One of the main challenges in magnet resonance imaging (MRI) is the limited signal-to-noise ratio (SNR). The University Hospital Erlangen is one of the few sites hosting a clinically approved Ultra High Field 7 Tesla MRI system. The increased field strength compared to conventional systems allows for a greatly improved SNR so that image resolutions of a few 100 micrometers can be reached.
As the wavelength of the transmit field becomes of the same size as the human head at 7 Tesla, one must face the challenge of minimizing the arising inhomogeneous image brightness (Fig. 1, left). Moreover, the electric field may become inhomogeneous potentially entailing hot spots of the specific absorption rate (SAR), which must be taken into account in SAR calculations.
In order to achieve a homogeneous transmit field, we shape the radio frequency (RF) excitation field using eight different transmission coils, which transmit in parallel (parallel Transmit, pTx) . For this, the transmission coils and the gradient coils are driven simultaneously but independently with individual voltage curves and pulse shapes. The basic underlying calculation approach is called ‘Transmit-SENSE’, in analogy to the established ‘SENSE’ algorithm using receive coil sensitivity profiles. During excitation, gradients are applied to influence the static magnetic field using the concept of ‘transmit k-space’. The optimal interaction of k-space trajectory and RF pulse shapes of all transmit coils is expressed as a minimization problem to be solved patient-specifically during an examination and as fast as possible to make it feasible in clinical routine.
Using this approach, images with little brightness inhomogeneity and low SAR exposure can be achieved (Fig. 1, right).
Figure 1. MP-Rage image at 7 Tesla.
Left: Image brightness drops in some regions owing to the inhomogeneous transmit field. Right: By using the pTX approach, we were able to homogenize the image brightness.
 Ladd ME, Bachert P, Meyerspeer M, Moser E, Nagel AM, Norris DG, Schmitter S, Speck O, Straub S, Zaiss M.
Pros and cons of ultra-high-field MRI/MRS for human application.
Prog Nucl Mag Res Sp 2018; 109: 1-50.