Contrast agent-free 3D ultrasound deep-depth vascular imaging with a 2D row column addressed Array: In vivo human clinical feasibility study.

Prevalence of left heart contrast with agitated saline contrast echocardiography in healthy resting dogs.

Optical imaging techniques, such as hyperspectral imaging combined with machine learning-based analysis, have the potential to revolutionize clinical surgical imaging. However, these modalities face a shortage of large-scale, representative clinical data for training machine learning-based algorithms. While preclinical animal data are abundantly available through standardized experiments and allow for controlled induction of pathological tissue states, it is not ethically possible to obtain similar data from patients. To leverage this situation, we propose 'xeno-learning', a cross-species knowledge-transfer concept inspired by xeno-transplantation. Here, using a total of 14,013 hyperspectral images from humans as well as porcine and rat models, we show that, although spectral signatures of organs differ substantially across species, relative changes resulting from pathologies or surgical manipulation such as malperfusion or injection of contrast agent are comparable. Such changes learnt in one species can be transferred to a new species through a 'physiology-based data augmentation' method, enabling the large-scale secondary use of preclinical animal data for human application. The resulting benefits promise a high impact of the proposed knowledge-transfer concept on future developments in the field.

Perfusion imaging is extensively utilized to assess hemodynamic status and tissue perfusion in various organs. Computed tomography perfusion (CTP) imaging plays a key role in the early assessment and planning of stroke treatment. While CTP provides essential perfusion parameters to identify abnormal blood flow in the brain, the use of contrast agents in CTP can lead to allergic reactions and adverse side effects, along with costing USD 4.9 billion worldwide in 2022. To address these challenges, we propose a novel deep learning framework called Multitask Automated Generation of Intermodal CT perfusion maps (MAGIC). This framework combines generative artificial intelligence and physiological information to map non-contrast computed tomography (CT) imaging to multiple contrast-free CTP imaging maps. We demonstrate enhanced image fidelity by incorporating physiological characteristics into the loss terms. Our network was trained and validated using CT image data from patients referred for stroke at UF Health and demonstrated robustness to abnormalities in brain perfusion activity. A double-blinded study was conducted involving seven experienced neuroradiologists and vascular neurologists. This study validated MAGIC's visual quality and diagnostic accuracy showing favorable performance compared to clinical perfusion imaging with intravenous contrast injection. Overall, MAGIC holds great promise in revolutionizing healthcare by offering contrast-free, cost-effective, and rapid perfusion imaging.

Background/Objectives: Cerebral angiography is a cornerstone diagnostic and therapeutic procedure for cerebrovascular diseases; however, its potential effects on vascular integrity and cellular homeostasis remain incompletely elucidated. This systematic review aims to comprehensively evaluate endothelial and histopathological alterations induced by cerebral angiographic procedures, with particular emphasis on oxidative stress, inflammation, endothelial dysfunction, and blood–brain barrier disruption. Methods: This systematic review was conducted in accordance with the PRISMA 2020 guidelines. PubMed, Scopus, and Web of Science databases were systematically searched for studies published between 1981 and 2025 using predefined keywords related to cerebral angiography, endothelial injury, oxidative stress, inflammation, and histopathological changes. A total of 1142 records were identified, and 216 duplicates were removed. Following title and abstract screening, 312 full-text articles were assessed for eligibility, of which 112 were excluded due to irrelevance or insufficient endothelial or histopathological data. Ultimately, 200 studies were included in the qualitative synthesis. The literature identification, screening, and selection process are summarized in the manuscript. The review protocol was not prospectively registered. Results: The included studies demonstrated that cerebral angiographic procedures induce endothelial and microvascular alterations through both mechanical and contrast-mediated mechanisms. Iodinated contrast agents were consistently associated with increased reactive oxygen species production, reduced endothelial nitric oxide bioavailability, mitochondrial dysfunction, and activation of pro-inflammatory signaling pathways, including nuclear factor kappa B (NF-κB). Histopathological findings revealed endothelial swelling, vacuolization, apoptosis, microthrombus formation, inflammatory cell infiltration, and disruption of endothelial junctions, leading to increased vascular permeability and blood–brain barrier impairment. Mechanical factors related to catheter manipulation and high-pressure contrast injection further exacerbated endothelial injury by altering shear stress and promoting leukocyte adhesion. The severity of endothelial damage and inflammatory responses was consistently greater in patients with comorbid conditions such as diabetes mellitus, hypertension, and atherosclerotic disease. Conclusions: Cerebral angiography may induce endothelial dysfunction and histopathological vascular injury predominantly through oxidative and inflammatory mechanisms. Optimization of contrast agent selection, refinement of procedural techniques, and implementation of endothelial-protective strategies may mitigate vascular injury and improve procedural safety. Further translational and clinical studies are warranted to identify biomarkers and protective interventions targeting angiography-induced endothelial damage.

