Cerebral perfusion CT: technique and clinical applications

Recent advances in endoluminal revascularization for intracranial atherosclerotic disease

More than 750,000 strokes occur annually in the United States. Of these, 8-10% are due to intracranial atherosclerosis. Less than 50% of patients with strokes from intracranial atherosclerosis will have a transient ischemic attack. For those patients with symptomatic intracranial atherosclerosis, the prognosis is poor; and the recent Warfarin-Aspirin Symptomatic Intracranial Stenosis (WASID) trial results have demonstrated the high risk of warfarin without clear benefit. Intracranial angioplasty and stenting is emerging as a viable and effective treatment alternative for patients with symptomatic intracranial stenosis. Advances in stent design, endovascular wires, and catheters and balloons are allowing endovascular surgeons to safely treat intracranial atherosclerosis. Wider clinical experience has led to refinement of patient selection and endoluminal techniques. Drug eluting-stents have the promise of decreasing the risk of restenosis. In this review, the most recent clinical, laboratory, and technical details for the treatment of intracranial angioplasty and stenting are discussed.

Comparison of aortic arch and intravenous contrast injection techniques for C-arm cone beam CT: implications for cerebral perfusion imaging in the angiography suite

RATIONALE AND OBJECTIVES

The ability to perform cerebral perfusion imaging (CPI) in the angiography suite has provided a new tool for diagnosis and treatment of neurovascular patients but requires comparable contrast perfusion to each cerebral hemisphere. In the angiography suite, contrast injection may be performed via an intra-arterial or intravenous (IV) route. The purpose of this study was to investigate whether a difference exists between contrast injection in the aortic arch (AA) and a peripheral vein (IV), particularly in the setting of stroke.

MATERIALS AND METHODS

Using three canines, both AA and IV injection protocols compatible with CPI were performed prospectively at three time points after creation of a stroke. The common carotid arteries in the resulting image data sets were segmented and the means and distributions of corresponding pixel intensities analyzed with Student's t-test. Using similar techniques, the internal carotid arteries of three patients (one female, two males, ages 69, 29, and 20) undergoing AA contrast injection with cone beam computed tomography (CBCT) cerebral imaging were analyzed and compared retrospectively with those of three random patients (one female, two males, ages 19, 57, and 35) undergoing standard head CT scans using IV contrast administration. All acquisitions followed institutionally approved protocols and informed consent.

RESULTS

No statistical significance (P < .05) was found when mean values for the right and left carotid artery pixel intensities were compared in the canine model or the clinical studies in which patients underwent imaging after AA or IV contrast administration.

CONCLUSIONS

No statistically significant difference exists between right and left carotid artery filling density using either AA or IV contrast injection methods, making both suitable for CPI in the angiography suite.

Cerebral CT Perfusion in Patients with Perimesencephalic and Those with Aneurysmal Subarachnoid Hemorrhage

Background

The cause of perimesencephalic hemorrhage is unknown, but a venous source is suggested. If perimesencephalic hemorrhage is of venous origin, less elevation of the intracranial pressure and less perfusion deficits are expected than after aneurysmal subarachnoid hemorrhage.

Aims

We compared perfusion in the acute stage after perimesencephalic hemorrhage and aneurysmal subarachnoid hemorrhage.

Methods

We included 45 perimesencephalic hemorrhage patients and 45 aneurysmal subarachnoid hemorrhage patients, who were matched on clinical condition at admission and underwent computerized tomographic scanning <72 h after subarachnoid hemorrhage. Cerebral blood flow was assessed in 12 predefined regions of interest. Differences in cerebral blood flow values with corresponding 95% confidence intervals were calculated. Sub-group analyses were performed stratified on comparable amounts of blood and location of blood (posterior circulation aneurysms and additionally in infratentorial and supratentorial aneurysms).

Results

Cerebral blood flow was higher in perimesencephalic hemorrhage patients (mean: 63·8) than in aneurysmal subarachnoid hemorrhage patients (mean: 55·9; difference of means: −7·9 [95% confidence interval: −10·7 to −5·2]) and also in the sub-group with comparable amounts of blood (mean cerebral blood flow: 56·4; difference of means: −7·4 [95% confidence interval: −10·4 to −4·3]). Cerebral blood flow was comparable with perimesencephalic hemorrhage patients for the sub-group with posterior circulation aneurysms (difference of means: −0·7 [95% confidence interval: −5·2 to 3·8]); however, differences diverged after stratifying posterior circulation aneurysms into supratentorial (difference of means −3·9 [95% confidence interval: −9·3 to 1·4]) and infratentorial aneurysms (difference of means 3·0 [95% confidence interval: −2·8 to 8·8]).

Conclusion

Perimesencephalic hemorrhage patients have a higher cerebral blood flow than aneurysmal subarachnoid hemorrhage patients. The findings of this study further support a venous origin of bleeding in perimesencephalic hemorrhage patients. Future studies should further elaborate on cerebral blood flow in posterior circulation aneurysms.

Towards robust deconvolution of low-dose perfusion CT: sparse perfusion deconvolution using online dictionary learning

Computed tomography perfusion (CTP) is an important functional imaging modality in the evaluation of cerebrovascular diseases, particularly in acute stroke and vasospasm. However, the post-processed parametric maps of blood flow tend to be noisy, especially in low-dose CTP, due to the noisy contrast enhancement profile and the oscillatory nature of the results generated by the current computational methods. In this paper, we propose a robust sparse perfusion deconvolution method (SPD) to estimate cerebral blood flow in CTP performed at low radiation dose. We first build a dictionary from high-dose perfusion maps using online dictionary learning and then perform deconvolution-based hemodynamic parameters estimation on the low-dose CTP data. Our method is validated on clinical data of patients with normal and pathological CBF maps. The results show that we achieve superior performance than existing methods, and potentially improve the differentiation between normal and ischemic tissue in the brain.

