Removal of local field gradient artifacts in T2*-weighted images at high fields by gradient-echo slice excitation profile imaging

Development of high magnetic field MRI techniques is hampered by the significant artifacts produced by B₀ field inhomogeneities in the excited slices. A technique, gradient-echo slice excitation profile imaging (GESEPI), is presented for recovering the signal lost caused by intravoxel phase dispersion in T₂*-weighted images. This technique superimposes an incremental gradient offset on the slice refocusing gradient to sample Jr-space over the full range of spatial frequencies of the excitation profile. A third Fourier transform of the initial two-dimensional image set generates an image set in which the artifacts produced by the low-order B₀ inhomogeneity field gradients in the sample are separated and removed from the high-order microscopic field gradients responsible for T₂* contrast. Application to high field brain imaging, at 3.0 T for human and at 9.4 T for immature rat imaging demonstrates the significant improvement in quality of the T₂*-weighted contrast images.     

Polarization of the RF field in a human head at high field: a study with a quadrature surface coil at 7.0 T.

The RF field intensity distribution in the human brain becomes inhomogeneous due to wave behavior at high field. This is further complicated by the spatial distribution of RF field polarization that must be considered to predict image intensity distribution. An additional layer of complexity is involved when a quadrature coil is used for transmission and reception. To study such complicated RF field behavior, a computer modeling method was employed to investigate the RF field of a quadrature surface coil at 300 MHz. Theoretical and experimental results for a phantom and the human head at 7.0 T are presented. The results are theoretically important and practically useful for high-field quadrature coil design and application.     

B1 field, SAR, and SNR comparisons for birdcage, TEM, and microstrip coils at 7T

PURPOSE: To compare the performance of birdcage, transverse electromagnetic (TEM) and microstrip volume coils at 7T under the same geometric conditions. MATERIALS AND METHODS: Birdcage, TEM, and microstrip coils are modeled with the same dimensions. The finite difference time domain (FDTD) method is adopted to calculate the electromagnetic fields of the coils. Further, B(1) field, specific absorption rate (SAR) and signal-to-noise ratio (SNR) are calculated for these coils. RESULTS: In the unloaded case, within the central axial plane, the variation of B(1) field magnitude over 18-cm distance is about 15% for the birdcage coil, 23% for the TEM coil, and 38% for the microstrip coil. In the loaded case, the percentages of the samples on the central axial plane, which have B(1) field magnitude within +/-20% of the average B(1) field magnitude, are about 57% for the birdcage, 72% for the TEM, and 59% for the microstrip coil. Average SAR levels are 11.4% and 42.9% higher in the birdcage than those in the TEM and microstrip coils, respectively. The average relative SNR on the central axial plane for the shielded birdcage, TEM, and microstrip coils are 1, 1.07, and 1.48, respectively. CONCLUSION: The birdcage coil has the best unloaded B(1) field homogeneity, and the TEM coil has the best loaded B(1) field homogeneity and the lowest radiation loss; while the microstrip coil is better in SAR and SNR at 7T than the birdcage and TEM coils.     

Implementation of vascular‐space‐occupancy MRI at 7T

Vascular‐space‐occupancy (VASO) MRI exploits the difference between blood and tissue T₁ to null blood signal and measure cerebral blood volume changes using the residual tissue signal. VASO imaging is more difficult at higher field because of sensitivity loss due to the convergence of tissue and blood T₁ values and increased contamination from blood‐oxygenation‐level‐dependent (BOLD) effects. In addition, compared to 3T, 7T MRI suffers from increased geometrical distortions, e.g., when using echo‐planar‐imaging, and from increased power deposition, the latter especially problematic for the spin‐echo‐train sequences commonly used for VASO MRI. Third, non‐steady‐state blood spin effects become substantial at 7T when only a head coil is available for radiofrequency transmit. In this study, the magnetization‐transfer‐enhanced‐VASO approach was applied to maximize tissue‐blood signal difference, which boosted signal‐to‐noise ratio by 149% ± 13% (n = 7) compared to VASO. Second, a 3D fast gradient‐echo sequence with low flip‐angle (7°) and short echo‐time (1.8 ms) was used to minimize the BOLD effect and to reduce image distortion and power deposition. Finally, a magnetization‐reset technique was combined with a motion‐sensitized‐driven‐equilibrium approach to suppress three types of non‐steady‐state spins. Our initial functional MRI results in normal human brains at 7T with this optimized VASO sequence showed better signal‐to‐noise ratio than at 3T. Magn Reson Med 69:1003–1013, 2013. © 2012 Wiley Periodicals, Inc.     

