Quantitative assessment of cerebral hemodynamics using CT: stability, accuracy, and precision studies in dogs

A distributed parameter model of cerebral blood-tissue exchange with account of capillary transit time distribution

Quantitative estimates of physiological parameters associated with cerebral blood flow can be derived from the analysis of dynamic contrast-enhanced (DCE) images, using an appropriate model of the underlying tissue impulse residue function. The theoretical formulation of a distributed parameter model of tissue microcirculation, which accounts for the effects of capillary permeability and transit time distribution, is presented here. This model considers a statistical distribution of capillary-tissue units, each described by a distributed parameter model that accounts for convective transport within the capillary and transcapillary axial diffusion. Monte Carlo simulations were performed to study the confidence of the parameter estimates, and the model was used to analyze DCE CT images of patient study cases with metastatic cerebral tumors. The tumors were found to yield significantly higher estimates than normal tissues for the parameters associated with the extravasation of tracer and for the standard deviation of capillary transit times. The proposed model can be used with DCE imaging to study the microcirculatory characteristics of cerebral tumors.

CT perfusion in acute stroke

As new treatments are developed for stroke, the potential clinical applications of CT perfusion (CTP) imaging in the diagnosis, triage, and therapeutic monitoring of these diseases are certain to increase. Technical advances in scanner hardware and software should no doubt continue to increase the speed, coverage, and resolution of CTP imaging. CTP offers the promise of efficient use of imaging resources and, potentially, of decreased morbidity. Most importantly, current CT technology already permits the incorporation of CTP as part of an all-in-one acute stroke examination to answer the four fundamental questions of stroke triage quickly and accurately, further increasing the contribution of imaging to the diagnosis and treatment of acute stroke.

Perfusion Computed Tomography for the Indication of Percutaneous Transluminal Reconstruction for Acute Stroke

Perfusion computed tomography (CT) was performed in patients with acute-stage stroke to assess the indications for percutaneous transluminal reconstruction (PTR). This study included 59 patients admitted within 8 hours of onset of stroke in whom initial CT demonstrated no ischemic changes. Multiple regions of interest (ROIs) were selected in the ischemic lesions, and the ratios of cerebral blood flow (CBF) and cerebral blood volume (CBV) in the ROIs were calculated and compared to with those in the same location in the opposite hemisphere. The ischemic boundaries for CBF and CBV were analyzed in 29 patients treated conservatively. PTR was performed in 30 patients without visually decreased CBV. Some of the patients with visually evaluated abnormal regional mean transit time, decreased regional CBF, and normal regional CBV developed infarction, but others did not. The statistical analysis for CBF using the mean ROI ratios of each patient was 0.413 +/- 0.272 (mean +/- SD) (median, 0.307) in regions with subsequent infarction and 0.750 +/- 0.221 (0.772) in regions without infarction (P < .005), and that for CBV was 0.837 +/- 0.367 (0.778) in regions with subsequent infarction and 1.137 +/- 0.324 (1.121) in regions without infarction (P < .005). The statistical analysis for CBF using the highest and lowest ROI ratios of each patient was 0.548 +/- 0.342 (0.428) in regions with infarction and 0.584 +/- 0.191 (0.636) in regions without infarction (P = .655), and that for CBV was 0.997 +/- 0.430 (0.927) in regions with infarction and 0.948 +/- 0.182 (0.948) in regions without infarction (P = .606). Four of the 24 patients with recanalization after PTA had poor outcome and a CBV ratio of 0.6-0.8. The present study indicates that the ischemic boundary is approximately 0.5 for regional CBF and 0.9 for regional CBV, providing appropriate indications for PTR. Even a slight decrease in CBV, which may not be detected visually, can affect the outcome, and so the regional CBV must be calculated for the correct diagnosis.

Functional cluster analysis of CT perfusion maps: a new tool for diagnosis of acute stroke?

