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

Reduced respiratory motion artifacts using structural similarity in fast 2D dynamic contrast enhanced MRI of liver lesions

The purpose of this work was to improve dynamic contrast enhanced MRI (DCE-MRI) of liver lesions by removing motion corrupted images as identified by a structural similarity (SSIM) algorithm, and to assess the effect of this correction on the pharmacokinetic parameter K^trans using automatically determined arterial input functions (AIFs). Fifteen patients with colorectal liver metastases were measured twice with a T₁ weighted multislice 2D FLASH sequence for DCE-MRI (time resolution 1.2 s). AIFs were automatically derived from contrast inflow in the aorta of each patient. Thereafter, SSIM identified motion corrupted images of the liver were removed from the DCE dataset. From this corrected data set K^trans and its reproducibility were determined. Using the SSIM algorithm a median fraction of 46% (range 37-50%) of the liver images in DCE time series was labeled as motion distorted. Rejection of these images resulted in a significantly lower median K^trans (p < 0.05) and lower coefficient of repeatability of K^trans in liver metastases compared with an analysis without correction. SSIM correction improves the reproducibility of the DCE-MRI parameter K^trans in liver metastasis and reduces contamination of K^trans values of lesions by that of surrounding normal liver tissue.

Magnetic resonance imaging of pediatric lung parenchyma, airways, vasculature, ventilation, and perfusion: state of the art

Magnetic resonance (MR) imaging is a noninvasive imaging modality, particularly attractive for pediatric patients given its lack of ionizing radiation. Despite many advantages, the physical properties of the lung (inherent low signal-to-noise ratio, magnetic susceptibility differences at lung-air interfaces, and respiratory and cardiac motion) have posed technical challenges that have limited the use of MR imaging in the evaluation of thoracic disease in the past. However, recent advances in MR imaging techniques have overcome many of these challenges. This article discusses these advances in MR imaging techniques and their potential role in the evaluation of thoracic disorders in pediatric patients.

A method of rapid robust respiratory synchronization for MRI

Respiratory motion degrades MRI exams. Adequate detection of respiratory motion with pneumatic respiratory belts in small children is challenging and time-consuming.

Analysis of signal dynamics in oxygen-enhanced magnetic resonance imaging

OBJECTIVES

Oxygen-enhanced MRI (O2-MRI) is frequently based on a block paradigm consisting of a series of consecutive T1-weighted scans acquired during alternating blocks with inhalation of room air and of pure oxygen. This design results in a complex signal-time course for each pixel, which displays the oxygen wash-in and wash-out processes and provides spatially resolved information about the lung function. The purpose of the present study was to optimize the signal-time-course analysis to extract (pixelwise) the maximum amount of information from the acquired data, and to introduce an appropriate cross-correlation approach for data sets containing the oxygen wash-in and wash-out periods.

MATERIALS AND METHODS

O2-MRI data of 11 healthy volunteers were acquired with a multislice inversion-recovery single-shot turbo-spin-echo sequence at 1.5 Tesla; lung and spleen were manually segmented on all 44 acquired slices. Six different model functions were pixelwise fitted to the data and compared using the Akaike information criterion. Four different reference functions were compared for cross-correlation analysis.

RESULTS

The optimal model function is a piecewise exponential function (median enhancement in lung/spleen: 16.3%/14.8%) with different time constants for wash-in (29.4 seconds/72.7 seconds) and wash-out (25.1 seconds/29.6 seconds). As a new parameter, it contains the delay between switching the gas supply and the onset of the signal change (4.8 seconds/24.5 seconds). Optimal cross-correlation results were obtained with a piecewise exponential reference function, which was temporally shifted to maximize the correlation, yielding median correlation coefficients of 0.694 and 0.878, median time delays of 7.5 seconds and 38.6 seconds, and median fractions of oxygen-activated pixels of 83.6% and 92.2% in the lung and the spleen, respectively.

CONCLUSIONS

It was demonstrated that the pixelwise assessment of O2-MRI data are optimally performed with piecewise exponential functions. Cross-correlation analysis with a piecewise exponential reference function results in significantly higher fractions of oxygen-activated pixels than with rectangular functions.

[Oxygen-enhanced functional MR lung imaging]

Current diagnostic tools for the assessment of lung function are limited by global measurements or the need for radioactive tracers. Ideally, these tools should allow quantitative, regional distinct analyses without exposure to radiation. The current paper presents oxygen-enhanced functional MRI for assessment of lung ventilation. First applied in humans in 1996, a considerable amount of experience is now available on 1.5T scanners. The generation of quantitative T1-maps shows a high clinical potential. Low-field MR scanners, which are mostly open-designed, are especially interesting for functional lung imaging. The open design has advantages in respect to patient comfort by lower noise production and easy access to the patients and the costs are lower (no need for helium cooling). Lower signal-to-noise ratios can be overcome by changing the relaxation times. New navigator techniques allow further compensations. This article focuses on the presentation of low-field scanners and the application of T1 and T2(*) maps is described for healthy volunteers and first patients.

