Radiol Oncol 2023; 57(3): 317-324. doi: 10.2478/raon-2023-0043 317 research article The effects of normobaric and hyperbaric oxygenation on MRI signal intensities in T 1 -weighted, T 2 -weighted and FLAIR images in human brain Vida Velej 1,2 , Ksenija Cankar 1 , Jernej Vidmar 1,3 1 Institute of Physiology, Faculty of Medicine, University of Ljubljana, Ljubljana, Slovenia 2 Kranj Community Health Center, Gorenjska Basic Healthcare, Kranj, Slovenia 3 Institute of Radiology, University Medical Center Ljubljana, Ljubljana, Slovenia Radiol Oncol 2023; 57(3): 317-324. Received 29 May 2023 Accepted 24 July 2023 Correspondence to: Prof. Ksenija Cankar, D.M.D., Ph.D., Institute of Physiology, Faculty of Medicine, University of Ljubljana, Zaloška cesta 4, SI-1000 Ljubljana, Slovenia. E-mail: ksenija.cankar@mf.uni-lj.si Disclosure: No potential conflicts of interest were disclosed. This is an open access article distributed under the terms of the CC-BY li-cense (https://creativecommons.org/licenses/by/4.0/). Background. Dissolved oxygen has known paramagnetic effects in magnetic resonance imaging (MRI). The aim of this study was to compare the effects of normobaric oxygenation (NBO) and hyperbaric oxygenation (HBO) on hu- man brain MRI signal intensities. Patients and methods. Baseline brain MRI was performed in 17 healthy subjects (mean age 27.8 ± 3.2). MRI was repeated after exposure to the NBO and HBO at different time points (0 min, 25 min, 50 min). Signal intensities in T 1 - weighted, T 2 -weighted images and fluid attenuated inversion recovery (FLAIR) signal intensities of several intracranial structures were compared between NBO and HBO. Results. Increased T 1 -weighted signal intensities were observed in white and deep grey brain matter, cerebrospinal fluid (CSF), venous blood and vitreous body after exposure to NBO as well as to HBO compared to baseline (Dunnett’s test, p < 0.05) without significant differences between both protocols. There was also no significant difference in T 2 - weighted signal intensities between NBO and HBO. FLAIR signal intensities were increased only in the vitreous body after NBO and HBO and FLAIR signal of caudate nucleus was decreased after NBO (Dunnett’s test, p < 0.05). The statistically significant differences in FLAIR signal intensities were found between NBO and HBO (paired t-test, p < 0.05) in most observed brain structures (paired t-test, p < 0.05). Conclusions. Our results show that NBO and HBO alters signal intensities T 1 -weighted and FLAIR images of human brain. The differences between NBO and HBO are most pronounced in FLAIR imaging. Key words: hyperbaric oxygen; normobaric oxygen; magnetic resonance; human brain Introduction Magnetic resonance imaging (MRI) of brain is a su- perior soft-tissue contrast method that is used for the assessment of a numerous neurological condi- tions such as multiple sclerosis and headaches, and used to characterize strokes and space-occupying lesions. Basic MRI brain screen protocol is a sim- ple non-contrast MRI comprising a group of basic MRI sequences when imaging the brain in cases of no particular condition is being sought (e.g. head- ache). The protocol is designed to obtain a good general overview of the brain. A standard screen- ing protocol might include T 1 weighted imaging for anatomical overview, T 2 weighted imaging to evaluate basal cisterns, ventricular system and Radiol Oncol 2023; 57(3): 317-324. Velej V et al. / Oxygen and brain MRI 318 subdural spaces, and good visualization of flow voids in vessels, fluid attenuated inversion recov- ery imaging (FLAIR) to assess white-matter, dif- fusion weighted imaging (DWI) for multiple pos- sible purposes (from the identification of ischemic stroke to the assessment of active demyelination). Dissolved oxygen can be used as a contrast agent in MRI due to the paramagnetic properties of the dioxygen molecule O 2 1 . Since O 2 as well as hy- droxyl and superoxide radicals contain unpaired electrons, they exhibit paramagnetic effect and may shorten the spin-lattice relaxation time (T 1 ) in magnetic resonance imaging (MRI). 