Session Item

Compressed SENSE (CS) and 3D VANE are MR techniques which recently became available for the 1.5 T Unity MR-linac (Elekta AB, Sweden). While 3D VANE scans are aimed at reducing motion artifacts by sharing k-space data between acquisitions, CS is used to decrease scan times by sparsely sampling k-space. In this phantom study we investigate the effect of using these techniques in relation to imaging moving structures under free breathing conditions for radiotherapy treatment planning.

A 4D phantom (ModusQA, Canada) with a moving insert containing a 30 mm spherical target was used. Imaging was performed with and without motion. The motion pattern was cos4-shaped with a peak-to-peak amplitude of 20 mm and period of 4 seconds, oriented in cranial/caudal (CC) direction. Three types of scans were tested (T2_3D_TSE: SENSE factor 3; T2_3D_TSE_CS: CS factor 5;  T1_3D_VANE XD Dixon: water image) with a reconstructed voxel of 0.64x0.64x2/0.64x0.64x2/0.78x0.78x1.5mm3 and a scan duration of 4:04/2:52/5:43 minutes, respectively. The spherical target was delineated. From the delineations the cranial/caudal (CC) length was extracted, which is a surrogate to measure if the GTV (30 mm), the ITV (30+20 = 50 mm) or a volume in between is shown. In addition, the center-of-volume (CoV) of the delineation was determined. A high and low contrast imprint of the sphere was visible in all motion images therefore a full (automatic using threshold) and tight (most prominent imprint of the spherical sphere only) delineation was performed (figure 1).

Static scans confirmed a CC sphere diameter of 30 mm for all 3 imaging techniques. For the motion scans, this full delineation diameter increased to 48/42/46.5 mm for T2 TSE/T2 TSE CS/VANE respectively (figure 2). The CoV deviated -1/0/-1 mm from the time-averaged (mid-)position. For the tight delineation a CC diameter of 30/36/28.5 mm was observed, while the CoV shifted to 5/1/7 mm compared to mid-position.

The observed CC diameter is largest in T2_3D_TSE and T1_3D_VANE and coincides with the definition of the ITV, while for T2_3D_TSE_CS a markedly decreased dimension was observed. CoV positions were close to mid-position for all techniques. For the tight delineation, the CC diameter of T2_3D_TSE and T1_3D_VANE were close to the actual sphere , however at the largest deviations from mid-position; especially T1_3D_VANE is positioned closer to end-expiration position. This end-expiration position is the most prevalent position (but not the time average position) in the cos4 signal which therefore prevailed in the T1_3D_VANE reconstruction. For CS reconstruction (T2_3D_TSE_CS), motion artifacts seem to be partly suppressed by the iterative reconstruction process which removes the non-coherent artifacts arising from sparse sampling.

Alterations in volume and position due to MRI acquisition/reconstruction techniques need to be considered in treatment planning and delivery.