Ultimate intrinsic signal-to-noise ratio in MRI

Optimizing the intrinsic signal-to-noise ratio of MRI strip detectors

An MRI detector is formed from a conducting strip separated by a dielectric substrate from a ground plane, and tuned to a quarter-wavelength. By distributing discrete tuning elements along the strip, the geometric design may be adjusted to optimize the signal-to-noise ratio (SNR) for a given application. Here a numerical electromagnetic (EM) method of moments (MoM) is applied to determine the length, width, substrate thickness, dielectric constant, and number of tuning elements that yield the best intrinsic SNR (ISNR) of the strip detector at 1.5 Tesla. The central question of how strip performance compares with that of a conventional optimized loop coil is also addressed. The numerical method is validated against the known ISNR performance of loop coils, and its ability to predict the tuning capacitances and performance of seven experimental strip detectors of varying length, width, substrate thickness, and dielectric constant. We find that strip detectors with low-dielectric constant, moderately thin-substrate, and length about 1.3 (+/-0.2) times the depth of interest perform best. The ISNR of strips is comparable to that of loops (i.e., higher close to the detector but lower at depth). The SNR improves with two inherently-decoupled strips, whose sensitivity profile is well-suited to parallel MRI. The findings are summarized as design "rules of thumb."

Ultrahigh-field MRI whole-slice and localized RF field excitations using the same RF transmit array

In this paper, a multiport driving mechanism is numerically implemented at ultra high-field (UHF) magnetic resonance imaging (MRI) to provide 1) homogenous whole-slice (axial, sagittal, or coronal) and 2) highly localized radio frequency (RF) field excitation within the same slices, all with the same RF transmit array (here chosen to be a standard transverse electromagnetic (TEM) resonator/coil). The method is numerically tested using a full-wave model of a TEM coil loaded with a high-resolution/18-tissue/anatomically detailed human head mesh. The proposed approach is solely based on electromagnetic and phased array antenna theories. The results demonstrate that both homogenous whole-slice as well as localized RF excitation can be achieved within any slice of the head at 7 T (298 MHz for proton imaging).

Potential and feasibility of parallel MRI at high field

This survey focuses on the fusion of two major lines of recent progress in MRI methodology: parallel imaging with receiver coil arrays and the transition to high and ultra-high field strength for human applications. As discussed in this paper, combining the two developments has vast potential due to multiple specific synergies. First, parallel acquisition and high field are highly complementary in terms of their individual advantages and downsides. As a consequence, the joint approach generally offers enhanced flexibility in the design of scanning strategies. Second, increasing resonance frequency changes the electrodynamics of the MR signal in such a way that parallel imaging becomes more effective in large objects. The underlying conceptual and theoretical considerations are reviewed in detail. In further sections, technical challenges and practical aspects are discussed. The feasibility of parallel MRI at ultra-high field is illustrated by current results of parallel human MRI at 7 T.

Automatic tuning of flexible interventional RF receiver coils

Microcontroller-based circuitry was built and tested for automatically tuning flexible RF receiver coils at the touch of a button. This circuitry is robust to 10% changes in probe center frequency, is in line with the scanner, and requires less than 1 s to tune a simple probe. Images were acquired using this circuitry with a varactor-tunable 1-inch flexible probe in a phantom and in an in vitro porcine knee model. The phantom experiments support the use of automatic tuning by demonstrating 30% signal-to-noise ratio (SNR) losses for 5% changes in coil center frequency, in agreement with theoretical calculations. Comparisons between patellofemoral cartilage images obtained using a 3-inch surface coil and the surgically-implanted 1-inch flexible coil reveal a worst-case local SNR advantage of a factor of 4 for the smaller coil. This work confirms that surgically implanted coils can greatly improve resolution in small-field-of-view (FOV) applications, and demonstrates the importance and feasibility of automatically tuning such probes.

Sensitivity enhancement and compensation of RF penetration artifact with planar actively detunable quadrature surface coil

An actively detunable planar quadrature surface coil for human body imaging at 4 T has been constructed and compared with a conventional linear surface coil. The coil could be used as a transmit/receive or a receive-only device in combination with a volume transmit coil. Transmission, reception profiles and the corresponding images acquired with each coil, as well as with both individual modes of the quadrature coil, are presented. Data collected using a tissue equivalent loaded phantom recorded with the linear surface coil demonstrated significant intensity distortions due to RF penetration artifact. The quadrature surface coil, on the other hand, provided compensation of the artifact, separately in its transmission and reception profiles as well as in the resultant images. Substantial sensitivity gain was also observed for the quadrature coil compared to the linear device. Significant advantages of using the quadrature surface coil over the linear device at 4 T have, therefore, been demonstrated.