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.     

7T vs. 4T: RF power, homogeneity, and signal-to-noise comparison in head images.

Signal-to-noise ratio (SNR), RF field (B(1)), and RF power requirement for human head imaging were examined at 7T and 4T magnetic field strengths. The variation in B(1) magnitude was nearly twofold higher at 7T than at 4T ( approximately 42% compared to approximately 23%). The power required for a 90 degrees pulse in the center of the head at 7T was approximately twice that at 4T. The SNR averaged over the brain was at least 1.6 times higher at 7T compared to 4T. These experimental results were consistent with calculations performed using a human head model and Maxwell's equations. Magn Reson Med 46:24-30, 2001.     

Cardiac and respiratory double self-gated cine MRI in the mouse at 7 T.

ECG-gated cardiac MRI in the mouse is hindered by many technical difficulties in ECG signal recording inside static and variable high magnetic scanner fields. The present study proposes an alternative robust method of acquiring auto-gated cardiac and respiratory cine images in mouse heart. In our approach, a motion synchronization signal is extracted from the echo peak MR signal of a non-triggered radial acquisition. This signal is then used for both cardiac and respiratory retrospective gating before cine image reconstruction. Highly asymmetric echoes were acquired to achieve the radial k-space sampling in order to avoid radial acquisition related artifacts and to increase auto-gating robustness. In vivo experiments demonstrated the feasibility and robustness of self-gated cine-MRI in the mouse heart at 7T. The signal-to-noise and contrast-to-noise ratios of the self-gated and ECG-gated images were comparable, all parameters being equal. Magn Reson Med, 2006. (c) 2006 Wiley-Liss, Inc.     

Intracellular bimodal nanoparticles based on quantum dots for high‐field MRI at 21.1 T

Multimodal, biocompatible contrast agents for high magnetic field applications represent a new class of nanomaterials with significant potential for tracking of fluorescence and MR in vitro and vivo. Optimized for high‐field MR applications—including biomedical imaging at 21.1 T, the highest magnetic field available for MRI—these nanoparticles capitalize on the improved performance of chelated Dy³⁺ with increasing magnetic field coupled to a noncytotoxic Indium Phosphide/Zinc Sulfide (InP/ZnS) quantum dot that provides fluorescence detection, MR responsiveness, and payload delivery. By surface modifying the quantum dot with a cell‐penetrating peptide sequence coupled to an MR contrast agent, the bimodal nanomaterial functions as a self‐transfecting high‐field MR/optical contrast agent for nonspecific intracellular labeling. Fluorescent images confirm sequestration in perinuclear vesicles of labeled cells, with no apparent cytotoxicity. These techniques can be extended to impart cell selectivity or act as a delivery vehicle for genetic or pharmaceutical interventions. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

