Eddy currents analysis methods for an MRI longitudinal gradient coil.

PURPOSE: The rapid switching of the gradient fields induces eddy currents in neighboring metallic structures, causing undesirable effects. Numerical computations are thus required to understand eddy-currents effects for designing/implementing mitigation (involving passive shielding) and compensating techniques (using pre-emphasis). Previously, the network-analysis (NA) method was introduced to compute z-gradient eddy currents, although limited to a circularly symmetric and unconnected coil. Multi-layer integral method (MIM) method was recently introduced, modifying the circuit equation involving stream functions. We tailor MIM (TMIM) for a more general eddy-currents analysis in thin structures. Z-gradient eddy currents are analyzed and then compared using three methods (NA, TMIM, and Ansys). The analysis helps to evaluate the efficiency of passive shielding and to compensate eddy currents. METHODS: NA and TMIM computational frameworks for harmonic and transient eddy-currents analysis were implemented and cross-validated against Ansys Maxwell. A pre-emphasis pulse was modeled for compensating eddy currents. RESULTS: Eddy-currents analysis of an unconnected z-gradient coil in both the passive shield and cryostat were computed, and results were comparable to the least computationally efficient Ansys simulations. Although NA computations are fast, TMIM is implemented with reasonable efficiency and applied to circularly unsymmetric geometries. TMIM computations were further validated against Ansys using a connected z-gradient. Our computations allowed the effective evaluation of the performance of three various passive-shielding configurations, non-capped, capped, and slitted (for the first time), and an effective pre-emphasis compensation model was computed. CONCLUSION: Three eddy-currents analysis methods were studied and compared. Computationally efficient TMIM allows both harmonic and transient eddy-currents analysis involving different/complex gradient configurations/situations as well as involved shielding structures. Eddy-currents pre-emphasis compensation was demonstrated.

Cardiac work is related to creatine kinase energy supply in human heart failure: a cardiovascular magnetic resonance spectroscopy study.

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. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT00181259 .

Two repetition time saturation transfer (TwiST) with spill-over correction to measure creatine kinase reaction rates in human hearts.

BACKGROUND: 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). METHODS: Bloch equation simulations that included the effect of spillover irradiation on PCr were used to derive formulae for T1 (intrinsic) and kf measured by both TRiST and TwiST methods. Spillover was quantified from an unsaturated PCr measurement used in the current protocol for determining PCr and ATP concentrations. Cardiac TRiST and TwiST data were acquired at 3 T from 12 healthy and 17 HF patients. RESULTS: Simulations showed that both kf measured by TwiST and T1 (intrinsic) require spill-over corrections. In human heart at 3 T, the spill-over corrected T1 (intrinsic) = 8.4 ± 1.4 s (mean ± SD) independent of study group. TwiST and TRiST kf measurements were the same, but TwiST was 9 min faster. Spill-over corrected TwiST kf was 0.33 ± 0.08 s(-1) vs. 0.20 ± 0.06 s(-1) in healthy vs HF hearts, respectively (p < 0.0001). CONCLUSION: TwiST was validated against TRiST in the human heart at 3 T, generating the same results 9 min faster. TwiST detected significant reductions in CK kf in HF compared to healthy subjects, consistent with prior 1.5 T studies using different methodology.