Objective: To assess the effectiveness of a fixed contrast injection protocol—75 mL of contrast followed by 40 mL saline at 5 mL/s with an injection duration of 23 s—in achieving diagnostic enhancement in coronary CT angiography (CCTA) using 64-slice detector CT scanner. Materials and Methods: 456 consecutive patients with suspected coronary disease who underwent CCTA on a 64-slice detector CT scanner between January 2023 and December 2024 and were retrospectively enrolled. Each patient received 75 mL of contrast medium followed by 40 mL of saline at a flow rate of 5 mL/s, with a total injection duration of 23 s. Two radiologists, blinded to patient information, independently measured the contrast enhancement (HU) values in the coronary segments, ascending and descending aorta, and left ventricle. Attenuation levels ≥250 HU were considered diagnostic. Patients were grouped by body weight into two categories: Group 1 (≤75 kg) and Group 2 (>75 kg). The independent t-test and Mann–Whitney U test were used to compare HU values in each vessel between the two groups, while the Chi-square test was applied to compare enhancement success rates (HU ≥ 250) between the groups per vessel. Results: A total of 281 patients (mean age: 51.88 years ± 11.15 [SD]; 167 male, 114 female), were included. Statistically significant differences in the HU enhancement measurements were found between groups (p < 0.001–0.007). However, all segments showed mean and median HU values above 250 HU. Enhancement success rates were significantly higher in Group 1 (p = 0.005–0.04), except in the ascending aorta, descending aorta, left main coronary artery, middle right coronary, distal right coronary artery, and middle left circumflex artery, where the rates were statistically comparable between the groups (p = 0.054–0.61). Conclusions: A fixed contrast protocol (75 mL of contrast medium followed by 40 mL of saline at a 5 mL/s flow rate with a total injection duration of 23 s) appears to be feasible for achieving diagnostic contrast enhancement in CCTA using a 64-slice multidetector CT scanner. This protocol may offer a simplified alternative to individualized, weight-based contrast dosing strategies.

To test the optimum flushing volume for power-injectable peripherally inserted central catheters after the injection of iodinated contrast agents. Pulsed infusion with saline solution was recommended for peripherally inserted central catheters flushing. However, evidence of flushing volume for power-injectable peripherally inserted central catheters after the injection of iodinated contrast agent was lacking. Empirical quantitative research in vitro experiment. Single lumen power-injectable peripherally inserted central catheters were filled with an iodinated contrast agent. Sixty catheters were randomized into four groups: 10 ml flushing volume of 0.9% sodium chloride (Group A), 15 ml flushing volume of normal saline (Group B), 20 ml flushing volume of 0.9% sodium chloride (Group C), and 25 ml flushing volume of 0.9% sodium chloride (Group D). The same pulsatile flushing technique was performed for all four groups. Efficacy was measured based on the amount of iodine recovered from the lumen of the tested catheters. The iodine average recovery proportion was 98.5% ± 2.1% in Group A, 99.3% ± 1.4% in Group B, 98.4% ± 1.8% in Group C, and 97.2% ± 3.2% in Group D. The difference in proportion of iodine recovery between the control group (Group A) and the three experimental groups (Groups B, C, and D) was not significant. We concluded that pulsed flushing with 10 ml normal saline immediately after the contrast agent injection was sufficient. Higher flushing volumes did not lead to a better flushing effect; rather, they would only bring unnecessary fluid burden and cost to patients.Patient or public contribution:No patient or public contribution.

