Quantitative assessment of regional cerebral blood flows by perfusion CT studies at low injection rates: a critical review of the underlying theoretical models

Abstract

IntroductionBrain metabolism is provided almost exclusively by glucose oxidation. The high metabolic demand of the brain, associated with its inability to store energy, requires an important and stable cerebral perfusion, in order to guarantee a normal cerebral function. Mean cerebral blood flow (CBF) values range around 50 cc per 100 g and per minute. They are, however, higher in young people and lower in the elderly. Regional cerebral blood flows (rCBF) are twice or thrice superior in the gray matter, compared with those in the white matter. rCBF are increased in activated cerebral areas. The rCBF changes relating to cerebral activity modifications may reach 10±20 %, and up to 40 % in case of extreme metabolic conditions, such as coma or convulsion [1, 2].Complex autoregulation processes ensure both the adjustment of rCBF to local energetic needs, determined by the activity level of local neurons and the stability of CBF despite changes in systemic arterial pressure. This rCBF autoregulation is controlled by intricate neurobiochemical mechanisms, involving sensitivity to blood pressure, blood pCO 2 and pH. Brain vascular autoregulation notably allows for a vascular dilatation when the systemic pressure tends to lower, in order to keep a constant CBF. This vascular dilatation leads in turn to an increased cerebral blood volume (CBV) [3, 4].The rCBF alterations are encountered in a variety of pathological conditions, the most frequent being strokes. Strokes affect 500,000 patients every year in the United States and represent the third cause of death in Eur. Radiol. (2001) Abstract Viability of the cerebral parenchyma is dependent on cerebral blood flow (CBF), which is usually kept in a very narrow range due to efficient autoregulation processes and can be altered in a variety of pathological conditions. An accurate method allowing for a quantitative assessment of regional cerebral blood flows (rCBF) and available for the routine clinical practice would, for sure, greatly contribute to improving the management of patients with cerebrovascular diseases. Different imaging techniques are now available to evaluate rCBF: positron emission tomography; single photon emission CT; stable-xenon CT; perfusion CT; and perfusion MRI. Each of these imaging techniques uses an indicator, with specific biological properties, and is supported by a model, which consists of a few simplifying assumptions, necessary to state and solve the equations giving access to rCBF. The obtained results are more or less reliable, depending on whether modeling hypotheses are fulfilled by the used indicator. The purpose of this article is to review the various supporting models in the assessment of rCBF, with special emphasis on perfusion CT studies at low injection rates and on iodinated contrast material used as an indicator.

Prognostic accuracy of cerebral blood flow measurement by perfusion computed tomography, at the time of emergency room admission, in acute stroke patients

“…Using a weighting factor of 0.0023mSv/(mGy ϫ cm) for the brain, the cerebral effective dose is 3.4mSv, which is quite similar to the dose level for a standard cerebral CT examination (2.5mSv). 44,46,47 Perfusion CT examination with data analysis according to the central volume principle 48 allows accurate quantitative assessment of both the rCBF and the rCBV 24 as well as definition of the cerebral infarct and penumbra, as described above. Our proposed method for the calculation of cerebral penumbra and infarct maps from the rCBF and rCBV maps provided by perfusion CT data analysis relies on (1) the reported rCBF threshold of ischemia and (2) preserved (penumbra) or altered (infarct) autoregulatory mechanisms.…”

The Cerebral Perfusion of Circulation Disturbances. Part Ii (Ct- And Mri Semiotics, Patterns of Pathological Changes). Review

“…Чувствительность метода при применении в первые часы после начала инсульта составля-ет 96%, а специфичность 98% [10]. Достаточно давно и по сегодняшний день ПКТ является наи-более адаптированной к повседневной клини-ческой практике универсальной [11] методикой оценки мозговой перфузии при острых и хро-нических нарушениях мозгового кровообраще-ния, доступна в большинстве крупных клиник [12,13]. Однако основная доля выполняемых на сегодняшний день процедур перфузионных исследований головного мозга при нарушениях мозгового кровообращения происходит в России в региональных специализированных центрах помощи сосудистым больным и научно-исследо-вательских учреждениях.…”

“…Выявление с помо-щью ПКТ зон потенциально жизнеспособной и необратимо повреждённой ткани при фор-мировании ишемического очага должно быть основано не только на определении мозгового кровотока, но и на оценке соотношения между кровотоком, объёмом крови и длительностью прохождения крови в повреждённой области, то есть всеми регистрируемыми параметрами перфузии [11,21,22]. В современной отечественной и мировой литературе имеются достаточно ограниченные данные, касающиеся повышения или понижения перфузионных показателей у больных с ишеми-ческими поражениями головного мозга, а также широкий диапазон их нормальных значений, в зависимости от способов обработки первичных данных полученных при применении различных аппаратов и диагностических методик [13,23].…”

Development of MRI Scanning Technique that is Comfortable for Patients with Anxiety Disorder

“…Furthermore, it has been demonstrated that the noise and characteristics of the operation of the machine, which suppresses helium evaporation, can cause extreme uneasiness. According to a report, people often experience greater anxiety than was once the case, due to the mental and psychological conflict induced by complex social structures, environment, and stress; at least 15% of the total population suffers from anxiety disorder [8]. In reality, a large number of examinees express anxiety during the MRI scanning procedure.…”