7 Tesla MRI: Proton imaging and RF pulse design

Motivation

Clinical Magnetic Resonance Imaging (MRI) it typically performed at main field strengths (B0) of 1.5 Tesla or 3 Tesla. However, since the beginning of MRI, there is a constant push to increase the field strength, because it increases the signal-to-noise ratio (SNR) and therefore allows for higher spatial resolution. Furthermore, it increases the spectral resolution in spectroscopic imaging, it allows for higher acceleration factors during image acquisition and – depending on the application – can provide stronger image contrast.

Due to these reasons, ultra-high-field (UHF) MRI systems operating at 7 Tesla and beyond are currently being investigated in a few research sites in Germany, including the DKFZ, and worldwide. In agreement with the advantages mentioned above, it has been shown that UHF imaging is beneficial for a large range of applications and physiological targets. Among those are vascular applications such as MR angiography and blood flow imaging, which are one focus of the group. Furthermore, studies are being conducted to demonstrate the benefit  for tumor diagnostic and characterization.

Despite the advantages of UHF MRI, acquiring images at this field strength is fairly challenging, particularly when targeting the human body. The challenges are mostly related to the proton Larmor frequency of approximately 300 MHz at 7 Tesla. In order to be able to image the protons a radiofrequency (RF) pulse is applied at Larmor frequency that excites the proton spins. However, the high frequency results in an RF wavelength in the human body of about 11 cm,  which matches dimensions of target organs. This leads to variations of the electromagnetic field, consisting of the electric (E) and magnetic (B1) component. The varying transmit B1 field, causes spatially dependent flip angles and thus spatially varying contrast, which is a substantial problem for diagnostic purposes. Furthermore, the varying E field results in local variations of the specific absorption rate (SAR), which is a safety relevant parameter.

Additional problems occur when the target organ is located in the thorax or abdomen due to physiological motion such as respiratory motion, bowel motion or cardiac motion. Those factors need to be taken into account for RF excitation, image acquisition and reconstruction.

The aim of this group is to address the above-mentioned challenges particularly by use of parallel transmission (pTX) techniques and dedicated reconstruction methods. A central goal of this group is that developments will be applied in-vivo at 7 Tesla in smaller sized studies. Among the different applications are quantitative imaging methods in cardiovascular acquisitions as well as acquisitions techniques for tumor applications.

Major topics:

-          PTX and RF pulse design for UHF head and body imaging

-          Quantitative blood flow imaging using 4D flow MRI

-          Accelerated Cardiac imaging using PTX at 7T

-          Quantitative imaging using MRF

-          Tumor imaging

MR angiogram obtain from 3-slab TOF acquisition with standard excitation (CP mode) (a) and 2-spoke pTX excitation (b). Corresponding flip angle maps at the center slice for each of the 3 slabs are shown in c+d).
© CMRR, University of Minnesota, Minneapolis, USA

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