Complex B1 mapping and electrical properties imaging of the human brain using a 16‐channel transceiver coil at 7T

The electric properties of biological tissue provide important diagnostic information within radio and microwave frequencies, and also play an important role in specific absorption rate calculation which is a major safety concern at ultrahigh field. The recently proposed electrical properties tomography (EPT) technique aims to reconstruct electric properties in biological tissues based on B₁ measurement. However, for individual coil element in multichannel transceiver coil which is increasingly utilized at ultrahigh field, current B₁‐mapping techniques could not provide adequate information (magnitude and absolute phase) of complex transmit and receive B₁ which are essential for electrical properties tomography, electric field, and quantitative specific absorption rate assessment. In this study, using a 16‐channel transceiver coil at 7T, based on hybrid B₁‐mapping techniques within the human brain, a complex B₁‐mapping method has been developed, and in vivo electric properties imaging of the human brain has been demonstrated by applying a logarithm‐based inverse algorithm. Computer simulation studies as well as phantom and human experiments have been conducted at 7T. The average bias and standard deviation for reconstructed conductivity in vivo were 28% and 67%, and 10% and 43% for relative permittivity, respectively. The present results suggest the feasibility and reliability of proposed complex B₁‐mapping technique and electric properties reconstruction method. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Recovery pattern of motor reflex after a single bout of neuromuscular electrical stimulation session

We aimed at determining the recovery pattern of neural properties of soleus muscle after a single bout of neuromuscular electrical stimulation (NMES) session. Thirteen subjects performed an NMES exercise (75 Hz, 40 contractions, 6.25 s per contraction). Maximal voluntary contraction (MVC), H‐reflex at rest and during voluntary contraction fixed at 60% of MVC (respectively, H_max and H_sup) and volitional (V) wave were measured before and during the recovery period following this exercise [i.e., immediately after, 2 h (H2), 2 days (D2) and 7 days (D7)]. MVC exhibited an immediate and a delayed declines at 2 days (respectively, ?29.8±4.6%, P<0.001; ?13.0±3.4%, P<0.05). Likewise, V/M_sup was decreased immediately and 2 days after NMES session (respectively, ?43.3±11.6%, P<0.05; 35.3±6.6%, P<0.05). The delayed decrements in MVC and V‐wave occurred concomitantly with muscle soreness peak (P<0.001). It could be concluded that motor command alterations after an NMES resistance session contributed to the immediate and also to the delayed decreases in MVC without affecting resting and active H‐reflex excitability. These results suggested that spinal circuitry function of larger motoneurons was inhibited by NMES (as indicated by the depressed V‐wave responses) contrary to the smaller one (indicated by the unchanged H‐reflex responses).     

Electrical conductivity imaging by magnetic resonance electrical impedance tomography (MREIT).

Magnetic resonance electrical impedance tomography (MREIT) is a recently developed imaging technique that combines MRI and electrical impedance tomography (EIT). In MREIT, cross-sectional electrical conductivity images are reconstructed from the internal magnetic field density data produced inside an electrically conducting subject when an electrical current is injected into the subject. In this work the results of an electrical conductivity imaging experiment are presented, along with some practical considerations regarding MREIT. The MREIT experiment was performed with a 0.3 Tesla MRI system on a phantom made of two compartments with different electrical conductivities. The current density inside the phantom was measured by the MR current density imaging (MRCDI) technique. The measured current density was then used for conductivity image reconstruction by the J-substitution algorithm. The conductivity phantom images obtained with an injection current of 28mA showed conductivity errors of about 25.5%.     

Imaging anisotropic and viscous properties of breast tissue by magnetic resonance-elastography.

