Magnetic resonance imaging and spectroscopic techniques are widely used in humans both for clinical diagnostic applications and in basic research areas such as cognitive neuroimaging. In recent ...years, new human MR systems have become available operating at static magnetic fields of 7 T or higher (≥300 MHz proton frequency). Imaging human-sized objects at such high frequencies presents several challenges including non-uniform radiofrequency fields, enhanced susceptibility artifacts, and higher radiofrequency energy deposition in the tissue. On the other side of the scale are gains in signal-to-noise or contrast-to-noise ratio that allow finer structures to be visualized and smaller physiological effects to be detected. This review presents an overview of some of the latest methodological developments in human ultra-high field MRI/MRS as well as associated clinical and scientific applications. Emphasis is given to techniques that particularly benefit from the changing physical characteristics at high magnetic fields, including susceptibility-weighted imaging and phase-contrast techniques, imaging with X-nuclei, MR spectroscopy, CEST imaging, as well as functional MRI. In addition, more general methodological developments such as parallel transmission and motion correction will be discussed that are required to leverage the full potential of higher magnetic fields, and an overview of relevant physiological considerations of human high magnetic field exposure is provided.
Combining proton and phosphorus magnetic resonance spectroscopy offers a unique opportunity to study the oxidative and glycolytic components of metabolism in working muscle. This paper presents a 7 T ...proton calf coil design that combines dipole and loop elements to achieve the high performance necessary for detecting metabolites with low abundance and restricted visibility, specifically lactate, while including the option of adding a phosphorus array. We investigated the transmit, receive, and parallel imaging performance of three transceiver dipoles with six pair-wise overlap-decoupled standard or twisted pair receive-only coils. With a higher SNR and more efficient transmission decoupling, standard loops outperformed twisted pair coils. The dipoles with standard loops provided a four-fold-higher image SNR than a multinuclear reference coil comprising two proton channels and 32% more than a commercially available 28-channel proton knee coil. The setup enabled up to three-fold acceleration in the right–left direction, with acceptable g-factors and no visible aliasing artefacts. Spectroscopic phantom measurements revealed a higher spectral SNR for lactate with the developed setup than with either reference coil and fewer restrictions in voxel placement due to improved transmit homogeneity. This paper presents a new use case for dipoles and highlights their advantages for the integration in multinuclear calf coils.
Magnetic resonance (MR) acquisitions of the torso are frequently affected by respiratory motion with detrimental effects on signal quality. The motion of organs inside the body is typically decoupled ...from surface motion and is best captured using rapid MR imaging (MRI). We propose a pipeline for prospective motion correction of the target organ using MR image navigators providing absolute motion estimates in millimeters. Our method is designed to feature multi-nuclear interleaving for non-proton MR acquisitions and to tolerate local transmit coils with inhomogeneous field and sensitivity distributions. OpenCV object tracking was introduced for rapid estimation of in-plane displacements in 2D MR images. A full three-dimensional translation vector was derived by combining displacements from slices of multiple and arbitrary orientations. The pipeline was implemented on 3 T and 7 T MR scanners and tested in phantoms and volunteers. Fast motion handling was achieved with low-resolution 2D MR image navigators and direct implementation of OpenCV into the MR scanner's reconstruction pipeline. Motion-phantom measurements demonstrate high tracking precision and accuracy with minor processing latency. The feasibility of the pipeline for reliable in-vivo motion extraction was shown on heart and kidney data. Organ motion was manually assessed by independent operators to quantify tracking performance. Object tracking performed convincingly on 7774 navigator images from phantom scans and different organs in volunteers. In particular the kernelized correlation filter (KCF) achieved similar accuracy (74%) as scored from inter-operator comparison (82%) while processing at a rate of over 100 frames per second. We conclude that fast 2D MR navigator images and computer vision object tracking can be used for accurate and rapid prospective motion correction. This and the modular structure of the pipeline allows for the proposed method to be used in imaging of moving organs and in challenging applications like cardiac magnetic resonance spectroscopy (MRS) or magnetic resonance imaging (MRI) guided radiotherapy.
