Dynamic computed tomography of the brain: techniques, data analysis, and applications

50th anniversary of computed tomography: past and future applications in clinical neuroscience

Purpose: It has been 50 years since computed tomography was introduced to the worldwide neurologic medical and surgical community. In that time, tremendous advances in computer software and hardware, as well as creative changes in computerized tomographic (CT) hardware and tube technology, have dramatically improved the temporal and spatial resolution of CT. In this paper, I address what I feel are some of the most important impacts of CT in the field of clinical neuroscience over the last 50 years, as well as potential applications of CT that are on the horizon.

Approach: I have recounted from literature, colleagues, and personal recollection the historical impact of CT on neuroradiology practice and what appear to be near-term future applications.

Conclusions: Therapeutic applications beyond diagnosis, such as image-guided procedures, radiation, and surgical planning, and development of the field of theranostics have emerged and further increased the need for faster and more precise CT imaging. The integration of machine learning into the acquisition chain and radiologist tool kit has great implications for standardization, analysis, and diagnosis worldwide.

A Novel Statistical Optimization Algorithm for Estimating Perfusion Curves in Susceptibility Contrast-Enhanced MRI

Dynamic susceptibility contrast-enhanced magnetic resonance imaging is an important tool for evaluating intravascular indicator dynamics, which in turn is valuable for understanding brain physiology and pathophysiology. This procedure usually involves fitting a gamma-variate function to observed concentration-time curves in order to eliminate undesired effects of recirculation and the leakage of contrast agents. Several conventional curve-fitting approaches are routinely applied. The nonlinear optimization methods typically are computationally expensive and require reliable initial values to guarantee success, whereas a logarithmic linear least-squares (LL-LS) method is more stable and efficient, and does not suffer from the initial-value problem, but it can show degraded performance, especially when a few data or outliers are present. In this paper, we demonstrate, that the original perfusion curve-fitting problem can be transformed into a gamma-distribution-fitting problem by treating the concentration-time curves as a random sample from a gamma distribution with time as the random variable. A robust maximum-likelihood estimation (MLE) algorithm can then be readily adopted to solve this problem. The performance of the proposed method is compared with the nonlinear Levenberg-Marquardt (L-M) method and the LL-LS method using both synthetic and real data. The results show that the performance of the proposed approach is far superior to those of the other two methods, while keeping the advantages of the LL-LS method, such as easy implementation, low computational load, and dispensing with the need to guess the initial values. We argue that the proposed method represents an attractive alternative option for assessing intravascular indicator dynamics in clinical applications. Moreover, we also provide valuable suggestions on how to select valid data points and set the initial values in the two traditional approaches (LL-LS and nonlinear L-M methods) to achieve more reliable estimations.

Applications of 256-Slice, Spiral Computed Tomography Perfusion Scanning in Limb Salvage After High-Voltage Electrical Injury

OBJECTIVE

This study aimed to investigate the applications of intelligent 256-slice computed tomography (iCT) perfusion imaging in high-voltage electrical injuries (HVEIs).

METHODS

256-slice iCT was used to perform perfusion scanning for 48 patients with HVEI to detect the perfusion parameters.

RESULTS

The blood flow (BF) and peak enhancement intensity (PEI) values of the plane lower than the amputation level of the diseased side (ALD) were smaller than those of the corresponding healthy side (P<0.05); therefore, the differences were statistically significant. The BF value of the plane beyond the ALD was bigger than that of the ALD (t=2.99 and P=0.042); therefore, the difference was statistically significant. The BF, PEI, and blood volume values of the plane below the ALD were smaller than those of the ALD (P<0.05); therefore, the differences were statistically significant.

CONCLUSIONS

The technique of 256-slice iCT perfusion imaging could provide richer and more comprehensive imaging data for the clinical treatment of HVEIs, thus exhibiting its benefit in reducing the disability of patients with HVEIs. (Disaster Med Public Health Preparedness. 2018;12:478-485).

How Can New Imaging Modalities Help in the Practice of Radiology?

The purpose of this article was to provide an up-to-date review on the spectrum of new imaging applications in the practice of radiology. New imaging techniques have been developed with the objective of obtaining structural and functional analyses of different body systems. Recently, new imaging modalities have aroused the interest of many researchers who are studying the applicability of these modalities in the evaluation of different organs and diseases. In this review article, we present the efficiency and utilization of current imaging modalities in daily radiological practice.

