Effect of respiratory phases on MR lung signal intensity and lung conspicuity using segmented multiple inversion recovery turbo spin echo (MIR-TSE)

Lung water estimation on cardiac magnetic resonance imaging for predicting adverse cardiovascular outcomes in patients with heart failure

OBJECTIVES

Pulmonary congestion is a central feature of heart failure (HF) seen in acute decompensated state as well as in chronic stable disease. The present study sought to determine whether simplified cardiac magnetic resonance imaging (CMR)-derived lung water density (LWD) measurement has prognostic relevance in predicting adverse cardiovascular outcomes in patients with HF and left ventricular ejection fraction (LVEF)<50%.

METHODS

Eighty consecutive patients referred for CMR with HF and LVEF<50% along with 22 healthy age- and sex-matched controls were prospectively recruited. LWD was the lung-to-liver signal intensity ratio multiplied by 70% (estimated hepatic water density). The primary endpoint was composite of all-cause mortality or HF-related hospitalization within 6 months from CMR.

RESULTS

The mean LWD was significantly higher in HF patients compared to healthy controls (19.78 ± 6.1 vs 13.6 ± 2.3; p < 0.001). The mean LWD was significantly different among patients with NYHA class I/II and NYHA class III/IV (17.88 ± 4.8 vs 21.77 ± 1.08; p = 0.004). At 6 months, the primary endpoint was reached in 12 (15%) patients. Patients with "wet lungs" (LWD > 18.1%) had higher incidence of adverse cardiovascular outcomes compared to patients with "dry lungs". LWD was an independent predictor of adverse cardiovascular outcomes in multivariable analysis. At the optimal cut-off of LWD > 23.38%, the sensitivity and specificity were 91.67 and 91.18%, respectively, to predict adverse cardiovascular outcomes.

CONCLUSION

LWD on CMR is independently associated with increased risk of mortality and HF-related hospitalization in HF patients with LVEF<50%.

ADVANCES IN KNOWLEDGE

Non-invasive quantitative estimation of LWD on CMR can improve risk stratification and guide management in HF patients.

Quantification of lung water in heart failure using cardiovascular magnetic resonance imaging

BACKGROUND

Pulmonary edema is a cardinal feature of heart failure but no quantitative tests are available in clinical practice. The goals of this study were to develop a simple cardiovascular magnetic resonance (CMR) approach for lung water quantification, to correlate CMR derived lung water with intra-cardiac pressures and to determine its prognostic significance.

METHODS

Lung water density (LWD, %) was measured using a widely available single-shot fast spin-echo acquisition in two study cohorts. Validation Cohort: LWD was compared to left ventricular end-diastolic pressure or pulmonary capillary wedge pressure in 19 patients with heart failure undergoing cardiac catheterization. Prospective Cohort: LWD was measured in 256 subjects, including 121 with heart failure, 82 at-risk for heart failure and 53 healthy controls. Clinical outcomes were evaluated up to 1 year.

RESULTS

Within the validation cohort, CMR LWD correlated to invasively measured left-sided filling pressures (R = 0.8, p < 0.05). In the prospective cohort, mean LWD was 16.6 ± 2.1% in controls, 17.9 ± 3.0% in patients at-risk and 19.3 ± 5.4% in patients with heart failure, p < 0.001. In patients with or at-risk for heart failure, LWD >  20.8% (mean + 2 standard deviations of healthy controls) was an independent predictor of death, hospitalization or emergency department visit within 1 year, hazard ratio 2.4 (1.1-5.1, p = 0.03).

CONCLUSIONS

In patients with heart failure, increased CMR-derived lung water is associated with increased intra-cardiac filling pressures, and predicts 1 year outcomes. LWD could be incorporated in standard CMR scans.