7 Tesla MRI with a transmit/receive loopless antenna and B1-insensitive selective excitation.

PURPOSE: Use of external coils with internal detectors or conductors is challenging at 7 Tesla (T) due to radiofrequency (RF) field (B1 ) penetration, B1 -inhomogeneity, mutual coupling, and potential local RF heating. The present study tests whether the near-quadratic gains in signal-to-noise ratio and field-of-view with field-strength previously reported for internal loopless antennae at 7T can suffice to perform MRI with an interventional transmit/receive antenna without using any external coils. METHODS: External coils were replaced by semi-rigid or biocompatible transmit/receive loopless antennae requiring only a few Watts of peak RF power. Slice selection was provided by spatially selective B1 -insensitive composite RF pulses that compensate for the antenna's intrinsically nonuniform B1 -field. Power was adjusted to maintain local temperature rise ≤1°C. Fruit, intravascular MRI of diseased human vessels in vitro, and MRI of rabbit aorta in vivo are demonstrated. RESULTS: Scout MRI with the transmit/receive antennae yielded a ≤10 cm cylindrical field-of-view, enabling subsequent targeted localization at ∼100 µm resolution in 10-50 s and/or 50 µm MRI in ∼2 min in vitro, and 100-300 µm MRI of the rabbit aorta in vivo. CONCLUSION: A simple, low-power, one-device approach to interventional MRI at 7T offers the potential of truly high-resolution MRI, while avoiding issues with external coil excitation and interactions at 7T.

Quantification of human high-energy phosphate metabolite concentrations at 3 T with partial volume and sensitivity corrections.

Practical noninvasive methods for the measurement of absolute metabolite concentrations are key to the assessment of the depletion of myocardial metabolite pools which occurs with several cardiac diseases, including infarction and heart failure. Localized 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 the detection of low-concentration metabolites with surface coils. These problems are exacerbated at higher magnetic field strengths by greater radiofrequency (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 1.5 T. Here, practical acquisition and analysis protocols are presented for the quantification of [PCr] and [ATP] with one-dimensionally resolved surface coil spectra and concentration referencing at 3 T. The effects of nonuniform sensitivity and partial tissue volumes are addressed at 3 T by the application of MRI-based three-dimensional sensitivity weighting and tissue segmentation. The method is validated 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 and [ATP] = 5.7 ± 1.3 µmol/g wet weight, whereas, in heart (n = 18), [PCr] = 10.4 ± 1.5 and [ATP] = 6.0 ± 1.1 µmol/g wet weight (all mean ± SD), 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 3 T with nonuniform excitation and detection.

MRI pyschophysics: an experimental framework relating image quality to diagnostic performance metrics.

PURPOSE: To determine the minimal image quality needed to preserve diagnostic performance relative to arthroscopy in the knee. MATERIALS AND METHODS: Synthetic noise was added to images from clinical MRI scans (three-dimensional SPACE pulse sequence; Siemens) from five patients who had undergone knee MRI with arthroscopic follow-up, resulting in 25 simulated sets of images with standardized signal-to-noise ratios (SNRs) of 1, 2, 5, 10, or 20. All cases were scored by four musculoskeletal radiologists progressing from low to high SNR and grading all cartilage surfaces, major ligaments and menisci on a 5-point scale. Receiver operator characteristic (ROC) curves were constructed for the detection of meniscal tears and cartilage abnormalities. The area under the ROC curve (AUC) was determined for each structure at each SNR level. In addition, reader confidence was measured and pairwise comparisons across SNR levels were performed. Results were compared with arthroscopy as the reference standard. RESULTS: ROC AUC was maximized for meniscal tears at SNR = 5 (structure specific CNR = 3.2) and for cartilage abnormalities at SNR = 10 (CNR = 4.2). Observer confidence was maximized for menisci at SNR = 5 (CNR = 8.0), for ligaments at SNR = 10 (CNR = 13.6) and cartilage at SNR = 10 (CNR = 8.2). CONCLUSION: For 3D isotropic imaging in the knee, images with SNR < 10 or CNR < 10 should be rejected as nondiagnostic.

A power efficient actively shielded two-channel transverse MRI gradient coil numerical design.

