Coronary artery stenoses: assessment with contrast-enhanced electron-beam CT and axial reconstructions.

PURPOSE: To evaluate the usefulness of electron-beam computed tomography (CT) for identification of coronary artery stenoses. MATERIALS AND METHODS: Coronary angiography and contrast material-enhanced, electrocardiographically triggered electron-beam CT of the heart were performed in 23 patients. With axial CT images and axial maximum intensity projection reconstructions, the coronary arteries were assessed by two observers blinded to the results of angiography. RESULTS: Cardiac motion artifact (unsharpness) precluded evaluation of the right coronary artery (RCA) in six subjects and the left circumflex coronary artery (LCX) in one patient. With the vessels degraded by motion artifact eliminated from analysis, overall sensitivity of electron-beam CT for hemodynamically significant stenoses was 88%, and specificity was 79%. In the left anterior descending coronary artery (LAD), sensitivity was 93% and specificity was 63%; in the LCX, sensitivity was 100% and specificity was 67%; and in the RCA, sensitivity was 67% and specificity was 77%. The presence of coronary artery calcification did not have an effect on sensitivity for stenoses, but it did decrease specificity. CONCLUSION: Electron-beam CT angiography can depict hemodynamically significant stenoses in the LAD and LCX with a sensitivity of more than 90%. The presence of coronary artery calcification resulted in decreased specificity but no appreciable change in sensitivity.

Evaluation of electron beam CT coronary angiography in healthy subjects.

OBJECTIVE: This study evaluated the performance characteristics of electrocardiographically triggered, contrast-enhanced electron beam CT (EBCT) in defining the coronary artery lumen in healthy subjects. SUBJECTS AND METHODS: The coronary arteries of 11 healthy young men (mean age, 24 years old) were evaluated by contrast-enhanced EBCT. Measured parameters included degree of luminal enhancement, intravascular contrast-to-noise ratio, apparent luminal diameter, and length of continuously visualized lumen (100-H threshold for diameter and length measurements). RESULTS: Aortic blood pool attenuation was 44 +/- 5 H (mean +/- SD) before and 278 +/- 35 H after IV injection of contrast material. Contrast-to-noise ratios ranged from a high of 10.0 +/- 2.6 in the proximal right coronary artery to a low of 3.2 +/- 2.7 in the distal left circumflex artery, decreasing from proximal to distal within each vessel. Apparent luminal diameters were as follows: left main coronary artery, 4.5 +/- 0.6 mm; left anterior descending artery, 3.7 +/- 0.5 mm; left circumflex artery, 2.9 +/- 0.6 mm; and right coronary artery, 3.5 +/- 0.5 mm. The mean lengths of visualized lumina were as follows: left main coronary artery, 10 +/- 4 mm; left anterior descending artery, 65 +/- 26 mm; left circumflex artery, 45 +/- 20 mm; and right coronary artery, 58 +/- 24 mm. CONCLUSION: EBCT angiography can reveal the lumen of long segments of the major coronary arteries.

Measurement of collateral blood flow in a porcine model of aortic coarctation by velocity-encoded cine MRI.

The purpose of this study was to investigate the time course of development of collateral blood flow in an animal model of aortic coarctation. A juxtaductal aortic stenosis (model coarctation) was surgically created in five juvenile pigs. MRI was performed preoperatively, 1 to 2 days postoperatively, and 2 to 10 weeks postoperatively. Aortic blood flow was measured by velocity-encoded cine MR (VENC-MR). The percent change in aortic blood flow (delta BF) from proximal to distal descending thoracic aorta was calculated, and a multiple-comparison paired t test used to assess changes in delta BF over time. Invasive flow measurements were obtained in one animal before sacrifice using an ultrasonic probe. delta BF preoperatively was -2 +/- 8% (mean +/- SE). delta BF increased to 32 +/- 7% (mean +/- SE, P = .022) 2 days postoperatively and 55 +/- 19% (P = .032) 2 to 8 weeks postoperatively. Invasive measurements were in qualitative agreement with the VENC-MR data. VENC-MR is an accurate noninvasive method of measuring collateral blood flow in aortic coarctation. Recruitment and development of collateral flow pathways occur rapidly in an animal model.

Detection of extracapsular extension of prostate carcinoma with endorectal and phased-array coil MR imaging: multivariate feature analysis.

