Errors in cerebral blood flow determinations by xenon-enhanced computed tomography due to estimation of arterial xenon concentrations

Anniversary paper. Development of x-ray computed tomography: the role of medical physics and AAPM from the 1970s to present

The AAPM, through its members, meetings, and its flagship journal Medical Physics, has played an important role in the development and growth of x-ray tomography in the last 50 years. From a spate of early articles in the 1970s characterizing the first commercial computed tomography (CT) scanners through the "slice wars" of the 1990s and 2000s, the history of CT and related techniques such as tomosynthesis can readily be traced through the pages of Medical Physics and the annals of the AAPM and RSNA/AAPM Annual Meetings. In this article, the authors intend to give a brief review of the role of Medical Physics and the AAPM in CT and tomosynthesis imaging over the last few decades.

Cerebral Vasoreactivity: A Comparison of Color Velocity Imaging Quantification and Stable Xenon-Enhanced CT

OBJECTIVE

Compromised cerebral vasoreactivity increases the risk of stroke. In this study, we sought to determine whether extracranial arterial blood flow volume measured on color velocity imaging quantification could be predictive of cerebral vasoreactivity after the administration of acetazolamide.

SUBJECTS AND METHODS

Cerebral blood flow and extracranial arterial blood flow volume of 35 patients with symptomatic carotid occlusive disease were measured before and after the administration of acetazolamide on stable xenon CT and color velocity imaging quantification, respectively. The changes in unilateral extracranial arterial blood flow volume and respective hemispheric cerebral blood flow were compared. The mean difference in the percentage of change in flow volume, the 95% limit of agreement, and Cohen's kappa coefficient were calculated.

RESULTS

A total of 64 unilateral extracranial arterial blood flow volume changes were successfully compared with the changes in the ipsilateral hemispheric cerebral blood flow. The mean difference in percentage of change in flow volume between the two techniques was 4.7%, with the 95% limit of agreement ranging from -90.2% to 99.7%. Cohen's kappa coefficient was 0.41 (95% confidence interval, 0.13-0.68; p = 0.001).

CONCLUSION

The performance of color velocity imaging quantification for evaluating cerebral vasoreactivity is comparable to that of transcranial Doppler sonography. Because color velocity imaging quantification is not as limited as transcranial Doppler sonography, it could be an ideal complementary tool to transcranial Doppler sonography. More studies are required to define its clinical value.

Pulmonary function affects the quantification of rCBF by non-invasive xenon methods

Estimates of regional cerebral blood flow (rCBF) by non-invasive xenon methods (133-xenon inhalation, xenon-enhanced computed tomography (Xe/CT) and 133-xenon iv injection) are frequently applied in the diagnosis and evaluation of patients suffering from diseases which cause disturbances in the cerebrovascular circulation. These methods all depend on an estimate of the arterial xenon concentration curve derived non-invasively from measurements of the end-tidal xenon concentration curve and used as brain input function in the Kety equation. We have studied the influence of impaired pulmonary gas exchange on the end-tidal and arterial xenon concentration curves in nine anaesthetized pigs by simultaneously measurements of both the end-tidal xenon and arterial xenon concentration curves. Computer simulations were performed to determine the deviations in the calculated rCBF values when using the end-tidal as compared to the arterial xenon concentration curve as brain input function. The results indicated that impairment of the pulmonary gas exchange caused a significant further 'delay' in the arterial xenon concentration curve in comparison to the end-tidal xenon concentration curve. The time constants of arterial curve delay were 11.9 s in the normal pulmonary group, 21 s in the right lung atelectasis group, and 19.7 s in the left pulmonary artery occlusion group. Accordingly, computer simulations indicated a statistically significant 'underestimation' of rCBF due to: (1) pulmonary gas exchange; (2) high or low levels of rCBF; (3) partition coefficient (lambda) of gray and white matter; and (4) xenon inhalation protocols. Our results indicate that quantitative measurements of rCBF by non-invasive xenon methods are markedly affected by deviations between the end-tidal and arterial xenon concentration curve, so that estimates of flow thresholds for infarction are problematic under conditions of impaired pulmonary gas exchange.