Advanced imaging modalities in the detection of cerebral vasospasm

The pathophysiology of cerebral vasospasm following aneurysmal subarachnoid hemorrhage (SAH) is complex and is not entirely understood. Mechanistic insights have been gained through advances in the capabilities of diagnostic imaging. Core techniques have focused on the assessment of vessel caliber, tissue metabolism, and/or regional perfusion parameters. Advances in imaging have provided clinicians with a multifaceted approach to assist in the detection of cerebral vasospasm and the diagnosis of delayed ischemic neurologic deficits (DIND). However, a single test or algorithm with broad efficacy remains elusive. This paper examines both anatomical and physiological imaging modalities applicable to post-SAH vasospasm and offers a historical background. We consider cerebral blood flow velocities measured by Transcranial Doppler Ultrasonography (TCD). Structural imaging techniques, including catheter-based Digital Subtraction Angiography (DSA), CT Angiography (CTA), and MR Angiography (MRA), are reviewed. We examine physiologic assessment by PET, HMPAO SPECT, ¹³³Xe Clearance, Xenon-Enhanced CT (Xe/CT), Perfusion CT (PCT), and Diffusion-Weighted/MR Perfusion Imaging. Comparative advantages and limitations are discussed.

Clinical feasibility of pulmonary perfusion analysis using dynamic computed tomography and a gamma residue function

PURPOSE

To create and determine the clinical feasibility of a model based on dynamic computed tomography (CT) and a bolus injection of iodine contrast medium for evaluation of pulmonary perfusion for healthy individuals and for patients with lung diseases.

MATERIALS AND METHODS

We analyzed pulmonary perfusion by means of dynamic 16-row multidetector CT scanning with a gamma residue function with adding a linear component (extended gamma function model) for 20 healthy individuals and in five patients.

RESULTS

Four types of the time-attenuation curve (TAC) were identified for the peripheral lung. Although the TACs of most pixels for the peripheral lung comprised a single peak or a single-peak followed by another increase, no peak was observed for a small proportion of pixels, which either increased linearly or resulted in a delayed peak for healthy subjects. The ratios of these linearly increasing or delayed peak types of lung fields increased for pathological lungs. The analytical results for pathological cases showed that changes in lung perfusion, difficult to identify using only CT imaging, could be detected.

CONCLUSIONS

The extended gamma function model adequately evaluated pulmonary perfusion not only for normal regions, but also for structurally abnormal regions. Regional changes in perfusion could be evaluated by use of our model, and we confirmed its clinical feasibility for pulmonary perfusion analysis.

Imaging in the diagnosis and prognosis of traumatic brain injury

INTRODUCTION

Traumatic brain injury (TBI) is a major cause of death and disability worldwide. Improved understanding of the impact of head injury and the extent and development of neuronal loss and cognitive dysfunction could lead to improved therapy and outcome for patients.

AREAS COVERED

This paper reviews the currently available imaging techniques and defines their role in the diagnosis, management and prediction of outcome following traumatic brain injury. These imaging techniques provide delineation of the structural, physiological and functional derangements that result following acute injury, and map their development and association with late functional deficits. Imaging tools also have a role in defining the pathophysiological mechanisms responsible for further neuronal loss following the primary injury. Finally, this paper provides an overview of the role of functional imaging in classifying unresponsive coma and defining functional reorganisation of the brain following injury.

EXPERT OPINION

Brain imaging is of key importance in TBI management, enabling efficient and accurate diagnoses to be made, informing management decisions and contributing to prognostication. Developments in imaging techniques promise to improve understanding of the structural and functional derangements, improve management and guide the development and implementation of novel neuroprotective strategies following head injury.

Iterative image reconstruction for cerebral perfusion CT using a pre-contrast scan induced edge-preserving prior

Cerebral perfusion x-ray computed tomography (PCT) imaging, which detects and characterizes the ischemic penumbra, and assesses blood-brain barrier permeability with acute stroke or chronic cerebrovascular diseases, has been developed extensively over the past decades. However, due to its sequential scan protocol, the associated radiation dose has raised significant concerns to patients. Therefore, in this study we developed an iterative image reconstruction algorithm based on the maximum a posterior (MAP) principle to yield a clinically acceptable cerebral PCT image with lower milliampere-seconds (mA s). To preserve the edges of the reconstructed image, an edge-preserving prior was designed using a normal-dose pre-contrast unenhanced scan. For simplicity, the present algorithm was termed as 'MAP-ndiNLM'. Evaluations with the digital phantom and the simulated low-dose clinical brain PCT datasets clearly demonstrate that the MAP-ndiNLM method can achieve more significant gains than the existing FBP and MAP-Huber algorithms with better image noise reduction, low-contrast object detection and resolution preservation. More importantly, the MAP-ndiNLM method can yield more accurate kinetic enhanced details and diagnostic hemodynamic parameter maps than the MAP-Huber method.

Multicentre imaging measurements for oncology and in the brain

Multicentre imaging studies of brain tumours (and other tumour and brain studies) can enable a large group of patients to be studied, yet they present challenging technical problems. Differences between centres can be characterised, understood and minimised by use of phantoms (test objects) and normal control subjects. Normal white matter forms an excellent standard for some MRI parameters (e.g. diffusion or magnetisation transfer) because the normal biological range is low (<2-3%) and the measurements will reflect this, provided the acquisition sequence is controlled. MR phantoms have benefits and they are necessary for some parameters (e.g. tumour volume). Techniques for temperature monitoring and control are given. In a multicentre study or treatment trial, between-centre variation should be minimised. In a cross-sectional study, all groups should be represented at each centre and the effect of centre added as a covariate in the statistical analysis. In a serial study of disease progression or treatment effect, individual patients should receive all of their scans at the same centre; the power is then limited by the within-subject reproducibility. Sources of variation that are generic to any imaging method and analysis parameters include MR sequence mismatch, B(1) errors, CT effective tube potential, region of interest generation and segmentation procedure. Specific tissue parameters are analysed in detail to identify the major sources of variation and the most appropriate phantoms or normal studies. These include dynamic contrast-enhanced and dynamic susceptibility contrast gadolinium imaging, T(1), diffusion, magnetisation transfer, spectroscopy, tumour volume, arterial spin labelling and CT perfusion.

