Cornelis A.T. van den Berg

Simultaneous B 1+ homogenization and specific absorption rate hotspot suppression using a magnetic resonance phased array transmit coil

In high‐field MRI severe problems with respect to B  1+ uniformity and specific absorption rate (SAR) deposition pose a great challenge to whole‐body imaging. In this study the potential of a phased array transmit coil is investigated to simultaneously reduce B  1+ nonuniformity and SAR deposition. This was tested by performing electromagnetic simulations of a phased array TEM coil operating at 128 MHz loaded with two different homogeneous elliptical phantoms and four dielectric patient models. It was shown that the wave interference of a circularly polarized RF field with an ellipse and a pelvis produces largely identical B  1+ and electric field patterns. Especially for obese patients, this results in large B  1+ nonuniformity and global areas with elevated SAR deposition. It is demonstrated that a phased array transmit coil can reduce these phenomena. The technique was especially successful in suppressing SAR hotspots with a decrease up to 50%. The application of optimized settings for an ellipse to the patient models leads to comparable results as obtained with the patient‐specific optimizations. This suggests that generic phase/amplitude port settings are possible, requiring no preinformation about patient‐specific RF fields. Such a scheme would, due to its simultaneous B  1+ homogenization and extra SAR margin, have many benefits for whole‐body imaging at 3 T. Magn Reson Med 57:577–586, 2007.

B 1+ Phase mapping at 7 T and its application for in vivo electrical conductivity mapping

In this study, a new approach to measure local electrical conductivity in tissue is presented, which is based on the propagating B  1+ phase and the homogeneous Helmholtz equation. This new MRI technique might open future opportunities for tumor and lesion characterization based on conductivity differences, while it may also find application in radio frequency safety assessment. Prerequisites for conductivity mapping using only the B  1+ phase (instead of the complex B  +1 field) are addressed. Furthermore it was found that the B  1+ phase can be derived directly from the measurable transceive phase arg(B  +1 B  −1 ) in the head. Validation for a human head excited by a 7 T‐birdcage coil using simulations and measurements showed that it is possible to measure in vivo conductivity patterns in the brain using B  1+ phase information only. Conductivity contrast between different brain tissues is clearly observed. The measured mean values for white matter, gray matter and cerebrospinal fluid differed 54%, 26%, and −13% respectively from literature values. The proposed method for B  1+ phase measurements is very suited for in vivo applications, as the measurement is short (less than a minute per imaged slice) and exposes the patient to low RF power, contrary to earlier proposed approaches. Magn Reson Med, 2012.

7 T body MRI: B1 shimming with simultaneous SAR reduction

The high frequency of the radiofrequency (RF) fields used in high field magnetic resonance imaging (MRI) results in electromagnetic field variations that can cause local regions to have a large specific absorption rate (SAR) and/or a low excitation. In this study, we evaluated the use of a B1 shimming technique which can simultaneously improve the B+1 homogeneity and reduce the SAR for whole body imaging at 7 T. Optimizations for four individual anatomies showed a reduction up to 74% of the peak SAR values with respect to a quadrature excitation and a simultaneous improvement of the B+1 homogeneity varying between 39 and 75% for different optimization parameters. The average SAR was reduced with approximately 50% for all optimizations. The optimized phase and amplitude settings from an elliptical phantom model were applied to four realistic human anatomy models to evaluate whether a generic application without prior knowledge of the detailed human anatomy is possible. This resulted in an average improvement of the B+1 homogeneity of 37% and an average reduction of the maximum and average SAR of 50 and 55%, respectively. It can be concluded that this generic method can be used as a simple method to improve the prospects of 7 T body imaging.

The fractionated dipole antenna: A new antenna for body imaging at 7 Tesla.

Dipole antennas in ultrahigh field MRI have demonstrated advantages over more conventional designs. In this study, the fractionated dipole antenna is presented: a dipole where the legs are split into segments that are interconnected by capacitors or inductors.

SAR and power implications of different RF shimming strategies in the pelvis for 7T MRI

To determine the best radiofrequency (RF) shimming method for 7 T body imaging that provides sufficient B1+ excitation inside the target region while energy deposition (SAR) and power demands are as low as possible and that does not incorporate anatomy specific electric field information inside the patient models, as this information is not available in practice.

