Regional oxygen extraction predicts border zone vulnerability to stroke in sickle cell disease

OBJECTIVE

To determine mechanisms underlying regional vulnerability to infarction in sickle cell disease (SCD) by measuring voxel-wise cerebral blood flow (CBF), oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen utilization (CMRO2) in children with SCD.

METHODS

Participants underwent brain MRIs to measure voxel-based CBF, OEF, and CMRO2. An infarct heat map was created from an independent pediatric SCD cohort with silent infarcts and compared to prospectively obtained OEF maps.

RESULTS

Fifty-six participants, 36 children with SCD and 20 controls, completed the study evaluation. Whole-brain CBF (99.2 vs 66.3 mL/100 g/min, p < 0.001), OEF (42.7% vs 28.8%, p < 0.001), and CMRO2 (3.7 vs 2.5 mL/100 g/min, p < 0.001) were higher in the SCD cohort compared to controls. A region of peak OEF was identified in the deep white matter in the SCD cohort, delineated by a ratio map of average SCD to control OEF voxels. CMRO2 in this region, which encompassed the CBF nadir, was low relative to all white matter (p < 0.001). Furthermore, this peak OEF region colocalized with regions of greatest infarct density derived from an independent SCD cohort.

CONCLUSIONS

Elevated OEF in the deep white matter identifies a signature of metabolically stressed brain tissue at increased stroke risk in pediatric patients with SCD. We propose that border zone physiology, exacerbated by chronic anemic hypoxia, explains the high risk in this region.

Red cell exchange transfusions lower cerebral blood flow and oxygen extraction fraction in pediatric sickle cell anemia

Blood transfusions are the mainstay of stroke prevention in pediatric sickle cell anemia (SCA), but the physiology conferring this benefit is unclear. Cerebral blood flow (CBF) and oxygen extraction fraction (OEF) are elevated in SCA, likely compensating for reduced arterial oxygen content (CaO2). We hypothesized that exchange transfusions would decrease CBF and OEF by increasing CaO2, thereby relieving cerebral oxygen metabolic stress. Twenty-one children with SCA receiving chronic transfusion therapy (CTT) underwent magnetic resonance imaging before and after exchange transfusions. Arterial spin labeling and asymmetric spin echo sequences measured CBF and OEF, respectively, which were compared pre- and posttransfusion. Volumes of tissue with OEF above successive thresholds (36%, 38%, and 40%), as a metric of regional metabolic stress, were compared pre- and posttransfusion. Transfusions increased hemoglobin (Hb; from 9.1 to 10.3 g/dL; P < .001) and decreased Hb S (from 39.7% to 24.3%; P < .001). Transfusions reduced CBF (from 88 to 82.4 mL/100 g per minute; P = .004) and OEF (from 34.4% to 31.2%; P < .001). At all thresholds, transfusions reduced the volume of peak OEF found in the deep white matter, a location at high infarct risk in SCA (P < .001). Reduction of elevated CBF and OEF, both globally and regionally, suggests that CTT mitigates infarct risk in pediatric SCA by relieving cerebral metabolic stress at patient- and tissue-specific levels.

Large-Vessel Vasculopathy in Children With Sickle Cell Disease: A Magnetic Resonance Imaging Study of Infarct Topography and Focal Atrophy

BACKGROUND

Large-vessel vasculopathy (LVV) increases stroke risk in pediatric sickle cell disease beyond the baseline elevated stroke risk in this vulnerable population. The mechanisms underlying this added risk and its unique impact on the developing brain are not established.

METHODS

We analyzed magnetic resonance imaging and angiography scans of 66 children with sickle cell disease and infarcts by infarct density heatmaps and Jacobian determinants, a metric utilized to delineate focal volume change, to investigate if infarct location, volume, frequency, and cerebral atrophy differed among hemispheres with and without LVV.

RESULTS

Infarct density heatmaps demonstrated infarct "hot spots" within the deep white matter internal border zone region in both LVV and non-LVV hemispheres, but with greater infarct density and larger infarct volumes in LVV hemispheres (2.2 mL versus 0.25 mL, P < 0.001). Additional scattered cortical infarcts in the internal carotid artery territory occurred in LVV hemispheres, but were rare in non-LVV hemispheres. Jacobian determinants revealed greater atrophy in gray and white matter of the parietal lobes of LVV compared with non-LVV hemispheres.

CONCLUSION

Large-vessel vasculopathy in sickle cell disease appears to increase ischemic vulnerability in the borderzone region, as demonstrated by the increased frequency and extent of infarction within deep white matter, and increased risk of focal atrophy. Scattered infarctions across the LVV-affected hemispheres suggest additional stroke etiologies of vasculopathy (i.e., thromboembolism) in addition to chronic hypoxia-ischemia.

