Abstract
The aim of this work was to demonstrate the feasibility of NMR tomography at 3 Tesla for selected medical applications and to investigate the advantages and disadvantages of the high field strength.
For cardiac imaging a fast, ECG gated, flow compensated gradient echo sequence was implemented and optimized. Within a breath-hold period (17 heartbeats) artifact-free images of slices of any desired orientation, e.g. short axis slices, and cine-sequences, i.e. the movement of the heart within a slice during the heart cycle, could be acquired. With the use of specially developed multi-element surface coils for receiving in combination with the whole body resonator for transmitting, an increase in signal-to-noise ratio (SNR) by a factor of 2 was achieved compared to a field strength of 1.5 T.
Regions of different magnetic susceptibility cause larger B0 inhomogeneities (±
1 ppm), leading to shorter T2* relaxation times within the left ventricle (<
20 ms). Therefore cardiac imaging, and especially the use of real-time imaging sequences, which are prone to artifacts, is likely to be ore difficult at 3 T compared to lower field strength. The decrease of T2* with increasing magnetic field strength is attributed to susceptibility effects; hence a higher BOLD (Blood Oxygenation Level Dependent) contrast is expected at 3 T, which can be exploited for tissue oxygenation and perfusion measurements.
At 3 T the RF-wavelength within the body is comparable to body dimensions, thus dielectric resonances influence the electromagnetic field distribution. This may lead to B1 field inhomogeneities. Furthermore, compared to 1.5 T, 4 fold higher RF-power is needed to achieve the same flip angle of the magnetization at the same pulse length. Therefore the use of spin echo sequences and various preparation sequences is more difficult at 3 T.
In the second part of the work a MR thermography technique (temperature probe method), using a paramagnetic Praseodymium complex (Pr-MOE-DO3A) as a contrast media, was investigated in combination with a fast spectroscopic imaging technique (Echo Planar Spectroscopic Imaging, EPSI) aiming at therapy control of regional hyperthermia treatment. Using the EPSI method, in a phantom the distribution of absolute temperature was measured in a volume of 24 ´
24 ´
24 cm3 (voxel size 1.5 ´
1.5 ´
1.5 cm3) within 14 s to an accuracy of ±
0,45 °C.
This work demonstrates that the use of higher field strengths is not only accompanied by advantages but also by substantial disadvantages. Although having great potential for medical research and special areas of medical diagnostics and therapy control, MR imaging and spectroscopy at high field strengths (?
3 T) is unlikely to replace the clinically well-established MR tomography at lower field strengths (1.0 – 1.5 T). | 