CT perfusion imaging constitutes an important contribution to the early diagnosis of acute stroke. Cerebral blood flow (CBF), cerebral blood volume (CBV) and time-to-peak (TTP) maps are used to estimate the severity of cerebral damage after acute ischemia. We introduce functional cluster analysis as a new tool to evaluate CT perfusion in order to identify normal brain, ischemic tissue and large vessels. CBF, CBV and TTP maps represent the basis for cluster analysis applying a partitioning (k-means) and density-based (density-based spatial clustering of applications with noise, DBSCAN) paradigm. In patients with transient ischemic attack and stroke, cluster analysis identified brain areas with distinct hemodynamic properties (gray and white matter) and segmented territorial ischemia. CBF, CBV and TTP values of each detected cluster were displayed. Our preliminary results indicate that functional cluster analysis of CT perfusion maps may become a helpful tool for the interpretation of perfusion maps and provide a rapid means for the segmentation of ischemic tissue.

Quantitative assessment of colorectal cancer perfusion using MDCT: inter- and intraobserver agreement

OBJECTIVE

The objective of our study was to determine inter- and intraobserver agreement of MDCT colorectal cancer perfusion measurements.

SUBJECTS AND METHODS

Thirty-one patients (17 men, 14 women; median age, 69 years) with proven colorectal cancer were examined prospectively using MDCT. A 65-sec dynamic study (cine mode, 4 x 5 mm collimation) was acquired through the tumor after i.v. contrast administration (100 mL of iopamidol 350, 5 mL/sec). Tumor blood volume, blood flow, mean transit time, and permeability measurements were determined by two independent observers using commercial software. Inter- and intraobserver agreement was assessed using the Bland-Altman test.

RESULTS

The mean difference for interobserver agreement (95% limits of agreement) was -0.81 mL/100 g tissue (-3.14 to 1.52); -9.94 mL/100 g tissue/min (-51.43 to 32.65); -1.09 sec (-7.05 to 4.86); and -2.90 mL/100 g tissue/min (-11.48 to 5.68) for blood volume, blood flow, mean transit time, and permeability, respectively. The intraclass correlation coefficient was 0.83, 0.89, 0.89, and 0.80, respectively. The mean difference for intraobserver agreement (95% limits of agreement) was 0.12 mL/100 g tissue (-1.90 to 2.14); 0.02 mL/100 g tissue/min (-13.13 to 13.17); -0.19 sec (-3.19 to 2.81); and 0.00 mL/100 g tissue/min (-2.45 to 2.45) for observer 1 and 0.26 mL/100 g tissue (-1.46 to 1.98); 4.47 mL/100 g tissue/min (-26.65 to 35.59); -0.21 sec (-2.48 to 2.06); 1.08 mL/100 g tissue/min (-4.92 to 7.08) for observer 2. The intraclass correlation coefficient was 0.86, 0.98, 0.97, 0.98 for observer 1 and 0.93, 0.96, 0.99, and 0.94, respectively, for observer 2.

CONCLUSION

There is greater inter- than intraobserver agreement for CT vascular perfusion measurements of primary colorectal cancer, which must be addressed for reliable clinical application in therapeutic monitoring.

Neuroimaging of ischemia and infarction

Since the introduction of thrombolytic therapy as the foundation of acute stroke treatment, neuroimaging has rapidly advanced to empower therapeutic decision making. Diffusion-weighted imaging is the most sensitive and accurate method for stroke detection, and, allied with perfusion-weighted imaging, provides information on the functional status of the ischemic brain. It can also help to identify a response to thrombolytic and neuroprotective therapies. Additionally, multimodal magnetic resonance imaging, including magnetic resonance angiography, offers information on stroke mechanism and pathophysiology that can guide long-term medical management. Multimodal computed tomography is a comprehensive, cost-effective, and safe stroke imaging modality that can be easily implemented in the emergency ward and that offers fast and reliable information with respect to the arterial and functional status of the ischemic brain. Accessibility, contraindications, cost, speed, and individual patient-determined features influence which is the best imaging modality to guide acute stroke management.