Evaluating Multisite rCBV Consistency from DSC-MRI Imaging Protocols and Postprocessing Software Across the NCI Quantitative Imaging Network Sites Using a Digital Reference Object (DRO)

Relative cerebral blood volume (rCBV) cannot be used as a response metric in clinical trials, in part, because of variations in biomarker consistency and associated interpretation across sites, stemming from differences in image acquisition and postprocessing methods (PMs). This study leveraged a dynamic susceptibility contrast magnetic resonance imaging digital reference object to characterize rCBV consistency across 12 sites participating in the Quantitative Imaging Network (QIN), specifically focusing on differences in site-specific imaging protocols (IPs; n = 17), and PMs (n = 19) and differences due to site-specific IPs and PMs (n = 25). Thus, high agreement across sites occurs when 1 managing center processes rCBV despite slight variations in the IP. This result is most likely supported by current initiatives to standardize IPs. However, marked intersite disagreement was observed when site-specific software was applied for rCBV measurements. This study's results have important implications for comparing rCBV values across sites and trials, where variability in PMs could confound the comparison of therapeutic effectiveness and/or any attempts to establish thresholds for categorical response to therapy. To overcome these challenges and ensure the successful use of rCBV as a clinical trial biomarker, we recommend the establishment of qualifying and validating site- and trial-specific criteria for scanners and acquisition methods (eg, using a validated phantom) and the software tools used for dynamic susceptibility contrast magnetic resonance imaging analysis (eg, using a digital reference object where the ground truth is known).

Normalization of cerebral hemodynamics after hematopoietic stem cell transplant in children with sickle cell disease

Children with sickle cell disease (SCD) demonstrate cerebral hemodynamic stress and are at high risk of strokes. We hypothesized that curative hematopoietic stem cell transplant (HSCT) normalizes cerebral hemodynamics in children with SCD compared with pre-transplant baseline. Whole-brain cerebral blood flow (CBF) and oxygen extraction fraction (OEF) were measured by magnetic resonance imaging 1 to 3 months before and 12 to 24 months after HSCT in 10 children with SCD. Three children had prior overt strokes, 5 children had prior silent strokes, and 1 child had abnormal transcranial Doppler ultrasound velocities. CBF and OEF of HSCT recipients were compared with non-SCD control participants and with SCD participants receiving chronic red blood cell transfusion therapy (CRTT) before and after a scheduled transfusion. Seven participants received matched sibling donor HSCT, and 3 participants received 8 out of 8 matched unrelated donor HSCT. All received reduced-intensity preparation and maintained engraftment, free of hemolytic anemia and SCD symptoms. Pre-transplant, CBF (93.5 mL/100 g/min) and OEF (36.8%) were elevated compared with non-SCD control participants, declining significantly 1 to 2 years after HSCT (CBF, 72.7 mL/100 g per minute; P = .004; OEF, 27.0%; P = .002), with post-HSCT CBF and OEF similar to non-SCD control participants. Furthermore, HSCT recipients demonstrated greater reduction in CBF (-19.4 mL/100 g/min) and OEF (-8.1%) after HSCT than children with SCD receiving CRTT after a scheduled transfusion (CBF, -0.9 mL/100 g/min; P = .024; OEF, -3.3%; P = .001). Curative HSCT normalizes whole-brain hemodynamics in children with SCD. This restoration of cerebral oxygen reserve may explain stroke protection after HSCT in this high-risk patient population.

Cerebral Oxygen Metabolic Stress, Microstructural Injury, and Infarction in Adults With Sickle Cell Disease

OBJECTIVE

To determine the patient- and tissue-based relationships between cerebral hemodynamic and oxygen metabolic stress, microstructural injury, and infarct location in adults with sickle cell disease (SCD).

METHODS

Control and SCD participants underwent brain MRI to quantify cerebral blood flow (CBF), oxygen extraction fraction (OEF), mean diffusivity (MD), and fractional anisotropy (FA) within normal-appearing white matter (NAWM), and infarcts on FLAIR. Multivariable linear regression examined the patient- and voxel-based associations between hemodynamic and metabolic stress (defined as elevated CBF and OEF, respectively), white matter microstructure, and infarct location.

RESULTS

Of 83 control and SCD participants, adults with SCD demonstrated increased CBF (50.9 vs 38.8 mL/min/100g, p<0.001), increased OEF (0.35 vs 0.25, p<0.001), increased MD (0.76 vs 0.72 x 10^-3mm² s^-1, p=0.005), and decreased FA (0.40 vs 0.42, p=0.021) within NAWM compared to controls. In multivariable analysis, increased OEF (β=0.19, p=0.035), but not CBF (β=0.00, p=0.340), independently predicted increased MD in the SCD cohort, while neither were predictors in controls. On voxel-wise regression, the SCD cohort demonstrated widespread OEF elevation, encompassing deep white matter regions of elevated MD and reduced FA, which spatially extended beyond high density infarct locations from the SCD cohort.

CONCLUSION

Elevated OEF, a putative index of cerebral oxygen metabolic stress, may provide a metric of ischemic vulnerability which could enable individualization of therapeutic strategies in SCD. The patient- and tissue-based relationships between elevated OEF, elevated MD, and cerebral infarcts suggest that oxygen metabolic stress may underlie microstructural injury prior to the development of cerebral infarcts in SCD.

Spatiotemporal uptake characteristics of [18]F-2-fluoro-2-deoxy-D-glucose in a rat middle cerebral artery occlusion model

BACKGROUND AND PURPOSE

Alterations of cerebral glucose metabolism are well anticipated during cerebral ischemia. However, detailed spatiotemporal characteristics of disturbed cerebral glucose metabolism during acute ischemia remain largely elusive. This study aims to delineate spatiotemporal distributions of [18]F-2-fluoro-2-deoxy-D-glucose (FDG) uptake using positron emission tomography imaging, particularly at the peri-ischemic zone, and its correlation with tissue outcome.

