Physiologic changes in regional cerebral blood flow defined by xenon-enhanced CT scanning

Anesthesia-related Ramifications of Benjamin Franklin's Ether-based Refrigeration

In 1758, Benjamin Franklin froze water by means of the evaporation of diethyl ether. Diethyl ether became the coolant in early mechanical refrigerators and ice makers. Refrigeration advances by Carl von Linde and others provide medical oxygen from the air, liquid nitrogen for cryopreservation and cryoablation, xenon for inhaled anesthesia, and liquid helium for supercooling of magnetic resonance image scanners.

Novel imaging approaches to cerebrovascular disease

Imaging techniques available to the physician treating neurovascular disease have substantially grown over the past several decades. New techniques as well as advances in imaging modalities continuously develop and provide an extensive array of modalities to diagnose, characterize, and understand neurovascular pathology. Modern noninvasive neurovascular imaging is generally based on computed tomography (CT), magnetic resonance (MR) imaging, or nuclear imaging and includes CT angiography, CT perfusion, xenon-enhanced CT, single-photon emission CT, positron emission tomography, magnetic resonance angiography, MR perfusion, functional magnetic resonance imaging with global and regional blood oxygen level dependent imaging, and magnetic resonance angiography with the use of the noninvasive optional vessel analysis software (River Forest, Ill). In addition to a brief overview of the technique, this review article discusses the clinical indications, advantages, and disadvantages of each of those modalities.

Effect of low-xenon and krypton supplementation on signal/noise of regional CT-based ventilation measurements

Xenon computed tomography (Xe-CT) is used to estimate regional ventilation by measuring regional attenuation changes over multiple breaths while rebreathing a constant Xe concentration ([Xe]). Xe-CT has potential human applications, although anesthetic properties limit [Xe] to ≤35%. We investigate effects of lower [Xe], including a low [Xe]-krypton (Kr) combination, on time constant (TC) determination. Six anesthetized sheep were scanned prone and supine using multidetector row CT. Lungs were imaged by respiratory gating during washin of a 30%, 40%, 55% Xe, and a 30% Xe/30% Kr mixture. Using Kr avoids unwanted effects of Xe. Mean TCs, coefficients of variation (CV), and half confidence intervals (CI)/mean served as indexes of sensitivity to noise. Mean supine and prone TCs of three [Xe] values were not significantly different. Average CVs of TCs increased from 57% (55% Xe), 58% (40% Xe), and 73% (30% Xe) ( P < 0.05: paired t-tests; 30% Xe vs. higher [Xe]). Monte Carlo simulation indicated a CV based on inherent image noise was 8% for 55% Xe and 17% for 30% Xe ( P < 0.05). Adding 30% Kr to 30% Xe gave a washin signal equivalent to 40% Xe. Half CI/mean using the 30% Xe/30% Kr mixture was not significantly different from 55 and 40% Xe. Although average TCs were not affected by changes in [Xe], the higher CV and half CI/mean suggested reduced signal-to-noise ratio at the 30% [Xe]. The 30% Xe/30% Kr mixture was comparable to that of 40% Xe, providing an important agent for CT-based assessment of regional ventilation in humans.

Computed tomography studies of lung ventilation and perfusion

With the emergence of multidetector-row computed tomography (CT) it is now possible to image both structure and function via use of a single imaging modality. Breath-hold spiral CT provides detail of the airway and vascular trees along with texture reflective of the state of the lung parenchyma. Use of stable xenon gas wash-in and/or wash-out methods using an axial mode of the CT scanner whereby images are acquired through gating to the respiratory cycle provide detailed images of regional ventilation with isotropic voxel dimensions now on the order of 0.4 mm. Axial scanning during a breath hold and gating to the electrocardiogram during the passage of a sharp bolus injection of iodinated contrast agent provide detailed images of regional pulmonary perfusion. These dynamic CT methods for the study of regional lung function are discussed in the context of other methods that have been used to study heterogeneity of lung function.

