Fig. 1: The new Bruker BiopSpec 94/20 with Avance III HD hardware went operational in April 2015. The system provides a wide range of MR coils for a wide range of applications, including a helium cooled, high performance 2ch quadrature coil with exceptional low noise performance.

In April 2015 the newly installed 9.4T small animal MRI scanner from Bruker went operational. Compared to small animal imaging at the clinical 3T system the new high field system provides a much smaller magnet bore of 20cm (inner diameter). Including the necessary gradient system and a volume coil the usable diameter is 86mm, which makes the scanner primarily suitable for rats, mice ad tissue samples. The high field strength provides a higher sample magnetization and thereby a stronger MRI signal. At the same time the gradient system provides a much higher performance (660 mT/m or 1000 mT/m with gradient insert) compared to clinical systems with typically 40-80 mT/m. This gradient performance allows very high resolution imaging as well as fast imaging techniques like EPI, which are not applicable to mice on a clinical 3T Scanner. The dedicated small animal scanner opens access to the full wide range of sequences and MRI contrast mechanisms for small animals.

High Resolution anatomical MR imaging

Fig. 2: High resolution (83µm isotropic), T2-weighted anatomical images of a mouse brain (fixated). From a single acquired isotropic 3D data set arbitrary slices positions can be viewed without reduced image quality.
Fig. 3: 3D in vivo imaging of the arterial vessels in a mouse brain (38µm isotropic resolution).

The dedicated small animal MR scanner (Fig. 1) enables very high resolution MRI in mice by combining the ultra high filed strength of 9.4T with a high performance gradient system and a highly efficient helium cooled coil. This allows anatomical imaging as shown in Fig. 2, but the very fast gradient system also permits hardware intensive sequence techniques like functional time-of-flight angiography to delineate the arterial blood vessels in the mouse brain (Fig. 3). However, even with the high performance system measurement time can exceed realistic anesthesia times and Fig. 2 shows data acquired from a formalin fixated mouse at an isotropic resolution of 83µm and an acquisition time of 8 hours. The employed sequence was a standard spin echo sequence without accelerations of any kind, leading to the very clean and artifact free images. One major advantage of the isotropic 3D resolution is the possibility to reslice the data and display arbitrary slice positions and directions without reducing the image quality.

Fig. 3: 3D in vivo imaging of the arterial vessels in a mouse brain (38µm isotropic resolution).

The rather long acquisition times of 3D sequences can be substantially reduced by accelerated sequence techniques (e.g. RARE or Turbo Spin Echo) or by choosing a different contrast like T1 weighted images. The time-of-flight angiography shown in Fig. 3 provides an isotropic resolution of 38µm and required only 1h 30 min of scan time, which was preformed in vivo on an anesthetized mouse. Since T1-weighting is created by a short repetition time TR, which doesn't allow the spins sufficient time for a full T1 Relaxation, the 3D acquisition can be performed much faster as in the T2-weighted example. The time-of-flight angiography is created by using a relatively high flipangle, which leads to saturation of all static tissues in the brain. In contrast, the blood in the mouse's body and heart is outside the coils transmit field, therefore the blood is fully T1 relaxed. When this fresh, unsaturated blood enters the imaging area of the head it will provide maximum signal, leading to the bright gray value of all inflowing arterial blood vessels.

Kontakt

Dr. rer. nat. Karl-Heinz Herrmann

Medizinische Physik / IDIR,
wissenschaftlicher Mitarbeiter,
S1 Projektleiter am Kleintier MRT

Philosophenweg 3
07743 Jena

phone: +49 3641 9-390736
fax: +49 3641 9-390728
Web

Small animal MRI @9.4T

High Resolution anatomical MR imaging

Kontakt