TY  - JOUR
A1  - Lagemaat, Miriam W.
A1  - Breukels, Vincent
A1  - Vos, Eline K.
A1  - B., Adam
A1  - Uden, Mark J. van
A1  - Orzada, Stephan
A1  - Bitz, Andreas
A1  - Maas, Marnix C.
A1  - Scheenen, Tom W. J.
T1  - ¹H MR spectroscopic imaging of the prostate at 7T using spectral-spatial pulses
JF  - Magnetic Resonance in Medicine
N2  - Purpose
To assess the feasibility of prostate ¹H MR spectroscopic imaging (MRSI) using low-power spectral-spatial (SPSP) pulses at 7T, exploiting accurate spectral selection and spatial selectivity simultaneously.

Methods
A double spin-echo sequence was equipped with SPSP refocusing pulses with a spectral selectivity of 1 ppm. Three-dimensional prostate ¹H-MRSI at 7T was performed with the SPSP-MRSI sequence using an 8-channel transmit array coil and an endorectal receive coil in three patients with prostate cancer and in one healthy subject. No additional water or lipid suppression pulses were used.

Results
Prostate ¹H-MRSI could be obtained well within specific absorption rate (SAR) limits in a clinically feasible time (10 min). Next to the common citrate signals, the prostate spectra exhibited high spermine signals concealing creatine and sometimes also choline. Residual lipid signals were observed at the edges of the prostate because of limitations in spectral and spatial selectivity.

Conclusion
It is possible to perform prostate ¹H-MRSI at 7T with a SPSP-MRSI sequence while using separate transmit and receive coils. This low-SAR MRSI concept provides the opportunity to increase spatial resolution of MRSI within reasonable scan times.
Y1  - 2016
U6  - http://dx.doi.org/10.1002/mrm.25569
SN  - 1522-2594
VL  - 75
IS  - 3
SP  - 933
EP  - 945
PB  - International Society for Magnetic Resonance in Medicine
ER  - 
TY  - JOUR
A1  - Orzada, Stephan
A1  - Ladd, Mark E.
A1  - Bitz, Andreas
T1  - A method to approximate maximum local SAR in multichannel transmit MR systems without transmit phase information
JF  - Magnetic Resonance in Medicine
N2  - Purpose
To calculate local specific absorption rate (SAR) correctly, both the amplitude and phase of the signal in each transmit channel have to be known. In this work, we propose a method to derive a conservative upper bound for the local SAR, with a reasonable safety margin without knowledge of the transmit phases of the channels.

Methods
The proposed method uses virtual observation points (VOPs). Correction factors are calculated for each set of VOPs that prevent underestimation of local SAR when the VOPs are applied with the correct amplitudes but fixed phases.

Results
The proposed method proved to be superior to the worst-case calculation based on the maximum eigenvalue of the VOPs. The mean overestimation for six coil setups could be reduced, whereas no underestimation of the maximum local SAR occurred. In the best investigated case, the overestimation could be reduced from a factor of 3.3 to a factor of 1.7.

Conclusion
The upper bound for the local SAR calculated with the proposed method allows a fast estimation of the local SAR based on power measurements in the transmit channels and facilitates SAR monitoring in systems that do not have the capability to monitor transmit phases
Y1  - 2016
U6  - http://dx.doi.org/10.1002/mrm.26398
SN  - 1522-2594
VL  - 78
IS  - 2
SP  - 805
EP  - 811
PB  - International Society for Magnetic Resonance in Medicine
ER  -