METHODS

The intraluminal suture middle cerebral artery occlusion model was used to induce focal cerebral ischemia in rats (n=48). All animals underwent sequential MRI and FDG positron emission tomography imaging at different times (30-150 minutes) after middle cerebral artery occlusion. MR and positron emission tomography images were coregistered. FDG uptake in the peri-ischemic zone was assessed in relation to middle cerebral artery occlusion duration, cerebral blood flow, apparent diffusion coefficient, and 24-hour T2 lesions.

RESULTS

Elevated FDG uptake was consistently observed at the peri-ischemic zone surrounding the presumed ischemic core with low FDG uptake. Both the spatial volume and the uptake level of the hyper-uptake region were inversely correlated with the duration of middle cerebral artery occlusion. The hyper-uptake regions exhibited a mild reduction of cerebral blood flow (28.2±3.2%) and apparent diffusion coefficient (9.1±1.4%) when compared with that in the contralateral hemisphere. Colocalization analysis revealed that, with reperfusion, an average of 12.1±1.7% of the hyper-uptake volume was recruited into final infarction.

CONCLUSIONS

Elevated FDG uptake at the peri-ischemic zone is consistently observed during acute cerebral ischemia. The region with elevated FDG uptake likely reflects viable tissues that can be salvaged with reperfusion. Therefore, acute FDG positron emission tomography imaging might hold promise in the management of patients with acute stroke.

Bulk volume susceptibility difference between deoxyhemoglobin and oxyhemoglobin for HbA and HbS: A comparative study

PURPOSE

Sickle cell anemia is a blood disorder that alters the morphology and the oxygen affinity of the red blood cells. Cerebral oxygen extraction fraction measurements using quantitative BOLD contrast have been used for assessing inadequate oxygen delivery and the subsequent risk of ischemic stroke in sickle cell anemia. The BOLD signal in MRI studies relies on Δ χ do , the bulk volume susceptibility difference between fully oxygenated and fully deoxygenated blood. Several studies have measured Δ χ do for normal hemoglobin A (HbA). However, it is not known whether the value is different for sickle hemoglobin. In this study, Δ χ do was measured for both HbA and sickle hemoglobin.

METHODS

Six sickle cell anemia patients and 6 controls were recruited. Various blood oxygenation levels were achieved through in vivo manipulations to keep the blood close to its natural state. To account for the differences in oxygen affinity, Hill's equations were used to translate partial pressure of oxygen to oxygen saturation for HbA, sickle hemoglobin, and fetal hemoglobin (HbF) separately. The pH and PCO₂ corrections were performed. Temperature and magnetic field drift were controlled for. A multivariate generalized linear mixed model with random participant effect was used.

RESULTS

Assuming that Δ χ do is similar for HbA and HbF and that Δ χ metHb is 5/4 of Δ χ do for HbA, it was found that the Δ χ do values for HbA and sickle hemoglobin were not statistically significantly different from each other.

CONCLUSION

The same Δ χ do value can be used for both types of hemoglobin in quantitative BOLD analysis.

CAPTURE: Consistently Acquired Projections for Tuned and Robust Estimation: A Self-Navigated Respiratory Motion Correction Approach

OBJECTIVES

In this study, we present a fully automated and robust self-navigated approach to obtain 4-dimensional (4-D) motion-resolved images during free breathing.

MATERIALS AND METHODS

The proposed method, Consistently Acquired Projections for Tuned and Robust Estimation (CAPTURE), is a variant of the stack-of-stars gradient-echo sequence. A 1-D navigator was consistently acquired at a fixed azimuthal angle for all stacks of spokes to reduce nonphysiological signal contamination due to system imperfections. The resulting projections were then "tuned" using complex phase rotation to adapt to scan-to-scan variations, followed by the detection of the respiratory curve. Four-dimensional motion-corrected and uncorrected images were then reconstructed via respiratory and temporal binning, respectively.This Health Insurance Portability and Accountability Act-compliant study was performed with Institutional Review Board approval. A phantom experiment was performed using a custom-made deformable motion phantom with an adjustable frequency and amplitude. For in vivo experiments, 10 healthy participants and 12 liver tumor patients provided informed consent and were imaged with the CAPTURE sequence.Two radiologists, blinded to which images were motion-corrected and which were not, independently reviewed the images and scored the image quality using a 5-point Likert scale.

RESULTS

In the respiratory motion phantom experiment, CAPTURE reversed the effects of motion blurring and restored edge sharpness from 36% to 78% of that observed in the images from the static scan.Despite large intra- and intersubject variability in respiration patterns, CAPTURE successfully detected the respiratory motion signal in all participants and significantly improved the image quality according to the subjective radiological assessments of 2 raters (P < 0.05 for both raters) with a 1 to 2-point improvement in the median Likert scores across the whole set of participants. Small lesions (<1 cm in size) which might otherwise be missed on uncorrected images because of motion blurring were more clearly depicted on the CAPTURE images.

CONCLUSIONS

CAPTURE provides a robust and fully automated solution for obtaining 4-D motion-resolved images in a free-breathing setting. With its unique tuning feature, CAPTURE can adapt to large intersubject and interscan variations. CAPTURE also enables better lesion delineation because of improved image sharpness, thereby increasing the visibility of small lesions.

