Multinuclear MRI at Ultrahigh Fields

Correlation between pelvic floor four-dimensional ultrasound parameters and POP-Q score

BackgroundPelvic organ prolapse (POP) is a prevalent ailment that lowers a woman's quality of life and for which precise diagnosis techniques are required for successful treatment. Advanced imaging techniques may be beneficial for traditional clinical assessments like the Pelvic Organ Prolapse Quantification (POP-Q) system.ObjectivesThe purpose of this study was to look into the relationship between the POP-Q score and four-dimensional pelvic floor ultrasonography parameters in women who have been diagnosed with pelvic organ prolapse.MethodsWe included 120 female patients who presented with symptoms consistent with pelvic floor dysfunction. Pelvic cancers, a history of hysterectomy, and the incapacity to execute Valsalva maneuvers were among the exclusion criteria. The pelvic floor was imaged in three dimensions using a color Doppler ultrasonography equipment (GE, Voluson E8 Edition) both at rest and during Valsalva maneuvers. Important variables, such as the posterior bladder angle and bladder neck distance, were examined and contrasted with POP-Q evaluations.ResultsAccording to the study, an ultrasonography was used to detect Green II prolapse in 63 individuals and Green III prolapse in 57 patients. The ultrasonography results and clinical POP-Q diagnosis agreed rather well (Kappa = 0.572). Furthermore, there was a substantial association found between the four-dimensional ultrasound characteristics and the POP-Q stages, suggesting that ultrasonography is a useful tool for visualizing pelvic floor structures.ConclusionsAccording to the results, four-dimensional ultrasonography has a strong correlation with the POP-Q score and is a useful method for evaluating pelvic floor dysfunction. This imaging modality advances our knowledge of the dynamics of the pelvic floor and might lead to more accurate diagnoses for female patients with prolapsed pelvic organs.

Motion and magnetic field inhomogeneity correction techniques for chemical exchange saturation transfer (CEST) MRI: A contemporary review

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) has emerged as a powerful imaging technique sensitive to tissue molecular composition, pH, and metabolic processes in situ. CEST MRI uniquely probes the physical exchange of protons between water and specific molecules within tissues, providing a window into physiological phenomena that remain invisible to standard MRI. However, given the very low concentration (millimolar range) of CEST compounds, the effects measured are generally only on the order of a few percent of the water signal. Consequently, a few critical challenges, including correction of motion artifacts and magnetic field (B0 and B1 +) inhomogeneities, have to be addressed in order to unlock the full potential of CEST MRI. Motion, whether from patient movement or inherent physiological pulsations, can distort the CEST signal, hindering accurate quantification. B0 and B1 + inhomogeneities, arising from scanner hardware imperfections, further complicate data interpretation by introducing spurious variations in the signal intensity. Without proper correction of these confounding factors, reliable analysis and clinical translation of CEST MRI remain challenging. Motion correction methods aim to compensate for patient movement during (prospective) or after (retrospective) image acquisition, reducing artifacts and preserving data quality. Similarly, B0 and B1 + inhomogeneity correction techniques enhance the spatial and spectral accuracy of CEST MRI. This paper aims to provide a comprehensive review of the current landscape of motion and magnetic field inhomogeneity correction methods in CEST MRI. The methods discussed apply to saturation transfer (ST) MRI in general, including semisolid magnetization transfer contrast (MTC) and relayed nuclear Overhauser enhancement (rNOE) studies.

Parameter optimization for proton density fat fraction quantification in skeletal muscle tissue at 7 T

OBJECTIVE

To establish an image acquisition and post-processing workflow for the determination of the proton density fat fraction (PDFF) in calf muscle tissue at 7 T.

MATERIALS AND METHODS

Echo times (TEs) of the applied vendor-provided multi-echo gradient echo sequence were optimized based on simulations of the effective number of signal averages (NSA*). The resulting parameters were validated by measurements in phantom and in healthy calf muscle tissue (n = 12). Additionally, methods to reduce phase errors arising at 7 T were evaluated. Finally, PDFF values measured at 7 T in calf muscle tissue of healthy subjects (n = 9) and patients with fatty replacement of muscle tissue (n = 3) were compared to 3 T results.

