Papers by Abdel-Monem El-Sharkawy
IEEE Access
is financially supported by the African Biomedical Engineering Mobility (ABEM) for his Ph.D. prog... more is financially supported by the African Biomedical Engineering Mobility (ABEM) for his Ph.D. program at Cairo University. The ABEM project is funded by the Intra-Africa Academic Mobility Scheme of the Education, Audiovisual and Culture Executive Agency of European Commission. Sadeq Alsharafi is partially financially supported for his Ph.D. program at Cairo University by the Yemeni ministry of higher education. All programming and simulations in this work were performed by Haile Baye Kassahun.
2020 IEEE 5th Middle East and Africa Conference on Biomedical Engineering (MECBME), 2020
MRI gradient coils are mainly designed using discrete wire techniques and continuous current dens... more MRI gradient coils are mainly designed using discrete wire techniques and continuous current density methods. The width of the coil tracks is not usually included upfront in the coil design optimization process. In this work, we use stream functions to compute the current density distribution on modular coil tracks which are consequently used to calculate resultant magnetic fields and coil resistances. This is then used to determine the number of turns plus locations for both primary and shielding coils using a constrained discrete wire optimization technique. Using this process, we achieved a design for a transverse self-shielded gradient coil. The stream functions for the final coil configuration are re-computed and used to calculate the final designed coil parameters including the coil’s inductance.
Supplementary material. (DOCX 79 kb)
INTRODUCTION. Phosphocreatine (PCr), is the main energy reservoir in both skeletal and cardiac mu... more INTRODUCTION. Phosphocreatine (PCr), is the main energy reservoir in both skeletal and cardiac muscles, supplying ATP energy for contraction via the creatine kinase (CK) reaction. Cardiac P MRS measurements of absolute ATP and PCr concentrations could benefit from higher magnetic field strengths, but is challenged by T2decay, increased RF power deposition, and field inhomogeneities. While these have been specifically addressed at 3T [1,2], approaches to concentration measurements employing water-referencing and H signal detection with a single tuned P coil [3] are compromised at 3T due to differences in spatial sensitivity at the higher field strengths. This work presents a new method and semi-automated tool for measuring PCr and ATP concentrations from 1D chemical shift imaging (CSI) P spectra that compensates for the nonuniform sensitivity. It is validated in phantoms and applied to the human heart. METHODS. The method utilizes a single 1D CSI acquisition with a 5ms duration, 7kHz...
Journal of Cardiovascular Magnetic Resonance, 2018
Background: It has been hypothesized that the supply of chemical energy may be insufficient to fu... more Background: It has been hypothesized that the supply of chemical energy may be insufficient to fuel normal mechanical pump function in heart failure (HF). The creatine kinase (CK) reaction serves as the heart's primary energy reserve, and the supply of adenosine triphosphate (ATP flux) it provides is reduced in human HF. However, the relationship between the CK energy supply and the mechanical energy expended has never been quantified in the human heart. This study tests whether reduced CK energy supply is associated with reduced mechanical work in HF patients. Methods: Cardiac mechanical work and CK flux in W/kg, and mechanical efficiency were measured noninvasively at rest using cardiac pressure-volume loops, magnetic resonance imaging and phosphorus spectroscopy in 14 healthy subjects and 27 patients with mild-to-moderate HF. Results: In HF, the resting CK flux (126 ± 46 vs. 179 ± 50 W/kg, p < 0.002), the average (6.8 ± 3.1 vs. 10.1 ± 1. 5 W/kg, p <0.001) and the peak (32 ± 14 vs. 48 ± 8 W/kg, p < 0.001) cardiac mechanical work-rates, as well as the cardiac mechanical efficiency (53% ± 16 vs. 79% ± 3, p < 0.001), were all reduced by a third compared to healthy subjects. In addition, cardiac CK flux correlated with the resting peak and average mechanical power (p < 0.01), and with mechanical efficiency (p = 0.002). Conclusion: These first noninvasive findings showing that cardiac mechanical work and efficiency in mild-to-moderate human HF decrease proportionately with CK ATP energy supply, are consistent with the energy deprivation hypothesis of HF. CK energy supply exceeds mechanical work at rest but lies within a range that may be limiting with moderate activity, and thus presents a promising target for HF treatment.