Abstract Aim Two-dimensional echocardiography remains the primary means of right ventricular (RV) evaluation. This study evaluated effects of ultrasound contrast, employed to improve left ventricular (LV) visualization, on assessment of RV size and function. Methods In 100 consecutive patients where an ultrasound enhancing agent (UEA) was indicated to improve LV visualization, apical 4-chamber (A4C) views were collected before and after contrast injection, maintaining transducer position. For the last 50 subjects, RV-focused (RVF) views were also obtained. Images were presented in random order to four echocardiographers with varying levels of experience. They were tasked with determining which ventricle formed the apex, whether the RV free wall was adequately visualized, assessing RV size and contractility and reporting their confidence level in their readings. Results In both A4C and RVF views the LV was less often determined to be apex-forming when UEA was used and RV size assessments moved in the direction of dilated for all readers. Similarly, a determination of reduced RV contractility occurred more often with UEA. Many of these differences were statistically significant. Interestingly, RV free wall visualization was not improved with contrast use. Moreover, reader confidence in interpretation was actually reduced when contrast was employed, with statistically significant differences for all four readers in the A4C view and two readers in the RVF view. Conclusions Opportunistic evaluation of the RV after employing UEA resulted in significant differences in visual assessment of various RV parameters. Reader confidence in interpretation was not improved when contrast was used.

ObjectiveThis study aimed to compare abdominopelvic computed tomography (APCT) image quality using low and high concentrations of a contrast agent in patients with cancer via a noninferiority design.MethodsThis study prospectively included 99 patients with malignancies who underwent dual-source APCT following injection of 1.5 mL/kg of iohexol 270 (low-concentration group). The control group included patients who were retrospectively matched 1:1 based on weight, body mass index, and sex. These patients (high-concentration group) received 1.1 or 1.2 mL/kg of iohexol 350, reconstructed at two tube voltages (80 kVp and 80/Sn150 kVp). The noninferiority margins were set at −0.21 for overall image quality. Two radiologists blindly and independently analyzed the subjective and objective image quality of matching focal lesions.ResultsA total of 198 patients with 102 focal lesions (44 liver lesions, 58 renal lesions) were assessed. The total iodine amount was slightly higher in the low-concentration group than in the high-concentration group without significant differences (24580.0 ± 3745.6 vs. 24190.2 ± 3954.3 mg I/mL; p = 0.051). Overall image quality did not differ significantly between the groups (2.6 ± 0.5 vs. 2.6 ± 0.5 on 80 kVp; p = 0.484; 3.4 ± 0.5 vs. 3.4 ± 0.6 on 80/Sn 150 kVp, p = 0.891). Margin sharpness and conspicuity also showed no significant differences between the low- and high-concentration groups at 80 kVp (p = 0.890, 0.103) and 80/Sn 150 kVp images (p = 0.278, 0.369). Liver attenuation was slightly higher in the low-concentration than in the high-concentration group (80 kVp, 127.3 ± 16.8 vs. 122.4 ± 13.0, p = 0.013; 80 kVp/Sn 150 kVp, 108.2 ± 13.3 vs. 104.5 ± 10.9, p = 0.025). No major or minor adverse reactions occurred during or immediately after contrast agent injection in either group. Five patients in the low-concentration group experienced below-average vascular pain.ConclusionsThe low-concentration group offered noninferior overall image quality compared to the high-concentration group among patients with cancer.

Early detection of coronary artery disease using low-dose CT perfusion: An experimental western fat diet study.

Comparison of precise imaging and iterative reconstruction techniques at low doses using the dose right index in 100-kVp cerebral CT angiography.

Fully personalized contrast injection optimized for weight and tube voltage in low-dose high-pitch coronary CT angiography.

Endoscopic retrograde cholangiopancreatography (ERCP) is essential for diagnosing and treating pancreaticobiliary diseases, but post-ERCP pancreatitis (PEP) is a common and potentially severe complication. Perfusion computed tomography (CT) is a method for analyzing blood flow by capturing sequential images after rapid intravenous contrast injection, however, its usefulness in the early detection of PEP is unclear. This study aimed to assess whether ERCP followed by perfusion CT could predict PEP at an early phase. This single-center prospective study included 49 patients (25 women, 24 men; median age, 70y) who underwent ERCP, who were at high risk for PEP, and who underwent perfusion CT within 2 hours after ERCP. All patients underwent perfusion assessment of blood volume (BV), blood flow (BF), mean transit time (MTT), and time to peak (TTP). PEP occurred in 13 patients (26.5%). The PEP group had higher BF and shorter MTT and TTP than the non-PEP group ( P < 0.01). Receiver operating characteristic analysis identified BV, BF, MTT, and TTP as good predictive markers for PEP (with areas under the curve of 0.624, 0.739, 0.735, and 0.748, respectively). The respective sensitivity and specificity for the diagnosis of PEP were as follows: BV, 84.6% and 50.0%; BF, 61.5% and 91.7%; MTT, 84.6% and 66.7%; TTP, 84.6% and 69.4%; serum amylase, 61.5% and 86.1%. The sensitivity and specificity of TTP combined with serum amylase increased to 76.9% and 88.9%, respectively. Early perfusion CT findings after ERCP could predict PEP development in high-risk patients.

