Simultaneous proton resonance frequency shift thermometry and T1 measurements using a single reference variable flip angle T1 method.

PURPOSE: Implement simultaneous proton resonance frequency (PRF) shift and T1 measurements with equivalent temporal resolution using a single reference variable flip angle method. This novel method allows for simultaneous thermometry in both aqueous and fatty tissue. METHODS: This method acquires a single reference image at the lower flip angle and all dynamic images at the higher angle. T1 is calculated using a single reference variable flip angle method, which accounts for the reference image temperature remaining constant. Monte Carlo simulations determined the optimal dynamic flip angle for combined PRF and T1 measurements. This method was evaluated in MR-guided focused ultrasound heating experiments using a gelatin phantom and human cadaver breasts. In vivo measurement precision was demonstrated in healthy female volunteers under nonheating conditions. RESULTS: Temperature rise during MR-guided focused ultrasound heating was measured in aqueous tissue with both PRF and T1 . Both measures show good qualitative agreement in both space and time in aqueous tissue. The T1 change due to temperature increase was measured in fat, demonstrating the expected temporal response. The dynamic flip angle that produces optimal SNR for PRF measurements is lower than the optimal angle for T1 measurements, necessitating the selection of a compromise angle. CONCLUSION: The single reference variable flip angle method provides a reliable way to simultaneously measure PRF temperature and T1 change and overcomes PRF's inability to simultaneously monitor temperature in aqueous and adipose tissues. Future work will calibrate T1 change to temperature, enabling real-time temperature in fat and increasing patient safety and treatment efficacy during thermal interventional treatments.

Effect of k-space-weighted image contrast and ultrasound focus size on the accuracy of proton resonance frequency thermometry.

PURPOSE: To construct a predictive model that describes how the duration and symmetry of a k-space-weighted image contrast (KWIC) window affects the temporal resolution of differently sized ultrasound foci when using a pseudo-golden angle stack-of-stars acquisition. METHODS: We performed a modulation analysis of proton resonance frequency temperature measurements to create the temporal modulation transfer function for KWIC windows of different symmetry and temporal duration. We reconstructed simulated ultrasound heating trajectories and stack-of-stars k-space data as well as experimental phantom data using the same trajectories. Images were reconstructed using symmetric and asymmetric KWIC windows of 3 different temporal durations. Simulated results were compared against temporal modulation transfer function predictions, experimental results, and the original simulated temperatures. RESULTS: The temporal modulation transfer function shows that temporal resolution with KWIC reconstructions depend on the object size. The KWIC window duration affected SNR and severity of undersampling artifacts. Accuracy and response delay improved as the KWIC window duration decreased or the size of the heated region within the KWIC plane increased. Precision worsened as the window duration decreased. Using a symmetric window eliminated the response delay to heated region size but introduced a large reconstruction delay. CONCLUSION: The accuracy and precision of proton resonance frequency temperature measurements from a stack-of-stars acquisition using a sliding KWIC window reconstruction are dependent on the size of the KWIC window and the size and shape of the heated region. The temporal modulation transfer function of KWIC reconstructions for any object size can predict the temporal response to changes in signal being acquired, such as temperature and contrast enhancement.

Multiecho pseudo-golden angle stack of stars thermometry with high spatial and temporal resolution using k-space weighted image contrast.

PURPOSE: Implement and evaluate a 3D MRI method to measure temperature changes with high spatial and temporal resolution and large field of view. METHODS: A multiecho pseudo-golden angle stack-of-stars (SOS) sequence with k-space weighted image contrast (KWIC) reconstruction was implemented to simultaneously measure multiple quantities, including temperature, initial signal magnitude M(0), transverse relaxation time ( T2*), and water/fat images. Respiration artifacts were corrected using self-navigation. KWIC artifacts were removed using a multi-baseline library. The phases of the multiple echo images were combined to improve proton resonance frequency precision. Temperature precision was tested through in vivo breast imaging (N = 5 healthy volunteers) using both coronal and sagittal orientations and with focused ultrasound (FUS) heating in a pork phantom using a breast specific MR-guided FUS system. RESULTS: Temperature measurement precision was significantly improved after echo combination when compared with the no echo combination case (spatial average of the standard deviation through time of 0.3-1.0 and 0.7-1.9°C, respectively). Temperature measurement accuracy during heating was comparable to a 3D seg-EPI sequence. M(0) and T2* values showed temperature dependence during heating in pork adipose tissue. CONCLUSION: A self-navigated 3D multiecho SOS sequence with dynamic KWIC reconstruction is a promising thermometry method that provides multiple temperature sensitive quantitative values. Magn Reson Med 79:1407-1419, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Technical Note: The effect of 2D excitation profile on T1 measurement accuracy using the variable flip angle method with an average flip angle assumption.