2-6 T 1 relaxation times shortening under the influence of increased partial pressure of oxygen (pO 2 ) in the inspired gas mixture is called tissue-oxygen-level-depend- ent effect (TOLD). It was observed in many tissues: arterial blood, myocardium, spleen, skeletal mus- cles, renal cortex, liver and fat. 4,7-9 TOLD effect was also detected in brain parenchyma (grey and white matter) and cerebrospinal fluid (CSF). 10-16 In addi- tion, pO 2 increase also affects spin-spin relaxation time (T 2 ) 13,17 ; however, results of available studies on the effect of pO 2 on T 2 relaxation times are con- troversial. 7,12 FLAIR images of healthy volunteers also showed increased CSF signal intensity during 100% oxygen breathing. 18 Concentration of dissolved oxygen is directly proportional to its partial pressure, pO 2 . 19 High pO 2 values in arterial blood as well as in brain parenchyma can be achieved with normobaric 100% oxygenation (NBO) compared to breathing normobaric air (NBA). Hyperbaric 100% oxygena- tion (HBO) causes a more pronounced increase of arterial and brain pO 2 compared to NBO as well as augments production of reactive oxygen species (ROS). 20-22 In few animal studies, it has been al- ready observed that HBO had a more pronounced effect on T 1 and T 2 relaxation times compared to breathing NBO or NBA. 11,23 To our knowledge, no human studies were per- formed studying the effect of HBO on MRI signal intensities. The aim of this study was to compare the effects of HBO and NBO on MRI signal intensi- ties (e.g. T 1 , T 2 and FLAIR). Patients and methods The study was approved by The National Ethics Committee (No. 0120-203/2019/4). Research was conducted at the Institute of Physiology (University of Ljubljana, Faculty of Medicine). Informed consent was obtained from each subject. 17 healthy volunteers (12 males and 5 females), age 20–40 years (mean age 27.8 ± 3.2), were enrolled in the study. Exclusion criteria were: history of a neurological disorder, non-MRI-compatible de- vices, a lung disease with FEV1/FVC < 60% and/or emphysema and/or pneumothorax, history of mid- dle ear trauma or disease, therapy with platinum complexes, doxorubicin, bleomycin, disulfiram of mafenide acetate, pregnancy or claustrophobia. Study protocol MRI examination was performed before oxygen breathing protocol (baseline state), after HBO and after NBO with subsequent MRI on separate visits. NBO protocol was performed using a non- rebreather oxygen mask connected to a large reser- voir supplied by 100% oxygen for 70 minutes. HBO protocol was performed in multiplace hyperbaric chamber (Kovinarska P&P, Slovenia) at 2.4 ATA with breathing of 100% oxygen for 70 minutes as shown in Figure 1. After each oxygen breathing protocol (NBO or HBO), MRI examination was re- peated three times, i.e. immediately after the end of HBO or NBO, after 25 min and after 50 min. MR image acquisition The MRI imaging was performed on a 3T MRI system (TX Achieva Philips Netherlands) with the use of a 32-channel head coil. The MR examination consisted of: • T 1 spin echo (SE) imaging in the transversal plane with imaging parameters: repetition time (TR) 1026 ms, echo time (TE) 10 ms, filed of view (FOV) 230 × 183 mm, matrix 256 × 163, voxel 0.9 × 1.12 mm, slice thickness 4 mm, gap 1 mm, number of slices 29, number of signals averaged (NSA) 2 with approximate duration of 5 min 38 s • T 2 turbo spin echo (TSE) imaging in the trans- versal plane with imaging parameters: TR 9179 ms, TE 100 ms, FOV 230 × 185 mm, matrix 384 × 229, voxel 0.6 × 0.75 mm, slice thickness 3 mm, gap 0 mm, number of slices 50, NSA 3, sensitiv- ity (SENS) 1.7 with approximate duration of 4 min 55 s. • FLAIR in transversal plane: TR 11000 ms, TE 125 ms, TI 2800 ms, FOV 230 x 183 mm, matrix 328 × 185, voxel 0.7 × 0.93 mm, slice thickness 3 mm, gap 1 mm, number of slices 36, NSA 2, SPIR tech- nique, SENS 2 with approximate duration: 3 min 51 s. The total MRI scanning time during one MR ex- amination was 25 min. Radiol Oncol 2023; 57(3): 317-324. Velej V et al. / Oxygen and brain MRI 319 MR data and statistical analysis The MRI images were analysed using ImageJ free image analysis software (National Institutes of Health, USA). Mean signal values in distinct re- gions of interest (ROI) on T 1 -weighted, T 2 -weighted and FLAIR images were obtained: frontal white matter, thalamus, caudate nucleus, putamen, hip- pocampus, superior sagittal sinus, vitreous body and cerebrospinal fluid (CSF). Statistical analysis was performed using SigmaPlot 14.0 (Systat Software, Inc., USA). The signal intensi- ties after NBO or HBO were compared to the base- line values. Shapiro-Wilk test and Brown-Forsythe were used to check for normality and equal vari- ance. One-way repeated measurements analysis of variance (RM ANOVA) was used to test for differences between signal intensities before, im- mediately, 25 min and 50 min after NBO or HBO. In cases when Shapiro-Wilk or Brown-Forsythe test failed, Friedman RM ANOVA on Ranks was performed. If RM ANOVA showed statistically significant differences between groups of data, Dunnett’s method for multiple comparisons was used to compare signal intensities at three time A B C D E F G 0 20 40 60 80 100 120 Breathing protocol in hyperbaric chamber A) room air, compression to 2.4 ATA, 10 min B) 100 % oxygen, 2.4 ATA, 25 min C) room air, 2.4 ATA, 5 min D) 100 % oxygen, 2.4 ATA, 25 min E) room air, 2.4 ATA, 5 min F) 100 % oxygen, 2.4 ATA, 20 min G) room air, decompression to 1.0 ATA, 10 min Time (min) FIGURE 1. Hyperbaric oxygenation (HBO) protocol. TABLE 1. Comparison of T 1 -weighted signal intensities before, immediately, 25 min and 50 min after normobaric oxygenation (NBO) (A) and hyperbaric oxygenation (HBO) (B) (mean ± standard deviation) A) NBO Structure Baseline 0 min 25 min 50 min p Frontal white matter 770.1 ± 251.2 791.0 ± 238.4 837.3 ± 328.4 851.3 ± 337.7* 0.044 Thalamus 827.9 ± 275.6 846.7 ± 244.7 894.6 ± 338.4 914.7 ± 356.5* 0.038 Head of caudate nucleus 731.9 ± 238.3 753.8 ± 227.0 798.7 ± 318.8 820.9 ± 331.3* 0.023 Putamen 800.2 ± 257.0 814.2 ± 239.2 860.7 ± 333.8 878.1 ± 350.2 0.059 Hippocampus 701.6 ± 232.0 712.1 ± 211.3 751.8 ± 285.5 771.6 ± 303.8* 0.038 Superior sagittal sinus 818.0 ± 375.6 748.2 ± 252.1 778.3 ± 288.6 860.9 ± 304.8 0.019 Cerebrospinal fluid 356.6 ± 126.8 362.7 ± 108.5 396.6 ± 152.9* 397.7 ± 163.0* 0.010 Vitreous body 281.3 ± 89.6 288.0 ± 92.1 296.8 ± 115.0 302.5 ± 116.3 0.264 B) HBO Structure Baseline 0 min 25 min 50 min p Frontal white matter 770.1 ± 251.2 834.7 ± 133.0 860.9 ± 158.9 886.6 ± 184.7* 0.026 Thalamus 827.9 ± 275.6 900.8 ± 147.1 933.3 ± 169.1 957.1 ± 193.9* 0.004 Head of caudate nucleus 731.9 ± 238.3 794.9 ± 124.4 819.9 ± 142.4 846.2 ± 173.7 0.013 Putamen 800.2 ± 257.0 867.1 ± 134.9 893.9 ± 154.6 922.5 ± 185.1* 0.007 Hippocampus 701.6 ± 232.0 764.8 ± 124.1 787.2 ± 142.1 807.5 ± 163.2 0.044 Superior sagittal sinus 818.0 ± 375.6 848.0 ± 227.8 911.9 ± 310.7 907.6 ± 335.0 0.127 Cerebrospinal fluid 356.6 ± 126.8 396.4 ± 60.2 400.7 ± 73.7 413.0 ± 98.9 0.256 Vitreous body 281.3 ± 89.6 361.2 ± 63.0* 335.9 ± 69.6 349.4 ± 82.8 0.040 * statistically significant difference compared to baseline value at p < 0.05; NBO = 100 % normobaric oxygen, HBO = 100 % hyperbaric oxygen Radiol Oncol 2023; 57(3): 317-324. Velej V et al. / Oxygen and brain MRI 320 points after oxygen breathing protocol with base- line values. Additionally, RM ANOV A or Friedman RM ANOV A on Ranks was used to check for differ- ences