Eberhard D. Pracht

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.

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.

MRI thermometry: Fast mapping of RF‐induced heating along conductive wires

Conductive implants are in most cases a strict contraindication for MRI examinations, as RF pulses applied during the MRI measurement can lead to severe heating of the surrounding tissue. Understanding and mapping of these heating effects is therefore crucial for determining the circumstances under which patient examinations are safe. The use of fluoroptic probes is the standard procedure for monitoring these heating effects. However, the observed temperature increase is highly dependent on the positioning of such a probe, as it can only determine the temperature locally. Temperature mapping with MRI after RF heating can be used, but cooling effects during imaging lead to a significant underestimation of the heating effect. In this work, an MRI thermometry method was combined with an MRI heating sequence, allowing for temperature mapping during RF heating. This technique may provide new opportunities for implant safety investigations. Magn Reson Med, 2008.

Multimodal MEMPRAGE, FLAIR, and R2* Segmentation to Resolve Dura and Vessels from Cortical Gray Matter

While widely in use in automated segmentation approaches for the detection of group differences or of changes associated with continuous predictors in gray matter volume, T1-weighted images are known to represent dura and cortical vessels with signal intensities similar to those of gray matter. By considering multiple signal sources at once, multimodal segmentation approaches may be able to resolve these different tissue classes and address this potential confound. We explored here the simultaneous use of FLAIR and apparent transverse relaxation rates (a signal related to T2* relaxation maps and having similar contrast) with T1-weighted images. Relative to T1-weighted images alone, multimodal segmentation had marked positive effects on 1. the separation of gray matter from dura, 2. the exclusion of vessels from the gray matter compartment, and 3. the contrast with extracerebral connective tissue. While obtainable together with the T1-weighted images without increasing scanning times, apparent transverse relaxation rates were less effective than added FLAIR images in providing the above mentioned advantages. FLAIR images also improved the detection of cortical matter in areas prone to susceptibility artifacts in standard MPRAGE T1-weighted images, while the addition of transverse relaxation maps exacerbated the effect of these artifacts on segmentation. Our results confirm that standard MPRAGE segmentation may overestimate gray matter volume by wrongly assigning vessels and dura to this compartment and show that multimodal approaches may greatly improve the specificity of cortical segmentation. Since multimodal segmentation is easily implemented, these benefits are immediately available to studies focusing on translational applications of structural imaging.

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.