Oxygen challenge magnetic resonance imaging in healthy human volunteers

Oxygen Challenge Imaging Reveals Differences in Metabolic Activity Between Kurtosis Lesion and Diffusion/Kurtosis Lesion Mismatch in a Rodent Model of Acute Stroke

OBJECTIVE

Accurate identification of potentially salvageable tissues is critical for improving acute stroke treatment. A previous study showed that the kurtosis lesion exhibited insignificant response after prompt reperfusion treatment, while the diffusion/kurtosis lesion mismatch could recover after reperfusion. We hypothesized that these 2 regions are in different metabolic states.

MATERIALS AND METHODS

Transient oxygen challenge (OC) is a procedure that uses oxygen as a metabolic bio-tracer and has been performed to explore metabolic activity in tissues. We combined OC with multiparameter magnetic resonance imaging (including diffusion kurtosis imaging and T2* mapping sequences) to study metabolic activity in the ischemic brain of Sprague Dawley rats.

RESULTS

Oxygen challenge image analysis revealed changes in T2* values, most significantly in the mean diffusivity (MD)/mean kurtosis (MK) lesion mismatch (22.3 ± 1.6%) and least significantly in the MK lesions (6.6 ± 0.6%). The MD images acquired within 138 ± 9 minutes after ischemia showed a larger ischemic lesion (45.5 ± 3.0% of the total area) than the MK images (33.2 ± 4.2% of the total area). The change rate of the MK value (53.0 ± 4.4%) was higher than that of the MD value (37.5 ± 3.2%).

CONCLUSIONS

The present study shows that MK lesion and MD/MK lesion mismatch exhibited different metabolic activity states. The MK lesion presented metabolic-related values close to the ischemic core, while at least part of the MD/MK mismatch area was comparable with ischemic penumbra metabolic activity. These findings are important to support image-guided individualized stroke therapies.

Clinical Imaging of the Penumbra in Ischemic Stroke: From the Concept to the Era of Mechanical Thrombectomy

The ischemic penumbra is defined as the severely hypoperfused, functionally impaired, at-risk but not yet infarcted tissue that will be progressively recruited into the infarct core. Early reperfusion aims to save the ischemic penumbra by preventing infarct core expansion and is the mainstay of acute ischemic stroke therapy. Intravenous thrombolysis and mechanical thrombectomy for selected patients with large vessel occlusion has been shown to improve functional outcome. Given the varying speed of infarct core progression among individuals, a therapeutic window tailored to each patient has recently been proposed. Recent studies have demonstrated that reperfusion therapies are beneficial in patients with a persistent ischemic penumbra, beyond conventional time windows. As a result, mapping the penumbra has become crucial in emergency settings for guiding personalized therapy. The penumbra was first characterized as an area with a reduced cerebral blood flow, increased oxygen extraction fraction and preserved cerebral metabolic rate of oxygen using positron emission tomography (PET) with radiolabeled O₂. Because this imaging method is not feasible in an acute clinical setting, the magnetic resonance imaging (MRI) mismatch between perfusion-weighted imaging and diffusion-weighted imaging, as well as computed tomography perfusion have been proposed as surrogate markers to identify the penumbra in acute ischemic stroke patients. Transversal studies comparing PET and MRI or using longitudinal assessment of a limited sample of patients have been used to define perfusion thresholds. However, in the era of mechanical thrombectomy, these thresholds are debatable. Using various MRI methods, the original penumbra definition has recently gained a significant interest. The aim of this review is to provide an overview of the evolution of the ischemic penumbra imaging methods, including their respective strengths and limitations, as well as to map the current intellectual structure of the field using bibliometric analysis and explore future directions.

Use of blood oxygen level-dependent magnetic resonance imaging to detect acute cellular rejection post-liver transplantation

OBJECTIVES

Acute cellular rejection (ACR) is a major immune occurrence post-liver transplant that can cause abnormal liver function. Blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) can be used to evaluate liver disease, but it has not been utilized in the diagnosis of ACR post-liver transplant. Therefore, the purpose of this study is to evaluate the diagnostic performance of BOLD MRI and to monitor treatment response in recipients with ACR.

METHODS

This prospective study was approved by the local institutional review board. Fifty-five recipients with highly suspected ACR were enrolled in this study. Each patient underwent hepatic BOLD MRI, blood biochemistry, and biopsy before treatment. Of 55 patients, 19 recipients with ACR received a follow-up MRI after treatment. After obtaining the R2* maps, five regions-of-interest were placed on liver parenchyma to estimate the mean R2* values for statistical analysis. Receiver operating characteristic curve (ROC) analysis was performed to assess the diagnostic performance of R2* values in detecting patients with ACR.