Validation of frontal near-infrared spectroscopy as noninvasive bedside monitoring for regional cerebral blood flow in brain-injured patients

OBJECT

Near-infrared spectroscopy (NIRS) offers noninvasive bedside measurement of direct regional cerebral arteriovenous (mixed) brain oxygenation. To validate the accuracy of this monitoring technique, the authors analyzed the statistical correlation of NIRS and CT perfusion with respect to regional cerebral blood flow (CBF) measurements.

METHODS

The authors retrospectively reviewed all cases in which NIRS measurements were obtained at a single, academic neurointensive care unit from February 2008 to June 2011 in which CT perfusion was performed at the same time as NIRS data was collected. Regions of interest were obtained 2.5 cm below the NIRS bifrontal scalp probe on CT perfusion with an average volume between 2 and 4 ml, with mean CBF values used for purposes of analysis. Linear regression analysis was performed for NIRS and CBF values.

RESULTS

The study included 8 patients (2 men, 6 women), 6 of whom suffered subarachnoid hemorrhage, 1 ischemic stroke, and 1 intracerebral hemorrhage and brain edema. Mean CBF measured by CT perfusion was 61 ml/100 g/min for the left side and 60 ml/100 g/min for the right side, while mean NIRS values were 75 on the right and 74 on the left. Linear regression analysis demonstrated a statistically significant probability value (p<0.0001) comparing NIRS frontal oximetry and CT perfusion-obtained CBF values.

CONCLUSIONS

The authors demonstrated a linear correlation for frontal NIRS cerebral oxygenation measurements compared with regional CBF on CT perfusion imaging. Thus, frontal NIRS cerebral oxygenation measurement may serve as a useful, noninvasive, bedside intensive care unit monitoring tool to assess brain oxygenation in a direct manner.

A pictorial essay of brain perfusion-CT: not every abnormality is a stroke!

Perfusion-CT (PCT) of the brain is a rapidly evolving imaging technique used to assess blood supply to the brain parenchyma. PCT is readily available at most imaging centers, resulting in steadily increasing use of this imaging technique. Though PCT was initially introduced and still most widely used to evaluate patients with acute ischemic stroke, a wide variety of other pathologic processes demonstrate abnormal perfusion maps. Therefore, it is important for the radiologist to recognize altered perfusion patterns observed in diseases other than typical ischemic stroke. The goal of this article is to show the perfusion maps and review the perfusion patterns observed in some subtypes of atypical stroke and in neurological entities other than stroke, so that they are recognized and not confused with the PCT patterns observed in patients with typical ischemic stroke.

Perfusion computed tomography assessments of peri-enhancing brain tissue in high-grade gliomas

PURPOSE

This study was undertaken to identify tumoural infiltration of peri-enhancing brain tissue in patients with glioblastoma by means of perfusion computed tomography (PCT) parameters, cerebral blood volume (CBV) and permeability surface (PS).

MATERIALS AND METHODS

Eight patients with surgically treated glioblastoma who were eligible for radiotherapy and nine patients with brain metastases from lung and breast cancer underwent CT before and after injection of contrast medium. CBV and PS were calculated in the contrast-enhancing lesion area, in the area of perilesional oedema and in the normal-appearing white matter (NAWM), normalised to contralateral symmetrical areas.

RESULTS

No significant differences were found for normalised CBV (nCBV) and nPS in NAWM regions between metastasis and glioma. Significant differences in nPS (p<0.005) were found between the typically vasogenic oedema surrounding the metastases and signal alteration surrounding the glial neoplasm. On the contrary, no significant differences were detected in the same areas for nCBV.

CONCLUSIONS

PCT can analyse the histopathological substrate underlying the hypodense peritumoural halo and differentiate between vasogenic oedema and neoplastic infiltration on the basis of the PS parameter. In our study, PS was more informative than CBV. These findings can be used to integrate plans for radiation therapy and/or surgery.

Multiwavelength time-resolved detection of fluorescence during the inflow of indocyanine green into the adult's brain

Optical technique based on diffuse reflectance measurement combined with indocyanine green (ICG) bolus tracking is extensively tested as a method for clinical assessment of brain perfusion in adults at the bedside. Methodology of multiwavelength and time-resolved detection of fluorescence light excited in the ICG is presented and advantages of measurements at multiple wavelengths are discussed. Measurements were carried out: 1. on a physical homogeneous phantom to study the concentration dependence of the fluorescence signal, 2. on the phantom to simulate the dynamic inflow of ICG at different depths, and 3. in vivo on surface of the human head. Pattern of inflow and washout of ICG in the head of healthy volunteers after intravenous injection of the dye was observed for the first time with time-resolved instrumentation at multiple emission wavelengths. The multiwavelength detection of fluorescence signal confirms that at longer emission wavelengths, probability of reabsorption of the fluorescence light by the dye itself is reduced. Considering different light penetration depths at different wavelengths, and the pronounced reabsorption at longer wavelengths, the time-resolved multiwavelength technique may be useful in signal decomposition, leading to evaluation of extra- and intracerebral components of the measured signals.

Regional cerebral perfusion and ischemic status after standard superficial temporal artery-middle cerebral artery (STA-MCA) bypass surgery in ischemic cerebrovascular disease

BACKGROUND AND PURPOSE

Standard superficial temporal artery-middle cerebral artery (STA-MCA) bypass surgery is an effective treatment for ischemic cerebrovascular diseases, including moyamoya disease and occlusive cerebrovascular disease. Our purpose in this study was to evaluate the ischemic status based on the imaging modality of computed tomographic perfusion (CTP) before and after STA-MCA bypass in patients with moyamoya disease and occlusive cerebrovascular disease.

METHODS

CTP was performed on 19 patients, comprising 10 patients with moyamoya disease and nine patients with occlusive cerebrovascular disease, preoperatively and on the third postoperative day. According to the regional cerebral microcirculatory change and modality of CTP, the regional cerebral ischemic status was graded into two stages with four substages (I1, I2, II1, and II2) to indicate the regional cerebral perfusion and ischemic status of the patients. The ischemic status was analyzed in all 19 patients according to those stages preoperatively and postoperatively.

