An eight-channel sodium/proton coil for brain MRI at 3 T

Triple-tuned birdcage and single-tuned dipole array for quadri-nuclear head MRI at 7 T

PURPOSE

The purpose of this work was to design and build a coil for quadri-nuclear MRI of the human brain at 7 T.

METHODS

We built a transmit/receive triple-tuned (45.6 MHz for  2 $$ {}^2 $$ H, 78.6 MHz for  23 $$ {}^{23} $$ Na, and 120.3 MHz for  31 $$ {}^{31} $$ P) quadrature four-rod birdcage that was geometrically interleaved with a transmit/receive four-channel dipole array (297.2 MHz for  1 $$ {}^1 $$ H). The birdcage rods contained passive, two-pole resonant circuits that emulated capacitors required for single-tuning at three frequencies. The birdcage assembly also included triple-tuned matching networks, baluns, and transmit/receive switches. We assessed the performance of the coil with quality factor (Q) and signal-to-noise ratio (SNR) measurements, and performed in vivo multinuclear MRI and MR spectroscopic imaging (MRSI).

RESULTS

Q measurements showed that the triple-tuned birdcage efficiency was within 33% of that of single-tuned baseline birdcages at all three frequencies. The quadri-tuned coil SNR was 78%, 59%, 44%, and 48% lower than that of single or dual-tuned reference coils for  1 $$ {}^1 $$ H,  2 $$ {}^2 $$ H,  23 $$ {}^{23} $$ Na, and  31 $$ {}^{31} $$ P, respectively. Quadri-nuclear MRI and MRSI was demonstrated in brain in vivo in about 30 min.

CONCLUSION

While the SNR of the quadruple tuned coil was significantly lower than dual- and single-tuned reference coils, it represents a step toward truly simultaneous quadri-nuclear measurements.

Wideband receive-coil array design using high-impedance amplifiers for broadband decoupling

PURPOSE

Multinuclear MRI/S is of increasing interest. Currently, most multinuclear receive array coils are constructed by nesting multiple single-tuned array coils or using switching elements to control the operating frequency, in which case more than one set of conventional isolation preamplifiers and associated decoupling circuits is required. These conventional configurations rapidly become complicated when greater numbers of channels or nuclei are needed. In this work, a novel coil decoupling mechanism is proposed to enable broadband decoupling for array coils with one set of preamplifiers.

METHODS

Instead of using conventional isolation preamplifiers, a high-input impedance preamplifier is proposed to create broadband decoupling of the array elements. A matching network consisting of a single inductor-capacitor-capacitor multi-tuned network and a wire-wound transformer was used to interface the surface coil to the high-impedance preamplifier. To validate the concept, the proposed configuration was compared to the conventional preamplifier decoupling configuration on both bench and scanner.

RESULTS

2 The approach can provide more than 15dB decoupling over a range of 25MHz, covering the Larmor frequencies of 23 Na and 2 H at 4.7T. This multi-tuned prototype obtained 61% and 76% of the imaging SNR at 2 H and 23 Na respectively, 76 and 89% in a higher loading test phantom, when compared to the conventional single-tuned preamplifier decoupling configuration.

CONCLUSION

With the multinuclear array operation and decoupling achieved using only one layer of array coil and preamplifiers, this work provides a simple approach of building high element-count arrays to enable accelerated imaging or SNR improvement from multiple nuclei.

Recent technical developments and clinical research applications of sodium (23Na) MRI

Sodium is an essential ion that plays a central role in many physiological processes including the transmembrane electrochemical gradient and the maintenance of the body's homeostasis. Due to the crucial role of sodium in the human body, the sodium nucleus is a promising candidate for non-invasively assessing (patho-)physiological changes. Almost 10 years ago, Madelin et al. provided a comprehensive review of methods and applications of sodium (23Na) MRI (Madelin et al., 2014) [1]. More recent review articles have focused mainly on specific applications of 23Na MRI. For example, several articles covered 23Na MRI applications for diseases such as osteoarthritis (Zbyn et al., 2016, Zaric et al., 2020) [2,3], multiple sclerosis (Petracca et al., 2016, Huhn et al., 2019) [4,5] and brain tumors (Schepkin, 2016) [6], or for imaging certain organs such as the kidneys (Zollner et al., 2016) [7], the brain (Shah et al., 2016, Thulborn et al., 2018) [8,9], and the heart (Bottomley, 2016) [10]. Other articles have reviewed technical developments such as radiofrequency (RF) coils for 23Na MRI (Wiggins et al., 2016, Bangerter et al., 2016) [11,12], pulse sequences (Konstandin et al., 2014) [13], image reconstruction methods (Chen et al., 2021) [14], and interleaved/simultaneous imaging techniques (Lopez Kolkovsky et al., 2022) [15]. In addition, 23Na MRI topics have been covered in review articles with broader topics such as multinuclear MRI or ultra-high-field MRI (Niesporek et al., 2019, Hu et al., 2019, Ladd et al., 2018) [16-18]. During the past decade, various research groups have continued working on technical improvements to sodium MRI and have investigated its potential to serve as a diagnostic and prognostic tool. Clinical research applications of 23Na MRI have covered a broad spectrum of diseases, mainly focusing on the brain, cartilage, and skeletal muscle (see Fig. 1). In this article, we aim to provide a comprehensive summary of methodological and hardware developments, as well as a review of various clinical research applications of sodium (23Na) MRI in the last decade (i.e., published from the beginning of 2013 to the end of 2022).

