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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC San Francisco Electronic Theses and Dissertations (2015)

Stroke and cardiovascular disease are major health threats in the United States. Treatment is typically done under X-ray fluoroscopy but MR imaging could provide physiological information and does not have ionizing radiation. An issue with MR is that tools have to be compatible in a magnetic field environment. Hence, the aim is to build an RC with satisfactory tip visualization at various orientations. It will be utilized in a stroke model involving swine where the catheter will be inserted in the femoral artery up to the carotid. There will also be testing the accuracy of the electromagnetic simulation software XFdtd.

A unique material, pyralux (by Dupont), was processed with photolithography to etch the design. The resonators were wrapped around catheters and tuning was done to achieve the proper resonant frequency for 3.0 T and 1.5 T MR scanners. Scans were made with vascular phantoms and mainly MRA (TOF GRE) protocols. Catheters were oriented perpendicular or parallel to the B0 magnetic field. OsiriX and MS excel were used to calculate the SNR of the catheter relative to the background.

Increasing flip angle, starting from 5 degrees and ending at 90 degrees, decreases the SNR and stronger signal is achieved at 1.5T than 3.0T with these parameters. There is not a significant different between SNR with the perpendicular and parallel orientations in the 3.0T. XFdtd successfully demonstrated the relationship between frequency, capacitance, and inductance with a single coil model but not with the double helix. A clot was successfully induced in the femoral artery of a pig and temperature mapping shows insignificant heating hazards.

The goal of the resonator is to visualize the tip, so wider bandwidth would be useful to account for frequency shifts caused by magnetic field disturbances. While choosing the ROI for analysis, selection was based off covering the catheter width and choosing the background such that any artifacts can be accounted for. Although the same TOF GRE sequence in previous successful cases were used, the new sequences tested this year were variable so they were not included in analysis.

From this study, it is shown that the resonator has similar SNR regardless of orientation in the GE 3.0 T scanner. It would be most beneficial to use low flip angles and currently the 1.5 T scanner.

    Cover page: Integrating Resonators to Catheters for MR-Guided Interventional Radiology
    • Article
    • Peer Reviewed
    UC San Francisco Previously Published Works (2017)

    Background

    It is technically challenging to design compact yet sensitive miniature catheter radio frequency (RF) coils for endovascular interventional MR imaging.

    Methods

    In this work, a new design method for catheter RF coils is proposed based on the coaxial transmission line resonator (TLR) technique. Due to its distributed circuit, the TLR catheter coil does not need any lumped capacitors to support its resonance, which simplifies the practical design and construction and provides a straightforward technique for designing miniature catheter-mounted imaging coils that are appropriate for interventional neurovascular procedures. The outer conductor of the TLR serves as an RF shield, which prevents electromagnetic energy loss, and improves coil Q factors. It also minimizes interaction with surrounding tissues and signal losses along the catheter coil. To investigate the technique, a prototype catheter coil was built using the proposed coaxial TLR technique and evaluated with standard RF testing and measurement methods and MR imaging experiments. Numerical simulation was carried out to assess the RF electromagnetic field behavior of the proposed TLR catheter coil and the conventional lumped-element catheter coil.

    Results

    The proposed TLR catheter coil was successfully tuned to 64 MHz for proton imaging at 1.5 T. B1 fields were numerically calculated, showing improved magnetic field intensity of the TLR catheter coil over the conventional lumped-element catheter coil. MR images were acquired from a dedicated vascular phantom using the TLR catheter coil and also the system body coil. The TLR catheter coil is able to provide a significant signal-to-noise ratio (SNR) increase (a factor of 200 to 300) over its imaging volume relative to the body coil.

    Conclusions

    Catheter imaging RF coil design using the proposed coaxial TLR technique is feasible and advantageous in endovascular interventional MR imaging applications.
      Cover page: Design of catheter radio frequency coils using coaxial transmission line resonators for interventional neurovascular MR imaging
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2014)
      Hypervariable loops of HIV-1 Env protein gp120 are speculated to play roles in the conformational transition of Env to the receptor binding-induced metastable state. Structural analysis of full-length Env-based immunogens, containing the entire V2 loop, displayed tighter association between gp120 subunits, resulting in a smaller trimeric diameter than constructs lacking V2. A prominent basal quaternary location of V2 and V3' that challenges previous reports would facilitate gp41-independent gp120-gp120 interactions and suggests a quaternary mechanism of epitope occlusion facilitated by hypervariable loops. Deletion of V2 resulted in dramatic exposure of basal, membrane-proximal gp41 epitopes, consistent with its predicted basal location. The structural features of HIV-1 Env characterized here provide grounds for a paradigm shift in loop exposure and epitope occlusion, while providing substantive rationale for epitope display required for elicitation of broadly neutralizing antibodies, as well as substantiating previous pertinent literature disregarded in recent reports.
        Cover page: Trimeric HIV Env provides epitope occlusion mediated by hypervariable loops
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