in signal values of each ROI in T 1 -weighted, T 2 -weighted and FLAIR images between baseline signal values and values after HBO/NBO at each time point (0 min, 25 min, 50 min). The signal in- tensity changes in T 1 -weighted, T 2 -weighted and FLAIR images compared to baseline in each ROI at each time point (0 min, 25 min, 50 min) after NBO and HBO were calculated. Paired t-test was used to compare the signal intensity changes at each time point between NBO and HBO. In cases when Shapiro-Wilk normality test failed, Wilcoxon signed rank test was performed. The α level was set at p < 0.05 for all statistical significances. Results The results of T 1 -weighted signal intensities before, immediately, 25 min and 50 min after NBO or HBO are presented in Table 1. After NBO there was a statistically significant increase in T 1 -weighted signal intensity in all studied structures except for vitreous body and putamen (RM ANOVA, Dunnett’s test, p < 0.05). In contrast, after HBO we observed a significant increase in T 1 -weighted sig- nal intensities except for the superior sagittal sinus and CSF (Dunnett’s test, p < 0,05). T 1 -weighted sig- nal intensity was significantly higher immediately (0 min) as well as 25 min after the end of the HBO compared to T 1 -weighted signal intensity imme- diately and 25 min after NBO in vitreous body (paired t-test, p < 0.05). In contrast, there was no difference in signal intensities in T 1 -weighted im- ages between HBO and NBO after 50 min. The results of T 2 -weighted signal intensities be- fore, immediately, 25 min and 50 min after NBO or HBO are presented in Table 2. T 2 -weighted sig- nal intensities were increased only in frontal white matter and thalamus after NBO and in the supe- rior sagittal sinus and vitreous body after HBO (Dunnett’s test, p < 0.05). There was also no signifi- cant difference in T 2 -weighted signal intensities between HBO and NBO. TABLE 2. Comparison of T 2 -weighted signal intensities before, immediately, 25 min and 50 min after normobaric oxygenation (NBO) (A) and hyperbaric oxygenation (HBO) (B) (mean ± standard deviation) A) NBO Structure Baseline 0 min 25 min 50 min p Frontal white matter 362.5 ± 33.1 367.8 ± 32.8 397.1 ± 80.4* 389.0 ± 51.3* 0.007 Thalamus 480.6 ± 49.6 485.1 ± 63.5 521.0 ± 114.8 508.0 ± 61.7 0.022 Head of caudate nucleus 621.8 ± 64.8 637.9 ± 54.0 673.6 ± 139.8 656.7 ± 91.4 0.631 Putamen 514.0 ± 57.0 527.7 ± 50.7 552.3 ± 103.4 542.2 ± 64.1 0.073 Hippocampus 694.9 ± 71.9 712.6 ± 83.5 756.9 ± 190.3 734.0 ± 98.2 0.281 Superior sagittal sinus 41.3 ± 6.8 42.7 ± 8.6 45.7 ± 12.5 45.0 ± 11.3 0.317 Cerebrospinal fluid 1996.8 ± 143.6 2059.7 ± 203.3 2171.2 ± 522.0 2107.9 ± 274.3 0.318 Vitreous body 1404.8 ± 114.8 1499.4 ± 159.1 1585.5 ± 396.5 1520.9 ± 224.1 0.080 B) HBO Structure Baseline 0 min 25 min 50 min p Frontal white matter 362.5 ± 33.1 359.4 ± 26.7 367.0 ± 35.5 375.1 ± 44.7 0.223 Thalamus 480.6 ± 49.6 473.6 ± 34.4 479.3 ± 33.3 489.1 ± 59.1 0.365 Head of caudate nucleus 621.8 ± 64.8 617.8 ± 47.3 620.9 ± 50.0 639.5 ± 77.1 0.390 Putamen 514.0 ± 57.0 510.9 ± 37.1 514.6 ± 39.8 532.9 ± 65.4 0.256 Hippocampus 694.9 ± 71.9 695.9 ± 57.2 688.1 ± 38.4 713.4 ± 81.4 0.378 Superior sagittal sinus 41.3 ± 6.8 48.3 ± 14.0* 47.4 ± 11.8 48.0 ± 15.0 0.047 Cerebrospinal fluid 1996.8 ± 143.6 1972.3 ± 79.2 1984.8 ± 102.3 2027.9 ± 195.4 0.482 Vitreous body 1404.8 ± 114.8 1524.0 ± 114.7* 1529.5 ± 143.2* 1530.9 ± 189.8* 0.001 * statistically significant difference compared to baseline value at p < 0.05; NBO = 100 % normobaric oxygen; HBO = 100 % hyperbaric oxygen Radiol Oncol 2023; 57(3): 317-324. Velej V et al. / Oxygen and brain MRI 321 The results of FLAIR signal intensities before, immediately, 25 min and 50 min after NBO or HBO are presented in Table 3. FLAIR signal inten- sities were increased only in the vitreous body af- ter NBO and HBO, signal