Diffusion and perfusion MR imaging of acute ischemic stroke

Intraventricular haemorrhage in premature infants: the role of immature neuronal salt and water transport

Intraventricular haemorrhage is a common complication of premature birth. Survivors are often left with cerebral palsy, intellectual disability and/or hydrocephalus. Animal models suggest that brain tissue shrinkage, with subsequent vascular stretch and tear, is an important step in the pathophysiology, but the cause of this shrinkage is unknown. Clinical risk factors for intraventricular haemorrhage are biomarkers of hypoxic-ischaemic stress, which causes mature neurons to swell. However, immature neuronal volume might shift in the opposite direction in these conditions. This is because immature neurons express the chloride, salt and water transporter NKCC1, which subserves regulatory volume increases in non-neural cells, whereas mature neurons express KCC2, which subserves regulatory volume decreases. When hypoxic-ischaemic conditions reduce active ion transport and increase the cytoplasmic membrane permeability, the effects of these transporters are diminished. Consequentially, mature neurons swell (cytotoxic oedema), whereas immature neurons might shrink. After hypoxic-ischaemic stress, in vivo and in vitro multi-photon imaging of perinatal transgenic mice demonstrated shrinkage of viable immature neurons, bulk tissue shrinkage and blood vessel displacement. Neuronal shrinkage was correlated with age-dependent membrane salt and water transporter expression using immunohistochemistry. Shrinkage of immature neurons was prevented by prior genetic or pharmacological inhibition of NKCC1 transport. These findings open new avenues of investigation for the detection of acute brain injury by neuroimaging, in addition to prevention of neuronal shrinkage and the ensuing intraventricular haemorrhage, in premature infants.

Early diffusion-weighted MRI at 3 Tesla detects ischemic changes of the optic nerve in anterior ischemic optic neuropathy

OBJECTIVES

To assess the impact of timing from visual symptoms' onset to diffusion-weighted (DW) 3 T MRI completion to detect ischemic changes of the optic disc and optic nerve in AION patients.

METHODS

This IRB-approved retrospective single-center study included 3 T MRI data from 126 patients with AION and 111 controls with optic neuritis treated between January 2015 and May 2020. Two radiologists blinded to all data individually analyzed imaging. A senior neuroradiologist resolved any discrepancies by consensus. The primary judgment criterion was the restricted diffusion of the optic disc and/or the optic nerve assessed subjectively on the ADC maps. ADC values were also measured. Spearman rank correlations were used to examine the relationships between timing from visual symptoms' onset to MRI completion and both the restricted diffusion and the ADC values.

RESULTS

One hundred twenty-six patients (47/126 [37.3%] women and 79/126 [62.7%] men, mean age 69.1 ± 13.7 years) with AION were included. Restricted diffusion of the optic disc in AION eyes was more frequent in the early MRI group than in the late MRI group: 35/49 (71.4%) eyes versus 3/83 (3.6%) eyes, p < 0.001. ADC values of the pathological optic discs and optic nerves were lower in the early MRI group than in the late MRI group: 0.61 [0.52-0.94] × 10^-3 mm²/s versus 1.28 [1.01-1.44] × 10^-3 mm²/s, p < 0.001, and 0.74 [0.61-0.88] × 10^-3 mm²/s versus 0.89 [0.72-1.10] × 10^-3 mm²/s, p < 0.001, respectively.

CONCLUSIONS

DWI MRI showed good diagnostic performance to detect AION when performed early after the onset of visual symptoms.

KEY POINTS

• Restricted diffusion of the optic disc in eyes affected by AION was significantly more likely to be observed in patients who had undergone MRI within 5 days after onset of visual symptoms. • ADC values of the pathological optic discs and optic nerves were significantly lower in patients who had undergone MRI within 5 days after onset of visual symptoms of AION: 0.61 × 10^-3 mm²/s versus 1.28 × 10^-3 mm²/s, p < 0.001, and 0.74 × 10^-3 mm²/s versus 0.89 × 10^-3 mm²/s, p < 0.001, respectively. • The optimal threshold for timing from visual symptoms' onset to MRI completion to detect restricted diffusion of the optic disc and/or optic nerve was 5 days, with an AUC of 0.88 (CI_95%: 0.82-0.94).