Early changes of tissue perfusion after tissue plasminogen activator in hyperacute ischemic stroke

BACKGROUND AND PURPOSE

it is hypothesized that tissue plasminogen activator rescues brain tissue by improving perfusion. In this study, we aimed to examine acute regional perfusion changes and how they influence infarction and clinical outcome.

METHODS

three sequential MR scans were performed in 15 tissue plasminogen activator-treated patients within 3.5 (tp1), at 6 hours (tp2), and at 1 month (tp3) after stroke onset. "Hypoperfusion" was defined if mean transit time prolongation was more than a threshold (4 thresholds: 3, 4, 5, and 6 seconds). Four regions of interest were classified: (1) "reperfusion"-hypoperfused at tp1, normal at tp2; (2) "nonreperfusion"-hypoperfused at tp1 and tp2; (3) "normal perfusion"-normal at tp1 and tp2; and (4) "new hypoperfusion"-normal at tp1 and hypoperfused at tp2. Risk of infarction was calculated within each region of interest. Associations between tissue perfusion changes and clinical variables were evaluated using stepwise multiple linear regressions. Moreover, the association between National Institutes of Health Stroke Scale changes and perfusion alterations was assessed using linear mixed effect models.

RESULTS

regardless of the mean transit time threshold chosen, the risk of infarction in nonreperfused regions (40% to 68%, thresholds 3 to 6 seconds) was higher than reperfused regions (9% to 30%, P<0.05), and it was higher in new hypoperfusion regions (9% to 33%) than normal perfusion regions (3% to 4%, P<0.05). Volume of new hypoperfusion was significantly associated with onset-to-treatment time and initial hypoperfused volume. Overall relative reperfusion was significantly associated with National Institutes of Health Stroke Scale improvement.

CONCLUSIONS

early tissue perfusion changes influenced final tissue fate. The development of new hypoperfusion may result from delay in tissue plasminogen activator and a large initial lesion.

Evaluating the Use of rCBV as a Tumor Grade and Treatment Response Classifier Across NCI Quantitative Imaging Network Sites: Part II of the DSC-MRI Digital Reference Object (DRO) Challenge

We have previously characterized the reproducibility of brain tumor relative cerebral blood volume (rCBV) using a dynamic susceptibility contrast magnetic resonance imaging digital reference object across 12 sites using a range of imaging protocols and software platforms. As expected, reproducibility was highest when imaging protocols and software were consistent, but decreased when they were variable. Our goal in this study was to determine the impact of rCBV reproducibility for tumor grade and treatment response classification. We found that varying imaging protocols and software platforms produced a range of optimal thresholds for both tumor grading and treatment response, but the performance of these thresholds was similar. These findings further underscore the importance of standardizing acquisition and analysis protocols across sites and software benchmarking.

Functional Connectivity Decreases with Metabolic Stress in Sickle Cell Disease

OBJECTIVE

Children with sickle cell disease (SCD) experience cognitive deficits even when unaffected by stroke. Using functional connectivity magnetic resonance imaging (MRI) as a potential biomarker of cognitive function, we tested our hypothesis that children with SCD would have decreased functional connectivity, and that children experiencing the greatest metabolic stress, indicated by elevated oxygen extraction fraction, would have the lowest connectivity.

METHODS

We prospectively obtained brain MRIs and cognitive testing in healthy controls and children with SCD.

RESULTS

We analyzed data from 60 participants (20 controls and 40 with sickle cell disease). There was no difference in global cognition or cognitive subdomains between cohorts. However, we found decreased functional connectivity within the sensory-motor, lateral sensory-motor, auditory, salience, and subcortical networks in participants with SCD compared with controls. Further, as white matter oxygen extraction fraction increased, connectivity within the visual (p = 0.008, parameter estimate = -0.760 [95% CI = -1.297, -0.224]), default mode (p = 0.012, parameter estimate = -0.417 [95% CI = -0.731, -0.104]), and cingulo-opercular (p = 0.009, parameter estimate = -0.883 [95% CI = -1.517, -0.250]) networks decreased.

INTERPRETATION

We conclude that there is diminished functional connectivity within these anatomically contiguous networks in children with SCD compared with controls, even when differences are not seen with cognitive testing. Increased white matter oxygen extraction fraction was associated with decreased connectivity in select networks. These data suggest that elevated oxygen extraction fraction and disrupted functional connectivity are potentially presymptomatic neuroimaging biomarkers for cognitive decline in SCD. ANN NEUROL 2020;88:995-1008.

Reperfusion Beyond 6 Hours Reduces Infarct Probability in Moderately Ischemic Brain Tissue

BACKGROUND AND PURPOSE

We aimed to examine perfusion changes between 3 and 6 and 6 and 24 hours after stroke onset and their impact on tissue outcome.

METHODS

Acute ischemic stroke patients underwent perfusion magnetic resonance imaging at 3, 6, and 24 hours after stroke onset and follow-up fluid-attenuated inversion recovery at 1 month to assess tissue fate. Mean transit time prolongation maps (MTTp=MTT-[median MTT of contralateral hemisphere]) were obtained at 3 (MTTp3 h), 6 (MTTp6 h), and 24 hours (MTTp24 h). Perfusion changes between 3 and 6 hours (ΔMTTp3_6) and 6 and 24 hours (ΔMTTp6_24) were calculated. A 2-step analysis was performed to evaluate the impact of ΔMTTp3_6 and ΔMTTp6_24 on tissue fate. First, a voxel-based multivariable logistic regression was performed for each individual patient with MTTp3 h, ΔMTTp3_6, and ΔMTT6_24 as independent variables and tissue fate as outcome. Second, Wilcoxon signed-rank tests on logistic regression coefficients were performed across patients to evaluate whether ΔMTTp3_6 and ΔMTT6_24 had significant impact on tissue fate for varying severities of baseline perfusion.

