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Temporal and spatial evaluation of pulmonary blood flow using multiple delay PCASL at 1.5 Tesla

Ferdinand Seith¹, Rolf Pohmann², Martin Schwartz^3,4, Thomas Küstner^3,4, Klaus Scheffler^2,5, Konstantin Nikolaou¹, Fritz Schick³, and Petros Martirosian³

¹Department of Diagnostic and Interventional Radiology, University of Tübingen, Tübingen, Germany, ²Max Planck Institute for Biological Cybernetics, Tübingen, Germany, ³Section on Experimental Radiology, University of Tübingen, Tübingen, Germany, ⁴Institute of Signal Processing and System Theory, University of Stuttgart, Stuttgart, Germany, ⁵Department of Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany

Synopsis

Pseudo-continuous-arterial-spin-labeling (PCASL) has been successfully applied in abdominal organs to image organ perfusion. The aim of this work was to evaluate the pulmonary blood flow in dependence on the cardiac cycle using PCASL at 1.5T. Labeling of pulmonary blood flow was achieved by ECG triggering and an labeling plane perpendicular to the pulmonary trunk (tagging duration 300ms). In five volunteers, eight measurements were acquired with fast True-FISP imaging (in-plane-resolution, 2.5×2.5mm², coronal view) with post-labeling delays between 100 and 1500ms. The PCASL-True-FISP technique was able to precisely assess blood flow of pulmonary arteries, as well as perfusion of the lung parenchyma.

Introduction and Purpose

The evaluation of the regional blood flow of pulmonary vessels and lung parenchyma in various phases of the cardiac cycle with high spatial resolution can be of importance for the clinical diagnosis of diseases affecting e.g. the pulmonary arteries or the lung interstitium. Arterial spin labeling (ASL) MRI techniques have been used for non-invasive evaluation of perfusion in brain as well as in many extracranial organs such as kidney¹, liver², and lung³. Moreover, temporal and spatial development of the distribution of tagged blood in an organ can be characterized by a variation of the delay time between blood preparation and image recording. Bolar et al.⁴ applied a multiple delay ASL to evaluate the entire magnetic bolus delivery curve in the lung and found out a strong cardiac-cycle dependence of pulmonary blood flow. They used a pulsed ASL approach and a single-shot turbo spin echo sequence for data acquisition. Recently, a combination of PCASL of pulmonary arteries and True-FISP imaging provided high-quality perfusion images of the lung at 3T.⁵ However, a prerequisite for imaging of lung perfusion by MRI is the capability of deriving a measurable signal from the lung parenchyma, which is hampered by extremely heterogeneous magnetic susceptibility of this tissue.⁶The total ASL signal might increased by measuring at low static magnetic field strength. The main goal of this work was to assess the potential of muti-delay PCASL with True-FISP data acquisition to measure the temporal and spatial development of pulmonary blood flow at 1.5T.

Methods

Measurements were performed on a 1.5T MR scanner (AvantoFit, Siemens Healthcare, Erlangen, Germany). Five healthy volunteers (32±10, one female) were examined using a PCASL sequence⁷ with a True-FISP imaging module. Eight measurements with post labeling delays (PLDs) between 100 and 1500 ms were conducted with the tagging duration of 300 ms, tagging flip angle of 25° and gradient strength 7 mT/m. The tagging plane was placed nearly perpendicular to the pulmonary trunk allowing simultaneous perfusion imaging of both lungs (Figure 1A). The tagging pulse was applied during the systolic period by ECG-triggering and the data acquisition was started after PLD time (Figure 1B). The True-FISP sequence was adapted to achieve short TE/TR (0.9/2.1 ms) using following parameters: flip angle, 70°; slice thickness, 10 mm; in-plane resolution, 2.5×2.5 mm²; partial Fourier, 0.75; matrix size, 144×192; readout bandwidth, 1260 Hz/Pixel. Ten label-control image pairs were acquired with a repetition delay of 5 s by employing a timed breathing protocol. A proton-density weighted True-FISP image was also acquired at the start of the sequence to estimate the M_0B of the blood. The overall measurement time for eight measurements was 17:25 min. PCASL images were processed by self-written code in MATLAB (The MathWorks, Inc., Natick, MA, USA). Free-hand regions of interest (ROIs) were carefully placed in the small pulmonary arteries, a large pulmonary arteries and the lung parenchyma to assess the development of signal intensity in the perfusion images with different post labeling delays (see Figure 2).