Physiologic implications of adding small amounts of carbon dioxide to the gas mixture during inhalation of xenon

In addition to being a physiologically active tracer of CBF, xenon (Xe) in subanesthetic concentrations produces a relatively mild lowering of carbon dioxide (CO2) in the blood and elevation of transcranial Doppler (TCD) velocity. The addition of small concentrations of CO2 (0.4-1.2%) to the inhaled mixture produced no measurable effect on end tidal (P(et)) CO2 or TCD velocity. Cerebral blood flow (CBF) alterations induced by Xe are minimized by allowing P(et)CO2 to fall, permitting quantitative measurement of CBF by the Xe/CT CBF method.

Acute cerebral revascularization

Fifteen patients evaluated for acute cerebral ischemia underwent acute cerebral revascularization between March, 1979, and May, 1983. Clinical presentation included crescendo transient ischemic attacks (TIA's) in eight cases, progressing neurological dysfunction in three cases, and completed nonfluctuating deficits in four cases. Nine patients received intravenous heparin but did not improve neurologically. The patients with crescendo TIA's were operated on within 4 hours of their last event; those with progressing deficits were operated on while the deficit was developing, and those with established deficits were operated on 4, 6, 9, and 12 hours, respectively, after the event began. The clinical picture for 10 patients had resolved within 10 hours after surgery. One patient with crescendo TIA's, two with progressing deficits, and two with established deficits had postoperative residual deficits, of which three were mild and two severe. One patient, who had a saphenous vein graft to the middle cerebral artery, developed an intracerebral hematoma. In this prospective noncontrolled nonrandomized study, acute cerebral revascularization was performed safely, had limited risks, and offered the potential to help some patients. Further controlled randomized studies are indicated.

Computed tomographic measurement of local cerebral blood flow by xenon enhancement

The present technique was developed to overcome some of the disadvantages of measuring cerebral blood flow by radionuclide methods, such as poor localization of flow values and errors that result if the brain is pathological and local partition coefficients are altered. Serial CT scanning in humans was carried out during and after inhalation of 50 to 70% non-radioactive xenon. This diffusible gas with high atomic number enhanced gray matter first by 19 +/- 4 Hounsfield Units (HU) and later white matter by 24 +/- 4 HU. The regionality of flow values were cursored on CT pictures with a high spatial resolution of 4 X 4 mm (64 pixels) or 0.16cm2 X 0.5cm. In seven normal subjects, blood flow in gray matter was 82 +/- 11 ml/100gm/min and that in white matter 24 +/- 5ml/100gm/min. The partition coefficient (lambda), which is not obtainable in vivo by radionuclide scanning, was 0.9 +/- 0.1 for normal gray matter, 1.4 +/- 0.2 for normal white matter. Reduced flow, 13% in gray matter and 46% in white matter, was found in a large infarct secondary to complete occlusion of middle cerebral artery. In edematous tissue, blood flow was not significantly impaired in gray matter but was reduced to 29 to 54% in white matter. Local lambda values were reduced to 0.6-0.9 in edematous tissue, and 0.3-0.7 in infarction. This method appears to have several advantages over conventional isotope methods of measuring cerebral blood flow and provides useful clinical and research information.

Local cerebral blood flow values as estimated with diffusible tracers: validity of assumptions in normal and ischemic tissue

A theoretical assessment is made of the validity of assumptions underlying the theory for estimating local cerebral blood flow with diffusible tracer in the tissue under normal and ischemic conditions. First, Kety's derivation of equations that have commonly been used for calculating local cerebral blood flow values is examined in order to define the problems and assumptions. Second, the brain:blood partition coefficient of diffusible tracer, lambda, and the diffusion-limited factor, m, under normal and ischemic conditions are reviewed. An examination of the literature suggested that contrary to common belief, lambda appears to change very little in ischemia if the tissue constituents remain unchanged, whereas m does change with ischemia if the diffusible tracer used is greatly diffusion-limited in the exchange between brain and blood. Even when a gas with an m value close to unity is used as the diffusible tracer, the prolonged mean transit time of blood through the ischemic tissue would make it difficult to maintain the exponential assumptions. As part of the ischemic tissue became infarcted, which is the case with most stroke patients, so the assumptions of homogeneous perfusion would become invalid. This inevitably renders it difficult to estimate local cerebral blood flow with diffusible tracer in ischemic tissue containing an infarcted mass.