RESULTS

Perfusion change was common during both time periods: 85% and 81% of patients had perfusion improvement during 3- to 6- and 6- and 24-hour time intervals, respectively. ΔMTT3_6 significantly influenced 1-month infarct probability across a wide range of baseline perfusion (MTTp 0-15 s). ΔMTT6_24 also impacted 1-month infarct probability, but its influence was restricted to tissue with milder baseline ischemia (MTTp 0-10 s).

CONCLUSIONS

Brain tissue with mild to moderate ischemia can be salvaged by reperfusion even after 6 hours. Such tissue could be targeted for intervention beyond current treatment windows.

3D pediatric cranial bone imaging using high-resolution MRI for visualizing cranial sutures: a pilot study

OBJECTIVE

There is an unmet need to perform imaging in young children and obtain CT-equivalent cranial bone images without subjecting the patients to radiation. In this study, the authors propose using a high-resolution fast low-angle shot golden-angle 3D stack-of-stars radial volumetric interpolated breath-hold examination (GA-VIBE) MRI sequence that is intrinsically robust to motion and has enhanced bone versus soft-tissue contrast.

METHODS

Patients younger than 11 years of age, who underwent clinical head CT scanning for craniosynostosis or other cranial malformations, were eligible for the study. 3D reconstructed images created from the GA-VIBE MRI sequence and the gold-standard CT scan were randomized and presented to 3 blinded reviewers. For all image sets, each reviewer noted the presence or absence of the 6 primary cranial sutures and recorded on 5-point Likert scales whether they recommended a second scan be performed.

RESULTS

Eleven patients (median age 1.8 years) underwent MRI after clinical head CT scanning was performed. Five of the 11 patients were sedated. Three clinicians reviewed the images, and there were no cases, either with CT scans or MR images, in which a reviewer agreed a repeat scan was required for diagnosis or surgical planning. The reviewers reported clear imaging of the regions of interest on 99% of the CT reviews and 96% of the MRI reviews. With CT as the standard, the sensitivity and specificity of the GA-VIBE MRI sequence to detect suture closure were 97% and 96%, respectively (n = 198 sutures read).

CONCLUSIONS

The 3D reconstructed images using the GA-VIBE sequence in comparison to the CT scans created clinically acceptable cranial images capable of detecting cranial sutures. Future directions include reducing the scan time, improving motion correction, and automating postprocessing for clinical utility.

TOWERS: T-One with Enhanced Robustness and Speed

PURPOSE

A new T1 mapping method is proposed that is accurate, rapid, and robust to motion. Considering these features, the method is dubbed "T-One with Enhanced Robustness and Speed (TOWERS)."

METHODS

TOWERS is composed of inversion recovery (IR) and saturation recovery (SR) acquisitions. In the IR acquisitions, a slice reordering scheme is used to sample all slices in an efficient manner, whereas the SR acquisitions serve as references for motion estimation. Furthermore, as opposed to the usual way of running generalized autocalibrating partially parallel acquisitions (GRAPPA) calibration only once at the beginning, GRAPPA coefficients are updated in the middle and at the end, and are later used for retrospectively correcting for motion artifacts. Finally, sub-voxel magnetization tracking is deployed to account for motion-induced signal evolution changes.

RESULTS

Whole-brain T1 mapping data with a spatial resolution of 1.56 × 1.56 × 2.00 mm can be collected within 2.5 min. TOWERS and the gold-standard IR method agree well in phantom, while high reproducibility is achieved in vivo. High-quality T1 maps in the presence of severe motion show the robustness of the method.

CONCLUSION

The proposed method, TOWERS, is shown to be rapid, accurate, and robust. Multiple GRAPPA calibrations and sub-voxel magnetization tracking make TOWERS unique. Magn Reson Med 76:118-126, 2016. © 2015 Wiley Periodicals, Inc.

Phase2Phase: Respiratory Motion-Resolved Reconstruction of Free-Breathing Magnetic Resonance Imaging Using Deep Learning Without a Ground Truth for Improved Liver Imaging

OBJECTIVES

Respiratory binning of free-breathing magnetic resonance imaging data reduces motion blurring; however, it exacerbates noise and introduces severe artifacts due to undersampling. Deep neural networks can remove artifacts and noise but usually require high-quality ground truth images for training. This study aimed to develop a network that can be trained without this requirement.

MATERIALS AND METHODS

This retrospective study was conducted on 33 participants enrolled between November 2016 and June 2019. Free-breathing magnetic resonance imaging was performed using a radial acquisition. Self-navigation was used to bin the k-space data into 10 respiratory phases. To simulate short acquisitions, subsets of radial spokes were used in reconstructing images with multicoil nonuniform fast Fourier transform (MCNUFFT), compressed sensing (CS), and 2 deep learning methods: UNet3DPhase and Phase2Phase (P2P). UNet3DPhase was trained using a high-quality ground truth, whereas P2P was trained using noisy images with streaking artifacts. Two radiologists blinded to the reconstruction methods independently reviewed the sharpness, contrast, and artifact-freeness of the end-expiration images reconstructed from data collected at 16% of the Nyquist sampling rate. The generalized estimating equation method was used for statistical comparison. Motion vector fields were derived to examine the respiratory motion range of 4-dimensional images reconstructed using different methods.

