C.A.T. Van den Berg

Design of a radiative surface coil array element at 7 T:the single-side adapted dipole antenna

Ultra high field MR imaging (≥7 T) of deeply located targets in the body is facing some radiofrequency‐field related challenges: interference patterns, reduced penetration depth, and higher Specific Absorbtion Ratio (SAR) levels. These can be alleviated by redesigning the elements of the transmit or transceive array. This is because at these high excitation field (B1) frequencies, conventional array element designs may have become suboptimal. In this work, an alternative design approach is presented, regarding coil array elements as antennas. Following this approach, the Poynting vector of the element should be oriented towards the imaging target region. The single‐side adapted dipole antenna is a novel design that fulfills this requirement. The performance of this design as a transmit coil array element has been characterized by comparison with three other, more conventional designs using finite difference time domain (FDTD) simulations and B  +1 measurements on a phantom. Results show that the B  +1 level at the deeper regions is higher while maintaining relatively low SAR levels. Also, the B  +1 field distribution is more symmetrical and more uniform, promising better image homogeneity. Eight radiative antennas have been combined into a belt‐like surface array for prostate imaging. T1‐weighted (T1W) and T2‐weighted (T2W) volunteer images are presented along with B  +1 measurements to demonstrate the improved efficiency. Magn Reson Med, 2011.

High-resolution MRI of the carotid arteries using a leaky waveguide transmitter and a high-density receive array at 7 T

A setup for 7T MRI of the carotid arteries in the neck was designed and constructed. Separate dedicated arrays were used for transmit and receive. For the transmit array, single‐side adapted dipole antennas were mounted on a dielectric pillow, which was shown to serve as a leaky waveguide, efficiently distributing B1 into the neck. Risk assessment was performed by simulations. Phantom measurements were performed to establish optimal positions of the antennas on the pillow. Using two antennas, a dual transmit setup was created. In vivo B1+ maps with different shim configurations were acquired to assess transmit performance. This effective transmit array was used in combination with a dedicated 30 channel small element receive coil. High‐resolution in vivo turbo spin echo images were acquired to demonstrate the excellent performance of the setup. Magn Reson Med 69:1186–1193, 2013.

Dipole antennas for ultrahigh-field body imaging: a comparison with loop coils

Although the potential of dipole antennas for ultrahigh‐field (UHF) MRI is largely recognized, they are still relatively unknown to the larger part of the MRI community. This article intends to provide electromagnetic insight into the general operating principles of dipole antennas by numerical simulations. The major part focuses on a comparison study of dipole antennas and loop coils at frequencies of 128, 298 and 400 MHz. This study shows that dipole antennas are only efficient radiofrequency (RF) coils in the presence of a dielectric and/or conducting load. In addition, the conservative electric fields (E‐fields) at the ends of a dipole are negligible in comparison with the induced E‐fields in the center. Like loop coils, long dipole antennas perform better than short dipoles for deeply located imaging targets and vice versa. When the optimal element is chosen for each depth, loop coils have higher B1+ efficiency for shallow depths, whereas dipole antennas have higher B1+ efficiency for large depths. The cross‐over point depth decreases with increasing frequency: 11.6, 6.2 and 5.0 cm for 128, 298 and 400 MHz, respectively. For single elements, loop coils demonstrate a better B1+/√SARmax ratio for any target depth and any frequency. However, one example study shows that, in an array setup with loop coil overlap for decoupling, this relationship is not straightforward. The overlapping loop coils may generate increased specific absorption rate (SAR) levels under the overlapping parts of the loops, depending on the drive phase settings. Copyright

On verification of hyperthermia treatment planning for cervical carcinoma patients.

