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Utrecht uses 7.0T MultiTransmit in oncology research

Best Practice
Klomp, Dennis Utrecht, University Medical Center Netherlands

7.0T MultiTransmit and other new technology used in oncology research

Researchers at University Medical Center Utrecht (UMCU, Utrecht, The Netherlands) are optimizing and developing Achieva 7.0T technology for MR imaging and spectroscopy studies related to oncology. MultiTransmit - standard or with eight channels - provides many opportunities to improve MR scanning. The group is using prostate spectroscopy to investigate biomarkers that might help determine treatment response. Phosphorous spectroscopy is explored for breast examination.
 Dennis Klomp, PhD The 7.0T team at University Medical Center Utrecht (missing: Peter Luijten, PhD)
Dennis Klomp, PhD
The 7.0T team at University Medical Center Utrecht (missing: Peter Luijten, PhD)

Developing 8-channel MultiTransmit for Achieva 7.0T

UMCU operates several Philips MRI systems for clinical use. In addition, multiple research projects are deployed on an Achieva 7.0T research system, installed in 2007, and an Achieva 3.0T system.

"The gains in 3.0T body imaging using MultiTransmit are substantial," says Dennis Klomp, PhD, of the Functional Medical Imaging group of Prof. Peter Luijten, Division of Radiology, UMCU. "Clinical results are demonstrating better and more consistent signal uniformity and faster scanning, particularly because of the optimized RF power deposition.

Considering the RF wavelength at 3.0T related to the human body size, similar dielectric shading effects might be expected for 7.0T imaging when restricted to the head. And, even more complicated, the body has the size of several wavelengths at 7.0T, requiring more steering capability to control image uniformity."

This is why he and his team developed an 8-channel MultiTransmit configuration for their 7.0T system. The RF signal from the Achieva 7.0T console is converted into eight amplitude and phase modulated signals that are fed to eight independent RF amplifiers. "We hope to achieve 7.0T imaging of the body that is as impressive as 7.0T brain MRI."

Dr. Klomp and his team use a coil that is close to the tissue to realize high flip angles with short pulses. Temperature probe measurements in phantoms and numerical simulations of the RF and electric fields in human models are used to assess the influence of MultiTransmit on local and global SAR values. "In close collaboration with the group of Dr. Nico van den Berg, Department of Radiotherapy, we have designed sophisticated calculation tools to optimize an RF pulse using eight independent RF channels for finding substantially reduced SAR values compared to conventionally fixed phase and amplitude settings of the RF pulse," he explains. "So, we not only make the RF field more uniform, but also spread heating - which generally occurs very locally - to a larger area, meaning that local SAR is reduced."

MultiTransmit assists in advancing other research applications at 7.0T

Dr. Klomp's team developed an approach that integrates MultiTransmit parallel excitation and SENSE parallel imaging using the same coil. Special in this setup is that the SENSE reference scan makes use of a traveling wave scan. "This way we can use MultiTransmit for steered excitation with minimized RF power deposition, while multiple coil elements are used for the highest acceleration of the acquisition." explains Dr. Klomp.

Apart from a reduction in local SAR and improved uniformity, MultiTransmit also accelerates volume selective excitation. "For instance we can inverse the B1 non-uniformities, resulting in images with uniform contrast," he says. "Or we can use a reduced field of view, which accelerates imaging even beyond the maximum acceleration that can be applied with conventional SENSE parallel imaging." " We hope to achieve 7.0T imaging of the body that is as impressive as 7.0T brain MRI."

"As volume selective RF pulses generally have relatively long durations, and are therefore sensitive to B0 nonuniformities and T2 relaxation effects, they have limited application," he adds. "With MultiTransmit, these pulse lengths can be reduced by a factor equal to the number of transmit channels, therefore making them applicable for practically any application."

"For brain spectroscopy, we built a MultiTransmit RF coil that fits very closely to the skull," explains Dr. Klomp. "MultiTransmit allows us to tune outer volume suppression to suppress the lipid signal from the skull while obtaining metabolic information from the brain. This is not possible with any fixed RF setting."

 Spectroscopy with MultiTransmit
Spectroscopy with MultiTransmit

MultiTransmit with 8 channels enables generation of two different optimized RF pulses in spectroscopic imaging, the first for outer volume suppression of subcutaneous fat (left image), the second for uniform excitation of the brain [1]. The spectra are from voxels of 2D Spectroscopic Imaging. Acquisition time is 6 minutes and pixel volume is 1 cc.

[1] VO Boer, I Voogt, H Kroeze, AH Westra, B van de Bank, PR Luijten, DWJ Klomp, Abstract ISMRM 2010.

MultiTransmit helps advance prostate spectroscopy research

In prostate spectroscopy, Dr. Klomp seeks to detect standard metabolites like choline, which is generally a marker for tumors. Now at 7.0T, polyamines are visible as well. At 3.0T or 1.5T, polyamines have a very low signal intensity due to the complex spin system, and overlap with choline and creatine signals. "At 7.0T, polyamine peaks are separated from choline and creatine. In addition, selective refocusing techniques further increase the signal intensity of polyamine by a factor of three." Analysis of biopsy materials has shown that polyamine levels are inversely related to the aggressiveness of the cancer, so the hope is that this marker may help to detect changes during radiotherapy in an earlier stage.

Dr. Klomp says he and his team employ a transmit/ receive endorectal coil for prostate spectroscopy. "MultiTransmit is tuned to realize a very efficient B1 field with only low power in the RF amplifiers, which is really not possible with a conventional volume coil. Combined with adiabatic RF pulses, uniformity in contrast can be maintained."

 MR Spectroscopy of the prostate
MR Spectroscopy of the prostate

Prostate spectroscopy at 7.0T in patients with a tumor using endorectal tranceivers for localized 1H MRS (left) and coil localized 31P MRS (right). RF shimming was applied with an 8-element external surface array to obtain T2-weighted TSE images for anatomy and tumor localization [2].

[2] C Arteaga, A Raaijmakers, N van de Berg, U van der Heide, PR Luijten, DWJ Klomp, Abstract ISMRM 2010.

31P spectroscopy in the breast at 7.0T

Whereas proton MR spectroscopy only gives information on total choline, Dr. Klomp is setting up phosphorus MR spectroscopy in the breast to look for multiple choline compounds. "With phosphorus spectroscopy, we hope to resolve four different metabolites involved in phospholipid metabolism, which may reveal more information about cell proliferation than proton spectroscopy, which only provides the total choline pool."


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Best Practice
Achieva 7.0T
1H, 7.0T, 7T, Body, Breast, MultiTransmit, Neuro, Prostate, Spectroscopy
 

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