Improved SNR of phased-array PERES coils via simulation study

A computational comparison of signal-to-noise ratio (SNR) was performed between a conventional phased array of two circular-shaped coils and a petal resonator surface array. The quasi-static model and phased-array optimum SNR were combined to derive an SNR formula for each array. Analysis of mutual inductance between coil petals was carried out to compute the optimal coil separation and optimum number of petal coils. Mutual interaction between coil arrays was not included in the model because this does not drastically affect coil performance. Phased arrays of PERES coils show a 114% improvement in SNR over that of the simplest circular configuration.

Optimal electric fields for different sample shapes in high resolution NMR spectroscopy

For many applications, reducing sample resistance, rather than increasing probe Q or filling factor, is the only way to further improve the signal-to-noise ratio of cryogenically cooled NMR probes. In this paper, bounds are calculated for the minimum sample resistance that can be achieved for various sample geometries. The sample resistance of 100 mM NaCl in H(2)O in 5 mm sample tubes was measured on a 600 MHz cold probe to be within 14% of the optimum value. The minimum sample resistance can however be lowered by altering the tube cross section. Rectangular tubes oriented with the long axis along the RF magnetic field are particularly favourable.

Transmit and receive transmission line arrays for 7 Tesla parallel imaging

Transceive array coils, capable of RF transmission and independent signal reception, were developed for parallel, 1H imaging applications in the human head at 7 T (300 MHz). The coils combine the advantages of high-frequency properties of transmission lines with classic MR coil design. Because of the short wavelength at the 1H frequency at 300 MHz, these coils were straightforward to build and decouple. The sensitivity profiles of individual coils were highly asymmetric, as expected at this high frequency; however, the summed images from all coils were relatively uniform over the whole brain. Data were obtained with four- and eight-channel transceive arrays built using a loop configuration and compared to arrays built from straight stripline transmission lines. With both the four- and the eight-channel arrays, parallel imaging with sensitivity encoding with high reduction numbers was feasible at 7 T in the human head. A one-dimensional reduction factor of 4 was robustly achieved with an average g value of 1.25 with the eight-channel transmit/receive coils.

In vitro validation of phase-contrast flow measurements at 3 T in comparison to 1.5 T: Precision, accuracy, and signal-to-noise ratios

PURPOSE

To evaluate the signal-to-noise ratio (SNR), precision, and accuracy of phase-contrast flow measurements at 3 T with the help of an in vitro model and to compare the results with data from two 1.5-T scanners.

MATERIALS AND METHODS

Using an identical setup of a laminar flow model and sequence parameters, measurements were done at one 3-T and at two 1.5-T systems. Precision, accuracy, and SNR were obtained for velocity encodings ranging from 55 up to 550 cm(-1). SNRs were calculated from the magnitude as well as the flow encoded images.

RESULTS

Precision and accuracy for the in vitro flow model were similarly high in all scanners with no significant difference. For velocity encodings from 55 cm(-1) up to 550 cm(-1), the SNR in magnitude as well as phase encoded images of the 3-T measurements was approximately 2.5 times higher than the SNR obtained from the two 1.5-T systems.

CONCLUSION

Even without optimization for the 3-T environment, flow measurements show the same high accuracy and precision as is known from clinical 1.5-T scanners. The superior SNR at 3 T will allow further improvements in temporal and spatial resolution. This will be of interest for small-size vessels like coronary arteries or for slow diastolic flow patterns.