Purpose: The aim of this study was to verify hyperthermia treatment planning calculations by means of measurements performed during hyperthermia treatments. The calculated specific absorption rate (SARcalc) was compared with clinically measured SAR values, during 11 treatments in seven cervical carcinoma patients. Methods: Hyperthermia treatments were performed using the 70 MHz AMC-4 waveguide system. Temperatures were measured using multisensor thermocouple probes. One invasive thermometry catheter in the cervical tumour and two non-invasive catheters in the vagina were used. For optimal tissue contact and fixation of the catheters, a gynaecological tampon was inserted, moisturized with distilled water (4 treatments), or saline (6 treatments) for better thermal contact. During one treatment no tampon was used. At the start of treatment the temperature rise (ΔTmeas) after a short power pulse was measured, which is proportional to SARmeas. The SARcalc along the catheter tracks was extracted from the calculated SAR distribution and compared with the ΔTmeas-profiles. Results: The correlation between ΔTmeas and SARcalc was on average R = 0.56 ± 0.28, but appeared highly dependent on the wetness of the tampon (preferably with saline) and the tissue contact of the catheters. Correlations were strong (R ∼ 0.85–0.93) when thermal contact was good, but much weaker (R ∼ 0.14–0.48) for cases with poor thermal contact. Conclusion: Good correlations between measurements and calculations were found when tissue contact of the catheters was good. The main difficulties for accurate verification were of clinical nature, arising from improper use of the gynaecological tampon. Poor thermal contact between thermocouples and tissue caused measurement artefacts that were difficult to correlate with calculations.

Characterization of transceive surface element designs for 7 tesla magnetic resonance imaging of the prostate: radiative antenna and microstrip

Ultra-high field magnetic resonance (≥7 tesla) imaging (MRI) faces challenges with respect to efficient spin excitation and signal reception from deeply situated organs. Traditional radio frequency surface coil designs relying on near-field coupling are suboptimal at high field strengths. Better signal penetration can be obtained by designing a radiative antenna in which the energy flux is directed to the target location. In this paper, two different radiative antenna designs are investigated to be used as transceive elements, which employ different dielectric permittivities for the antenna substrate. Their transmit and receive performances in terms of B(+)(1), local SAR (specific absorption rate) and SNR (signal-to-noise ratio) were compared using extensive electromagnetic simulations and MRI measurements with traditional surface microstrip coils. Both simulations and measurements demonstrated that the radiative element shows twofold gain in B(+)(1) and SNR at 10 cm depth, and additionally a comparable SAR peak value. In terms of transmit performance, the radiative antenna with a dielectric permittivity of 37 showed a 24% more favorable local SAR(10g avg)/(B(+)(1))(2) ratio than the radiative antenna with a dielectric permittivity of 90. In receive, the radiative element with a dielectric permittivity of 90 resulted in a 20% higher SNR for shallow depths, but for larger depths this difference diminished compared to the radiative element with a dielectric permittivity of 37. Therefore, to image deep anatomical regions effectively, the radiative antenna with a dielectric permittivity of 37 is favorable.

In vivo electric conductivity of cervical cancer patients based on B₁⁺ maps at 3T MRI.

The in vivo electric conductivity (σ) values of tissue are essential for accurate electromagnetic simulations and specific absorption rate (SAR) assessment for applications such as thermal dose computations in hyperthermia. Currently used σ-values are mostly based on ex vivo measurements. In this study the conductivity of human muscle, bladder content and cervical tumors is acquired non-invasively in vivo using MRI. The conductivity of 20 cervical cancer patients was measured with the MR-based electric properties tomography method on a standard 3T MRI system. The average in vivo σ-value of muscle is 14% higher than currently used in human simulation models. The σ-value of bladder content is an order of magnitude higher than the value for bladder wall tissue that is used for the complete bladder in many models. Our findings are confirmed by various in vivo animal studies from the literature. In cervical tumors, the observed average conductivity was 13% higher than the literature value reported for cervical tissue. Considerable deviations were found for the electrical conductivity observed in this study and the commonly used values for SAR assessment, emphasizing the importance of acquiring in vivo conductivity for more accurate SAR assessment in various applications.

Fast thermal simulations and temperature optimization for hyperthermia treatment planning, including realistic 3D vessel networks.

