Emergency Lane-Change Maneuvers of Autonomous Vehicles

This paper addresses the issue of collision avoidance using lane-change maneuvers. Of particular interest is to determine the minimum distance beyond which an obstacle cannot be avoided at a given initial speed. Using a planar bicycle model, we first compute the sharpest dynamically feasible maneuver by minimizing the longitudinal distance of a lane transition, assuming given initial and free final speeds. The minimum distance to an obstacle is then determined from the path traced by the optimal maneuver. Plotting the minimum distance in the phase plane establishes the clearance curve, a valuable tool for planning emergency maneuvers. For the bicycle model, the clearance curve is shown to closely correlate with the straight line produced by a point mass model. Examples demonstrate the use of the clearance curve for planning safe avoidance maneuvers.

Comparative Kinetics of Microvascular Inulin and 99mTc-Labelled Diethylenetriaminepenta-Acetic Acid Exchange

1. After simultaneous intravenous injection as a mixture, 99mTc-labelled diethylenetriaminepentaacetic acid (99mTc-DTPA; molecular mass 492 Da) and inulin (∼6000 Da) gave arterial plasma clearance curves consisting of three exponentials, the time courses of which were not significantly different between the two solutes. 2. The ratio of 99mTc-DTPA to inuiin concentration in antecubital venous plasma (normalized to the ratio in arterial plasma at 30 s) was 0.6, significantly less than unity, within 2 min after intravenous injection, but increased to reach unity by 60 min. The minimun concentration ratio of 99mTc-DTPA to inulin in arterial plasma was 0.75 at 4 min, also rising to just above unity at 60 min. 3. The extraction fraction from plasma to interstitial space was higher for 99mTc-DTPA (∼0.5) than for inuiin (∼0.2). For both solutes, the net extraction fraction decreased with time, becoming negative at about 25 min after injection. Thereafter, the net extraction fractions remained negative, between −0.05 and −0.1, and not significantly different between the two solutes. 4. 99mTc-DTPA time—activity curves recorded over the limbs with scintillation probes were biphasic, with an initial phase corresponding closely in time with the first exponential of the arterial 99mTc-DTPA plasma clearance curve. The second phase corresponded in time to the intermediate exponential of the arterial 99mTc-DTPA plasma clearance curve. 5. The time course of net 99mTcm-DTPA extraction fraction across the forearm vascular bed was bi-exponential, with phases corresponding in time with the two phases of the limb uptake curves. 6. Deconvolution analysis of the limb time—activity curves, using the arterial time—concentration curve as the input function, gave bi-exponential 99mTc-DTPA impulse response curves in which the time courses of the exponentials corresponded with the first and intermediate exponentials of the arterial 99mTc-DTPA clearance curve. 7. The bi-exponential nature of the equilibrium of 99mTc-DTPA between vascular and interstitial compartments suggests the presence of two separate functional volumes within the interstitial space. Although 99mTc-DTPA and inuiin clearly diffuse at different rates across the endothelium, as would be expected from their disparate sizes, the similarity in the time courses of their initial exponentials and simultaneous equalization of transfer rates (i.e. when net extraction fraction was zero) is consistent with the hypothesis that inuiin moves initially into a smaller functional interstitial fluid volume than 99mTc-DTPA. The total distribution volumes, however, are not significantly different between the two solutes.

Validation of 133Xe Clearance as a Cerebral Blood Flow Measurement Technique during Cardiopulmonary Bypass

133Xe clearance to measure cerebral blood flow (CBF) was examined in 10 dogs during cardiopulmonary bypass. As a reference method, a continuous Kety–Schmidt technique (CBFKS) with 133Xe as indicator was used. Extracranial tissue was removed to directly place the 133Xe detectors on the skull, and the head was covered with a 3 mm lead shield to minimize contamination of the 133Xe clearance curve with extracranial radiation. 133Xe detectors for the Kety–Schmidt technique were embedded in a shielded brass block to minimize interference with radiation from the animal's body. 133Xe clearance data were analyzed using stochastic (CBF10, CBF15, and CBFINF) and initial slope methods (CBFIS), and the results were compared with CBFKS using linear regression. CBF15 and CBFINF yielded similar CBF values as CBFKS (CBFKS = 0.97 · CBF15 − 2.08, r = 0.92, p < 0.01; CBFKS = 1.13 · CBFINF − 1.21, r = 0.92, p < 0.01). CBF10 slightly overestimated CBFKS but still showed a close correlation to CBFKS (CBFKS = 0.89 · CBF10 − 2.58, r = 0.92, p < 0.01) and CBFIS considerably overestimated CBFKS (CBFKS = 0.60 · CBFIS − 1.27, r = 0.87, p < 0.01). With extracranial contamination of the 133Xe clearance curve minimized, all 133Xe clearance techniques used to measure CBF were consistently related to CBFKS in a constant, significant manner. 133Xe clearance therefore is a valid method to assess CBF during cardiopulmonary bypass.

Monoexponential Analysis of 133Xe Clearance Curves for Regional Cerebral Blood Flow Measurements

The theoretical properties of a monoexponential flow index, analogous to the one used earlier by other investigators for regional CBF (rCBF) measured after intraarterial injection, were investigated after the administration of 133Xe intraarterially, intravenously, and by inhalation under high and low flow conditions. The sensitivity of the flow index to changes in fast flow components or changes in the weight ratio between the fast and the slow flow compartments was found to be dependent on whichever part of the 133Xe clearance curve was used for the flow calculation and on the shape of the input function for 133Xe. Since biexponential analysis of the clearance curves includes a monoexponential approximation for each of the two components of the clearance curve corresponding to the high and the low flow “families” in the brain, the limitations of the monoexponential flow index observed are in principle also valid for the results of biexponential analysis of the clearance curves.