Challenges in Pharmacokinetic Modelling of [18F]fluoro-PEG-folate PET/CT Imaging in Epithelial Ovarian Cancer Patients.

PURPOSE: To describe the pharmacokinetic properties of the [18F]fluoro-polyethylene glycol(PEG)-folate radiotracer in PET/CT imaging of patients with advanced stage epithelial ovarian cancer (EOC). PROCEDURES: In five patients with advanced EOC (FIGO stage IIIB/IIIC, Fédération Internationale de Gynécologie et d'Obstétrique), a 90-min dynamic PET acquisition of the pelvis was performed directly after i.v. administration of 185 MBq [18F]fluoro-PEG6-folate. Arterial blood samples collected at nineteen timepoints were used to determine the plasma input function. A static volume of interest (VOI) for included tumor lesions was drawn manually on the PET images. Modelling was performed using PMOD software. Three different models (a 1-tissue compartment model (1T2k) and two 2-tissue compartment models, irreversible (2T3k) and reversible (2T4k)) were compared in goodness of fit with the time activity curves by means of the Akaike information criterion. RESULTS: The pharmacokinetic analysis in the pelvic area has proven to be much more challenging than expected. Only four out of 22 tumor lesions in five patients were considered suitable to perform modelling on. The remaining tumor lesions were inapt due to either low tracer uptake, small size, proximity to other [18F]fluoro-PEG6-folate -avid structures and/or displacement by abdominal organ motion in the dynamic scan. Data from the four analyzed tumor lesions suggest that the irreversible 2T3k may best describe the pharmacokinetics. All 22 lesions were immunohistochemically stained positive for the folate receptor alpha (FRα) after resection. CONCLUSION: Performing pharmacokinetic analysis in the abdominal pelvic region is very challenging. This brief article describes the challenges and pitfalls in pharmacokinetic analysis of a tracer with high physiological accumulation in the intestines, in case of lesions of limited size in the abdominal pelvic area.

Twisted pair transmission line coil - a flexible, self-decoupled and robust element for 7 T MRI.

OBJECTIVE: This study evaluates the performance of a twisted pair transmission line coil as a transceive element for 7 T MRI in terms of physical flexibility, robustness to shape deformations, and interelement decoupling. METHODS: Each coil element was created by shaping a twisted pair of wires into a circle. One wire was interrupted at the top, while the other was interrupted at the bottom, and connected to the matching circuit. Electromagnetic simulations were conducted to determine the optimal number of twists per length (in terms of B1+ field efficiency, SAR efficiency, sensitivity to elongation, and interelement decoupling properties) and for investigating the fundamental operational principle of the coil through fields streamline visualisation. A comparison between the twisted pair coil and a conventional loop coil in terms of B1+ fields, maxSAR10g, and stability of S11 when the coil was deformed was performed. Experimentally measured interelement coupling between individual elements of multichannel arrays was also investigated. RESULTS: Increasing the number of twists per length resulted in a more physically robust coil. Poynting vector streamline visualisation showed that the twisted pair coil concentrated most of the energy in the near field. The twisted pair coil exhibited comparable B1+ fields and improved maxSAR10g to the conventional coil but demonstrated exceptional stability with respect to coil deformation and a strong self-decoupling nature when placed in an array configuration. DISCUSSION: The findings highlight the robustness of the twisted pair coil, showcasing its stability under shape variations. This coil holds great potential as a flexible RF coil for various imaging applications using multiple-element arrays, benefiting from its inherent decoupling.

A flexible five-channel shielded-coaxial-cable (SCC) transceive neck coil for high-resolution carotid imaging at 7T.

PURPOSE: Imaging the carotid arteries at 7T ideally requires a flexible multichannel array that allows B1-shimming and conforms to different neck sizes. The major challenge is to minimize coupling between closely spaced coils and to make the coupling relatively insensitive to loading conditions. METHODS: We have designed a five-channel flexible transceive array composed of shielded-coaxial-cable coils placed on the anterior part of the neck and conforming to the anatomy. In vivo imaging of the carotid arteries in three subjects has been performed. RESULTS: The measured noise correlation matrices show the decoupling level between the individual elements to be -12.5 dB and better. Anatomical localizer imaging of the carotids shows both carotids in every subject well visualized after B1-shimming. In vivo black-blood, carotid images were acquired with very high in-plane spatial resolution (0.25 × 0.25 mm2 ) with clear depiction of the vessel walls. CONCLUSIONS: The flexibility of the proposed coil has been demonstrated by imaging subjects with different neck circumferences. To the best of our knowledge, the in-plane resolution of 0.25 × 0.25 mm2 is the highest reported at 7T.

Design and characterization of receive-only surface coil arrays at 3T with integrated solid high permittivity materials.

