Detectability and structural stability of a liquid fiducial marker in fresh ex vivo pancreas cancer resection specimen on CT and 3T MRI

Objectives
The aim of this study was to test the visibility of a new liquid fiducial marker injected in ex vivo pancreas tissue on magnetic resonance imaging (MRI) and computed tomography (CT). Furthermore, its injection performance using different needle sizes was investigated as well as its structural stability after fixation in formaldehyde.
Material and Methods
Liquid fiducial markers with a volume of 20-100 µL were injected into the freshly resected pancreas of three patients (two males age 69 and 72, one female age 68) with suspected adenocarcinoma of the pancreatic head. Injection was performed under X-ray guidance using a high precision unit dose injector with different needle sizes (18G, 22G, 25G). While cooled on ice, the specimens were scanned on MRI and CT with routine clinical sequences. Signal threshold based segmentation was performed manually on CT. The marker volume visible on CT was compared to the actually injected volume as a measure of potential marker backflow. After rigid registration of the MR images to the CT data set, marker detectability was assessed by searching for the corresponding hypointense structure in the respective segmentation.
Results
Markers with a volume of ≥ 20 µL were easily detected as hyperintense structures on X-ray and CT. In clinically used T1- and T2-weighted 3T MRI sequences, all marker sizes ranging from 20µL – 100µL were visible as hypointensity. Since most markers were non-spherical however, MRI visibility was relatively poor and their differentiation from hypointensities caused by air cavities or surgical clips was challenging and only feasible with a reference CT. Marker backflow was observed when injected with an 18G needle, which was prevented by injection using a smaller 22G and 25G needle. The marker was stable after 24h fixation in formaldehyde where only small volume degradations were observed (6.6±13.0%) and with the exception of one instance no wash out occurred.
Conclusion
The liquid fiducial marker with injected volumes of 20µL – 100µL, injected in an ex vivo pancreatic cancer resection specimen, was visible as hyperintensity on kV X-ray, CT and hypointensity on MRI and stable over a period of 24 hours in formaldehyde. Since most injected markers were non-spherical, a marker size of ≥50µL is recommended for the clinically used MRI sequences. Most likely, in vivo marker injection will result in more spherical forms due to persisting metabolism, and this in turn will enhance MRI visibility in an hyper intense structure.