Parallel imaging performance as a function of field strength--an experimental investigation using electrodynamic scaling

In this work, the dependence of parallel MRI performance on main magnetic field strength is experimentally investigated. Using the general framework of electrodynamic scaling, the B0-dependent behavior of the relevant radiofrequency fields at a single physical field strength of 7 T is studied. In the chosen implementation this is accomplished by adjusting the permittivity and conductivity of a homogeneous spherical phantom. With different mixing ratios of decane, ethanol, purified water, N-methylformamide, and sodium chloride, field strengths in the range of 1.5 to 11.5 T are mimicked. Based on sensitivity maps of an eight-coil receiver array, the field-dependent performance of parallel imaging is assessed in terms of the geometry factor g, which reflects noise enhancement in parallel imaging reconstruction. At low field strengths the SNR penalty was nearly independent of B0 and favorably low for 1D reduction factors up to between 3 and 4. At higher field strengths the transition between favorable and prohibitive parallel imaging conditions was found to shift toward higher feasible reduction factors. These findings are in good agreement with previous theoretical predictions. From this agreement it is concluded that parallel MRI at high B0 benefits specifically from onsetting far-field behavior of the involved radiofrequency fields.

Central brightening due to constructive interference with, without, and despite dielectric resonance

PURPOSE

To aid in discussion about the mechanism for central brightening in high field magnetic resonance imaging (MRI), especially regarding the appropriateness of using the term dielectric resonance to describe the central brightening seen in images of the human head.

MATERIALS AND METHODS

We present both numerical calculations and experimental images at 3 T of a 35-cm-diameter spherical phantom of varying salinity both with one surface coil and with two surface coils on opposite sides, and further numerical calculations at frequencies corresponding to dielectric resonances for the sphere.

RESULTS

With two strategically placed surface coils it is possible to create central brightening even when one coil alone excites an image intensity pattern either bright on one side only or bright on both sides with central darkening. This central brightening can be created with strategic coil placement even when the resonant pattern would favor central darkening. Results in a conductive sample show that central brightening can similarly be achieved in weakly conductive dielectric materials where any true resonances would be heavily damped, such as in human tissues.

CONCLUSION

Constructive interference and wavelength effects are likely bigger contributors to central brightening in MR images of weakly conductive biological samples than is true dielectric resonance.

Evaluation of internal MRI coils using ultimate intrinsic SNR

The upper bounds of the signal-to-noise ratio (also known as the "ultimate intrinsic signal-to-noise ratio" (UISNR)) for internal and external coils were calculated. In the calculation, the body was modeled as a dielectric cylinder with a small coaxial cylindrical cavity in which internal coils could be placed. The calculated UISNR values can be used as reference solutions to evaluate the performance of internal MRI coils. As examples, we evaluated the performance of a loopless antenna and an endourethral coil design by comparing their ISNR with the UISNR.

Parallel imaging at high field strength: synergies and joint potential

MRI faces fundamental limitations in terms of sensitivity and speed. These limitations can be effectively tackled by the transition to higher field strengths and parallel imaging technology. Owing to largely independent physics, the two approaches can be readily combined. Considering the specific advantages and disadvantages of high field strength and parallel imaging, it is found that the combination is particularly synergistic. In the joint approach, the two concepts play different roles. Higher field strength acts as a source of higher baseline signal-to-noise ratio (SNR), while parallelization acts as a means of converting added SNR into a variety of alternative benefits. This interplay holds promise for a broad range of clinical applications, as recently illustrated by several imaging studies at 3 T. As a consequence, clinical MRI at 3 T and higher is expected to rely more on parallel acquisition than at lower field strength. The specific synergy with parallel imaging may even make 3 T the field strength of choice for a range of exams that conventionally work best at 1.5 T or less.

Lumped-element planar strip array (LPSA) for parallel MRI

The recently introduced planar strip array (PSA) can significantly reduce scan times in parallel MRI by enabling the utilization of a large number of RF strip detectors that are inherently decoupled, and are tuned by adjusting the strip length to integer multiples of a quarter-wavelength (lambda/4) in the presence of a ground plane and dielectric substrate. In addition, the more explicit spatial information embedded in the phase of the signals from the strip array is advantageous (compared to loop arrays) for limiting aliasing artifacts in parallel MRI. However, losses in the detector as its natural resonance frequency approaches the Larmor frequency (where the wavelength is long at 1.5 T) may limit the signal-to-noise ratio (SNR) of the PSA. Moreover, the PSA's inherent lambda/4 structure severely limits our ability to adjust detector geometry to optimize the performance for a specific organ system, as is done with loop coils. In this study we replaced the dielectric substrate with discrete capacitors, which resulted in both SNR improvement and a tunable lumped-element PSA (LPSA) whose dimensions can be optimized within broad constraints, for a given region of interest (ROI) and MRI frequency. A detailed theoretical analysis of the LPSA is presented, including its equivalent circuit, electromagnetic fields, SNR, and g-factor maps for parallel MRI. Two different decoupling schemes for the LPSA are described. A four-element LPSA prototype was built to test the theory with quantitative measurements on images obtained with parallel and conventional acquisition schemes.