Large and fast electrical current pulses are typically applied to conventional single-channel transverse MR gradient coils. However, these pulses result in a significant amount of power losses and heating of the coils. Previously, we presented a cylindrical multi-channel Z-gradient coil design that has better power efficiency compared to the single-channel design. In this work, we further investigate the DC power advantage for a two-channel actively-shielded transverse cylindrical gradient coil over the single-channel design. The conventional coil quadrants are radially divided into two sections, one for each channel, for both the primary and shielding surfaces. The symmetric inner sections of both the primary and shielding coils are assigned to the first channel, while the outer enclosing sections for each quadrant are assigned to the second channel. Discrete wire design is employed, where quasi-elliptic functions are used to parameterize the turns of each section. The coil geometric parameters, section size, number of turns, and turn locations are used as the design optimization parameters. The coils are optimized to maximize the coil's efficiency while keeping the linearity error less than 10% and the shielding ratio above 85%. The design procedure is employed to design both the single and two-channel transverse gradient coils for comparison. Eleven different two-channel configurations having different section sizes were investigated. Results show that the power used to drive the most power-efficient two-channel coil is less than that of the single-channel design by ∼25%. Moreover, the two-channel configuration showed slightly better shielding efficiency.

Transient eddy current analysis for a spiral gradient pulse.

Eddy currents are induced in the metallic structures of MRI machines due to the rapid switching of gradient fields generated by gradient coils. Several undesirable effects are associated with the induced eddy currents such as heat, acoustic noise, and MR image distortions. Accurate transient eddy currents numerical computations are required to predict and ameliorate such effects. Spiral gradient waveforms are of importance, particularly for fast MRI acquisition applications. For mathematical convenience, previously published work is mostly concerned with transient eddy currents computations associated with trapezoidal gradient waveforms; where spiral gradient waveforms were not considered. We recently presented preliminary transient eddy currents computations induced by an amplitude-modulated sinusoidal pulse in the scanner's cryostat. In this work, we present a full computational framework for transient eddy currents induced by a spiral gradient waveform. A mathematical model for transient eddy currents involving the spiral pulse was derived and presented in detail using the circuit equation. Computations were implemented using a tailored multilayer integral method (TMIM) and results were compared to Ansys eddy currents analysis for cross-validation. The transient response of resultant fields generated by both an unshielded transverse coil driven by a spiral waveform was computed showing high agreement between Ansys and TMIM; albeit with high computational efficiency concerning time and memory. For further validation, computations for a shielded transverse coil were performed showing how eddy currents effects are reduced.

Interventional loopless antenna at 7 T.

The loopless antenna magnetic resonance imaging detector is comprised of a tuned coaxial cable with an extended central conductor that can be fabricated at submillimeter diameters for interventional use in guidewires, catheters, or needles. Prior work up to 4.7 T suggests a near-quadratic gain in signal-to-noise ratio with field strength and safe operation at 3 T. Here, for the first time, the signal-to-noise ratio performance and radiofrequency safety of the loopless antenna are investigated both theoretically, using the electromagnetic method-of-moments, and experimentally in a standard 7 T human scanner. The results are compared with equivalent 3 T devices. An absolute signal-to-noise ratio gain of 5.7 ± 1.5-fold was realized at 7 T vs. 3 T: more than 20-fold higher than at 1.5 T. The effective field-of-view area also increased approximately 10-fold compared with 3 T. Testing in a saline gel phantom suggested that safe operation is possible with maximum local 1-g average specific absorption rates of <12 W kg(-1) and temperature increases of <1.9°C, normalized to a 4 W kg(-1) radiofrequency field exposure at 7 T. The antenna did not affect the power applied to the scanner's transmit coil. The signal-to-noise ratio gain enabled magnetic resonance imaging microscopy at 40-50 µm resolution in diseased human arterial specimens, offering the potential of high-resolution large-field-of-view or endoscopic magnetic resonance imaging for targeted intervention in focal disease.

MRI acoustic noise can harm experimental and companion animals.