MR-elastography is a new technique for assessing the viscoelastic properties of tissue. One current focus of elastography is the provision of new physical parameters for improving the specificity in breast cancer diagnosis. This analysis describes a technique to extend the reconstruction to anisotropic elastic properties in terms of a so-called transversely isotropic model. Viscosity is treated as being isotropic. The particular model chosen for the anisotropy is appealing because it is capable of describing elastic shear anisotropy of parallel fibers. The dependence of the reconstruction on the particular choice of Poisson's ratio is eliminated by extracting the compressional displacement contribution using the Helmholtz-Hodge decomposition. Results are presented for simulations, a polyvinyl alcohol breast phantom, excised beef muscle, and measurements in two patients with breast lesions (invasive ductal carcinoma and fibroadenoma). The results show enhanced anisotropic and viscous properties inside the lesions and an indication for preferred fiber orientation.     

Experimental performance evaluation of multi‐echo ICNE pulse sequence in magnetic resonance electrical impedance tomography

Latest experimental results in magnetic resonance electrical impedance tomography (MREIT) demonstrated high‐resolution in vivo conductivity imaging of animal and human subjects using imaging currents of 5 to 9 mA. Externally injected imaging currents induce magnetic flux density distributions, which are affected by a conductivity distribution. Since we extract the induced magnetic flux density images from MR phase images, it is essential to reduce noise in the phase images. In vivo human and disease model animal experiments require reduction of imaging current amplitudes and scan times. In this article, we investigate a multi‐echo based MREIT pulse sequence where we utilize a remaining time after the first echo within one TR to obtain more echo signals. It also allows us to prolong the total current injection time. From phantom and animal imaging experiments, we found that this method significantly reduces the noise level in measured magnetic flux density images. We describe experimental validation of the multi‐echo sequence by comparing its performance with a single‐echo method using 3 mA imaging currents. The proposed method will be advantageous for an imaging region with long T2 values such as the brain and knee. Depending on T2 values, we suggest using two or three echoes in future experimental studies. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

The influence of physical activity during youth on structural and functional properties of the Achilles tendon

Achilles tendinopathy is a highly prevalent sports injury. Animal studies show a growth response in tendons in response to loading in the immature phase but not after puberty maturation. The aim of this investigation was to examine the structural and material properties in long distance runners who were either physically active (HAY) or inactive (LAY) in young age. Twelve men in HAY group and eight men in LAY group participated. Structural, functional, and biochemical properties of Achilles tendon were estimated from magnetic resonance imaging, ultrasound video recordings, mechanical tests, and tendon biopsies, respectively. There was no difference between the groups with respect to tendon cross‐sectional area or tendon free length. There was no difference between the groups with respect to maximal force or mechanical properties. The collagen content, enzymatic and nonenzymatic cross‐link density did not differ between the groups, nor did collagen fibril density, diameter, and area. There was a correlation between age and pentosidine/collagen within the groups [(HAY: P < 0.05 and r² = 0.47) and (LAY: P < 0.05 and r² = 0.52)]. The data suggest that high or low activity during youth did not appreciably influence the mechanical, structural, or biochemical properties of the Achilles tendon in adult long distance runners.     

The dependence of radiofrequency induced pacemaker lead tip heating on the electrical conductivity of the medium at the lead tip

Radiofrequency induced pacemaker lead tip heating is one of the main reasons magnetic resonance imaging (MRI) is contraindicated for patients with pacemakers. The objective of this work was to evaluate the dependence of pacemaker lead tip heating during MRI scanning on the electrical conductivity of the medium surrounding the pacemaker lead tip. The effect of conductivity was measured using hydroxyethyl cellulose, polyacrylic acid, and saline with conductivities ranging from 0 to 3 S/m which spans the range of human tissue conductivity. The maximum lead tip heating observed in polyacrylic acid was 50.4 °C at 0.28 S/m, in hydroxyethyl cellulose the maximum was 36.8 °C at 0.52 S/m, and in saline the maximum was 12.5 °C at 0.51 S/m. The maximum power transfer theorem was used to calculate the relative power deposited in the solution based on the characteristic impedance of the pacemaker lead and test solution impedance. The results demonstrate a strong correlation between the relative power deposited and pacemaker lead tip heating for hydroxyethyl cellulose and saline solutions. Maximum power deposition occurred when the impedance of the solution matched the pacemaker lead impedance. Pacemaker lead tip heating is dependent upon the electrical conductivity of the solution at the lead tip and should be considered when planning in vitro gel or saline experiments.     