Lung imaging under free-breathing conditions

Respiratory motion and pulsatile blood flow can generate artifacts in morphological and functional lung imaging. Total acquisition time, and thus the achievable signal to noise ratio, is limited when performing breath-hold and/or electrocardiogram-triggered imaging. To overcome these limitations, imaging during free respiration can be performed using respiratory gating/triggering devices or navigator echoes. However, these techniques provide only poor gating resolution and can induce saturation bands and signal fluctuations into the lung volume. In this work, acquisition schemes for nonphase encoded navigator echoes were implemented into different sequences for morphological and functional lung imaging at 1.5 Tesla (T) and 0.2T. The navigator echoes allow monitoring of respiratory motion and provide an ECG-trigger signal for correction of the heart cycle without influencing the imaged slices. Artifact free images acquired during free respiration using a 3D GE, 2D multislice TSE or multi-Gradient Echo sequence for oxygen-enhanced T(2)(*) quantification are presented.

Oxygen-enhanced magnetic resonance imaging: influence of different gas delivery methods on the T1-changes of the lungs

OBJECTIVE

The clinical feasibility of oxygen-enhanced magnetic resonance imaging (MRI) of the lung may benefit from the use of a simple gas delivery method. In this study, the oxygen-induced T1 change of the lung obtained using a closed O(2) delivery system was compared with that obtained by a conventional nontight face mask.

MATERIAL AND METHODS

Twenty-three healthy subjects (15 men, 8 women, mean age = 25 years, age range = 20-35 years) underwent oxygen-enhanced MRI of the lung using a closed O(2) delivery system composed by a tightly fitting face mask and a 60-L reservoir bag (equipment type A: n = 13, 9 men, 4 women, mean age = 24.4 years, age range = 20-32 years), or a clinically available nontight face mask (equipment type B: n = 10; 6 men, 4 women, mean age = 25.8 years, age range = 20-35 years). The effect of 100%-oxygen inhalation was assessed using a Snapshot FLASH T1-mapping technique (repetition time/echo time = 1.5-1.6/0.56 milliseconds; matrix = 128 x 90; acquisition time = 3.3-3.7 seconds; slice thickness = 15-20 mm; number of images = 40). By nonlinear curve fitting, the mean T1 values of the left and right lung at room air and 100%-oxygen ventilation were calculated (T1(room air, right); T1(oxygen, right); T1(room air, left); T1(oxygen, left)). The average T1 differences (DeltaT1 = T1(room air) - T1(oxygen)) of the 2 volunteer groups were compared (Wilcoxon signed rank test, Mann-Whitney U test).

RESULTS

The mean T1 values obtained using the 2 respiratory equipments at room air or oxygen ventilation were not significantly different (A vs. B at room air ventilation: P = 0.85 for the right lung, P = 0.27 for the left lung; A vs. B at oxygen ventilation: P = 0.55 for the left lung, P = 0.29 for the right lung). With both systems, the mean T1 values decreased significantly after oxygen inhalation (P = 0.03-0.0002). For both lungs, the DeltaT1 obtained using the equipment type A was statistically equivalent to that obtained using the equipment type B: DeltaT1A, right = 96 +/- 19 milliseconds versus DeltaT1B, right = 97 +/- 34 milliseconds (P = 0.82); DeltaT1A, left = 74 +/- 47 milliseconds versus DeltaT1B, left = 68 +/- 63 milliseconds (P = 0.85).

CONCLUSION

Gas delivery in oxygen-enhanced MRI of the lung can be performed with a clinically available standard face mask, without the need for closed sophisticated equipments.

Advances in magnetic resonance (2007)

Advances in clinical magnetic resonance (MR) are discussed in this review in the context of publications from Investigative Radiology during 2006 and 2007. The articles relevant to this topic, published during this 2 year time period, are considered as organized by anatomic region. An additional final focus of discussion is in regards to those studies involving MR contrast media.

Basics concepts and clinical applications of oxygen-enhanced MR imaging

Oxygen-enhanced MR imaging is a new technique, and its physiological significance has not yet been fully elucidated. This review article covers (1) the theory of oxygen enhancement and its relationship with respiratory physiology; (2) design for oxygen-enhanced MR imaging sequencing; (3) a basic study of oxygen-enhanced MR imaging in animal models and humans; (4) a clinical study of oxygen-enhanced MR imaging; and (5) a comparison of advantages and disadvantages of this technique with those of hyperpolarized noble gas MR ventilation imaging. Oxygen-enhanced MR imaging provides not only the ventilation-related, but also respiration-related information. Oxygen-enhanced MR imaging has the potential to replace nuclear medicine studies for the identification of regional pulmonary function, and many investigators are now attempting to adapt this technique for routine clinical studies. We believe that further basic studies as well as clinical applications of this new technique will define the real significance of oxygen-enhanced MR imaging for the future of pulmonary functional imaging and its usefulness for diagnostic radiology and pulmonary medicine.