Nonlinear estimation of BOLD signals with the aid of cerebral blood volume imaging

BACKGROUND

The hemodynamic balloon model describes the change in coupling from underlying neural activity to observed blood oxygen level dependent (BOLD) response. It plays an increasing important role in brain research using magnetic resonance imaging (MRI) techniques. However, changes in the BOLD signal are sensitive to the resting blood volume fraction (i.e., [Formula: see text]) associated with the regional vasculature. In previous studies the value was arbitrarily set to a physiologically plausible value to circumvent the ill-posedness of the inverse problem. These approaches fail to explore actual [Formula: see text] value and could yield inaccurate model estimation.

METHODS

The present study represents the first empiric attempt to derive the actual [Formula: see text] from data obtained using cerebral blood volume imaging, with the aim of augmenting the existing estimation schemes. Bimanual finger tapping experiments were performed to determine how [Formula: see text] influences the model estimation of BOLD signals within a single-region and multiple-regions (i.e., dynamic causal modeling). In order to show the significance of applying the true [Formula: see text], we have presented the different results obtained when using the real [Formula: see text] and assumed [Formula: see text] in terms of single-region model estimation and dynamic causal modeling.

RESULTS

The results show that [Formula: see text] significantly influences the estimation results within a single-region and multiple-regions. Using the actual [Formula: see text] might yield more realistic and physiologically meaningful model estimation results.

CONCLUSION

Incorporating regional venous information in the analysis of the hemodynamic model can provide more reliable and accurate parameter estimations and model predictions, and improve the inference about brain connectivity based on fMRI data.

Perfusion-CT for early assessment of traumatic cerebral contusions

INTRODUCTION

To investigate the value of perfusion-CT (PCT) for assessment of traumatic cerebral contusions (TCC) and to compare the abilities of early noncontrast CT and PCT modalities to evaluate tissue viability.

METHODS

PCT studies performed in 30 patients suffering from TCC during the acute phase of their illness were retrospectively reviewed. Cerebral blood flow (CBF), volume (CBV) and mean transit time (MTT) were measured in three different areas: the hemorrhagic core of the TCC, the surrounding hypodense area and the perilesional normal-appearing parenchyma. TCC area was measured on CBF-, CBV- and MTT-derived maps and compared with the areas measured using the same slice obtained with CT scans performed on admission, at the time of PCT (follow-up CT) and at 1 week.

RESULTS

TCC were characterized by low CBF and CBV values (9.2+/-6.6 ml/100 g per min and 0.9+/-0.7 ml/100 g, respectively) and a significant prolongation of MTT (11.9+/-10.7 s) in the hemorrhagic core whereas PCT parameters were more variable in the hypodense area. The TCC whole area showed a noticeable growth of the lesions during the first week of admission. In comparison with early noncontrast CT, CBV and CBF maps proved to be more congruent with the findings of noncontrast CT scans at 1 week.

CONCLUSION

PCT confirmed the results of xenon-CT studies and was shown to allow better evaluation of tissue viability than noncontrast CT. These findings suggest that PCT could be implemented in the future for the early assessment of patients with traumatic brain injury.

Diffusion-weighted and perfusion MR imaging for brain tumor characterization and assessment of treatment response

Diffusion-weighted magnetic resonance (MR) imaging and perfusion MR imaging are advanced techniques that provide information not available from conventional MR imaging. In particular, these techniques have a number of applications with regard to characterization of tumors and assessment of tumor response to therapy. In this review, the authors describe the fundamental principles of diffusion-weighted and perfusion MR imaging and provide an overview of the ways in which these techniques are being used to characterize tumors by helping distinguish tumor types, assess tumor grade, and attempt to determine tumor margins. In addition, the role of these techniques for evaluating response to tumor therapy is outlined.

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.

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.

Measurement of cerebral mean transit time by dynamic susceptibility contrast magnetic resonance imaging

OBJECTIVE

To determine whether direct measurement of mean transit time from pixels over in-plane vessels on high spatial resolution echo planar imaging is a reliable method for quantitative assessment of cerebral circulation.

METHODS AND MATERIALS

Dynamic susceptibility contrast studies were performed using high spatial resolution echo planar imaging (echo time, 60 ms; field of view, 256 x 192-270 x 203 mm; matrix size, 256 x 192; slice thickness, 4 mm) in ten healthy subjects. Forty sequential measurements of five images between the level of the middle cerebral arteries and that of the centrum semiovale were acquired every 1.5 s before, during, and after intravenous injection of 0.12 mmol/kg of gadopentetate dimeglumine. Mean transit times were calculated from the results of gamma variate fitting to the measured deltaR2* data of the middle cerebral arteries, cerebral cortex and white matter.