A radially interleaved sodium and proton coil array for brain MRI at 7 T

The objective of the current study was to design and build a dual-tuned coil array for simultaneous ²³ Na/¹ H MRI of the human brain at 7 T. Quality factor, experimental B₁⁺ measurements, and electromagnetic simulations in prototypes showed that setups consisting of geometrically interleaved ¹ H and ²³ Na loops performed better than or similar to ¹ H or ²³ Na loops in isolation. Based on these preliminary findings, we built a transmit/receive eight-channel ²³ Na loop array that was geometrically interleaved with a transmit/receive eight-channel ¹ H loop array. We assessed the performance of the manufactured array with mononuclear signal-to-noise ratio (SNR) and B₁⁺ measurements, along with multinuclear magnetic resonance fingerprinting maps and images. The ²³ Na array within the developed dual-tuned device provided more than 50% gain in peripheral SNR and similar B₁⁺ uniformity and coverage as a reference birdcage coil of similar size. The ¹ H array provided good B₁⁺ uniformity in the brain, excluding the cerebellum and brain stem. The integrated ²³ Na and ¹ H arrays were used to demonstrate truly simultaneous quantitative ¹ H mapping and ²³ Na imaging.

Sodium Radiofrequency Coils for Magnetic Resonance: From Design to Applications

Sodium (23Na) is the most abundant cation present in the human body and is involved in a large number of vital body functions. In the last few years, the interest in Sodium Magnetic Resonance Imaging (23Na MRI) has considerably increased for its relevance in physiological and physiopathological aspects. Indeed, sodium MRI offers the possibility to extend the anatomical imaging information by providing additional and complementary information on physiology and cellular metabolism with the heteronuclear Magnetic Resonance Spectroscopy (MRS). Constraints are the rapidly decaying of sodium signal, the sensitivity lack due to the low sodium concentration versus 1H-MRI induce scan times not clinically acceptable and it also constitutes a challenge for sodium MRI. With the available magnetic fields for clinical MRI scanners (1.5 T, 3 T, 7 T), and the hardware capabilities such as strong gradient strengths with high slew rates and new dedicated radiofrequency (RF) sodium coils, it is possible to reach reasonable measurement times (~10–15 min) with a resolution of a few millimeters, where it has already been applied in vivo in many human organs such as the brain, cartilage, kidneys, heart, as well as in muscle and the breast. In this work, we review the different geometries and setup of sodium coils described in the available literature for different in vivo applications in human organs with clinical MR scanners, by providing details of the design, modeling and construction of the coils.

Frontiers of Sodium MRI Revisited: From Cartilage to Brain Imaging

Sodium magnetic resonance imaging (²³ Na-MRI) is a highly promising imaging modality that offers the possibility to noninvasively quantify sodium content in the tissue, one of the most relevant parameters for biochemical investigations. Despite its great potential, due to the intrinsically low signal-to-noise ratio (SNR) of sodium imaging generated by low in vivo sodium concentrations, low gyromagnetic ratio, and substantially shorter relaxation times than for proton (¹ H) imaging, ²³ Na-MRI is extremely challenging. In this article, we aim to provide a comprehensive overview of the literature that has been published in the last 10-15 years and which has demonstrated different technical designs for a range of ²³ Na-MRI methods applicable for disease diagnoses and treatment efficacy evaluations. Currently, a wider use of 3.0T and 7.0T systems provide imaging with the expected increase in SNR and, consequently, an increased image resolution and a reduced scanning time. A great interest in translational research has enlarged the field of sodium MRI applications to almost all parts of the body: articular cartilage tendons, spine, heart, breast, muscle, kidney, and brain, etc., and several pathological conditions, such as tumors, neurological and degenerative diseases, and others. The quantitative parameter, tissue sodium concentration, which reflects changes in intracellular sodium concentration, extracellular sodium concentration, and intra-/extracellular volume fractions is becoming acknowledged as a reliable biomarker. Although the great potential of this technique is evident, there must be steady technical development for ²³ Na-MRI to become a standard imaging tool. The future role of sodium imaging is not to be considered as an alternative to ¹ H MRI, but to provide early, diagnostically valuable information about altered metabolism or tissue function associated with disease genesis and progression. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY STAGE: 1.

A dual-tuned 17 O/1 H head array for direct brain oximetry at 3 Tesla

PURPOSE

To design and build a dual-tuned ¹⁷ O/¹ H coil for direct brain oximetry at 3T.

METHODS

A dual-tuned ¹⁷ O/¹ H coil comprising 2 degenerate mode birdcage coils was constructed to facilitate high-sensitivity ¹⁷ O and ¹ H imaging. In vivo ¹⁷ O brain images were acquired in a healthy volunteer using a fermat looped orthogonally encoded trajectories sequence, together with high-resolution structural brain ¹ H images.

