Session Item

Radiotherapy preparation MRI is often based on 3D sequences. However, the use of immobilization devices and flexible coils lead to reduced Signal to Noise ratio (SNR). Compressed Sensing is a novel acceleration technique for 3D acquisitions. We investigated Compressed Sensing (CS) acceleration and compared it with CAIPIRINHA k-space based acceleration.

The goal was to obtain improved resolution and contrast in the same time with equivalent Signal-to-Noise Ratio (SNR). All acquisitions were performed with the patient in treatment position using radiotherapy immobilization devices on a 1.5T Sola MRI (Siemens). T1 and T2 FLAIR acquisitions were investigated for brain lesions and T2 acquisitions for pelvic purposes.

Sequences were first optimized on phantom using NEMA subtraction method in order to obtain equivalent SNR, as there were no real guidelines yet for compressed sensing factors and denoising factors. After, patient acquisitions were evaluated quantitatively (post-contrast T1 and T2 FLAIR) by a student t test and qualitatively through a randomized alternative choice test (1=CS preferred, 0 = equal, -1 = CAIPIRINHA) by three expert radiologists and two radiation oncologists through a Wilcoxon signed rank test. Rater agreement was evaluated by a pairwise Kappa Cohen test.

CS 3D T1 brain acquisitions were evaluated as superior both quantitatively and qualitatively (rater value = 0.6, p < 0.05) and benefited from improved lesion contrast of 7% (p=0.017, 17 lesions) due to reduced repetition time which would otherwise lead to insufficient SNR for CAIPIRINHA acceleration. T2 FLAIR acquisitions were quantitatively superior (resolution), but qualitatively evaluated as equivalent with rater value 0.3 (p > 0.05). 3D T2 pelvic acquisitions were evaluated as inferior with CS acceleration: no significant gain in resolution/SNR was obtained and mean rater value was -0.5 (p<0.05). The results of the raters suffered however from large interrater variability with pairwise Cohen’s Kappa < 0.33.

Compressed sensing is an useful technique to overcome the flexible coil SNR issues for radiotherapy preparation for intracranial T1 SPACE and T2 FLAIR. However for pelvic 3D T2 images, k-space based acceleration techniques should be used.