Stretchable self-tuning MRI receive coils based on liquid metal technology (LiquiTune)

Magnetic resonance imaging systems rely on signal detection via radiofrequency coil arrays which, ideally, need to provide both bendability and form-fitting stretchability to conform to the imaging volume. However, most commercial coils are rigid and of fixed size with a substantial mean offset distance of the coil from the anatomy, which compromises the spatial resolution and diagnostic image quality as well as patient comfort. Here, we propose a soft and stretchable receive coil concept based on liquid metal and ultra-stretchable polymer that conforms closely to a desired anatomy. Moreover, its smart geometry provides a self-tuning mechanism to maintain a stable resonance frequency over a wide range of elongation levels. Theoretical analysis and numerical simulations were experimentally confirmed and demonstrated that the proposed coil withstood the unwanted frequency detuning typically observed with other stretchable coils (0.4% for the proposed coil as compared to 4% for a comparable control coil). Moreover, the signal-to-noise ratio of the proposed coil increased by more than 60% as compared to a typical, rigid, commercial coil.

Evaluation of a Flexible 12-Channel Screen-printed Pediatric MRI Coil

Background Screen-printed MRI coil technology may reduce the need for bulky and heavy housing of coil electronics and may provide a better fit to patient anatomy to improve coil performance. Purpose To assess the performance and caregiver and clinician acceptance of a pediatric-sized screen-printed flexible MRI coil array as compared with conventional coil technology. Materials and Methods A pediatric-sized 12-channel coil array was designed by using a screen-printing process. Element coupling and phantom signal-to-noise ratio (SNR) were assessed. Subjects were scanned by using the pediatric printed array between September and November 2017; results were compared with three age- and sex-matched historical control subjects by using a commercial 32-channel cardiac array at 3 T. Caregiver acceptance was assessed by asking nurses, technologists, anesthesiologists, and subjects or parents to rate their coil preference. Diagnostic quality of the images was evaluated by using a Likert scale (5 = high image quality, 1 = nondiagnostic). Image SNR was evaluated and compared. Results Twenty study participants were evaluated with the screen-printed coil (age range, 2 days to 12 years; 11 male and nine female subjects). Loaded pediatric phantom testing yielded similar noise covariance matrices and only slightly degraded SNR for the printed coil as compared with the commercial coil. The caregiver acceptance survey yielded a mean score of 4.1 ± 0.6 (scale: 1, preferred the commercial coil; 5, preferred the printed coil). Diagnostic quality score was 4.5 ± 0.6. Mean image SNR was 54 ± 49 (paraspinal muscle), 78 ± 51 (abdominal wall muscle), and 59 ± 35 (psoas) for the printed coil, as compared with 64 ± 55, 65 ± 48, and 57 ± 43, respectively, for the commercial coil; these SNR differences were not statistically significant (P = .26). Conclusion A flexible screen-printed pediatric MRI receive coil yields adequate signal-to-noise ratio in phantoms and pediatric study participants, with similar image quality but higher preference by subjects and their caregivers when compared with a conventional MRI coil. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Lamb in this issue.

Depletion-Mode GaN HEMT Q-Spoil Switches for MRI Coils

Q-spoiling is the process of decoupling an MRI receive coil to protect the equipment and patient. Conventionally, Q-spoiling is performed using a PIN diode switch that draws significant current. In this work, a Q-spoiling technique using a depletion-mode Gallium Nitride HEMT device was developed for coil detuning at both 1.5 T and 3 T MRI. The circuits with conventional PIN diode Q-spoiling and the GaN HEMT device were implemented on surface coils. SNR was measured and compared for all surfaces coils. At both 1.5 T and 3 T, comparable SNR was achieved for all coils with the proposed technique and conventional Q-spoiling. The GaN HEMT device has significantly reduced the required power for Q-spoiling. The GaN HEMT device also provides useful safety features by detuning the coil when unpowered.

A semiflexible 64-channel receive-only phased array for pediatric body MRI at 3T

PURPOSE

To design, construct, and validate a semiflexible 64-channel receive-only phased array for pediatric body MRI at 3T.