RESULTS

A total of 15 healthy participants and 18 patients with hepatic malignancy (50 ± 15 years, 6 women) were enrolled. Both reviewers found that the UNet3DPhase and P2P images had higher contrast (P < 0.01) and fewer artifacts (P < 0.01) than the CS images. The UNet3DPhase and P2P images were reported to be sharper than the CS images by 1 reviewer (P < 0.01) but not by the other reviewer (P = 0.22, P = 0.18). UNet3DPhase and P2P were similar in sharpness and contrast, whereas UNet3DPhase had fewer artifacts (P < 0.01). The motion vector lengths for the MCNUFFT800 and P2P800 images were comparable (10.5 ± 4.2 mm and 9.9 ± 4.0 mm, respectively), whereas both were significantly larger than CS2000 (7.0 ± 3.9 mm; P < 0.0001) and UNnet3DPhase800 (6.9 ± 3.2; P < 0.0001) images.

CONCLUSIONS

Without a ground truth, P2P can reconstruct sharp, artifact-free, and high-contrast respiratory motion-resolved images from highly undersampled data. Unlike the CS and UNet3DPhase methods, P2P did not artificially reduce the respiratory motion range.

Deep-learning synthesized pseudo-CT for MR high-resolution pediatric cranial bone imaging (MR-HiPCB)

PURPOSE

CT is routinely used to detect cranial abnormalities in pediatric patients with head trauma or craniosynostosis. This study aimed to develop a deep learning method to synthesize pseudo-CT (pCT) images for MR high-resolution pediatric cranial bone imaging to eliminating ionizing radiation from CT.

METHODS

3D golden-angle stack-of-stars MRI were obtained from 44 pediatric participants. Two patch-based residual UNets were trained using paired MR and CT patches randomly selected from the whole head (NetWH) or in the vicinity of bone, fractures/sutures, or air (NetBA) to synthesize pCT. A third residual UNet was trained to generate a binary brain mask using only MRI. The pCT images from NetWH (pCTNetWH ) in the brain area and NetBA (pCTNetBA ) in the nonbrain area were combined to generate pCTCom . A manual processing method using inverted MR images was also employed for comparison.

RESULTS

pCTCom (68.01 ± 14.83 HU) had significantly smaller mean absolute errors (MAEs) than pCTNetWH (82.58 ± 16.98 HU, P < 0.0001) and pCTNetBA (91.32 ± 17.2 HU, P < 0.0001) in the whole head. Within cranial bone, the MAE of pCTCom (227.92 ± 46.88 HU) was significantly lower than pCTNetWH (287.85 ± 59.46 HU, P < 0.0001) but similar to pCTNetBA (230.20 ± 46.17 HU). Dice similarity coefficient of the segmented bone was significantly higher in pCTCom (0.90 ± 0.02) than in pCTNetWH (0.86 ± 0.04, P < 0.0001), pCTNetBA (0.88 ± 0.03, P < 0.0001), and inverted MR (0.71 ± 0.09, P < 0.0001). Dice similarity coefficient from pCTCom demonstrated significantly reduced age dependence than inverted MRI. Furthermore, pCTCom provided excellent suture and fracture visibility comparable to CT.

CONCLUSION

MR high-resolution pediatric cranial bone imaging may facilitate the clinical translation of a radiation-free MR cranial bone imaging method for pediatric patients.

Respiratory motion management using a single rapid MRI scan for a 0.35 T MRI-Linac system

BACKGROUND

MRI has a rapidly growing role in radiation therapy (RT) for treatment planning, real-time image guidance, and beam gating (e.g., MRI-Linac). Free-breathing 4D-MRI is desirable in respiratory motion management for therapy. Moreover, high-quality 3D-MRIs without motion artifacts are needed to delineate lesions. Existing MRI methods require multiple scans with lengthy acquisition times or are limited by low spatial resolution, contrast, and signal-to-noise ratio.

PURPOSE

We developed a novel method to obtain motion-resolved 4D-MRIs and motion-integrated 3D-MRI reconstruction using a single rapid (35-45 s scan on a 0.35 T MRI-Linac.

METHODS

Golden-angle radial stack-of-stars MRI scans were acquired from a respiratory motion phantom and 12 healthy volunteers (n = 12) on a 0.35 T MRI-Linac. A self-navigated method was employed to detect respiratory motion using 2000 (acquisition time = 5-7 min) and the first 200 spokes (acquisition time = 35-45 s). Multi-coil non-uniform fast Fourier transform (MCNUFFT), compressed sensing (CS), and deep-learning Phase2Phase (P2P) methods were employed to reconstruct motion-resolved 4D-MRI using 2000 spokes (MCNUFFT2000) and 200 spokes (CS200 and P2P200). Deformable motion vector fields (MVFs) were computed from the 4D-MRIs and used to reconstruct motion-corrected 3D-MRIs with the MOtion Transformation Integrated forward-Fourier (MOTIF) method. Image quality was evaluated quantitatively using the structural similarity index measure (SSIM) and the root mean square error (RMSE), and qualitatively in a blinded radiological review.