Effect of alpine skiing training on tendon mechanical properties in older men and women

Strain is one of the parameters determining tendon adaptation to mechanical stimuli. The aim of this study was to test whether the patellar tendon strain induced during recreational alpine skiing would affect tendon mechanical properties in older individuals. Twenty-two older males and females (67 ± 2 years) were assigned to a 12-week guided skiing programme (IG) and 20 aged-matched volunteers served as controls (CG). Patellar tendon mechanical properties and cross-sectional area (CSA) were measured before and after training, with combined dynamometry and ultrasonography scanning. None of the variables changed significantly in the CG after training. In the IG, tendon stiffness and Young's modulus were increased (respectively, 14% and 12%, P<0.01), without any significant change in tendon CSA. In addition, changes in tendon stiffness were blunted in women (9%) compared with men (19%). Serum IGF-1 concentration tended to be lower in women (-19%, P=0.07). These results demonstrate that the mechanical stimulus induced by alpine skiing is sufficient to elicit adaptive changes in patellar tendon mechanical and material properties in older subjects. Furthermore, the present sex-specific adaptations are consistent with previous reports of lower collagen metabolic responsiveness in women and may be underpinned by anthropometric and metabolic differences.     

Cerebral electrical phenomena elicited by alcohol

Summary The authors investigated the EEG changes in young clinically healthy individuals for some hours after the administration of various quantities and qualities of alcohol during and after the absorption and excretion phase. During the period of EEG examination, hyperventilation and light stimuli of different frequencies were employed.It was stablished that — in accordance with literary data — a slowing of the cerebral electric activity and an increase of the amplitudes ensued, which changed diversely under the influence of hyperventilation and light stimulus according to the individual sensitivity.In the cases where pathological cerebral electric phenomena not associated with clinical disease symptoms were observed — as e.g. wave activity of pathological (epileptic-like) character affected by light stimuli — a significant impairment of the electric activity ensued already before the administration of alcohol being most marked chiefly in the hours following alcohol excretion. The authors call attention to the dangers of the post-alcohol state, mainly in the case of individuals who are clinically healthy, but who show pathological cerebral electroactivity during the EEG examination if subjected to physiological stimuli (hyperventilation, light stimuli). On the basis of their results, they recommend the routine introduction of EEG at the aptitude tests for drivers.

---

Zusammenfassung Die Verfasser haben nach der Eingabe von Alkohol verschiedener Qualität und Quantität die EEG-Veränderungen in der Resorptions- bzw. in der Exkretions-phase sowie in den folgenden Stunden an klinisch gesunden Personen untersucht. Bei den EEG-Untersuchungen wurden Hyperventilation und Lichtreize mit verschiedenen Frequenzen angewendet.Im Einklang mit Literaturangaben wurde eine Verminderung der elektrischen Aktivität, jedoch ein Anstieg der Amplituden festgestellt, die sich unter der Wirkung von Lichtreizen und HV auf verschiedene Weise — der persönlichen Empfindlichkeit gemäß — veränderten.In solchen Fällen, bei denen pathologische Phänomene auftraten, z. B. pathologische Wellenaktivität (vom epileptischen Typ) auf Lichtreize, ist eine wesentliche Veränderung der elektrischen Aktivität schon vor der Einnahme des Alkoholgetränkes, besonders in den Stunden nach der Exkretion, eingetreten.Wichtig sind die Gefahren der postalkoholischen Phase, besonders bei solchen klinisch gesunden Personen, die während der EEG-Untersuchungen — physiologischen Reizen ausgesetzt (Hyperventilation, Lichtreize) — eine pathologische cerebrale Aktivität zeigen.Auf Grund der Ergebnisse erscheint es empfehlenswert, die routineartige Anwendung der EEG-Untersuchungen bei den Fahrtauglichkeitsprüfungen der Kraftwagenführer einzuführen.