Lung Parenchymal Analysis on Dynamic MRI in Thoracic Insufficiency Syndrome to Assess Changes Following Surgical Intervention

General surgeons, orthopedists, and pulmonologists individually treat patients with thoracic insufficiency syndrome (TIS). The benefits of growth-sparing procedures such as Vertical Expandable Prosthetic Titanium Rib (VEPTR)insertionfor treating patients with TIS have been demonstrated. However, at present there is no objective assessment metricto examine different thoracic structural components individually as to their roles in the syndrome, in contributing to dynamics and function, and in influencing treatment outcome. Using thoracic dynamic MRI (dMRI), we have been developing a methodology to overcome this problem. In this paper, we extend this methodology from our previous structural analysis approaches to examining lung tissue properties. We process the T2-weighted dMRI images through a series of steps involving 4D image construction of the acquired dMRI images, intensity non-uniformity correction and standardization of the 4D image, lung segmentation, and estimation of the parameters describing lung tissue intensity distributions in the 4D image. Based on pre- and post-operative dMRI data sets from 25 TIS patients (predominantly neuromuscular and congenital conditions), we demonstrate how lung tissue can be characterized by the estimated distribution parameters. Our results show that standardized T2-weighted image intensity values decrease from the pre- to post-operative condition, likely reflecting improved lung aeration post-operatively. In both pre- and post-operative conditions, the intensity values decrease also from end-expiration to end-inspiration, supporting the basic premise of our results.

DC-gated high resolution three-dimensional lung imaging during free-breathing

PURPOSE

To use the acquisition of the k-space center signal (DC signal) implemented into a Cartesian three-dimensional (3D) FLASH sequence for retrospective respiratory self-gating and, thus, for the examination of the whole human lung in high spatial resolution during free breathing.

MATERIALS AND METHODS

Volunteer as well as patient measurements were performed under free breathing conditions. The DC signal is acquired after the actual image data acquisition within each excitation of a 3D FLASH sequence. The DC signal is then used to track respiratory motion for retrospective respiratory gating.

RESULTS

It is shown that the acquisition of the DC signal after the imaging module can be used in a 3D FLASH sequence to extract respiratory motion information for retrospective respiratory self-gating and allows for shorter echo times (TE) and therefore increased lung parenchyma SNR.

CONCLUSION

The acquisition of the DC signal after image signal acquisition allows successful retrospective gating, enabling the reconstruction of high resolution images of the whole human lung under free breathing conditions.

Navigator-triggered oxygen-enhanced MRI with simultaneous cardiac and respiratory synchronization for the assessment of interstitial lung disease

PURPOSE

To evaluate an optimized method for oxygen-enhanced MRI of the lung, using simultaneous electrocardiograph (ECG) and navigator triggering. To correlate oxygen-enhanced MRI with lung function tests assessing alveolar-capillary gas exchange.

MATERIALS AND METHODS

A total of 12 healthy volunteers (aged 20-32 years) and 10 patients (aged 37-87 years) with interstitial lung diseases (ILD) underwent oxygen-enhanced MRI and pulmonary functional tests (PFTs) assessing alveolar-capillary gas exchange. The paradigm room-air-oxygen-room-air was acquired with a nonselective inversion-recovery half-Fourier single-shot turbo spin-echo sequence (inversion time = 1200 msec; acquisition time = 134.5 msec; slice thickness = 20 mm; matrix size = 128 x 128), using simultaneous double triggering (navigator plus ECG trigger). Cross-correlation was performed in regions of interest (ROIs) encompassing both lungs. The number of oxygen-activated pixels over the total number of pixels in the ROIs (OAP%) of volunteers and patients was compared. OAP%s were correlated with PFTs.

RESULTS

The mean OAP% of patients was significantly lower than that of volunteers (36.7 vs. 81.7, P = 0.001). OAP% correlated with the transfer lung factor for carbon monoxide (Tlco) (r = 0.64; P = 0.002), the transfer coefficient (Kco) (r = 0.75; P = 0.001), the arterial partial pressure (r = 0.77; P < 0.001), and the saturation (r = 0.70; P < 0.001) of oxygen.

CONCLUSION

Navigator-triggered oxygen-enhanced MRI of the lung may have a potential role in the quantitative assessment of lung function in ILD.

