7 Tesla MRI with a transmit/receive loopless antenna and B₁-insensitive selective excitation

Abstract

Purpose Use of external coils with internal detectors or conductors is challenging at 7 Tesla (T) due to radiofrequency (RF) field (B1) penetration, B1-inhomogeneity, mutual coupling, and potential local RF heating. The present study tests whether the near-quadratic gains in signal-to-noise ratio and field-of-view with field-strength previously reported for internal loopless antennae at 7T can suffice to perform MRI with an interventional transmit/receive antenna without using any external coils. Methods External coils were replaced by semi-rigid or biocompatible transmit/receive loopless antennae requiring only a few Watts of peak RF power. Slice selection was provided by spatially selective B1-insensitive composite RF pulses that compensate for the antenna’s intrinsically nonuniform B1-field. Power was adjusted to maintain local temperature rise ≤1° C. Fruit, intravascular MRI of diseased human vessels in vitro, and MRI of rabbit aorta in vivo are demonstrated. Results Scout MRI with the transmit/receive antennae yielded a ≤10 cm cylindrical field-of-view, enabling subsequent targeted localization at ~100 μm resolution in 10-50 s and/or 50 μm MRI in ~2 min in vitro, and 100–300 μm MRI of the rabbit aorta in vivo. Conclusion A simple, low-power, one-device approach to interventional MRI at 7T offers the potential of truly high-resolution MRI, while avoiding issues with external coil excitation and interactions at 7T.

Acceleration and motion‐correction techniques for high‐resolution intravascular MRI

“…As in any undersampling scheme, there is some SNR loss. A possible remedy is 7T IVMRI which potentially offers a quadratic SNR improvement with field strength . In quantifying reconstruction artifacts, the simple PSF analysis presented here closely approximates the more sophisticated analysis of nonlinear CS reconstruction reported elsewhere .…”

Monitoring local heating around an interventional MRI antenna with RF radiometry

“…Moreover, due to the reciprocity principle, the loopless antenna radiometer is most sensitive to temperature in the region that heats the most: that near the cable-whip junction. 17 It is indeed surprising that the peak 1 g average local temperature in the sample at the antenna is only 1.36 times the temperature measured by the radiometer from the entire antenna, as given by the peak H-factor introduced herein. This attests to the sensitivity of the radiometer for detecting local peak temperature changes associated with the presence of the antenna and provides a potential "selfmonitoring" function that could be used for routine assessment of device safety.…”

“…17 Therefore, the radiometric temperature measured by the loopless antenna is expected to be most sensitive to temperature changes at locations with high P d that lie nearest the cable-whip junction. We introduce a "H-factor," H(r), defined as the coefficient relating the local ∆T at an arbitrary location to the measured radiometric temperature rise, ∆T radiometer .…”

“…11 Interventional MRI (iMRI) employing active small-diameter catheters, guidewires, needles, etc., as MRI detectors is an application of MRI technology that poses potential safety risks due to RF heating close to the conducting elements. [12][13][14][15] This is compounded when the internal device is used for both MRI transmission and reception in intravascular applications 16,17 and at ultrahigh magnetic field strengths (B 0 > 3 T) 17,18 where RF penetration effects may limit MRI access to deep tissue. [19][20][21] In addition, when MRI devices are used for targeted thermal therapy, for example, to enhance gene therapy, 22 or for RF ablation as a treatment for cardiac arrhythmias, 23 or for tumor ablation, 24 having an independent means of measuring local temperature for assessing safety and titrating thermal therapy in real-time is key.…”

2D radially compensating excitation pulse in combination with an internal transceiver antenna for 3D MRI of the rectum at 7 T

“…Recently, promising results in rabbits have been demonstrated, using an internal antenna with nonadiabatic (composite) RF pulses. 17 In this work, we use multidimensional RF pulses [18][19][20] which are designed to compensate the inhomogeneous B 1 + field of an internal single channel transmit and receive antenna to obtain a homogeneous flip angle in the human rectum in vivo. We compare the 2D radially compensating excitation (RACE) pulse with an adiabatic BIR-4 RF pulse 21 and show that we can increase the efficiency of the pulse and substantially decrease the SAR, hence enabling high resolution MRI of the rectum.…”