PURPOSE Accurate thermal simulations in hyperthermia treatment planning require discrete modeling of large blood vessels. The very long computation time of the finite difference based DIscrete VAsculature model (DIVA) developed for this purpose is impractical for clinical applications. In this work, a fast steady-state thermal solver was developed for simulations with realistic 3D vessel networks. Additionally, an efficient temperature-based optimization method including the thermal effect of discrete vasculature was developed. METHODS The steady-state energy balance for vasculature and tissue was described by a linear system, which was solved with an iterative method on the graphical processing unit. Temperature calculations during optimization were performed by superposition of several precomputed temperature distributions, calculated with the developed thermal solver. The thermal solver and optimization were applied to a human anatomy, with the prostate being the target region, heated with the eight waveguide 70 MHz AMC-8 system. Realistic 3D pelvic vasculature was obtained from angiography. Both the arterial and venous vessel networks consisted of 174 segments and 93 endpoints with a diameter of 1.2 mm. RESULTS Calculation of the steady-state temperature distribution lasted about 3.3 h with the original DIVA model, while the newly developed method took only ≈ 1-1.5 min. Temperature-based optimization with and without taking the vasculature into account showed differences in optimized waveguide power of more than a factor 2 and optimized tumor T90 differed up to ≈ 0.5°C. This shows the necessity to take discrete vasculature into account during optimization. CONCLUSIONS A very fast method was developed for thermal simulations with realistic 3D vessel networks. The short simulation time allows online calculations and makes temperature optimization with realistic vasculature feasible, which is an important step forward in hyperthermia treatment planning.

Improved RF performance of travelling wave MR with a high permittivity dielectric lining of the bore

Application of travelling wave MR to human body imaging is restricted by the limited peak power of the available RF amplifiers. Nevertheless, travelling wave MR advantages like a large field of view excitation and distant location of transmit elements would be desirable for whole body MRI. In this work, improvement of the B1+ efficiency of travelling wave MR is demonstrated. High permittivity dielectric lining placed next to the scanner bore wall effectively reduces attenuation of the travelling wave in the longitudinal direction and at the same time directs the radial power flow toward the load. First, this is shown with an analytical model of a metallic cylindrical waveguide with the dielectric lining next to the wall and loaded with a cylindrical phantom. Simulations and experiments also reveal an increase of B1+ efficiency in the center of the bore for travelling wave MR with a dielectric lining. Phantom experiments show up to a 2‐fold gain in B1+ with the dielectric lining. This corresponds to a 4‐fold increase in power efficiency of travelling wave MR. In vivo experiments demonstrate an 8‐fold signal‐to‐noise ratio gain with the dielectric lining. Overall, it is shown that dielectric lining is a constructive method to improve efficacy of travelling wave MR. Magn Reson Med 70:885–894, 2013.

Recommendations for MRI-based contouring of gross tumor volume and organs at risk for radiation therapy of pancreatic cancer

PURPOSE Local recurrence is a common and morbid event in patients with unresectable pancreatic adenocarcinoma. A more conformal and targeted radiation dose to the macroscopic tumor in nonmetastatic pancreatic cancer is likely to reduce acute toxicity and improve local control. Optimal soft tissue contrast is required to facilitate delineation of a target and creation of a planning target volume with margin reduction and motion management. Magnetic resonance imaging (MRI) offers considerable advantages in optimizing soft tissue delineation and is an ideal modality for imaging and delineating a gross tumor volume (GTV) within the pancreas, particularly as it relates to conformal radiation planning. Currently, no guidelines have been defined for the delineation of pancreatic tumors for radiation therapy treatment planning. Moreover, abdominal MRI sequences are complex and the anatomy relevant to the radiation oncologist can be challenging. The purpose of this study is to provide recommendations for delineation of GTV and organs at risk (OARs) using MRI and incorporating multiple MRI sequences. METHODS AND MATERIALS Five patients with pancreatic cancer and 1 healthy subject were imaged with MRI scans either on 1.5T or on 3T magnets in 2 separate institutes. The GTV and OARs were contoured for all patients in a consensus meeting. RESULTS An overview of MRI-based anatomy of the GTV and OARs is provided. Practical contouring instructions for the GTV and the OARs with the aid of MRI were developed and included in these recommendations. In addition, practical suggestions for implementation of MRI in pancreatic radiation treatment planning are provided. CONCLUSIONS With this report, we attempt to provide recommendations for MRI-based contouring of pancreatic tumors and OARs. This could lead to better uniformity in defining the GTV and OARs for clinical trials and in radiation therapy treatment planning, with the ultimate goal of improving local control while minimizing morbidity.

Thermal noise variance of a receive radiofrequency coil as a respiratory motion sensor.

Development of a passive respiratory motion sensor based on the noise variance of the receive coil array.