B1 mapping and angiography methods for magnetic resonance imaging [electronic resource]
Keywords: Electrical Engineering.

Magnetic resonance imaging (MRI) is a safe and reliable technology for medical imaging. Different types of image contrast can be generated using radiofrequency (RF) pulses. One commonly desired application is imaging the variation in the transmit RF field itself. The RF field excitation amplitude (B1) is often obtained by acquiring a B1 map. A new phase-based B1 mapping method is designed. This method uses two adiabatic full-passage pulses with different magnitudes that are applied as successive refocusing pulses. These pulses result in a linear relationship between image phase and B1 field strength that is insensitive to the repetition time, off-resonance effects, T1, and T2. Using this method, B1 mapping can be localized to a slice or 3D volume, with a spin-echo acquisition that is appropriate for fast projection measurements. This new method is shown to agree well with the existing Bloch-Siegert B1 mapping method for both phantom and in vivo B1 measurements at 1.5T, 3T and 7T. The method is further designed to use flattened hyperbolic secant (HSn) pulses, which have lower adiabatic thresholds. This substitution of HSn pulses for the original adiabatic pulses requires the HSn pulse parameters to be optimized to minimize phase sensitivity to off-resonance frequency. The performance of the method using HSn pulses is validated via simulation and in vivo at 3T, showing that the adiabatic threshold of the method is reduced, which improves the method's applicability for measurement of low B1. Furthermore, the phase sensitivity of the method increases with increasing n. Finally, the presented B1 mapping method's ability to acquire fast projections make it suitable for acquiring B1 distribution data by encoding in B1 instead of image space. To encode in B1, multiple projections of a volume are acquired along the same direction, each using a different phase sensitivity to B1. Using a convex optimization formulation, histograms of the B1 distribution estimates of the imaging volume are reconstructed. The B1 distribution measurement is verified by comparing measured B1 distributions to distributions calculated from reference spatial B1 maps. Phantom measurements using a surface coil and wire show that when there is a high dynamic B1 range, measured B1 distributions using the proposed method more accurately estimate the B1 distribution than a low-resolution spatial B1 map of the same volume. The method may provide faster estimates of a B1 field when there is high spatial B1 variation, such as when imaging with a guidewire. Another desired contrast in MRI is between arterial blood and other sources of signal in the body. Magnetic resonance angiography (MRA) is a technique used to diagnose arterial disease. Often the goal of MRA is to suppress any signal that comes from the veins and surrounding background tissues, while enhancing the signal in the arteries. RF pulses can be designed to generate arterial contrast directly, without the need for introducing external contrast agents into the body. A new method for non-contrast-enhanced MRA that uses acceleration-selective excitation is designed and tested. An acceleration-selective (AS) pulse is designed by interspersing RF sub-pulses with tri-lobed gradient pulses to create acceleration-selective excitation, allowing for signal from tissue, fat, and venous blood to be suppressed without the use of subtractive imaging techniques, while blood that is accelerating in the arteries remains untouched. The AS pulse uses phase-cycled refocusing pulses to increase robustness to off-resonance frequency and B1 variations. The method is validated in phantoms and in vivo at 1.5T. Two acceleration-selective pulses were designed, each using a different MLEV phase-cycling scheme for refocusing, and in vivo results from each sequence were compared. Both sequences demonstrated the ability of the method to selectively image the arteries based on arterial acceleration selectivity. Because background suppression is achieved directly, this method offers better immunity to motion over subtractive techniques. The results are comparable to previously attained in vivo results using a velocity-selective sequence; however, acceleration-selectivity may prove more robust for patient imaging because the pulse passband can be centered around zero acceleration, and does not need to be shifted with changes in arterial acceleration.