Reflex-mediated reduction in human cerebral blood volume

Adrenergic nerves innervate the human cerebrovasculature, yet the functional role of neurogenic influences on cerebral hemodynamics remains speculative. In the current study, regional cerebrovascular responses to sympathoexcitatory reflexes were evaluated. In eight volunteers, contrast-enhanced computed tomography was performed at baseline, -40 mmHg lower body negative pressure (LBNP), and a cold pressor test (CPT). Cerebral blood volume (CBV), mean transit time (MTT), and cerebral blood flow (CBF) were evaluated in cortical gray matter (GM), white matter (WM), and basal ganglia/thalamus (BGT) regions. Lower body negative pressure resulted in tachycardia and decreased central venous pressure while mean arterial pressure was maintained. Cold pressor test resulted in increased mean arterial pressure concomitant with tachycardia but no change in central venous pressure. Neither reflex altered end-tidal carbon dioxide. Cerebral blood volume was reduced in GM during both LBNP and CPT (P<0.05) but was unchanged in WM and BGT. Mean transit time was reduced in WM and GM during CPT (P<0.05). Cerebral blood flow was only modestly affected with either reflex (P<0.07). The combined reductions in GM CBV (approximately -25%) and MTT, both with and without any change in central venous pressure, with small CBF changes (approximately -11%), suggest that active venoconstriction contributed to the volume changes. These data demonstrate that CBV is reduced during engagement of sympathoexcitatory reflexes and that these cerebrovascular changes are heterogeneously distributed.

Perfusion CT for the assessment of tumour vascularity: which protocol?

Perfusion CT is a technique that can be readily incorporated into the existing CT protocols that continue to provide the mainstay for anatomical imaging in oncology to provide an in vivo marker of tumour angiogenesis. By capturing physiological information reflecting the tumour vasculature, perfusion CT can be useful for diagnosis, risk-stratification and therapeutic monitoring. However, a wide range of perfusion CT techniques have evolved and the various commercial implementations advocate different acquisition protocols and processing methods. Acquisition choices include first pass studies or delayed imaging, temporal resolution versus image noise, and single location sequences or multiple spiral acquisitions. Data processing may be semi-quantitative or, using either compartmental analysis or deconvolution, produce results that are quantified in absolute physiological terms such as perfusion, blood volume and permeability. This article discusses the advantages and disadvantages of the more common CT perfusion protocols and offers proposals that could allow for easier comparison between studies employing different techniques.

Functional CT imaging in oncology

The two-compartment pharmacokinetics exhibited by iodinated contrast media makes these agents well suited to the study of tumour angiogenesis in which new vessels are not only produced in greater number but also are abnormally permeable to circulating molecules. The temporal changes in contrast enhancement of tumours on CT have been shown to correlate with histopathological assessments of angiogenesis with the intravascular and extravascular phases of contrast enhancement reflecting microvessel density and vascular permeability, respectively. By quantifying tumour contrast enhancement to capture physiological information about the vascular system, functional CT can provide a useful adjunct to the anatomical information afforded by MDCT in oncology, aiding with tumour diagnosis, risk stratification and therapy monitoring. By simultaneously assessing tumour vascularity and metabolic demand, the broader expansion of integrated MDCT/PET imaging will support highly sophisticated assessments of tumour biology within a single examination.

Absolute cerebral blood volume and blood flow measurements based on synchrotron radiation quantitative computed tomography

Synchrotron radiation computed tomography opens new fields by using monochromatic x-ray beams. This technique allows one to measure in vivo absolute contrast-agent concentrations with high accuracy and precision, and absolute cerebral blood volume or flow can be derived from these measurements using tracer kinetic methods. The authors injected an intravenous bolus of an iodinated contrast agent in healthy rats, and acquired computed tomography images to follow the temporal evolution of the contrast material in the blood circulation. The first image acquired before iodine infusion was subtracted from the others to obtain computed tomography slices expressed in absolute iodine concentrations. Cerebral blood volume and cerebral blood flow maps were obtained after correction for partial volume effects. Mean cerebral blood volume and flow values (n = 7) were 2.1 +/- 0.38 mL/100 g and 129 +/- 18 mL. 100 g-1. min-1 in the parietal cortex; and 1.92 +/- 0.32 mL/100 g and 125 +/- 17 mL. 100 g-1. min-1 in the caudate putamen, respectively. Synchrotron radiation computed tomography has the potential to assess these two brain-perfusion parameters.

