Martin Christa1, Ibrahim A. Elabyad2, Wolfgang Rudolf Bauer1, and Maxim Terekhov2
1Comprehensive Heart Failure Center (CHFC), Department of Internal Medicine I, University Hospital Wuerzburg, Wuerzburg, Germany, 2Comprehensive Heart Failure Center (CHFC), Chair of Cellular and Molecular Imaging, University Hospital Wuerzburg, Wuerzburg, Germany
Synopsis
With higher field strength and, thus, better SNR available, 23Na-MRI
is becoming increasingly popular in (pre-) clinical studies. Improving accuracy
and repeatability will help to translate 23Na-MRI into routine. Therefore,
we investigated the effect of respiratory and cardiac gating, as well as the
influence of B1-field correction on cardiac total sodium quantification with a
2D radial UTE sequence in mice using a surface coil at 7T. We found that
ungated and uncorrected measurements overestimate cardiac sodium concentration.
Combined respiratory and cardiac gating and B1-field correction significantly
decreased the measured cardiac tissue sodium concentration (-40.6%).
Introduction
Sodium is regarded as a marker of ion homeostasis and driver of biological
processes.1,2 With higher field strength
available there is a renaissance of sodium MRI as it allows the non-invasive
quantification of tissue sodium concentration (TSC). In order to improve
accuracy of TSC measurements, previous studies investigated the effects of
various factors on the sodium signal using birdcage coils.3 However, with regard to the
physical sensitivity, surface coils would be prefereable. Therefore, we
investigated the influence of respiratory and cardiac gating, as well as B1-field
distribution on the cardiac sodium signal using an in-house-built dual-tune 23Na/1H
surface coil (Setup, see Figure 1). The effects of each gating setup and
B1-field correction were compared to baseline uncorrected data to demonstrate
the scope of TSC signal changes.Methods
Mouse experiments were performed in accordance with
local laws and official permit (TVA RUF-55.2.2-2532-2-735-16) of the
responsible authorities at a 7T Bruker small animal scanner (PharmaScan 70/16
Bruker BioSpin GmbH, Ettlingen, Germany). We used an in-house build dual-tuned 23Na/1H
spiral surface coil, which was inspired by the design from Wetterling et al.4 To
monitor ECG and respiratory motion, and also for prospective triggering, a
Model 1030 animal monitoring and gating system (SA Instruments Inc., Stony
Brook, NY) was used. For imaging, a short axis view at the midventricular level
was selected in 1H-MRI. Subsequently, a 2D radial UTE sequence was
used to assess the sodium signal (TE: 0.35 ms; TR: 100 ms, Treadout:
0.365 ms, AQbandwidth: 65 kHz, Flip angle: 90°, 150
projections, FOV 35x35 mm², Slice thickness 4 mm, Acquisition-Matrix
48x48 resulting in a nominal resolution of 1.96 mm³, approx. 2 µl/Voxel, 70
Averages, acquisition time TAQ: ~26 min). Three measurements were
performed for each mouse (n=4). One without gating, one with respiratory gating,
and one with combined cardiac and respiratory gating to image the fully relaxed
heart (diastolic state). For B1-field correction, B1 maps were acquired using
the double angle method (DAM).5
DAM Maps were acquired using a gradient echo sequence with TE 0.8 ms, TR 200 ms, Treadout: 0.365 ms, FA 45°
and 90°, 70 Averages, same resolution as the 2D-UTE, TAQ ~14 min for
both). An external vial with known sodium concentration was used as a reference
for total sodium quantification.
B1-field correction and sodium quantification were done using a custom MATLAB
script (Matlab2015b, MathWorks, Massachusetts, United States).Results
Quantitative sodium imaging showed a sodium signal of 76.1±19.6 mmol/l
(mean±SD) without gating or B1-field correction, 64.0±16.8 with respiratory
gating and 52.7±10.9 with cardiac and respiratory gating to diastolic state.
Applying B1-field correction further reduced the signal to 45.2±1.8 mmol/l. Hence,
respiratory gating reduced the measured cardiac TSC by 15.8 %, combining
cardiac and respiratory gating additionally reduced the TSC by 14.9% and B1-field
correction added another 9.8% of signal reduction.
Thus, uncorrected data overestimates cardiac TSC and the applied correction
methods reduced the signal in total by 40.6%. (Figure 2)Discussion
Our results demonstrate the effect of different gating set-ups and the
applied B1-field correction on the sodium signal measured with a surface coil
at 7T. In previous studies with birdcage resonators, B1-field correction only
reduced the signal by 3%.
In contrast when using surface coils B1-field correction has a major influence
on the TSC quantification reducing the signal by 9.8% alone. A greater impact on
data acquisition showed the proper gating on the diastolic phase of the heart
(combined prospective respiratory and cardiac gating) with a signal reduction
of -30.7%, assumedly by reducing partial volume and motion related overlapping
effects. Calculating the theoretical cardiac TSC of mice using a formula
proposed by Bottomly6 and values from literature,
estimated TSC is 43 mmol/l; thus our results after correction (45.2±1.8 mmol/l)
are in good agreement. Conclusion
With higher field strength available, sodium MRI is becoming feasible and
more commonly used in preclinical and clinical studies. Our results demonstrate
the importance of proper gating and B1-field correction to increase the
accuracy of TSC measurements when using surface coils. Higher accuracy and
repeatability will help to establish sodium MRI in (pre-) clinical routine.Acknowledgements
Financial support from the German Ministry of
Education and Research (BMBF grant #01EO1504
) is appreciated.References
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