Two-dimensional sixteen channel transmit/receive coil array for cardiac MRI at 7.0 T: design, evaluation, and application

How bold is blood oxygenation level-dependent (BOLD) magnetic resonance imaging of the kidney? Opportunities, challenges and future directions

Renal tissue hypoperfusion and hypoxia are key elements in the pathophysiology of acute kidney injury and its progression to chronic kidney disease. Yet, in vivo assessment of renal haemodynamics and tissue oxygenation remains a challenge. Many of the established approaches are invasive, hence not applicable in humans. Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) offers an alternative. BOLD-MRI is non-invasive and indicative of renal tissue oxygenation. Nonetheless, recent (pre-) clinical studies revived the question as to how bold renal BOLD-MRI really is. This review aimed to deliver some answers. It is designed to inspire the renal physiology, nephrology and imaging communities to foster explorations into the assessment of renal oxygenation and haemodynamics by exploiting the powers of MRI. For this purpose, the specifics of renal oxygenation and perfusion are outlined. The fundamentals of BOLD-MRI are summarized. The link between tissue oxygenation and the oxygenation-sensitive MR biomarker T2∗ is outlined. The merits and limitations of renal BOLD-MRI in animal and human studies are surveyed together with their clinical implications. Explorations into detailing the relation between renal T2∗ and renal tissue partial pressure of oxygen (pO2 ) are discussed with a focus on factors confounding the T2∗ vs. tissue pO2 relation. Multi-modality in vivo approaches suitable for detailing the role of the confounding factors that govern T2∗ are considered. A schematic approach describing the link between renal perfusion, oxygenation, tissue compartments and renal T2∗ is proposed. Future directions of MRI assessment of renal oxygenation and perfusion are explored.

Skin sodium measured with ²³Na MRI at 7.0 T

Skin sodium (Na(+) ) storage, as a physiologically important regulatory mechanism for blood pressure, volume regulation and, indeed, survival, has recently been rediscovered. This has prompted the development of MRI methods to assess Na(+) storage in humans ((23) Na MRI) at 3.0 T. This work examines the feasibility of high in-plane spatial resolution (23) Na MRI in skin at 7.0 T. A two-channel transceiver radiofrequency (RF) coil array tailored for skin MRI at 7.0 T (f = 78.5 MHz) is proposed. Specific absorption rate (SAR) simulations and a thorough assessment of RF power deposition were performed to meet the safety requirements. Human skin was examined in an in vivo feasibility study using two-dimensional gradient echo imaging. Normal male adult volunteers (n = 17; mean ± standard deviation, 46 ± 18 years; range, 20-79 years) were investigated. Transverse slices of the calf were imaged with (23) Na MRI using a high in-plane resolution of 0.9 × 0.9 mm(2) . Skin Na(+) content was determined using external agarose standards covering a physiological range of Na(+) concentrations. To assess the intra-subject reproducibility, each volunteer was examined three to five times with each session including a 5-min walk and repositioning/preparation of the subject. The age dependence of skin Na(+) content was investigated. The (23) Na RF coil provides improved sensitivity within a range of 1 cm from its surface versus a volume RF coil which facilitates high in-plane spatial resolution imaging of human skin. Intra-subject variability of human skin Na(+) content in the volunteer population was <10.3%. An age-dependent increase in skin Na(+) content was observed (r = 0.78). The assignment of Na(+) stores with (23) Na MRI techniques could be improved at 7.0 T compared with current 3.0 T technology. The benefits of such improvements may have the potential to aid basic research and clinical applications designed to unlock questions regarding the Na(+) balance and Na(+) storage function of skin.

High spatial resolution coronary magnetic resonance angiography at 7 T: comparison with low spatial resolution bright blood imaging

OBJECTIVES

The aim of this study was to compare bright blood high spatial resolution (HR) coronary magnetic resonance angiography (MRA) with low spatial resolution (LR) bright blood coronary MRA at 7 T.

MATERIALS AND METHODS

Twenty-four healthy volunteers underwent navigator-gated 3-dimensional imaging of the right coronary artery at 7 T using 2 sequences: HR bright blood and LR bright blood. Image postprocessing involved newly developed multiplanar reformatting to straighten the right coronary artery. Image quality was determined by vessel edge sharpness, signal-to-noise ratio, contrast-to-noise ratio, visible vessel length, and vessel diameter.

