Measurement of the Charge-Averaged Elastic Lepton-Proton Scattering Cross Section by the OLYMPUS Experiment.

We report the first measurement of the average of the electron-proton and positron-proton elastic scattering cross sections. This lepton charge-averaged cross section is insensitive to the leading effects of hard two-photon exchange, giving more robust access to the proton's electromagnetic form factors. The cross section was extracted from data taken by the OLYMPUS experiment at DESY, in which alternating stored electron and positron beams were scattered from a windowless gaseous hydrogen target. Elastic scattering events were identified from the coincident detection of the scattered lepton and recoil proton in a large-acceptance toroidal spectrometer. The luminosity was determined from the rates of Møller, Bhabha, and elastic scattering in forward electromagnetic calorimeters. The data provide some selectivity between existing form factor global fits and will provide valuable constraints to future fits.

Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering Determined by the OLYMPUS Experiment.

The OLYMPUS Collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, R_{2γ}, a direct measure of the contribution of hard two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01 GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight scintillators detected elastically scattered leptons in coincidence with recoiling protons over a scattering angle range of ≈20° to 80°. The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved gas electron multiplier and multiwire proportional chamber detectors at 12°, as well as symmetric Møller or Bhabha calorimeters at 1.29°. A total integrated luminosity of 4.5 fb^{-1} was collected. In the extraction of R_{2γ}, radiative effects were taken into account using a Monte Carlo generator to simulate the convolutions of internal bremsstrahlung with experiment-specific conditions such as detector acceptance and reconstruction efficiency. The resulting values of R_{2γ}, presented here for a wide range of virtual photon polarization 0.456<ε<0.978, are smaller than some hadronic two-photon exchange calculations predict, but are in reasonable agreement with a subtracted dispersion model and a phenomenological fit to the form factor data.

Predictors of patient response to pulmonary thromboendarterectomy.

OBJECTIVE: We sought to identify imaging features that help predict surgical success in patients undergoing thromboendarterectomy. MATERIALS AND METHODS: Thirty-nine consecutive patients who underwent pulmonary angiography and thromboendarterectomy during 1995 and 1996 were included. Thirty-four underwent helical CT angiography. Measurements of postoperative pulmonary vascular resistance were compared with preoperative imaging features and preoperative pulmonary vascular resistance. RESULTS: The best imaging indicators of a relatively high postoperative pulmonary vascular resistance were the extent of small vessel disease identified on CT angiograms as segments with abnormal perfusion but normal segmental arteries (p = 0.005) and the extent of central disease (p = 0.015). Combined with preoperative pulmonary vascular resistance, these features had a strong correlation with postoperative outcome (p = 0.0005). Segmental arterial disease seen on both conventional angiography and CT angiography correlated poorly with surgical outcome. CONCLUSION: In patients with chronic thromboembolic pulmonary hypertension, CT angiographic evidence of extensive central vessel disease and limited small vessel involvement indicates a favorable surgical outcome.

Identifying the cause of unilateral hypoperfusion in patients suspected to have chronic pulmonary thromboembolism: diagnostic accuracy of helical CT and conventional angiography.

PURPOSE: To determine the prevalence of unilateral hypoperfusion in patients suspected to have chronic thromboembolism (CTE), to identify the most common cause of hypoperfusion, and to compare the accuracy of helical computed tomographic (CT) angiography with that of conventional angiography in helping to determine the cause. MATERIALS AND METHODS: Radionuclide lung scan reports showed asymmetric hypoperfusion in 47 of 410 consecutive patients referred because of suspected CTE. Twenty-seven patients had unilateral or predominantly unilateral perfusion abnormalities. Each pulmonary angiogram and CT angiogram in these patients was interpreted independently by two readers blinded to clinical information and surgical outcome. Surgical confirmation of the diagnosis was available in 39 of the 47 patients with asymmetric hypoperfusion. RESULTS: Unilateral (n = 11) or predominantly unilateral hypoperfusion (n = 16) was found in 6.6% (27 of 410 patients) of patients referred, and CTE was the most common cause. The accuracies of CT angiogram readers (reader 1, 83%; reader 2, 89%) were greater than those of conventional angiogram readers (reader 1, 73%; reader 2, 65%) for distinguishing CTE from other causes. CONCLUSION: Unilateral hypoperfusion occurred in 6.6% of our study population, most frequently because of CTE. CT angiography is an excellent diagnostic alternative to conventional angiography for distinguishing patients with CTE from those with other causes.

Intravascular gas as an incidental finding at US after blunt abdominal trauma.