PURPOSE: To assess possible damage to the hearing of experimental and companion animal subjects of magnetic resonance imaging (MRI) scans. MATERIALS AND METHODS: Using animal hearing threshold data and sound level measurements from typical MRI pulse sequences, we estimated "equivalent loudness" experienced by several experimental and companion animals commonly subjects of MRI scans. We compared the equivalent loudness and exam duration to safe noise standards set by the National Institute for Occupational Safety and Health (NIOSH). RESULTS: Monkeys, dogs, cats, pigs, and rabbits are frequently exposed to equivalent loudness levels during MRI scans beyond what is considered safe for human exposure. The sensitive frequency ranges for rats and mice are shifted substantially upward and their equivalent loudness levels fall within the NIOSH safe zone. CONCLUSION: MRI exposes many animals to levels of noise and duration that would exceed NIOSH human exposure limits. Researchers and veterinarians should use hearing protection for animals during MRI scans. Experimental research animals used in MRI studies are frequently kept and reimaged, and hearing loss could result in changed behavior. Damage to companion animals' hearing could make them less sensitive to commands and generally worsen interactions with family members. Much quieter MRI scanners would help decrease stress and potential harm to scanned animals, normalize physiology during MRI, and enable MRI of awake animals.

A multichannel, real-time MRI RF power monitor for independent SAR determination.

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.

High-energy phosphate transfer in human muscle: diffusion of phosphocreatine.

The creatine kinase (CK) reaction is central to muscle energetics, buffering ATP levels during periods of intense activity via consumption of phosphocreatine (PCr). PCr is believed to serve as a spatial shuttle of high-energy phosphate between sites of energy production in the mitochondria and sites of energy utilization in the myofibrils via diffusion. Knowledge of the diffusion coefficient of PCr (D(PCr)) is thus critical for modeling and understanding energy transport in the myocyte, but D(PCr) has not been measured in humans. Using localized phosphorus magnetic resonance spectroscopy, we measured D(PCr) in the calf muscle of 11 adults as a function of direction and diffusion time. The results show that the diffusion of PCr is anisotropic, with significantly higher diffusion along the muscle fibers, and that the diffusion of PCr is restricted to a ∼28-µm pathlength assuming a cylindrical model, with an unbounded diffusion coefficient of ∼0.69 × 10(-3) mm(2)/s. This distance is comparable in size to the myofiber radius. On the basis of prior measures of CK reaction kinetics in human muscle, the expected diffusion distance of PCr during its half-life in the CK reaction is ∼66 µm. This distance is much greater than the average distances between mitochondria and myofibrils. Thus these first measurements of PCr diffusion in human muscle in vivo support the view that PCr diffusion is not a factor limiting high-energy phosphate transport between the mitochondria and the myofibrils in healthy resting myocytes.

A Comparative Study for Evaluating Passive Shielding of MRI Longitudinal Gradient Coil.

Gradient coils are vital for Magnetic Resonance Imaging (MRI). Their rapid switching generates eddy currents in the surrounding metallic structures of the MRI scanner causing undesirable thermal, acoustic, and field distortion effects. The use of actively shielded gradient coils does not eliminate such undesirable effects totally. Use of passive shielding was proposed lately to particularly help in mitigating eddy currents and loud acoustic noise. Numerical computations are necessary for calculating eddy currents and evaluating the efficacy of passive shielding. Harmonic and temporal eddy current analysis caused by gradient coil(s) using network analysis (NA) can be faster and more flexible than the traditional FDTD and FEM methods. NA was used more than a decade ago but was limited to analyzing eddy currents resulting from z-gradient coils of separated turns. NA with stream function was recently modified resulting in the more general Multilayer Integral Method (MIM) for simulation of eddy currents in thin structures of arbitrary geometries. In this work, we compared the performance of the NA method and an adapted MIM method to analyze eddy current in both the passive shielding and cryostat to the Ansys Maxwell 3D analysis thus evaluating the performance of gradient configurations with and without passive shielding. Both an unconnected and a connected z-gradient coil configuration were used. Our analysis showed high agreement in the profiles of eddy ohmic losses in metallic structures using the three methods. The NA method is the most computationally efficient however, it is limited to specific symmetries unlike the more general MIM and Ansys methods. Our implementation of the adapted MIM method showed computational efficiency relative to Ansys with comparable values. We have developed a computationally efficient eddy current analysis framework that can be used to evaluate more designs for passive shielding using different configurations of MRI gradient coils.