High-field proton MRS of human brain

Proton magnetic resonance spectroscopy (1H-MRS) of the brain reveals specific biochemical information about cerebral metabolites, which may support clinical diagnoses and enhance the understanding of neurological disorders. The advantages of performing 1H-MRS at higher field strengths include better signal to noise ratio (SNR) and increased spectral, spatial and temporal resolution, allowing the acquisition of high quality, easily quantifiable spectra in acceptable imaging times. In addition to improved measurement precision of N-acetylaspartate, choline, creatine and myo-inositol, high-field systems allow the high-resolution measurement of other metabolites, such as glutamate, glutamine, gamma-aminobutyric acid, scyllo-inositol, aspartate, taurine, N-acetylaspartylglutamate, glucose and branched amino acids, thus extending the range of metabolic information. However, these advantages may be hampered by intrinsic field-dependent technical difficulties, such as decreased T2 signal, chemical shift dispersion errors, J-modulation anomalies, increased magnetic susceptibility, eddy current artifacts, limitations in the design of homogeneous and sensitive radiofrequency (RF) coils, magnetic field instability and safety issues. Several studies demonstrated that these limitations could be overcome, suggesting that the appropriate optimization of high-field 1H-MRS would expand the application in the fields of clinical research and diagnostic routine.

Ultimate intrinsic signal-to-noise ratio for parallel MRI: electromagnetic field considerations

A method is described for establishing an upper bound on the spatial encoding capabilities of coil arrays in parallel MRI. Ultimate intrinsic signal-to-noise ratio (SNR), independent of any particular conductor arrangement, is calculated by expressing arbitrary coil sensitivities in terms of a complete set of basis functions that satisfy Maxwell's equations within the sample and performing parallel imaging reconstructions using these basis functions. The dependence of the ultimate intrinsic SNR on a variety of experimental conditions is explored and a physically intuitive explanation for the observed behavior is provided based on a comparison between the electromagnetic wavelength and the distance between aliasing points. Imaging at high field strength, with correspondingly short wavelength, is shown to offer advantages for parallel imaging beyond those already expected due to the larger available spin polarization. One-dimensional undersampling of k-space yields a steep drop in attainable SNR for more than a 5-fold reduction of scan time, while 2D undersampling permits access to much higher degrees of acceleration. Increased tissue conductivity decreases baseline SNR, but improves parallel imaging performance. A procedure is also provided for generating the optimal coil sensitivity pattern for a given acceleration, which will serve as a useful guide for future coil designs.

Electrodynamics and ultimate SNR in parallel MR imaging

The purpose of this article is to elucidate inherent limitations to the performance of parallel MRI. The study focuses on the ultimate signal-to-noise ratio (SNR), which refers to the maximum SNR permitted by the electrodynamics of the signal detection process. Using a spherical model object, it is shown that the behavior of the ultimate SNR imposes distinct limits on the acceleration rate in parallel imaging. For low and moderate acceleration, the ultimate SNR performance is nearly optimal, with geometry factors close to 1. However, for high reduction factors beyond a critical value, the ultimate performance deteriorates rapidly, corresponding to exponential growth of the geometry factor. The transition from optimal to deteriorating performance depends on the electrodynamic characteristics of the detected RF fields. In the near-field regime, i.e., for low B0 and small object size, the critical reduction factor is constant and approximately equal to four for 1D acceleration in the sphere. In the far-field wave regime the critical reduction factor is larger and increases both with B0 and object size. Therefore, it is concluded that parallel techniques hold particular promise for human MR imaging at very high field.