RESULTS

The calculated true mean transit times for cerebral cortex and white matter varied greatly among individuals and from side to side even in a given individual. The fitness of regression models for the deltaR2* curves of the middle cerebral arteries was significantly lower than those for cerebral cortex and white matter.

CONCLUSION

Direct measurement of mean transit time from pixels over in-plane vessels was not sufficiently accurate for quantitative assessment of cerebral circulation, probably because the echo planar imaging we used had spatial resolution and dynamic range insufficient for determination of mean transit time for in-plane vessels.

Perfusion measurements of the brain: using dynamic CT for the quantitative assessment of cerebral ischemia in acute stroke

OBJECTIVE

Perfusion CT has been successfully used as a functional imaging technique for the differential diagnosis of patients with hyperacute stroke. We investigated to what extent this technique can also be used for the quantitative assessment of cerebral ischemia.

METHODS AND MATERIAL

We studied linearity, spatial resolution and noise behaviour of cerebral blood flow (CBF) determination with computer simulations and phantom measurements. Statistical ROI based analysis of CBF images of a subset of 38 patients from a controlled clinical stroke study with currently more than 75 patients was done to check the power of relative cerebral blood flow (rCBF) values to predict definite infarction and ischemic penumbra. Classification was performed using follow-up CT and MR data.

RESULTS

Absolute CBF values were systematically underestimated, the degree depended on the cardiac output of the patients. Phantom measurements and simulations indicated very good linearity allowing reliable calculation of rCBF values. Infarct and penumbra areas in 19 patients receiving standard heparin therapy had mean rCBF values of 0.19 and 0.62, respectively. The corresponding values for 19 patients receiving local intraarterial fibrinolysis were 0.18 and 0.57. The difference between infarct and penumbra values was highly significant (P < 0.0001) in both groups. No penumbra area was found with an rCBF value of less than 0.20. While in the heparin group only 25% of all areas with an rCBF between 0.20 and 0.35 survived, in the fibrinolytic group 61% of these areas could be saved (P < 0.05).

CONCLUSION

Perfusion CT is a fast and practical technique for routine clinical application. It provides substantial and important additional information for the selection of the optimal treatment strategy for patients with hyperacute stroke. Relative values of cerebral blood flow discriminate very well between areas of reversible and irreversible ischemia; an rCBF value of 0.20 appears to be a definite lower limit for brain tissue to survive an ischemic injury.

Advanced imaging concepts: a pictorial glossary of CT and MRI technology

This article serves as an illustrative glossary of concepts related to computed tomography (CT) and magnetic resonance imaging (MRI) technology. The principles of tomography, digital processing, image resolution, CT windowing, CT gray levels, contrast enhancement, and MRI spin echo pulse sequences are reviewed. Techniques not commonly described for use in animal patients are also introduced, and include gradient echo, short time of inversion recovery, fluid attenuated inversion recovery and fat saturation pulse sequences, fast imaging, MRI angiography, perfusion and diffusion imaging, brain activation, CT angiography/functional CT, interventional procedures, and three-dimensional CT.

Regional blood flow, capillary permeability, and compartmental volumes: measurement with dynamic CT--initial experience

Sequential computed tomographic scanning was performed in patients with neck tumors after contrast material administration. For data analysis, a pharmacokinetic two-compartment model was employed that takes into account both capillary blood supply and bidirectional diffusion of the contrast agent across the capillary wall. This approach offers the possibility to quantitatively characterize tissue microcirculation with regional blood flow, capillary permeability, and relative compartmental volumes.

Dual dynamic contrast-enhanced MR imaging

A method was devised for obtaining dynamic contrast-enhanced T1-weighted and relaxation rate (delta R2*) images simultaneously to evaluate regional hemodynamics of the brain tumors. On a 1.5-T MR system, dual dynamic contrast-enhanced images were obtained using a gradient echo (dual echo fast field echo) pulse sequence with the keyhole technique to improve temporal and spatial resolution during a rapid bolus injection of gadopentetate dimeglumine. The dynamic T1 contrast images were obtained from the first echo: moreover. integral delta R2*dt values were calculated from the first and the second echo images. The dynamic T1 contrast images provided information about characteristic enhancement pattern (vascularization and disruption of blood-brain barrier), and the integral delta R2*dt values provided a map of regional blood pool in tumor site, peritumoral edema, and other surrounding regions of the brain. The ability to obtain dynamic contrast-enhanced T1 contrast and delta R2* imaging at the same time allows optimization of the advantages of each and thereby more information about the microvascular circulation of the brain lesions.