Brain perfusion CT in acute stroke: current status

Dynamic perfusion CT has become a widely accepted imaging modality for the diagnostic workup of acute stroke patients. Although compared with standard spiral CT the use of multislice CT has broadened the range from which perfusion data may be derived in a single scan run. The advent of multidetector row technology has not really overcome the limited 3D capability of this technique. Multidetector CT angiography (CTA) of the cerebral arteries may in part compensate for this by providing additional information about the cerebrovascular status. This article describes the basics of cerebral contrast bolus scanning with a special focus on optimization of contrast/noise in order to ensure high quality perfusion maps. Dedicated scan protocols including low tube voltage (80 kV) as well as the use of highly concentrated contrast media are amongst the requirements to achieve optimum contrast signal from the short bolus passage through the brain. Advanced pre and postprocessing algorithms may help reduce the noise level, which may become critical in unconscious stroke victims. Two theoretical concepts have been described for the calculation of tissue perfusion from contrast bolus studies, both of which can be equally employed for brain perfusion imaging. For each perfusion model there are some profound limitations regarding the validity of perfusion values derived from ischemic brain areas. This makes the use of absolute quantitative cerebral blood flow (CBF) values for the discrimination of the infarct core from periinfarct ischemia questionable. Multiparameter imaging using maps of CBF, cerebral blood volume (CBV), and a time parameter of the local bolus transit enables analyzing of the cerebral perfusion status in detail. Perfusion CT exceeds plain CT in depicting cerebral hypoperfusion at its earliest stage yielding a sensitivity of about 90% for the detection of embolic and hemodynamic lesions within cerebral hemispheres. Qualitative assessment of brain perfusion can be further enhanced by adding relative perfusion indices from regions of interest. Multislice CTA using a collimation of 4 x 1 mm and high pitch factors allows for isotropic scanning of the brain supplying arteries from the aortic arch to the vertex in a single run. Various image processing modalities such as multiplanar reformations, curved planar reconstructions, maximum intensity projections, and volume rendering techniques are available to deal with the extensive data and to bring out those vascular lesions, which are of relevance for individual stroke. With the advent of multidetector CT advanced stroke protocols combining plain CT, perfusion CT and CTA can routinely be accomplished within a very short timespan thus ensuring the role of CT in the diagnostic workup of acute stroke.

Dynamic single-section CT demonstrates reduced cerebral blood flow in acute intracerebral hemorrhage

Optimum blood pressure (BP) management in acute intracerebral hemorrhage (ICH) remains controversial. BP reduction may limit hematoma expansion, but may also exacerbate ischemia. Reduced regional cerebral blood flow (rCBF) has been reported in ICH. Its extent and precise pattern, however, remain uncertain. Dynamic single-section CT perfusion (CTP) is rapid, easily performed and offers superior spatial resolution to PET, SPECT and MRI. It may be the most applicable method for assessing the effects of BP management on rCBF in ICH. We sought to assess whether CTP can identify rCBF abnormalities in acute ICH in 5 patients with ICH who underwent CTP within 24 h of symptom onset. rCBF was measured in serially expanded 2-mm rings around the hematoma and compared with rCBF in the uninvolved hemisphere. Mean time to CTP was 9 h (range 3-23). Mean ICH volume was 25 ml (range 9-64). Perihematoma perfusion was reduced in all patients compared with contralateral hemisphere rCBF. rCBF reduction was most pronounced immediately adjacent to the hematoma (p < 0.05 at 2 mm, p = 0.084 at 4 mm, p > 0.2 at 6 and 8 mm). Perihematoma rCBF increased as a function of the distance from hematoma perimeter. Rate of rCBF increase over distance correlated with time from onset (p = 0.006). We conclude that CTP identifies a rim of reduced rCBF in ICH. A gradient of hypoperfusion appears to extend at least 4 mm beyond the hematoma edge and may be time dependent. Whether reduced CBF is associated with perihematoma ischemia requires additional study.

MOSAIC: Multimodal Stroke Assessment Using Computed Tomography: novel diagnostic approach for the prediction of infarction size and clinical outcome

BACKGROUND AND PURPOSE

With new CT technologies, including CT angiography (CTA), perfusion CT (PCT), and multidetector row technique, this method has regained interest for use in acute stroke assessment. We have developed a score system based on Multimodal Stroke Assessment Using CT (MOSAIC), which was evaluated in this prospective study.

