Phase enhanced PSIR T1 weighted imaging improves contrast resolution of the nucleus basalis of Meynert at 7 T: a preliminary study

Background: The nucleus basalis of Meynert (NBM) provides the majority of cortical cholinergic innervation which is required for memory formation, maintaining attention and promoting learning. Neuronal loss within this area is implicated in a number of neurodegenerative disorders. Imaging the NBM is however limited by its small size and suboptimal contrast resolution at the base of the brain.

Purpose: To develop a novel method of processing T1 weighted MRI data for improving contrast resolution and delineation of the NBM.

Study type: Technical development, case series.

Subjects: Five healthy volunteers.

Field strength, sequence, analysis: Volunteers were scanned on a Philips 7 T Achieva imaging system. T1-weighted images were constructed from a double inversion phase sensitive inversion recovery (PSIR) sequence. Inversion recovery data were combined with the filtered phase data from the long inversion time image to produce a novel susceptibility weighted-PSIR (SW-PSIR) map. This process is similar to that used to combine T2* weighted image and phase maps to create susceptibility weighted images (SWI), but with the processing parameters optimized in terms of contrast-to-noise ratio to the NBM in the final SW-PSIR maps. Average NBM thickness was reported as mean ± standard deviation (SD). Intra-observer and inter-observer reliability were tested using intra-class correlation coefficient (ICC).

Results: 0.7mm³ isotropic resolution images were acquired in a 5 min and 50 s scan. The mean thickness ± SD of the left (right) NBM was 3.5 ± 0.4 mm and 3.8 ± 0.5 mm (3.6 ± 0.5 mm and 3.7 ± 0.5 mm) by the first and second observers respectively with excellent intra-observer and inter-observer agreement (>0.90).

Conclusion: In this pilot study the SW-PSIR imaging approach improves delineation of the NBM between the ventral pallidum and chiasmatic cistern allowing accurate thickness measurement. The role of this sequence, in enabling robust morphometry of the NBM in health and disease, can be tested further in larger studies.