption rate (SAR) in 3-D numerical breast models due to microwave radiation in the 1-11 GHz frequency band

Virtual population-based assessment of the impact of 3 Tesla radiofrequency shimming and thermoregulation on safety and B1 + uniformity.

To assess the effect of radiofrequency (RF) shimming of a 3 Tesla (T) two‐port body coil on B1+ uniformity, the local specific absorption rate (SAR), and the local temperature increase as a function of the thermoregulatory response.

Heating and Safety Concerns of the Radio-Frequency Field in MRI

Magnetic resonance imaging (MRI) is a widely used and powerful imaging technique for non-invasive clinical diagnosis. The absorbed radiofrequency (RF) energy must be carefully managed, as MRI presents one of the highest RF exposures to humans. Temperature increases in the patient caused by high-level RF exposure is a major safety concern in MRI, potentially causing local thermal tissue damage or systemic overheating. This review article summarizes recent findings in MR safety research, including the clear distinction between exposures of patients with and without implants; evaluates the advantages and limitations of numerical simulations for RF safety assessment in MRI; and discusses the need for additional research at high RF exposure levels and in novel MRI systems.

Assessment of local RF-induced heating of AIMDs during MR exposure

Patients with active implantable medical devices (AIMDs) are generally excluded from magnetic resonance imaging (MRI) diagnostics because the interaction of the AIMD with MRI-induced radiofrequency (RF) electromagnetic fields (EMFs) can lead to hazardous localized heating in surrounding tissues. In this work, two safety assessment methods, based on Tiers 3 and 4 of the Technical Specification ISO/IEC 10974, for a generic AIMD, for which a cardio application is assumed, are implemented, and the results from the assessments are illustrated.

Experimental phantoms for the assessment of medical implant leads induced SAR under a linear-phase incident field condition

The induced SAR at the lead tip of medical implant leads can exceed the value reached under resonance conditions if the phase of the incident electric field tangential to the lead is varied even if the field amplitude remains constant. An approximately linear-phase (i.e. constant phase gradient) incident field condition is found to maximize the energy deposition at the tip. Here, we develop experimental phantoms for the assessment of induced SAR on medical implant leads under a constant-amplitude and linear-phase incident field condition. Two different phantoms are developed to accommodate lead testing for a wide range of constant phase gradients. The incident fields along the lead paths in the phantoms are numerically validated prior to construction of the phantoms.

Novel mechanistic model and computational approximation for electromagnetic safety evaluations of electrically short implants

This paper addresses unresolved issues related to the safety of persons with conductive medical implants exposed to electromagnetic (EM) fields. When exposed to EM fields compatible with the reference limits-in particular  <100 MHz-implants may enhance local fields and energy absorption to values much higher than the basic restrictions that are considered safe. A mechanistic model based on transfer functions has been postulated for elongated active implants at magnetic resonance imaging (MRI) frequencies and used as a basis for standards dealing with MRI implant safety. However, this mechanistic model is inconsistent with the behavior observed for electrically short implants, such as abandoned leads in MRI or active implants under low-frequency exposure conditions (e.g. wireless power transfer). In this paper, a new mechanistic model for electrically short implants is proposed that allows implant safety assessment to be decomposed into separate steps. Per tip-shape, it requires only a single simulation or measurement of the implant exposed under (semi-)homogeneous conditions. To validate the approach, predictions of the mechanistic model were compared to results of numerical simulations for electric- and magnetic-field exposures. The impact of parameters such as tissue properties, length, tip shape, and insulation thickness on safety- and compliance-relevant quantities was studied. Validation involving an anatomically detailed computational human body model with a realistic implant at multiple locations under electric and magnetic exposures resulted in prediction agreement on the order of 7% (maximal deviation  <15%). The approach was found to be applicable for electrical lengths up to 20% of the effective wavelength and can be used to derive suitable testing procedures as well as to develop safety guidelines and standards.

Practical considerations in experimental evaluations of RF-induced heating of leaded implants

This study illustrates some considerations to accommodate the non-ideality of experimental conditions needed in the Tier 3 assessment of RF-induced heating of leaded implants. The method, based on Tier 3 safety assessment during MRI exposure of ISO/TS 10974, is currently used for the rapid modeling of RF-implant interactions. We summarize the theoretical accuracy of Tier 3 method and the practical considerations for its actual implementation.