3D resolved human cardiac creatine kinase rate by 31P-MRS at 7T.
William Thomas Clarke1, Matthew D Robson1, and Christopher T Rodgers1

1Oxford Centre for Clinical Magnetic Resonance Research, University of Oxford, Oxford, United Kingdom

Synopsis

The creatine kinase (CK) forward rate constant kf is a sensitive biomarker for heart failure. However, the low SNR of 31P-MRS at 1.5T and 3T has only allowed it to be measured at low spatial resolution by 1D-CSI. Here, we show how cardiac 7T 31P-MRS permits 3D resolved measurements for the first time. A 3D variant of the FAST kfCK method was combined with 31P Bloch-Siegert B1+ mapping to enable 3D-resolved measurements at 7T. The first measurements of the creatine kinase rate in myocardium in the interventricular septum are obtained from four subjects. Our mean kf = 0.36±0.04 s-1 was consistent with literature values.

Purpose

The creatine kinase enzyme regulates the rate of creation of adenosine triphosphate (ATP) from phosphocreatine (PCr) in myocardial myofibrils.1 The creatine kinase forward rate constant kfCK is a sensitive biomarker for heart failure.2

Phosphorus magnetic resonance spectroscopy (31P-MRS) at 1.5 and 3T has previously been used to measure kfCK in the human myocardium.3 At 1.5T and 3T 31P-MRS suffers from low SNR, so kfCK had to be measured with a low spatial resolution, e.g. using 1D chemical shift imaging (CSI).

The SNR for cardiac 31P-MRS increases by 2.8x at 7T compared to 3T.4 Our aim is to use this increased SNR to obtain the first 3D-localised measurements of kfCK in the human heart. Higher resolution decreases confounding signal contamination, and is the first step towards kfCK measurements in localised myocardial disease.

Methods

Saturation transfer (FAST3, TRiST5 and TwiST6) and progressive saturation7 methods have been implemented for human cardiac kfCK measurements. The FAST method, with no restraint on the sequence TR, is the most extensible to 3D localisation.

FAST makes two dual-angle T1 measurements: one while saturating the γ-ATP peak, and one with control saturation. Each measurement comprises identical acquisitions, with different flip angles: α (15o) and β (60o).8 The original 1D-CSI FAST implementation, using BIR-4 pulses for uniform excitation, comprised 4 acquisitions.

After measuring T1s, with and without saturation (T1’ and T1), the corresponding magnetisations with and without saturation (M0’ and M0) are calculated. kfCK is given by: $$k_{\mathrm{f}}^{\mathrm{CK}}=\frac{1}{T_{1}'}(1-\frac{M_{0}'}{M_{0}}).$$

Unfortunately, the $$$B_{1,\mathrm{peak}}^{+}$$$ on a Siemens Magnetom 7T scanner falls below the adiabatic onset of BIR-4 pulses. Therefore, we replaced BIR-4 excitation with conventional pulses and added a $$$B_{1}^{+}$$$-mapping step to the protocol, using a 31P-MRS Bloch-Siegert method.9

The precision of the kfCK measured by FAST was estimated, using Monte Carlo simulations, in the presence of errors in flip angle. The error from the uncertainty in the B1 maps was compared with the error from imperfect excitation by BIR-4 pulses.10

This “FAST+Bloch-Siegert” method was validated in one subject’s calf muscle, which has high SNR and homogeneous tissue, at 3T, with a volume coil for uniform excitation, and at 7T, with a surface coil. The 10-cm loop transmit-receive surface coil was also used for the cardiac protocol.

Four healthy subjects (male, 29±6yrs and 75±7Kg) underwent the cardiac protocol. Subjects were positioned and localizers and calibration scans were acquired as previously described.4 The protocol comprised 3D-CSI 31P-MRS acquisitions with a 16x16x8 acquisition weighted CSI matrix (8 along the heart long-axis), 240x240x200mm3 FOV, 6000Hz bandwidth and shaped excitation pulse.4

The scan protocol and processing is described in Figure 1. In the Bloch-Siegert prepared acquisitions, a Fermi pulse 9, at ±2000Hz, was added after the excitation. The $$$B_{1}^{+}$$$ is used to scale the following, four constituent, FAST acquisitions’ excitation flip angles, to 15o for α and 60o for β in the interventricular septum. Selective saturation was achieved by DANTE (TR=330μs, TP=100μs). The total 31P-MRS acquisition time was 80 min. Data was processed and fitted using AMARES in Matlab.

Results

The error in kfCK from the uncertainty in $$$B_{1}^{+}$$$ maps was computed as being <±20% which is comparable to that from BIR-4 pulses (6.1-23.4%).

The kfCK measured in skeletal muscle was 0.27±0.10 s-1 (3T: all purely muscle containing voxels), and 0.31±0.07 s-1 (7T: all voxels within 10% of target flip angles).

The kfCK in the target myocardial voxels of the 4 subjects was 0.39±0.15 s-1, 0.31±0.17 s-1, 0.39±0.21 s-1 and 0.35±0.09 s-1 (value ± SD). The study average was 0.37±0.05 s-1.