A receive-only surface coil array for 3 Tesla integrating a high-permittivity material (HPM) with a relative permittivity of 660 was designed and constructed and subsequently its performance was evaluated and compared in terms of transmit field efficiency and specific absorption ratio (SAR) during transmission, and signal-to-noise ratio during reception, with a conventional identically-sized surface coil array. Finite-difference time-domain simulations, bench measurements and in-vivo neck imaging on three healthy volunteers were performed using a three-element surface coil array with integrated HPMs placed around the larynx. Simulation results show an increase in local transmit efficiency of the body coil of ~10-15% arising from the presence of the HPM. The receiver efficiency also increased by approximately 15% close to the surface. Phantom experiments confirmed these results. In-vivo scans using identical transmit power resulted in SNR gains throughout the laryngeal area when compared with the conventional surface coil array. In particular specifically around the carotid arteries an average SNR gain of 52% was measured averaged over the three subjects, while in the spine an average of 20% SNR gain was obtained.

Shielded-coaxial-cable coils as receive and transceive array elements for 7T human MRI.

PURPOSE: To investigate the use of shielded-coaxial-cable (SCC) coils as elements for multi-channel receive-only and transceive arrays for 7T human MRI and to compare their performance with equivalently sized conventional loop coils. METHODS: The SCC coil element consists of a coaxial loop with interrupted central conductor at the feed-point side and an interrupted shield at the opposite point. Inter-element decoupling, transmit efficiency, and sample heating were compared with results from conventional capacitively segmented loop coils. Three multichannel arrays (a 4-channel receive-only array and 8- and 5-channel transceive arrays) were constructed. Their inter-element decoupling was characterized via measured noise correlation matrices and additionally under different flexing conditions of the coils. Thermal measurements were performed and in vivo images were acquired. RESULTS: The measured and simulated B1+ maps of both SCC and conventional loops were very similar. For all the arrays constructed, the inter-element decoupling was much greater for the SCC elements than the conventional ones. Even under high degrees of flexion, the coupling coefficients were lower than -10 dB, with a much smaller frequency shift than for the conventional coils. CONCLUSION: Arrays constructed from SCC elements are mechanically flexible and much less sensitive to changes of the coil shape from circular to elongated than arrays constructed from conventional loop coils, which makes them suitable for construction of size adjustable arrays.

Improvements in High Resolution Laryngeal Magnetic Resonance Imaging for Preoperative Transoral Laser Microsurgery and Radiotherapy Considerations in Early Lesions.

As the benefits, limitations, and contraindications of transoral laser microsurgery (TLM) in glottic carcinoma treatments become better defined, pretreatment imaging has become more important to assess the case-specific suitability of TLM and to predict functional outcomes both for treatment consideration and patient counseling. Magnetic resonance imaging (MRI) is the preferred modality to image such laryngeal tumors, even though imaging the larynx using MRI can be difficult. The first challenge is that there are no commercial radiofrequency (RF) coils that are specifically designed for imaging the larynx, and performance in terms of coverage and signal-to-noise ratio is compromised using general-purpose RF coils. Second, motion in the neck region induced by breathing, swallowing, and vessel pulsation can induce severe image artifacts, sometimes rendering the images unusable. In this paper, we design a dedicated RF coil array, which allows high quality high-resolution imaging of the larynx. In addition, we show that introducing respiratory-triggered acquisition improves the diagnostic quality of the images by minimizing breathing and swallowing artifacts. Together, these developments enable robust, essentially artifact-free images of the full larynx with an isotropic resolution of 1 mm to be acquired within a few minutes.

Modular transmit/receive arrays using very-high permittivity dielectric resonator antennas.

PURPOSE: Dielectric resonator antenna (DRAs) are compact structures that exhibit low coupling between adjacent elements and therefore can be used as MRI transmit arrays. In this study, we use very high permittivity materials to construct modular flexible transceive arrays of a variable numbers of elements for operation at 7T. METHODS: DRAs were constructed using rectangular blocks of ceramic (lead zirconate titanate, εr = 1070) with the transverse electric (TE)01 mode tuned to 298 MHz. Finite-difference time-domain simulations were used to determine the B1 and specific absorption rate distributions. B1+ maps were acquired in a phantom to validate the simulations. Performance was compared to an equally sized surface coil. In vivo images were acquired of the wrist (four elements), ankle (seven elements), and calf muscle (16 elements). RESULTS: Coupling between DRAs spaced 5 mm apart on a phantom was -18.2 dB compared to -9.1 dB for equivalently spaced surface coils. DRAs showed a higher B1+ intensity close to the antenna but a lower penetration depth compared to the surface coil. CONCLUSION: DRAs show very low coupling compared to equally sized surface coils and can be used in transceive arrays without requiring decoupling networks. The penetration depth of the current DRA geometry means they are ideally suited to imaging of extremities. Magn Reson Med 79:1781-1788, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Design of a dielectric resonator receive array at 7 Tesla using detunable ceramic resonators.

Ceramic-based dielectric resonators can be used for high frequency magnetic resonance imaging and microscopy. When used as elements in a transmit array, the intrinsically low inter-element coupling allows flexibility in designing different geometric arrangements for different regions-of-interest. However, without being able to detune such resonators, they cannot be used as elements in a receive-only array. Here, we propose and implement a method, based on mode-disruption, for detuning ceramic-based dielectric resonators to enable them to be used as receive-only elements.