Feasibility of dual-energy CT in the arterial phase: Imaging after endovascular aortic repair.

OBJECTIVE: The purpose of this study was to investigate replacing unenhanced and arterial single-energy CT acquisitions after endovascular aneurysm repair with one dual-energy CT arterial acquisition. SUBJECTS AND METHODS: Thirty patients underwent arterial dual-energy CT (80 and 140 kVp) and venous single-energy CT (120 kVp) after endovascular aneurysm repair, and the radiation doses were compared with those of a standard triple-phase protocol. Both virtual unenhanced and arterial images were generated with dual-energy CT. Images were reviewed clinically for detection of endoleaks and evaluation of stent and calcium appearance. The aortic luminal attenuation on virtual unenhanced CT images was compared with that on previously acquired true unenhanced images. Virtual unenhanced, arterial, and venous images were compared for thrombus attenuation. Single-energy CT and dual-energy CT images were compared for noise. RESULTS: Replacement of two (unenhanced, arterial) of three single-energy CT acquisitions with one dual-energy CT acquisition resulted in 31% radiation dose savings. All images were clinically interpretable. Thoracic (32 +/- 2 vs 35 +/- 4 HU) and abdominal (30 +/- 3 vs 35 +/- 5 HU) aortic attenuation was similar on virtual unenhanced and true unenhanced images. Thrombus attenuation was similar on virtual unenhanced (32 +/- 6 HU), arterial phase (33 +/- 7 HU), and venous phase (34 +/- 6 HU) images. Decreased stent and calcium attenuation was observed at some locations on virtual unenhanced images. Noise in the thoracic (10 +/- 1 HU) and abdominal (12 +/- 2 HU) aorta was lower on virtual unenhanced images than on true unenhanced images (13 +/- 4 HU, 19 +/- 5 HU). Noise was comparable for dual-energy and single-energy CT (thorax, 16 +/- 2 vs 13 +/- 2 HU; abdomen, 21 +/- 3 vs 23 +/- 5 HU). CONCLUSION: Virtual unenhanced and arterial phase images derived from dual-energy CT can replace true unenhanced and arterial phase single-energy CT images in follow-up after endovascular aneurysm repair (except immediately after the procedure), providing comparable diagnostic information with substantial dose savings.

CT-measured regional specific volume change reflects regional ventilation in supine sheep.

Computer tomography (CT) imaging techniques permit the noninvasive measurement of regional lung function. Regional specific volume change (sVol), determined from the change in lung density over a tidal breath, should correlate with regional ventilation and regional lung expansion measured with other techniques. sVol was validated against xenon (Xe)-CT-specific ventilation (sV) in four anesthetized, intubated, mechanically ventilated sheep. Xe-CT used expiratory gated axial scanning during the washin and washout of 55% Xe. sVol was measured from the tidal changes in tissue density (H, houndsfield units) of lung regions using the relationship sVol = [1,000(Hi - He)]/[He(1,000 + Hi)], where He and Hi are expiratory and inspiratory regional density. Distinct anatomical markings were used to define corresponding lung regions of interest between inspiratory, expiratory, and Xe-CT images, with an average region of interest size of 1.6 +/- 0.7 ml. In addition, sVol was compared with regional volume changes measured directly from the positions of implanted metal markers in an additional animal. A linear relationship between sVol and sV was demonstrated over a wide range of regional sV found in the normal supine lung, with an overall correlation coefficient (R(2)) of 0.66. There was a tight correlation (R(2) = 0.97) between marker-measured volume changes and sVol. Regional sVol, which involves significantly reduced exposure to radiation and Xe gas compared with the Xe-CT method, represents a safe and efficient surrogate for measuring regional ventilation in experimental studies and patients.

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

Static versus prospective gated non-breath hold volumetric MDCT imaging of the lungs.

