Calculation of induced current densities and specific absorption rates (SAR) for pregnant women exposed to hand-held metal detectors

Searching for prohibited items in mental-health hospitals: A randomised controlled trial of two metal-detecting technologies

Weapons and other items with potential to cause harm are usually prohibited in mental-health hospitals and other psychiatric facilities. Detecting such prohibited items (PIs) can be problematic, particularly if concealed, and metal detectors are commonly used to search for such items. Our study compared two types of metal detection: continuous wave detection (CWD) by hand-held metal detector (HHMD) and magnetic anomaly detection (MAD) by a static pole device. In the study, real and dummy PIs were hidden on test subjects and in a simulated body cavity. The results showed MAD to be significantly superior to CWD in detecting small concealed PIs containing ferrous metal. The MAD pole found 100% of the real PIs on the test subjects and in the simulated body cavity. The CWD HHMD found only 5.2% of the real PIs, and these were limited to those on the test subjects, as it detected none in the simulated body cavity. In addition, the time taken to search by MAD pole was shorter than time taken to search by CWD HHMD.

The utility of a handheld metal detector in detection and localization of pediatric metallic foreign body ingestion

OBJECTIVE

To test the ability of a handheld metal detector (HHMD) to identify the presence and location of ingested metallic foreign bodies (MFBs) in children.

METHODS

Prospective case series enrolling children suspected of metallic foreign body ingestion presenting to the Emergency Department. Thirty-eight children were enrolled and the HHMD was used to detect the presence and location of a MFB. Results were compared to standard radiographic studies.

RESULTS

Thirty-seven of the 38 ingested foreign bodies were MFBs. Of the 37 MFBs, the HHMD positively identified 33, and 4 were missed by HHMD but identified on radiography. When positive, the location indicated by HHMD correlated 100% with radiograph. There were 33 true positives, 0 false positives, 4 false negatives, and 1 true negative. This resulted in a sensitivity of 89% (95% CI of 75%-96%) and specificity of 100% (95% CI of 2.5%-100%).

CONCLUSION

Our study demonstrates the accuracy of HHMD in the identification and localization of metallic foreign bodies. We propose an emergency room foreign body protocol that uses HHMD as an early screening tool in triage in order to expedite the process of obtaining Otolaryngology consultation and potentially shorten the wait time to the operating room or discharge. In instances were outside films are previously performed, HHMD use may be able to minimize the overall radiation exposure to children by obviating the need for repeat radiographs. As the sensitivity is not 100%, a negative HHMD screening does not negate the need for a standard radiograph in order to avoid missed MFBs. HHMD is best suited for detection of coins, which accounts for the majority of the MFB ingestions, and may not be suitable for all metallic objects since the amount of metal may decrease its sensitivity.

Assessing the Electromagnetic Fields Generated By a Radiofrequency MRI Body Coil at 64 MHz: Defeaturing Versus Accuracy

GOAL

This study aims at a systematic assessment of five computational models of a birdcage coil for magnetic resonance imaging (MRI) with respect to accuracy and computational cost.

METHODS

The models were implemented using the same geometrical model and numerical algorithm, but different driving methods (i.e., coil "defeaturing"). The defeatured models were labeled as: specific (S2), generic (G32, G16), and hybrid (H16, [Formula: see text]). The accuracy of the models was evaluated using the "symmetric mean absolute percentage error" ("SMAPE"), by comparison with measurements in terms of frequency response, as well as electric ( ||→E||) and magnetic ( || →B ||) field magnitude.

RESULTS

All the models computed the || →B || within 35% of the measurements, only the S2, G32, and H16 were able to accurately model the ||→E|| inside the phantom with a maximum SMAPE of 16%. Outside the phantom, only the S2 showed a SMAPE lower than 11%.

CONCLUSIONS

Results showed that assessing the accuracy of || →B || based only on comparison along the central longitudinal line of the coil can be misleading. Generic or hybrid coils - when properly modeling the currents along the rings/rungs - were sufficient to accurately reproduce the fields inside a phantom while a specific model was needed to accurately model ||→E|| in the space between coil and phantom.

SIGNIFICANCE

Computational modeling of birdcage body coils is extensively used in the evaluation of radiofrequency-induced heating during MRI. Experimental validation of numerical models is needed to determine if a model is an accurate representation of a physical coil.