RESULTS

Simulations, phantom and in vivo measurements showed the importance of using optimized TEs for the fat-water separation at 7 T. Fat-water swaps could be mitigated using a phase demodulation with an additional B0 map, or by shifting the TEs to longer values. Muscular PDFF values measured at 7 T were comparable to measurements at 3 T in both healthy subjects and patients with increased fatty replacement.

CONCLUSION

PDFF determination in calf muscle tissue is feasible at 7 T using a chemical shift-based approach with optimized acquisition and post-processing parameters.

Frequency-independent dual-tuned cable traps for multi-nuclear MRI and MRS

Magnetic Resonance Imaging (MRI) and Magnetic Resonance Spectroscopy (MRS) of non-proton nuclei (X-nuclei) typically require additional proton imaging for anatomical reference and B0 shimming. Therefore, two RF systems exist, necessitating cable traps to block the unwanted common-mode current at both Larmor frequencies of 1H and X-nuclei. This study introduces a frequency-independent dual-tuned cable trap that combines a standard solenoid cable trap with a float solenoid trap to independently tune high and low frequencies without compromising performance. The methods involved theoretical analysis, electromagnetic simulations, and bench tests. Two design approaches were evaluated: a float cable trap for 1H, a non-float cable trap for X-nuclei, and vice versa. Results showed that the design with the float trap for X-nuclei and non-float for 1H had superior performance, with high common-mode current suppression ability at both frequencies. Bench tests confirmed these findings, demonstrating effectiveness across various static fields and X-nuclei. The proposed frequency-independent dual-tuned cable trap provides a compact and efficient solution for multinuclear MRI and MRS, enhancing safety, image quality, and flexibility.

Inverse shielding and mutual exclusion for PET-MR hybrid imaging concerning induced positronium hyperfine splits radiations

Prevalent PET imaging reconstructs 2γ-photon pairs emitted after an annihilation from para-positronium (p-Ps) and rejects 3γ events from ortho-positronium (o-Ps) as noises. The 3γ/2γ decay ratio is ~ 3/7 in human body theoretically but in fact significantly lower due to pick-off process, hence PET imaging quality is well controlled. In a PET-MR hybrid unit, the MR magnetic field alters positronium decay patterns through magnetic quenching: all o-Ps and excited p-Ps states are split into finer quantum states under strong magnetic field, thus transitions between some triplet and singlet finer states (mz = 0) were no longer forbidden, thus some o-Ps converts to p-Ps spontaneously by emitting hyperfine split (HFS) photons, which also drops 3γ/2γ ratio hence helps PET imaging quality. However, inverse magnetic quenching might also occur if any external source of HFS frequencies is nearby, thus many p-Ps convert to o-Ps by absorbing those HFS photons (induced HFS transitions). This will dramatically increase 3γ/2γ ratio and hence degrade PET imaging quality instantaneously. The HFS spectrum lies in a broad range of microwaves, from 0.02 to 200 GHz. To prevent inverse magnetic quenching, it is necessary to block external microwave sources outside the hybrid vault, by adding a thin metal layer at all directions of the vault. This could be achieved by adopting the metallic Faraday Cage, which was originally for MR shielding, with possible amendment if necessary. The frequencies of excitation pulses in MR imaging overlap with HFS spectrum, however, the chance for mutual interference during hybrid imaging is small, hence there seems no need to veto each other during hybrid scans.

Neurofluids and the glymphatic system: anatomy, physiology, and imaging

First described in 2012, the glymphatic system is responsible for maintaining homeostasis within the central nervous system, including nutrient delivery, waste clearance, and consistency of the ionic microenvironment. It is comprised of glial cells and barrier systems that modulate neurofluid production, circulation, and exchange. Experimental interrogation of neurofluid dynamics is restricted to ex vivo and in vitro studies in animals and humans, therefore diagnostic imaging plays an important role in minimally invasive evaluation. This review article will synthesize current knowledge and theories regarding neurofluid circulation and implications for neuroimaging. First, we will discuss the anatomy of the neurogliovascular unit, including paravascular and perivascular pathways of fluid exchange. In addition, we will summarize the structure and function of barrier systems including the blood-brain, blood-cerebrospinal fluid, and brain-cerebrospinal fluid barriers. Next, we will mention physiologic factors that yield normal variations in neurofluid circulation, and how various disease pathologies can disrupt glymphatic drainage pathways. Lastly, we will cover the spectrum of diagnostic imaging and interventional techniques with relevance to glymphatic structure, flow, and function. We conclude by highlighting current barriers and future directions for translational imaging and applications to neurologic disorders.