Acetylcarnitine is an essential metabolite for maintaining metabolic flexibility and glucose homeostasis. The in vivo behavior of muscle acetylcarnitine content during exercise has not been shown ...with magnetic resonance spectroscopy. Therefore, this study aimed to explore the behavior of skeletal muscle acetylcarnitine during rest, plantar flexion exercise, and recovery in the human gastrocnemius muscle under aerobic conditions. Ten lean volunteers and nine overweight volunteers participated in the study. A 7 T whole-body MR system with a double-tuned surface coil was used to acquire spectra from the gastrocnemius medialis. An MR-compatible ergometer was used for the plantar flexion exercise. Semi-LASER-localized 1H MR spectra and slab-localized 31P MR spectra were acquired simultaneously in one interleaved exercise/recovery session. The time-resolved interleaved 1H/31P MRS acquisition yielded excellent data quality. A between-group difference in acetylcarnitine metabolism over time was detected. Significantly slower τPCr recovery, τPCr on-kinetics, and lower Qmax in the overweight group, compared to the lean group was found. Linear relations between τPCr on-kinetics, τPCr recovery, VO2max and acetylcarnitine content were identified. In conclusion, we are the first to show in vivo changes of skeletal muscle acetylcarnitine during acute exercise and immediate exercise recovery with a submaximal aerobic workload using interleaved 1H/31P MRS at 7 T.
The heart's geometry and its metabolic activity vary over the cardiac cycle. The effect of these fluctuations on phosphorus (
P) magnetic resonance spectroscopy (MRS) data quality and metabolite ...ratios was investigated. 12 healthy volunteers were measured using a 7 T MR scanner and a cardiac
P-
H loop coil.
P chemical shift imaging data were acquired untriggered and at four different times during the cardiac cycle using acoustic triggering. Signals of adenosine-triphosphate (ATP), phosphocreatine (PCr), inorganic phosphate (Pi) and 2,3-diphosphoglycerate (2,3-DPG) and their fit quality as Cramér-Rao lower bounds (CRLB) were quantified including corrections for contamination by
P signals from blood, flip angle, saturation and total acquisition time. The myocardial filling factor was estimated from cine short axis views. The corrected signals of PCr and Formula: see text-ATP were higher during end-systole and lower during diastasis than in untriggered acquisitions (Formula: see text). Signal intensities of untriggered scans were between those with triggering to end-systole and diastasis. Fit quality of PCr and Formula: see text-ATP peaks was best during end-systole when blood contamination of ATP and Pi signals was lowest. While metabolite ratios and pH remained stable over the cardiac cycle, signal amplitudes correlated strongly with myocardial voxel filling. Triggering of cardiac
P MRS acquisitions improves signal amplitudes and fit quality if the trigger delay is set to end-systole. We conclude that triggering to end-systole is superior to triggering to diastasis.
Magnetic resonance elastography (MRE) is a non-invasive method to quantify biomechanical properties of human tissues. It has potential in diagnosis and monitoring of kidney disease, if established in ...clinical practice. The interplay of flow and volume changes in renal vessels, tubule, urinary collection system and interstitium is complex, but physiological ranges of
viscoelastic properties during fasting and hydration have never been investigated in all gross anatomical segments simultaneously.
Ten healthy volunteers underwent two imaging sessions, one following a 12-hour fasting period and the second after a drinking challenge of >10 mL per kg body weight (60-75 min before the second examination). High-resolution renal MRE was performed using a novel driver with rotating eccentric mass placed at the posterior-lateral wall to couple waves (50 Hz) to the kidney. The biomechanical parameters, shear wave speed (c
in m/s), storage modulus (G
in kPa), loss modulus (G
in kPa), phase angle
and attenuation (α in 1/mm) were derived. Accurate separation of gross anatomical segments was applied in post-processing (whole kidney, cortex, medulla, sinus, vessel).
High-quality shear waves coupled into all gross anatomical segments of the kidney (mean shear wave displacement: 163 ± 47 μm, mean contamination of second upper harmonics <23%, curl/divergence: 4.3 ± 0.8). Regardless of the hydration state, median G
of the cortex and medulla (0.68 ± 0.11 kPa) was significantly higher than that of the sinus and vessels (0.48 ± 0.06 kPa), and consistently, significant differences were found in c
,
, and G
(all
< 0.001). The viscoelastic parameters of cortex and medulla were not significantly different. After hydration sinus exhibited a small but significant reduction in median G
by -0.02 ± 0.04 kPa (
= 0.01), and, consequently, the cortico-sinusoidal-difference in G
increased by 0.04 ± 0.07 kPa (
= 0.05). Only upon hydration, the attenuation in vessels became lower (0.084 ± 0.013 1/mm) and differed significantly from the whole kidney (0.095 ± 0.007 1/mm,
= 0.01).
