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First heart images with the 7 Tesla MRT

At the 7 Tesla research tomograph, which is jointly operated by the Max Delbrück Centre for Molecular Medicine, the Charité Universitätsmedizin Berlin and the PTB in Berlin-Buch, first successful imaging experiments of the human heart were performed. Similarly sensitive measurements have been successfully performed so far only by two other research groups. The results open up a new horizon for non-invasive tissue differentiation and the diagnosis of tissue modifications of the cardiac muscle.

Heart images recorded at 7 Tesla (late diastole). In the image on the left (without B1+ shimming), artefacts caused by field inhomogeneities can be seen which can be eliminated by optimized field homogeneization (on the right).

Magnetic resonance imaging (MRI) offers the unique possibility of representing the anatomy and the function of the heart with high tissue and bloodmuscle contrast and with good spatial and temporal resolution. In the case of the MRI field strengths between 1.5 Tesla and maximally 3 Tesla which are usual today, the spatial resolution is limited to slice thicknesses from 7 mm to 10 mm. Ultra-high field systems (e. g. 7 Tesla) allow, however, a new diagnostic quality to be achieved. To utilize this potential, fundamental difficulties of ultra-high field imaging have to be overcome: in this way, one would like to reduce the heating of the tissue caused by the absorbed radio-frequency (RF) power and to generate, at the same time, as homogeneous a RF field as possible which furnishes sharp images.

For this purpose, a 4-channel transceiver coil has been developed which allows – in the case of an acceptable power deposition – a good illumination of the thorax. The field inhomogeneity in the target volume could be optimized by a procedure in which – for each coil element – relative cards of the RF excitation field B1+ are measured, and the respective amplitudes and phases are entered into the calculation of RF excitation (so-called B1+shimming). Finally, triggering to the cardiac cycle, which is required for imaging of the different phases of a heart beat, could be achieved with the aid of an acoustic method, as the ECG usually applied at lower field strengths is – due to strong signal disturbances – unsuitable in the ultra-high field.

With this measuring technique, sufficiently homogeneous heart images with a slice thickness of only 4 mm can be obtained. This allows a better diagnosis of tissue changes of the myocardium to be made, for example in the case of scar tissue after a myocardial infarction, of diseases of the cardiac valves and irregularities of the blood flow in the cardiac muscle. Now, especially the diagnostic imaging of the thin-walled right ventricle seems possible. The partners of the Siemens AG (4-channel coil), of the Charité-Buch (heart imaging), of the MDC (acoustic triggerung) and of PTB (B1+shimming) made contributions to the jointly achieved result.


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