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.

Free-breathing radial imaging using a pilot-tone radiofrequency transmitter for detection of respiratory motion.

PURPOSE: To describe an approach for detection of respiratory signals using a transmitted radiofrequency (RF) reference signal called Pilot-Tone (PT) and to use the PT signal for creation of motion-resolved images based on 3D stack-of-stars imaging under free-breathing conditions. METHODS: This work explores the use of a reference RF signal generated by a small RF transmitter, placed outside the MR bore. The reference signal is received in parallel to the MR signal during each readout. Because the received PT amplitude is modulated by the subject's breathing pattern, a respiratory signal can be obtained by detecting the strength of the received PT signal over time. The breathing-induced PT signal modulation can then be used for reconstructing motion-resolved images from free-breathing scans. The PT approach was tested in volunteers using a radial stack-of-stars 3D gradient echo (GRE) sequence with golden-angle acquisition. RESULTS: Respiratory signals derived from the proposed PT method were compared to signals from a respiratory cushion sensor and k-space-center-based self-navigation under different breathing conditions. Moreover, the accuracy was assessed using a modified acquisition scheme replacing the golden-angle scheme by a zero-angle acquisition. Incorporating the PT signal into eXtra-Dimensional (XD) motion-resolved reconstruction led to improved image quality and clearer anatomical depiction of the lung and liver compared to k-space-center signal and motion-averaged reconstruction, when binned into 6, 8, and 10 motion states. CONCLUSION: PT is a novel concept for tracking respiratory motion. Its small dimension (8 cm), high sampling rate, and minimal interaction with the imaging scan offers great potential for resolving respiratory motion.

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.

A rigid, stand-off hybrid dipole, and birdcage coil array for 7 T body imaging.

PURPOSE: To design a robust and patient friendly radiofrequency coil array (8-channel transmit and 16-channel receive) for cross-sectional body imaging at 7 T, and to improve our understanding of the combination of dipole and loop like elements for ultra high field strengths. METHODS: The hybrid coil array was optimized in eletromagnetic simulations. Considered array candidates were the dipole, loop and birdcage array. The winning design was constructed and the signal-to-noise (SNR) was compared to a close fitting array at 3 T. Transmit and receive properties for different body sizes were assessed, and multi-parametric maps were acquired with the Plug-and-Play MRF method. RESULTS: The winning design consists of a dipole array for transceive combined with a birdcage array for receive only. The central SNR improved by a factor of 3 as compared to a 3 T system with a local receive array. A transmit efficiency between 2.4 and 3.9 µT/kW, a specific absorption rate efficiency of 0.25 to 0.53 µT/W/kg, and a high SNR was achieved in the center for the targeted patient population. CONCLUSION: The constructed coil array is easy to handle, safe, and patient friendly, allowing further development of abdominal imaging at 7 T. Quantitative MRI in the abdomen is possible with Plug-and-Play MRF using the designed coil array. Magn Reson Med 80:822-832, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Travelling-wave nuclear magnetic resonance.

Nuclear magnetic resonance (NMR) is one of the most versatile experimental methods in chemistry, physics and biology, providing insight into the structure and dynamics of matter at the molecular scale. Its imaging variant-magnetic resonance imaging (MRI)-is widely used to examine the anatomy, physiology and metabolism of the human body. NMR signal detection is traditionally based on Faraday induction in one or multiple radio-frequency resonators that are brought into close proximity with the sample. Alternative principles involving structured-material flux guides, superconducting quantum interference devices, atomic magnetometers, Hall probes or magnetoresistive elements have been explored. However, a common feature of all NMR implementations until now is that they rely on close coupling between the detector and the object under investigation. Here we show that NMR can also be excited and detected by long-range interaction, relying on travelling radio-frequency waves sent and received by an antenna. One benefit of this approach is more uniform coverage of samples that are larger than the wavelength of the NMR signal-an important current issue in MRI of humans at very high magnetic fields. By allowing a significant distance between the probe and the sample, travelling-wave interaction also introduces new possibilities in the design of NMR experiments and systems.

A Hybrid Volume-Surface Integral Equation Method for Rapid Electromagnetic Simulations in MRI.

OBJECTIVE: We developed a hybrid volume surface integral equation (VSIE) method based on domain decomposition to perform fast and accurate magnetic resonance imaging (MRI) simulations that include both remote and local conductive elements. METHODS: We separated the conductive surfaces present in MRI setups into two domains and optimized electromagnetic (EM) modeling for each case. Specifically, interactions between the body and EM waves originating from local radiofrequency (RF) coils were modeled with the precorrected fast Fourier transform, whereas the interactions with remote conductive surfaces (RF shield, scanner bore) were modeled with a novel cross tensor train-based algorithm. We compared the hybrid-VSIE with other VSIE methods for realistic MRI simulation setups. RESULTS: The hybrid-VSIE was the only practical method for simulation using 1 mm voxel isotropic resolution (VIR). For 2 mm VIR, our method could be solved at least 23 times faster and required 760 times lower memory than traditional VSIE methods. CONCLUSION: The hybrid-VSIE demonstrated a marked improvement in terms of convergence times of the numerical EM simulation compared to traditional approaches in multiple realistic MRI scenarios. SIGNIFICANCE: The efficiency of the novel hybrid-VSIE method could enable rapid simulations of complex and comprehensive MRI setups.