of caudate nucleus was decreased after NBO (Dunnett’s test, p < 0.05). The statistically significant differences in FLAIR signal intensities were found between NBO and HBO (paired t-test, p < 0.05) in caudate nucle- us, thalamus, hippocampus and vitreous body at each time point (0 min, 25 min, 50 min). In addi- tion, the differences were also observed between NBO in HBO in putamen and frontal white matter at 0 min and 25 min and in superior sagittal sinus at 25 min (paired t-test, p < 0.05). Discussion In the present study we observed increased signal intensity in T 1 -weighted imaging in frontal white matter, thalamus, caudate nucleus and hippocam- pus after NBO as well as HBO, in superior sagittal sinus and CSF after NBO and in vitreous body and putamen after HBO. Additionally, signal intensity was increased in T 2 -weighted imaging in frontal white matter and thalamus after NBO as well as in superior sagittal sinus and vitreous body af- ter HBO. FLAIR signal intensities were increased only in the vitreous body after NBO and HBO. In contrast, FLAIR signal of caudate nucleus was decreased after NBO. Statistically significant dif- ferences between HBO and NBO were observed in FLAIR signal intensities of caudate nucleus, vitreous body, putamen, frontal white matter, hip- pocampus and thalamus and also in T 1 -weighted signal intensity of vitreous body. In our study, T 1 -weighted signal intensity of brain structures increased progressively with time after NBO/HBO and was the highest 50 min after the end of both, HBO and NBO. This finding is in agreement with the paramagnetic effect of O 2 . Increased level of dissolved paramagnetic molecu- lar O 2 shortens T 1 -relaxation times due to dipol- dipol interactions and increases signal intensity on T 1 -weighted images. 4,7-16 23 24 Relaxation rate (R 1 = 1/T 1 ) increases proportionally with increasing pO 2 in inspired gas mixture, the increase being linear or logarithmic when in normobaric or hyperbaric conditions, respectively. 23,24 The various increase of T 1 -weighted signal intensities in the observed tissues might be explained by increased microvas- cular pO 2 as well as by differences in tissue oxy- genation. 7 The sustained increase in T 1 -weighted signal intensity is further supported by a study of Rockswold et al. which showed significantly elevated brain tissue pO 2 30 min after the end of HBO and NBO. 25 In contrast to Rockswold et al., we failed to observe a peak in T 1 -weighted signal intensity immediately after the end of oxygen ther- apy. A possible explanation is that the time delay between HBO/NBO and MRI was too long to de- tect the peak. We observed progressive increase in T 1 - weighted signal intensities after both NBO and HBO along with MRI imaging time, with the high- est signal increase at the end of imaging protocol. This phenomenon could not be attributed solely to changes in pO 2 , but also to the effect of ROS on FIGURE 2. Representative MRI images in healthy subject at baseline, immediately after the end, after 25 min and after 50 min of NBO or HBO. Radiol Oncol 2023; 57(3): 317-324. Velej V et al. / Oxygen and brain MRI 322 T 1 and T 2 -weighted images. Since ROS such as hy- droxyl and superoxide radicals contain unpaired electrons, they also exhibit strong paramagnetic effect (a strong T 1 relaxation times shortening) and only a small, statistically insignificant reduction of T 2 relaxation times. 