Journal of cardiovascular magnetic resonance : official journal of the Society for Cardiovascular Magnetic Resonance, Jan 8, 2015
Phosphorus saturation transfer (ST) magnetic resonance spectroscopy can measure the rate of ATP g... more Phosphorus saturation transfer (ST) magnetic resonance spectroscopy can measure the rate of ATP generated from phosphocreatine (PCr) via creatine kinase (CK) in the human heart. Recently, the triple-repetition time ST (TRiST) method was introduced to measure the CK pseudo-first-order rate constant kf in three acquisitions. In TRiST, the longitudinal relaxation time of PCr while γ-ATP is saturated, T1`, is measured for each subject, but suffers from low SNR because the PCr signal is reduced due to exchange with saturated γ-ATP, and the short repetition time of one of the acquisitions. Here, a two-repetition time ST (TwiST) method is presented. In TwiST, the acquisition with γ-ATP saturation and short repetition time is dropped. Instead of measuring T1`, an intrinsic relaxation time T1 for PCr, T1 (intrinsic), is assumed. The objective was to validate TwiST measurements of CK kinetics in healthy subjects and patients with heart failure (HF). Bloch equation simulations that included th...
Medical physics, 2015
Radiofrequency (RF) radiometry uses thermal noise detected by an antenna to measure the temperatu... more Radiofrequency (RF) radiometry uses thermal noise detected by an antenna to measure the temperature of objects independent of medical imaging technologies such as magnetic resonance imaging (MRI). Here, an active interventional MRI antenna can be deployed as a RF radiometer to measure local heating, as a possible new method of monitoring device safety and thermal therapy. A 128 MHz radiometer receiver was fabricated to measure the RF noise voltage from an interventional 3 T MRI loopless antenna and calibrated for temperature in a uniformly heated bioanalogous gel phantom. Local heating (ΔT) was induced using the antenna for RF transmission and measured by RF radiometry, fiber-optic thermal sensors, and MRI thermometry. The spatial thermal sensitivity of the antenna radiometer was numerically computed using a method-of-moment electric field analyses. The gel's thermal conductivity was measured by MRI thermometry, and the localized time-dependent ΔT distribution computed from the ...
Proceedings of the International Society for Magnetic Resonance in Medicine ... Scientific Meeting and Exhibition. International Society for Magnetic Resonance in Medicine. Scientific Meeting and Exhibition, 2012
Proceedings of the International Society for Magnetic Resonance in Medicine ... Scientific Meeting and Exhibition. International Society for Magnetic Resonance in Medicine. Scientific Meeting and Exhibition, 2013
Correct determination of patient whole-body average SAR during MRI scans is essential for assessi... more Correct determination of patient whole-body average SAR during MRI scans is essential for assessing MR safety [1]. We have developed an RF dosimeter that can measure power deposition independent of the MR scanner. The dosimeter has an RF transducer with two tuned, orthogonal, lossy loops that mimics the loading by an average human subject, and a spherical phantom for the scanner to set the RF field, B1. However, currents induced on the transducer loops by the scanner can change the net B1, causing the scanner to incorrectly set the RF power output required to produce a prescribed flip angle. By a particular loop tuning, we make the final, resultant B1 the same as the original B1, thus ensuring that our RF transducer produces minimal B 1 disturbance. Power calibration curves obtained for eight 3T scanners including Philips, GE and Siemens models show that the dosimeter is independent of scanner or model operating at the same MR frequency. The results can be used to estimate whole-body SAR. METHODS The RF transducer comprises two orthogonal copper strip rectangular loops (70 cm × 50 cm) fixed on a polycarbonate frame (Fig 1a). The loops' loading and tuning were adjusted by a series of distributed capacitors and resistors. A lossless 10 cm diameter mineral oil ball was positioned at the transducer center to give an MR signal for the scanner's B1 calibration. Two 3 T RF transducers were constructed: one that operated at 127.8 MHz for Philips and GE scanners, and one at 123.8 MHz for Siemens. The effect of currents induced in the transducer loops and their resulting contribution to B1 was analyzed analytically. An algebraic solution was found for the reactance X in each loop that keeps the net B1 equal to the applied B1:
Proceedings of the International Society for Magnetic Resonance in Medicine ... Scientific Meeting and Exhibition. International Society for Magnetic Resonance in Medicine. Scientific Meeting and Exhibition, 2012
T1 and T2 are typically determined by separate partial saturation (PS) or inversion recovery and ... more T1 and T2 are typically determined by separate partial saturation (PS) or inversion recovery and spin-echo (SE) experiments. We propose a new method to measure both T1 and T2 in just three acquisitions, without using echoes or varying the repetition period TR. Instead, T2 is measured by varying the pulse length (τ) of an adiabatic B1-independent rotation (BIR-4) pulse in two of the acquisitions, based on the fact that long adiabatic excitation pulses are prone to T2 decay [1,2]. T1 is determined by varying the flip-angle in two acquisitions, analogous to the dual-angle method [3]. Thus, this 3-acquisition “Tri-τ” method employs an α hard pulse excitation, a β short-duration BIR-4 pulse, and a β long-duration BIR4 excitation. The method is validated with T1 and T2 SE and PS measurements on phantoms.