RESULTS

Vessel edge sharpness was statistically significantly higher in HR as compared with LR (0.57 ± 0.1 vs 0.46 ± 0.06; P < 0.001), at the cost of lower signal-to-noise ratio (HR, 32.9 ± 11.0 vs LR, 112.5 ± 48.9; P < 0.001) and contrast-to-noise ratio (HR, 17.9 ± 7.4 vs LR, 50.5 ± 26.1; P < 0.001). Visible vessel length and vessel diameter were similar for both sequences (P > 0.05).

CONCLUSIONS

High spatial resolution bright blood coronary MRA at 7 T is feasible and improves vessel edge sharpness as compared with LR bright blood imaging.

Real-time magnetic resonance imaging of cardiac function and flow-recent progress

Cardiac structure, function and flow are most commonly studied by ultrasound, X-ray and magnetic resonance imaging (MRI) techniques. However, cardiovascular MRI is hitherto limited to electrocardiogram (ECG)-synchronized acquisitions and therefore often results in compromised quality for patients with arrhythmias or inabilities to comply with requested protocols-especially with breath-holding. Recent advances in the development of novel real-time MRI techniques now offer dynamic imaging of the heart and major vessels with high spatial and temporal resolution, so that examinations may be performed without the need for ECG synchronization and during free breathing. This article provides an overview of technical achievements, physiological validations, preliminary patient studies and translational aspects for a future clinical scenario of cardiovascular MRI in real time.

Dual optimization method of radiofrequency and quasistatic field simulations for reduction of eddy currents generated on 7T radiofrequency coil shielding

PURPOSE

To optimize the design of radiofrequency (RF) shielding of transmit coils at 7T and reduce eddy currents generated on the RF shielding when imaging with rapid gradient waveforms.

METHODS

One set of a four-element, 2 × 2 Tic-Tac-Toe head coil structure was selected and constructed to study eddy currents on the RF coil shielding. The generated eddy currents were quantitatively studied in the time and frequency domains. The RF characteristics were studied using the finite difference time domain method. Five different kinds of RF shielding were tested on a 7T MRI scanner with phantoms and in vivo human subjects.

RESULTS

The eddy current simulation method was verified by the measurement results. Eddy currents induced by solid/intact and simple-structured slotted RF shielding significantly distorted the gradient fields. Echo-planar images, B1+ maps, and S matrix measurements verified that the proposed slot pattern suppressed the eddy currents while maintaining the RF characteristics of the transmit coil.

CONCLUSION

The presented dual-optimization method could be used to design RF shielding and reduce the gradient field-induced eddy currents while maintaining the RF characteristics of the transmit coil.

On the RF heating of coronary stents at 7.0 Tesla MRI

PURPOSE

Examine radiofrequency (RF) induced heating of coronary stents at 7.0 Tesla (T) to derive an analytical approach which supports RF heating assessment of arbitrary stent geometries and RF coils.

METHODS

Simulations are performed to detail electromagnetic fields (EMF), local specific absorption rates (SAR) and temperature changes. For validation E-field measurements and RF heating experiments are conducted. To progress to clinical setups RF coils tailored for cardiac MRI at 7.0T and coronary stents are incorporated into EMF simulations using a human voxel model.

RESULTS

Our simulations of coronary stents at 297 MHz were confirmed by E-field and temperature measurements. An analytical solution which describes SAR(1g tissue voxel) induced by an arbitrary coronary stent interfering with E-fields generated by an arbitrary RF coil was derived. The analytical approach yielded a conservative estimation of induced SAR(1g tissue voxel) maxima without the need for integrating the stent into EMF simulations of the human voxel model.

CONCLUSION

The proposed analytical approach can be applied for any patient, coronary stent type, RF coil configuration and RF transmission regime. The generalized approach is of value for RF heating assessment of other passive electrically conductive implants and provides a novel design criterion for RF coils.