5,6 Additional point to consider is that distinct neurons respond to oxidative stress differently 26,27 , which leads us to presumption that the effect of HBO-induced oxidative stress would lead to different levels of ROS and thus different effect on T 1 and T 2 weighted signal intensities in various brain regions. The increase of T 1 -weighted signal intensities was more pronounced in frontal white matter and thalamus after HBO compared to NBO. This could be explained by altered O 2 diffusion after HBO. We observed increased signal intensity in superior sagittal sinus and CSF only after NBO, but not after HBO. Longer time delay between HBO and MRI most likely lowered pO 2 in the aforementioned fluids before the beginning of MRI. We showed that T 1 -weighted signal intensity of vitreous body was significantly increased immediately after the end of HBO exposure and then decreased in subsequent imaging blocks. This is in accordance with expected pO 2 dynamics in vitreous body, de- scribed by Shui et al. 28 Surprisingly, after the expo- sure to NBO, no increase in vitreous T 1 -weighted signal intensity was observed. A possible explana- tion is that lower vitreous pO 2 (as achieved during NBO compared to HBO) dropped to baseline level before the beginning of the MRI. In our study, there were statistical differences in T 2 -weighted signal intensities between baseline and after NBO in frontal white matter and thala- mus. This is in accordance with Wu et al. who ob- served significant differences in T 1 and T 2 between grey and white matter after inhalation of NBO. 12 According to Wu et al., T 2 relaxation time increases in rat brain with hyperoxia. In contrast, Tadamura et al. did not observe this effect in human “non- brain” tissues (myocardium, spleen, liver, subcu- taneous fat, skeletal muscle and bone marrow). 7 Therefore, it is possible that the effect of hyperoxia on T 2 -weighted signal intensities appears to vary in different tissues. In the present study, a signifi- TABLE 3. Comparison of FLAIR signal intensities before, immediately, 25 min and 50 min after normobaric oxygenation (NBO) (A) and hyperbaric oxygenation (HBO) (B) (mean ± standard deviation) A) NBO Structure Baseline 0 min 25 min 50 min p Frontal white matter 731.7 ± 77.0 700.8 ± 89.8 713.7 ± 125.1 717.4 ± 128.7 0.615 Thalamus 897.1 ± 101.7 851.5 ± 96.7 868.6 ± 164.3 861.5 ± 153.6 0.399 Head of caudate nucleus 1119.2 ± 131.9 1083.0 ± 129.4 1070.0 ± 213.0 1030.0 ± 172.5* 0.039 Putamen 928.1 ± 116.5 875.8 ± 129.0 893.2 ± 184.3 893.0 ± 171.4 0.354 Hippocampus 1216.2 ± 135.7 1162.1 ± 144.8 1173.5 ± 223.0 1174.1 ± 239.1 0.490 Superior sagittal sinus 91.2 ± 23.6 84.6 ± 23.5 78.2 ± 27.2 86.0 ± 33.2 0.299 Cerebrospinal fluid 139.6 ± 33.3 140.5 ± 39.8 147.0 ± 51.8 157.8 ± 53.2 0.228 Vitreous body 127.2 ± 29.4 170.4 ± 49.1* 157.3 ± 45.0* 147.3 ± 45,9 0.002 B) HBO Structure Baseline 0 min 25 min 50 min p Frontal white matter 731.7 ± 77.0 755.8 ± 113.1 794.7 ± 135.2 782.2 ± 115.3 0.508 Thalamus 897.1 ± 101.7 927.1 ± 104.6 691.3 ± 127.3 949.7 ± 110.5 0.973 Head of caudate nucleus 1119.2 ± 131.9 1180.6 ± 183.3 1208.0 ± 175.8 1190.8 ± 152.3 0.508 Putamen 928.1 ± 116.5 958.3 ± 143.4 993.0 ± 148.2 978.9 ± 133.4 0.567 Hippocampus 1216.2 ± 135.7 1269.4 ± 171.3 1294.4 ± 184.7 1286.7 ± 160.4 0.771 Superior sagittal sinus 91.2 ± 23.6 93.4 ± 25.8 105.1 ± 30.5 106.4 ± 37.0 0.193 Cerebrospinal fluid 139.6 ± 33.3 134.5 ± 27.0 139.4 ± 20.2 138.3 ± 21.8 0.909 Vitreous body 127.2 ± 29.4 691.4 ± 142.9* 523.6 ± 122.9* 378.1 ± 88.8 < 0.001 * statistically significant difference compared to baseline value at p < 0.05; NBO = 100 % normobaric oxygen; HBO = 100 % hyperbaric oxygen Radiol Oncol 2023; 57(3): 317-324. Velej V et al. / Oxygen and brain MRI 323 cant increase in T 2 -weighted signal intensities was also observed after HBO in superior sagittal sinus and vitreous body. Oxygen affects spin-spin relax- ation time (T 2 ) by two competing mechanisms, i.e. T 2 shortening analogous to effect on T 1 (although the effect on T 2 is much smaller) and T 2 lengthen- ing due to diffusion of water protons through field inhomogeneities induced by deoxyhemoglobin generated field gradients (blood-oxygen-level- dependent (BOLD) effect). 