Brain perfusion: computed tomography applications

Imaging blood-brain barrier dysfunction: A state-of-the-art review from a clinical perspective

The blood-brain barrier (BBB) consists of specialized cells that tightly regulate the in- and outflow of molecules from the blood to brain parenchyma, protecting the brain's microenvironment. If one of the BBB components starts to fail, its dysfunction can lead to a cascade of neuroinflammatory events leading to neuronal dysfunction and degeneration. Preliminary imaging findings suggest that BBB dysfunction could serve as an early diagnostic and prognostic biomarker for a number of neurological diseases. This review aims to provide clinicians with an overview of the emerging field of BBB imaging in humans by answering three key questions: (1. Disease) In which diseases could BBB imaging be useful? (2. Device) What are currently available imaging methods for evaluating BBB integrity? And (3. Distribution) what is the potential of BBB imaging in different environments, particularly in resource limited settings? We conclude that further advances are needed, such as the validation, standardization and implementation of readily available, low-cost and non-contrast BBB imaging techniques, for BBB imaging to be a useful clinical biomarker in both resource-limited and well-resourced settings.

Potential effects of low-dose average CT on cardiac implantable electronic devices

BACKGROUND

Average CT has been shown to be more accurate than conventional helical CT in quantitation of the PET data. The risk of CT irradiation of a cardiac implantable electronic device (CIED) causing an adverse event is low and is generally outweighed by the clinical benefit of a medically indicated examination. However, irradiation of CIED over one breath cycle in cine CT scan for average CT could impose risks on a patient who is pacing dependent. The purpose of this study was to demonstrate that low-dose average CT can be safe for CIED.

METHODS

A Medtronic CIED of model Protecta VR was submerged in a saline bath for a series of 4-s cine CT scans on a GE CT scanner programmed to deliver a 2-cm-wide radiation at a dose rate of 0.9 to 41.2 mGy/s to the CIED. The number of over-sensings was recorded as the interference of radiation to the CIED.

RESULTS

Dose rates ≥ 1.9 mGy/s caused over-sensing. The higher the dose rate, the more over-sensings there were. The lowest dose rate of 0.9 mGy/s did not cause any over-sensing.

CONCLUSIONS

Low-dose average CT at 0.9 mGy/s can be safe for a CIED patient who is pacing dependent.

Clinical evaluation of post-operative cerebral infarction in traumatic epidural haematoma

BACKGROUND

Patients with traumatic epidural haematoma, undergoing the prompt and correct treatment, usually have favourable outcomes. However, secondary cerebral infarction may be life-threatening condition, as it is difficult to be identified before neurological impairment occurs.

OBJECTIVE

To evaluate the clinical data of patients with traumatic EDH and assess potential risk factors for post-operative cerebral infarction.

METHODS

The clinical data of patients with traumatic EDH were collected and analysed retrospectively.

RESULTS

The univariate analysis revealed 10 potential risk factors (the haematoma location, volume, the largest thickness and mid-line shift, basal cisterns compression, traumatic subarachnoid haemorrhage, pupil dilatation, pre-operative Glasgow Coma Scale score, ∆GCS and intraoperative brain pressure) for cerebral infarction with statistically significant difference. Of these factors, haematoma volume and basal cistern compression turned out to be the most significant risk factors through final multivariate logistic regression analysis.

CONCLUSION

The findings of this study can provide predictive factors for development of cerebral infarction and information for clinical decision-making and future studies.

Brain perfusion CT compared with ¹⁵O-H₂O PET in patients with primary brain tumours

Purpose

Perfusion CT (PCT) measurements of regional cerebral blood flow (rCBF) have been proposed as a fast and easy method for identifying angiogenically active tumours. In this study, quantitative PCT rCBF measurements in patients with brain tumours were compared to the gold standard PET rCBF with ¹⁵O-labelled water (¹⁵O-H₂O).

Methods

On the same day within a few hours, rCBF was measured in ten adult patients with treatment-naïve primary brain tumours, twice using ¹⁵O-H₂O PET and once with PCT performed over the central part of the tumour. Matching rCBF values in tumour and contralateral healthy regions of interest were compared.