Evaluation of hydrocephalic periventricular radiolucency by dynamic computed tomography and xenon-computed tomography

OBJECTIVE

A common finding of computed tomography in a case of normal-pressure hydrocephalus (NPH) is periventricular radiolucency (PVL). We analyzed PVL for patients with hydrocephalus, using dynamic computed tomographic and xenon-computed tomographic techniques to differentiate NPH from similar diseases.

METHODS

Dynamic computed tomography was evaluated as a method of diagnosing NPH in 14 patients with computed tomographic findings of both PVL and ventricular dilatation. Of the 14 patients, varying degrees of clinical improvement after shunt surgery were observed in 10 (shunt-effective group) but not in the remaining 4 (shunt-ineffective group). The difference in arrival time between PVL and thalamus, the difference in peak time between PVL and anterior cerebral artery, and cerebral blood flow in PVL by xenon-computed tomographic study were analyzed.

RESULTS

The difference in arrival time between PVL and thalamus was significantly longer in the effective group than among the remaining patients. There was no significant difference in PVL/cerebral blood flow and the difference in peak time between PVL and the anterior cerebral artery between the two groups.

CONCLUSION

Dynamic computed tomographic analysis of the difference in arrival time between PVL and thalamus is useful for diagnosing NPH and predicting response to shunting.

Temporal correlation analysis of penumbral dynamics in focal cerebral ischemia

A novel temporal correlation technique was used to map the first-pass transit of iodinated contrast agents through the brain. Transit profiles after bolus injections were measured with dynamic computed tomography (CT) scanning (1 image/s over 50 s). A rabbit model of focal cerebral ischemia (n = 6) was used, and dynamic CT scans were performed at 30, 60, 90, and 120 min postocclusion. Within the ischemic core, no bolus transit was detectable, demonstrating that complete ischemia was present after arterial occlusion. In the periphery of the ischemic distribution, transit dynamics showed smaller peaks, broadened profiles, and overall delay in bolus transit. A cross-correlation method was used to generate maps of delays in ischemic transit profiles compared with normal transit profiles from the contralateral hemisphere. These maps showed that penumbral regions surrounding the ischemic core had significantly delayed bolus transit profiles. Enlargement of the ischemic core over time (from 30 to 120 min postocclusion) was primarily accomplished by the progressive deterioration of the penumbral regions. These results suggest that (a) temporal correlation methods can define regions of abnormal perfusion in focal cerebral ischemia, (b) peripheral regions of focal cerebral ischemia are characterized by delays in bolus transit profiles, and (c) these regions of bolus transit delay deteriorate over time and thus represent a hemodynamic penumbra.

Microscopic susceptibility variation and transverse relaxation: theory and experiment

Microscopic susceptibility variations invariably increase apparent transverse relaxation rates. In this paper, we present comparisons between Monte Carlo simulations and experiments with polystyrene microspheres to demonstrate that this enhanced relaxation can be explained quantitatively for both spin echo and gradient echo imaging experiments. The spheres used (1 to 30 microms), and degree of susceptibility variation (caused by 0-12 mM Dy-DTPA) covered a wide range of biologically relevant compartment sizes and contrast agent concentrations. These results show that several regimes of behavior exist, and that contrast dependence is quite different in these regimes. For a given susceptibility, delta chi, a small range of particle sizes show peak transverse relaxation. For the range of susceptibilities found in the first pass of a clinical IV contrast agent bolus, this size range is 5 to 10 microns, or roughly capillary sized compartments. In both our simulations and experiments, smaller spheres showed quadratic relaxation versus concentration curves, and larger particles showed sublinear behavior. For particles corresponding to the peak relaxivity, the relaxation-concentration curves were linear. In addition, we demonstrated that increasing the diffusion coefficient can increase, decrease, or, paradoxically, leave unaffected the apparent relaxation rate. The regime for which the diffusion coefficient is relatively unimportant corresponds to the region of peak relaxivity. By using the Bloch-Torrey equation to produce scaling rules, the specific Monte Carlo simulations were extended to more general cases. We use these scaling rules to demonstrate why we often find that susceptibility-induced relaxation rates vary approximately linearly with concentration of injected agent.