2007 IEEE International Symposium on Circuits and Systems, 2007
2012 Cairo International Biomedical Engineering Conference (CIBEC), 2012
ABSTRACT De-noising techniques can improve the signal to noise ratio (SNR), and quality of magnet... more ABSTRACT De-noising techniques can improve the signal to noise ratio (SNR), and quality of magnetic resonance (MR) images. In this work, we introduce a spectral subtraction de-noising (SSD) method that operates directly on the acquired raw MR signals and then we reconstruct images using the de-noised signals to improve the SNR. MR images acquired using coil arrays and reconstructed using parallel imaging techniques exhibit spatially varying noise distribution, which hampers the performance of image de-noising techniques applied in the image domain. The proposed SSD method is applied in the k-space (Fourier) domain of each of the individual coil array elements and is thus not affected by non-uniform noise distribution. Using numerical simulations and experimental data, we show that up to 45% improvements in SNR in both single and multi-channel coil data can be achieved.
NMR in Biomedicine, 2013
Practical noninvasive methods for measuring absolute metabolite concentrations are key to assessi... more Practical noninvasive methods for measuring absolute metabolite concentrations are key to assessing depletion of myocardial metabolite pools which occurs with several cardiac diseases including infarction and heart failure. Localized magnetic resonance spectroscopy (MRS) offers unique noninvasive access to many metabolites, but is often confounded by nonuniform sensitivity and partial volume effects in the large, poorly-defined voxels commonly used for detecting lowconcentration metabolites with surface coils. These problems are exacerbated at higher magnetic field-strengths by greater radio frequency (RF) field inhomogeneity and differences in RF penetration with heteronuclear concentration referencing. An example is the 31 P measurement of cardiac adenosine triphosphate (ATP) and phosphocreatine (PCr) concentrations, which although central to cardiac energetics, have not been measured at field-strengths above 1.5T. Here, practical acquisition and analysis protocols are presented for quantifying [PCr] and [ATP] with onedimensionally resolved surface coil spectra and concentration referencing at 3T. The effects of non-uniform sensitivity and partial tissue volumes are addressed at 3T by applying MRI-based three-dimensional sensitivity-weighting and tissue segmentation. The method is validated at 3T in phantoms of different sizes and concentrations, and used to measure [PCr] and [ATP] in healthy subjects. In calf-muscle (n=8), [PCr]=24.7±3.4 (mean±SD) and [ATP]=5.7±1.3 µmol/g wet wt, while [PCr]=10.4±1.5 and [ATP]=6.0±1.1 µmol/g in heart (n=18), consistent with previous reports at lower fields. The method enables for the first time, the efficient, semi-automated quantification of high energy phosphate metabolites in humans at 3T with non-uniform excitation and detection.