Results

PCT overestimated intratumoural blood flow in all patients with volume-weighted mean rCBF values of 28.2 ± 18.8 ml min⁻¹ 100 ml⁻¹ for PET and 78.9 ± 41.8 ml min⁻¹ 100 ml⁻¹ for PCT. There was a significant method by tumour grade interaction with a significant tumour grade rCBF difference for PCT of 32.9 ± 15.8 ml min⁻¹ 100 ml⁻¹ for low-grade (WHO I + II) and 81.5 ± 15.4 ml min⁻¹ 100 ml⁻¹ for high-grade (WHO III + IV) tumours, but not for PET. The rCBF PCT and PET correlation was only significant within tumours in two patients.

Conclusion

Although intratumoural blood flow measured by PCT may add valuable information on tumour grade, the method cannot substitute quantitative measurements of blood flow by PET and ¹⁵O-H₂O PET in brain tumours.

Comparison between acetazolamide challenge and 10% carbon dioxide challenge perfusion CT in rat C6 glioma

RATIONALE AND OBJECTIVES

The aim of this study was to investigate the effect of perfusion computed tomography (PCT) with acetazolamide (ACZ) challenge and compare it to 10% carbon dioxide (CO(2)) challenge in rat C6 glioma.

MATERIALS AND METHODS

PCT was performed on 32 rats, including 20 with orthotopically implanted C6 gliomas and 12 serving as controls. Ten rats with gliomas and six normal rats underwent PCT with ACZ challenge. The other 10 rats with gliomas and six normal rats underwent PCT with 10% CO(2) challenge. The raw data were processed using Philips computed tomographic brain perfusion software. Perfusion parameters before and after the challenge were recorded. Percentage changes due to ACZ administration and 10% CO(2) challenge were calculated. Pearson's correlation coefficients were used to investigate relationships between percentage changes in perfusion parameters and vascular endothelial growth factor and microvessel density.

RESULTS

In C6 gliomas, percentage change in cerebral blood flow was significantly different between ACZ (72.73%) and 10% CO(2) (28.47%) challenge (P < .01). Percentage change in cerebral blood volume was 37.85% with ACZ and 24.69% with 10% CO(2) challenge (P = .02). In controls, percentage change in cerebral blood flow was significantly different between ACZ (117.42%) and 10% CO(2) (65.86%) challenge (P < .01). For percentage change in cerebral blood volume, there was no significant difference between ACZ (107.51%) and 10% CO(2) (92.95%) challenge. Significant correlations were observed among percentage changes in vascular endothelial growth factor, microvessel density, and cerebral blood volume (P < .01). Percentage change in cerebral blood flow correlated well with vascular endothelial growth factor.

CONCLUSIONS

The results of this study indicate that PCT with ACZ challenge is a more reliable technique compared to 10% CO(2) challenge for the quantitative evaluation of microcirculation in gliomas.

Brain perfusion CT compared with15O-H2O-PET in healthy subjects

Background

Regional cerebral blood flow [rCBF] measurements are valuable for identifying angiogenically active tumours, and perfusion computed tomography [CT] has been suggested for that purpose. This study aimed to validate rCBF measurements by perfusion CT with positron-emission tomography [PET] and¹⁵O-labelled water [¹⁵O-H₂O] in healthy subjects.

Methods

RCBF was measured twice in 12 healthy subjects with¹⁵O-H₂O PET and once with perfusion CT performed over the basal ganglia. Matching rCBF values in regions of interest were compared.

Results

Measured with perfusion CT, rCBF was significantly and systematically overestimated. White matter rCBF values were 17.4 ± 2.0 (mean ± SD) mL min^-1 100 g^-1 for PET and 21.8 ± 3.4 mL min^-1 100 g^-1 for perfusion CT. Grey matter rCBF values were 48.7 ± 5.0 mL min^-1 100 g^-1 for PET and 71.8 ± 8.0 mL min^-1 100 g^-1 for perfusion CT. The overestimation of grey matter rCBF could be reduced from 47% to 20% after normalization to white matter rCBF, but the difference was still significant.