Gravity-dependent signal gradients on MR images of the lung in supine and prone positions: a comparison with isogravitational signal variability

PURPOSE

To investigate the tendency of proton MR signal intensity (SI) gradients to be steeper in the supine than in the prone body position, and to quantify the relation between gravity-related and isogravitational changes of SI on proton MR images of the lung.

MATERIALS AND METHODS

In eight healthy volunteers, MR images were obtained in the supine and prone positions using a multiple inversion recovery turbo spin-echo (TSE) sequence. The variation in SI along the gravity-dependent direction and within isogravitational planes was measured on a pixel-by-pixel basis. Ratios of slopes were calculated for comparisons among volunteers. Comparisons of ratios were made using Fisher's exact test. Isogravitational variability was compared with the mean SI, the signal-to-noise ratio (SNR), and the image noise.

RESULTS

The average ratios of slopes showed that the overall SI gradient was steeper in the supine than the prone position, with a substantial difference in the supine/prone ratios between inspiration (1.21) and expiration (1.72). In both the supine and prone positions, gravity-dependent gradients were steeper in expiration than in inspiration (P = 0.001). The SI variability along the gravitational direction was larger than the isogravitational variability. The isogravitational variability in turn was larger than the image noise but smaller than the mean SI of the MR images.

CONCLUSION

Gravity-dependent gradients in proton MR SI are steeper in the supine than in the prone position. The magnitudes of these gradients were larger than the isogravitational signal variability, showing that MRI is sensitive to gravitationally induced effects.

Simultaneous Cardiac and Respiratory Synchronization in Oxygen-Enhanced Magnetic Resonance Imaging of the Lung Using a Pneumotachograph for Respiratory Monitoring

OBJECTIVES

We sought to evaluate an optimized method for oxygen-enhanced magnetic resonance imaging of the lung, using electrocardiogram-trigger and a pneumotachograph for simultaneous cardiac and respiratory synchronization.

MATERIALS AND METHODS

Five series of IR-SSFSE images (echo time = 28.2 milliseconds; inversion time = 1,200 milliseconds) were obtained in 6 volunteers during the ventilation-paradigm room-air/oxygen/room-air: series 1, respiratory-triggered; series 2, cardiac-triggered; series 3, cardiac-triggered and respiratory-synchronized using the signal of the pneumatic belt; series 4, cardiac-triggered and respiratory-synchronized using the external signal of the pneumotachograph; and series 5, not cardiac-triggered and respiratory-synchronized using the signal of the pneumotachograph. Standard deviations of the lung (SI(var)) and diaphragm mismatch (DM) were measured. The relative SI change (DeltaSI) was computed from room-air and oxygen-enhanced images. Parametric maps were obtained from cross-correlation analysis of the ventilation paradigm. Mean correlation coefficients (cc) and the percentage of activated pixels over the lung (Act%) were calculated from these maps. All 5 parameters were compared among the 5 series (Friedman-analysis of variance, Dunn's posthoc test).

RESULTS

In series 4, DM and SI(var) were significantly lower than in respiratory and cardiac-triggered series (DM = 4.7 vs. 14.3 and 18.4; SI(var) = 4.9 vs. 10 and 11). In the same series cc and Act% also were significantly higher than in series 1 and 2 (cc = 0.86 vs. 0.7 and 0.6; Act% = 71.3 vs. 44.7 and 41.2). DeltaSI was not significantly different among all series.

CONCLUSIONS

Effective respiratory and cardiac synchronization can be achieved in oxygen-enhanced magnetic resonance imaging of the lung, using a pneumotachograph for real-time targeting of end-expiration.

Sauerstoff-MRT der Lunge: Optimierte Berechnung von Differenzbildern

BACKGROUND

In oxygen-enhanced lung MRI, difference maps of acquisitions during inhalation of room air and pure oxygen are calculated to assess lung function. The purpose of this study was to analyze how the calculation of these difference maps depends on the delayed signal change after switching the gas supply.