3T MRI in paediatrics: challenges and clinical applications

3T MRI is being increasingly performed for clinical purposes in paediatrics, primarily because of the potential to improve spatial and temporal resolution - these can assist in overcoming the unique anatomic, physiologic and behavioural challenges of imaging children. The increased spatial resolution improves the capacity to image small patients; with particular reference to smaller structures such as the inner ear, brachial plexus, biliary system and the vascular system. The challenges inherent to imaging at high field strength remain pertinent especially, the altered T1 contrast, artefacts (susceptibility, chemical shift and B1 inhomogeneity) and safety issues, including specific absorption rate - several of these are circumvented due to software and hardware advances, or by trade off of some of the increased signal. The above mentioned challenges also create opportunities at 3T, with improvement in MR angiography, arterial spin labelling, functional MRI, susceptibility weighted imaging, and MR spectroscopy - all of which have distinctive applications in paediatrics. Whole body imaging also becomes more practical because of the capacity for faster scans. 3T MRI has the potential to image all the systems in paediatrics. However, neonatal brain and paediatric spine imaging have specific challenges at 3T. Several factors also limit cardiac imaging at present. Further improvements in coil technology and newer sequences may help overcome the challenges that remain.     

A measurement setup for direct 17O MRI at 7 T

An efficient breathing system was designed for direct ¹⁷O MRI to perform oxygen metabolism studies of the human brain. The breathing system consists of a demand oxygen delivery device for ¹⁷O₂ supply and a custom‐built re‐breathing circuit with pneumatic switching valve. To efficiently deliver the ¹⁷O gas to the alveoli of the lungs, the system applies short gas pulses upon an inspiration trigger via a nasal cannula. During and after ¹⁷O₂ administration, the exhaled gas volumes are stored and filtered in the re‐breathing section to make the most efficient use of the rare ¹⁷O gas. In an inhalation experiment, 2.2 ± 0.1 L of 70%‐enriched ¹⁷O₂ were administered to a healthy volunteer and direct ¹⁷O MRI was performed for a total imaging time of 38 min with a temporal resolution of 50 s per 3D data set. Mapping of the maximum signal increase was carried out showing regional variations of oxygen concentration of up to 30% over the natural abundance of ¹⁷O water. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

In vivo sodium imaging of human patellar cartilage with a 3D cones sequence at 3 T and 7 T

Purpose:

To compare signal‐to‐noise ratios (SNRs) and T*₂ maps at 3 T and 7 T using 3D cones from in vivo sodium images of the human knee.

Materials and Methods:

Sodium concentration has been shown to correlate with glycosaminoglycan content of cartilage and is a possible biomarker of osteoarthritis. Using a 3D cones trajectory, 17 subjects were scanned at 3 T and 12 at 7 T using custom‐made sodium‐only and dual‐tuned sodium/proton surface coils, at a standard resolution (1.3 × 1.3 × 4.0 mm³) and a high resolution (1.0 × 1.0 × 2.0 mm³). We measured the SNR of the images and the T*₂ of cartilage at both 3 T and 7 T.

Results:

The average normalized SNR values of standard‐resolution images were 27.1 and 11.3 at 7 T and 3 T. At high resolution, these average SNR values were 16.5 and 7.3. Image quality was sufficient to show spatial variations of sodium content. The average T*₂ of cartilage was measured as 13.2 ± 1.5 msec at 7 T and 15.5 ± 1.3 msec at 3 T.

Conclusion:

We acquired sodium images of patellar cartilage at 3 T and 7 T in under 26 minutes using 3D cones with high resolution and acceptable SNR. The SNR improvement at 7 T over 3 T was within the expected range based on the increase in field strength. The measured T*₂ values were also consistent with previously published values. J. Magn. Reson. Imaging 2010;32:446–451. © 2010 Wiley‐Liss, Inc.     