MIDA: A Multimodal Imaging-Based Detailed Anatomical Model of the Human Head and Neck

Computational modeling and simulations are increasingly being used to complement experimental testing for analysis of safety and efficacy of medical devices. Multiple voxel- and surface-based whole- and partial-body models have been proposed in the literature, typically with spatial resolution in the range of 1-2 mm and with 10-50 different tissue types resolved. We have developed a multimodal imaging-based detailed anatomical model of the human head and neck, named "MIDA". The model was obtained by integrating three different magnetic resonance imaging (MRI) modalities, the parameters of which were tailored to enhance the signals of specific tissues: i) structural T1- and T2-weighted MRIs; a specific heavily T2-weighted MRI slab with high nerve contrast optimized to enhance the structures of the ear and eye; ii) magnetic resonance angiography (MRA) data to image the vasculature, and iii) diffusion tensor imaging (DTI) to obtain information on anisotropy and fiber orientation. The unique multimodal high-resolution approach allowed resolving 153 structures, including several distinct muscles, bones and skull layers, arteries and veins, nerves, as well as salivary glands. The model offers also a detailed characterization of eyes, ears, and deep brain structures. A special automatic atlas-based segmentation procedure was adopted to include a detailed map of the nuclei of the thalamus and midbrain into the head model. The suitability of the model to simulations involving different numerical methods, discretization approaches, as well as DTI-based tensorial electrical conductivity, was examined in a case-study, in which the electric field was generated by transcranial alternating current stimulation. The voxel- and the surface-based versions of the models are freely available to the scientific community.

A High-Order, Symplectic, Finite-Difference Time-Domain Scheme for Bioelectromagnetic Applications within the Mother/Fetus Model

An explicit high-order, symplectic, finite-difference time-domain (SFDTD) scheme is applied to a bioelectromagnetic simulation using a simple model of a pregnant woman and her fetus. Compared to the traditional FDTD scheme, this scheme maintains the inherent nature of the Hamilton system and ensures energy conservation numerically and a high precision. The SFDTD scheme is used to predict the specific absorption rate (SAR) for a simple model of a pregnant female woman (month 9) using radio frequency (RF) fields from 1.5 T and 3 T MRI systems (operating at approximately 64 and 128 MHz, respectively). The results suggest that by using a plasma protective layer under the 1.5 T MRI system, the SAR values for the pregnant woman and her fetus are significantly reduced. Additionally, for a 90 degree plasma protective layer, the SAR values are approximately equal to the 120 degree layer and the 180 degree layer, and it is reduced relative to the 60 degree layer. This proves that using a 90 degree plasma protective layer is the most effective and economical angle to use.

Estimate of the fetal temperature increase due to UHF RFID exposure

Exposure from electromagnetic (EM) devices has increased during the last decades due to the rapid development of new technologies. Among them, radiofrequency identification (RFID) applications are used in almost every aspect of everyday life, which could expose people unselectively. This scenario could pose potential risks for certain groups of general population, such as pregnant women, who are more sensitive to thermal effects produced by EM exposure. In this paper, the temperature rise at the steady state in two pregnant women models exposed to UHF RFID has been assessed. Results show that heating of tissues is far from the threshold of biological effects indicated by radiation protection guidelines.

Specific absorbed fractions for internal electron emitters derived for a set of anatomically realistic reference pregnant female models

The specific absorbed fraction (Phi), defined by the Medical Internal Radiation Dose Committee, is generally applied to evaluate the average absorbed dose in a target organ as a result of radioactive materials deposited in a source organ. This paper reports a new set of Phi values for internal electron emitters ranging from 10 keV to 4 MeV from various internal organs of the mother to the fetus based on three newly developed pregnant female tomographic models, called RPI-P3, RPI-P6 and RPI-P9. The results show a linear log relationship between Phi values and electron energy. The linear log coefficients have been derived and reported. The relationship between Phi values and mean distances between source organs and the fetus were also determined to allow for individual dosimetry. Since the RPI-P models have finer details of human anatomy and more realistic organ volumes and geometries, which follow the latest ICRP reference values, the newly derived Phi values could be used as reference values in determination of the dose to the fetus from internal electron emitters.

SAF values for internal photon emitters calculated for the RPI-P pregnant-female models using Monte Carlo methods

Estimates of radiation absorbed doses from radionuclides internally deposited in a pregnant woman and her fetus are very important due to elevated fetal radiosensitivity. This paper reports a set of specific absorbed fractions (SAFs) for use with the dosimetry schema developed by the Society of Nuclear Medicine's Medical Internal Radiation Dose (MIRD) Committee. The calculations were based on three newly constructed pregnant female anatomic models, called RPI-P3, RPI-P6, and RPI-P9, that represent adult females at 3-, 6-, and 9-month gestational periods, respectively. Advanced Boundary REPresentation (BREP) surface-geometry modeling methods were used to create anatomically realistic geometries and organ volumes that were carefully adjusted to agree with the latest ICRP reference values. A Monte Carlo user code, EGS4-VLSI, was used to simulate internal photon emitters ranging from 10 keV to 4 MeV. SAF values were calculated and compared with previous data derived from stylized models of simplified geometries and with a model of a 7.5-month pregnant female developed previously from partial-body CT images. The results show considerable differences between these models for low energy photons, but generally good agreement at higher energies. These differences are caused mainly by different organ shapes and positions. Other factors, such as the organ mass, the source-to-target-organ centroid distance, and the Monte Carlo code used in each study, played lesser roles in the observed differences in these. Since the SAF values reported in this study are based on models that are anatomically more realistic than previous models, these data are recommended for future applications as standard reference values in internal dosimetry involving pregnant females.