PURPOSE: To determine which magnetic resonance (MR) imaging features are most predictive of extracapsular extension of prostate carcinoma. MATERIALS AND METHODS: In 77 patients who had stage pT2 or pT3 prostate carcinoma, MR images were retrospectively reviewed by three readers with varying experience in interpretation of endorectal coil images of the prostate gland. MR imaging features assessed were broad tumor contact, smooth capsular bulge, irregular bulge, obliteration of the rectoprostatic angle, and asymmetry of the neurovascular bundle. Multivariate logistic regression analysis was performed to determine the relative value of each MR imaging feature. RESULTS: At multivariate analysis, obliteration of the rectoprostatic angle and asymmetry of the neurovascular bundle were most predictive of extracapsular extension, with a specificity of up to 95% and sensitivity of 38%. Poor-quality images reduced accuracy for all readers. The most experienced reader demonstrated overall accuracy of 77% in determination of extracapsular extension. CONCLUSION: Obliteration of the rectoprostatic angle and asymmetry of the neurovascular bundle were most indicative of extracapsular extension. Reader experience plays an important role in the ability to interpret prostate MR images and is an important contributor to interobserver variability.

Asymptomatic functional popliteal artery entrapment: demonstration at MR imaging.

PURPOSE: To determine if functional popliteal entrapment can occur in healthy subjects and to define the mechanism of vascular compression. MATERIALS AND METHODS: The right lower extremities were examined in 13 subjects (nine men 27-34 years of age, mean age 31 years; four women 29-44 years of age, mean age 34 years) with no symptoms of popliteal artery entrapment. Magnetic resonance (MR) and Doppler ultrasound images were obtained while the subjects were at rest and while they performed plantar flexion against resistance. RESULTS: Blood flow during plantar flexion ceased in nine of 13 subjects (69%) and was impaired in three of 13 (23%). MR images showed muscular compression of the popliteal artery at two levels: between the plantaris muscle and the medial head of the gastrocnemius muscle and between the plantaris and popliteus muscles. CONCLUSION: Functional impairment of popliteal arterial flow during plantar flexion occurs in subjects who have no symptoms of popliteal entrapment syndrome.

Three-dimensional imaging and display of renal tumors using spiral CT: a potential aid to partial nephrectomy.

OBJECTIVE: A new technique for creating three-dimensional (3D) images of renal tumors using contrast-enhanced spiral computed tomography (CT) is described and preliminarily investigated. METHODS: 3D spiral CT was employed in 2 patients before radical nephrectomy and in 5 patients before partial nephrectomy. Preoperative and postoperative image analyses were conducted to evaluate the ability of the images to depict key anatomic relationships in planning partial nephrectomies. RESULTS: 3D spiral CT defined the tumor's location and relationship to the kidney surface better than the tumor's proximity to renal hilar vessels and collecting system. Negative surgical margins were obtained in all 4 patients with renal cell carcinoma, and post-operative serum creatinine remained less than 2 mg/dL in all 5 patients after partial nephrectomy. CONCLUSIONS: This early experience suggests that 3D spiral CT can help in the planning of partial nephrectomy and in attaining complete resection of renal cell carcinoma while conserving normal renal tissue.

Membrane potential determination in large unilamellar vesicles with hexakis(2-methoxyisobutylisonitrile)technetium(I).

The accumulation of the lipophilic cation hexakis (2-methoxyisobutylisonitrile)technetium (99mTc-MIBI) within large unilamellar vesicles made from egg phosphatidylcholine was examined as a function of time and membrane potential (Em). Equilibrium distribution occurred within minutes at 30 degrees C. The transmembrane distribution of Tc-MIBI was measured at Em = 0 mV and at a series of negative membrane potentials. The distribution of Tc-MIBI was in close agreement with the Nernst equation for passive distribution of a permeant ion across a bilayer, permitting the membrane potential to be predicted from Tc-MIBI distribution. In this respect, Tc-MIBI behaves similarly to other radioprobes of membrane potential, but with unique properties including high specific activity (10(9) Ci/mol), rapid kinetics of distribution, low potential-independent binding, and short half-life (6.02 h). The results indicate a mechanism for tissue accumulation of Tc-MIBI in vivo that may in part account for its utility in clinical imaging of ischemic myocardium.

Common carotid artery bifurcation: evaluation with spiral CT. Work in progress.

To determine the utility of spiral computed tomography (CT) in evaluation of carotid artery stenosis, spiral CT images of 20 patients were compared with images obtained with conventional angiography (20 patients), ultrasound (US) (15 patients), and magnetic resonance (MR) angiography (six patients). The category of stenosis was determined for each internal carotid artery on the basis of the percentage of narrowing: mild = less than 30%, moderate = 30%-69%, and severe = 70%-99%. Occlusions were also noted. The degree of carotid stenosis determined with spiral CT correlated with that determined with conventional angiography in 92% of cases, with that determined with US in 97% of cases, and with that determined with MR angiography in 100% of cases. Calcifications and large ulcers were also well delineated. Spiral CT provided an accurate anatomic depiction of the carotid bifurcation, which could be helpful in preoperative evaluation. The major disadvantage of the technique was the need to postprocess data to remove veins, calcifications, and bone structures from the images.

Kinetic analysis of phasic inhibition of neuronal sodium currents by lidocaine and bupivacaine.