Triple repetition time saturation transfer (TRiST) 31P spectroscopy for measuring human creatine kinase reaction kinetics.

Human cardiac phosphorus MR saturation transfer experiments to quantify creatine kinase forward rate constants (k(f)) have previously been performed at 1.5 T. Such experiments could benefit from increased signal-to-noise ratio (SNR) and spectral resolution at 3 T. At 1.5 T, the four-angle saturation transfer method was applied with low-angle adiabatic pulses and surface coils. However, low-angle adiabatic pulses are potentially problematic above 1.5 T due to bandwidth limitations, power requirements, power deposition, and intrapulse spin-spin relaxation. For localized metabolite spin-lattice relaxation time (T(1)) measurements, a dual repetition time approach with adiabatic half-passage pulses was recently introduced to solve these problems at 3 T. Because the saturation transfer experiment requires a T(1) measurement performed while one reacting moiety is saturated, we adapt the dual repetition time approach to measure k(f) using a triple repetition time saturation transfer (TRiST) method. A new pulsed saturation scheme with reduced sensitivity to static magnetic field inhomogeneity and compatibility with cardiac triggering is also presented. TRiST measurements of k(f) are validated in human calf muscle against conventional saturation transfer and found to agree within 3%. The first 3-T TRiST measurements of creatine kinase k(f) in the human calf (n = 6), chest muscle, and heart (n = 8) are 0.26 +/- 0.04 s(-1), 0.23 +/- 0.03 s(-1), and 0.32 +/- 0.07 s(-1), respectively, consistent with prior 1.5 T values.

Interventional MRI: tapering improves the distal sensitivity of the loopless antenna.

The "loopless antenna" is an interventional MRI detector consisting of a tuned coaxial cable and an extended inner conductor or "whip". A limitation is the poor sensitivity afforded at, and immediately proximal to, its distal end, which is exacerbated by the extended whip length when the whip is uniformly insulated. It is shown here that tapered insulation dramatically improves the distal sensitivity of the loopless antenna by pushing the current sensitivity toward the tip. The absolute signal-to-noise ratio is numerically computed by the electromagnetic method-of-moments for three resonant 3-T antennae with no insulation, uniform insulation, and with linearly tapered insulation. The analysis shows that tapered insulation provides an approximately 400% increase in signal-to-noise ratio in trans-axial planes 1 cm from the tip and a 16-fold increase in the sensitive area as compared to an equivalent, uniformly insulated antenna. These findings are directly confirmed by phantom experiments and by MRI of an aorta specimen. The results demonstrate that numerical electromagnetic signal-to-noise ratio analysis can accurately predict the loopless detector's signal-to-noise ratio and play a central role in optimizing its design. The manifold improvement in distal signal-to-noise ratio afforded by redistributing the insulation should improve the loopless antenna's utility for interventional MRI.

The performance of interventional loopless MRI antennae at higher magnetic field strengths.

Interventional, "loopless antenna" MRI detectors are currently limited to 1.5 T. This study investigates whether loopless antennae offer signal-to-noise ratio (SNR) and field-of-view (FOV) 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 (EM) 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 3.8 +/- 0.2- and 9.7 +/- 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 approximately 1 degrees 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 MRI-guided intervention.

Monitoring local heating around an interventional MRI antenna with RF radiometry.

