Voxel-wise assessment of lung aeration changes on CT images using image registration: application to acute respiratory distress syndrome (ARDS).

PURPOSE: (1) To improve the accuracy of global and regional alveolar-recruitment quantification in CT scan pairs by accounting for lung-tissue displacements and deformation, (2) To propose a method for local-recruitment calculation. METHODS: Recruitment was calculated by subtracting the quantity of non-aerated lung tissues between expiration and inspiration. To assess global recruitment, lung boundaries were first interactively delineated at inspiration, and then they were warped based on automatic image registration to define the boundaries at expiration. To calculate regional recruitment, the lung mask defined at inspiration was cut into pieces, and these were also warped to encompass the same tissues at expiration. Local-recruitment map was calculated as follows: For each voxel at expiration, the matching location at inspiration was determined by image registration, non-aerated voxels were counted in the neighborhood of the respective locations, and the voxel count difference was normalized by the neighborhood size. The methods were evaluated on 120 image pairs of 12 pigs with experimental acute respiratory distress syndrome. RESULTS: The dispersion of global- and regional-recruitment values decreased when using image registration, compared to the conventional approach neglecting tissue motion. Local-recruitment maps overlaid onto the original images were visually consistent, and the sum of these values over the whole lungs was very close to the global-recruitment estimate, except four outliers. CONCLUSIONS: Image registration can compensate lung-tissue displacements and deformation, thus improving the quantification of alveolar recruitment. Local-recruitment calculation can also benefit from image registration, and its values can be overlaid onto the original image to display a local-recruitment map. They also can be integrated over arbitrarily shaped regions to assess regional or global recruitment.

Reliability of the nitrogen washin-washout technique to assess end-expiratory lung volume at variable PEEP and tidal volumes.

BACKGROUND: End-expiratory lung volume measurement by the nitrogen washin-washout technique (EELVWI-WO) may help titrating positive end-expiratory pressure (PEEP) during acute respiratory distress syndrome (ARDS). Validation of this technique has been previously performed using computed tomography (EELVCT), but at mild PEEP levels, and relatively low fraction of inspired oxygen (FiO2), which may have insufficiently challenged the validity of this technique. The aims of this study were (1) to evaluate the reliability of EELVWI-WO measurements at different PEEP and V T during experimental ARDS and (2) to evaluate trending ability of EELVWI-WO to detect EELV changes over time. METHODS: ARDS was induced in 14 piglets by saline lavage. Optimal PEEP was selected during a decremental PEEP trial, based on best compliance, best EELVWI-WO, or a PEEP-FiO2 table. Eight V T (4 to 20 mL · kg(-1)) were finally applied at optimal PEEP. EELVWI-WO and EELVCT were determined after ARDS onset, at variable PEEP and V T. RESULTS: EELVWI-WO underestimated EELVCT with a non-constant linear bias, as it decreased with increasing EELV. Limits of agreement for bias were ±398 mL. Bias between methods was greater at high PEEP, and further increased when high PEEP was combined with low V T. Concordance rate of EELV changes between consecutive measurements was fair (79%). Diagnostic accuracy was good for detection of absolute EELV changes above 200 mL (AUC = 0.79). CONCLUSIONS: The reliability of the WI-WO technique is critically dependent on ventilatory settings, but sufficient to accurately detect EELV change greater than 200 mL.

Assessment of age modulated vascular inflammation in ApoE-/- mice by USPIO-enhanced magnetic resonance imaging.