RESULTS

Among the 19 patients, nine (47.4%) showed improvement in the regional cerebral ischemic status, six (31.6%) showed no change, and four (21.1%) showed deterioration. Both improvement and no change in the regional cerebral ischemic status were regarded as effective, while deterioration was regarded as ineffective, meaning that 15 (78.9%) patients were effective and four (21.1%) patients were ineffective. The effective rate for moyamoya disease (one of ten, 90.0%) was significantly higher than that for occlusive cerebrovascular disease (three of nine, 66.7%). Postoperatively, only one patient (case N11) exhibited deterioration of symptoms, which presented as right hemiplegia and aphasia, and no obvious changes in symptoms were observed for the other patients in the transient period.

CONCLUSIONS

This study has shown that STA-MCA bypass is an effective and safe way to improve the regional cerebral perfusion and ischemic status in ischemic cerebrovascular diseases. The stage based on the regional cerebral microcirculatory change and presentation of CTP can directly reflect the pathological mechanism underlying the regional cerebral ischemic status.

Selection of arterial input function for postprocessing of cerebral ct perfusion in chronic unilateral high-grade stenosis or occlusion of the carotid or middle cerebral artery

RATIONALE AND OBJECTIVES

We evaluated the effect of the arterial input function (AIF) on computed tomography perfusion (CTP) in patients with unilateral high-grade stenosis or occlusion in the carotid artery or middle cerebral artery without acute stroke.

MATERIALS AND METHODS

CTP datasets were retrospectively postprocessed using the same venous output function and different AIF selections: the second segment of the anterior cerebral artery (A2 AIF), the second segment of the middle cerebral artery (MCA) on the lesion side (affected M2 AIF), and M2 on the contralateral side (nonaffected M2 AIF). We measured CTP values in the region of interest (ROI) in the bilateral MCA territory and evaluated the lesion-to-contralateral ratios.

RESULTS

The mean and standard deviations of cerebral blood flow (CBF) on the normal side were similar to previously reported data only when using "non-affected M2 AIF." Selecting an "affected M2 AIF" overestimated the CBF and shortened the mean transit time (MTT) in normal and lesion areas. Selecting an "A2 AIF" may cause overestimation of CBF in the normal side in patients with nonaffected-side A1 hypoplasia or occlusion. The sensitivity of the CBF ratio or MTT ratio to detect these unilateral cerebrovascular diseases was 100% using "nonaffected M2 AIF for bilateral MCA ROIs" and 70% (CBF ratio) and 90% (MTT ratio) using "respective AIF."

CONCLUSION

The use of "nonaffected AIF for the bilateral MCA ROIs" was found to be the best of these AIF-ROI combinations in patients with chronic unilateral carotid or M1 severe stenosis or occlusion.

Porcine ex vivo liver phantom for dynamic contrast-enhanced computed tomography: development and initial results

OBJECTIVES

: To demonstrate the feasibility of developing a fixed, dual-input, biologic liver phantom for dynamic contrast-enhanced computed tomography (CT) imaging and to report initial results of use of the phantom for quantitative CT perfusion imaging.

MATERIALS AND METHODS

: Porcine livers were obtained from completed surgical studies and perfused with saline and fixative. The phantom was placed in a body-shaped, CT-compatible acrylic container and connected to a perfusion circuit fitted with a contrast injection port. Flow-controlled contrast-enhanced imaging experiments were performed using 128-slice and 64-slice dual-source multidetector CT scanners. CT angiography protocols were used to obtain portal venous and hepatic arterial vascular enhancement, reproduced over a period of 4 to 6 months. CT perfusion protocols were used at different input flow rates to correlate input flow with calculated tissue perfusion, to test reproducibility, and to determine the feasibility of simultaneous dual-input liver perfusion. Histologic analysis of the liver phantom was also performed.

RESULTS

: CT angiogram 3-dimensional reconstructions demonstrated homogenous tertiary and quaternary branching of the portal venous system to the periphery of all lobes of the liver as well as enhancement of the hepatic arterial system to all lobes of the liver and gallbladder throughout the study period. For perfusion CT, the correlation between the calculated mean tissue perfusion in a volume of interest and input pump flow rate was excellent (R = 0.996) and color blood flow maps demonstrated variations in regional perfusion in a narrow range. Repeat perfusion CT experiments demonstrated reproducible time-attenuation curves, and dual-input perfusion CT experiments demonstrated that simultaneous dual input liver perfusion is feasible. Histologic analysis demonstrated that the hepatic microvasculature and architecture appeared intact and well preserved at the completion of 4 to 6 months of laboratory experiments and contrast-enhanced imaging.

CONCLUSIONS

: We have demonstrated successful development of a porcine liver phantom using a flow-controlled extracorporeal perfusion circuit. This phantom exhibited reproducible dynamic contrast-enhanced CT of the hepatic arterial and portal venous system over a 4- to 6-month period.

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 relative effect of vendor variability in CT perfusion results: a method comparison study

OBJECTIVE

There are known interoperator, intraoperator, and intervendor software differences that can influence the reproducibility of quantitative CT perfusion values. The purpose of this study was to determine the relative impact of operator and software differences in CT perfusion variability.

MATERIALS AND METHODS

CT perfusion imaging data were selected for 11 patients evaluated for suspected ischemic stroke. Three radiologists each independently postprocessed the source data twice, using four different vendor software applications. Results for cerebral blood volume (CBV), cerebral blood flow (CBF), and mean transit time (MTT) were recorded for the lentiform nuclei in both hemispheres. Repeated variables multivariate analysis of variance was used to assess differences in the means of CBV, CBF, and MTT. Bland-Altman analysis was used to assess agreement between pairs of vendors, readers, and read times.

