Synopsis

Evaluation of the effect of B0 and B1 correction on region-of-interest (ROI) analysis in forty-one 3T brain 23Na-MRI scans shows average change from B0, B1 and B0B1 correction of 1.0% (p=0.05), -3.7% (p<0.001) and -4.7% (p<0.001) respectively. Effect is more for B1 than B0 correction, and depending on the anticipated effect size of the neurologic disease of interest, B0 +/- B1 correction may not be required, potentially reducing scan time by 66% and thus encourage quantitative 23Na-MRI research in a clinical environment.

INTRODUCTION

Quantitative sodium 23Na-MRI has potential applications in Alzheimer’s disease, Huntington’s disease, multiple sclerosis, brain tumour, and stroke^1-5, but accurate quantitation conventionally includes B0 and B1 field inhomogeneity correction, which can triple acquisition time⁶. This increased scan time, particularly when added to the scan time of clinical proton 1H-MRI, can be difficult for patients with these diseases to tolerate, and when performed on high throughput clinical MR scanners, is another factor prohibiting translation of quantitative 23Na-MRI towards wider clinical application.

We aim to evaluate the magnitude of B0 and B1 correction on sodium signal intensities (SI) in different brain regions (ROI) from a 3T clinical scanner using a commercially available dual-tuned 1H/23Na birdcage head coil.

METHODS

23Na-MRI scans were obtained (n=41,M=14,F=27, age=75.8±5.8 years) on a 3T Trio scanner (Siemens, Erlangen, Germany) using a 1H/23Na quadrature transmit/receive head coil tuned at 123.2 and 32.6 MHz (Model V-XQ-HQ-303, Rapid Biomedical, Rimpar, Germany). Proton T1-MPRAGE scans were acquired with TR=1900ms, TE=2.13ms, FA=9 degrees, NEX=1, TA=5:40 minutes at 1mm isotropic resolution. After shimming on 23Na FID, 23Na-MRI was performed using the flexible twisted projection imaging (FlexTPI) sequence⁶ with TR=160ms,TE=0.3ms,FA=90 degrees, readout time 12ms, NEX=2,TA=8 minutes at nominal 5mm isotropic resolution. For B0 correction using the frequency-segmented conjugate phase reconstruction method⁷, the above FlexTPI sequence was repeated, but with TE=2.6ms. For B1 correction using the double flip angle method⁸, the FlexTPI sequence was again performed with TE=0.3ms but FA=45 degrees.

Data processing: (i) Custom built automated software pipeline⁹ (Matlab, Natick, MA, USA) was used to generate B0, B1 maps and sodium images of the whole brain, without correction (uncorrected), with B0 correction only (B0 corrected), and with B0 and B1 correction (B0B1 corrected) (ii) 1H-MPRAGE images were segmented using FSL v5.0 and co-registered with sodium images to improve the skull-strip. (iii) Uncorrected, B0 corrected and B0B1 corrected sodium images of each subject were registered into MNI space using the JHU-T2w-MNI template. (iii) Harvard-Oxford cortical and subcortical atlases^10-13 were used for ROI parcellation to extract average SI and its standard deviation(SD).

The uncorrected, B0 corrected and B0B1 corrected 23Na-SI of each region were compared. SI change for B0, B1 and combined B0B1 correction effect were assessed by (B0-uncorrected)/uncorrected, (B0B1-B0)/B0 and (B0B1-uncorrected)/uncorrected) respectively.

Table1 lists the ROIs analyzed. Descriptive statistics performed on Excel. 2 tailed paired t-test performed with significance defined as p<0.05.

RESULTS

Table1 shows the SI changes of various regions. Average change from B0, B1 and B0B1 correction 1.0%(p=0.05), -3.7% (p<0.001) and -4.7% (p<0.001) respectively.

B0 correction: Most affects lateral ventricles (5%). Structures near bone interfaces including hippocampi, parahippocampal, fronto-orbital and inferior temporal gyri are minimally changed (0.4-1.9%). Structures furthest from isocentre are also minimally changed (1.8-2.6%). SD for all ROIs increased with B0 correction (1.3-18%), most at the parahippocampal gyri.

B1 correction: Most affects cerebellum (-8.2%) and brainstem (-6.3%), and least in structures near the vertex (-1.9% superior frontal gyrus). SD for all ROIs decreased.

B0B1 correction: Most affects cerebellum(-6.4%) and brainstem(-8.7%), and least in structures near the vertex (-1.5% superior frontal gyrus). SD for most ROIs decreased.

DISCUSSION

Overall, SI change from the corrections are not large (<5%), B1>B0 correction.

SI change from B0 correction is <2% for almost all ROIs, except for lateral ventricles and brainstem. As a typical B0 map shows a -70Hz maximum frequency shift, at nominal 5mm voxel size at 3T and a pixel bandwidth of 2000Hz, the findings are consistent. The lateral ventricles may be most affected because of long boundaries with CSF. The effect on the brainstem is also expected, with B0 inhomogeneity increasing away from isocentre. The magnitude of effect suggests B0 correction may not be needed for quantitative ROI analysis of supratentorial brain structures.

SI change from B1 correction is <10% for all ROIs; all supratentorial ROIs except for parahippocampal gyri are <6%. The slightly larger changes in the brainstem and cerebellum are not unexpected, being near the edge of the coil. Depending on the effect size anticipated and the ROI for the disease of interest (ie. SI increase 37% caudate in Huntington’s disease², 32% and 23% in normal appearing white and grey matter respectively in MS³, and 8% hippocampi in mild cognitive impairment¹), B1 correction may also not be needed.

CONCLUSION

B0 correction may not be necessary for quantitative ROI analysis on 3T 23Na-MRI, and B1 correction may also not be necessary depending on the ROI of interest and the anticipated effect size of the pathology of interest.

An up to 66% reduction in scan time would help encourage translation of 23Na-MRI research to clinical patients in clinical environments.

Acknowledgements

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