Advancements in Positron Emission Tomography/Magnetic Resonance Imaging and Applications to Diagnostic Challenges in Neuroradiology

Since the clinical adoption of magnetic resonance (MR) in medical imaging, MR has proven to be a workhorse in diagnostic neuroradiology, with the ability to provide superb anatomic detail as well as additional functional and physiologic data, depending on the techniques utilized. Positron emission tomography/computed tomography has also shown irreplaceable diagnostic value in certain disease processes of the central nervous system by providing molecular and metabolic information through the development of numerous disease-specific PET tracers, many of which can be utilized as a diagnostic technique in and of themselves or can provide a valuable adjunct to information derived from MR. Despite these advances, many challenges still remain in neuroradiology, particularly in malignancy, neurodegenerative disease, epilepsy, and cerebrovascular disease. Through improvements in attenuation correction, motion correction, and PET detectors, combining the 2 modalities of PET and MR through simultaneous imaging has proven feasible and allows for improved spatial and temporal resolution without compromising either of the 2 individual modalities. The complementary information offered by both technologies has provided increased diagnostic accuracy in both research and many clinical applications in neuroradiology.

Reduced diffusion in white matter after radiotherapy with photons and protons

BACKGROUND AND PURPOSE

Radio(chemo)therapy is standard in the adjuvant treatment of glioblastoma. Inevitably, brain tissue surrounding the target volume is also irradiated, potentially causing acute and late side-effects. Diffusion imaging has been shown to be a sensitive method to detect early changes in the cerebral white matter (WM) after radiation. The aim of this work was to assess possible changes in the mean diffusivity (MD) of WM after radio(chemo)therapy using Diffusion-weighted imaging (DWI) and to compare these effects between patients treated with proton and photon irradiation.

MATERIALS AND METHODS

70 patients with glioblastoma underwent adjuvant radio(chemo)therapy with protons (n = 20) or photons (n = 50) at the University Hospital Dresden. MRI follow-ups were performed at three-monthly intervals and in this study were evaluated until 33 months after the end of therapy. Relative white matter MD changes between baseline and all follow-up visits were calculated in different dose regions.

RESULTS

We observed a significant decrease of MD (p < 0.05) in WM regions receiving more than 20 Gy. MD reduction was progressive with dose and time after radio(chemo)therapy (maximum: -7.9 ± 1.2% after 24 months, ≥50 Gy). In patients treated with photons, significant reductions of MD in the entire WM (p < 0.05) were seen at all time points. Conversely, in proton patients, whole brain MD did not change significantly.

CONCLUSIONS

Irradiation leads to measurable MD reduction in white matter, progressing with both increasing dose and time. Treatment with protons reduces this effect most likely due to a lower total dose in the surrounding white matter. Further investigations are needed to assess whether those MD changes correlate with known radiation induced side-effects.

Unusual images of ischemic stroke with hyperacute spontaneous recanalization: a case report

Acute ischemic stroke commonly presents as hypodensity on computed tomography (CT), and as high diffusion-weighted magnetic resonance imaging (DWI) signal and low apparent diffusion coefficient (ADC) signal on MRI. However, the radiographic characters of the infarct with hyperacute recanalization have received little attention. This paper reports a case of an ischemic stroke patient with middle cerebral artery (MCA) occlusion and hyperacute spontaneous recanalization. A series of radiological exams were applied for dynamic observation of the infarct regions. CT showed a high hyperdense sign at the second segment of MCA and various kinds of images of infarcted lenticula in several phases, namely hypodensity at the third hour from onset, increased density at the fourth hour, significant hyperdensity on the seventh day and equal density on the fourteenth day. MRI showed a slightly low DWI signal and a high ADC signal in both the infarcted lenticula and caudate at the twelfth hour. The underlying mechanisms for explaining the evolution of infarct images are discussed in this article. Both ischemic injury and reperfusion affect the process of cerebral edema resulting from ischemia, and subsequently contribute to the imaging of ischemic stroke on CT and MRI scans. Reperfusion promotes the development of cerebral edema and also accelerates the evolution of infarct images. Consequently, acute ischemic stroke could manifest as hyperdensity on CT, and slightly lower DWI signal and higher ADC signal on MRI in case of recanalization.