RESULTS

Choice of vendor software, but not interoperator or intraoperator disagreement, was associated with significant variability (p < 0.001) in CBV, CBF, and MTT. The mean difference in CT perfusion values was greater for pairs of vendors than for pairs of operators.

CONCLUSION

Different vendor software applications do not generate quantitative perfusion results equivalently. Intervendor difference is, by far, the largest cause of variability in perfusion results relative to interoperator and intraoperator difference. Caution should be exercised when interpreting quantitative CT perfusion results because these values may vary considerably depending on the postprocessing software.

Intracranial Masses with Perilesional Edema: Differential Diagnosis with Perfusion CT

Perilesional edema (PE) is commonly observed in association with an intracranial mass. PE is thought to be determined by vasogenic effects in the cerebral parenchyma surrounding the mass due to the loss or absence of the blood-brain barrier (BBB) inside the lesion. Alterations in capillary permeability induce extrusion of fluids into the extravascular space around the mass. On Computed Tomography (CT) PE corresponds to an area of low density for the increased water content, outside the margins of the lesion. It is difficult to differentiate PE from areas of parenchymal compressive ischemia and sometimes the two events could be associated. A solitary mass with PE is more commonly discovered on a non-enhanced computed tomography (NECT) study performed for the onset of stable or rapidly progressive neurological symptoms. In such cases, a supplementary CT scan with contrast (CECT) is generally indicated to complete the baseline imaging before MRI. Contrast enhancement is generally present in a mass with PE and it is not specific for differential diagnosis. Perfusion computed tomography (PCT) requires a few minutes in addition to the time needed for CECT. PCT may give information on regional microvascular density, permeability and blood flow, thus it may play a role when tumoral neo-angiogenesis or non-neoplastic altered haemodynamics are suspected. We therefore investigated the utility of PCT in the differential diagnosis of the intracranial solitary masses with PE.

TIPS bilateral noise reduction in 4D CT perfusion scans produces high-quality cerebral blood flow maps

Cerebral computed tomography perfusion (CTP) scans are acquired to detect areas of abnormal perfusion in patients with cerebrovascular diseases. These 4D CTP scans consist of multiple sequential 3D CT scans over time. Therefore, to reduce radiation exposure to the patient, the amount of x-ray radiation that can be used per sequential scan is limited, which results in a high level of noise. To detect areas of abnormal perfusion, perfusion parameters are derived from the CTP data, such as the cerebral blood flow (CBF). Algorithms to determine perfusion parameters, especially singular value decomposition, are very sensitive to noise. Therefore, noise reduction is an important preprocessing step for CTP analysis. In this paper, we propose a time-intensity profile similarity (TIPS) bilateral filter to reduce noise in 4D CTP scans, while preserving the time-intensity profiles (fourth dimension) that are essential for determining the perfusion parameters. The proposed TIPS bilateral filter is compared to standard Gaussian filtering, and 4D and 3D (applied separately to each sequential scan) bilateral filtering on both phantom and patient data. Results on the phantom data show that the TIPS bilateral filter is best able to approach the ground truth (noise-free phantom), compared to the other filtering methods (lowest root mean square error). An observer study is performed using CBF maps derived from fifteen CTP scans of acute stroke patients filtered with standard Gaussian, 3D, 4D and TIPS bilateral filtering. These CBF maps were blindly presented to two observers that indicated which map they preferred for (1) gray/white matter differentiation, (2) detectability of infarcted area and (3) overall image quality. Based on these results, the TIPS bilateral filter ranked best and its CBF maps were scored to have the best overall image quality in 100% of the cases by both observers. Furthermore, quantitative CBF and cerebral blood volume values in both the phantom and the patient data showed that the TIPS bilateral filter resulted in realistic mean values with a smaller standard deviation than the other evaluated filters and higher contrast-to-noise ratios. Therefore, applying the proposed TIPS bilateral filtering method to 4D CTP data produces higher quality CBF maps than applying the standard Gaussian, 3D bilateral or 4D bilateral filter. Furthermore, the TIPS bilateral filter is computationally faster than both the 3D and 4D bilateral filters.

Role of CT perfusion imaging in the diagnosis and treatment of vasospasm

The current role of CT perfusion (CTP) imaging in the diagnosis and treatment of vasospasm in the setting of aneurysmal subarachnoid hemorrhage is discussed in this article, with specific attention directed towards defining the terminology of vasospasm and delayed cerebral ischemia. A commonly used CTP technique in clinical practice is described. A review of the literature regarding the usefulness of CTP for the diagnosis of vasospasm and its role in guiding treatment are discussed. Recent research advances in the utilization of CTP and associated ongoing challenges are also presented.

Early changes measured by CT perfusion imaging in tumor microcirculation following radiosurgery in rat C6 brain gliomas

Object

In this paper, the authors' aim was to use CT perfusion imaging to evaluate the early changes in tumor microcirculation following radiosurgery in rat C6 brain gliomas.

Methods

C6 glioma cells were inoculated into the right caudate nucleus of 25 Wistar rats using a stereotactic procedure. Tumor-bearing rats were randomly divided into 2 groups (tumor group and treatment group). Rats in the treatment group received maximal doses of 20 Gy delivered by the X-knife unit 16 days postimplantation. Computed tomography perfusion imaging was performed in all rats 3 weeks after tumor implantation prior to death and histopathological analysis.

Results

Hypocellular regions and tumor edema were increased in the treatment group compared with the tumor group. Parameters of CT perfusion imaging including cerebral blood volume (CBV) and mean transit time (MTT) of the tumors as well as the permeability surface area (PSA) product in the tumor-brain districts were decreased in the treatment group compared with the tumor group (p < 0.05). Although microvascular density (MVD) in the periphery of the tumors in the treatment group was higher than that in the normal contralateral brain (p < 0.05), MVD of the tumors in the treatment group was less than that in the tumor group (p < 0.01). There was a positive correlation between cerebral blood flow (CBF) and MVD as well as CBV and MVD in the center and periphery of tumors in both groups (p < 0.05).