METHODS

Forty-four acute stroke patients (mean age, 63.8 years) were enrolled within a mean of 3.0+/-1.9 hours after symptom onset. The MOSAIC score (0 to 8 points) was generated by results of the 3 sequential CT investigations: (1) presence and amount of early signs of infarction on noncontrast CT (NCCT; 0 to 2 points), (2) stenosis (>50%) or occlusion of the distal internal carotid or middle cerebral artery on CTA (0 to 2 points), and (3) presence and amount of reduced cerebral blood flow on 2 adjacent PCT slices (0 to 4 points). The predictive value of the MOSAIC score was compared with each single CT component with respect to the final size of infarction and the clinical outcome 3 months after stroke by use of the modified Rankin Scale (mRS) and the Barthel Index (BI).

RESULTS

Among the CT components, PCT showed the best correlation to infarction size (r=0.75) and clinical outcome (r=0.60 to 0.62) compared with NCCT (r=0.43 to 0.58) and CTA (r=0.47 to 0.71). The MOSAIC score showed consistently higher correlation factors (r=0.67 to 0.78) and higher predictive values (0.73 to 1.0) than all single CT components with respect to outcome measures. A MOSAIC score <4 predicted independence with 89% to 96% likelihood (mRS </=2, BI >/=90); a MOSAIC score <5 predicted fair outcome with 96% to 100% likelihood (mRS </=3, BI >/=60).

CONCLUSIONS

The MOSAIC score based on multidetector row CT technology is superior to NCCT, CTA, and PCT in predicting infarction size and clinical outcome in hyperacute stroke.

Functional computed tomography in oncology

Functional Computed Tomography (CT) describes the use of existing technologies and conventional contrast agents to capture physiological parameters that reflect the vasculature within tumours and other tissues. The technique is readily incorporated into routine conventional CT examinations and, in tumours, the physiological parameters obtained provide an in-vivo marker of angiogenesis. As well as providing a research tool, functional CT has clinical applications in tumour diagnosis, staging, risk stratification and therapy monitoring, including the characterisation of pulmonary nodules, detection of occult hepatic metastases, grading of cerebral glioma and monitoring of anti-angiogenesis drugs. With the recent commercial availability of appropriate software and the development of multislice CT systems, functional CT is poised to make a significant impact upon the imaging of patients with cancer.

Cerebral blood volume measurements by rapid contrast infusion and T2*-weighted echo planar MRI

Cerebral blood volume (CBV) provides information complementary to that of cerebral blood flow in cerebral ischemia, tumors, and other conditions. We have developed an alternative theory and method for measuring CBV based on dynamic imaging by MRI or CT during a short contrast infusion. This method avoids several limitations of traditional approaches that involve waiting for steady state or measuring the area under the curve (AUC) during bolus contrast injection. Anesthetized dogs were studied by T2*-weighted echo planar imaging during gadolinium-DTPA infusions lasting 30-60 sec. CBV was calculated from the ratio of the signal changes in tissue and artery. Method responsiveness was compared to AUC measurements using the vasodilator acepromazine. The ratio of signal change in tissue to that in artery rapidly approached an asymptotic value even while the amount of contrast in artery continued to increase. Using 30-sec infusions, the mean (+/- SD) of CBV for control animals was 3.6 +/- 0.9 ml blood/100 g tissue in gray matter and 2.3 +/- 0.8 ml blood/100 g tissue in white matter (ratio = 1.6). Acepromazine increased CBV to 5.7 +/- 1.5 ml blood/100 g tissue in gray matter and 3.1 +/- 0.8 ml blood/100 g tissue in white matter (ratio = 2.0). AUC measurements after bolus injection yielded similar values for control animals but failed to demonstrate any change after acepromazine. It is possible to measure CBV using dynamic MRI or CT during 30-60-sec contrast infusions. This method may be more sensitive to changes in CBV than traditional AUC methods.

A numerical method for estimating blood flow by dynamic functional imaging

We present a numerical deconvolution scheme for estimating regional blood flow and tissue retention functions by dynamic functional imaging. The present approach implements the Tikhonov-Miller regularization in general form, which allows for prior knowledge or assumptions to be incorporated during the deconvolution process, so as to stabilize the solution against variations due to noise. Appropriate approximations and simplifications in the context of functional imaging, were also introduced to ease numerical computations. Monte Carlo simulation experiments were carried out to study the applicability of the present approach and to compare with other deconvolution techniques previously studied.