An anatomically realistic voxel model of the pregnant woman and numerical dosimetry for a whole-body exposure to RF electromagnetic fields

The numerical dosimetry of pregnant women is one of the most important issues in electromagnetic-field safety. We have recently developed a whole-body numerical female model of an adult Japanese (non-pregnant) average figure. Therefore, a new fetus model including inherent tissues of pregnant women was constructed based on abdominal MRI data of a 7-month pregnant woman. A whole-body pregnant woman model was developed by combining the new fetus and the female models. The anatomical details of the developed pregnant woman model and basic SAR characteristics for whole-body exposure to RF electromagnetic fields are demonstrated.

An anatomically realistic whole-body pregnant-woman model and specific absorption rates for pregnant-woman exposure to electromagnetic plane waves from 10 MHz to 2 GHz

The numerical dosimetry of pregnant women is an important issue in electromagnetic-field safety. However, an anatomically realistic whole-body pregnant-woman model for electromagnetic dosimetry has not been developed. Therefore, we have developed a high-resolution whole-body model of pregnant women. A new fetus model including inherent tissues of pregnant women was constructed on the basis of abdominal magnetic resonance imaging data of a 26-week-pregnant woman. The whole-body pregnant-woman model was developed by combining the fetus model and a nonpregnant-woman model that was developed previously. The developed model consists of about 7 million cubical voxels of 2 mm size and is segmented into 56 tissues and organs. This pregnant-woman model is the first completely anatomically realistic voxel model that includes a realistic fetus model and enables a numerical simulation of electromagnetic dosimetry up to the gigahertz band. In this paper, we also present the basic specific absorption rate characteristics of the pregnant-woman model exposed to vertically and horizontally polarized electromagnetic waves from 10 MHz to 2 GHz.

Possible overexposure of pregnant women to emissions from a walk through metal detector

This paper presents a systematic procedure to evaluate the induced current densities and electric fields due to walk-through metal detector (WTMD) exposure. This procedure is then used to assess the exposure of nine pregnant women models exposed to one WTMD model. First, we measured the magnetic field generated by the WTMD, then we extracted the equivalent current source to represent the WTMD emissions and finally we calculated the induced current densities and electric fields using the impedance method. The WTMD emissions and the induced fields in the pregnant women and fetus models are then compared to the ICNIRP Guidelines and the IEEE C95.6 exposure safety standard. The results prove the consistency between maximum permissible exposure (MPE) levels and basic restrictions for the ICNIRP Guidelines and IEEE C95.6. We also found that this particular WTMD complies with the ICNIRP basic restrictions for month 1-5 models, but leads to both fetus and pregnant women overexposure for month 6-9 models. The IEEE C95.6 restrictions (MPEs and basic restrictions) are not exceeded. The fetus overexposure of this particular WTMD calls for carefully conducted safety evaluations of security systems before they are deployed.

Prediction of specific absorption rate in mother and fetus associated with MRI examinations during pregnancy

There is uncertainty regarding the risk posed by magnetic resonance imaging (MRI) examinations to pregnant patients. The most frequently used methods, such as single-shot fast spin echo (ssFSE), often require operation at the specific absorption rate (SAR) limits imposed by safety guidelines. With the introduction of higher-field systems, such limits will be even more significant for fetal imaging. An electromagnetic solver based on the time domain finite integration technique (FIT) was used to predict SAR in an anatomically realistic model of a pregnant patient (28 weeks' gestation) associated with the radiofrequency (RF) fields from birdcage body coils typical of 1.5 T and 3 T MRI systems (i.e., operating at approximately 64 and 127 MHz, respectively). The results suggest that 1) the highest local SAR is in the mother, with the fetus being exposed to a peak of approximately 40-60% of that value at 64 MHz, increasing to approximately 50-70% at 127 MHz; 2) compliance with U.S. Food and Drug Administration (FDA) and International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines requires control of SAR values averaged over 1 g or 10 g of tissue, respectively; and 3) compliance with Medical Device Agency (MDA) guidelines requires control of the maximum SAR(10g) within the fetus.

On the safety assessment of human exposure in the proximity of cellular communications base-station antennas at 900, 1800 and 2170 MHz

In this work, the procedures for safety assessment in the close proximity of cellular communications base-station antennas at three different frequencies (900, 1800 and 2170 MHz) are analysed. For each operating frequency, we have obtained and compared the distances to the antenna from the exposure places where electromagnetic fields are below reference levels and the distances where the specific absorption rate (SAR) values in an exposed person are below the basic restrictions, according to the European safety guidelines. A high-resolution human body model has been located, in front of each base-station antenna as a worst case, at different distances, to compute whole body averaged SAR and maximum 10 g averaged SAR inside the exposed body. The finite-difference time-domain method has been used for both electromagnetic fields and SAR calculations. This paper shows that, for antenna-body distances in the near zone of the antenna, the fact that averaged field values be below the reference levels could, at certain frequencies, not guarantee guidelines compliance based on basic restrictions.