BackgroundIn gadoxetic acid-enhanced liver magnetic resonance imaging (MRI), an inadequate breath-holding strategy is an independent risk factor for image quality degradation in single-phase arterial acquisition. Therefore, optimizing breath-holding training protocols is critical. This study aimed to assess the effectiveness of a nongradual breath training protocol combined with contrast dilution for reducing arterial phase (AP) respiratory artifacts in upper abdomen gadoxetic acid-enhanced MRI.MethodsIn this retrospective study, we enrolled 126 patients (91 males; mean age 52.65±11.07 years) who underwent gadoxetic acid-enhanced upper abdomen MRI for the first and only time from May 2019 to February 2020. All patients were divided into four groups according to different breath-holding training strategies and contrast injection patterns (group 1: gradual strategy + diluted injection; group 2: nongradual strategy + diluted injection; group 3: gradual strategy + undiluted injection; group 4: nongradual strategy + undiluted injection). Two radiologists evaluated AP images in a blinded manner using a 5-point scale. Semiquantitative scores, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) of the hepatic artery and portal vein to the liver parenchyma, and CNR of the hepatic artery to the portal vein were compared between the four groups via Mann-Whitney and Kruskal-Wallis tests. The differences in transient severe motion (TSM) incidence rates and respiratory artifact scores between the gradual and nongradual groups were calculated via the adjusted the Chi-squared test and the Mann-Whitney, respectively.ResultsThe results of the 5-point scale evaluation showed that respiratory artifacts significantly differed between groups (group 1: 2.56±1.16; group 2: 1.53±0.62; group 3: 2.12±1.08; group 4: 1.79±0.89; P=0.002): groups 2 and 4 had smaller artifacts, and group 2 had the fewest number of artifacts. However, no statistically significant differences were observed in the scores for the hepatic artery (P=0.177), portal vein (P=0.214), abdominal aorta (P=0.599), and liver parenchyma (P=0.243) or for the total score (P=0.235). The groups significantly differed terms of portal vein SNR (P<0.001), liver parenchyma SNR (P=0.035), and the CNR of the portal vein to the liver parenchyma (P=0.008). These parameters were optimal in group 2. The incidence of TSM and the respiratory artifact score were significantly higher in the gradual group (groups 1 and 3) compared to the nongradual group (groups 2 and 4).ConclusionsThe nongradual breath-holding training strategy combined with an injection rate of 2 mL/s with 50% diluted gadoxetic acid could significantly reduce respiratory artifacts and improve AP image quality.

Endoscopic ultrasound (EUS) with fine-needle biopsy (FNB) is one of the techniques applied for sampling subepithelial lesions (SELs) of the gastrointestinal tract. Elastography and contrast-enhanced evaluation could permit identification of different patterns among areas of the lesions, depending on their consistence and the presence of vital cells or necrosis. Targeting a specific area when performing FNB in the case of large lesions could potentially permit an increase in accuracy and reduce the need for re-sampling. A 61-year-old woman was admitted reporting severe abdominal pain. The patient underwent cholecystectomy many years ago. She had no known family history of gastrointestinal, hepatic, biliary, or pancreatic disease. Laboratory tests were normal. A computed tomography scan showed a large lesion between the stomach and the pancreatic body, suspected to originate from the gastric wall. An endoscopic view showed a large bulging into the gastric lumen and EUS identified a lesion originating from the muscular layer of the gastric wall. Elastography and contrast-enhanced EUS identified two different areas, one softer with lower enhancement (A) and the other harder with higher enhancement after contrast injection (B). FNB was performed targeting both the areas, sending samples for separate histological evaluation. Histology showed a gastrointestinal stromal tumor (GIST), finding differences in amount of necrotic and neoplastic cells between the two areas. EUS-FNB guided by elastography and/or contrast-enhanced EUS could identify differences within large SELs, allowing targeting of areas more likely to collect diagnostic samples.