Use of positron emission tomography/ computed tomography in the initial staging of head and neck squamous cell carcinoma: Accuracy in evaluation of the primary site of the tumor, metastases to cervical lymph nodes, and distant metastases.

AIM: Our study aimed to evaluate the use of positron emission tomography/computed tomography (PET/CT) in the initial staging of head and neck squamous cell carcinoma (HNSCC), including assessment of local and distant spread of the disease. We also aimed to compare the accuracy of PET/CT in the evaluation of human papillomavirus (HPV) positive and HPV-negative oropharyngeal carcinoma. MATERIAL AND METHODS: This single-center, prospective study was conducted between August 2016 and September 2021. A total of 198 patients with HNSCC who underwent PET/CT within the primary staging were included. We compared PET/CT results with histological findings. We calculated the accuracy, sensitivity, specificity, and positive and negative predictive values to assess the primary tumor, cervical lymph nodes, and distant metastases. RESULTS: PET/CT showed a high success rate (32%) in revealing the primary site of carcinoma of unknown primary (CUP). The accuracy of PET/CT in displaying the primary tumor, cervical lymph node metastases, and distant metastases was 89.4%, 85.4%, and 87.4%, respectively. The method provided high sensitivity but lower specificity in all three areas. Specifically, PET/CT showed low specificity in the assessment of small tumors (75%), metastatic involvement of cervical lymph nodes (69.6%), and HPV-positive oropharyngeal carcinoma (55.6%). CONCLUSIONS: The high accuracy of PET/CT to identify distant metastases and whole-body staging in one diagnostic step accelerated primary staging and resulted in earlier commencement of therapy. However, it also led to an overestimation of clinical findings and thus to extensive surgical treatment, especially in patients with small tumors, metastatic involvement of cervical lymph nodes, and HPV- positive oropharyngeal carcinoma. PET/CT is also useful for CUP diagnostics.

A 13C/31P surface coil to visualize metabolism and energetics in the rodent brain at 3 Tesla.

PURPOSE: We constructed a 13C/31P surface coil at 3 T for studying cancer metabolism and bioenergetics. In a single scan session, hyperpolarized 13C-pyruvate MRS and 31P MRS was carried out for a healthy rat brain. METHODS: All experiments were carried out at 3 Tesla. The multinuclear surface coil was designed as two coplanar loops each tuned to either the 13C or 31P operating frequency with an LCC trap on the 13C loop. A commercial volume proton coil was used for anatomical localization and B0 shimming. Single tuned coils operating at either the 13C or 31P frequency were built to evaluate the relative performance of the multinuclear coil. Coil performance metrics consisted of measuring Q factor ratio, calculating system input power using a single-pulse acquisition, and acquiring SNR and flip angle maps using 2D CSI sequences. To observe in vivo spectra, a bolus of hyperpolarized [1-13C] pyruvate was administered via tail vein. In vivo13C and endogenous 31P spectra were obtained in a single scan session using 1D slice selective acquisitions. RESULTS: When compared with single tuned surface coils, the multinuclear coil performance showed a decrease in Q factor ratio, SNR, and transmit efficiency. Flip angle maps showed adequate flip angles within the phantom when the transmit voltage was set using an external phantom. Results show good detection of 13C labeled lactate, alanine, and bicarbonate in addition to ATP from 31P MRS. CONCLUSIONS: The coil enables obtaining complementary information within a scan session, thus reducing the number of trials and minimizing biological variability for studies of metabolism and bioenergetics.

Traveling-wave RF shimming and parallel MRI.

At sufficiently high Larmor frequencies, traveling electromagnetic waves along a magnet bore can be used for remote magnetic resonance excitation and detection, effectively using the bore as a waveguide. So far, this approach has relied only on the lowest waveguide modes and thus has not supported multiple-channel operation for radiofrequency shimming and parallel imaging. In this work, this limitation is addressed by establishing a larger number of propagating modes and tapping their spatial field diversity with multiple waveguide ports. The number of available modes is increased by loading with dielectric inserts; the ports are implemented by stub and loop couplers at the end of a waveguide extension. The resulting traveling-wave array, operated at 298 MHz in a 7T whole-body magnet, is shown to enable radiofrequency shimming as well as parallel imaging with commonly used acceleration factors. The last part of the study concerns the amount of dielectric loading that is required. For the given Larmor frequency and bore dimensions, it is found that rather few water-filled inserts, occupying ∼5% of the bore cross-section, are sufficient for effective parallel imaging.

[The occurrence of microcarcinomas in the patients after thyroidectomy--retrospective analysis]. / Výskyt mikrokarcinomu stítné zlázy u operovaných pacientu--retrospektivní analýza.

