Magnetic Resonance Materials in Physics, Biology and Medicine 11 (2000) 36–38

Methodological advances in cardiac 31P-MR spectroscopy

Markus von Kienlin *Department of Biophysics, Uni6ersity of Wurzburg, Wurzburg, Germany

Received 4 August 2000; accepted 4 August 2000

Keywords: Cardiac; MRS; Human

www.elsevier.com/locate/magma

1. Introduction

Some pioneering studies have demonstrated the im-portant clinical potential of cardiac spectroscopy forthe diagnosis of patients with coronary artery disease[1,2], with myocardial infarction [3,4] or with dilatedcardiomyopathy [5–7]. Nevertheless, as of today, thismodality does not have any clinical impact, mainly dueto its inherent technological limitations [8,9]. The mainproblem for 31P-MR spectroscopy is its low sensitivity,which restricts the achievable spatial resolution andimposes a long experimental duration. The sensitivitycan be increased using better instrumentation such asphased-array coils [10,11] or higher magnetic fields.This contribution rather deals with methodological ap-proaches to improve the accuracy of human cardiacspectroscopy. In particular, the SLOOP method is pre-sented, which has been demonstrated to yield quantita-tive 31P-spectra from the human myocardium with highreproducibility. When no a priori information is avail-able (or when one does not want to use it), acquisition-weighted spectroscopic imaging can provide accurateimages of the distribution of the high-energy phosphatemetabolites Figs. 1 and 2.

2. Quantitative 31P-spectroscopy using SLOOP

The method of choice to conduct human cardiac31P-spectroscopy nowadays clearly is chemical shiftimaging (CSI). Compared with the earlier techniques

such as DRESS [12] or ISIS [13,14], CSI has theadvantage to be a multi-voxel technique with a gooddelineation of the sensitive volume. For quantitativeexaminations, CSI again stumbles about the issue ofpoor sensitivity. Either the size of the sensitive volumeis chosen small enough to fit completely into the my-ocardium, and one only obtains a poor signal-to-noiseratio (SNR), or a large size of the sensitive volumeimplies significant contamination of the myocardialspectrum by signals originating from adjacent tissue.To some extent, this issue can be circumvented by thetechnique (spectral localization with optimalpointspread function) SLOOP [15].

The data reconstruction in SLOOP is based on theSLIM algorithm [16] introduced by Hu et al. It takesadvantage of all a priori information that is availableabout the heart, for example, from 1H morphologicalimages. This allows one to define some anatomicalstructure — for instance the left ventricular my-ocardium — as a compartment, and to reconstruct theMR spectrum for this sensitive volume that is matchedin shape and size to the anatomy. In addition to SLIM,SLOOP also allows one to determine the optimal exper-imental parameters for the data acquisition, and itevaluates the spatial response function to assess thequality of localization and the efficiency of signal detec-tion. Finally, SLOOP intrinsically can provide absolutequantitation: all known parameters such as (locallyvarying) excitation flip angle, B1-field distribution orT1-relaxation are taken into account when modeling theexperiment, and therefore, are automatically accountedfor in the reconstruction.

R. Loeffler et al. have implemented SLOOP on aclinical instrument and could demonstrate the superiorperformance of SLOOP over CSI [17]. In a volunteerstudy, M. Meininger et al. showed that the concentra-

* Present address: MR Imaging and Spectroscopy, F. Hoffmann-La Roche AG Pharma, PRBT-S 68/05B, CH-4070 Basel, Switzerland.Tel.: +41-61-6871411; fax: +41-61-6871910.

E-mail address: markus–f–m.von–kienlin@roche.com (M. vonKienlin).

1352-8661/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S 1 3 5 2 -8661 (00 )00106 -X

M. 6on Kienlin / Magnetic Resonance Materials in Physics, Biology and Medicine 11 (2000) 36–38 37

tion of high-energy phosphates could be measured withbetter reproducibility than with other methods [18].

SLOOP has recently been used in a larger study toevaluate patients with DCM or after myocardial infarc-tion. The concept behind SLOOP will be presented inmore detail, and first clinical results will be shown.

3. Acquisition-weighted 31P-spectroscopic imaging

In some circumstances, one is less interested in thelocal concentration of some metabolites, but rather inthe (non-) uniformity of their distribution. Such a situa-tion can arise for instance after myocardial infarction,where a non-viable region would present itself by theabsence of high-energy phosphates. In this case, theinclusion of a priori information in a data evaluationprotocol might lead to erroneous conclusions. Withconventional spectroscopic imaging protocols, however,there is far too much ‘cross-talk’ between adjacentvoxels to generate reliable metabolic maps of the hu-

man myocardium. This problem of ‘cross-talk’ is partic-ularly severe under circumstances where the spatialresolution of the experiment is similar to the size ofanatomical structures. With conventional CSI, signalsoriginating in the breast muscle (which has very highconcentrations of PCr, and which lies in the mostsensitive region of the surface coil) can spread acrossthe whole metabolic image. This difficulty has lead to alarge variability in the published results about metabo-lite ratios or concentrations in the human heart. Webelieve that these discrepancies are to a large extent dueto the varying experimental protocols and to the con-comitantly varying degree of signal contamination.

It is well known that this signal contamination can besignificantly reduced with no loss in sensitivity or spa-tial resolution using acquisition weighted CSI [19]. Un-fortunately, until today, such methods with threespatial dimensions have not been routinely available onhuman instruments. We now have implemented 3Dacquisition weighted CSI on an experimental human 2Tsystem and could generate metabolic maps in a number

Fig. 1. Spectra reconstructed from human left ventricular myocardium with a conventional CSI reconstruction (box on left panel, top spectrum)and with SLOOP (outlined compartment on left panel, bottom spectrum). The SLOOP spectrum has less blood contamination and better SNR.

Fig. 2. Patient with myocardial infraction in anterior wall. Left panel: anatomical 1H-image acquired with a surface coil. The white squarerepresents the region depicted in the right panel. Right panel: corresponding ATP-image acquired in ca. 30 min on a 2T-instrument. The infarctedregion in the anterior wall can be discerned from the reduced level of ATP.

M. 6on Kienlin / Magnetic Resonance Materials in Physics, Biology and Medicine 11 (2000) 36–3838

of human volunteers and also in a patient with an oldmyocardial infarction in the anterior wall.

The reduced contamination in acquisition weightedCSI again leads to systematically different values thatare observed, i.e. for the PCr/ATP ratios. The methodshow to accurately control the spatial resolution inacquisition-weighted CSI and first results from humanmeasurements will be presented.

Acknowledgements

All the human cardiac work was conducted in closeand excellent cooperation with S. Neubauer from theUniversity Hospital. We are very grateful to M. Beer,T. Pabst and D. Hahn from the Institute of DiagnosticRadiology, where the various SLOOP projects werecarried out. The 31P-CSI study was conducted on the2T MR-instrument in the Department of Biophysics,and I am indebted to my numerous colleagues therewho participated in these projects, W. Landschutz, M.Meininger, R. Pohmann, A. Greiser and A. Haase.

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