RESULTS

Natural abundance ¹⁷ O images with a nominal resolution of 8 mm³ were acquired in under 20 minutes exhibiting clear delineation of the physiological ¹⁷ O distribution. One-millimeter isotropic ¹ H structural brain images demonstrated excellent quality and anatomical detail using routine clinical imaging sequence parameters and parallel acceleration.

CONCLUSION

A dual-tuned ¹⁷ O/¹ H array was constructed to enable high-sensitivity ¹⁷ O and ¹ H imaging under standard clinical 3 T scanning conditions.

An 8-channel receive array for improved 31 P MRSI of the whole brain at 3T

PURPOSE

To demonstrate a ¹ H/³¹ P whole human brain volume coil configuration for 3 Tesla with separate ³¹ P transmit and receive components that maintains ¹ H MRS performance and delivers optimal ³¹ P MRSI with ¹ H decoupling.

METHODS

We developed an 8-channel ³¹ P receive array coil covering the head to be used as an insert for a commercial double-tuned ¹ H/³¹ P birdcage transmit-receive coil. This retains the possibility of using low-power rectangular pulses for ¹ H-decoupled 3D ³¹ P MRSI (nominal resolution 17.6 cm³ ; acquisition duration 13 min) but increases the SNR with the receive sensitivity of ³¹ P surface coils. The performance of the combined coil setup was evaluated by measuring ¹ H and ³¹ P SNR with and without the ³¹ P receive array and by assessing the effect of the receive array on the transmit efficiencies of the birdcage coil.

RESULTS

Compared to the birdcage coil alone, the ³¹ P insert in combination with the birdcage achieved an average ³¹ P SNR gain of 1.4 ± 0.4 in a center partition of the brain. The insert did not cause losses in ¹ H MRS performance and transmit efficiency, whereas for ³¹ P approximately 20% more power was needed to achieve the same γB1.

CONCLUSION

The new coil configuration allows ¹ H MRSI and optimal ¹ H-decoupled 3D ³¹ P MRSI, with increased SNR of the human brain without patient repositioning, for clinical and research purposes at 3 Tesla.

Feasibility study of a double resonant 8-channel 1H/ 8-channel 23Na receive-only head coil at 3 Tesla

Sodium (²³Na) magnetic resonance imaging (MRI), especially brain applications are increasingly interesting since sodium MRI can provide additional information about tissue viability and vitality. In order to include sodium MRI in the clinical routine, a single RF setup is preferable which provides high sodium sensitivity and full proton performance in terms of signal-to-noise ratio (SNR) and parallel imaging performance. The aim of this work was to evaluate the feasibility of a double resonant receive (Rx) coil array for proton and sodium head MRI. The coil was designed to provide high sodium SNR and full proton performance comparable to commercial coils which are optimized for sodium MRI or for proton MRI, respectively. A measurement setup was built which comprised an 8-channel Rx degenerate Birdcage for sodium imaging and an 8-channel Rx array for proton imaging. The performance of the coil was evaluated against commercial sodium and proton coils using phantom and in-vivo measurements of two healthy volunteers.

MRI Evidence of Altered Callosal Sodium in Mild Traumatic Brain Injury

BACKGROUND AND PURPOSE

Mild traumatic brain injury is a leading cause of death and disability worldwide with 42 million cases reported annually, increasing the need to understand the underlying pathophysiology because this could help guide the development of targeted therapy. White matter, particularly the corpus callosum, is susceptible to injury. Animal models suggest stretch-induced mechanoporation of the axonal membrane resulting in ionic shifts and altered sodium ion distribution. The purpose of this study was to compare the distribution of total sodium concentration in the corpus callosum between patients with mild traumatic brain injury and controls using sodium (²³Na) MR imaging.

MATERIALS AND METHODS

Eleven patients with a history of mild traumatic brain injury and 10 age- and sex-matched controls underwent sodium (²³Na) MR imaging using a 3T scanner. Total sodium concentration was measured in the genu, body, and splenium of the corpus callosum with 5-mm ROIs; total sodium concentration of the genu-to-splenium ratio was calculated and compared between patients and controls.

RESULTS

Higher total sodium concentration in the genu (49.28 versus 43.29 mmol/L, P = .01) and lower total sodium concentration in the splenium (which was not statistically significant; 38.35 versus 44.06 mmol/L, P = .08) was seen in patients with mild traumatic brain injury compared with controls. The ratio of genu total sodium concentration to splenium total sodium concentration was also higher in patients with mild traumatic brain injury (1.3 versus 1.01, P = .001).

CONCLUSIONS

Complex differences are seen in callosal total sodium concentration in symptomatic patients with mild traumatic brain injury, supporting the notion of ionic dysfunction in the pathogenesis of mild traumatic brain injury. The total sodium concentration appears to be altered beyond the immediate postinjury phase, and further work is needed to understand the relationship to persistent symptoms and outcome.