Johannes F.T. Arnold

Oxygen-enhanced proton imaging of the human lung using T2

Magnetic susceptibility gradients caused by tissue/air interfaces lead to very short T2* times in the human lung. These susceptibility gradients are dependent on the magnetic susceptibility of the respiratory gas and therefore should influence T2* relaxation. In this work, a technique for quantitative T2* mapping of the human lung during one breath hold is presented. Using this method, the lung T2* relaxation time was measured under normoxic (room air, 21% O2) and hyperoxic (100% O2) conditions to verify this assumption. The mean T2* difference between room air and 100% O2 is about 10% and contains ventilation information, since only ventilated regions contribute to signal change due to different susceptibility gradients. Magn Reson Med 53:1193–1196, 2005.

Imaging lung function using rapid dynamic acquisition of T1-maps during oxygen enhancement

This paper describes imaging of lung function with oxygen-enhanced MRI using dynamically acquired T1 parameter maps, which allows an accurate, quantitative assessment of time constants of T1-enhancement and therefore lung function. Eight healthy volunteers were examined on a 1.5-T whole-body scanner. Lung T1-maps based on an IR Snapshot FLASH technique (TE = 1.4 ms, TR = 3.5 ms, FA = 7 ∘) were dynamically acquired from each subject. Without waiting for full relaxation between subsequent acquisition of T1-maps, one T1-map was acquired every 6.7 s. For comparison, all subjects underwent a standard pulmonary function test (PFT). Oxygen wash-in and wash-out time course curves of T1 relaxation rate (R1)-enhancement were obtained and time constants of oxygen wash-in (win) and wash-out (wout) were calculated. Averaged over the whole right lung, the mean wout was 43.90 ± 10.47 s and the mean (win) was 51.20 ± 15.53 s, thus about 17% higher in magnitude. Wash-in time constants correlated strongly with forced expired volume in one second in percentage of the vital capacity (FEV1 % VC) and with maximum expiratory flow at 25% vital capacity (MEF25), whereas wash-out time constants showed only weak correlation. Using oxygen-enhanced rapid dynamic acquisition of T1-maps, time course curves of R1-enhancement can be obtained. With win and wout two new parameters for assessing lung function are available. Therefore, the proposed method has the potential to provide regional information of pulmonary function in various lung diseases.

Quantitative regional oxygen transfer imaging of the human lung.

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.

Quantitative and O2 Enhanced MRI of the Pathologic Lung: Findings in Emphysema, Fibrosis, and Cystic Fibrosis

Purpose: beyond the pure morphological visual representation, MR imaging offers the possibility to quantify parameters in the healthy, as well as, in pathologic lung parenchyma. Gas exchange is the primary function of the lung and the transport of oxygen plays a key role in pulmonary physiology and pathophysiology. The purpose of this review is to present a short overview of the relaxation mechanisms of the lung and the current technical concepts of T1 mapping and methods of oxygen enhanced MR imaging. Material and Methods: molecular oxygen has weak paramagnetic properties so that an increase in oxygen concentration results in shortening of the T1 relaxation time and thus to an increase of the signal intensity in T1 weighted images. A possible way to gain deeper insights into the relaxation mechanisms of the lung is the calculation of parameter Maps. T1 Maps based on a snapshot FLASH sequence obtained during the inhalation of various oxygen concentrations provide data for the creation of the so-called oxygen transfer function (OTF), assigning a measurement for local oxygen transfer. T1 weighted single shot TSE sequences also permit expression of the signal changing effects associated with the inhalation of pure oxygen. Results: the average of the mean T1 values over the entire lung in inspiration amounts to 1199 +/− 117 milliseconds, the average of the mean T1 values in expiration was 1333 +/− 167 milliseconds. T1 Maps of patients with emphysema and lung fibrosis show fundamentally different behavior patterns. Oxygen enhanced MRT is able to demonstrate reduced diffusion capacity and diminished oxygen transport in patients with emphysema and cystic fibrosis. Discussion: results published in literature indicate that T1 mapping and oxygen enhanced MR imaging are promising new methods in functional imaging of the lung and when evaluated in conjunction with the pure morphological images can provide additional valuable information.