Conclusion

RCBF measured with perfusion CT does contain perfusion information, but neither quantitative nor relative values can substitute rCBF measured by¹⁵O-H₂O PET yet. This, however, does not necessarily preclude a useful role in patient management.

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).

Cerebral perfusion imaging in hemodynamic stroke: be aware of the pattern

SUMMARY

Reduction of the cerebral perfusion pressure caused by vessel occlusion or stenosis is a cause of neurological symptoms and border-zone infarctions. The aim of this article is to describe perfusion patterns in hemodynamic stroke, to give a practical approach for the assessment of colour encoded CT- and MR-perfusion maps and to demonstrate the clinical use of comprehensive imaging in the workup of patients with hemodynamic stroke. Five patients with different duration cause and degree of hemodynamic stroke were selected. The patients shared the typical presentation with fluctuating and transient symptoms. All were examined by MR or CT angiography and MR or CT perfusion in the symptomatic phase. All patients were examined with diffusion weighted imaging. All five cases showed the altered perfusion patterns of hemodynamic insufficiency with a slight or marked increase in CBV in the supply area of the affected vessel and only slightly reduced or maintained CBF. The perfusion disturbances were most easily detected on the MTT maps. Border-zone infarctions were seen in all cases. The typical pattern for hemodynamic insufficiency is characterized by increased CBV, normal or decreased CBF and prolonged MTT in the affected areas. The increased CBV is the hallmark of stressed autoregulation. Reading the color-encoded perfusion maps enables a quick and robust assessment of the cerebral perfusion and its characteristic patterns. Internal border-zone infarctions can be regarded as a marker for hemodynamic insufficiency. Finding of the typical rosary-like pattern of DWI lesions should call for further work up.

Radiation dose from multidetector row CT imaging for acute stroke

INTRODUCTION

The objective of this study was to determine the radiation dose delivered during comprehensive computed tomography (CT) imaging for acute stroke.

METHODS

All CT examinations performed over 18 months using our acute stroke protocol were included. Protocol includes an unenhanced CT head, CT angiography from the arch to vertex, CT perfusion/permeability, and an enhanced CT head. All imaging was acquired with a 64-MDCT. Examinations where any element of the protocol was repeated or omitted due to mistimed injection or patient motion were excluded. Dose-length products (DLP) for all components of each examination were obtained from dose reports generated at the time of acquisition, separating neck, and head calculations. Effective doses for each examination were calculated using the DLP and normalized values of effective dose per DLP appropriate for the body regions imaged.

RESULTS

Ninety-five examinations were included. Mean DLP was 6,790.0 mGy x cm. Effective doses ranged from 11.8 to 27.3 mSv, mean effective dose of 16.4 mSv. Mean effective dose for acquisition of the unenhanced head was 2.7 mSv. Largest contribution to effective dose was the CTA with a mean effective dose of 5.4 mSv. Mean effective dose for the CT perfusion was 4.9 mSv.

CONCLUSION

A comprehensive CT acute stroke protocol delivered a mean effective dose of 16.4 mSv, which is approximately six times the dose of an unenhanced CT head. These high-dose results must be balanced with the benefits of the detailed anatomic and physiologic data obtained. Centers should implement aggressive dose reduction strategies and freely use MR as a substitute.

Structural and functional neuroimaging in mild-to-moderate head injury

Head injury is a major cause of disability and death in adults. Significant developments in imaging techniques have contributed to the knowledge of the pathophysiology of head injury. Although extensive research is available on severe head injury, less is known about mild-to-moderate head injury despite the fact that most patients sustain this type of injury. In this review, we focus on structural and functional imaging techniques in patients with mild-to-moderate head injury. We discuss CT and MRI, including different MRI sequences, single photon emission computed tomography, perfusion-weighted MRI, perfusion CT, PET, magnetic resonance spectroscopy, functional MRI and magnetic encephalography. We outline the advantages and limitations of these various techniques in the contexts of the initial assessment and identification of brain abnormalities and the prediction of outcome.