13,17 An increased T 2 - weighted signal intensity after NBO and HBO in our study suggests that in human brain structures and vitreous body the paramagnetic effect of oxy- gen on T 2 relaxation times shortening prevails over BOLD effect. Our results show statistically significant differ- ences between HBO and NBO were observed in FLAIR signal intensities in different brain struc- tures particularly those that are close to CSF spac- es. These results are in accordance with previous studies which showed that in patients receiving 100% NBO elevated pO 2 leads to incomplete sig- nal suppression of CSF in FLAIR imaging. 29,30 The elevated pO 2 most likely favors O 2 entry into the CSF not through the choroid plexus but directly through the walls of arteries and arterioles on the brain surface. 30 Since in HBO there is up to 2.5 times higher pO 2, this effect in FLAIR imaging is more pronounced. We observed increase in FLAIR signal of vitreous body immediately after HBO/ NBO exposure and then a subsequent decrease in time – again, this is in accordance with expected pO 2 dynamics in vitreous body, as described by Shui et al. 28 The results of the present study could have also some clinical implications. Namely, the prolonged intubation induces changes of signal intensities in T 1 -weighted and FLAIR images of brain MRI 31 similar as those observed in our study after NBO. Knowing that prolonged oxygenation induces par- amagnetic effects in brain tissues as observed in our study, it is important to take this into account when interpreting brain MRI in intubated patients or in patients after HBO therapy. The present study has several limitations. First, we failed to show significant differences in MRI signal intensities in brain structures after HBO compared to NBO. It would be expected that brain tissue pO 2 is significantly higher after HBO com- pared to NBO 21 due to higher concentration of dissolved O 2 during HBO. 19 The only exception was T 1 -weighted signal intensity of vitreous body immediately and 25 min after HBO compared to NBO. One possible explanation is that MRI was performed with time delay of 15 minutes after the end of HBO due to logistics. Perhaps with shorter time delay the peak in T 1 -weighted signal intensi- ties could be observed similarly as in the study of Rockswold et al. 25 Additionally, the present study was semiquantitative using clinical head MRI pro- tocol and the next step would be more quantita- tive approach using T 1 mapping and T 2 mapping. Furthermore, we did not measure brain tissue pO 2 nor levels of ROS, which would help to explain the observed changes in signal intensities in T 1 - weighted and T 2 –weighted images. Since our study was performed in vivo in a group of volunteers measuring of brain tissue pO 2 seems rather con- troversial. We could only measure pO 2 in arterial blood, however these results do not reflect brain tissue pO 2 directly. However, according to the ref- erence, at 3 ATA pO 2 in arterial blood increases to nearly 270 kPa and in tissue to above 53 kPa. 32 In contrast, in NBO conditions, partial pressure of pO 2 in the brain is expected to be only between 4 - 6.4 kPa according to study of Meixensberger et al. 33 These values are much lower than during HBO. Therefore, we expected similar tissue pO 2 differ- ences between HBO and NBO in the present study protocol. In conclusion, the increased T 1 -weighted signal intensities were observed in white and grey brain tissues, brain fluids and vitreous body after NBO as well as HBO, without significant differences between both protocols. In addition, the structure limited and diverse signal intensity increase was observed in T 2 -weighted imaging and FLAIR after NBO and HBO. However, the prospective quanti- tative studies are needed to further clarify the ef- fects of NBO and HBO breathing on MRI in hu- man brain. References 1. 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