Discussion

kfCK values recorded using the "FAST + Bloch-Siegert" method fall within the range of literature values for cardiac and skeletal muscle. The values in this study have been obtained from voxels centred on the myocardium, with voxel sizes (FWHM of the point-spread-function) <50% those obtained in any previous study.

Simulations show that the precision and accuracy of kfCK drop to <50% if the flip angles deviate from their target values by more than 20%. The inhomogeneous flip angles produced by surface coils mean that the target flip angles are only obtained across a few voxels in the left ventricle. A coil that produces more uniform excitation, e.g. a larger loop or a volume coil, would allow more voxels to be quantified simultaneously.

The acquisition time is limited by the SNR of the method. Receive array coils offer a 1.8x increase in SNR for cardiac 31P-MRS at 7T; equivalent to a 3.2x shorter scan duration.11

Conclusion

3D resolved creatine kinase rate measurements have been recorded from human myocardium in vivo for the first time using 7T cardiac 31P-MRS.

Acknowledgements

Funded by a Sir Henry Dale Fellowship from the Royal Society and the Wellcome Trust [098436/Z/12/Z].

WTC is funded by the MRC.

References

1. Neubauer S. Mechanisms of disease - The failing heart - An engine out of fuel. N Engl J Med 2007;356(11):1140-1151.

2. Weiss RG, Gerstenblith G, Bottomley PA. ATP flux through creatine kinase in the normal, stressed, and failing human heart. Proc Natl Acad Sci U S A 2005;102(3):808-813.

3. Bottomley PA, Ouwerkerk R, Lee RF, Weiss RG. Four-angle saturation transfer (FAST) method for measuring creatine kinase reaction rates in vivo. Magn Reson Med 2002;47(5):850-863.

4. Rodgers CT, Clarke WT, Snyder C, Vaughan JT, Neubauer S, Robson MD. Human Cardiac P-31 Magnetic Resonance Spectroscopy at 7 Tesla. Magn Reson Med 2014;72(2):304-315.

5. Schar M, El-Sharkawy AMM, Weiss RG, Bottomley PA. Triple Repetition Time Saturation Transfer (TRiST) (31)P Spectroscopy for Measuring Human Creatine Kinase Reaction Kinetics. Magn Reson Med 2010;63(6):1493-1501.

6. Schär M, Gabr RE, El-Sharkawy AM, Steinberg A, Bottomley PA, Weiss RG. Two repetition time saturation transfer (TwiST) with spill-over correction to measure creatine kinase reaction rates in human hearts. J Cardiov Magn Reson 2015;17(70).

7. Bashir A, Gropler R. Reproducibility of creatine kinase reaction kinetics in human heart: a 31P time-dependent saturation transfer spectroscopy study. NMR Biomed 2014;27(6):663-671.

8. Bottomley PA, Ouwerkerk R. The Dual-Angle Method for Fast, Sensitive T-1 Measurement in-Vivo with Low-Angle Adiabatic Pulses. J Magn Reson, Ser B 1994;104(2):159-167.

9. Clarke WT, Robson MD, Rodgers CT. Bloch-Siegert B1+-mapping for human cardiac 31P-MRS at 7 Tesla. Magn Reson Med 2015.

10. Bottomley PA, Ouwerkerk R. Birp, an Improved Implementation of Low-Angle Adiabatic (Bir-4) Excitation Pulses. J Magn Reson, Ser A 1993;103(2):242-244.

11. Rodgers CT, Clarke WT, Berthel D, Neubauer S, Robson MD. A 16-element receive array for human cardiac 31P MR spectroscopy at 7T. Proceedings of the 22th Annual Meeting of ISMRM, Milan, Italy 2014.

Figures

Description of the constituent acquisitions of the “FAST+Bloch-Siegert” protocol. The 7th scan is optional, but has been collected in this validation to confirm the absence of spill-over saturation.

Percentage error in the kfCK measured by the FAST method as a function of error in the paired flip angles (α=β/4). The black and red lines illustrate the range of error present in a tanh modulated BIR-4 pulse (8) and the Cramér-Rao bounds in a typical Bloch-Siegert B1 measurement.

a: Nominal and point-spread-function (PSF) volumes of voxels in this work (3D CSI) and previous determinations of kfCK. Broadened voxels are defined by the full-width-at-half-maximum of the PSF and by the 50% limit of coil sensitivity in non-localized dimensions. b: Example 1D-CSI grid. c: 3D-CSI grid used in this study.

Spectra, localized to the interventricular septum of a single subject, used in the determination of kfCK. a: Bloch-Siegert prepared spectra. b: Spectra collected using the “β” flip angle. There is little spill-over saturation and no residual γ-ATP peak. c: Spectra collected using the lower “α” flip angle.

Measured kfCK values in skeletal muscle at 3T and 7T (a) and in the myocardium at 7T (b). In skeletal muscle values were taken from voxels which experience an excitation flip angle within 10% of the nominal value. In the heart values are taken from the target interventricular voxel.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
1101