Simultaneous MRI diffusion and perfusion imaging for tumor delineation in prostate cancer patients

BACKGROUND AND PURPOSE A study was performed to investigate if we can quantify if the two imaging modalities diffusion weighted imaging (DWI) and dynamic contrast-enhanced (DCE)-MRI are consistent in what voxels they determine as being suspicious of tumor tissue. MATERIAL AND METHODS Twenty-one patients with biopsy proven prostate cancer underwent a DWI and a DCE-MRI scan. These scans were compared using a receiver operating curve (ROC) analysis, where either one of the two imaging modalities was thresholded and taken as a reference. The resulting area under the curve (AUC) reflects the consistency between target delineations based on the two imaging techniques. This analysis was performed for the complete prostate and the peripheral zone (PZ). RESULTS Consistency between DWI and DCE-MRI parameter maps varied greatly between patients. Values of the AUC up to 0.90 were found. However, on average AUC values were 0.60. The AUC values were related to the patients PSA and clinical stage. CONCLUSIONS Large variation in consistency between the two imaging modalities was found. This did not depend on the precise thresholds used. For making decisions on dose painting in the prostate, the knowledge about the inconsistency must be taken into account.

Focal Salvage Guided by T2-Weighted and Dynamic Contrast-Enhanced Magnetic Resonance Imaging for Prostate Cancer Recurrences

PURPOSE Salvage treatment of the entire prostate for local recurrent cancer after primary radiotherapy is associated with high toxicity rates. Our goal was to show that, using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) for the visualization of a recurrence, focal salvage treatment can be performed, with, potentially, a reduction in toxicity. METHODS AND MATERIALS We performed MRI, including a DCE sequence, in 7 patients with biopsy-proven locally recurrent prostate cancer. The specific regions of interest suspect for containing tumor were delineated using DCE and T(2)-weighted MRI scans. Subsequently, focal salvage high-dose-rate brachytherapy plans were created to illustrate the principle of focal salvage. Total salvage treatment plans were also created for comparison. RESULTS The transfer constant (K(trans)) values from the DCE were 0.33-0.67 min(-1) for areas suspect for tumor and 0.07-0.25 min(-1) for normal tissue. In 4 cases, a focal salvage plan could be generated; 93-100% of the gross tumor volume was covered with the prescribed dose, with relative sparing of the bladder, rectum, and urethra. In the total salvage plans, 24-53% of the gross tumor volume was covered, and the organs at risk received high doses. In 3 cases, a focal salvage plan could not be created because of multifocal tumor, seminal vesicle extension, or capsular extension. CONCLUSION Focal salvage treatment plans can be created in patients with local recurrent prostate cancer after radiotherapy. DCE-MRI supports the localization of the target area. This could lead to less toxicity in patients with local recurrent prostate cancer.

Electrical Properties Tomography in the Human Brain at 1.5, 3, and 7T: A Comparison Study

To investigate the effect of magnetic field strength on the validity of two assumptions (namely, the “transceive phase assumption” and the “phase‐only reconstruction”) for electrical properties tomography (EPT) at 1.5, 3, and 7T.

Enhancement of Magnetic Resonance Imaging with Metasurfaces

It is revealed that the unique properties of ultrathin metasurface resonators can improve magnetic resonance imaging dramatically. A metasurface formed when an array of metallic wires is placed inside a scanner under the studied object and a substantial enhancement of the radio-frequency magnetic field is achieved by means of subwavelength manipulation with the metasurface, also allowing improved image resolution.

Specific absorption rate intersubject variability in 7T parallel transmit MRI of the head

Patient‐specific radiofrequency shimming in high‐field MRI strengthens the need for online, patient‐specific specific absorption rate (SAR) monitoring. Numerical simulation is currently most effective for this purpose but may require a patient‐specific dielectric model. To investigate whether a generic model may be combined with a safety factor to account for variation within the population, generic SAR behavior is studied for 7T MRI of the head. For six detailed head models, radiofrequency fields were simulated for an eight‐channel parallel transmit array. SAR behavior is studied through comparison of the eigenvalues/eigenvectors of the local Q‐matrices. Furthermore, numerical radiofrequency shimming experiments without and with SAR constraints were performed where SAR during optimization was evaluated on a generic model. In both cases, the ability of different generic models to predict actual SAR levels was evaluated. The largest eigenvalue distribution is comparable between models. Radiofrequency shimming without constraints improves the |B  +1 | homogeneity while the SAR increases substantially. Imposing constraints on SAR during optimization, estimating SAR on a generic model, was effective. A safety factor of 1.4 was found to be sufficient. Generic SAR behavior makes a generic head model a practical alternative to patient‐specific models and allows effective |B  +1 | shimming with SAR constraints. Magn Reson Med, 2013.