Functional perfusion and blood-brain barrier permeability images in the diagnosis of cerebral tumors by Angio CT

We performed rapid sequential CT scanning following iv injection of a bolus of contrast medium and generated three functional images relating to intravascular circulation time (rABCT), vascular volume (Vv) density and blood-brain barrier (BBB) unidirectional constant uptake rate (Ki), respectively. This was accomplished by calculating the first mathematical moment of the monitored time-density curves about the injection time and from the multiple time graph analysis described by Patlack and co-workers. A satisfactory resolution was achieved, allowing separate appreciation of changes in rABCT both in large vessels and in tissue small vessels. Combined evaluation of rABCT and Vv images allowed us to differentiate between tumors.

Functional image of dynamic computed tomography in diagnostic and prognostic evaluation of ischemic stroke within the first 6 hours

BACKGROUND AND PURPOSE

It is important to make a diagnosis before a low-density area appears on computed tomography for appropriate management of acute ischemic stroke. We report the diagnostic and prognostic usefulness of functional image of dynamic computed tomography for acute ischemic stroke.

METHODS

Forty-seven patients with ischemic strokes within 6 hours of ictus underwent dynamic computed tomography in which functional images were obtained. These findings were compared with angiographic findings, follow-up computed tomography, and clinical outcome.

RESULTS

The functional images were categorized into three groups: (1) cortical type: abnormalities on time to peak image and/or corrected mean transit time image involving mainly cortical structures (29 cases); (2) noncortical type: abnormalities on either or both images limited to noncortical structures (7 cases); and (3) normal type: no abnormalities on both images (11 cases). Cortical type as a diagnostic test for arterial trunk occlusion had a good sensitivity (100%), specificity (95%), and accuracy (98%). Infarction volume on follow-up computed tomography correlated with extension of prolonged time-to-peak area (r = .80, P < .01) and that of prolonged corrected mean transit time area (r = .81, P < .01). Cortical type was associated with significantly unfavorable outcome (P < .01).

CONCLUSIONS

Functional image of dynamic computed tomography findings predicted arterial trunk occlusion, infarction volume, and clinical outcome. Therefore, this technique would be useful not only for indicating definitive angiography and subsequent therapy but for evaluating the effectiveness of surgical or medical recanalization.

Dynamic computed tomographic imaging of regional cerebral blood flow and blood volume. A clinical pilot study

BACKGROUND AND PURPOSE

The advent of faster computed tomography scanners has evoked considerable interest in using this technology as a more practical method of regional cerebral hemodynamic evaluation than the currently available positron emission and single-photon emission computed tomography. The theoretical concepts have been worked out and validated in the laboratory by several groups. The aim of the present study was the development of a clinically useful system.

METHODS

Software was developed for dynamic computed tomography-based calculation and color-coded representation of regional cerebral blood flow and blood volume. Normal values, reproducibility, and sensitivity to acetazolamide challenge were established in 13 volunteers. The method was applied to an additional three patients with internal carotid artery occlusion and known decreased vascular reserve capacity as diagnosed by transcranial Doppler ultrasonography.

RESULTS

Normal regional cerebral blood flow was determined as 50 +/- 13 ml/100 ml per minute and normal fractional cerebral blood volume as 58 +/- 12 ml/1,000 ml (mean +/- SD). In five volunteers, two examinations were performed within 15 minutes for determination of reproducibility. Intermeasurement variability of hemispheric blood flow and blood volume was determined as +/- 23% and +/- 16%, respectively. Intravenous administration of 1 g acetazolamide resulted on the average in a 75% increase of blood flow and a 65% increase of fractional blood volume. In the patients with decreased cerebrovascular reserve capacity, baseline fractional blood volume in the ischemic hemispheres was significantly increased. Baseline regional cerebral blood flow in the ischemic territories was overestimated. Reactivity to acetazolamide of both regional blood flow and fractional blood volume was clearly reduced in the ischemic hemispheres.

CONCLUSIONS

The present results demonstrate that the method is a simple and effective means of determining regional cerebral blood volume. Spatial resolution is superior to that of the radioactive tracer methods. Hemodynamic evaluation of ischemic conditions can be performed on the basis of increased resting cerebral blood volume and a diminished increase after acetazolamide. Accuracy of cerebral blood flow measurements, on the other hand, is affected by abnormal cerebral blood volume, and corresponding adjustments need to be made in pathological conditions.

Neurological improvement after cranioplasty. Analysis by dynamic CT scan

The authors studied the changes in neurological signs and cerebral circulation by using dynamic CT scans before and after cranioplasty in six externally decompressed patients. Five of the 6 patients showed some improvements in neurological signs. The results of the dynamic CT scans in 6 cases suggested that increases of bilateral cerebral blood flow may play a role in their neurological improvement.