Lung MRI using an MR-compatible active breathing control (MR-ABC)

This work introduces an MR‐compatible active breathing control device (MR‐ABC) that can be applied to lung imaging. An MR‐ABC consists of a pneumotachograph for respiratory monitoring and an airway‐sealing unit. Using an MR‐ABC, the subjects were forced to suspend breathing for short time intervals, which were used in turn for data acquisition. While the breathing flow was stopped, data acquisition was triggered by ECG to achieve simultaneous cardiac and respiratory synchronization and thus avoid artifacts from blood flow or heart movement. The flow stoppage allowed a prolonged acquisition window of up to 1.5 sec. To evaluate the potential of an MR‐ABC for segmented k‐space acquisition, diaphragm displacement was investigated in five volunteers and compared with images acquired using breath‐holding, a respiratory belt, and free breathing. Respiratory movement was comparatively low using the breath‐hold approach, a respiratory belt or an MR‐ABC. During free‐breathing diaphragm displacement was comparatively large. To demonstrate the potential of an MR‐ABC, lung MRI was performed using whole‐chest 3D gradient‐echo imaging, multislice turbo spin‐echo (TSE) imaging, and short tau inversion recovery TSE (STIR‐TSE). Cardiorespiratory synchronization was used for each sequence. None of the volunteers reported any discomfort or inconvenience when using an MR‐ABC. Flow stoppage of up to 2.5 sec per breathing cycle was well tolerated, therefore allowing for a reduction of the total imaging time as compared to usage of a respiratory belt or MR navigator. Magn Reson Med, 2007.

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. Magn Reson Med, 2009.

Single‐shot quantitative perfusion imaging of the human lung

The major drawback to quantitative perfusion imaging using arterial spin labeling (ASL) techniques is the need to acquire two images (tag and control), which must be subtracted in order to obtain a perfusion‐weighted image. This can potentially result in misregistration artifacts, especially in lung imaging, due to varying lung inflation levels in different breath‐holds. In this work a double inversion recovery (DIR) imaging technique that yields perfusion‐weighted images of the human lung in a single shot is presented. This technique ensures the complete suppression of background tissue while it preserves signal from the blood. Furthermore, the perfusion‐weighted images and an additional (independent) acquired reference scan can be used to obtain quantitative perfusion information from the lungs. Magn Reson Med, 2006.

Assessment of pulmonary perfusion in a single shot using SEEPAGE.

To present a single‐shot perfusion imaging sequence that does not require contrast agents or a subtraction of a tag and a control image to create the perfusion‐weighted contrast. The proposed method is based on SEEPAGE.

Potential of magnetization transfer MRI for target volume definition in patients with non-small-cell lung cancer.

To develop a magnetization transfer (MT) module in conjunction with a single‐shot MRI readout technique and to investigate the MT phenomenon in non‐small‐cell lung cancer (NSCLC) as an adjunct for radiation therapy planning.

Oxygen-enhanced Proton Magnetic Resonance Imaging of the Human Lung

Publisher Summary This chapter presents the most important developments in 1H magnetic resonance (1H MR) lung imaging using oxygen as contrast agent and highlights the concept and theory of oxygen-enhanced imaging. The use of molecular oxygen as a contrast agent leads to an increase in the oxygen concentration in the alveoli, as well as to an increase in oxygen physically dissolved in arterial blood. An increase in the oxygen concentration leads to changes in the local magnetic field, as oxygen molecules are weakly paramagnetic. Therefore, a change in MR-relaxation parameters of the lung is expected in the presence of different oxygen concentrations. Two main setups that can be used to deliver oxygen to the subjects during oxygen-enhanced (OE) imaging experiments include a mask system and a mouthpiece system. The mask system is more comfortable for the volunteer or patient, because it simply lies on the face. A pressure regulator with which the researcher can control the oxygen concentration and breathing gas flow rate is employed. The signal is detected using a body array, which has the advantage of increased signal-to-noise ratio (SNR) in comparison to a single coil. The oxygen-enhanced images are acquired while the mean pressure of oxygen in the arterial blood is at equilibrium. This avoids wash-in and wash-out effects, which can be seen directly after inhalation gas exchange when the concentration of oxygen in the lungs, and therefore in the blood, changes during the measurements. In dynamic oxygen-enhanced imaging, the images are acquired not only before and well after changing the breathing gas, but also during the time when the partial pressure of oxygen in the blood is not in equilibrium.