METHODS

Ten healthy volunteers were examined with an ECG and respiratory-triggered T1-weighting inversion recovery HASTE sequence with parallel imaging. Four blocks with 20 repetitions of up to 6 coronal slices were continuously acquired; in blocks 1 and 3 room air was supplied, in blocks 2 and 4 oxygen. Data were postprocessed, discarding between 0 and 19 repetitions after each change of gas supply before calculating the relative signal difference.

RESULTS

The averaged relative signal difference increases from 9.4 to 17.4% when the number of discarded acquisitions increases; the ratio of signal difference and spatial standard deviation reaches a maximum at 5-8 discarded acquisitions.

CONCLUSIONS

An optimized ratio of signal difference and statistical error is found if about 5-8 of 20 respiratory-triggered repetitions are discarded after each change of gas supply for the calculation of difference maps.

Assessment of pulmonary air trapping and obstruction in expiration: an experimental MRI study

PURPOSE

The aim of this experimental study was to evaluate the potential of a simple expiration technique by means of magnetic resonance imaging (MRI) in an animal model to detect pulmonary air-trapping areas after artificial bronchial obstruction.

MATERIAL AND METHODS

Sixteen pigs were evaluated by means of a modified T1-weighted FLASH with fat saturation in respiratory arrest (TR=4.6 ms, TE=1.8 ms, alpha=10 degrees, S.D.=3-5 mm). A measurement of the signal intensity (SI) in the peripheral lung tissue was made in both inspiration and expiration before and after inhalation of 2 ml of 0.5% acetylcholine to simulate a bronchial obstruction. A final measurement of the lung SI was also made after bronchospasmolytic induction through salbutamol (beta2-mimetic bronchodilator).

RESULTS

In expiration, a mean SI increase in peripheral lung tissue of about 183% was seen in comparison to inspiration (mean SI increase of 11-32). After inhalation of 0.5% acetylcholine, the expirational signal increase in peripheral lung tissue was only 114% of the original SI. The expirational signal homogeneity decreased after inhalation of acetylcholine. After inhalation of salbutamol, the lung tissue signal elevation in expiration was 193%.

CONCLUSION

We interpret the low expiratory signal elevation after acetylcholine inhalation as a result of an air-trapped bronchial constriction in certain areas. The simple expiratory technique in an animal model showed that it is suitable to demonstrate obstructive air trapping using MRI.

Fast oxygen-enhanced multislice imaging of the lung using parallel acquisition techniques

The purpose of this study was to evaluate an optimized multislice acquisition technique for oxygen-enhanced MRI of the lung using slice-selective inversion and refocusing pulses in combination with parallel imaging. An inversion recovery HASTE sequence was implemented with respiratory triggering to perform imaging in end-expiration and with ECG triggering to avoid image acquisition during the systolic phase. Inversion pulses and the readout of echo trains could be interleaved to decrease acquisition time. The sequence was evaluated in 15 healthy volunteers, comparing three acquisition schemes: (1) acquisition of four slices without parallel imaging; (2) acquisition of four slices with parallel imaging; (3) acquisition of six slices with parallel imaging. These multislice acquisitions were repeated 80 times with alternating inhalation of room air and oxygen. The oxygen-induced signal increase showed no significant difference with and without parallel imaging. However, only with parallel imaging did the interleaved acquisition of six or more slices become possible, thus enabling a more complete anatomic coverage of the lung. The average required end-expiration time per repetition to acquire six slices could be significantly reduced from 4112 ms without to 2727 ms with parallel imaging. Total acquisition time varied between 8 and 13 min depending on the respiratory frequency.

Impact of lung volume on MR signal intensity changes of the lung parenchyma

PURPOSE

To test the hypothesis that, in magnetic resonance (MR) imaging of healthy individuals, equal relative changes in lung volume cause equal relative changes in MR signal intensity of the lung parenchyma.

MATERIALS AND METHODS

In two experimental runs, 10 volunteers underwent spirometrically monitored MR imaging of the lungs, with MR images acquired at 10 incremental lung volumes ranging from total lung capacity to 10% above residual volume. Average signal intensity, signal variability, and signal intensity integrals were calculated for each volunteer and for each lung volume. The effect of lung volume on signal intensity was quantified using linear regression analysis complemented by the runs test. Slopes and intercepts of regression lines were compared with an analysis of covariance. Slopes of the lines of best fit for lung volumes and signal intensities from the two runs were compared to the slope of the line of identity. Comparisons between the two runs were visualized using Bland and Altman plots.