PURPOSE: To describe the occurrence and importance of intravascular gas at ultrasonography (US) during the initial examination of patients after blunt abdominal trauma. MATERIALS AND METHODS: Reports of all abdominal US examinations performed at one institution for blunt trauma from October 1995 through June 1996 were reviewed. The charts of patients with intravascular gas were examined to determine the mechanism of injury, associated findings, and clinical outcome. RESULTS: A total of 730 patients underwent abdominal US examinations during the 9 months of study; five had intravascular gas demonstrated. Two patients had portal venous gas, one had hepatic venous gas, and two had inferior vena caval gas. Four of the five patients were involved in motor vehicle accidents, and one had been assaulted. In patients in whom follow-up studies were obtained, there was no evidence of intravascular gas at US or computed tomography. No cause was found at imaging or clinical examination. CONCLUSION: Intravascular gas may occur as a transient incidental finding after blunt abdominal trauma.

Chronic thromboembolism: diagnosis with helical CT and MR imaging with angiographic and surgical correlation.

PURPOSE: To evaluate the accuracy of identification of central and segmental chronic thromboembolic disease on helical computed tomographic (CT) scans and on magnetic resonance (MR) images. MATERIALS AND METHODS: Radiologic findings in 55 patients suspected of having chronic thromboembolic pulmonary hypertension were analyzed; these included findings from angiography (n = 55), helical CT (n = 47), and MR imaging (n = 26). Forty patients underwent thromboendarterectomy. CT and MR images were independently interpreted by two readers for the presence of thromboembolic material in central and segmental vessels. Surgical findings and angiographic findings were the reference standards for disease in central and segmental vessels, respectively. RESULTS: Central vessel disease was determined more accurately with helical CT scans (accuracy of 0.79 for each of the two readers) than with angiograms (accuracy of 0.74) or with MR images (accuracy of 0.39 and 0.46 for two readers). Segmental vessel disease was also more accurately determined with CT scans (accuracy of 0.75 and 0.76 for two readers) than with MR images (accuracy of 0.61 and 0.57 for two readers). CONCLUSION: Helical CT is a useful alternative to conventional angiography for diagnosis of chronic thromboembolism but may not be sufficient for selecting candidates for surgery in all cases.

Accuracy of MR angiography compared with radionuclide scanning in identifying the cause of pulmonary arterial hypertension.

OBJECTIVE: MR imaging has proven accurate in identifying patients with chronic thromboembolic pulmonary hypertension (CTEPH). However, we know of no attempt to distinguish patients with CTEPH from patients with other causes of pulmonary arterial hypertension (PAH). Primary pulmonary hypertension (PPH) is the disease most frequently confused with CTEPH. We examined patients with CTEPH or PPH and control subjects to identify MR imaging features that might distinguish CTEPH from PPH, to compare the accuracy of MR angiography (MRA) with that of radionuclide scanning, and to determine the cardiac and pulmonary vascular measurements in these groups. SUBJECTS AND METHODS: T1-weighted and two-dimensional multiplanar spoiled gradient-recalled scans were obtained in 30 patients with CTEPH who had undergone conventional pulmonary angiography, 10 patients with PPH, and 13 control subjects with no known vascular disease. Ventilation-perfusion scans were available in all patients with CTEPH and in six of the 10 patients with PPH. The MR scans were assessed independently by two radiologists who evaluated the appearance of segmental vessels and noted the presence of mosaic perfusion. Cardiac chambers and pulmonary vessels were measured on T1-weighted spin-echo scans. RESULTS: The two radiologists used MR angiograms to reliably distinguish between patients with CTEPH and those with PPH. The accuracy of MR angiograms matched that of ventilation-perfusion scans (92%). On T1-weighted scans, the two radiologists used cardiac and pulmonary vascular measurements to distinguish patients with PAH from control subjects but failed to distinguish between patients with CTEPH and those with PPH. CONCLUSION: MRA is useful in distinguishing patients with CTEPH from those with PPH. In this population, MRA had an accuracy that was identical to that of radionuclide scanning. Vascular and cardiac measurements made on MR scans reliably identified patients with PAH but did not distinguish between patients with CTEPH and those with PPH.

Magnetic resonance detection of acute pulmonary emboli in a canine model with pathologic correlation.

RATIONALE AND OBJECTIVES: The authors evaluated the accuracy of magnetic resonance (MR) imaging in depicting acute pulmonary emboli at the lobar, segmental, and subsegmental levels. METHODS: The authors induced 29 autologous emboli in five dogs and confirmed their location with angiography and anatomic dissection. MR images obtained with four sequences were independently evaluated by two radiologists to detect emboli in each vascular segment. Sensitivities, specificities, and accuracies were calculated at segmental and lobar levels. RESULTS: The fast short-tau inversion-recovery images provided the greatest conspicuity and highest overall accuracy (reader 1 = 74.3%, reader 2 = 80%). Accuracy of two-dimensional fast multiplanar spoiled gradient-recalled-echo images was limited by spatial resolution (reader 1 = 71.4%, reader 2 = 74.3%). The fast spin-echo T2-weighted and spin-echo T1-weighted sequences were intermediate in their depiction of acute emboli. Similar results were seen at the lobar level. CONCLUSION: MR images depict acute pulmonary embolism at the segmental and lobar levels with reasonable accuracy. Fast short-tau inversion-recovery sequences provided the greatest sensitivity and accuracy.