High-resolution renal MRE with an innovative driver and well-defined 3D segmentation can resolve all renal segments, especially when including the sinus in the analysis. Even after a prolonged hydration period the approach is sensitive to small hydration-related changes in the sinus and in the cortico-sinusoidal-difference.
Mechanism of Amino Acid-Induced Skeletal Muscle Insulin Resistance in Humans
Michael Krebs 1 ,
Martin Krssak 1 ,
Elisabeth Bernroider 1 ,
Christian Anderwald 1 ,
Attila Brehm 1 ,
Martin Meyerspeer 1 ...2 ,
Peter Nowotny 1 ,
Erich Roth 3 ,
Werner Waldhäusl 1 and
Michael Roden 1
1 Division of Endocrinology and Metabolism, Department of Internal Medicine III, University of Vienna Medical School, Vienna,
Austria
2 NMR-Group, Institute of Medical Physics, University of Vienna Medical School, Vienna, Austria
3 Department of Surgery, University of Vienna Medical School, Vienna, Austria
Abstract
Plasma concentrations of amino acids are frequently elevated in insulin-resistant states, and a protein-enriched diet can
impair glucose metabolism. This study examined effects of short-term plasma amino acid (AA) elevation on whole-body glucose
disposal and cellular insulin action in skeletal muscle. Seven healthy men were studied for 5.5 h during euglycemic (5.5 mmol/l),
hyperinsulinemic (430 pmol/l), fasting glucagon (65 ng/l), and growth hormone (0.4 μg/l) somatostatin clamp tests in the presence
of low (∼1.6 mmol/l) and increased (∼4.6 mmol/l) plasma AA concentrations. Glucose turnover was measured with d -6,6- 2 H 2 glucose. Intramuscular concentrations of glycogen and glucose-6-phosphate (G6P) were monitored using 13 C and 31 P nuclear magnetic resonance spectroscopy, respectively. A ∼2.1-fold elevation of plasma AAs reduced whole-body glucose disposal
by 25% ( P < 0.01). Rates of muscle glycogen synthesis decreased by 64% (180–315 min, 24 ± 3; control, 67 ± 10 μmol · l −1 · min −1 ; P < 0.01), which was accompanied by a reduction in G6P starting at 130 min (ΔG6P 260–300 min , 18 ± 19; control, 103 ± 33 μmol/l; P < 0.05). In conclusion, plasma amino acid elevation induces skeletal muscle insulin resistance in humans by inhibition of
glucose transport/phosphorylation, resulting in marked reduction of glycogen synthesis.
Footnotes
Address correspondence and reprint requests to Michael Roden, MD, Division of Endocrinology and Metabolism, Department of
Internal Medicine III, University of Vienna Medical School, Währinger Gürtel 18-20, A-1090 Vienna, Austria. E-mail: michael.roden{at}akh-wien.ac.at .
Received for publication 18 September 2001 and accepted in revised form 26 November 2001.
AA, amino acid; APE, atom percent excess; EGP, endogenous glucose production; FFA, free fatty acid; G6P, glucose-6-phosphate;
GIR, glucose infusion rate; HPLC, high-performance liquid chromatography; NMR, nuclear magnetic resonance; PCr, phosphocreatine;
Pi, inorganic phosphate; R a , rate of glucose appearance; R d , rate of glucose disappearance; RIA, radioimmunoassay.
DIABETES
Background
PET/MRI phantom studies are challenged by the need of phantom-specific attenuation templates to account for attenuation properties of the phantom material. We present a PET/MRI phantom ...built from MRI-visible material for which attenuation correction (AC) can be performed using the standard MRI-based AC.
Methods
A water-fillable phantom was 3D-printed with a commercially available MRI-visible polymer. The phantom had a cylindrical shape and the fillable compartment consisted of a homogeneous region and a region containing solid rods of different diameters. The phantom was filled with a solution of water and 18FFDG. A 30 min PET/MRI acquisition including the standard Dixon-based MR-AC method was performed. In addition, a CT scan of the phantom was acquired on a PET/CT system.