Conclusions

A decrease in the perfusion volume of rat C6 brain gliomas was observed during the acute stage following X-knife treatment, and CBF and CBV were positively correlated with MVD of rat C6 brain gliomas. Thus, CT perfusion imaging can be used to evaluate the early changes in tumor microcirculation following radiosurgery.

Cognitive outcome after EC-IC bypass surgery in hemodynamic cerebral ischemia

OBJECTIVE

The purpose of this study was to evaluate cognitive functions in patients undergoing extracranial-intracranial (EC-IC) bypass surgery for cerebral ischemia.

POPULATION AND METHODS

From August 2003 to January 2009, 276 patients with occluded internal carotid arteries (ICA) were screened. Forty of these met the criteria for a low-flow EC-IC bypass. These patients were identified based on evidence of exhausted vasomotor reactivity (VMR) using the Doppler CO(2) test and CT perfusion. These patients were invited to have a complete battery of neuropsychological tests preoperatively and 12 months after surgery. Complete neurocognitive testing was finished in 20 patients.

RESULTS

This group of 20 patients showed preoperative cognitive impairment ranging from mild to medium-severe. There were no cases of stroke ipsilateral to the operated side during the follow-up period. VMR improvement was seen in all patients within 6 months of surgery. A comparison using a paired t-test demonstrated significant improvement 12 months after surgery in the following neuropsychological tests: WAIS-R (p = 0.01), Number Collection Test (p = 0.02), Trail Making Test (p = 0.03), and Benton Visual Retention Test (p = 0.05). Repeat analysis of variance (ANOVA) suggested the following predictors associated with cognitive improvement:the presence of ophthalmic collateral flow (p = 0.04), preoperative amaurosis fugax (p = 0.02), and external watershed infarction detected by MRI (p = 0.04).

CONCLUSION

Patients with occlusion of the ICA and exhausted VMR have cognitive impairment prior to EC-IC bypass surgery. Twelve months after surgery, there is significant improvement in various areas of cognition.

Sources of variability in computed tomography perfusion: implications for acute stroke management

Object

Although dynamic, first-pass cerebral CT perfusion is used in the evaluation of acute ischemic stroke, a lack of standardization restricts the value of this imaging modality in clinical decision-making. The purpose of this study was to comprehensively review the reported sources of variability and error in cerebral CT perfusion results.

Methods

A systematic literature review was conducted, 120 articles were reviewed, and 23 published original research articles were included. Sources of variability and error were thematically categorized and presented within the context of the 3 stages of a typical CT perfusion study: data acquisition, postprocessing, and results interpretation.

Results

Seven factors that caused variability were identified and described in detail: 1) contrast media, the iodinated compound injected intravascularly to permit imaging of the cerebral vessels; 2) data acquisition rate, the number of images obtained by CT scan per unit time; 3) user inputs, the subjective selections that operators make; 4) observer variation, the failure of operators to repeatedly measure a perfusion parameter with precision; 5) software operational mode, manual, semiautomatic, or automatic; 6) software design, the mathematical algorithms used to perform postprocessing; and 7) value type, absolute versus relative values.

Conclusions

Standardization at all 3 stages of the CT perfusion study cycle is warranted. At present, caution should be exercised when interpreting CT perfusion results as these values may vary considerably depending on a variety of factors. Future research is needed to define the role of CT perfusion in clinical decision-making for acute stroke patients and to determine the clinically acceptable limits of variability in CT perfusion results.

Advanced imaging assessment of posterior circulation stroke before and after endovascular intervention

BACKGROUND

Perfusion-computed tomography (CTP) is a relatively new technique that allows rapid semiquantitative noninvasive evaluation of cerebral perfusion aiding in the diagnosis of cerebral ischemia and infarction. There is a paucity of data on its application within the posterior circulation, especially, the brainstem. We describe a case of basilar artery thrombosis with cerebellar and brainstem CTP mismatch and discuss possible future applications of CTP for acute posterior fossa circulation and infarction.

METHODS

Case report.

RESULTS

Successful use of CTP to aid in decision to proceed with neurointervention in acute basilar artery occlusion and confirm its resolution after mechanical clot retrieval.

CONCLUSION

Perfusion-computed tomography can successfully be used to define cerebral ischemia and infarction within the posterior fossa and aid in decisions to proceed with neurointervention.

Role of quantitative chest perfusion computed tomography in detecting diabetic pulmonary microangiopathy

AIMS

Aim of the study was to determine the role of perfusion chest computed tomography (pCT) in evaluation of pulmonary diabetic angiopathy.

METHODS

18 never-smoking patients (10 diabetic patients and 8 healthy controls) underwent chest high resolution CT (HRCT) and then pCT scanning. In both groups, blood tests, biochemical analysis, fibrinogen, HbA(1c), spirometry, diffusion capacity for carbon monoxide (DLCO) and body pletysmography were performed.Following parameters of pulmonary perfusion have been analysed: blood volume (BV), blood flow (BF), mean transit time (MTT), time to peak (TTP) and permeability surface (PS).

RESULTS

there were no statistically significant differences between groups in terms of age, sex, BMI, forced expiratory volume in one second (FEV(1)), DLCO. Chest HRCT revealed no pathologies. Significantly higher values of chest pCT for BF (p=0.05), BV (p=0.05) and PS (p=0.01) have been found in diabetics in comparison to controls. No differences were found in MTT.

CONCLUSIONS

significant increase of perfusion parameters in diabetes seems to confirm pulmonary microangiopathy. The results indicate that further studies on application of pCT in diabetic patients may be beneficial for better understanding of lung microangiopathy, its diagnosing and monitoring.