BACKGROUND: Microcarcinomas, minimum carcinomas, are tumours, which are in clinical practice defined as tumours < or = 1 cm in size. WHO defines thyroid microcarcinomas as tumours < or = 2 cm in size, which have different biological behaviour. The aim of the study was to analyze the occurrence of MC in post-operative patients. METHODS: Using retrospective analysis we evaluated the occurrence of thyroid microcarcinoma in post-operative patients. Except for basic demographic data, carcinoma size and histological variance, the occurrence of bilateral impairment, presence of multi-focuses and occurrence of regional throat metastases were considerd. RESULTS: From 2004 to 2008 thyroid surgeries were performed in 400 patients. Microcarcinoma was diagnosed in 34 patients (8.5%), 5 men and 29 women. The average age of patients with microcarcinoma was 52 years, similarly to other patients undergoing surgery. Histologically, 32 cases (94%) were papillary carcinoma, from which 4 cases were papillary follicular and 2 were follicular carcinomas. There were multifocal findings of microcarcinomas in 5 patients (15%), and 4 patients (12%) had bilateral involvement. The average size of the tumours was 5 mm, sd 2.6. Two patients (6%) had metastases in the lymph nodes of the neck. Total thyroidectomies were carried out in 32 patients (94%) and hemithyroidectomies in 2 patients (6%). Five patients (15%), i. e. both patients with metastases in the lymph nodes of the neck and three patients with bilateral multifocal carcinomas underwent postoperative adjuvant radioiodine 131I ablation therapy. CONCLUSIONS: Due to the possibility of the future growth, metastasizing andreoccurrence, microcarcinomas cannot be considered harmless or almost insignificant findings. The increased risk of the MC occurrence was found in chronic lymphoplasmocellular thyroiditis (17%).

Autologous blood injection for the treatment of chronic recurrent temporomandibular joint dislocation.

PURPOSE: Many different surgical techniques for the treatment of chronic recurrent temporomandibular joint (TMJ) dislocation have been described. This article discusses a technique of autologous blood injection to the TMJ for treatment of chronic recurrent TMJ dislocation. MATERIALS AND METHODS: Twenty-five patients diagnosed with chronic recurrent TMJ dislocation were treated by bilateral injections of autologous blood into the upper joint space and around the TMJ capsules bilaterally. RESULTS: Eighty percent had a successful outcome and required no further treatment at their 1-year follow-up. CONCLUSION: This procedure has proven to be safe, simple, and cost effective for the treatment of chronic recurrent TMJ dislocation.

Standard surgical versus percutaneous dilatational tracheostomy in intensive care patients.

OBJECTIVE: The present prospective randomized trial compared surgical tracheostomy (ST) and percutaneous dilatational tracheostomy (PDT) in intensive care unit (ICU) patients in terms of outcomes and complications. METHODS: Between January 2003 and December 2005 tracheostomies were performed on critically ill ICU patients in Medical Faculty Hospital in Prague, with a random allocation of 105 patients for ST and 100 for PDT. RESULTS: The 2 groups did not differ significantly in terms of basic demographic characteristics or length of endotracheal intubation prior to the procedure. Following the procedures, the 2 groups did not differ significantly in terms of the time required for decannulation, decannulated patients or mortalities. Post-mortem examination showed that both groups were similar in terms of placement of the tracheostomy tube. Surgical tracheostomy was found to take longer time to perform than PDT (p<0.001). In terms of early postoperative complications, PDT was associated with a higher rate of postoperative bleeding compared to ST (p=0.0302). CONCLUSION: Percutaneous dilatational tracheostomy is a simpler and faster technique to perform, but is associated with a higher occurrence of early complications, particularly postoperative bleeding.

Laryngotracheal stenosis in critically ill patients.

CONCLUSIONS: Prolonged artificial pulmonary ventilation by tracheostomy tube (>30 days) doubled the risk of stenosis (relative risk, RR = 2.04, p = 0.002). Critically ill patients with repeated tracheotomies were more than six times likely to experience stenosis (RR = 6.44, p< 0.001) than other critically ill patients. OBJECTIVE: In this retrospective study, we describe the occurrence of laryngotracheal stenosis (LTS) in critically ill patients after elective tracheostomy who had undergone treatment for LTS at the Na Homolce Hospital in Prague, Czech Republic. METHODS: We studied 28 patients who were diagnosed with symptomatic LTS. Basic major demographic data, duration of mechanical ventilation, onset of tracheal stenosis after decannulation, and tracheostomy type (percutaneous dilatational or surgical tracheostomy) were recorded. The number of patients requiring repeated tracheostomies was also recorded. RESULTS: Neither the demographic data nor the type of tracheostomy represented statistically significant risk factors. The risk factors for LTS were prolonged artificial pulmonary ventilation using the tracheostomy tube (p = 0.005) and repeated tracheostomy (p< 0.001). The mean onset of stenosis symptoms was 53.7 days (range 2-300 days), with a median of 58 days. Stenosis involved the trachea in 20 patients, subglottis in five cases, and glottis and subglottis in three cases. Seven patients (25%) underwent a tracheal resection and primary end-to-end reconstruction. One patient underwent laryngotracheoplasty with dilatation. The procedure was endoscopic in 18 patients (64.3%). Two patients (7.1%) received permanent tracheostomies.