Quantitative and qualitative MR imaging using Ultra short TE at 3T for image guided radioembolization of liver metastases

Purpose: Local radiation therapy with neutron-activated holmium-loaded microspheres (HoMS) is a novel and promising treatment technique for both unresectable liver metastases and hepatocellular carcinoma [1]. In embolization therapies with radioactive microspheres, accurate assessment and quantification of the biodistribution is of major importance for treatment planning, dosimetry and follow up [2]. Since holmium is strongly paramagnetic it can be used as a T₂ ^* contrast agent [3]. In this work we utilize ultrashort TE (UTE) imaging in combination with multiple gradient echo (MGE) acquisition to allow the quantification of a wide range of T₂ ^* including sub-millisecond T₂ ^* values [4]. Furthermore, UTE images are used to allow selective depiction of ultrashort T2* species, as introduced by HoMS.

Materials and Methods: HoMS Phantom: For T₂ ^* relaxometry an agarose gel (2%) HoMS dilution series with HoMS concentrations ranging from 4 to 15mg/ml was created, providing a wide range of R₂ ^* values. Ex vivo rabbit liver: Qualitative MR imaging was done after administration of successive doses of 10mg of HoMS to the hepatic artery of an excised rabbit liver. MRI: MRI was performed on a 3T whole body scanner (Achieva, Philips Medical Systems, The Netherlands). T₂ ^* relaxometry was done using quantitative UTE (qUTE). QUTE was performed by varying the minimal echo time in successive UTE scans in an interleaved manner, allowing sampling of fast decaying signals with minimal echo times of 0.08, 0.15, 0.3, 0.6 and 1.2ms. Post-processing: T₂ ^* relaxometry was applied using weighted least squares fitting. Selective depiction of HoMS was realized by taking the difference of image between UTE images and later echoes.

Results T₂ ^* relaxometry using qUTE increased the maximum R2* value possible to estimate to at least 2500s^-1. Furthermore, difference UTE images allowed the selective depiction of HoMS in liver tissue, even at very small doses.

Conclusions The increased R₂ ^* range possible to quantify in combination with its capabilities to selectively depict low concentrations of HoMS makes UTE imaging a valuable tool for for treatment planning, dosimetry and follow up of local radiation therapy of hepatic malignancies with radioctive HoMS.

Literature:

[1] Nijsen et al. Radiology 2004;231:491–499.

[2] Vente et al. Anticancer Agents Med Chem 2007; 7:441–459.

[3] Seppenwoolde et al. Magn Reson Med. 2005; 53:76–84.

[4] Seevinck et al. Proc. 16th ISMRM 2008. p 1419.