Multi-parametric neuroimaging reproducibility: a 3-T resource study

Modern MRI image processing methods have yielded quantitative, morphometric, functional, and structural assessments of the human brain. These analyses typically exploit carefully optimized protocols for specific imaging targets. Algorithm investigators have several excellent public data resources to use to test, develop, and optimize their methods. Recently, there has been an increasing focus on combining MRI protocols in multi-parametric studies. Notably, these have included innovative approaches for fusing connectivity inferences with functional and/or anatomical characterizations. Yet, validation of the reproducibility of these interesting and novel methods has been severely hampered by the limited availability of appropriate multi-parametric data. We present an imaging protocol optimized to include state-of-the-art assessment of brain function, structure, micro-architecture, and quantitative parameters within a clinically feasible 60-min protocol on a 3-T MRI scanner. We present scan-rescan reproducibility of these imaging contrasts based on 21 healthy volunteers (11 M/10 F, 22-61 years old). The cortical gray matter, cortical white matter, ventricular cerebrospinal fluid, thalamus, putamen, caudate, cerebellar gray matter, cerebellar white matter, and brainstem were identified with mean volume-wise reproducibility of 3.5%. We tabulate the mean intensity, variability, and reproducibility of each contrast in a region of interest approach, which is essential for prospective study planning and retrospective power analysis considerations. Anatomy was highly consistent on structural acquisition (~1-5% variability), while variation on diffusion and several other quantitative scans was higher (~<10%). Some sequences are particularly variable in specific structures (ASL exhibited variation of 28% in the cerebral white matter) or in thin structures (quantitative T2 varied by up to 73% in the caudate) due, in large part, to variability in automated ROI placement. The richness of the joint distribution of intensities across imaging methods can be best assessed within the context of a particular analysis approach as opposed to a summary table. As such, all imaging data and analysis routines have been made publicly and freely available. This effort provides the neuroimaging community with a resource for optimization of algorithms that exploit the diversity of modern MRI modalities. Additionally, it establishes a baseline for continuing development and optimization of multi-parametric imaging protocols.

Intravascular functional maps of common neurovascular lesions derived from volumetric 4D CT data

PURPOSE

Current computed tomography angiography (CTA) postprocessing tools do not support quantitative assessment of intravascular physiology. Dynamic volumetric CT, acquired at a sufficiently high temporal resolution, is ideal for such analysis. Following preliminary experiments in flow phantoms, we examine the segmentation of blood vessels from 4D CT angiography by curve fit and encoding of functional blood flow information into the resulting functional intravascular maps.

MATERIALS AND METHODS

Flow phantoms were constructed consisting of a single pipe input and 4 simultaneous outputs of varying flow rates. Two outflow pipe diameters were tested. Bolus transit time (TT), time to peak (TTP), and time of arrival (TOA) were analyzed using contrast bolus profiles generated from 4D volumetric CT examinations on a 320 detector scanner in regions of interest placed 10 cm apart in all outflow pipes. Six subjects with various neurovascular lesions were next examined using a volumetric contrast-enhanced 4D CT angiography protocol. Segmentation was performed by quadratic curve fit after comparative analysis and optimization of the segmentation technique using quadratic curves, the gamma variate function, and a simplified formulation of the gamma variate function. After segmentation, quantitative analysis of spatially congruent intravascular voxels including TTP, rise, TT, and slope of the contrast upstroke was employed to encode physiologic information into the segmentations and produce intravascular functional maps. Comparison was made in each case to the patient's routine imaging.

RESULTS

Increasing volumetric flow rates correspond to reduction of bolus TT in flow phantoms. TT elongation was observed as the contrast bolus moved distally in all pipes, with greater elongation seen at slower flow rates and larger pipe diameters. A greater difference was observed between TTP proximally and distally in pipes compared with TOA, an effect most prominent at slower flow rates and larger pipe lumens, and thus TTP was chosen for functional encoding into segmentations of the clinical series. In vivo, the quadratic function demonstrated the lowest coefficient of variation when fit to intravascular time density series and outperformed 2 formulations of the gamma variate function. After segmentation with quadratic curves, Gaussian distributions were chosen over gamma variate functions to characterize contrast bolus profiles while neglecting recirculation and to calculate functional parameters for spatial encoding. Intravascular functional maps free of bone artifacts were created in every case that demonstrated all appropriate vessels and showed agreement with conventional imaging modalities in terms of vessel delineation and the diagnosis of vascular pathology. The most useful and interesting functional maps are discussed in each case.

CONCLUSIONS

The above approach to quantitative CT angiography provides a method of evaluating dynamic CTA data by means of intravascular functional maps. The techniques are broadly applicable in the clinical assessment of a variety of vascular diseases.

Utility of CT perfusion with 64-row multi-detector CT for acute ischemic brain stroke

We investigated the utility of computed tomographic (CT) perfusion (CTP) with 64-row multi-detector row CT (MDCT) to diagnose acute infarction and ischemic penumbra. We reviewed 58 clinical cases with acute ischemic stroke with CTP, compared the size of the area with long mean transit time (MTT) to that with abnormal intensity in magnetic resonance (MR) diffusion-weighted imaging (DWI) to diagnose penumbra, and compared the size of the area with reduced cerebral blood volume (CBV) in CTP to that in MR DWI to evaluate sensitivity for infarction. The total sensitivity of MTT to acute ischemic lesions was 81% (47/58). Sensitivity of MTT to segmental lesions was 100% (42/42) and for spot and focal lesions, 31% (5/16). In 13 patients, penumbra was diagnosed as lesions mismatched between MTT in CTP and MR DWI. When we regarded a lesion with decreased CBV as infarction, the sensitivity of CBV to segmental lesions was 85% (11/13), and the sensitivity to small infarction was 14% (4/28). Use of 64-row MDCT improves coverage and radiation exposure in head CTP. The combination of plain CT, CT angiography, and CTP with MDCT can demonstrate all segmental ischemic lesions and most large segmental infarctions, and their combined application is useful in considering indication and contraindication for thrombolysis. The problem of low sensitivity for small lesions remains, and MR DWI may be required to assess small infarctions when findings from combined plain CT, CT angiography, and CTP are negative in patients with suspected acute brain stroke.

Perfusion CT scanning and CT angiography in the evaluation of extracranial-intracranial bypass grafts

OBJECT

Extracranial-intracranial (EC-IC) bypass surgery remains an important treatment alternative for patients with occlusive cerebrovascular disease. The aim of the present study was to use perfusion CT and CT angiography (CTA) to evaluate cerebral hemodynamics and bypass patency in patients with occlusive cerebrovascular disease before and after EC-IC bypass surgery.