ObjectiveThis study aimed to compare the diagnostic performance of conventional fat-suppression T1-weighted imaging (T1WI) and fat-suppression three-dimensional T1-weighted imaging with sampling perfection with application-optimized contrasts using different flip angle evolution (3D-T1WI-SPACE) sequences in detecting glenoid labral injuries during direct shoulder magnetic resonance arthrography (MRA).MethodsA retrospective analysis was conducted on imaging data from 60 individuals presenting with shoulder symptoms. Both imaging sequences were performed following intra-articular contrast injection. The intra-articular contrast-to-noise ratio (CNR) of both sequences was calculated to evaluate image quality. The images were analyzed to identify and classify glenoid labral injuries. Diagnositc sensitivity, specificity, and accuracy of the two sequences were calculated using arthroscopy as the reference standard. Agreement between MRA findings (from the two sequences) and arthroscopic results was assessed using the Kappa consistency test.ResultsThe fat-suppression 3D-T1WI-SPACE sequence reduced scanning time by 52%. However, its intra-articular CNR was significantly lower than that of the conventional fat-suppression T1WI sequence (p < 0.001). When using arthroscopy as the reference standard, the sensitivity, specificity, and accuracy for diagnosing glenoid labral injury were 97.5%, 95%, and 96.7%, respectively, surpassing those of the conventional fat-suppression T1WI sequence (90.0%, 90.0%, and 90.0%). Despite these differences, the diagnostic performance of both sequences was comparable. The Kappa values indicated high consistency between MRA findings and arthroscopy, with values of 0.925 for the 3D-T1WI-SPACE sequence and 0.78 for the conventional T1WI sequence.ConclusionWhile the fat-suppression 3D-T1WI-SPACE sequence demonstrated inferior image quality compared to the conventional fat-suppression T1WI sequence, its diagnostic accuracy was similar. Its substantially reduced scanning time supports its utility as a supplementary option for evaluating glenoid labral injuries during shoulder joint MRA.

ABSTRACTVessel size imaging (VSI) to provide a measure of vessel radius in the brain has been demonstrated using an injection of contrast agent. Venous vessel radius imaging of brain microvasculature is also possible by exploiting hypercapnia and hyperoxia. However, these respiratory challenges need external devices like masks, monitors, and gas application. In this study, we employ a breath‐hold task to mimic hypercapnia for venous VSI. Breath‐hold experiments of brain scans were performed on 14 subjects on a 3‐T scanner. Parametric maps of mean venous vessel radius were calculated from the changes in R2* and R2, ΔR2* and ΔR2, respectively, which were measured by simultaneous acquisition of gradient‐echo and spin‐echo signals using a spin‐ and gradient‐echo (SAGE) echo‐planar imaging sequence. In addition, we numerically simulated the expected transverse relaxation in voxels with different vessel radii based on randomly distributed cylinders to eventually obtain vessel size index q (ΔR2*/ΔR2) and associated average vessel radii. With this empirical relation of vessel size index q and vessel radius, the mean measured vessel size index was determined, and venous vessel radii in breath–hold were found to be 7.18 ± 0.49 μm in gray matter and 6.06 ± 0.22 μm in white matter. This study demonstrates the feasibility of venous VSI using a simple breath–hold task. The approach avoids contrast agents and specialized gas delivery, providing a practical alternative for assessing vascular properties. Our results show good agreement with previous hypercapnia– and contrast–based studies, supporting the validity of this noninvasive method.