Intravascular extended sensitivity (IVES) MRI antennas

The design and application of an intravascular extended sensitivity (IVES) MRI antenna is described. The device is a loopless antenna design that incorporates both an insulating, dielectric coating and a winding of the antenna whip into a helical shape. Because this antenna produces a broad region of high SNR and also allows for imaging near the tip of the device, it is useful for imaging long, luminal structures. To elucidate the design and function of this device, the effects of both insulation and antenna winding were characterized by theoretical and experimental studies. Insulation broadens the longitudinal region over which images can be collected (i.e., along the lumen of a vessel) by increasing the resonant pole length. Antenna winding, conversely, allows for imaging closer to the tip of the antenna by decreasing the resonant pole length. Over a longitudinal region of 20 cm, the IVES imaging antenna described here produces a system SNR of approximately 40,000/r (mL(-1)Hz(1/2)), where r is the radial distance from the antenna axis in centimeters. As opposed to microcoil antenna designs, these antennas do not require exact positioning and allow for imaging over broad tissue regions. While focusing on the design of the IVES antenna, this work also serves to enhance our overall understanding of the properties and behavior of the loopless antenna design.

Phased-array MRI of canine prostate using endorectal and endourethral coils

A four-channel phased array consisting of one surface coil, two endorectal coils, and one flexible endourethral loop coil was designed for MRI of the canine prostate. The endorectal coils provide high signal in the posterior region of the prostate, while the endourethral and surface coils are sensitive to the central and anterior regions of the prostate. Gel phantom experiments indicate that the proposed phased-array configuration generates 15 times more signal-to-noise ratio (SNR) than a combination of two surface coils and one endorectal coil within the posterior region of the prostate; the performance of the two configurations is comparable near the anterior prostate surface. Ultimate intrinsic SNR (UISNR) analysis was used to compare the proposed phased array's performance to the best possible SNR for external coils. This analysis showed that the proposed phased array outperforms the best-case external coil within the posterior and central regions of the prostate by up to 20 times. In canine experiments in vivo, high-resolution fast spin-echo (FSE) images of the prostate were obtained with a pixel size of 230 microm obtained in 3 min 12 s. The proposed phased-array design potentially can be used to increase the accuracy of prostate cancer staging and the feasibility of MR-guided prostate interventions.

Intravascular MR imaging and intravascular MR-guided interventions

Intravascular MR technology, using an intravascularly placed MR receiver probe to acquire high-resolution angiographic MR images (i.e. intravascular MR imaging) and to guide cardiovascular interventional therapies (i.e. intravascular MR-guided interventions), is a new, very attractive development in the field of MR imaging. The new technology offers unique advantages for cardiovascular imaging and interventions, including superior contrast capability and multiplanar imaging capabilities without the use of contrast agents and with no risk of ionizing radiation. Thecombination of intravascular MR techniques with other advanced MR imaging techniques, such as functional MR imaging, will open new avenues for the future comprehensive management of cardiovascular atherosclerotic disease. Further improvements in intravascular MR fluoroscopy with true real-time display, analogous to X-ray fluoroscopy, will dramatically establish the role of intravascular MR technology in modern medicine.

Comments on "theoretical model for an MRI radio frequency resonator"

In contrast to a previous report [Baertlein et al. (2000)], the transverse electomagnetic resonator used in magnetic resonance imaging is shown to be similar to the high-pass "birdcage" resonator in having an electric field minimum in correspondence with the maximum of the magnetic field. The noise performance of each resonator will, in consequence, be comparable, since at high frequencies patient conductive losses are predominant.

Analysis of wave behavior in lossy dielectric samples at high field

Radiofrequency (RF) field wave behavior and associated nonuniform image intensity at high magnetic field strengths are examined experimentally and numerically. The RF field produced by a 10-cm-diameter surface coil at 300 MHz is evaluated in a 16-cm-diameter spherical phantom with variable salinity, and in the human head. Temporal progression of the RF field indicates that the standing wave and associated dielectric resonance occurring in a pure water phantom near 300 MHz is greatly dampened in the human head due to the strong decay of the electromagnetic wave. The characteristic image intensity distribution in the human head is the result of spatial phase distribution and amplitude modulation by the interference of the RF traveling waves determined by a given sample-coil configuration. The numerical calculation method is validated with experimental results. The general behavior of the RF field with respect to the average brain electrical properties in a frequency range of 42-350 MHz is also analyzed.