From the Dixon in-phase, opposed-phase and fat images, a phantom-specific AC map (Phantom MR-AC) was produced by separating the phantom material from the water compartment using a thresholding-based method and assigning fixed attenuation coefficients to the individual compartments. The PET data was reconstructed using the Phantom MR-AC, the original Dixon MR-AC, and an MR-AC just containing the water compartment (NoWall-AC) to estimate the error of ignoring the phantom walls. CT-based AC was employed as the reference standard. Average %-differences in measured activity between the CT corrected PET and the PET corrected with the other AC methods were calculated.
Results
The phantom housing and the liquid compartment were both visible and distinguishable from each other in the Dixon images and allowed the segmentation of a phantom-specific MR-based AC. Compared to the CT-AC PET, average differences in measured activity in the whole water compartment in the phantom of −0.3%, 9.4%, and −24.1% were found for Dixon phantom MR-AC, MR-AC, and NoWall-AC based PET, respectively. Average differences near the phantom wall in the homogeneous region were −0.3%, 6.6%, and −34.3%, respectively. Around the rods, activity differed from the CT-AC PET by 0.7%, 8.9%, and −45.5%, respectively.
Conclusion
The presented phantom material is visible using standard MR sequences, and thus, supports the use of standard, phantom-independent MR measurements for MR-AC in PET/MRI phantom studies.
OBJECTIVE: Impaired muscular mitochondrial function is related to common insulin resistance in type 2 diabetes. Mitochondrial diseases frequently lead to diabetes, which is mostly attributed to ...defective β-cell mitochondria and secretion. RESEARCH DESIGN AND METHODS: We assessed muscular mitochondrial function and lipid deposition in liver (hepatocellular lipids HCLs) and muscle (intramyocellular lipids IMCLs) using ³¹P/¹H magnetic resonance spectroscopy and insulin sensitivity and endogenous glucose production (EGP) using hyperinsulinemic-euglycemic clamps combined with isotopic tracer dilution in one female patient suffering from MELAS (myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) syndrome and in six control subjects. RESULTS: The MELAS patient showed impaired insulin sensitivity (4.3 vs. 8.6 ± 0.5 mg · kg⁻¹ · min⁻¹) and suppression of EGP (69 vs. 94 ± 1%), and her baseline and insulin-stimulated ATP synthesis were reduced (7.3 and 8.9 vs. 10.6 ± 1.0 and 12.8 ± 1.3 μmol · l⁻¹ · min⁻¹) compared with those of the control subjects. HCLs and IMCLs were comparable between the MELAS patient and control subjects. CONCLUSIONS: Impairment of muscle mitochondrial fitness promotes insulin resistance and could thereby contribute to the development of diabetes in some patients with the MELAS syndrome.
Quality assurance (QA) in magnetic resonance imaging (MRI) requires test objects. ‘Phantoms’ provided by MR manufacturers are homogeneously filled spheres or cylinders, and commercially available ...products are often too small for abdominal imaging, particularly for enlarged polycystic kidneys. Here we present the design, manufacturing and testing of a dedicated, yet versatile, abdominal MRI phantom, that can be reproduced with relatively low costs. The phantom mimics a human abdomen in size and shape and comprises seven test fluids, representing various tissue types at 3 T. The conductivity and permittivity of the test fluids match the average abdomen and kidney with a relative permittivity (ε) 65 and 72 as well as conductivity 0.6 and 0.7 S/m, respectively. The T
1
and T
2
relaxation times cover healthy average abdomen and kidney tissue values (T
1(abd)
: 856 ms and T
1(kid)
: 1,106 ms; T
2(abd)
: 52 ms and T
2(kid)
: 67 ms), intermediate (T
1
: 1,183 ms and 1,271 ms; T
2
: 128 and 189 ms), and disease values for (polycystic) kidney (T
1
: 1,428 ms, 1,561 ms and 1763 ms; T
2
: 319 ms, 424 and 647 ms). T
1
and T
2
relaxation times were stable over 73 weeks. Our reasonably priced, durable and reproducible abdominal phantom enables single and multi-center QA for future collaborative studies aiming for various challenges around abdominal and, particularly, kidney imaging.
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