Browsing by Author "Mehtonen, Jonna"

In MRI, the diffusion of water molecules can be imaged using Diffusion Weighted Imaging (DWI). Increasing cellularity in tumor tissue causes the restriction of water diffusion to increase, which will show as a high contrast in DW-image. Due to this advantage, the DWI shows great promise as a biomarker for cancer treatment. However, currently the most commonly used DWI technique is an Echo-Planar-Imaging (EPI) based sequence which suffers greatly from geometric distortion. Therefore, it is not applicable for radiation therapy planning. Turbo-Spin-Echo (TSE) based DWI sequences are proposed due to their great geometric accuracy. However, Signal-to-Noise Ratio (SNR) of DWI-TSE is poor compared to DWI-EPI. The purpose of this work is to evaluate the image quality of different DWI sequences for the use of radiation therapy planning. The evaluation is done by comparing SNR, patient-induced susceptibility effect on geometric distortion, and Apparent Diffusion Coefficient (ADC) correctness of DWI-TSE sequences to the most commonly used DWI-EPI. The selected TSE based sequences are SPLICE and Alsop. The image quality comparison is also done between two radiation oncology products for MRI: Philips Ingenia 1.5T MR-RT and Elekta Unity 1.5T (MR-Linac). In addition, the aim is to see how Compressed SENSE (CS-SENSE) technique affects the image quality of SPLICE. The SNR comparison of DWI-EPI, Alsop, SPLICE, and SPLICE with Compressed SENSE were done both with a phantom and with a volunteer. Same results were observed from both studies. The DWI-EPI has significantly higher SNR than TSE-based DWI. However, the SPLICE achieved nearly square root of two times better SNR compared to the Alsop sequence. The CS-SENSE improved the SNR of SPLICE notably both in a volunteer and phantom studies. In volunteer studies, the SNR of SPLICE with CS-SENSE achieved 45% of the SNR of diagnostic DWI-EPI with Ingenia MR-RT setup, and only 27% with MR-Linac. Therefore, the image quality of diagnostic DWI-EPI can be reached with Ingenia MR-RT setup for SPLICE with CS-SENSE by optimizing the sequence. However, with MR-Linac the same image quality cannot be reached with acceptable acquisition time. The susceptibility-induced geometric distortion of DWI-EPI and SPLICE were analyzed using the same volunteer as in SNR measurements. As a result, the median distortion value for DWI-EPI was 1.5 mm in Ingenia 1.5T and 1.7 mm in MR-Linac 1.5T. Whereas, the median distortion of SPLICE was 0.02 mm in both systems. Therefore, the subject-induced susceptibility effect has an insignificant impact on the geometric accuracy of TSE-based DWI. Thus, DWI-TSE can be used for RTP in favor of geometric accuracy. The ADC value correctness of DWI-EPI, DWI-TSE, and SPLICE were measured using a standardized diffusion phantom. All ADC values of TSE-based DWI were within 14% of the reported literature values. Whereas, the maximum difference in DWI-EPI was 50%. The large variety in the ADC values of DWI-EPI were caused by geometric inaccuracy in the anterior side of the phantom. The minor changes in the ADC values of TSE-based DWI proves that the ADC values are reliable for clinical use.