High-concentration contrast media (HCCM) in CT angiography of the carotid system: impact on therapeutic decision making

BACKGROUND

CT angiography (CTA) is a minimally invasive technique that enables precise delineation of extracranial and intracranial vascular anatomy and pathology based on high intravascular density. With a 64-slice MDCT scanner, improved first-pass vascular visualization can be obtained with HCCM (400 mg I/ml) using 25 ml for combined intra-and extracranial studies and 20 ml for intracranial examinations alone. We reviewed 23 patients with extra-cranial stenoocclusive disease and 12 patients with intra-cranial aneurysms. Two additional patients presented with a mycotic aneurysm and a micro arteriovenous malformation (micro-AVM).

RESULTS

In 23 patients with 27 significant (> or =70%) vascular stenoses, high intraluminal contrast density and optimal projection of the stenosis profile facilitated precise delineation of the residual lumen in all vessels affected. Pseudoocclusion was present in 3 of the 27 vessels (11%) and ulceration in 5 (18.5%). CTA, in contrast to digital subtraction angiography (DSA) and magnetic resonance angiography (MRA), was able to delineate the mural constituents of stenoses, particularly marked calcification (present in 17 of 27 vessels, 62.9%). Eccentric vessel wall narrowing indicated dissection in three vessels (11%) and circular postradiation fibrosis was found in two vessels (7.4%). High-quality CTA obviated the need for DSA for diagnostic purposes in every patient. The decision regarding surgical treatment in nine vessels (33%) was influenced by the location of the stenosis relative to the carotid bifurcation, the length of the stenosis, and the level of the carotid bifurcation. Endovascular treatment in 12 vessels (44.4%) and the appropriate choice and placement of endovascular devices was affected by the anatomic configuration of the supraaortic vessels, and by the intrastenotic dimensions relative to the dimensions of the vessel proximal and distal to the stenosis. The anatomic availability of collaterals, relevant for both treatments, was determined via the anterior communicating artery (ACoA) in 91.3% of patients and the posterior communicating artery (PCoA) in 80.4% of patients. Intracranial CTA displayed the lesion location, configuration, size, and orientation in each of the 12 patients with intracranial aneurysms and in the two patients with a micro-AVM and a mycotic aneurysm. The correct diagnosis was established prior to DSA in each patient. Following CTA, the role of DSA was relegated to endovascular therapy in three aneurysms. For surgical candidates, DSA served to exclude potential multiplicity in ten saccular aneurysms and in one mycotic aneurysm. CTA provided information supplementary to DSA in 11 of the 13 aneurysms (84.6%). Delineation of blebs, calcification, thrombus, or incorporation of branches facilitated risk stratification with respect to rupture and to surgical or endovascular treatment.

CONCLUSION

Advantages of CTA, such as virtual independence from the hemodynamic situation, delineation of landmarks and vessel wall calcification, and the ability to quantify vessels and aneurysms, distinguish this technique from other noninvasive vascular imaging techniques and DSA. Acquired with the use of iodinated HCCM, CT angiograms permit excellent recognition of appropriate diagnostic and interventional treatment paths, thereby facilitating improved decision-making regarding endovascular or surgical treatment.

Imaging after brain injury

Head injury remains an important cause of death and disability in young adults. This review will discuss the role of structural imaging using computed tomography (CT) and magnetic resonance imaging (MRI) and physiological imaging using CT perfusion, 131Xe CT, MRI and spectroscopy (MRS), single photon emission computed tomography, and positron emission tomography (PET) in the assessment, management, and prediction of outcome after head injury. CT allows rapid assessment of brain pathology which ensures patients who require urgent surgical intervention receive appropriate care. Although MRI provides greater spatial resolution, particularly within the posterior fossa and deep white matter, a complete assessment of the burden of injury requires imaging of cerebral physiology. Physiological imaging techniques can only provide 'snap shots' of physiology within the injured brain, but they can be repeated, and such data can be used to assess the impact of therapeutic interventions. Perfusion imaging based on CT techniques (xenon CT and CT perfusion) can be implemented easily in most hospital centres, and provide quantitative perfusion data in addition to structural images. PET imaging provides unparalleled insights into cerebral physiology and pathophysiology, but is not widely available and is primarily a research tool. MR technology continues to develop and is becoming generally available. Using a complex variety of sequences, MR can provide data concerning both structural and physiological derangements. Future developments with such imaging techniques should improve understanding of the pathophysiology of brain injury and provide data that should improve management and prediction of functional outcome.

