In vivo evaluation of population-specific inversion pulses in parallel transmission

Abstract

Purpose: The aim of the study was to conduct in vivo evaluation of population-specific pulses in the context of the MP2RAGE sequence for brain imaging at 7T.

Methods: Five clusters were identified in a cohort of 39 volunteers. One cluster centered around the average head shape and position, whereas four others were located toward the extremes of the population distribution. Additionally, the one-size-fits-all solution was considered, using a standard universal pulse (UP) approach. Head shapes were characterized using lateral head breadth (HB), anterior-posterior head length (HL), and Y-shift metrics. For each group, a 5kT-points universal inversion pulse was computed and evaluated on four new test anatomies. A Python pipeline was integrated into the image reconstruction routine, using localizer scans to classify head shapes and positions. The pipeline selected one of five precomputed population-specific pulses or defaulted to the generic UP without extending scan time.

Results: The pipeline accurately classified head shapes and selected suitable pulses, enhancing the contrast of MP2RAGE images. Population-specific pulses helped mitigate some of the performance loss associated with using a one-size-fits-all UP, bringing performance closer to that of fully tailored solutions. This approach was particularly beneficial for individuals with smaller head sizes. However, the performance was worse in larger, more challenging head shapes.

Conclusion: The novel head clustering and pulse selection pipeline facilitates the implementation of population-specific pulses in clinical practice by allowing pulse selection tailored to each head shape and position without increasing scan time.

Keywords: k T $$ {\mathrm{k}}_{\mathrm{T}} $$ ‐points; MRI; RF pulse design; UHF; parallel transmission; universal pulse.

FIGURE 1

(A) Slice of interest through the anterior commissure—posterior commissure (AC‐PC) line orthogonal to the sagittal plane. The head shapes were characterized by three parameters: lateral head breadth (HB), anterior–posterior head length (HL), and Y‐shift (displacement along the anterior–posterior axis). The HL and HB were measured as the major and minor axis lengths of the ellipse with the same normalized second central moments as the thresholded slice of interest. The Y‐shift was calculated as the distance between that region's center of mass and the coil's isocenter. (B) Scatter plot illustrating the head anatomies considered in this study as measured by the automated pipeline. Five clusters are shown, including C1—red, C2—green, C3—blue, C4—cyan and C5—magenta. The set of anatomies, referred to as the “full population”, was utilized to design a generic one‐size‐fits‐all UP. Additionally, the nine anatomies used to evaluate the performance of the clustering algorithm and three test subjects used for in vivo performance evaluation are shown.

FIGURE 2

A system diagram illustrating the automated head measurement and clustering add‐in integrated into the primary image reconstruction pipeline.

FIGURE 3

Accuracy of the automated pipeline measured by (A) the standard deviation across 20 runs and (B) the range between the highest and lowest measurements.

FIGURE 4

Scatter plot of normalized root mean square errors (NRMSE) of the target longitudinal magnetization profile within the brain mask after inversion. The performance of HS4 adiabatic pulse (A) and tailored pulses (B) is compared to that of generic full‐population UP (C), and population‐specific pulses C1‐C5 (D‐H) are applied. Colored circles highlight cluster locations and threshold levels used in automated pulse selection; stars indicate test subjects. The Y‐axis (third dimension) was flattened for simplicity.

FIGURE 5

Performance of inversion pulses in MP2RAGE UNI‐DEN images for subjects #1‐4 (A‐D). “N, M J/kg” indicates a pulse designed for cluster N with specific energy deposition (SED) of M J/kg. Highlighted inversion pulses: C1 (red), C2 (green), C5 (magenta). Subject automated classifications: #1 in C1, #2 in C5, #3 in full population (closest cluster C2), and #4 in C2. Significant improvements were observed in smaller head shapes, where population‐specific pulses effectively recovered signal around sinuses.

FIGURE 6

Relative difference in T1 values compared to the reference HS4 adiabatic inversion for subjects #1‐4 (A‐D). “N, M” indicates a pulse designed for cluster N with NRMSE of M across T1 values. Highlighted inversion pulses: C1 (red), C2 (green), and C5 (magenta). Subject automated classifications: #1 in C1, #2 in C5, #3 in full population (closest cluster C2), and #4 in C2.