A transmit/receive radiofrequency array for imaging the carotid arteries at 7 Tesla: coil design and first in vivo results.

OBJECTIVE: To develop a transmit/receive radiofrequency (RF) array for magnetic resonance imaging (MRI) of the carotid arteries at 7 T. The prototype is characterized in numerical simulations and bench measurements, and the feasibility of plaque imaging at 7 T is demonstrated in first in vivo images. MATERIALS AND METHODS: The RF phased array coil consists of 8 surface loop coils. To allow imaging of both sides of the neck, the RF array is divided into 2 coil clusters, each with 4 overlapping loop elements. For safety validation, numerical computations of the RF field distribution and the corresponding specific absorption rate were performed on the basis of a heterogeneous human body model. To validate the coil model, maps of the transmit B1(+) field were compared between simulation and measurement. In vivo images of a healthy volunteer and a patient (ulcerating plaque and a 50% stenosis of the right internal carotid artery) were acquired using a 3-dimensional FLASH sequence with a high isotropic spatial resolution of 0.54 mm as well as using pulse-triggered proton density (PD)/T2-weighted turbo spin echo sequences. RESULTS: Measurements of the S-parameters yielded a reflection and isolation of the coil elements of better than -18 and -13 dB, respectively. Measurements of the g-factor indicated good image quality for parallel imaging acceleration factors up to 2.4. A similar distribution and a very good match of the absolute values were found between the measured and simulated B1(+) transmit RF field for the validation of the coil model. In vivo images revealed good signal excitation of both sides of the neck and a high vessel-to-background image contrast for the noncontrast-enhanced 3-dimensional FLASH sequence. Imaging at 7 T could depict the extent of stenosis, and revealed the disruption and ulcer of the plaque. CONCLUSIONS: This study demonstrates that 2 four-channel transmit/receive RF arrays for each side of the neck is a suitable concept for in vivo MRI of the carotid arteries at 7 Tesla. Further studies are needed to explore and exploit the full potential of 7 T high-field MRI for carotid atherosclerotic plaque imaging.

An eight-channel phased array RF coil for spine MR imaging at 7 T.

OBJECTIVE: To develop a transmit/receive radiofrequency (RF) array for MR imaging of the human spine at 7 T. The prototype is characterized in simulations and bench measurements, and the feasibility of high-resolution spinal cord imaging at 7 T is demonstrated in in vivo images of volunteers. MATERIALS AND METHODS: The RF phased array consists of 8 overlapping surface loop coils with a dimension of 12 cm x 12 cm each. Bench measurements were obtained with a phantom made of body-simulating liquid and assessed with a network analyzer. For safety validation, numerical computations of the RF field distribution and the corresponding specific absorption rate were performed on the basis of 3 different human body models. In vivo images of 3 volunteers (2 with a documented scoliosis) were acquired using a 3D-FLASH sequence with a high spatial resolution of 0.57 mm isotropic. RESULTS: The 7 T transmit/receive RF coil could be easily integrated into the patient table for examinations of the cervicothoracic or thoracolumbosacral spine. Comparable results were found for all 3 numerical calculations using different human body models. Measurements of the g-factor indicated good image quality for parallel imaging acceleration factors up to 2.7 along the head-feet direction, which could be validated in the in vivo images. The in vivo images demonstrated very fine anatomic features such as the longitudinal ligaments or the venous drainage through the vertebral bodies. A largely homogeneous excitation over an extensive field-of-view of 40 cm could be obtained. CONCLUSIONS: These early results indicate that a multichannel transmit/receive phased array RF coil can be used for in vivo spine imaging at 7 T, thereby rendering high-resolution spine imaging a promising new application in 7 T clinical research.

High-resolution MRI of the human parotid gland and duct at 7 Tesla.

OBJECTIVES: MR techniques have been reported as an alternative to conventional sialography. High-field systems (7 T) provide new contrasts coupled with increased signal-to-noise ratio, and hence higher spatial resolution. To our knowledge, no measurements of the parotid gland at 7 T have been reported. Therefore, our study aimed to optimize sequences for high-field MR imaging of the parotid gland and duct, as well as the facial nerve at 7 T and show the potential of high field imaging. MATERIALS AND METHODS: A 7 T whole-body scanner was used together with a 10-cm-diameter loop coil. Various GRE (MEDIC, DESS) and TSE (PD/T2, STIR) sequences were optimized and subsequently tested on 4 healthy volunteers and 4 patients. High-resolution images were compared with 1.5 T images both quantitatively (signal-to-noise ratio, contrast-to-noise) and qualitatively (visual rating of 2 independent readers). RESULTS: The high 0.6 mm isotropic resolution of the 3D DESS sequence was very useful for defining an oblique orientation with most of the duct being in-plane for subsequent imaging. With the MEDIC sequence, very fine branches of the duct were visible; furthermore, MEDIC yielded a very good depiction of lymph nodes. Severe specific absorption rate problems were observed with the STIR sequence at 7 T. Gland tissue in tumor patients can be well characterized with the PD/T2 TSE. Highest contrast-to-noise between duct and gland was achieved with the 7 T DESS. At 1.5 T, only the STIR sequence showed comparable quality to the overall superiority of the 7 T sequences. The facial nerve could only be depicted close to the skull base. CONCLUSION: MR imaging at 7 T provides excellent image contrast and resolution of the parotid gland and duct. The proposed protocol offers a noninvasive examination within about 30 minutes.