Omnipaque

The candidates for this study were patients with suspected iNPH. After obtaining written informed consent, the eligible patients were pre-registered and received lumbar puncture. The inclusion criteria were (1) age between 60 and 85 years, (2) presence of one or more symptom(s) of the triad (gait disturbance, cognitive impairment, and urinary symptoms), which were measurable on the iNPH Grading Scale (iNPHGS) [14 (link)], (3) MRI features of iNPH, i.e., both ventriculomegaly of Evans' index > 0.3 and tight high-convexity and medial subarachnoid spaces on coronal T1-weighted MRI (Figure 1) [10 (link)], (4) absence of known disorders causing ventriculomegaly, and (5) normal cerebrospinal fluid (CSF) content (protein ≤ 50 mg/dl and cell count ≤ 3 μm³) and pressure (≤ 20 cmH₂O). Exclusion criteria were (1) presence of musculoskeletal, cardiopulmonary, renal, hepatic, or mental disorders that would make it difficult to evaluate changes of symptoms, (2) obstacles to one-year follow-up, and (3) hemorrhagic diathesis or anticoagulant medication. For the evaluation of the MRIs, Evans' index, size of the Sylvian fissures rated according to the protocol of Kitagaki et al. [10 (link)], presence or absence of focal dilatation of the cerebral sulci, and white-matter changes according to scale of Fazekas et al. [15 (link)], were assessed on each site and recorded.
The candidates were pre-registered before CSF examination via a web-based case report system. MRI was reviewed by each site in the pre-registration phase, and the final eligibility of the subjects was judged by the central MRI review committee, which consist of neurosurgeons, neurologists, and a neuroradiologist. The central MRI review committee excluded those whose MRI did not fulfil the inclusion criteria. After the confirmation of normal CSF content and pressure, the investigator was notified of registration via the web system. Tap test was carried out in all subjects with 30 ml CSF removal via lumbar puncture. CT cisternography was carried out 1 week after the tap test with iohexol (Omnipaque^®: 180 mg/ml) 30 mg/kg. Cerebral blood flow was measured using ¹²³I-Iodoamphetamine and single photon emission computed tomography at baseline. However, the results of these measures were not considered for the eligibility.

Evaluation of the free water volume fraction inside the phantoms with microbeads was performed based on percentage of the contrast-filled volume, quantified in 8 bits gray scale axial images. First, the central portion of the phantom volume, reflecting that from DWI analysis, was manually separated in ImageJ (v. 1.53 j, NIH, USA). The free fluid volume regions (filled with Omnipaque 350 solution) were of clearly higher Hounsfield Units (HU) intensities compared to those from the microbeads. Thus, the voxels belonging to the free fluid were empirically verified occupying >75^th percentile of the cumulative HU distribution in all the separated volume images, from all phantoms. Therefore, the voxels belonging to the free fluid were counted three times, considering only voxels above 75^th, 80^th, and 85^th percentile of HU distribution. For each phantom, the free fluid volume fraction was estimated as a mean voxel count from all thresholds, divided by the voxels count in the separated volume image. As objective separation of the fluid space was impossible in the phantom with the fine microbeads and lower HU thresholds would consider voxels affected by partial volume from the microbeads, proposed free fluid volume fraction estimation was found sufficient to correct for the changes in shape of voxels HU distribution from different phantoms.

To confirm that the free fluid volume surrounding the microbeads increases with increasing particles size, additional measurements were performed employing MR T₁ mapping and contrast-enhanced micro computed-tomography (µCT).
For T₁ mapping, all phantoms were prepared de novo and scanned jointly using spin-echo sequence (2D-RARE; Table 1B) with variable repetition times. For each phantom, dry microbeads were put for 24 hr directly into the phantom lumen filled with 0.001 mM/ml gadobutrol solution. Before MRI, any possible air bubbles were removed, and the phantom was sealed (as above). Microbeads of different size were expected to differently infiltrate surrounding water, and partially gadobutrol. Thus, the relative concentration of gadobutrol was expected to be altered compared to original solution, and shorter T₁ relaxations were expected to be observed for phantoms with decreasing microbeads size. For reference, T₁ mapping was performed in the phantom filled solely with gadobutrol solution. To correct T₁ maps for B₁-filed inhomogeneities using double-angle method (Insko and Bolinger, 1993 (link); Cunningham et al., 2006 (link)), all phantoms were scanned with the same spin-echo sequence with a maximal repetition time used for T₁ mapping, and using two different flip angles (Table 1B).
For free fluid space estimation using µCT, all phantoms previously scanned for DWI were unsealed and flushed with a distilled water solution via 5 ml syringe to remove gadobutrol. Subsequently, distilled water was replaced with a 1:1 dilute of non-ionic iodine Omnipaque 350 contrast agent (Iohexol, 350 mg iodine/ml; GE Healthcare AS) in normal saline via 3 ml syringe. Each phantom was scanned using Vector4uCT system (MILabs, Utrecht, Netherlands) using the scan parameters of 15 μm isomteric resolution, 50 kVp, 0.24 mA (75ms exposure), 360 degrees rotation, 0.2 degree rotation step, 2 frames for averaging, 0.5 mm thick beam aluminum filter, Hann filter with cone-beam reconstruction.
To verify the relation between MR diffusion values and the T₁ relaxation times as well as free fluid estimates using µCT, DWI was performed 4 times in each phantom (as above).

All patients underwent triphasic CT scanning on gemstone spectral CT (GE, Discovery HD750), including a TNC scan, a routine arterial contrast-enhanced scan, and a gemstone spectral venous contrast-enhanced scan. The scanning range was from the basis cranii to the upper edge of the aortic arch, according to the recommended thyroid scanning protocol in our institution. During the scanning, the patients were asked to raise the inferior jaw without swallowing and lower the shoulders to avoid clavicle interference. TNC and routine arterial contrast-enhanced scans were both performed with the following scan parameters: tube voltage: 120 kVp, automatic tube current modulation (auto mA), pitch: 0.984, rotation time: 0.8 s, detector coverage: 40.0 mm, slice thickness: 0.625 mm, and slice interval: 0.625 mm. The parameters used for the subsequent gemstone spectral venous contrast-enhanced scan were as follows: fast kV switching between 80 and 140 kVp, tube current: 260 mA, rotation time: 0.8 s, pixel spacing: 0.625 mm, field of view: 25 cm × 25 cm, pitch: 0.984, slice thickness: 5.0 mm, and slice interval: 5.0 mm. After the intravenous injection of contrast medium (3.0–3.5 ml/s, 1.5 ml/kg, Omnipaque, 350 mg·I/ml, GE) via a syringe pump (Bayer Healthcare), routine arterial contrast-enhanced and gemstone spectral venous contrast-enhanced were performed with delays of 30 s and 60 s, respectively. Adaptive statistical iterative reconstruction and automatic tube current modulation techniques were used for dose reduction. Axial images with a slice thickness and slice interval of 1.25 mm were reconstructed. In each scan, the volume CT dose index (CTDIvol, mGy) and dose-length product (DLP, mGy.cm) for each scan were obtained directly from the dosimetry metrics displayed in the CT scanner to estimate the radiation dose.