Table 1.
Inclusion of pediatric patients (see footnote for abbreviations).
Fig 1.
Schematic overview of the MR protocols and measurements for both patients and phantoms.
Patients: (left) A head coil was used preoperatively, whereas a combination of two flex coils was used for the intraoperative setting. For patients, up to 16 ROIs were placed in 3D-QALAS images in each patient, in normal-appearing white matter (NAWM) and the thalamus. Data were compared using Bland-Altman plots. Phantoms: (right) Overview of the control experiments using four different homogenous relaxometric spherical phantoms. Measurements were performed using both a head array coil and the combination of two flex coils, replicating the measurements performed on patients. The positions of the four different phantoms (and flex coils) were moved into three different spatial positions (A/P direction) in the bore of the magnet, resulting in a total of 60 different ROIs in the phantoms. Fifteen ROIs were then placed in the A/P direction of the images, at varying distances from the centre of the phantom (and isocentre). Data were compared using both correlation and Bland-Altman plots, including a comparison of 3D-QALAS with gold standard pulse sequences (IR and ME).
Fig 2.
Schematic overview of the positioning of the patient.
(A) Patient in the preoperative, and (B) in the intraoperative settings. The outer circle illustrates the bore of the magnet. Note that different MR detection coils were used (conventional head array coil versus a combination of two flex coils). In addition, the patient positions with comparison to the isocentre of the magnet were very different in the preoperative versus the intraoperative settings due to the spatial requirements of the headrest, and due to the patient-specific determined rotation and height of the patient’s head. The headrest was used to fix the patient’s head in the appropriate position. Red: Tumour; Blue: Tumour cavity. ISO: Isocentre of the MR scanner.
Fig 3.
Example of 3D-QALAS patient measurements.
(A) 3D-QALAS pulse sequence signal pathway. The 3D-QALAS sequence consists of five so called ‘dynamics’ (see top panel where the five points for data sampling are marked with green squares). Each dynamic consists of a preparation phase and an acquisition phase. In the first dynamic, a T2 preparation pulse is used to make the first volume T2 weighted. The second dynamic starts with a 180 degree-inversion pulse to encode R1 relaxation. In all dynamics, the acquisitions are read with a 4-degree flash readout. (B) Signal intensities. Residuals on top and the signal amplitudes in the preop- (green) and intraop situations (blue). (C). Preoperative images. Sagittal images of a patient at different time points in the pulse sequence. (6 mm diameter ROI, containing ca. 14 voxels). The dynamic scale represents signal intensities. (D) Intraoperative images. Corresponding images, but in the intraoperative situation. (6 mm diameter ROI, same anatomical location as in (C), containing ca. 11 voxels). See Fig 4 for examples of ROIs in axial slices. The dynamic scale represents signal intensities.
Fig 4.
Typical examples of ROI-placements in patient.
Examples of the placement of ROIs in three different preoperative slices of a patient. Peduncles (LEFT), corpus callosum splenium (MIDDLE), frontal left and parietal right (RIGHT).
Fig 5.
Correlation between R1 and R2 in patients.
(A) Samples of normal-appearing matter in preoperative vs intraoperative settings. The same structure should have identical values in both settings, everything else being equal. However, a difference was observed between the pre- and intraoperative R2 clusters. In contrast, this was not obvious for R1. Cf. the Discussion for a comprehensive analysis. (B and C) Bland-Altman plots of the difference between determined R1 (B) and R2 (C) between the intraoperative and preoperative examinations. The difference was insignificant for R1, but not for R2. An R2 difference of about 2 s-1 was observed.
Fig 6.
Quality control and quantification using four different relaxation phantoms (10, 15, 20 and 30 mM). The accuracy of the R1 and R2 measurements using 3D-QALAS sequences compared to gold standard measurements (of R1 and R2) showed a slight difference for R2 and a small difference but only for R1 in the 30 mM phantom. The relaxation values were determined at 15 different positions in the phantoms. Green line shows the maximum value found in vivo.