RESULTS

The slopes of the 10 individual regression lines yielded no significant differences (F = 1.703, P = 0.101; F = 1.321, P = 0.239). The common slopes were -0.556 +/- 0.027 (P = 0.0001) for the first and -0.597 +/- 0.0031 (P = 0.0001) for the second experimental run. Both slopes displayed no significant nonlinearity (P = 0.419 and P = 0.067). There was a strong association between changes in lung volumes (rs = 0.991, P = 0.0001) and changes in signal intensity (rs = 0.889, P = 0.0001) in the two experimental runs. Lines of best fit for lung volume and signal intensities were not significantly different from the slope of the line of identity (P = 0.321 and P = 0.212, respectively).

CONCLUSION

Equal changes in lung volume cause equal changes in MR signal intensity of the lung parenchyma. This linear and reproducible phenomenon could be helpful in comparing pulmonary MR signal intensity between individuals.

T1 mapping of the entire lung parenchyma: Influence of the respiratory phase in healthy individuals

PURPOSE

To determine the effect of respiratory phase on the T1 values of the entire lung.

MATERIALS AND METHODS

We calculated T1 relaxation time on a pixel-by-pixel analysis of a series of Snapshot FLASH tomograms acquired after a single inversion pulse. Two sets of coronal T1 maps were acquired, one in full suspended inspiration and one in suspended expiration.

RESULTS

The average of the mean T1 values over the entire lung in inspiration was 1199+/-117 msec, the average of the mean T1 values in expiration was 1333+/-167 msec. This difference was statistically significant, at P=0.005. In inspiration the average coefficient of variation was 182+/-56 msec, in expiration it was 153+/-41 msec. This difference was not statistically significant.

CONCLUSION

T1 measurements of the entire lung are feasible. Our findings underline the need for a close monitoring of the inspirational status during MR examinations of the thorax, notably when T1 measurements of the lung are performed.

Magnetization transfer short inversion time inversion recovery enhanced 1H MRI of the human lung

The unique characteristics of the human lung arising from low proton density and multiple air-tissue interfaces of the alveoli cause difficulty in 1H lung magnetic resonance imaging. In addition, the dominating signal from sources such as the thoracic muscle and subcutaneous fat hampers the visualization of the lung parenchyma. In this contribution, an efficient tissue suppression technique is presented which allows one to significantly enhance lung parenchyma visibility. A short inversion time inversion recovery (STIR) experiment combined with a magnetization transfer (MT) experiment was used for magnetization preparation in order to suppress the signal from muscle. A half-Fourier single-shot turbo spin-echo sequence was used as acquisition module. This approach was used to perform lung anatomical imaging in eight healthy human subjects and five patients with cystic fibrosis. The results obtained demonstrate that with MT-STIR approach high quality human lung images can be obtained and that this approach has the potential for the evaluation of lung pathologies.

Temporal dynamics of blood flow effects in half-Fourier fast spin echo (1)H magnetic resonance imaging of the human lungs

A cardiac-triggered half-Fourier single-shot turbo spin echo (HASTE) technique is currently the method of choice for MR imaging of the lung parenchyma without the use of exogenous contrast agents. In this study, we specifically examined the effects of the cardiac cycle on the HASTE signal intensity of the lungs. Images were obtained from six healthy human volunteers at an end expiration breath-hold using a HASTE sequence and a variable cardiac-triggered delay time. Analysis of the data sets showed a 30% decrease in the lung signal intensity during systole, and a 15% decrease during mid-diastole. These decreases correlate with phases of the cardiac cycle when the blood flow in the lungs is expected to be greatest. Misregistration artifacts, particularly from the pulmonary arteries and aorta, are worse during these periods of signal decrease. To minimize cardiac dependent signal losses, HASTE lung imaging should be performed after systole but before rapid filling of the ventricles.