METHODS

Ten patients underwent perfusion CT and CTA before and after bypass surgery. Preoperative and postoperative digital subtraction angiography served as the diagnostic gold standard. An artery bypass was established from the superficial temporal artery to a cortical branch of the middle cerebral artery. Perfusion CT scanning was performed at the level of the basal ganglia. Color-coded perfusion maps of cerebral blood volume, cerebral blood flow, and time to peak were calculated.

RESULTS

Preoperative perfusion CT showed significant prolonged time to peak and reduced cerebral blood flow of the affected hemisphere. Postoperative neurological deterioration did not develop in any patient. Computed tomography angiography provided adequate evaluation of the anastomoses as well as the course and caliber of the bypass and confirmed bypass patency in all patients. Postoperative perfusion CT showed improved cerebral hemodynamics with a return to nearly normal perfusion parameters.

CONCLUSIONS

Computed tomography angiography is a noninvasive and reliable tool for evaluating patients with EC-IC bypass. Perfusion CT allows monitoring of hemodynamic changes after bypass surgery. The combination of both modalities enables noninvasive anatomical and functional analysis of superficial temporal artery-middle cerebral artery anastomoses using a single CT protocol. Hemodynamic evaluation of patients with occlusive cerebrovascular disease before and after surgery may improve the prediction of outcome and may help identify patients in whom a bypass procedure can be performed.

A spatio-temporal deconvolution method to improve perfusion CT quantification

Perfusion imaging is a useful adjunct to anatomic imaging in numerous diagnostic and therapy-monitoring settings. One approach to perfusion imaging is to assume a convolution relationship between a local arterial input function and the tissue enhancement profile of the region of interest via a "residue function" and subsequently solve for this residue function. This ill-posed problem is generally solved using singular-value decomposition based approaches, and the hemodynamic parameters are solved for each voxel independently. In this paper, we present a formulation which incorporates both spatial and temporal correlations, and show through simulations that this new formulation yields higher accuracy and greater robustness with respect to image noise. We also show using rectal cancer tumor images that this new formulation results in better segregation of normal and cancerous voxels.

Dose exposure of patients undergoing comprehensive stroke imaging by multidetector-row CT: comparison of 320-detector row and 64-detector row CT scanners

BACKGROUND AND PURPOSE

Recently introduced 320-detector row CT enables whole brain perfusion imaging compared to a limited scanning area in 64-detector row CT. Our aim was to evaluate patient radiation exposure in comprehensive stroke imaging by using multidetector row CT consisting of standard CT of the head, CTA of cerebral and cervical vessels, and CTP.

MATERIAL AND METHODS

Organ doses were measured by using LiF-TLDs located at several organ sites in an Alderson-Rando phantom. Effective doses were derived from these measurements. Stroke protocols including noncontrast head CT, CTA of cerebral and cervical vessels, and CTP were performed on 320- and 64-detector row scanners.

RESULTS

Measured effective doses for the different scanning protocols ranged between 1.61 and 4.56 mSv, resulting in an effective dose for complete stroke imaging of 7.52/7.54 mSv (m/f) for 64-detector row CT and 10.56/10.6 mSv (m/f) for 320-detector row CT. The highest organ doses within the area of the primary beam were measured in the skin (92 mGy) and cerebral hemispheres (69.91 mGy). Use of an eye-protection device resulted in a 54% decrease of the lens dose measured for the combo protocol for whole-brain perfusion with the 320-detector row CT scanner.

CONCLUSIONS

Phantom measurements indicate that comprehensive stroke imaging with multidetector row CT may result in effective radiation doses from 7.52 mSv (64-detector row CT) to 10.6 mSv (320-detector row CT). The technique of 320-detector row CT offers additional information on the time course of vascular enhancement and whole-brain perfusion. Physicians should weigh the potential of the new technique against the higher radiation dose that is needed. Critical doses that would cause organ damage were not reached.

Comparison of 4 cm Z-axis and 16 cm Z-axis multidetector CT perfusion

OBJECTIVE

The aim of the study was to compare 4 cm with 16 cm Z-axis coverage in the assessment of brain CT perfusion (CTP) using. 320 slice multidetector CT METHODS: A retrospective non-randomised review of CTP performed on MD320 CT between September 2008 and January 2009 was undertaken. Two experienced readers reviewed the studies along with the 4 cm and 16 cm Z-axis CTP image data set. The outcome parameters assessed were the extent of the original finding, any additional findings and a change of diagnosis.

RESULTS

14 out of 27 patients were found to have abnormal CTP (mean age 58.1 years, 9 male). The 16 cm Z-axis increased the accuracy of the infarct core in 78% and ischaemic penumbra quantification in 100% of the cases. It also diagnosed additional infarcts in the same vascular territory in 28% of cases and in a different vascular territory in 14%.

CONCLUSIONS

The increased field of view with MD320 better defines the true extent of the infarct core and ischaemic penumbra. It also identified other areas of infarction that were not identified on the 4 cm Z-axis.

Magnetic resonance imaging in the evaluation of acute stroke

The ability to use physiologic imaging with either magnetic resonance (MR) or computed tomography to help define irreversibly injured brain (the infarct core) and tissue at risk of infarct (reversible ischemic penumbra) holds great promise in the future treatment of stroke. The physiologic principles and concepts underlying the evaluation for mismatch between injured tissue and tissue at risk are similar for the 2 imaging techniques. Multimodal MR imaging (diffusion-weighted imaging/perfusion-weighted imaging/MR angiography) provides a validated penumbral selection criteria based on the results of 2 clinical trials (diffusion and perfusion imaging evaluation for understanding stroke evolution and echoplanar imaging thrombolysis evaluation). Computed tomographic perfusion parameters have also been calculated to optimize final infarct prediction. Both techniques await further study to prove their capability of selecting cases for short-term recanalization/reperfusion therapy.