Ultrafast B1 mapping with RF‐prepared 3D FLASH acquisition: Correcting the bias due to T₁‐induced k‐space filtering effect

Abstract

Purpose:The traditional radiofrequency (RF)-prepared B 1 mapping technique consists of one scan with an RF preparation module for flip angle-encoding and a second scan without this module for normalizing. To reduce the T 1 -induced k-space filtering effect, this method is limited to 2D FLASH acquisition with a two-parameter method. A novel 3D RF-prepared three-parameter method for ultrafast B 1 -mapping is proposed to correct the T 1 -induced quantification bias. Theory:The point spread function analysis of FLASH shows that the prepared longitudinal magnetization before the FLASH acquisition and the image signal obeys a linear (not proportional) relationship. The intercept of the linear function causes the quantification bias and can be captured by a third saturated scan. Methods: Using the 2D double-angle method (DAM) as the reference, a 3D RF-prepared three-parameter protocol with 9 s duration was compared with the two-parameter method, as well as the saturated DAM (SDAM) method, the dual refocusing echo acquisition mode (DREAM) method, and the actual flip-angle imaging (AFI) method, for B 1 mapping of brain, breast, and abdomen with different orientations and shim settings at 3T. Results:The 3D RF-prepared three-parameter method with complex-subtraction delivered consistently lower RMS error, error mean, error standard deviation, and higher concordance correlation coefficients values than the two-parameter method, the three-parameter method with magnitude-subtraction, the multi-slice DREAM and the 3D AFI, and were close to the results of 2D or multi-slice SDAM. Conclusion:The proposed ultrafast 3D RF-prepared three-parameter method with complex-subtraction was demonstrated with high accuracy for B 1 mapping of brain, breast, and abdomen.

Direct angiographic comparison of different velocity‐selective saturation, inversion, and DANTE labeling modules on cerebral arteries

“…In addition, the suppressed tissue signal by FT‐VSS would experience T1 recovery during the extended TFL readout based on theoretical analysis of the point spread functions 56 . The bias due to T1‐induced k‐space filtering effect would be canceled out by subtracting the control counterpart as done in the other VSMRA methods evaluated in this study or by subtracting a separate acquisition prepared with nonselective saturation 57–59 …”

Evaluation of 3D stack‐of‐spiral turbo FLASH acquisitions for pseudo‐continuous and velocity‐selective ASL–derived brain perfusion mapping

“…As a result, the subtraction between control and label images fully eliminates the bias. A quick Sat image captured this bias to be subtracted from PD and DIR images, as utilized in an ultrashort B 1 mapping protocol 23 …”

“…The order of the three labeling techniques was also random. For each different acquisition, three additional scans were performed for perfusion quantification: a proton-density weighted image (PD, TR = 10 s, for normalization), a double inversion recovery (DIR, TR/TI 1 /TI 2 = 10/3.58/0.48 s, to visualize gray matter), and an image acquired immediately after global saturation (Sat) (to record the T 1 -induced bias of TFL acquisitions for correction of PD and DIR, 15,23 with more explanation in the Discussion section).…”

“…A quick Sat image captured this bias to be subtracted from PD and DIR images, as utilized in an ultrashort B 1 mapping protocol. 23…”

Rapid 3D absolute B1+ mapping using a sandwiched train presaturated TurboFLASH sequence at 7 T for the brain and heart

“…In Figure 10 , both satTFL and the sandwich scheme show similar B 1 + map profiles in the heart. Recent work has applied 3D FLASH readouts to satTFL 30 , 31 to enable single‐shot acquisitions of an entire brain volume. Although this works well in the brain, this is not suitable for the heart because the readout is much longer than diastole and T 1 effects become a concern.…”