METHODS

A 64-channel receive-only phased array was developed and constructed. The designed flexible coil can easily conform to different patient sizes with nonoverlapping coil elements in the transverse plane. It can cover a field of view of up to 44 × 28 cm(2) and removes the need for coil repositioning for body MRI patients with multiple clinical concerns. The 64-channel coil was compared with a 32-channel standard coil for signal-to-noise ratio and parallel imaging performances on different phantoms. With IRB approval and informed consent/assent, the designed coil was validated on 21 consecutive pediatric patients.

RESULTS

The pediatric coil provided higher signal-to-noise ratio than the standard coil on different phantoms, with the averaged signal-to-noise ratio gain at least 23% over a depth of 7 cm along the cross-section of phantoms. It also achieved better parallel imaging performance under moderate acceleration factors. Good image quality (average score 4.6 out of 5) was achieved using the developed pediatric coil in the clinical studies.

CONCLUSION

A 64-channel semiflexible receive-only phased array has been developed and validated to facilitate high quality pediatric body MRI at 3T. Magn Reson Med 76:1015-1021, 2016. © 2015 Wiley Periodicals, Inc.

Clinical performance of contrast enhanced abdominal pediatric MRI with fast combined parallel imaging compressed sensing reconstruction

PURPOSE

To deploy clinically, a combined parallel imaging compressed sensing method with coil compression that achieves a rapid image reconstruction, and assess its clinical performance in contrast-enhanced abdominal pediatric MRI.

MATERIALS AND METHODS

With Institutional Review Board approval and informed patient consent/assent, 29 consecutive pediatric patients were recruited. Dynamic contrast-enhanced MRI was acquired on a 3 Tesla scanner using a dedicated 32-channel pediatric coil and a three-dimensional SPGR sequence, with pseudo-random undersampling at a high acceleration (R = 7.2). Undersampled data were reconstructed with three methods: a traditional parallel imaging method and a combined parallel imaging compressed sensing method with and without coil compression. The three sets of images were evaluated independently and blindly by two radiologists at one siting, for overall image quality and delineation of anatomical structures. Wilcoxon tests were performed to test the hypothesis that there was no significant difference in the evaluations, and interobserver agreement was analyzed.

RESULTS

Fast reconstruction with coil compression did not deteriorate image quality. The mean score of structural delineation of the fast reconstruction was 4.1 on a 5-point scale, significantly better (P < 0.05) than traditional parallel imaging (mean score 3.1). Fair to substantial interobserver agreement was reached in structural delineation assessment.

CONCLUSION

A fast combined parallel imaging compressed sensing method is feasible in a pediatric clinical setting. Preliminary results suggest it may improve structural delineation over parallel imaging.

Measuring mitochondrial metabolism in rat brain in vivo using MR Spectroscopy of hyperpolarized [2-¹³C]pyruvate

Hyperpolarized [1-(13) C]pyruvate ([1-(13) C]Pyr) has been used to assess metabolism in healthy and diseased states, focusing on the downstream labeling of lactate (Lac), bicarbonate and alanine. Although hyperpolarized [2-(13) C]Pyr, which retains the labeled carbon when Pyr is converted to acetyl-coenzyme A, has been used successfully to assess mitochondrial metabolism in the heart, the application of [2-(13) C]Pyr in the study of brain metabolism has been limited to date, with Lac being the only downstream metabolic product reported previously. In this study, single-time-point chemical shift imaging data were acquired from rat brain in vivo. [5-(13) C]Glutamate, [1-(13) C]acetylcarnitine and [1-(13) C]citrate were detected in addition to resonances from [2-(13) C]Pyr and [2-(13) C]Lac. Brain metabolism was further investigated by infusing dichloroacetate, which upregulates Pyr flux to acetyl-coenzyme A. After dichloroacetate administration, a 40% increase in [5-(13) C]glutamate from 0.014 ± 0.004 to 0.020 ± 0.006 (p = 0.02), primarily from brain, and a trend to higher citrate (0.002 ± 0.001 to 0.004 ± 0.002) were detected, whereas [1-(13) C]acetylcarnitine was increased in peripheral tissues. This study demonstrates, for the first time, that hyperpolarized [2-(13) C]Pyr can be used for the in vivo investigation of mitochondrial function and tricarboxylic acid cycle metabolism in brain.

