Abstract
When using ultrahigh-field MR systems (7T), the variations in the RF magnetic field can lead to significant loss in image uniformity. To optimize the overall MR image quality, the image region is divided into multiple smaller regions of interest (the ROIs), which can be independently optimized using transmit array optimization techniques including RF shimming, to improve RF magnetic fields and image intensity. Electromagnetic numerical simulations and corresponding transverse magnetization (|Mt|) acquired using the Bloch equation-based MRI simulator are used to evaluate the proposed method. Compared to the simulation results of quadrature driving method, mean and standard deviation (SD) of |Mt| in the full image (an inner diameter of 500 mm) were improved 47% (mean) and 48% (SD), whereas 94% (max) and 97% (mean) improved in the unaveraged SAR using the proposed method. The uniformity of |Mt| acquired using the method was especially improved in the peripheral region of the selected phantom image compared to that of other methods. The proposed method using multiple independently optimized ROIs and numerical simulations significantly improved the uniformity of |Mt| body images at 7T. This technique would be generally applicable to any high-field strength MR systems, which generate short RF wavelengths compared to the field of view.
Highlights
To acquire magnetic resonance (MR) images of high resolution and increased signal-to-noise ratio (SNR), a higher static magnetic field (|B0|) is needed [1,2,3,4,5,6]. is is because the intrinsic SNR (ISNR) in magnetic resonance imaging (MRI) is proportional to the square of |B0|(ISNR ∝ |B0|2) [1, 2]
Each region of multiple regions of interest (ROIs) was optimized individually starting with reference data and combined into a full MR image using individually calculated |Mt| of each region and MR simulator [14, 16, 17]. e reference data, i.e., initial values used for optimization of amplitude and phase of each channel, were selected based on the numerical simulations
Note that the mean and standard deviation (SD) of |Mt| were changed less than 10% until the inner diameter (ID) was 100 mm (Table 1). e optimization of |Mt| uniformity using RF shimming for each IDs was performed based on the previous research [17]
Summary
To acquire magnetic resonance (MR) images of high resolution and increased signal-to-noise ratio (SNR), a higher static magnetic field (|B0|) is needed [1,2,3,4,5,6]. is is because the intrinsic SNR (ISNR) in magnetic resonance imaging (MRI) is proportional to the square of |B0|(ISNR ∝ |B0|2) [1, 2]. Collins et al [11] showed that optimization of amplitude and phase of current at each transmit channel; i.e., RF shimming could significantly improve flip angle uniformity for highfield human head MRIs with operating frequencies between 300 MHz and 600 MHz. Setsompop et al [18] showed that custom designed spatially selective RF waveforms, called spoke pulses, with an 8-channel transmit array could improve image uniformity compared to the standard slice selective method for 3.0 T human head MRI. Erturk et al [21] designed a 16-channel transmit and receive array, consisting of eight combined loop-dipole elements, which improved
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