Impact of Oxygen Inhalation on the Pulmonary Circulation

PURPOSE

Oxygen-enhanced magnetic resonance (MR)-ventilation imaging of the lung is based on the inhalation of a high concentration of oxygen (hyperoxia). However, the effect of hyperoxia on the pulmonary circulation is not yet fully understood. In this study the impact of hyperoxia on the pulmonary circulation was evaluated.

MATERIALS AND METHODS

Ten healthy volunteers were examined in a 1.5 T MRI system with contrast-enhanced perfusion MRI (saturation recovery 2D turboFLASH) of the lung and phase-contrast flow measurements in the pulmonary trunk. Both measurements were performed breathing room air (RA) and, subsequently, 100% oxygen (15 L/min) (O(2)).

RESULTS

The perfusion measurements showed a significant difference between RA and O(2) for the pulmonary blood flow (181 vs. 257 mL/min/100 mL, P = 0.04) and blood volume (14 vs. 21 mL/100 mL, P = 0.008). The mean transit time of the contrast bolus was not changed (P = 0.4) in the dorsal part of the lung, whereas it was significantly prolonged (P = 0.006) in the central part. The mean heart rate during flow measurements breathing RA (67 +/- 11 beats/min) and O(2) (61 +/- 12 beats/min) were not significantly different (P = 0.055). The average cardiac output (pulmonary trunk) was not significantly lower while breathing O(2) (RA: 5.9 vs. O(2): 5.5 L/min, P = 0.054).

CONCLUSION

Hyperoxia causes a significant increase and redistribution of the pulmonary perfusion, whereas it leads to a not significant decrease in cardiac output. Thus, for MR-perfusion and MR-flow measurements oxygen inhalation should be avoided, if possible. In the context of oxygen-enhanced MR-ventilation imaging of the lung the contribution of this effect needs to be further evaluated.

Quantitative regional oxygen transfer imaging of the human lung

PURPOSE

To demonstrate that the use of nonquantitative methods in oxygen-enhanced (OE) lung imaging can be problematic and to present a new approach for quantitative OE lung imaging, which fulfills the requirements for easy application in clinical practice.

MATERIALS AND METHODS

A total of 10 healthy volunteers and three non-small-cell lung cancer (NSCLC) patients were examined using a 1.5T scanner. OE imaging was performed using a snapshot fast low-angle shot (FLASH) T(1)-mapping technique (TE = 1.4 msec, TR = 3.5 msec) as well as a series of T(1)-weighted inversion recovery (IR) half- Fourier acquisition single-shot turbo spin-echo (HASTE) (TE(effective) = 43 msec, TE(inter) = 4.2 msec, and inversion time [TI] = 1200 msec) images. Semiquantitative relative signal enhancement ratios (RER) of T(1)-weighted images before and after inhalation of oxygen-enriched gas were compared to the quantitative change in T(1). A hybrid method is proposed that combines the advantages of T(1)-weighted imaging with the quantification provided by T(1)-mapping. To this end, the IR-HASTE images were transformed into quantitative parameter maps. To prevent mismatching and incorrect parameter maps, retrospective image selection was performed using a postprocessing navigator technique.

RESULTS

The RER was dependent on the intrinsic values of T(1) in the lung. Quantitative parameters, such as the decrease of T(1) after switching the breathing gas, were more suited to oxygen transfer quantification than to relative signal enhancement. The mean T(1) value during inhalation of room air (T(1,room)) for the volunteers was 1260 msec. This value decreased by about 10% after switching the breathing gas to carbogen. For the patients, the mean T(1,room) value was 1182 msec, which decreased by about 7% when breathing carbogen. The parameter maps generated using the proposed hybrid method deviated, on average, only about 1% from the T(1)-maps.

CONCLUSION

For the purpose of intersubject comparison, OE lung imaging should be performed quantitatively. The proposed hybrid technique produced reliable quantitative results in a short amount of time and, therefore, is suited for clinical use.

Advances in Magnetic Resonance (2006)

Advances in the field of magnetic resonance (MR) as it pertains to clinical diagnostic radiology are examined in this review on the basis of publications in Investigative Radiology over the past 2 years (2005-2006). The articles published during that timeframe are discussed, organizationally wise, by anatomic region with an additional focus on studies involving MR contrast media.