Reversible parenchymal ischemic injury on fetal brain MRI following fetoscopic laser coagulation-Implication on parental counseling

We present a case of reversible extensive ischemic injury seen on fetal-brain MRI in a fetus following laser coagulation performed for treatment of severe twin-twin transfusion syndrome twin-twin transfusion syndrome. A 32-year-old pregnant mother presented with twin-twin transfusion syndrome. Following fetoscopic laser coagulation, intrauterine fetal death of the donor fetus was diagnosed. On fetal-brain MRI, multiple areas of restricted diffusion were noted, consistent with acute infarctions. Nevertheless, follow-up MRI showed only subtle parenchymal injury, also confirmed on postnatal brain MRI. Our case illustrates that ischemic injury, as depicted on diffusion-weighted imaging, might be reversible, possibly with reperfusion before irreversible insult follows. Two to 3 weeks follow-up fetal MRI might provide additional information on the extent of irreversible injury in cases of restricted diffusion seen on initial fetal-brain MRI and might assist in parental counseling regarding long-term sequela.

Bilateral effects of unilateral cerebellar lesions as detected by voxel based morphometry and diffusion imaging

Over the last decades, the importance of cerebellar processing for cortical functions has been acknowledged and consensus was reached on the strict functional and structural cortico-cerebellar interrelations. From an anatomical point of view strictly contralateral interconnections link the cerebellum to the cerebral cortex mainly through the middle and superior cerebellar peduncle. Diffusion MRI (dMRI) based tractography has already been applied to address cortico-cerebellar-cortical loops in healthy subjects and to detect diffusivity alteration patterns in patients with neurodegenerative pathologies of the cerebellum. In the present study we used dMRI-based tractography to determine the degree and pattern of pathological changes of cerebellar white matter microstructure in patients with focal cerebellar lesions. Diffusion imaging and high-resolution volumes were obtained in patients with left cerebellar lesions and in normal controls. Middle cerebellar peduncles and superior cerebellar peduncles were reconstructed by multi fiber diffusion tractography. From each tract, measures of microscopic damage were assessed, and despite the presence of unilateral lesions, bilateral diffusivity differences in white matter tracts were found comparing patients with normal controls. Consistently, bilateral alterations were also evidenced in specific brain regions linked to the cerebellum and involved in higher-level functions. This could be in line with the evidence that in the presence of unilateral cerebellar lesions, different cognitive functions can be affected and they are not strictly linked to the side of the cerebellar lesion.

Brain Vascular Imaging Techniques

Recent major improvements in a number of imaging techniques now allow for the study of the brain in ways that could not be considered previously. Researchers today have well-developed tools to specifically examine the dynamic nature of the blood vessels in the brain during development and adulthood; as well as to observe the vascular responses in disease situations in vivo. This review offers a concise summary and brief historical reference of different imaging techniques and how these tools can be applied to study the brain vasculature and the blood-brain barrier integrity in both healthy and disease states. Moreover, it offers an overview on available transgenic animal models to study vascular biology and a description of useful online brain atlases.

Characteristics of Cerebral Hemodynamics in Patients with Ischemic Leukoaraiosis and New Ultrasound Indices of Ischemic Leukoaraiosis

OBJECTIVE

The diagnosis of ischemic leukoaraiosis (ILA) is based on head magnetic resonance imaging (MRI) and exclusion of other causes of white matter hyperintensities (WMHs). Recent studies have shown increased arterial stiffness and diminished carotid flow in ILA patients. So far, there are very little data on intracerebral hemodynamic parameters in ILA. Due to the specific structure of the intracranial arteries, our aim was to investigate intracerebral hemodynamic parameters in ILA patients and, possibly, to find a reliable ultrasound index of combined intra- and extracranial cerebral arteries.

