Abstract
Computed tomography (CT) is the standard imaging modality in radiation therapy treatment planning (RTP). However, magnetic resonance (MR) imaging provides superior soft tissue contrast, increasing the precision of target volume selection. We present MR-only based RTP for a rat brain on a small animal radiation research platform (SARRP) using probabilistic voxel classification with multiple MR sequences. Six rat heads were imaged, each with one CT and five MR sequences. The MR sequences were: T1-weighted, T2-weighted, zero-echo time (ZTE), and two ultra-short echo time sequences with 20 μs (UTE1) and 2 ms (UTE2) echo times. CT data were manually segmented into air, soft tissue, and bone to obtain the RTP reference. Bias field corrected MR images were automatically segmented into the same tissue classes using a fuzzy c-means segmentation algorithm with multiple images as input. Similarities between segmented CT and automatic segmented MR (ASMR) images were evaluated using Dice coefficient. Three ASMR images with high similarity index were used for further RTP. Three beam arrangements were investigated. Dose distributions were compared by analysing dose volume histograms. The highest Dice coefficients were obtained for the ZTE-UTE2 combination and for the T1-UTE1-T2 combination when ZTE was unavailable. Both combinations, along with UTE1-UTE2, often used to generate ASMR images, were used for further RTP. Using 1 beam, MR based RTP underestimated the dose to be delivered to the target (range: 1.4%-7.6%). When more complex beam configurations were used, the calculated dose using the ZTE-UTE2 combination was the most accurate, with 0.7% deviation from CT, compared to 0.8% for T1-UTE1-T2 and 1.7% for UTE1-UTE2. The presented MR-only based workflow for RTP on a SARRP enables both accurate organ delineation and dose calculations using multiple MR sequences. This method can be useful in longitudinal studies where CT’s cumulative radiation dose might contribute to the total dose.
Highlights
Small animal models are widely used for the development of new cancer treatment strategies
The highest Dice coefficient was obtained for the zero echo time (ZTE)-UTE2 combination with a value of 0.617 ±0.013
The highest Dice coefficient for a combination without ZTE was obtained for the T1-UTE1-T2 combination with a value of 0.608±0.019
Summary
Small animal models are widely used for the development of new cancer treatment strategies. The recently developed 3D conformal small animal micro-irradiators have significantly reduced the technological gap between laboratory radiation research and human treatment methods [1,2]. These devices are using kV X-ray sources that combine irradiation with highresolution cone-beam (CB) computed tomography (CT) [3]. There is no contrast between air, lung and bone To solve this problem, novel MR sequences have been developed that acquire the MR signal directly after radio-frequency excitation, such as ultra-short echo time (UTE) and/or zero echo time (ZTE) sequences. This is the first study that investigates the use of an MR-only based workflow in pre-clinical RTP and the first study that uses ZTE sequence for image segmentation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