An eight-channel transmit/receive array of TE01 mode high permittivity ceramic resonators for human imaging at 7T

This study describes the design, construction and operation of a new type of transmit/receive array using ceramic resonators operating in a transverse electromagnetic (TE) mode. Single element function and performance at 298.1MHz (7T) are analyzed and compared to a lumped element design loop coil with comparable geometry. The results show that ceramic resonators working in the TE01δ mode configuration produce similar efficiency, defined as the transmit magnetic field (B1(+)) per square root of the specific absorption rate (SAR), to conventional surface coils. An array consisting of eight ceramic elements was then designed to operate in transmit/receive mode. This array was driven via power/phase splitters by two independent transmit channels and functional cardiac images were produced from a number of healthy volunteers.

[Cardiovascular ultrahigh field magnetic resonance imaging : challenges, technical solutions and opportunities]

CLINICAL/METHODICAL ISSUE

This involves high spatial resolution cardiac imaging with ultrahigh magnetic fields (7 T) and clinically acceptable image quality.

STANDARD RADIOLOGICAL METHODS

Cardiovascular magnetic resonance imaging (MRI) at a field strength of 1.5 T using a spatial resolution of (2 × 2 × 6-8) mm(3).

METHODICAL INNOVATIONS

Cardiac MRI at ultrahigh field strength makes use of multitransmit/receive radiofrequency (RF) technology and development of novel technology that utilizes the traits of ultrahigh field MRI.

PERFORMANCE

Enhanced spatial resolution which is superior by a factor of 6-10 to what can be achieved by current clinical cardiac MRI. The relative spatial resolution (pixels per anatomical structure) comes close to what can be accomplished by current cardiac MRI in small rodents.

ACHIEVEMENTS

Feasibility studies demonstrate the gain in spatial resolution at 7.0 T due to the sensitivity advantage inherent to ultrahigh magnetic fields.

PRACTICAL RECOMMENDATIONS

Please stay tuned and please put further weight behind the solution of the remaining technical problems of cardiac MRI at 7.0 T.

Modular 32-channel transceiver coil array for cardiac MRI at 7.0T

PURPOSE

To design and evaluate a modular transceiver coil array with 32 independent channels for cardiac MRI at 7.0T.

METHODS

The modular coil array comprises eight independent building blocks, each containing four transceiver loop elements. Numerical simulations were used for B1 (+) field homogenization and radiofrequency (RF) safety validation. RF characteristics were examined in a phantom study. The array's suitability for accelerated high spatial resolution two-dimensional (2D) FLASH CINE imaging of the heart was examined in a volunteer study.

RESULTS

Transmission field adjustments and RF characteristics were found to be suitable for the volunteer study. The signal-to-noise intrinsic to 7.0T together with the coil performance afforded a spatial resolution of 1.1 × 1.1 × 2.5 mm(3) for 2D CINE FLASH MRI, which is by a factor of 6 superior to standardized CINE protocols used in clinical practice at 1.5T. The 32-channel transceiver array supports one-dimensional acceleration factors of up to R = 4 without impairing image quality significantly.

CONCLUSION

The modular 32-channel transceiver cardiac array supports accelerated and high spatial resolution cardiac MRI. The array is compatible with multichannel transmission and provides a technological basis for future clinical assessment of parallel transmission techniques at 7.0T.

Progress and promises of human cardiac magnetic resonance at ultrahigh fields: a physics perspective

A growing number of reports eloquently speak about explorations into cardiac magnetic resonance (CMR) at ultrahigh magnetic fields (B0≥7.0 T). Realizing the progress, promises and challenges of ultrahigh field (UHF) CMR this perspective outlines current trends in enabling MR technology tailored for cardiac MR in the short wavelength regime. For this purpose many channel radiofrequency (RF) technology concepts are outlined. Basic principles of mapping and shimming of transmission fields including RF power deposition considerations are presented. Explorations motivated by the safe operation of UHF-CMR even in the presence of conductive implants are described together with the physics, numerical simulations and experiments, all of which detailing antenna effects and RF heating induced by intracoronary stents at 7.0 T. Early applications of CMR at 7.0 T and their clinical implications for explorations into cardiovascular diseases are explored including assessment of cardiac function, myocardial tissue characterization, MR angiography of large and small vessels as well as heteronuclear MR of the heart and the skin. A concluding section ventures a glance beyond the horizon and explores future directions. The goal here is not to be comprehensive but to inspire the biomedical and diagnostic imaging communities to throw further weight behind the solution of the many remaining unsolved problems and technical obstacles of UHF-CMR with the goal to transfer MR physics driven methodological advancements into extra clinical value.