METHODS

We compared different hemodynamic parameters in the middle cerebral artery (MCA) and local carotid stiffness parameters in 53 ILA patients to 40 gender and risk factor-matched controls with normal head MRI. The ILA diagnosis was based on head MRI and exclusion of other causes of WMH. In addition, we introduced new ischemic leukoariosis indices (ILAi) that are ratios of carotid stiffness parameters and MCA mean blood flow velocity. The diagnostic significance of ILAi for the prediction of ILA was analyzed.

RESULTS

We found significantly lower diastolic, systolic, and mean MCA blood flow velocities and increased carotid stiffness in the ILA group (P ≤ .05). All ILAi significantly differed between the groups (P < .05), were significantly associated with ILA (P < .01), and were sensitive and specific for predicting ILA (P < .05).

CONCLUSION

MCA blood flow velocities in ILA patients are lower compared to risk factor-matched controls. A combination of lower velocities and increased carotid stiffness represented as ILAi could have a potential diagnostic value for ILA.

Carotid Arterial Hemodynamic in Ischemic Levkoaraiosis Suggests Hypoperfusion Mechanism

BACKGROUND

Leukoaraiosis (ILA) is believed to be ischaemic in origin due to its similar location as that of lacunar infarctions and its association with cerebrovascular risk factors. However, its pathophysiology is not well understood. The ischaemic injuries may be a result of increased pulsatility or cerebral hypo-perfusion. We used carotid duplex ultrasound to prove that the underlying mechanism is hypo-perfusion.

METHODS

We compared 55 ILA patients to 44 risk factor-matched controls with normal magnetic resonance imaging (MRI) of the head. ILA diagnosis was based on MRI and was further categorised according to the Fazekas scale. We measured carotid artery blood flow velocity and diameter and calculated carotid blood flow and resistance indexes.

RESULTS

Blood flow velocities and blood flows were significantly lower in the ILA group, including diastolic, systolic and mean pressures (p ≤ 0.05). The resistance indices were higher in the ILA group, but the differences were not statistically significant. All the velocities and blood flows showed a decreasing trend with higher Fazekas score, whereas resistance indexes showed an increasing trend.

CONCLUSIONS

Lower blood flow and higher resistance of carotid arteries are consistent with the hypo-perfusion theory of ILA. Carotid ultrasound could have a diagnostic and prognostic role in ILA patients.

Multiphase arterial spin labeling assessment of cerebral perfusion changes associated with middle cerebral artery stenosis

RATIONALE AND OBJECTIVES

To assess the sensitivity and utility of multiphase pulsed arterial spin labeling (PASL) in detecting dynamic changes of cerebral perfusion in the presence of a middle cerebral artery (MCA) severe stenosis.

MATERIALS AND METHODS

Seventeen patients with severe stenoses in unilateral M1 segment of MCA were involved in this study. All patients underwent multiphase PASL imaging. Bilateral basal ganglia were drawn by hand as regions of interest (ROIs) on eight-phase images of each patient. The signal intensities of ROIs were measured and the time-intensity curves (TICs) were acquired through postprocessing on a magnetic resonance workstation. Whether there was a significant difference in the peak signal intensities of ROIs between the narrowed and normal sides was determined by the paired samples t test.

RESULTS

Three types of TICs were observed: eight cases with platform type, five cases with two-peak type, and four cases with single-peak type. There was a significant difference in the peak signal intensities of ROIs between the narrowed and normal sides.

CONCLUSIONS

Different types of TICs represent different cerebral hemodynamic changes. Multiphase PASL can sensitively detect the dynamic characteristics of cerebral perfusion and provide important dynamic perfusion information for clinical treatment of arterial stenosis.

Tips and traps in brain MRI: applications to vascular disorders

The French Society of Radiology's guide to good use of medical imaging examinations recommends MRI as the first-line examination for exploring cerebrovascular events or disorders. This paper will discuss the main traps in the images when stroke is suspected and provide the technical tips or knowledge necessary for an optimal radiological report.