A PC-based neural network for on-line measurement of regional cerebral blood flow

The measurement of regional cerebral blood flow has been used in diagnoses for many cerebral disorders. However, the complexity of data processing makes the technique only possible for off-line study. This paper explores a PC-based neural net technique for on-line determination of the blood flow rate. Experimental results justify the efficiency of the proposed methodology and raise the possibility of designing a portable system for time-critical applications.

A simplified method for measuring cerebral blood flow with xenon-enhanced computed tomography

The measurement of cerebral blood flow using the xenon-enhanced computed tomography (XECT) technique requires that the build-up of xenon in both brain tissue and end-tidal expired air be determined as a function of time. Monitoring of the former is carried out using CT scanning and the latter, most often, using a thermoconductivity analyser or mass spectrometer. This paper examines the possibility of greatly simplifying the XECT technique by eliminating the need for either thermoconductivity analyser or mass spectrometer. In the proposed approach, the patient's expired air is channelled through the scan field using a flexible plastic tube and sampled by the CT scanner in conjunction with the build-up of xenon in brain tissue. Phantom measurements have demonstrated the ability of the CT scanner to detect variations in the xenon concentration in expired air while computer simulations have shown that errors arising as a result of the proposed methodology are small compared to other inherent sources in the XECT technique.

The role of noise in the measurement of cerebral blood flow and partition coefficient using xenon-enhanced computed tomography

Monte Carlo simulations have been used to study the accuracy which can be expected in the quantification of blood flow and the partition coefficient using xenon-enhanced computed tomography in the presence of noise. We have demonstrated that the markedly asymmetric frequency distribution of estimates increases in size rapidly with an increase in the standard error of the input CT data. On the basis of our results, we recommend that controllable sources of noise (eg. CT number drift) be corrected and that estimates be obtained by averaging CT data and then fitting, rather than averaging blood flow and partition coefficients derived from individual pixels, as the latter procedure results in the introduction of considerable bias.

Xenon computed tomography measuring cerebral blood flow in the determination of brain death in children

Local cerebral blood flow was measured using stable xenon computed tomography in 21 children, 10 of whom were clinically brain dead and had electrocerebral silence as determined by electroencephalography. Radioisotopic brain scanning in 9 patients showed no visible cerebral activity in all patients and minimal residual sagittal sinus activity in 4. In this population, mean cerebral blood flow as measured by xenon computed tomography was 1.3 +/- 1.6 ml/min/100 gm. Respiratory support was discontinued in 8 patients, and 2 patients had cardiac arrest. Eleven profoundly comatose children who did not meet all clinical criteria for brain death and who had markedly suppressed but not isoelectric electroencephalograms had an average cerebral blood flow of 33.5 +/- 16.3 ml/min/100 gm. There was no difference in cerebral blood flow in those children who survived (30.4 +/- 16.3 ml/min/100 gm; n = 7) compared with those who died acutely (38.3 +/- 14.3 ml/min/100 gm; n = 4). Two patients who survived had average total flows of only 11.8 and 12.1 ml/min/100 gm. Our findings suggest that in infants and children older than 1 month, (1) cerebral blood flow below approximately 10 ml/min/100 gm is consistent with clinical brain death, (2) cerebral blood flow of less than 5 ml/min/100 gm is consistent with no flow as demonstrated by radionuclide techniques, and (3) flow of more than 10 to 15 ml/min/100 gm is associated with the potential for survival.

Stable xenon enhanced computed tomography in the study of clinical and pathologic correlates of focal ischemia in baboons

When the lateral striate arteries of baboons are occluded, an immediate cessation of blood flow followed by a transient, minimal restitution of flow occurs in that vascular distribution. These findings are evident from serial xenon/computed tomography cerebral blood flow imaging. In our study, infarction consistently accompanied arterial occlusion for 6 hours or more. The xenon/computed tomography method provides a sensitive, noninvasive technique for examining sequential alterations of cerebral blood flow in small regions deep within the brain. This methodology for recording cerebral blood flow permits correlative studies of cerebral infarction, clinically and experimentally, and allows reasonable inferences about the probabilities of neural tissue damage.