Phasic ("use-dependent") inhibition of sodium currents by the tertiary amine local anesthetics, lidocaine and bupivacaine, was observed in voltage-clamped node of Ranvier of the toad, Bufo marinus. Local anesthetics were assumed to inhibit sodium channels through occupation of a binding site with 1:1 stoichiometry. A three-parameter empirical model for state-dependent anesthetic binding to the Na channel is presented: this model includes two discrete parameters that represent the time integrals of binding and unbinding reactions during a depolarizing pulse, and one continuous parameter that represents the rate of unbinding of drug between pulses. The change in magnitude of peak sodium current during a train of depolarizing pulses to 0 mV was used as an assay of the extent of anesthetic binding at discrete intervals; estimates of model parameters were made by applying a nonlinear least-squares algorithm to the inhibition of currents obtained at two or more depolarizing pulse rates. Increasing the concentration of drug increased the rate of binding but had little or no effect on unbinding, as expected for a simple bimolecular reaction. The dependence of the model parameters on pulse duration was assessed for both drugs: as the duration of depolarizing pulses was increased, the fraction of channels binding drug during each pulse became significantly larger, whereas the fraction of occupied channels unbinding drug remained relatively constant. The rate of recovery from block between pulses was unaffected by pulse duration or magnitude. The separate contributions of open (O) and inactivated (I) channel binding of drug to the net increase in block per pulse were assessed at 0 mV: for lidocaine, the forward binding rate ko was 1.3 x 10(5) M-1 s-1, kl was 2.4 x 10(4) M-1 s-1; for bupivacaine, ko was 2.5 x 10(5) M-1 s-1, kl was 4.4 x 10(4) M-1 s-1. These binding rates were similar to those derived from time-dependent block of maintained Na currents in nodes where inactivation was incomplete due to treatment with chloramine-T. The dependence of model parameters on the potential between pulses (holding potential) was examined. All three parameters were found to be nearly independent of holding potential from -70 to -100 mV. These results are discussed with respect to established models of dynamic local anesthetic-Na channel interactions.

Kinetics of local anesthetic inhibition of neuronal sodium currents. pH and hydrophobicity dependence.

This study assesses the importance of local anesthetic charge and hydrophobicity in determining the rates of binding to and dissociation from neuronal Na channels. Five amide-linked local anesthetics, paired either by similar pKa or hydrophobicity, were chosen for study: lidocaine, two tertiary amine lidocaine homologs, a neutral lidocaine homolog, and bupivacaine. Voltage-clamped nodes of Ranvier from the sciatic nerve of Bufo marinus were exposed to anesthetic externally, and use-dependent ("phasic") block of Na current was observed. Kinetic analysis of binding (blocking) rates was performed using a three parameter, piecewise-exponential binding model. Changes in extracellular pH (pHo) were used to assess the role of drug protonation in determining the rate of onset of, and recovery from, phasic block. For those drugs with pKa's in the range of pHo tested (6.2-10.4), the forward binding rate during a depolarizing pulse increased at higher pH, consistent with an increase in either intracellular or intramembrane concentration of drug. The rate for unbinding during depolarization was independent of pHo. The dissociation rate between pulses also increased at higher pHo. The pHo dependence of the dissociation rate was not consistent with a model in which the cation is trapped relentlessly within a closed channel. Quantitative estimates of dissociation rates show that the cationic form of lidocaine dissociates at a rate of 0.1 s-1 (at 13 degrees C); for neutral lidocaine, the dissociation rate is 7.0 s-1. Furthermore, the apparent pKa of bound local anesthetic was found to be close to the pKa in aqueous solution, but different than the pKa for "free" local anesthetic accessible to the depolarized channel.

Tonic and phasic block of neuronal sodium currents by 5-hydroxyhexano-2',6'-xylide, a neutral lidocaine homologue.

The effects of a neutral lidocaine homologue, 5-hydroxyhexano-2',6'-xylidide (5-HHX), on the kinetics and amplitude of sodium currents in voltage-clamped amphibian nerve fibers are described. 5-HHX produced two types of sodium current inhibition: (a) tonic block, in resting fibers (IC50 approximately 2 mM), and (b) phasic block, an additional, incremental inhibition, in repetitively depolarized fibers (frequency greater than 1 Hz). The kinetics of phasic block were characterized by a single-receptor, switched-affinity model, in which binding increases during a depolarizing pulse and decreases between pulses. In the presence of 4 mM 5-HHX, binding increased during pulses from -80 to 0 mV, with an apparent rate constant of 6.4 +/- 1.4 s-1. Binding decreased between pulses with an apparent rate constant of 1.1 +/- 0.3 s-1. There was little effect of extracellular pH on the kinetics of phasic block. These findings demonstrate that neither the presence of a terminal amine nor a net charge on a local anesthetic is required for phasic block of sodium channels.