PURPOSE: 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. METHODS: 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 bioheat transfer equation and compared with radiometry measurements. A "H-factor" relating the 1 g-averaged ΔT to the radiometric temperature was introduced to estimate peak temperature rise in the antenna's sensitive region. RESULTS: The loopless antenna radiometer linearly tracked temperature inside a thermally equilibrated phantom up to 73 °C to within ±0.3 °C at a 2 Hz sample rate. Computed and MRI thermometric measures of peak ΔT agreed within 13%. The peak 1 g-average temperature was H = 1.36 ± 0.02 times higher than the radiometric temperature for any media with a thermal conductivity of 0.15-0.50 (W/m)/K, indicating that the radiometer can measure peak 1 g-averaged ΔT in physiologically relevant tissue within ±0.4 °C. CONCLUSIONS: Active internal MRI detectors can serve as RF radiometers at the MRI frequency to provide accurate independent measures of local and peak temperature without the artifacts that can accompany MRI thermometry or the extra space needed to accommodate alternative thermal transducers. A RF radiometer could be integrated in a MRI scanner to permit "self-monitoring" for assuring device safety and/or monitoring delivery of thermal therapy.

Minimum acquisition methods for simultaneously imaging T(1), T(2), and proton density with B(1) correction and no spin-echoes.

The spin lattice (T(1)) and spin-spin (T(2)) relaxation times, along with the proton density (PD) contain almost all of the information that (1)H MRI routinely uses in clinical diagnosis and research, but are seldom imaged directly. Here, three methods for directly imaging T(1), T(2), and PD with the least possible number of acquisitions - three, are presented. All methods utilize long 0° self-refocusing adiabatic pre-pulses instead of spin-echoes to encode the T(2) information prior to a conventional gradient-echo MRI sequence. T(1) information is encoded by varying the flip-angle (FA) in the 'Dual-τ Dual-FA' and 'Four-FA' methods, or the sequence repetition period, TR, in the 'Dual-τ Dual-TR' method. Inhomogeneity in the FA distribution and slice-selection profile are recognized as the main error sources for T(1) measurements. The former is remedied by integrating an extra FA-dependent acquisition into the 'Four-FA' method to provide self-corrected T(1), T(2), PD, and FA in just four acquisitions - again, the minimum possible. Slice profile errors - which manifest as differences between 2D and 3D T(1) measurements, can be addressed by Bloch equation analysis and experimental calibration. All three methods are validated in phantom studies, and the 'Dual-τ Dual-FA' and 'Four-FA' methods are validated in human brain studies using standard partial saturation and spin-echo methods for reference. The new methods offer a minimum-acquisition option for imaging single-component T(1), T(2), and PD. 'Four-FA' performs best overall in accuracy, with high efficiency per unit accuracy vs. existing methods when B(1)-inhomogeneity is appropriately addressed.

Denoising MRI using spectral subtraction.

Improving the signal-to-noise-ratio (SNR) of magnetic resonance imaging (MRI) using denoising techniques could enhance their value, provided that signal statistics and image resolution are not compromised. Here, a new denoising method based on spectral subtraction of the measured noise power from each signal acquisition is presented. Spectral subtraction denoising (SSD) assumes no prior knowledge of the acquired signal and does not increase acquisition time. Whereas conventional denoising/filtering methods are compromised in parallel imaging by spatially dependent noise statistics, SSD is performed on signals acquired from each coil separately, prior to reconstruction. Using numerical simulations, we show that SSD can improve SNR by up to ~45% in MRI reconstructed from both single and array coils, without compromising image resolution. Application of SSD to phantom, human heart, and brain MRI achieved SNR improvements of ~40% compared to conventional reconstruction. Comparison of SSD with anisotropic diffusion filtering showed comparable SNR enhancement at low-SNR levels (SNR = 5-15) but improved accuracy and retention of structural detail at a reduced computational load.

An RF dosimeter for independent SAR measurement in MRI scanners.

PURPOSE: 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 (B1) 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 body-equivalent loads and provide scanner-independent assessments of whole-body RF power deposition for establishing safety compliance useful for MRI sequence and device testing.