Acute-onset migrainous aura mimicking acute stroke: MR perfusion imaging features

BACKGROUND AND PURPOSE

In a very limited number of cases, acute migrainous aura may mimic acute brain infarction. The aim of this study was to recognize patterns of MR perfusion abnormalities in this presentation.

MATERIALS AND METHODS

One thousand eight hundred fifty MR imaging studies performed for the suspicion of acute brain infarction were analyzed retrospectively to detect patients with acute migrainous aura not from stroke. All patients were examined clinically by 2 neurologists and underwent a standard stroke MR imaging protocol, including PWI. Two radiologists reviewed the perfusion maps visually and quantitatively for the presence, distribution, and grade of perfusion abnormalities.

RESULTS

Among 1850 MR imaging studies, 20 (1.08%) patients were found to have acute migrainous aura. Hypoperfusion was found in 14/20 patients (70%) with delayed rMTT and TTP, decreased rCBF, and minimal decrease in rCBV. In contrast to the typical pattern in stroke, perfusion abnormalities were not limited to a single vascular territory but extended to >1. Bilateral hypoperfusion was seen in 3/14 cases. In 11/14 cases, hypoperfusion with a posterior predominance was found. TTP and rMTT were the best maps to depict perfusion changes at visual assessment, but also rCBF maps demonstrated significant hypoperfusion in quantitative analysis. In all patients, clinical and imaging follow-up findings were negative for stroke.

CONCLUSIONS

Acute migrainous aura is rare but important in the differential diagnosis among patients with the suspicion of acute brain infarction. Atypical stroke perfusion abnormalities can be seen in these patients.

The role of imaging studies for evaluation of stroke in children

Stroke is a major cause of morbidity and mortality in children and long-term neurological deficits. Although cerebrovascular disorders occur less often in children than in adults, recognition of stroke in children has probably increased because of the widespread application of noninvasive diagnostic studies such as magnetic resonance imaging and computed tomography.Computed tomography (CT) should be the first imaging choice in the emergency setting when stroke is suspected. It will show the presence of hemorrhage (eg, bleeding from arteriovenous malformation). It is often normal within the first hours in arterial ischemic stroke. As in adults, magnetic resonance imaging is the neuroimaging modality to confirm the clinical diagnosis of ischemic stroke. In children, however, magnetic resonance imaging requires sedation and may not be as readily available as CT. Perfusion imaging demonstrates flow within the brain and can detect areas that are at risk of ischemia; however, further studies in the pediatric population need to be validated for this technique in children. Angiography detects arterial disease (eg, aneurysm); however, its use has been largely superseded by better magnetic resonance angiography, which is sensitive enough to visualize lesions in the proximal anterior cerebral artery, middle cerebral artery, and distal internal carotid artery (ICA). Magnetic resonance imaging using diffusion- weighted imaging is the most versatile and sensitive imaging technique for identifying ischemic lesions. In the future, we need to identify the pediatric patient presenting to the emergency department with an acute stroke and develop a pathway for the use of particular imaging techniques (eg, CT vs magnetic resonance imaging).

Arterial spin labeling in neuroimaging

Arterial spin labeling (ASL) is a magnetic resonance imaging technique for measuring tissue perfusion using a freely diffusible intrinsic tracer. As compared with other perfusion techniques, ASL offers several advantages and is now available for routine clinical practice in many institutions. Its noninvasive nature and ability to quantitatively measure tissue perfusion make ASL ideal for research and clinical studies. Recent technical advances have increased its sensitivity and also extended its potential applications. This review focuses on some basic knowledge of ASL perfusion, emerging techniques and clinical applications in neuroimaging.

Fetal MR imaging evidence of prolonged apparent diffusion coefficient decrease in fetal death

We report 2 fetal MR imaging cases at 22 wkGA with cerebral bright DWI and low ADC, 8 and 19 days after documented fetal death. These observations illustrate that decreased diffusion can be present weeks after injury onset, and its presence cannot be used to time injury onset within 1 week, which could significantly impact determination of the proximate cause of fetal brain injury in future cases.