Design of a radiative surface coil array element at 7 T: The single‐side adapted dipole antenna

Ultra high field MR imaging (≥7 T) of deeply located targets in the body is facing some radiofrequency‐field related challenges: interference patterns, reduced penetration depth, and higher Specific Absorbtion Ratio (SAR) levels. These can be alleviated by redesigning the elements of the transmit or transceive array. This is because at these high excitation field (B₁) frequencies, conventional array element designs may have become suboptimal. In this work, an alternative design approach is presented, regarding coil array elements as antennas. Following this approach, the Poynting vector of the element should be oriented towards the imaging target region. The single‐side adapted dipole antenna is a novel design that fulfills this requirement. The performance of this design as a transmit coil array element has been characterized by comparison with three other, more conventional designs using finite difference time domain (FDTD) simulations and B measurements on a phantom. Results show that the B level at the deeper regions is higher while maintaining relatively low SAR levels. Also, the B field distribution is more symmetrical and more uniform, promising better image homogeneity. Eight radiative antennas have been combined into a belt‐like surface array for prostate imaging. T₁‐weighted (T1W) and T₂‐weighted (T2W) volunteer images are presented along with B measurements to demonstrate the improved efficiency. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

High-resolution fast spin echo imaging of the human brain at 4.7 T: implementation and sequence characteristics.

In this work, a number of important issues associated with fast spin echo (FSE) imaging of the human brain at 4.7 T are addressed. It is shown that FSE enables the acquisition of images with high resolution and good tissue contrast throughout the brain at high field strength. By employing an echo spacing (ES) of 22 ms, one can use large flip angle refocusing pulses (162 degrees ) and a low acquisition bandwidth (50 kHz) to maximize the signal-to-noise ratio (SNR). A new method of phase encode (PE) ordering (called "feathering") designed to reduce image artifacts is described, and the contributions of RF (B(1)) inhomogeneity, different echo coherence pathways, and magnetization transfer (MT) to FSE signal intensity and contrast are investigated. B(1) inhomogeneity is measured and its effect is shown to be relatively minor for high-field FSE, due to the self-compensating characteristics of the sequence. Thirty-four slice data sets (slice thickness = 2 mm; in-plane resolution = 0.469 mm; acquisition time = 11 min 20 s) from normal volunteers are presented, which allow visualization of brain anatomy in fine detail. This study demonstrates that high-field FSE produces images of the human brain with high spatial resolution, SNR, and tissue contrast, within currently prescribed power deposition guidelines.     

Imaging brain function in humans at 7 Tesla.

This article describes experimental studies performed to demonstrate the feasibility of BOLD fMRI using echo-planar imaging (EPI) at 7 T and to characterize the BOLD response in humans at this ultrahigh magnetic field. Visual stimulation studies were performed in normal subjects using high-resolution multishot EPI sequences. Changes in R(*)(2) arising from visual stimulation were experimentally determined using fMRI measurements obtained at multiple echo times. The results obtained at 7 T were compared to those at 4 T. Experimental data indicate that fMRI can be reliably performed at 7 T and that at this field strength both the sensitivity and spatial specificity of the BOLD response are increased. This study suggests that ultrahigh field MR systems are advantageous for functional mapping in humans. Magn Reson Med 45:588-594, 2001.     

7T head volume coils: Improvements for rostral brain imaging

Purpose

To improve the performance of 7T head coils over the rostral head regions. Due to radiofrequency (RF) field/tissue interactions, the RF magnetic field profile produced by 7T volume head coils is very inhomogeneous, with enhanced sensitivity near the center of the human brain and substantially reduced in the periphery.

Materials and Methods

Two head‐sized quadrature volume coils of similar diameters but substantially different lengths (17 and 10 cm) were constructed and tested using a 7T Varian Inova system.

Results

Experimental data demonstrated that by using a shorter volume head‐sized coil or simply by partially moving a head out of the coil, coil efficiency near the top of a head can be improved by 20%. The homogeneity also improved, largely resulting from an increase in peripheral B₁ values. This resulted in 10%–20% variation in axial slices located near the top of a head.

Conclusion

We have demonstrated a less deeply positioned head or substantially shorter volume coil can significantly improve coil performance and homogeneity for the rostral head at ultrahigh magnetic fields (7T and above). For studies that target superior brain regions, this coil arrangement can be highly effective. J. Magn. Reson. Imaging 2009;29:461–465. © 2009 Wiley‐Liss, Inc.