Medical Physics, 2013
The monitoring and management of radio frequency (RF) exposure is critical for ensuring magnetic ... more The monitoring and management of radio frequency (RF) exposure is critical for ensuring magnetic resonance imaging (MRI) safety. Commercial MRI scanners can overestimate specific absorption rates (SAR) and improperly restrict clinical MRI scans or the application of new MRI sequences, while underestimation of SAR can lead to tissue heating and thermal injury. Accurate scanner-independent RF dosimetry is essential for measuring actual exposure when SAR is critical for ensuring regulatory compliance and MRI safety, for establishing RF exposure while evaluating interventional leads and devices, and for routine MRI quality assessment by medical physicists. However, at present there are no scanner-independent SAR dosimeters. Methods: An SAR dosimeter with an RF transducer comprises two orthogonal, rectangular copper loops and a spherical MRI phantom. The transducer is placed in the magnet bore and calibrated to approximate the resistive loading of the scanner's whole-body birdcage RF coil for human subjects in Philips, GE and Siemens 3 tesla (3T) MRI scanners. The transducer loop reactances are adjusted to minimize interference with the transmit RF field (B 1) at the MRI frequency. Power from the RF transducer is sampled with a high dynamic range power monitor and recorded on a computer. The deposited power is calibrated and tested on eight different MRI scanners. Whole-body absorbed power vs weight and body mass index (BMI) is measured directly on 26 subjects. Results: A single linear calibration curve sufficed for RF dosimetry at 127.8 MHz on three different Philips and three GE 3T MRI scanners. An RF dosimeter operating at 123.2 MHz on two Siemens 3T scanners required a separate transducer and a slightly different calibration curve. Measurement accuracy was ∼3%. With the torso landmarked at the xiphoid, human adult whole-body absorbed power varied approximately linearly with patient weight and BMI. This indicates that whole-body torso SAR is on average independent of the imaging subject, albeit with fluctuations. Conclusions: Our 3T RF dosimeter and transducers accurately measure RF exposure in bodyequivalent loads and provide scanner-independent assessments of whole-body RF power deposition for establishing safety compliance useful for MRI sequence and device testing.
Medical Physics, 2008
Interventional, "loopless antenna" MRI detectors are currently limited to 1.5 T. This study inves... more Interventional, "loopless antenna" MRI detectors are currently limited to 1.5 T. This study investigates whether loopless antennae offer signal-to-noise ratio ͑SNR͒ and field-of-view ͑FOV͒ advantages at higher fields, and whether device heating can be controlled within safe limits. The absolute SNR performance of loopless antennae from 0.5 to 5 T is investigated both analytically, using electromagnetic ͑EM͒ dipole antenna theory, and numerically with the EM method of moments, and found to vary almost quadratically with field strength depending on the medium's electrical properties, the noise being dominated by direct sample conduction losses. The prediction is confirmed by measurements of the absolute SNR of low-loss loopless antennae fabricated for 1.5, 3, and 4.7 T, immersed in physiologically comparable saline. Gains of 3.8Ϯ 0.2-and 9.7Ϯ 0.3-fold in SNR, and approximately 10-and 50-fold gains in the useful FOV area are observed at 3 and 4.7 T, respectively, compared to 1.5 T. Heat testing of a 3 T biocompatible nitinol-antenna fabricated with a redesigned decoupling circuit shows maximum heating of ϳ1°C for MRI operating at high MRI exposure levels. Experiments in the rabbit aorta confirm the SNR and FOV advantages of the 3 T antenna versus an equivalent commercial 1.5 T device in vivo. This work is the first to study the performance of experimental internal MRI detectors above 1.5 T. The large SNR and FOV gains realized present a major opportunity for high-resolution imaging of vascular pathology and MRIguided intervention.
Medical Physics, 2012
Purpose: Accurate measurements of the RF power delivered during clinical MRI are essential for sa... more Purpose: Accurate measurements of the RF power delivered during clinical MRI are essential for safety and regulatory compliance, avoiding inappropriate restrictions on clinical MRI sequences, and for testing the MRI safety of peripheral and interventional devices at known RF exposure levels. The goal is to make independent RF power measurements to test the accuracy of scanner-reported specific absorption rate (SAR) over the extraordinary range of operating conditions routinely encountered in MRI. Methods: A six channel, high dynamic range, real-time power profiling system was designed and built for monitoring power delivery during MRI up to 440 MHz. The system was calibrated and used in two 3 T scanners to measure power applied to human subjects during MRI scans. The results were compared with the scanner-reported SAR. Results: The new power measurement system has highly linear performance over a 90 dB dynamic range and a wide range of MRI duty cycles. It has about 0.1 dB insertion loss that does not interfere with scanner operation. The measurements of whole-body SAR in volunteers showed that scanner-reported SAR was significantly overestimated by up to about 2.2 fold. Conclusions: The new power monitor system can accurately and independently measure RF power deposition over the wide range of conditions routinely encountered during MRI. Scanner-reported SAR values are not appropriate for setting exposure limits during device or pulse sequence testing.
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Papers by Abdel-Monem El-Sharkawy