RESULTS

Evaluation using the respiratory motion phantom experiment showed that the proposed method reversed the effects of motion blurring and restored edge sharpness. In the human study, P2P200 had smaller inaccuracy in MVFs estimation than CS200. P2P200 had significantly greater SSIMs (p < 0.0001) and smaller RMSEs (p < 0.001) than CS200 in motion-resolved 4D-MRI and motion-corrected 3D-MRI. The radiological review found that MOTIF 3D-MRIs using MCNUFFT2000 exhibited the highest image quality (scoring > 8 out of 10), followed by P2P200 (scoring > 5 out of 10), and then motion-uncorrected (scoring < 3 out of 10) in sharpness, contrast, and artifact-freeness.

CONCLUSIONS

We have successfully demonstrated a method for respiratory motion management for MRI-guided RT. The method integrated self-navigated respiratory motion detection, deep-learning P2P 4D-MRI reconstruction, and a motion integrated reconstruction (MOTIF) for 3D-MRI using a single rapid MRI scan (35-45 s) on a 0.35 T MRI-Linac system.

Superimposed video disambiguation for increased field of view

Many infrared optical systems in wide-ranging applications such as surveillance and security frequently require large fields of view (FOVs). Often this necessitates a focal plane array (FPA) with a large number of pixels, which, in general, is very expensive. In a previous paper, we proposed a method for increasing the FOV without increasing the pixel resolution of the FPA by superimposing multiple sub-images within a static scene and disambiguating the observed data to reconstruct the original scene. This technique, in effect, allows each sub-image of the scene to share a single FPA, thereby increasing the FOV without compromising resolution. In this paper, we demonstrate the increase of FOVs in a realistic setting by physically generating a superimposed video from a single scene using an optical system employing a beamsplitter and a movable mirror. Without prior knowledge of the contents of the scene, we are able to disambiguate the two sub-images, successfully capturing both large-scale features and fine details in each sub-image. We improve upon our previous reconstruction approach by allowing each sub-image to have slowly changing components, carefully exploiting correlations between sequential video frames to achieve small mean errors and to reduce run times. We show the effectiveness of this improved approach by reconstructing the constituent images of a surveillance camera video.

Absolute oxygenation metabolism measurements using magnetic resonance imaging

Cerebral oxygen metabolism plays a critical role in maintaining normal function of the brain. It is the primary energy source to sustain neuronal functions. Abnormalities in oxygen metabolism occur in various neuro-pathologic conditions such as ischemic stroke, cerebral trauma, cancer, Alzheimer’s disease and shock. Therefore, the ability to quantitatively measure tissue oxygenation and oxygen metabolism is essential to the understanding of pathophysiology and treatment of various diseases. The focus of this review is to provide an introduction of various blood oxygenation level dependent (BOLD) contrast methods for absolute measurements of tissue oxygenation, including both magnitude and phase image based approaches. The advantages and disadvantages of each method are discussed.

MR-assisted PET respiratory motion correction using deep-learning based short-scan motion fields

PURPOSE

We evaluated the impact of PET respiratory motion correction (MoCo) in a phantom and patients. Moreover, we proposed and examined a PET MoCo approach using motion vector fields (MVFs) from a deep-learning reconstructed short MRI scan.

METHODS

The evaluation of PET MoCo was performed in a respiratory motion phantom study with varying lesion sizes and tumor to background ratios (TBRs) using a static scan as the ground truth. MRI-based MVFs were derived from either 2000 spokes (MoCo2000 , 5-6 min acquisition time) using a Fourier transform reconstruction or 200 spokes (MoCoP2P200 , 30-40 s acquisition time) using a deep-learning Phase2Phase (P2P) reconstruction and then incorporated into PET MoCo reconstruction. For six patients with hepatic lesions, the performance of PET MoCo was evaluated using quantitative metrics (SUVmax , SUVpeak , SUVmean , lesion volume) and a blinded radiological review on lesion conspicuity.

RESULTS

MRI-assisted PET MoCo methods provided similar results to static scans across most lesions with varying TBRs in the phantom. Both MoCo2000 and MoCoP2P200 PET images had significantly higher SUVmax , SUVpeak , SUVmean and significantly lower lesion volume than non-motion-corrected (non-MoCo) PET images. There was no statistical difference between MoCo2000 and MoCoP2P200 PET images for SUVmax , SUVpeak , SUVmean or lesion volume. Both radiological reviewers found that MoCo2000 and MoCoP2P200 PET significantly improved lesion conspicuity.

CONCLUSION

An MRI-assisted PET MoCo method was evaluated using the ground truth in a phantom study. In patients with hepatic lesions, PET MoCo images improved quantitative and qualitative metrics based on only 30-40 s of MRI motion modeling data.

Quantification of myocardial oxygen extraction fraction: A proof-of-concept study

PURPOSE

To demonstrate a proof of concept for the measurement of myocardial oxygen extraction fraction (mOEF) by a cardiovascular magnetic resonance technique.

METHODS

The mOEF measurement was performed using an electrocardiogram-triggered double-echo asymmetric spin-echo sequence with EPI readout. Seven healthy volunteers (22-37 years old, 5 females) were recruited and underwent the same imaging scans at rest on 2 different days for reproducibility assessment. Another 5 subjects (23-37 years old, 4 females) underwent cardiovascular magnetic resonance studies at rest and during a handgrip isometric exercise with a 25% of maximal voluntary contraction. Both mOEF and myocardial blood volume values were obtained in septal regions from respective maps.