Sensitivity calculations and comparisons for shielded elliptical and circular birdcage coils

An analytical framework is presented which justifies the use of a confocal elliptical shield (previously adopted on empirical or numerical grounds) to limit the spatial extent of the radiofrequency (RF) field produced by an elliptical birdcage coil, while concurrently minimizing field inhomogeneities. From this analytical capability it is further shown that the relative linear mode geometric sensitivities depend only on the shield dimensions and can be readily evaluated, thus providing an upper bound to the improvement in signal-to-noise ratio (SNR) of the damped elliptical birdcage over a circular coil of similar dimensions. The mismatch in the linear mode sensitivities can also be evaluated, and a method for the design of an appropriate feed configuration that ensures quadrature operation is given.

Single-voxel proton MRS of the human brain at 1.5T and 3.0T

Single-voxel proton spectra of the human brain were recorded in five subjects at both 1.5T and 3.0T using the STEAM pulse sequence. Data acquisition parameters were closely matched between the two field strengths. Spectra were recorded in the white matter of the centrum semiovale and in phantoms. Spectra were compared in terms of resolution and signal-to-noise ratio (SNR), and transverse relaxation times (T(2)) were estimated at both field strengths. Spectra at 3T demonstrated a 20% improvement in sensitivity compared to 1.5T at short echo times (TE = 20 msec), which was lower than the theoretical 100% improvement. Spectra at long echo times (TE = 272 msec) exhibited similar SNR at both field strengths. T(2) relaxation times were almost twofold shorter at the higher field strength. Spectra in phantoms demonstrated significantly improved resolution at 3T compared to 1.5T, but resolution improvements in in vivo spectra were almost completely offset by increased linewidths at higher field.

Signal-to-noise ratio and absorbed power as functions of main magnetic field strength, and definition of "90 degrees " RF pulse for the head in the birdcage coil

Calculations of the RF magnetic (B(1)) field as a function of frequency between 64 and 345 MHz were performed for a head model in an idealized birdcage coil. Absorbed power (P(abs)) and SNR were calculated at each frequency with three different methods of defining excitation pulse amplitude: maintaining 90 degrees flip angle at the coil center (center alpha = pi/2), maximizing FID amplitude (Max. A(FID)), and maximizing total signal amplitude in a reconstructed image (Max. A(image)). For center alpha = pi/2 and Max. A(image), SNR increases linearly with increasing field strength until 260 MHz, where it begins to increase at a greater rate. For these two methods, P(abs) increases continually, but at a lower rate at higher field strengths. Above 215 MHz in MRI of the human head, the use of FID amplitude to set B(1) excitation pulses may result in apparent decreases in SNR and power requirements with increasing static field strength. Magn Reson Med 45:684-691, 2001.

The NMR reciprocity theorem for arbitrary probe geometry

It is shown that the NMR reciprocity theorem is a variant of a problem considered by Lorentz in 1895. This formulation is quite general and applies to electric-dipole-based as well as coil-based or resonator-based magnetic resonance probes. The reasoning is related to, but different from, the proof of the reciprocity theorem for radiofrequency networks and for transmit/receive antenna systems in telecommunications. The signal-to-noise ratio of the NMR experiment is also discussed in very general terms.

Computation of the signal-to-noise ratio of high-frequency magnetic resonance imagers

A method of computing the signal-to-noise ratio (SNR) in high-frequency magnetic resonance (MR) imaging systems is presented. The method uses a numerical solution to Maxwell's equations which can capture all relevant electrodynamic effects at high B0-field strengths. Using this method, the intrinsic SNR of both volume and surface coils loaded with the human head is calculated as a function of frequency. It is shown that although the available SNR from any point scales favorably with frequency, phase inhomogeneity over the volume of the head may pose a challenge to achieving improved SNR with traditional imaging techniques at high B0-field strengths.

Sensitivity and power deposition in a high-field imaging experiment

Image signal-to-noise ratio and power dissipation are investigated theoretically up to 400 MHz. While the text is mathematical, the figures give insights into predictions. Hertz potential is introduced for probe modeling where charge separation cannot be ignored. Using a spherical geometry, the potential from current loops that would produce a homogeneous static B1 field is calculated; at high frequency it is shown to create an unnecessarily inhomogeneous field. However, a totally homogeneous field is shown to be unattainable. Boundary conditions are solved for circularly polarized fields, and strategies for limited shimming of the sample B1 field are then presented. A distinction is drawn between dielectric resonance and spatial field focusing. At high frequency, the region of maximum specific absorption is shown to move inside the sample and decrease. From the fields in both rotating frames, the signal-to-noise ratio is derived and compared with the traditional, low-frequency formulation. On average, it is mostly found to be slightly larger at high frequency. Nevertheless, the free induction decay is sometimes found to be annulled.

