Purpose

Prior work at 3T has focused on the development of flexible pads with permittivities up to 550¹, or monolithic ceramics with permitivies in the range of 800 – 4700². Each design offers a trade-off: pads are more conformal at the expense of lower permittivity and reduced enhancement, while monolithic ceramics offer greater enhancement though it is more difficult to shape the material conformal to human body. To date, most designs have only offered regional enhancement of the B fields, whereas a more global enhancement throughout the brain would be desirable for many neuroimaging applications. Here we present a brain imaging study using a conformal helmet with high permittivity (er ~ 1000) that can be inserted into a commercial receive coil.

Methods

Figure 1 displays the uHDC helmet that conforms to the upper human head to the level above eye and ear and can be fitted into the standard Siemens 20-Channel head array. Acquisition: All data was acquired on a Siemens 3T PrismaFit (Siemens Healthineers, Erlangen, Germany) with a 2-Ch pTX transmit coil. B₁⁺ maps were acquired with a Bloch-Siegert³ based sequence, with 100 mm of longitudinal coverage from the apex to the cerebellum. A small-tip angle GRE acquistion with parameters matched to the B₁⁺ scan was acquired for calculation of B₁⁺-normalized SNR maps⁴. Each subject (n = 5) was scanned under 3 reception configurations: 20-Ch Coil, 20-Ch Coil with uHDC Helmet (V1), 64-Ch Coil. One subject was scanned with the uHDC helmet in a higher permittivity configuration (V2) (er ~ 1200). Additionally, on a single subject, absolute B₁⁺ magnitude and relative transmit phase maps were acquired on both channels of the transmit coil with the aforementioned protocols. Analysis: A distance map was calculated over the brain, the distance metric defined as the shortest Euclidean distance between a voxel and the edge of the brain mask. This distance map was utilized in a histogram analysis to quantify SNR of each receive configuration. A RF pulse optimization⁵ that utilized the B₁⁺ magnitude and transmit phase maps was applied to calculate a blipped-spokes pulse off-line. This RF pulse was designed with a target flip angle of 70 degrees, 5 spokes and a duration of 7.38 ms.

Results & Discussion

Figure 2 displays representative transmit efficiency maps from a single subject. The helmet increased efficiency by up to a factor of 2 to 2.5 within the brain, though the overall homogeneity of the field pattern suffered. In Fig. 3, SNR maps from a single subject and 3 of the receive configurations are displayed. The figure displays a subset of the acquired slices. The SNR of the 20-Ch Coil + uHDC helmet (V1) was enhanced strongly above the baseline configuration, almost reaching the performance of the 64-Ch Coil. Figure 4 displays the results of the histogram analysis. Fig 4a demonstrates that most of the brain voxels of the 20-Ch coil fell within the SNR range 0–138 (95.3%), while only 4.7% of the brain voxels occurred in the 138–282 range. In order, the 20-Ch Coil + uHDC helmet (V1 & V2) and 64-Ch Coil’s percentage values were 27.13%, 34.15%, and 31.14% in the range of 138–210, and 3.34%, 8.97%, and 6.54% in the range of 210–282 respectively. Figure 4b displays the mean SNR at a given depth from the brain surface. Over the entire range, the 20-Ch Coil + uHDC helmet (V2) offered the best performance. From 10 mm onward, the 20-Ch Coil + uHDC helmet (V1) closely matched that of the 64-Ch Coil. Values in Fig 4. represent mean and standard deviation among the 5 subjects, except for V2 which was acquired on a single subject. In Fig. 5, results of the blipped-spokes calculation are shown, with the coefficient of variation displayed in white above each map. The use of this RF pulse greatly mitigated the transmit inhomogeneity induced by the uHDC helmet.

Conclusion

We have presented a study of a conformal helmet constructed with uHDC materials that enhances the SNR of a 20-Ch Coil to a comparable level with the 64-Ch Coil. This helmet design represents a proof-of-concept for a uHDC helmet, and with further optimization could potentially achieve further increases in SNR of standard receive coils. The transmit field was greatly increased but become extremely inhomogeneous due to the uHDC helmet. However, this issue was resolvable with the use of a patient-specific pTX blipped-spokes pulse.

Acknowledgements

This work was supported by grants from the NIH and Penn State Hershey Neuroscience Institute.