Imaging of cerebral blood flow and metabolism

PURPOSE OF REVIEW

To review the techniques for imaging cerebral blood flow and metabolism following injury to the brain.

RECENT FINDINGS

Xenon enhanced computerized tomography (Xenon CT), CT perfusion and single photon emission CT provide measurements of cerebral perfusion, while positron emission tomography (PET), and magnetic resonance imaging and spectroscopy (MRI and MRS) are able to assess both perfusion and cerebral metabolism. Xenon CT and CT perfusion are readily available and have proved useful in a variety of causes of brain injury. PET is an extremely useful research tool for defining cerebral physiology, but is limited in its availability. Despite the continuing development of MRI and MRS imaging, the scanning environment remains hostile for critically ill patients, and further research is required before the techniques become generally available.

SUMMARY

Imaging of cerebral blood flow and metabolism has been shown to be useful following a variety of causes of brain injury, as it can help to define the cause and extent of injury, identify appropriate treatments and predict outcome. Imaging based on CT techniques (Xenon CT and CT perfusion) can be implemented easily in most hospital centres, and are able to provide quantitative perfusion data in addition to structural images.

Godfrey Hounsfield and the dawn of computed tomography

OBJECTIVE

To provide a historical account of the events surrounding the development of the computed tomography scanner.

METHODS

Information was obtained by interviewing people who worked with Sir Godfrey Hounsfield and Dr. James Ambrose at Atkinson Morley's Hospital in the 1970s, and from published books, articles, and several web sites, including the Nobel web site.

RESULTS

The computed tomography scanner was successfully developed because of the collaboration between an imaginative engineer, Godfrey Hounsfield, who created the machine, and a brilliant neuroradiologist, James Ambrose, who demonstrated its wide clinical significance.

CONCLUSION

The computed tomography scanner represents one of the most important contributions to neurosurgical practice in the past 100 years, and its development is a remarkable story of scientific endeavor.

Cerebral perfusion computerized tomography: influence of reference vessels, regions of interest and interobserver variability

INTRODUCTION

There are still no standardized guidelines for perfusion computerized tomography (PCT) analysis.

METHODS

A total of 61 PCT studies were analyzed using either the anterior cerebral artery (ACA) or the middle cerebral artery (MCA) as the arterial reference, and the superior sagittal sinus (SSS) or the vein of Galen (VG) as the venous reference. The sizes of regions of interest (ROI) were investigated comparing PCT results obtained using a hemispheric ROI combined with vascular pixel elimination with those obtained using five smaller ROIs located over the cortex and basal ganglia. In addition, interobserver variations were explored using a standardized protocol.

RESULTS

MCA-based measurements of cerebral blood flow (CBF) and blood volume (CBV) were in accordance with those obtained with the ACA except in 16 patients with ischemic stroke, in whom CBF was overestimated by the ipsilateral MCA. Venous maximal intensity was significantly lower with the VG when compared with the SSS, resulting in overestimation of CBF and CBV. However, in 13.3% of patients the VG ROI yielded higher maximal intensities than the SSS ROI. There was no difference in PCT results between hemispheric ROI and averaged separate ROI when vascular pixel elimination was used. Finally, interobserver variations were as high as 11% for CBF and 12% for CBV.

CONCLUSION

The present results suggest that pathological rather than anatomical considerations should dictate the choice of the arterial ROI. For venous ROI, although SSS seems to be adequate in most instances, deep cerebral veins may occasionally generate higher maximal intensities and should therefore be selected. Importantly, significant user-dependency should be taken into account.