RESULTS

The histopathologic results showed that 27 recipients had ACR (14 mild, 11 moderate, and 2 severe) and their hepatic R2* values were significantly lower than those of patients without ACR. ROC analysis revealed that the sensitivity and specificity of the R2* values for detection of ACR were 82.1% and 89.9%, respectively. Moreover, the R2* values and liver function in patients with ACR significantly increased after immunosuppressive treatment.

CONCLUSION

The non-invasive BOLD MRI technique may be useful for assessment of hepatic ACR and monitoring of treatment response after immunosuppressive therapy.

KEY POINTS

• Patients with acute cellular rejection post-liver transplant exhibited significantly decreased R2* values in liver parenchyma. • R2* values and liver function were significantly increased after immunosuppressive therapy. • R2* values were constructive indicators in detecting acute cellular rejection due to their high sensitivity and specificity.

Effect of Drinking Oxygenated Water Assessed by in vivo MRI Relaxometry

GRANT SUPPORT

This project was funded by the Research Council of Norway.

BACKGROUND

Oxygen uptake through the gastrointestinal tract after oral administration of oxygenated water in humans is not well studied and is debated in the literature. Due to the paramagnetic properties of oxygen and deoxyhemoglobin, MRI as a technique might be able to detect changes in relaxometry values caused by increased oxygen levels in the blood.

PURPOSE

To assess whether oxygen dissolved in water is absorbed from the gastrointestinal tract and transported into the bloodstream after oral administration.

STUDY TYPE

A randomized, double-blinded, placebo-controlled crossover trial.

POPULATION/SUBJECTS

Thirty healthy male volunteers age 20-35.

FIELD STRENGTH/SEQUENCE

3T/Modified Look-Locker inversion recovery (MOLLI) T₁ -mapping and multi fast field echo (mFFE) T₂ *-mapping.

ASSESSMENT

Each volunteer was scanned in two separate sessions. T₁ and T₂ * maps were acquired repeatedly covering the hepatic portal vein (HPV) and vena cava inferior (VCI, control vein) before and after intake of oxygenated or control water. Assessments were done by placing a region of interest in the HPV and VCI.

STATISTICAL TEST

A mixed linear model was performed to the compare control vs. oxygen group.

RESULTS

Drinking caused a mean 1.6% 95% CI (1.1-2.0% P < 0.001) increase in T₁ of HPV blood and water oxygenation attributed another 0.70% 95% confidence interval (CI) (0.07-1.3% P = 0.028) increase. Oxygenation did not change T₁ in VCI blood. Mean T₂ * increased 9.6% 95% CI (1.7-17.5% P = 0.017) after ingestion of oxygenated water and 1.2% 95% CI (-4.3-6.8% P = 0.661) after ingestion of control water. The corresponding changes in VCI blood were not significant.

DATA CONCLUSION

Ingestion of water caused changes in T₁ and T₂ * of HPV blood compatible with dilution due to water absorption. The effects were enhanced by oxygen. Assessment of oxygen enrichment of HPV blood was not possible due to the dilution effect.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 2 J. Magn. Reson. Imaging 2020;52:720-728.

Preclinical Validation of the Therapeutic Potential of Glasgow Oxygen Level Dependent (GOLD) Technology: a Theranostic for Acute Stroke

In acute stroke patients, penumbral tissue is non-functioning but potentially salvageable within a time window of variable duration and represents target tissue for rescue. Reperfusion by thrombolysis and/or thrombectomy can rescue penumbra and improve stroke outcomes, but these treatments are currently available to a minority of patients. In addition to the utility of Glasgow Oxygen Level Dependent (GOLD) as an MRI contrast capable of detecting penumbra, its constituent perfluorocarbon (PFC) oxygen carrier, combined with normobaric hyperoxia, also represents a potential acute stroke treatment through improved oxygen delivery to penumbra. Preclinical studies were designed to test the efficacy of an intravenous oxygen carrier, the perfluorocarbon emulsion Oxycyte® (O-PFC), combined with normobaric hyperoxia (50% O₂) in both in vitro (neuronal cell culture) and in vivo rat models of ischaemic stroke. Outcome was assessed through the quantification of lipid peroxidation and oxidative stress levels, mortality, infarct volume, neurological scoring and sensorimotor tests of functional outcome in two in vivo models of stroke. Additionally, we investigated evidence for any positive or negative interactions with the thrombolytic recombinant tissue plasminogen activator (rt-PA) following embolus-induced stroke in rats. Treatment with intravenous O-PFC + normobaric hyperoxia (50% O₂) provided evidence of reduced infarct size and improved functional recovery. It did not exacerbate oxidative stress and showed no adverse interactions with rt-PA. The positive results and lack of adverse effects support human trials of O-PFC + 50% O₂ normobaric hyperoxia as a potential therapeutic approach. Combined with the diagnostic data presented in the preceding paper, O-PFC and normobaric hyperoxia is a potential theranostic for acute ischaemic stroke.