Virtual unenhanced dual-energy computed tomography for photon radiotherapy: The effect on dose distribution and cone-beam computed tomography based position verification.

Background and Purpose: Virtual Unenhanced images (VUE) from contrast-enhanced dual-energy computed tomography (DECT) eliminate manual suppression of contrast-enhanced structures (CES) or pre-contrast scans. CT intensity decreases in high-density structures outside the CES following VUE algorithm application. This study assesses VUE's impact on the radiotherapy workflow of gynecological tumors, comparing dose distribution and cone-beam CT-based (CBCT) position verification to contrast-enhanced CT (CECT) images. Materials and Methods: A total of 14 gynecological patients with contrast-enhanced CT simulation were included. Two CT images were reconstructed: CECT and VUE. Volumetric Modulated Arc Therapy (VMAT) plans generated on CECT were recalculated on VUE using both the CECT lookup table (LUT) and a dedicated VUE LUT. Gamma analysis assessed 3D dose distributions. CECT and VUE images were retrospectively registered to daily CBCT using Chamfer matching algorithm.. Results: Planning target volume (PTV) dose agreement with CECT was within 0.35% for D2%, Dmean, and D98%. Organs at risk (OARs) D2% agreed within 0.36%. A dedicated VUE LUT lead to smaller dose differences, achieving a 100% gamma pass rate for all subjects. VUE imaging showed similar translations and rotations to CECT, with significant but minor translation differences (<0.02 cm). VUE-based registration outperformed CECT. In 24% of CBCT-CECT registrations, inadequate registration was observed due to contrast-related issues, while corresponding VUE images achieved clinically acceptable registrations. Conclusions: VUE imaging in the radiotherapy workflow is feasible, showing comparable dose distributions and improved CBCT registration results compared to CECT. VUE enables automated bone registration, limiting inter-observer variation in the Image-Guided Radiation Therapy (IGRT) process.

Phase contrast MRI measurements of net cerebrospinal fluid flow through the cerebral aqueduct are confounded by respiration.

BACKGROUND: Net cerebrospinal fluid (CSF) flow through the cerebral aqueduct may serve as a marker of CSF production in the lateral ventricles, and changes that occur with aging and in disease. PURPOSE: To investigate the confounding influence of the respiratory cycle on net CSF flow and stroke volume measurements. STUDY TYPE: Cross-sectional study. SUBJECTS: Twelve young, healthy subjects (seven male, age range 19-39 years, average age 28.3 years). FIELD STRENGTH/SEQUENCE: Phase contrast MRI (PC-MRI) measurements were performed at 7T, with and without respiratory gating on expiration and on inspiration. All measurements were repeated. ASSESSMENT: Net CSF flow and stroke volume in the aqueduct, over the cardiac cycle, was determined. STATISTICAL TESTS: Repeatability was determined using the intraclass correlation coefficient (ICC) and linear regression analysis between the repeated measurements. Repeated measures analysis of variance (ANOVA) was performed to compare the measurements during inspiration/expiration/no gating. Linear regression analysis was performed between the net CSF flow difference (inspiration minus expiration) and stroke volume. RESULTS: Net CSF flow (average ± standard deviation) was 0.64 ± 0.32 mL/min (caudal) during expiration, 0.12 ± 0.49 mL/min (cranial) during inspiration, and 0.31 ± 0.18 mL/min (caudal) without respiratory gating. Respiratory gating did not affect stroke volume measurements (41 ± 18, 42 ± 19, 42 ± 19 µL/cycle for expiration, no respiratory gating and inspiration, respectively). Repeatability was best during inspiration (ICC = 0.88/0.56/-0.31 for gating on inspiration/expiration/no gating). A positive association was found between average stroke volume and net flow difference between inspiration and expiration (R = 0.678/0.605, P = 0.015/0.037 for the first/second repeated measurement). DATA CONCLUSION: Measured net CSF flow is confounded by respiration effects. Therefore, net CSF flow measurements with PC-MRI cannot in isolation be directly linked to CSF production. LEVEL OF EVIDENCE: 1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;49:433-444.

T 2 mapping of cerebrospinal fluid: 3 T versus 7 T.

OBJECT: Cerebrospinal fluid (CSF) T 2 mapping can potentially be used to investigate CSF composition. A previously proposed CSF T 2-mapping method reported a T 2 difference between peripheral and ventricular CSF, and suggested that this reflected different CSF compositions. We studied the performance of this method at 7 T and evaluated the influence of partial volume and B 1 and B 0 inhomogeneity. MATERIALS AND METHODS: T 2-preparation-based CSF T 2-mapping was performed in seven healthy volunteers at 7 and 3 T, and was compared with a single echo spin-echo sequence with various echo times. The influence of partial volume was assessed by our analyzing the longest echo times only. B 1 and B 0 maps were acquired. B 1 and B 0 dependency of the sequences was tested with a phantom. RESULTS: T 2,CSF was shorter at 7 T compared with 3 T. At 3 T, but not at 7 T, peripheral T 2,CSF was significantly shorter than ventricular T 2,CSF. Partial volume contributed to this T 2 difference, but could not fully explain it. B 1 and B 0 inhomogeneity had only a very limited effect. T 2,CSF did not depend on the voxel size, probably because of the used method to select of the regions of interest. CONCLUSION: CSF T 2 mapping is feasible at 7 T. The shorter peripheral T 2,CSF is likely a combined effect of partial volume and CSF composition.