An analytical SMASH procedure (ASP) for sensitivity-encoded MRI

The simultaneous acquisition of spatial harmonics (SMASH) method of imaging with detector arrays can reduce the number of phase-encoding steps, and MRI scan time several-fold. The original approach utilized numerical gradient-descent fitting with the coil sensitivity profiles to create a set of composite spatial harmonics to replace the phase-encoding steps. Here, an analytical approach for generating the harmonics is presented. A transform is derived to project the harmonics onto a set of sensitivity profiles. A sequence of Fourier, Hilbert, and inverse Fourier transform is then applied to analytically eliminate spatially dependent phase errors from the different coils while fully preserving the spatial-encoding. By combining the transform and phase correction, the original numerical image reconstruction method can be replaced by an analytical SMASH procedure (ASP). The approach also allows simulation of SMASH imaging, revealing a criterion for the ratio of the detector sensitivity profile width to the detector spacing that produces optimal harmonic generation. When detector geometry is suboptimal, a group of quasi-harmonics arises, which can be corrected and restored to pure harmonics. The simulation also reveals high-order harmonic modulation effects, and a demodulation procedure is presented that enables application of ASP to a large numbers of detectors. The method is demonstrated on a phantom and humans using a standard 4-channel phased-array MRI system.

A broadband phased-array system for direct phosphorus and sodium metabolic MRI on a clinical scanner

Despite their proven gains in signal-to-noise ratio and field-of-view for routine clinical MRI, phased-array detection systems are currently unavailable for nuclei other than protons (1H). A broadband phased-array system was designed and built to convert the 1H transmitter signal to the non-1H frequency for excitation and to convert non-1H phased-array MRI signals to the 1H frequency for presentation to the narrowband 1H receivers of a clinical whole-body 1.5 T MRI system. With this system, the scanner operates at the 1H frequency, whereas phased-array MRI occurs at the frequency of the other nucleus. Pulse sequences were developed for direct phased-array sodium (23Na) and phosphorus (31P) MRI of high-energy phosphates using chemical selective imaging, thereby avoiding the complex processing and reconstruction required for phased-array magnetic resonance spectroscopy data. Flexible 4-channel 31P and 23Na phased-arrays were built and the entire system tested in phantom and human studies. The array produced a signal-to-noise ratio improvement of 20% relative to the best-positioned single coil, but gains of 300-400% were realized in many voxels located outside the effective field-of-view of the single coil. Cardiac phosphorus and sodium MRI were obtained in 6-13 min with 16 and 0.5 mL resolution, respectively. Lower resolution human cardiac 23Na MRI were obtained in as little as 4 sec. The system provides a practical approach to realizing the advantages of phased-arrays for nuclei other than 1H, and imaging metabolites directly.

Error-tolerant RF litz coils for NMR/MRI

A new class of NMR RF volume coils is being developed that permits improved tuning range, B(1) homogeneity, tuning stability, and sensitivity compared to birdcages over a wide range of practical conditions, especially for microscopy and wraparound flexible applications. They are denoted litz coils, as their flux transparency and current distribution is obtained from woven foil patterns with insulated crossovers. Contrary to the design criteria of phased arrays, the parallel routes in litz coils use high coupling coefficients to achieve optimal current distribution, which is highly independent of tuning, balancing, and matching adjustments and is compatible with multiple capacitive segmentation. Magnetic filling factors, loaded Q, and inhomogeneity measurements and calculations are presented for a variety of litz coils with frequency-diameter products from 7 to 20 MHz-m and are compared to similar birdcages.

Practical aspects of birdcage coils

Numerical modeling and experimental results are presented for a variety of birdcages for high-field MRI microscopy. The data include the first published numerical calculations and experimental measurements of magnetic filling factors of birdcages or other MRI coils. Fast, 3D calculations for shielded coils are demonstrated using the Biot-Savart law along with energy minimization. It is shown that the near-field, higher-order inhomogeneity effects remaining after a first-order, asymmetry correction may easily exceed 20% in situations where it is desirable to maximize filling factor. It is also found that an order-of-magnitude improvement in the accuracy of predicted capacitor values may be achieved by using a more detailed circuit model of the birdcage resonator.