RESULTS

The reproducibility was excellent for the measurements of mOEF in septal myocardium (coefficient of variation: 3.37%) and moderate for myocardial blood volume (coefficient of variation: 19.7%). The average mOEF and myocardial blood volume of 7 subjects at rest were 0.61 ± 0.05 and 11.0 ± 4.3%, respectively. The mOEF agreed well with literature values that were measured by PET in healthy volunteers. In the exercise study, there was no significant change in mOEF (0.61 ± 0.06 vs 0.62 ± 0.07) or myocardial blood volume (12 ± 6% vs 13 ± 4%) from rest to exercise, as expected.

CONCLUSION

The implemented cardiovascular magnetic resonance method shows potential for the quantitative assessment of mOEF in vivo. Future technical work is needed to improve image quality and to further validate mOEF measurements.

Deformation-Compensated Learning for Image Reconstruction Without Ground Truth

Deep neural networks for medical image reconstruction are traditionally trained using high-quality ground-truth images as training targets. Recent work on Noise2Noise (N2N) has shown the potential of using multiple noisy measurements of the same object as an alternative to having a ground-truth. However, existing N2N-based methods are not suitable for learning from the measurements of an object undergoing nonrigid deformation. This paper addresses this issue by proposing the deformation-compensated learning (DeCoLearn) method for training deep reconstruction networks by compensating for object deformations. A key component of DeCoLearn is a deep registration module, which is jointly trained with the deep reconstruction network without any ground-truth supervision. We validate DeCoLearn on both simulated and experimentally collected magnetic resonance imaging (MRI) data and show that it significantly improves imaging quality.

Cranial vault imaging for pediatric head trauma using a radial VIBE MRI sequence

OBJECTIVE

Head trauma is the most common indication for a CT scan. In this pilot study, the authors assess the feasibility of a 5-minute high-resolution 3D golden-angle (GA) stack-of-stars radial volumetric interpolated breath-hold examination (VIBE) MRI sequence (GA-VIBE) to obtain clinically acceptable cranial bone images and identify cranial vault fractures compared to CT.

METHODS

Patients younger than 18 years of age presenting after head trauma were eligible for the study. Three clinicians reviewed and assessed 1) slice-by-slice volumetric CT and inverted MR images, and 2) 3D reconstructions obtained from inverted MR images and the gold standard (CT). For each image set, reviewers noted on 5-point Likert scales whether they recommended that a repeat scan be performed and the presence or absence of cranial vault fractures.

RESULTS

Thirty-one patients completed MRI after a clinical head CT scan was performed. Based on CT imaging, 8 of 31 patients had cranial fractures. Two of 31 patients were sedated as part of their clinical MRI scan. In 30 (97%) of 31 MRI reviews, clinicians agreed (or strongly agreed) that the image quality was acceptable for clinical diagnosis. Overall, comparing MRI to acceptable gold-standard CT, sensitivity and specificity of fracture detection were 100%. Furthermore, there were no discrepancies between CT and MRI in classification of fracture type or location.

CONCLUSIONS

When compared with the gold standard (CT), the volumetric and 3D reconstructed images using the GA-VIBE sequence were able to produce clinically acceptable cranial images with excellent ability to detect cranial vault fractures.

Resting-state MRI reveals spontaneous physiological fluctuations in the kidney and tracks diabetic nephropathy in rats

The kidneys maintain fluid-electrolyte balance and excrete waste in the presence of constant fluctuations in plasma volume and systemic blood pressure. The kidneys perform these functions to control capillary perfusion and glomerular filtration by modulating the mechanisms of autoregulation. An effect of these modulations are spontaneous, natural fluctuations in glomerular perfusion. Numerous other mechanisms can lead to fluctuations in perfusion and flow. The ability to monitor these spontaneous physiological fluctuations in vivo could facilitate the early detection of kidney disease. The goal of this work was to investigate the use of resting-state magnetic resonance imaging (rsMRI) to detect spontaneous physiological fluctuations in the kidney. We performed rsMRI of rat kidneys in vivo over 10 min, applying motion correction to resolve time series in each voxel. We observed spatially variable, spontaneous fluctuations in rsMRI signal between 0 and 0.3 Hz, in frequency bands associated with autoregulatory mechanisms. We further applied rsMRI to investigate changes in these fluctuations in a rat model of diabetic nephropathy. Spectral analysis was performed on time series of rsMRI signals in the kidney cortex and medulla. The power from spectra in specific frequency bands from the cortex correlated with severity of glomerular pathology caused by diabetic nephropathy. Finally, we investigated the feasibility of using rsMRI of the human kidney in two participants, observing the presence of similar, spatially variable fluctuations. This approach may enable a range of preclinical and clinical investigations of kidney function and facilitate the development of new therapies to improve outcomes in patients with kidney disease.NEW & NOTEWORTHY This work demonstrates the development and use of resting-state MRI to detect low-frequency, spontaneous physiological fluctuations in the kidney consistent with previously observed fluctuations in perfusion and potentially due to autoregulatory function. These fluctuations are detectable in rat and human kidneys, and the power of these fluctuations is affected by diabetic nephropathy in rats.