OBJECTIVE: Inflammation within atherosclerotic lesions increases the risk for plaque rupture and thrombosis. A functional approach to plaque analysis is the intravenous administration of ultrasmall superparamagnetic particles of iron oxide (USPIO) that enables visualization of macrophages residing in the plaques. In this study, we sought to characterize the age-related inflammatory status associated with atherosclerosis lesion progression in ApoE mice using USPIO-enhanced magnetic resonance imaging (MRI). MATERIALS AND METHODS: A total of 24 ApoE mice were divided in 4 groups (N = 6) and were given a high cholesterol diet from 6 weeks of age to the end of the protocol. One group per MR time point was investigated at 10, 16, 24, and 34 weeks of age. Each MR examination was performed on a 4.7 T scanner and consisted of baseline and 48 hours post-USPIO administration imaging sessions. P904, a USPIO contrast agent (Guerbet, Paris, France) with a potential for plaque macrophage targeting, was used.Vessel wall area measurements were performed on high resolution spin echo transverse images. Multi-echo gradient-echo images acquired with the same geometry were used to calculate T2* maps of the vessel wall using a pixel-by-pixel monoexponential fit. A one-way analysis of variance was performed to characterize the temporal variation of vessel wall area, susceptibility artifact area, baseline, and post-USPIO T2* values. MR measurements were correlated with the histologic findings. RESULTS: A significant increase was found in the aortic wall area from 1.4 ± 0.2 at 10 weeks to 2.0 ± 0.3 mm at 34 weeks of age (P < 0.05). Concerning the post-USPIO MRI, signal loss regions, with patterns spanning from focal to the complete disappearance of the vessel wall, were observed on all postcontrast images. A significant increase in the size of the susceptibility artifact was observed from 0.5 ± 0.2 to 2.4 ± 1.0 at 24 weeks (P < 0.05) and to 2.0 ± 0.9 mm at 34 weeks (P < 0.05).The T2* values calculated on the 48 hours post-USPIO images were shorter compared with baseline. The decrease was 34% ± 16% at 10 weeks, 57% ± 11% at 16 weeks, 57% ± 16% at 24 weeks, and 48% ± 13% at 34 weeks.The Pearson's correlation test between measurement of aortic wall area performed on both MR images and histologic analysis showed a statistically significant correlation (r = 0.695 and P < 0.05). A correlation was also obtained between the signal loss area and the macrophages covered area (r = 0.68 and P < 0.05). CONCLUSIONS: This study demonstrated the feasibility of USPIO-enhanced MRI in assessing the inflammatory status related to the temporal progression of the atherosclerosis plaque in ApoE transgenic mice model of atherosclerosis. In our experimental conditions, the vascular inflammation peak, for the ApoE mice feeding high-fat/high-cholesterol diet is measured between 16 and 24 weeks of age.

Free breathing hyperpolarized 3He lung ventilation spiral MR imaging.

OBJECTIVES: Current clinical hyperpolarized He lung ventilation MR imaging protocols rely on the patient's ability to control inhalation and exhalation and hold their breath on demand. This is impractical for intensive care unit patients under ventilation or for pediatric populations under the age of 3 to 4 years. To address this problem, we propose a free-breathing protocol for hyperpolarized He lung ventilation spiral imaging. This approach was evaluated in vitro and on rabbits. MATERIALS AND METHODS: The protocol was implemented on a clinical 1.5-T magnetic resonance imaging scanner. Ventilation images were acquired using a spiral sequence, in vitro on a lung phantom and in vivo on rabbits, the animal breathing freely from a gas reservoir. Dynamic spiral ventilation images were reconstructed using retrospective Cine synchronization. Magnetic resonance (MR) signal dynamics was modeled taking account of gas inflow and outflow, radiofrequency depolarization and oxygen-induced relaxation. RESULTS: Cine ventilation images acquired in spontaneously breathing rabbits were reconstructed with a temporal resolution of 50 milliseconds. Gas volume variations and time-to-maximum maps were obtained. The numerical model was validated in vitro and in vivo with various gas mixtures. Ventilation parameters (functional residual capacity, tidal volume, and alveolar pO2) were extracted from the MR signal dynamics. CONCLUSIONS: Ventilation imaging can be performed at tidal volume using a simple experimental protocol, without any ventilation device or breath-hold period. Acquisition time, SNR and pO2 decay can be optimized using the developed numerical model. Free-breathing ventilation images can be obtained without artifacts related to motion or gas flow. Lastly, parametric maps can be derived from the time-resolved ventilation images and physiological parameters extracted from the global signal dynamics.

In vivo quantification of regional myocardial blood flow: validity of the fast-exchange approximation for intravascular T1 contrast agent and long inversion time.