Custom-fitted 16-channel bilateral breast coil for bidirectional parallel imaging

A 16-channel receive-only, closely fitted array coil is described and tested in vivo for bilateral breast imaging at 3 T. The primary purpose of this coil is to provide high signal-to-noise ratio and parallel imaging acceleration in two directions for breast MRI. Circular coil elements (7.5-cm diameter) were placed on a closed "cup-shaped" platform, and nearest neighbor coils were decoupled through geometric overlap. Comparisons were made between the 16-channel custom coil and a commercially available 8-channel coil. SENSitivity Encoding (SENSE) parallel imaging noise amplification (g-factor) was evaluated in phantom scans. In healthy volunteers, we compared signal-to-noise ratio, parallel imaging in one and two directions, Autocalibrating Reconstruction for Cartesian sampling (ARC) g-factor, and high spatial resolution imaging. When compared with a commercially available 8-channel coil, the 16-channel custom coil shows 3.6× higher mean signal-to-noise ratio in the breast and higher quality accelerated images. In patients, the 16-channel custom coil has facilitated high-quality, high-resolution images with bidirectional acceleration of R = 6.3.

Multi-channel metabolic imaging, with SENSE reconstruction, of hyperpolarized [1-(13)C] pyruvate in a live rat at 3.0 tesla on a clinical MR scanner

We report metabolic images of (13)C, following injection of a bolus of hyperpolarized [1-(13)C] pyruvate in a live rat. The data were acquired on a clinical scanner, using custom coils for volume transmission and array reception. Proton blocking of all carbon resonators enabled proton anatomic imaging with the system body coil, to allow for registration of anatomic and metabolic images, for which good correlation was achieved, with some anatomic features (kidney and heart) clearly visible in a carbon image, without reference to the corresponding proton image. Parallel imaging with sensitivity encoding was used to increase the spatial resolution in the SI direction of the rat. The signal to noise ratio in was in some instances unexpectedly high in the parallel images; variability of the polarization among different trials, plus partial volume effects, are noted as a possible cause of this.

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 x 1.3 x 4.0 mm(3)) and a high resolution (1.0 x 1.0 x 2.0 mm(3)). 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.

Three-dimensional prepolarized magnetic resonance imaging using rapid acquisition with relaxation enhancement

Prepolarized MRI (PMRI) with pulsed electromagnets has the potential to produce diagnostic quality 0.5- to 1.0-T images with significantly reduced cost, susceptibility artifacts, specific absorption rate, and gradient noise. In PMRI, the main magnetic field cycles between a high field (B(p)) to polarize the sample and a homogeneous, low field (B(0)) for data acquisition. This architecture combines the higher SNR of the polarizing field with the imaging benefits of the lower field. However, PMRI can only achieve high SNR efficiency for volumetric imaging with 3D rapid imaging techniques, such as rapid acquisition with relaxation enhancement (RARE) (FSE, TSE), because slice-interleaved acquisition and longitudinal magnetization storage are both inefficient in PMRI. This paper demonstrates the use of three techniques necessary to achieve efficient, artifact-free RARE in PMRI: quadratic nulling of concomitant gradient fields, electromotive force cancelation during field ramping, and phase compensation of CPMG echo trains. This paper also demonstrates the use of 3D RARE in PMRI to achieve standard T(1) and fat-suppressed T(2) contrast in phantoms and in vivo wrists. These images show strong potential for future clinical application of PMRI to extremity musculoskeletal imaging and peripheral angiography.

Rapid polarizing field cycling in magnetic resonance imaging

We describe the electronics for controlling the independently pulsed polarizing coil in a prepolarized magnetic resonance imaging (PMRI) system and demonstrate performance with free induction decay measurements and in vivo imaging experiments. A PMRI scanner retains all the benefits of acquiring MRI data at low field, but with the higher signal of the polarizing field. Rapidly and efficiently ramping the polarizing coil without disturbing the data acquisition is one of the major challenges of PMRI. With our modular hardware design, we successfully ramp the 0.4-T polarizing coil of a wrist-sized PMRI scanner at up to 100 T/s without causing image artifacts or otherwise degrading data acquisition.