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).

Variability by region and method in human brain sodium concentrations estimated by 23Na magnetic resonance imaging: a meta-analysis

Sodium imaging (23Na-MRI) is of interest in neurological conditions given potential sensitivity to the physiological and metabolic status of tissues. Benchmarks have so far been restricted to parenchyma or grey/white matter (GM/WM). We investigate (1) the availability of evidence, (2) regional pooled estimates and (3) variability attributable to region/methodology. MEDLINE literature search for tissue sodium concentration (TSC) measured in specified 'healthy' brain regions returned 127 reports, plus 278 retrieved from bibliographies. 28 studies met inclusion criteria, including 400 individuals. Reporting variability led to nested data structure, so we used multilevel meta-analysis and a random effects model to pool effect sizes. The pooled mean from 141 TSC estimates was 40.51 mM (95% CI 37.59-43.44; p < 0.001, I2Total=99.4%). Tissue as a moderator was significant (F214 = 65.34, p-val < .01). Six sub-regional pooled means with requisite statistical power were derived. We were unable to consider most methodological and demographic factors sought because of non-reporting, but each factor included beyond tissue improved model fit. Significant residual heterogeneity remained. The current estimates provide an empirical point of departure for better understanding in 23Na-MRI. Improving on current estimates supports: (1) larger, more representative data collection/sharing, including (2) regional data, and (3) agreement on full reporting standards.

Multinuclear MRI in Drug Discovery

The continuous development of magnetic resonance imaging broadens the range of applications to newer areas. Using MRI, we can not only visualize, but also track pharmaceutical substances and labeled cells in both in vivo and in vitro tests. ¹H is widely used in the MRI method, which is determined by its high content in the human body. The potential of the MRI method makes it an excellent tool for imaging the morphology of the examined objects, and also enables registration of changes at the level of metabolism. There are several reports in the scientific publications on the use of clinical MRI for in vitro tracking. The use of multinuclear MRI has great potential for scientific research and clinical studies. Tuning MRI scanners to the Larmor frequency of a given nucleus, allows imaging without tissue background. Heavy nuclei are components of both drugs and contrast agents and molecular complexes. The implementation of hyperpolarization techniques allows for better MRI sensitivity. The aim of this review is to present the use of multinuclear MRI for investigations in drug delivery.

Ultra-High-Field Imaging of the Pediatric Brain and Spinal Cord

Neuroimaging with ultra-high field magnets (≥7T) provides superior signal-to-noise, spatial resolution and tissue contrast; but also greater safety concerns, longer scanning times, and increased distortion and field inhomogeneity. Brain and spinal cord anatomic microstructure and function imaged in greater detail offers improved lesion detection, delineation, and characterization. The ongoing development of novel imaging contrasts and translation of cutting-edge sequences will aid more accurate, sensitive, and precise diagnosis, interventional planning, and follow-up for a variety of pathologic conditions.

7 Tesla and Beyond: Advanced Methods and Clinical Applications in Magnetic Resonance Imaging

ABSTRACT

Ultrahigh magnetic fields offer significantly higher signal-to-noise ratio, and several magnetic resonance applications additionally benefit from a higher contrast-to-noise ratio, with static magnetic field strengths of B0 ≥ 7 T currently being referred to as ultrahigh fields (UHFs). The advantages of UHF can be used to resolve structures more precisely or to visualize physiological/pathophysiological effects that would be difficult or even impossible to detect at lower field strengths. However, with these advantages also come challenges, such as inhomogeneities applying standard radiofrequency excitation techniques, higher energy deposition in the human body, and enhanced B0 field inhomogeneities. The advantages but also the challenges of UHF as well as promising advanced methodological developments and clinical applications that particularly benefit from UHF are discussed in this review article.