Assessment of the right ventricle with cardiovascular magnetic resonance at 7 Tesla

BACKGROUND

Functional and morphologic assessment of the right ventricle (RV) is of clinical importance. Cardiovascular magnetic resonance (CMR) at 1.5T has become gold standard for RV chamber quantification and assessment of even small wall motion abnormalities, but tissue analysis is still hampered by limited spatial resolution. CMR at 7T promises increased resolution, but is technically challenging. We examined the feasibility of cine imaging at 7T to assess the RV.

METHODS

Nine healthy volunteers underwent CMR at 7T using a 16-element TX/RX coil and acoustic cardiac gating. 1.5T served as gold standard. At 1.5T, steady-state free-precession (SSFP) cine imaging with voxel size (1.2 x 1.2 x 6) mm3 was used; at 7T, fast gradient echo (FGRE) with voxel size (1.2 x 1.2 x 6) mm3 and (1.3 x 1.3 x 4) mm3 were applied. RV dimensions (RVEDV, RVESV), RV mass (RVM) and RV function (RVEF) were quantified in transverse slices. Overall image quality, image contrast and image homogeneity were assessed in transverse and sagittal views.

RESULTS

All scans provided diagnostic image quality. Overall image quality and image contrast of transverse RV views were rated equally for SSFP at 1.5T and FGRE at 7T with voxel size (1.3 x 1.3 x 4)mm3. FGRE at 7T provided significantly lower image homogeneity compared to SSFP at 1.5T. RVEDV, RVESV, RVEF and RVM did not differ significantly and agreed close between SSFP at 1.5T and FGRE at 7T (p=0.5850; p=0.5462; p=0.2789; p=0.0743). FGRE at 7T with voxel size (1.3 x 1.3 x 4) mm3 tended to overestimate RV volumes compared to SSFP at 1.5T (mean difference of RVEDV 8.2 ± 9.3 ml) and to FGRE at 7T with voxel size (1.2 x 1.2 x 6) mm3 (mean difference of RVEDV 9.3 ± 8.6 ml).

CONCLUSIONS

FGRE cine imaging of the RV at 7T was feasible and provided good image quality. RV dimensions and function were comparable to SSFP at 1.5T as gold standard.

High spatial resolution and temporally resolved T2* mapping of normal human myocardium at 7.0 Tesla: an ultrahigh field magnetic resonance feasibility study

Myocardial tissue characterization using T(2)(*) relaxation mapping techniques is an emerging application of (pre)clinical cardiovascular magnetic resonance imaging. The increase in microscopic susceptibility at higher magnetic field strengths renders myocardial T(2)(*) mapping at ultrahigh magnetic fields conceptually appealing. This work demonstrates the feasibility of myocardial T(2)(*) imaging at 7.0 T and examines the applicability of temporally-resolved and high spatial resolution myocardial T(2)(*) mapping. In phantom experiments single cardiac phase and dynamic (CINE) gradient echo imaging techniques provided similar T(2)(*) maps. In vivo studies showed that the peak-to-peak B(0) difference following volume selective shimming was reduced to approximately 80 Hz for the four chamber view and mid-ventricular short axis view of the heart and to 65 Hz for the left ventricle. No severe susceptibility artifacts were detected in the septum and in the lateral wall for T(2)(*) weighting ranging from TE = 2.04 ms to TE = 10.2 ms. For TE >7 ms, a susceptibility weighting induced signal void was observed within the anterior and inferior myocardial segments. The longest T(2)(*) values were found for anterior (T(2)(*) = 14.0 ms), anteroseptal (T(2)(*) = 17.2 ms) and inferoseptal (T(2)(*) = 16.5 ms) myocardial segments. Shorter T(2)(*) values were observed for inferior (T(2)(*) = 10.6 ms) and inferolateral (T(2)(*) = 11.4 ms) segments. A significant difference (p = 0.002) in T(2)(*) values was observed between end-diastole and end-systole with T(2)(*) changes of up to approximately 27% over the cardiac cycle which were pronounced in the septum. To conclude, these results underscore the challenges of myocardial T(2)(*) mapping at 7.0 T but demonstrate that these issues can be offset by using tailored shimming techniques and dedicated acquisition schemes.