Functional vascular volume and blood-brain barrier permeability images by angio-CT in the diagnosis of cerebral lesions

We performed angio-CT following i.v. administration of an iodinated contrast medium and analyzed the pixel contrast time-density curves from 4 to 10 min postinjection to derive vascular and volume BBB permeability images. This was performed by applying multiple regression analysis of pixel contrast curves versus blood contrast and integrated blood contrast curves. The two regression coefficients, mapped pixel by pixel, correspond respectively to vascular volume and BBB unidirectional transport rate. Initial application in a small number of normal subjects and patients allowed us to characterize areas of steady-state contrast enhancement according to changes of vascular volume and BBB permeability.

Ultrafast MR imaging of water mobility: animal models of altered cerebral perfusion

"Single shot" magnetic resonance (MR) diffusion imaging was used to study the details of signal decay curves in experimental perturbations of cerebral perfusion induced by hypercapnia or death. Despite large perfusion increases observed with dynamic susceptibility-contrast MR imaging, no correlation with these changes was seen in either the diffusion coefficient or any other intravoxel incoherent motion (IVIM) model parameters in dog gray matter as arterial carbon dioxide pressure increased. Non-monoexponential signal decay in cat gray matter was seen both before and after death. In addition, cat gray matter demonstrated a steady decrease in the diffusion coefficient after death. These data are strong evidence that the fast component of the non-monoexponential diffusion-related signal decay is not due solely to perfusion. The authors believe that a second compartment of nonexchanging spins, most likely cerebrospinal fluid, accounts for the non-monoexponential decay.

Hemodynamic assessment of vascular malformations by angio CT with generation of functional transit time images

We performed a CT dynamic study during the first pass of an intravenously injected bolus of a iodinated contrast medium, followed by generation of the regional arm-brain circulation time (rABCT) image, in 11 patients with vascular malformations. All lesions could be detected as changes of rABCT, and the comparison with values of normal arteries and veins allowed the deduction of the hemodynamic conditions of the lesions. Seven cases showed evidence of altered distribution of rABCT in the corresponding brain hemisphere, suggesting a perfusion reserve impairment.

Evaluation of hepatic density change by dynamic CT in healthy humans and in patients with chronic liver diseases

Dynamic computed tomography using 0.4 ml/kg of 65% meglumine diatrizoate was performed to estimate pharmacokinetics of contrast media in the liver in healthy controls (N = 11), in patients with chronic viral hepatitis (N = 17), posthepatic liver cirrhosis (N = 21), and alcoholic liver cirrhosis (N = 23). The time of peak enhancement (the time interval between peak aortic and liver enhancement) was significantly different between each group. Alcoholic liver cirrhosis was the most prolonged, followed by posthepatic liver cirrhosis, chronic viral hepatitis, and finally controls. A peak enhancement time of 28 sec had a diagnostic accuracy of 97% for chronic liver diseases. A time greater than 44 sec had a diagnostic accuracy for cirrhosis of 96%. The decay time (the time from peak enhancement of the liver to the curve's center of gravity) was also significantly different between each group. Again, alcoholic liver cirrhosis was the longest, followed by posthepatic liver cirrhosis, chronic viral hepatitis, and then the controls. Dynamic computed tomography has many potential applications in studying intrahepatic physiologic events and may contain diagnostic information for chronic liver diseases.

Functional circulation time images in the diagnosis of cerebral lesions by angio CT

An application of angio CT as an adjunct to contrast enhancement evaluation of cerebral lesions is presented. We performed rapid sequential scanning following intravenous bolus injection of an iodinated contrast medium, analyzed the time contrast curves and generated a functional circulation time image. This was accomplished by calculating on a pixel-by-pixel basis the first mathematical moment of the monitored time-density curves about injection time which corresponds to regional arm brain circulation time (rABCT). This method increased the rate of detection of cerebral lesions by contrast enhancement from 30 to 90%.

Contrast agents and cerebral hemodynamics

Contrast-enhanced magnetic resonance imaging of regional cerebral hemodynamics is discussed. Techniques for measuring cerebral blood volume (CBV) have been validated in animal models and have recently been applied to human studies. Factors affecting CBV measurement in pathologic tissue are addressed. Extension of these techniques to the measurement of cerebral blood flow is presented.