In the present study we investigated the effects of water exchange between intra- and extravascular compartments on absolute quantification of regional myocardial blood flow (rMBF) using a saturation-recovery sequence with a rather long inversion time (TI, 176 ms) and a T1-shortening intravascular contrast agent (CMD-A2-Gd-DOTA). Data were acquired in normal and ischemically injured pigs, with radiolabeled microsphere flow measurements used as the gold standard. Five water exchange rates (fast, 6 Hz, 3 Hz, 1 Hz, and no exchange) were tested. The results demonstrate that the fast-exchange approximation may be appropriate for rMBF quantification using the described experimental setting. Relaxation rate change (DeltaR1) analysis improved the accuracy of the analysis of rMBF compared to the MR signal. In conclusion, the current protocol could provide sufficient accuracy for estimating rMBF assuming fast exchange and a linear relationship between signal and tissue concentration when quantification of precontrast T1 is not an option.

Importance of parametric mapping and deconvolution in analyzing magnetic resonance myocardial perfusion images.

AIM: : We sought to improve the clinical interpretation of first-pass myocardial magnetic resonance perfusion. Parametric analyses of the myocardial distribution of the contrast agent have been proposed. The objective of the present study was to compare the effectiveness of visual analysis and of a parametric approach in an animal model under acquisition conditions as close as possible to clinical reality. METHOD: : Experiments were conducted in vivo with various kinds of pharmacological stimulation in normal pigs and in pigs with stenosis of the left circumflex coronary artery. First-pass MR images and parametric maps were first assessed by medical experts. MR parameters, the myocardial signal intensity variation DeltaSI, ascending up-slope, and rMBF (blood flow calculated by fast discrete ARMA deconvolution) were then compared with blood flow measurements using radioactive microspheres. RESULTS AND CONCLUSIONS: : Interobserver agreement was 57% and 81% and accuracy 53% and 81%, for visual and for parametric map analysis, respectively. For deconvolution parameters, a linear relationship y = 371 + 1.27x, r = 0.78 was obtained between rMBF calculated by ARMA and the radioactive microsphere blood flow. Moreover, the fast and robust parametric mapping of rMBF by the discrete ARMA method allows MR evaluation of myocardial perfusion independently of hemodynamic conditions.

Quantification of multicontrast vascular MR images with NLSnake, an active contour model: in vitro validation and in vivo evaluation.

Vessel-wall measurements from multicontrast MRI provide information on plaque structure and evolution. This requires the extraction of numerous contours. In this work a contour-extraction method is proposed that uses an active contour model (NLSnake) adapted for a wide range of MR vascular images. This new method employs length normalization for the purpose of deformation computation and offers the advantages of simplified parameter tuning, fast convergence, and minimal user interaction. The model can be initialized far from the boundaries of the region to be segmented, even by only one pixel. The accuracy and reproducibility of NLSnake endoluminal contours were assessed on vascular phantom MR angiography (MRA) and high-resolution in vitro MR images of rabbit aorta. An in vivo evaluation was performed on rabbit and clinical data for both internal and external vessel-wall contours. In phantoms with 95% stenoses, NLSnake measured 94.3% +/- 3.8%, and the accuracy was even better for milder stenoses. In the images of rabbit aorta, variability between NLSnake and experts was less than interobserver variability, while the maximum intravariability of NLSnake was equal to 1.25%. In conclusion, the NLSnake technique successfully quantified the vessel lumen in multicontrast MR images using constant parameters.

Mapping myocardial perfusion with an intravascular MR contrast agent: robustness of deconvolution methods at various blood flows.

Evaluation of quantitative parameters such as regional myocardial blood flow (rMBF), blood volume (rMBV), and mean transit time (rMTT) by MRI is gaining acceptance for clinical applications, but still lacks robust postprocessing methods for map generation. Moreover, robustness should be preserved over the full range of myocardial flows and volumes. Using experimental data from an isolated pig heart preparation, synthetic MR kinetics were generated and four deconvolution approaches were evaluated. These methods were then applied to the first-pass T(1) images of the isolated pig heart using an intravascular contrast agent and rMBF, rMBV and rMTT maps were generated. In both synthetic and experimental data, the fit between calculated and original data reached equally good results with the four techniques. rMBV was the only parameter estimated correctly in numerical experiments. Moreover, using the algebraic method ARMA, abnormal regions were well delineated on rMBV maps. At high flows, rMBF was underestimated at the experimental noise level. Finally, rMTT maps appeared noisy and highly unreliable, especially at high flows. In conclusion, over the myocardial flow range, i.e., 0-400 ml/min/100g, rMBF identification was biased in presence of noise, whereas rMBV was correctly identified. Thus, rMBV mapping could be a fast and robust way to detect abnormal myocardial regions.