RATIONALE AND OBJECTIVES: The study's aim is to establish lung-imaging methods that provide for the ability to image the lung under dynamic non-breath hold conditions while providing "virtual breath hold" quantifiable volumetric image data sets. Static breath hold images are used as the gold standard for evaluating these virtual breath hold images in both a phantom and sheep. MATERIALS AND METHODS: Axial methods for gating image acquisition to multiple points in the respiratory cycle interleaved with incremental table stepping during multidetector-row computed tomographic (MDCT) scanning were developed. Data sets are generated over multiple breaths, providing volume images representative of multiple points within a respiratory cycle. To determine the reproducibility and accuracy of the methods, six anesthetized sheep were studied by means of MDCT in nongated and airway-pressure (P(awy))-gated modes in which P(awy) was 0, 7, and 15 cm H2O. RESULTS: No significant differences were found between coefficients of variation in air volume measured from repeated static scans (1.74% +/- 1.78%), gated scans: inspiratory (1.2% +/- 0.44%) or expiratory gated (1.39% +/- 0.98%), or between static (1.74% +/- 1.78%) and gated (1.39% +/- 0.98%) scanning at similar P(awy) (P > .1). Measured air volumes were larger from static versus gated scans by 5.85% +/- 3.77% at 7 cm H2O and 4.45% +/- 3.6% at 15 cm H2O of P(awy) (P < .05), consistent with hysteresis. Differences between air volumes at 7 and 15 cm H2O measured from either static or gated scans or that delivered by a super syringe were insignificant (P < .05). Visual accuracy of three-dimensional anatomic geometry was achieved, and landmark certainty was within 1 mm across respiratory cycles. CONCLUSIONS: A method has been shown that provides for accurate gating to respiratory signals during axial scanning. High-resolution volumetric image data sets are achievable while the scanned subject is breathing. Images are quantitatively similar to breath hold images, with differences likely explained by known pressure-volume hysteresis effects.

Differences in regional wash-in and wash-out time constants for xenon-CT ventilation studies.

UNLABELLED: Xenon-enhanced computed tomography (Xe-CT) has been used to measure regional ventilation by determining the wash-in (WI) and wash-out (WO) rates of stable Xe. We tested the common assumption that WI and WO rates are equal by measuring WO-WI in different anatomic lung regions of six anesthetized, supine sheep scanned using multi-detector-row computed tomography (MDCT). We further investigated the effect of tidal volume, image gating (end-expiratory EE versus end-inspiratory EI), local perfusion, and inspired Xe concentration on this phenomenon. RESULTS: WO time constant was greater than WI in all lung regions, with the greatest differences observed in dependent base regions. WO-WI time constant difference was greater during EE imaging, smaller tidal volumes, and with higher Xe concentrations. Regional perfusion did not correlate with WI-WO. We conclude that Xe-WI rate can be significantly different from the WO rate, and the data suggest that this effect may be due to a combination of anatomic and fluid mechanical factors such as Rayleigh-Taylor instabilities set up at interfaces between two gases of different densities.

Maximizing quantitative accuracy of lung airway lumen and wall measures obtained from X-ray CT imaging.

To objectively quantify airway geometry from three-dimensional computed tomographic (CT) images, an idealized (circular cross section) airway model is parameterized by airway luminal caliber, wall thickness, and tilt angle. Using a two-dimensional CT slice, an initial guess for the airway center, and the full-width-half-maximum principle, we form an estimate of inner and outer airway wall locations. We then fit ellipses to the inner and outer airway walls via a direct least squares fit and use the major and minor axes of the ellipses to estimate the tilt and in-plane rotation angles. Convolving the airway model, initialized with these estimates, with the three-dimensional scanner point-spread function forms the predicted image. The difference between predicted and actual images is minimized by refining the model parameter estimates via a multidimensional, unconstrained, nonlinear minimization routine. When optimization converges, airway model parameters estimate the airway inner and outer radii and tilt angle. Results using a Plexiglas phantom show that tilt angle is estimated to within +/-4 degrees and both inner and outer radii to within one-half pixel when a "standard" CT reconstruction kernel is used. By opening up the ability to measure airways that are not oriented perpendicular to the scanning plane, this method allows evaluation of a greater sampling of airways in a two-dimensional CT slice than previously possible. In addition, by combining the tilt-angle compensation with the deconvolution method, we provide significant improvement over the previous full-width-half-maximum method for assessing location of the luminal edge but not the outer edge of the airway wall.