PURPOSE: To study the accuracy and precision of T1 estimates using the Variable Flip Angle (VFA) method in 2D and 3D acquisitions. METHODS: Excitation profiles were simulated using numerical implementation of the Bloch equations for Hamming-windowed sinc excitation pulses with different time-bandwidth products (TBP) of 2, 6, and 10 and for T1 values of 295 ms and 1045 ms. Experimental data were collected in 5° increments from 5° to 90° for the same T1 and TBP values. T1 was calculated for every combination of flip angle with and without a correction for B1 and slice profile variation. Calculations were also made for flat slice profile such as obtained in 3D acquisition. Monte Carlo simulations were performed to obtain T1 measurement uncertainty. RESULTS: VFA T1 measurements in 2D without correction can result in a 40-80% underestimation of true T1 . Flip angle correction can reduce the underestimation, but results in accurate measurements of T1 only within a narrow band of flip angle combinations. The narrow band of accuracy increases with TBP, but remains too narrow for any practical range of T1 values or B1 variation. Simulated noisy VFA T1 measurements in 3D were accurate as long as the two angles chosen are on either side of the Ernst angle. CONCLUSIONS: Accurate T1 estimates from VFA 2D acquisitions are possible, but only a narrow range of T1 values within a narrow range of flip angle combinations can be accurately calculated using a 2D slice. Unless a better flip angle correction method is used, these results demonstrate that accurate measurements of T1 in 2D cannot be obtained robustly enough for practical use and are more likely obtained by a thin slab 3D VFA acquisition than from multiple-slice 2D acquisitions. VFA T1 measurements in 3D are accurate for wide ranges of flip angle combinations and T1 values.

Focal point determination in magnetic resonance-guided focused ultrasound using tracking coils.

PURPOSE: To develop a method for rapid prediction of the geometric focus location in MR coordinates of a focused ultrasound (US) transducer with arbitrary position and orientation without sonicating. METHODS: Three small tracker coil circuits were designed, constructed, attached to the transducer housing of a breast-specific MR-guided focused US (MRgFUS) system with 5 degrees of freedom, and connected to receiver channel inputs of an MRI scanner. A one-dimensional sequence applied in three orthogonal directions determined the position of each tracker, which was then corrected for gradient nonlinearity. In a calibration step, low-level heating located the US focus in one transducer position orientation where the tracker positions were also known. Subsequent US focus locations were determined from the isometric transformation of the trackers. The accuracy of this method was verified by comparing the tracking coil predictions to thermal center of mass calculated using MR thermometry data acquired at 16 different transducer positions for MRgFUS sonications in a homogeneous gelatin phantom. RESULTS: The tracker coil predicted focus was an average distance of 2.1 ± 1.1 mm from the thermal center of mass. The one-dimensional locator sequence and prediction calculations took less than 1 s to perform. CONCLUSION: This technique accurately predicts the geometric focus for a transducer with arbitrary position and orientation without sonicating. Magn Reson Med 77:2424-2430, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Respiration artifact correction in three-dimensional proton resonance frequency MR thermometry using phase navigators.

PURPOSE: To develop reliable three-dimensional (3D) segmented echo planar imaging (seg-EPI) proton resonance frequency (PRF) temperature monitoring in the presence of respiration-induced B0 variation. METHODS: A free induction decay (FID) phase navigator was inserted into a 3D seg-EPI sequence before and after EPI readout to monitor B0 field variations. Using the field change estimates, the phase of each k-space line was adjusted to remove the additional phase from the respiratory induced off-resonance. This correction technique was evaluated while heating with MR-guided focused ultrasound (MRgFUS) in phantoms with simulated breathing and during nonheating conditions in healthy in vivo breasts. RESULTS: With k-space phase correction, the standard deviation of magnitude images and PRF temperature measurements in breast from five volunteers improved by an average factor of 1.5 and 2.1, respectively. Improved accuracy of temperature estimates was observed after correction while heating with MRgFUS in phantoms. CONCLUSION: Phase correction based on two FID navigators placed before and after the echo train provides promising results for implementing 3D monitoring of thermal therapy treatments in the presence of field variations due to respiration. Magn Reson Med 76:206-213, 2016. © 2015 Wiley Periodicals, Inc.