Measurement of regional cerebral blood flow using ultrafast computed tomography. Theoretical aspects

Theoretical and practical limitations have prevented the measurement of regional cerebral blood flow using dynamic x-ray computed tomography. Development of the ultrafast computed tomography scanner has made it possible to overcome the practical limitations and measure changes in contrast concentration in the brain with excellent time and spatial resolution. By applying modifications of indicator dilution theory, we have derived a method to use these changes in contrast concentration determined using ultrafast computed tomography to measure the fractional vascular volume, mean transit time of blood, and blood flow within specific regions of the brain in a relatively simple and practical manner. This method could theoretically be used in the evaluation of physiological and pathophysiological alterations in cerebral blood flow.

Functional cerebral imaging by susceptibility-contrast NMR

In vivo measurement of cerebral physiology by dynamic contrast-enhanced NMR is demonstrated. Time-resolved images of the cerebral transit of paramagnetic contrast agent were acquired using a new ultrafast NMR imaging technique and a novel mechanism of image contrast based on microscopic changes in tissue magnetic susceptibility. Global hypercapnia in dogs was used to establish the relationship between susceptibility-induced signal change and brain blood volume, and the response of gray and white matter to this microvascular stimulus was measured.

Functional image of dynamic computed tomography for the evaluation of cerebral hemodynamics

We report the usefulness of functional image of dynamic computed tomography as a technique for evaluating cerebral hemodynamics. Although the technique itself has been reported, the advance of computer technology has made it possible to obtain high-resolution functional images within only a few minutes. We conducted 70 examinations on 57 patients with cerebral ischemia and correlated the findings with clinical outcome. Those patients whose abnormalities were detected on all corrected mean transit time, time to peak, and peak value images developed massive cortical infarcts. On the other hand, patients with abnormalities detected only on the corrected mean transit time image had only partial low-density lesions on follow-up computed tomograms. Patients with abnormalities detected only on the time to peak image suffered repeated transient ischemic attacks, but follow-up computed tomograms showed no low-density lesion in most cases. Single-photon emission computed tomography with N-isopropyl-p-(123I)iodoamphetamine performed at the time of functional image of dynamic computed tomography showed a high concordance of the findings in many cases, especially those with hyperacute-stage cerebral ischemia, in which the concordance rate was 90.5% (19 of 21). Comparing images constructed from different parameters, functional image of dynamic computed tomography can delineate other than hemodynamic factors, such as extent of the vascular bed or the degree of collateral circulation. Thus, functional image of dynamic computed tomography is a potentially important and useful technique for the further analysis of cerebral hemodynamics.

Perfusion imaging with NMR contrast agents

Knowledge of regional hemodynamics has widespread application for both physiological research and clinical assessment. Here we review the use of MR contrast agents to measure tissue perfusion. Two primary mechanisms of image contrast are discussed: relaxivity and susceptibility effects. Relaxivity effects result from dipolar enhancement of T1 and T2 rates. Because tissue T1 rates are intrinsically smaller, the dominant effect is shortening of T1 relaxation times. The second mechanism of image contrast is the variation in tissue magnetic field produced by heterogeneous distribution of high magnetic susceptibility agents. Quantitation of tissue perfusion requires a detailed understanding of the relation between contrast agent concentration and associated MR signal changes. Studies to date show a linear relationship between contrast agent concentration and rate change in most organs. The exact nature of this relationship in the dynamic setting of rapid contrast agent passage through the microcirculatory bed is less well established. If this relationship is known, tracer kinetic modeling can be used to calculate regional blood flow and blood volume. Data are presented which indicate that this approach is feasible, and suggest the potential of contrast-enhanced NMR for high resolution in vivo mapping of both physiology and anatomy.

Dynamic CT of cranio-cervical vascular occlusive disease

Occlusive disease of the major vascular structures of the head and neck produce classic findings on conventional angiography that are mirrored in the sequences obtained on intravenous dynamic computed tomography (IVDCT). As a complement of the initial computed tomographic (CT) workup, IVDCT provides immediate, important diagnostic information, helps to direct appropriate clinico-radiologic triage with minimum delay, and may obviate more invasive studies in selected clinical settings.

Dynamic CT brain scanning in the haemodynamic evaluation of cerebral arterial occlusive disease

Dynamic cerebral CT scanning (DCT) was used to quantitatively analyse the haemodynamic effects of extracranial and intracranial arterial occlusive lesions in 17 patients with TIA's or minor cerebral infarcts. Using DCT and gamma variate curve fitting, mean transit times were determined for the terminal internal carotid arteries, middle cerebral arteries and middle cerebral-supplied Sylvian cortex at the level of the Circle of Willis. Six patients were studied sequentially, four before and after transcranial bypass surgery. No arterial or tissue delays were found in patients without haemodynamic arterial lesions or cortical infarcts. Seven of nine patients with haemodynamic, extracranial carotid lesions showed ipsilateral delays in arterial or tissue transit times. Tissue delays usually correlated with CT or clinical evidence of infarction. Improved haemodynamics in patients re-studied correlated with the effects of surgery or clinical recovery. DCT has several important limitations but has the potential to provide additional haemodynamic information about the cerebral circulation in selected patients with cerebral arterial occlusive disease.