7 Tesla Chlorine (35Cl) and Sodium (23Na) MR Imaging of an Enchondroma

We demonstrated the feasibility of 7 Tesla sodium (²³Na) and chlorine (³⁵Cl) MRI of a solitary enchondroma. For this, we established dedicated sequences on a 7-Tesla whole-body system with the following key parameters for ³⁵Cl MRI: TE/TR = 0.35/60 ms, T_RO = 5 ms, α = 90°, Δx³ = (6 mm)³, 3 averages, T_acq = 30 min and for ²³Na MRI: TE/TR = 0.4/101 ms, T_RO = 10ms; α = 90°; Δx³ = (1.9 mm)³, 3 averages, T_acq = 30 min 18 s. The measured apparent Na⁺ concentration was 255 mmol/l and was approximately 7-fold higher than the apparent Cl^- concentration with about 36 mmol/l. Additionally, repeated proton MRI examinations demonstrated constant but subtle growth (≈ 0.65 ml/year) over 14 years. In conclusion, enchondromas obviously have a high contrast-to-noise ratio when compared with the normal bone marrow in ²³Na and ³⁵Cl MRI, which may contribute to detection and differentiation in unclear or subtle cases. KEY POINTS:: · High magnetic field strengths (e. g., 7 Tesla) enable sodium (23Na) and chlorine (35Cl) MRI of solitary cartilage-forming tumors like enchondromas with nominal spatial resolutions of (1.9 mm)3 (23Na MRI) and (6 mm)3 (35Cl MRI).. · Measured median tumoral apparent Na+ and Cl- concentrations were nearly 13 times higher and 3 times higher than in normal muscle tissue, respectively.. · Enchondromas have a high contrast-to-noise-ratio when compared with the normal bone marrow in 23Na and 35Cl MRI, which may contribute to detection and differentiation in unclear or subtle cases.. CITATION FORMAT: · Weber M, Seyler L, Nagel AM. 7 Tesla Chlorine (35Cl) and Sodium (23Na) MR Imaging of an Enchondroma. Fortschr Röntgenstr 2021; DOI: 10.1055/a-1472-6730.

19F Magnetic Resonance Imaging and Spectroscopy in Neuroscience

¹H magnetic resonance imaging (MRI) has established itself as a key diagnostic technique, affording the visualization of brain anatomy, blood flow, activity and connectivity. The detection of other atoms (e.g. ¹⁹F, ²³Na, ³¹P), so called hetero-nuclear MRI and spectroscopy (MRS), provides investigative avenues that complement and extend the richness of information that can be gained from ¹H MRI. Especially ¹⁹F MRI is increasingly emerging as a multi-nuclear (¹H/¹⁹F) technique that can be exploited to visualize cell migration and trafficking. The lack of a ¹⁹F background signal in the brain affords an unequivocal detection suitable for quantification. Fluorine-based contrast material can be engineered as nanoemulsions, nanocapsules, or nanoparticles to label cells in vitro or in vivo. Fluorinated blood substitutes, typically nanoemulsions, can also carry oxygen and serve as a theranostic in poorly perfused brain regions. Brain tissue concentrations of fluorinated pharmaceuticals, including inhalation anesthetics (e.g. isoflurane) and anti-depressants (e.g. fluoxetine), can also be measured using MRS. However, the low signal from these compounds provides a challenge for imaging. Further methodological advances that accelerate signal acquisition (e.g. compressed sensing, cryogenic coils) are required to expand the applications of ¹⁹F MR imaging to, for instance, determine the regional pharmacokinetics of novel fluorine-based drugs. Improvements in ¹⁹F signal detection and localization, combined with the development of novel sensitive probes, will increase the utility of these multi-nuclear studies. These advances will provide new insights into cellular and molecular processes involved in neurodegenerative disease, as well as the mode of action of pharmaceutical compounds.

Ultra-high field magnetic resonance imaging in human epilepsy: A systematic review

RATIONALE

Resective epilepsy surgery is an evidence-based curative treatment option for patients with drug-resistant focal epilepsy. The major preoperative predictor of a good surgical outcome is detection of an epileptogenic lesion by magnetic resonance imaging (MRI). Application of ultra-high field (UHF) MRI, i.e. field strengths ≥ 7 Tesla (T), may increase the sensitivity to detect such a lesion.