Quantification of cerebral perfusion with "Real-Time" contrast-enhanced ultrasound

BACKGROUND

No noninvasive technique is currently capable of "real-time" assessment and monitoring of cerebral blood flow (CBF). We hypothesized that cerebral perfusion could be accurately measured and monitored in "real time" with contrast-enhanced ultrasound (CEU).

METHODS AND RESULTS

Cerebral perfusion was assessed in 9 dogs through a craniotomy with CEU at baseline and during hypercapnia and hypocapnia while normoxia was maintained. Cerebral microvascular blood volume (A), microbubble velocity (beta), and blood flow (Axbeta) were calculated from time-versus-acoustic intensity relations. Compared with baseline, hypercapnia and hypocapnia significantly increased and decreased CBF, respectively, as measured by CEU. These changes in blood flow were mediated by changes in both A and beta. A good correlation was found between Axbeta derived from CEU and CBF measured by radiolabeled microspheres (y=0.67x-0.04, r=0.91, P<0.001).

CONCLUSIONS

Changes in both cerebral microvascular blood volume and red blood cell velocity can be accurately assessed with CEU. Thus, CEU has the potential for bedside measurement and monitoring of cerebral perfusion in real time in patients with craniotomies or burr holes.

Perfusion mapping using computed tomography allows accurate prediction of cerebral infarction in experimental brain ischemia

BACKGROUND AND PURPOSE

We have developed a dynamic CT method to measure absolute cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT). In this study we evaluated the ability of CT-derived functional maps to detect infarction in a rabbit model of focal cerebral ischemia.

METHODS

Sequential dynamic CT studies were performed at 2 different slices in 5 control rabbits and another 8 after induction of focal cerebral ischemia. The size of critically ischemic tissue was correlated to size of infarction measured by postmortem 2,3,5-triphenyltetrazolium chloride staining. In the control rabbits, short-term variability of the parameters was assessed by ANOVA analysis.

RESULTS

In 7 of 8 animals of the ischemia group, cerebral infarction was visible on 2,3, 5-triphenyltetrazolium chloride staining, constituting 16.7+/-10.6% of the ipsilateral hemisphere. Good agreement of CBF functional maps with tissue specimens was found with respect to size and location of infarction. Best prediction of infarction was found for thresholds of CBF <10 mL/100 g per minute (mean size, 17.5+/-13.4%; r=0.95) and MTT >6 seconds (mean size, 15.6+/-13.5%; r=0.85), with regression slopes close to unity. CBV maps were less predictive of occurrence of infarction, especially in cases of small infarction. The short-term variability of CBF, CBV, and MTT in the control group was 10.9%, 15.2%, and 19.9%, respectively.

CONCLUSIONS

Functional CT measurements of absolute CBF and MTT early after onset of ischemia allow prediction of the size and location of cerebral infarction with good accuracy.

Dynamic contrast-enhanced brain perfusion imaging: technique and clinical applications

Magnetic resonance (MR) and computed tomographic (CT) perfusion imaging are evolving noninvasive imaging techniques that, unlike conventional MR and CT angiographic methods, can be used to evaluate capillary level tissue perfusion. These techniques can provide early, highly accurate delineation of ischemic tissue, allowing the underlying hemodynamic disturbances of disorders such as stroke and vasospasm to be further analyzed, as well as defining abnormal regions of blood pool in brain tumors. Because MR perfusion (MRP) and CT perfusion (CTP) imaging can assess physiologic parameters such as cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT), they offer additional data that can be useful in the detection and characterization of entities such as tumor, infection, inflammation, and infarction, which all can have similar appearances on both contrast and noncontrast enhanced conventional CT and MR images. They can also facilitate the further evaluation of processes such as early dementia, psychiatric illnesses, and migraine headaches, which may appear normal on routine CT and MR imaging. MRP and CTP might also be of value in distinguishing residual or recurrent tumor from treatment effects such as radiation-induced necrosis. This article reviews the background principles, scanning techniques, and clinical applications of noninvasive cerebral perfusion imaging.