Introduction: Base excess (BE) is an indicator of non-respiratory acid-base imbalances, which can impact coronary artery disease (CAD). This study evaluated the association between the severity of CAD and peripheral blood BE. Methods: This cross-sectional study included patients aged 18 and older who were candidates for coronary angiography. Demographic and clinical data were collected from medical records. Blood gas analysis was performed on a 2-millilitre arterial blood sample taken from the access artery before contrast injection. All patients underwent coronary angiography, and the Gensini score was calculated. Results: A total of 351 patients (194 males, 55.3%) were included in the study. The study population had a mean age of 60.79±9.5 and a mean BMI of 29.4±4.85. Coronary angiography revealed normal or minimal (&lt;50% stenosis) findings in 51.3% of cases (15.4% with normal coronary arteries and 35.9% with minimal non-obstructive lesions), single-vessel disease in 17.4%, two-vessel disease in 14.5%, and three-vessel disease in 16.8%. Median Gensini score was 13.0, with an IQR of 3.5 and 49. The findings indicated that a decrease in BE was significantly correlated with elevated Gensini scores (β: -0.04; 95% CI: -0.08 to -0.01; P=0.027). However, BE did not significantly affect the Gensini score of 0 (P=0.843). Moreover, negative values of BE were significantly and inversely associated with the Gensini score (β=-0.05; 95% CI: -0.07 to -0.02, P&lt;0.001). Conclusion: This study revealed an association between BE and CAD, suggesting that BE tending to acidosis is potentially associated with CAD.

Introduction Augmented reality (AR) has been increasingly applied to surgical procedures in fixed anatomical organs like brain, bones, aorta and kidneys, enabling image-guided precision, but sparingly to mobile organs such as the intestines. We report our initial experience with AR-guided intestinal stoma creation using an “image-guided” minimally invasive approach. Methods Adult patients requiring elective or urgent stoma creation for colonic decompression or diversion were included. Patient-specific 3D reconstructions of the relevant portion of the GI tract and reference organs (skin, bones, vessels) from a preoperative CT were co-registered intraoperatively via a head-mounted Augmented reality device (HoloLens2) onto the patient’s body using surface landmarks visible such as the umbilicus, bones, and prior surgical scars. A trajectory to the target bowel loop based on AR was marked on the skin, and stoma creation was performed at this site. Targeting of the correct bowel loop was confirmed with intraoperation fluoroscopy using intralumenal contrast injection. Technical success was defined as completion at the targeted site without open surgery. Results Fourteen patients underwent AR-guided stoma creation (9 colostomies, 5 ileostomies). Indications were bowel obstruction (n = 6), fistula (n = 5), anastomotic leak (n = 1), perforation (n = 1) and gastrointestinal bleeding (n = 1). Median age was 76 years, median BMI 23.8 kg/m 2 . The median (range) number of prior abdominal surgeries was 2 (0–11). The median operative time was 131 min (interquartile range [IQR]: 96–143). The approach was either cut down directly over the stoma site (n = 11) or laparoscopic assisted (n = 3). AR permitted precise identification of the bowel loop required for stoma creation in all cases and help to avoid need for standard open surgery. Median postoperative stay was 7 days (interquartile range: 3–10). No Clavien-Dindo grade III or IV complications, reoperations, or unplanned readmissions were observed. Two postoperative deaths occurred in ASA 4 patients, both due to the underlying malignancy and multiorgan failure preoperatively, unrelated to the surgical procedure. Conclusion This early experience suggests AR methods may identify and target a loop of bowel, play a useful role in intestinal stoma creation, with potential to avoid need for laparoscopy or extensive open surgery. Further clinical application and refinement are warranted.

The goal of this article is to introduce a technique to measure the velocity distribution of water inside each voxel of an MR image. The method is based on the use of motion sensitizing gradients with changing first moment to encode velocity. As such, it is completely non-invasive and requires no contrast injections. The technique consists of acquiring a series of images preceded by preparatory RF pulses that encode velocity information, analogously to k-space encoding. The velocity distribution can be decoded via the Fourier transform. We demonstrate its use on a simple flow phantom with known flow characteristics. We demonstrate the technique on the brains of five human participants from whom we collected the velocity distribution along each of the three laboratory axes. Velocity distribution measurements on simple phantoms yielded velocity distributions consistent with theory. Human velocity spectra identified specific anatomical features at different velocity bins. The largest fraction of spins was in the lowest velocity bands. Movement in the CSF spaces could be clearly identified at different velocity bands. Velocity Spectrum Imaging has great potential as a tool to study the movement of fluids in the human body without contrast agents. In addition to a useful tool for validating computational fluid dynamic models in vivo, it can be used to study the complex movement of water in the glymphatic system and its involvement in neurodegenerative disorders. However, further development is needed to probe the velocity spectrum in the ultra-low velocity regime of the perivascular spaces.