METHODS

A keyword search strategy was submitted to Pubmed, EMBASE, Cochrane Database and clinicaltrials.gov to select studies on UHF MRI in patients with epilepsy. Follow-up study selection and data extraction were performed following PRISMA guidelines. We focused on I) diagnostic gain of UHF- over conventional MRI, II) concordance of MRI-detected lesion, seizure onset zone and surgical decision-making, and III) postoperative histopathological diagnosis and seizure outcome.

RESULTS

Sixteen observational cohort studies, all using 7T MRI were included. Diagnostic gain of 7T over conventional MRI ranged from 8% to 67%, with a pooled gain of 31%. Novel techniques to visualize pathological processes in epilepsy and lesion detection are discussed. Seizure freedom was achieved in 73% of operated patients; no seizure outcome comparison was made between 7T MRI positive, 7T negative and 3T positive patients. 7T could influence surgical decision-making, with high concordance of lesion and seizure onset zone. Focal cortical dysplasia (54%), hippocampal sclerosis (12%) and gliosis (8.1%) were the most frequently diagnosed histopathological entities.

SIGNIFICANCE

UHF MRI increases, yet variably, the sensitivity to detect an epileptogenic lesion, showing potential for use in clinical practice. It remains to be established whether this results in improved seizure outcome after surgical treatment. Prospective studies with larger cohorts of epilepsy patients, uniform scan and sequence protocols, and innovative post-processing technology are equally important as further increasing field strengths. Besides technical ameliorations, improved correlation of imaging features with clinical semiology, histopathology and clinical outcome has to be established.

[Perspectives of X-nuclei magnetic resonance imaging in neuro-oncology]

BACKGROUND

X‑nuclei magnetic resonance imaging (MRI) yields a broad spectrum of metabolic and functional imaging techniques with increasing clinical feasibility.

OBJECTIVE

Current X‑nuclei techniques in (neuro)oncology with emphasis on potential clinical applications of sodium and oxygen MRI are described and discussed.

MATERIALS AND METHODS

Review with discussion of state-of-the-art literature on X‑nuclei imaging.

RESULTS

X‑nuclei MRI employs NMR-sensitive nonproton nuclei to enable both anatomical visualization as well as noninvasive imaging and quantification of physiological processes in the human body. At the moment, sodium MRI represents the most common application of X‑nuclei MRI because of its comparatively high NMR signal. Moreover, its sensitivity to pathological cellular proliferation renders sodium MRI a good candidate for oncological imaging, yielding additional biochemical information to proton MRI. Oxygen MRI is currently primarily investigational, requiring high technical efforts and costs. However, preliminary results show a huge potential of this technique for metabolic characterization of tumors.

CONCLUSIONS

X‑nuclei MRI is a rapidly evolving field in metabolic and functional imaging. In coming years, sodium MRI is expected to be increasingly used as an additional clinical tool in oncology to enhance diagnostic accuracy.

Quantitative Imaging in Muscle Diseases with Focus on Non-proton MRI and Other Advanced MRI Techniques

The role of neuromuscular imaging in the diagnosis of inherited and acquired muscle diseases has gained clinical relevance. In particular, magnetic resonance imaging (MRI), especially whole-body applications, is increasingly being used for the diagnosis and monitoring of disease progression. In addition, they are considered as a powerful outcome measure in clinical trials. Because many muscle diseases have a distinct muscle involvement pattern, whole-body imaging can be of diagnostic value by identifying this pattern and thus narrowing the differential diagnosis and supporting the clinical diagnosis. In addition, more advanced MRI applications including non-proton MRI, diffusion tensor imaging, perfusion MRI, T2 mapping, and magnetic resonance spectroscopy provide deeper insights into muscle pathophysiology beyond the mere detection of fatty degeneration and/or muscle edema. In this review article, we present and discuss recent data on these quantitative MRI techniques in muscle diseases, with a particular focus on non-proton imaging techniques.