Results

A series of perfusion-weighted images (ΔM/M_0B) of the lung of a 29 year-old female healthy volunteer using PCASL with PLD values ranging from 100 ms to 1500 ms is depicted in Figure 3A. It is obvious that large blood vessels in the lung, containing considerable amounts of tagged blood, are already visible at short PLDs, whereas the perfusion signal from lung parenchyma is first increased at PLD of 900 ms. The temporal perfusion signal development in lung parenchyma as well as in small and large pulmonary arteries are shown in Figure 3B. The time point of the signal increasing in the lung parenchyma is nearly corresponded to the cardiac cycle of the examined subject (approx. 1000 ms). In Figure 4A, perfusion-weighted images of a 26 year-old male healthy volunteer with cardiac cycle of approx. 700 ms are displayed. The perfusion signal time course (Figure 4B) reveals an increasing of the lung parenchymal signal at PLDs of approx. 400 and 1300 ms, which is in good agreement with a shorter cardiac cycle of this subject.

Discussion

We could demonstrate that ECG-triggered PCASL-True-FISP imaging at 1.5T has the potential to precisely image the perfusion-related signal of pulmonary arteries and lung parenchyma in dependence on the cardiac cycle. The combination of the PCASL sequence with fast True-FISP imaging provides perfusion images of the lung in high spatial resolution and with high signal intensity without the application of contrast media. Further prospective studies in patients are needed to evaluate the robustness of this technique in clinical routine.

Acknowledgements

No acknowledgement found.

References

1. Robson PM et al. Strategies for reducing respiratory motion artifacts in renal perfusion imaging with arterial spin labeling. Magn Reson Med 2009;61:1374–1387.

2. Schalkx HJ, et al. Arterial and portal venous liver perfusion using selective spin labelling MRI. Eur Radiol. 2015;25:1529-1540.

3. Mai VM, Berr SS. MR perfusion imaging of pulmonary parenchyma using pulsed arterial spin labeling techniques: FAIRER and FAIR. J Magn Reson Imaging 1999;9:483–487.

4. Bolar DS et al. Quantification of Regional Pulmonary Blood Flow Using ASL-FAIRER. Magn Reson Med 2006;55:1308–1317.

5. Martirosian P et al. Measurement of lung perfusion using optimized pseudo-continuous arterial spin labeling of pulmonary arteries and fast True-FISP imaging at 3 Tesla. In Proc. Intl Soc Mag Reson Med 2017, p. 1889.

6. Bieri O. Ultra-fast steady state free precession and its application to in vivo 1H morphological and functional lung imaging at 1.5 Tesla. Mag Reson Med 2013;70:657–663.

7. Pohmann R et al. Theoretical and experimental evaluation of continuous arterial spin labeling techniques. Magn Reson Med 2010;63:438–446.

Figures

Figure 1: (A) Positioning of labeling and imaging plane using the PCASL sequence on the sagittal and coronal images acquired with ECG-triggered dark-blood single-shot TSE sequence. The labeling plane was positioned nearly perpendicular to the pulmonary trunk. Perfusion images were acquired in coronal orientation. (B) Time course of ECG-triggered pCASL sequence: The tagging duration (TD) was limited to the systolic period and the imaging was performed at a PLD time. Eight measurements were conducted with PLDs of 100, 300, 500, 700, 900, 1100, 1300, and 1500 ms.

Figure 2: PCASL images of a 29 year-old healthy female volunteer acquired in multiple breath-holds with TD of 300ms, tagging FA of 25° and PLDs in the range of 100-1500ms. (A) Proton density weighted True-FISP image. For calculation of blood M_0B, the signal from a white ROI in aorta was used. Perfusion-weighted, ΔM/M_0B, images of the lung obtained at PLD of (B) 700ms and (C) 1300ms right and left lung. The three ROIs are representative of those used for perfusion signal evaluation of the lung (see Figure 3 and 4): red for the parenchyma, green for small pulmonary arteries and blue for large pulmonary arteries.

Figure 3: (A) Perfusion-weighted images, ΔM/M_0B, of a 29 year-old healthy female volunteer (with cardiac cycle of approx. 1000 ms) acquired at different PLDs. (B) Temporal development of normalized perfusion signal obtained from ROIs placed in the lung parenchyma (red line), small pulmonary artery (green line) and large pulmonary artery (blue line) at different PLDs.The time point of the signal increasing in the lung parenchyma is nearly corresponded to the cardiac cycle.

Figure 4: (A) Perfusion-weighted images, ΔM/M_0B, of a 26 year-old healthy male volunteer (with cardiac cycle of approx. 700 ms) acquired at different PLDs. (B) Temporal development of normalized perfusion signal obtained from ROIs placed in the lung parenchyma (red line), small pulmonary artery (green line) and large pulmonary artery (blue line) at different PLDs. The perfusion signal time course reveals an increasing of the lung parenchymal signal at PLDs of approx. 400 and 1300 ms, which is in good agreement with cardiac cycle of this subject.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)

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