Cerebral peak time mapping uniformity of the value of peak time

By studying patients with clinically suspected cerebrovascular occlusive disease using dynamic CT technique, we have shown that contrast peak attenuation of the whole normal cerebral tissue was uniformly reached at the same time within a given individual. On the other hand, the delay of peak time was shown to correlate with some vascular occlusive mechanism. The main implications of delayed contrast peak time can be summarized as follows: Reduced regional blood flow due to stenosis or occlusion of main vessel without tissue damage. Delayed contrast arrival time due to occlusion of main vessel, but with sufficient circulation. Tissue organically injured by cerebrovascular occlusion.

Dynamic computed tomography of the brain

Dynamic computed tomography (CT) scanning was performed on five normal subjects, 38 patients with cerebrovascular disorder, and 20 patients with brain tumours. It consisted of performing six rapid sequential scans after intravenous bolus injection of iodinated contrast medium. By gamma variate fit technique, five features (corrected first moment, area, peak, time to peak, and per cent terminal height) were obtained from a time-density curve in a region of interest. We compared these features in one side of the brain with those in the corresponding contralateral side and tried to get information about cerebral perfusion. These results showed that by this technique we are able to detect arterial occlusions or Moya moya diseases non-invasively and know earlier and more clearly the re-establishment of circulation in the occluded arteries and of extravasation of contrast medium in ischaemic regions than by conventional CT scanning. In arteriovenous malformations, serial images of dynamic CT scanning demonstrated the anatomical details of the nidus, and the afferent and efferent vessels. We can also identify an exact extent of tumoral invasion of the brain in patients with malignant brain tumor.

Dynamic computerized tomography in the assessment of hemangioma

Three hemangiomas of the face and orbit are studied with dynamic computerized tomography (CT). This noninvasive technique has the advantage of simultaneous bone and soft tissue visualization at the time of peak contrast enhancement, and it precludes the potential complications of arteriography and biopsy. The hemangiomas are precisely delineated with intense contrast; the sharp peak and rapid uninterrupted runoff of the computer-generated plot of contrast concentration (CT number) versus time is characteristic of this lesion. It is suggested that the progress of hemangioma involution may be documented by the change in contrast distribution in sequential dynamic CT studies. Dynamic CT is recommended for the evaluation of head and neck vascular tumors.

Dynamic computed tomographic scans in experimental brain abscess

Dynamic computed tomographic scans were performed in an experimental brain abscess model to establish criteria that could be utilized in abscess staging. The vascular phase of the time-density curves did not differentiate cerebritis and capsule stages. The amount of residual enhancement after the first pass of an intra-arterial contrast bolus differed between major abscess stages, the greater residual enhancement being noted in the capsule stage.

Progress in cerebrovascular disease: local cerebral blood flow by xenon enhanced CT

A noninvasive technique for measuring local cerebral blood flow (LCBF) by xenon enhanced x-ray transmission computed tomography (CT) has been developed an reported quite extensively in recent years. In this method, nonradioactive xenon gas in inhaled and the temporal changes in radiographic enhancement produced by the inhalation are measured by sequential computed tomography. Time dependent xenon concentrations within various tissue segments in the brain are used to derive both local partition coefficient (lambda) and LCBF. An assessment of this method reveals that although it provides functional mapping of blood flow with excellent anatomic specificity, there are distinct limitations. The assumptions underlying this methodology are examined and problems associated with various potential applications of this technique are discussed.

In vivo mapping of local cerebral blood flow by xenon-enhanced computed tomography

A noninvasive technique has been developed to measure and display local cerebral blood flow (LCBF) in vivo. In this procedure, nonradioactive xenon gas is inhaled and the temporal changes in radiographic enhancement produced by the inhalation are measured by sequential computerized tomography. The time-dependent xenon concentrations in various anatomical units in the brain are used to derive both the local partition coefficient and the LCBF. Functional mapping of blood flow with excellent anatomical specificity has been obtained in the baboon brain. The response of LCBF to stimuli such as changes in carbon dioxide concentrations as well as the variability in LCBF in normal and diseased tissue can be easily demonstrated. This method is applicable to the study of human physiology and pathologic blood flow alterations.